FEMTOSECOND STRUCTURAL DYNAMICS

Chapter 1

Femtosecond Condensed Phase Spectroscopy: Structural Dynamics

NIBBERING

2 NIBBERING

FEMTOSECOND STRUCTURAL DYNAMICS 3

FEMTOSECOND CONDENSED PHASE SPECTROSCOPY: STRUCTURAL DYNAMICS

Erik T. J. Nibbering

Max Born Institut fuer Nichtlineare Optik und Kurzzeitspektroskopie, D-12489 Berlin, Germany; email:

nibberin@mbi-berlin.de

Key Words femtosecond optical spectroscopy, ultrafast vibrational spectroscopy, pump-probe spectroscopy,

photon echo spectroscopy, ultrafast photochemistry, femtochemistry, dephasing dynamics, spectral

diffusion, solvation dynamics, time-resolved Stokes shift in fluorescence emission, wavepacket dynamics,

vibrational structural marker mode, anharmonic coupling between vibrational modes, intramolecular

vibrational redistribution (IVR), vibrational cooling Based on a contribution published in Encyclopedia of Modern Optics, Volume 5, R. D. Guenther, D. G. Steel

and L. Bayvel, eds., pp. 253-263 (Elsevier, Oxford, 2004).

ULTRAFAST CHEMISTRY

Chemistry means rearrangement of atoms in/between molecules. Chemical bonds are broken and/or formed,

leading to a change in chemical composition from reactants to products during a reaction. Following the

temporal behaviour of the reaction all the way from reactant – via possible intermediates – towards the product

state(s) amounts to deciphering the molecular processes that lead to these chemical reaction dynamics. Molecular

rearrangements from reactant to product may encompass many orders of magnitude in time (see Fig. 1).

Typically the larger and more involved the rearrangements are, the more time it takes for a molecular species to

reach the final state. Rearrangements of biomolecular structure occur in micro- to milliseconds (spontaneous

folding of small proteins consisting of several hundreds of amino acid units),[1, 2] or longer (DNA multiplication,

protein synthesis in ribosomes).[3, 4] In these conformational dynamics hundreds to thousands of chemical bonds

change their nature. Bimolecular reaction dynamics in liquid solution occur on time scales of nanoseconds or

longer, being controlled by the relatively slow diffusional motions of the reaction partners to each other.[5] Many

elementary processes in chemistry, however, occur on much faster time scales.[6-9] In fact, when one considers

the dynamical event of a single bond rearrangement the relocation of the (relatively few) atoms appear to take

place as ultrafast events. Bond fission,[8, 10-12] hydrogen and proton transfer,[13-20] electron transfer,[21-25] and

cis/trans-isomerizations [26-29] have been found to occur on femto- to picosecond time scales (“femtochemistry” [8, 30-34]). The dynamics of these elementary processes are not only determined by the energy landscapes of the

reacting partners. The energy landscapes (potential energy surfaces) are determined by molecular parameters

4 NIBBERING

such as relative orientation (distances, angles) of the reaction partners.[35-37] the energy levels of the reactant,

intermediate and product species, and – very important – the energy barriers represented by transition states.[38-

40] When considering condensed phase reaction dynamics the important role of the surrounding solvent has been

recognized.[41-45] The solvent shells modulate energy levels of the reactant, intermediate and product states

through electrostatic interactions. In the liquid phase the solvent fluctuations lead through these interactions to

fluctuating energy levels of the reaction partners. In addition, the solvent may facilitate chemical reactions

through energy exchange with the reaction partners, leading to efficient dissipation of excess energy, making

chemical reactions often irreversible. The processes that are extremely dominated by these solvent interactions

(such as molecular collisions,[46, 47] electronic [48-54] and vibrational dephasing,[55] and vibrational relaxation [56-59])

are also often found to occur on femto- to picosecond time scales. The outcome of a bond fission may be a null

effect, when the dissociating fragments are forced by the surrounding solvent to recombine and relax (“cage

effect” [60, 61]).

Spectroscopy has been a prime tool to obtain insight into these key molecular processes. Reactant,

intermediate and products contribute to the spectra with their respective molecular resonances. With spectral

domain spectroscopy one may in principle obtain a precise determination of structural information. Fluctuations

of energy levels of these species by the solvent interactions, and finite lifetimes contribute to a broadening of

these molecular resonances.[62-64] Using steady-state spectroscopy one could try to determine these effects

through a lineshape analysis. [65-67] Using this approach, the results have to be analysed with an a priori defined

model representing the dynamics. The disadvantage lies in the averaging over all time scales inherent in spectral

domain spectroscopy, where details of transient states may be lost in this averaging procedure. In contrast, in

experiments designed to grasp the molecular dynamics in real time the temporal resolution is the prerequisite

COLLISION TIME

IN LIQUIDSMOLECULAR ROTATION

SOLVENTRELAXATION

ELECTRONICDEPHASING

VIBRATIONAL DEPHASING

INTRAMOLECULAR VIBRATIONAL

REDISTRIBUTION

VIBRATIONAL MOTIONINTERNAL CONVERSION

INTERSYSTEM CROSSING

PHYSICS

femto nano10-14 10-13 10-12 10-11 10-10 10-9 10-8 10-7 10-6 10-5 10-4 10-3

pico micro milli

10-15

RADIATIVE DECAY

CHEMISTRYAND BIOLOGY

HYDROGEN AND PROTON TRANSFER

PROTEIN INTERNAL MOTIONS

ELECTRON TRANSFERIN PHOTOSYNTHESIS

TORSIONALDYNAMICS

OF DNA

PHOTODISSOCIATION

PHOTOCHEMICAL ISOMERIZATION

CAGERECOMBINATION

HELIX COIL TRANSITION

PROTEIN FOLDINGTERTIAIRY STRUCTURE

VIBRATIONAL COOLING

ELECTRON TRANSFER

ION PUMPING THROUGH MEMBRANES

ENZYME CATALYSIS

COLLISION TIMEIN LIQUIDS

MOLECULAR ROTATION

SOLVENTRELAXATION

ELECTRONICDEPHASING

VIBRATIONAL DEPHASING

INTRAMOLECULAR VIBRATIONAL

REDISTRIBUTION

VIBRATIONAL MOTIONINTERNAL CONVERSION

INTERSYSTEM CROSSING

PHYSICS

femto nano10-14 10-13 10-12 10-11 10-10 10-9 10-8 10-7 10-6 10-5 10-4 10-3

pico micro milli

10-15

femto nano10-14 10-13 10-12 10-11 10-10 10-9 10-8 10-7 10-6 10-5 10-4 10-3

pico micro milli

10-15

RADIATIVE DECAY

CHEMISTRYAND BIOLOGY

HYDROGEN AND PROTON TRANSFER

PROTEIN INTERNAL MOTIONS

ELECTRON TRANSFERIN PHOTOSYNTHESIS

TORSIONALDYNAMICS

OF DNA

PHOTODISSOCIATION

PHOTOCHEMICAL ISOMERIZATION

CAGERECOMBINATION

HELIX COIL TRANSITION

PROTEIN FOLDINGTERTIAIRY STRUCTURE

VIBRATIONAL COOLING

ELECTRON TRANSFER

ION PUMPING THROUGH MEMBRANES

ENZYME CATALYSIS

Figure 1 Chart showing the ultrafast time scales of fundamental physical and chemical processes (inspired by charts

of G. R. Fleming (1986) [6] and A. H. Zewail (2000) [9]).

FEMTOSECOND STRUCTURAL DYNAMICS 5

UV-VISPump

UV/VISProbe

UV/VISProbe

UV/VISProbe

UV-VISPump

UV-VISProbe

UV-VISProbe UV-VIS

Probe

n

n

1

1

0

n

10

n

1

Figure 2 Ultrafast time-resolved electronic spectroscopy. A UV/vis-pump pulse excited an electronic resonance, and

a probe pulse determines, depending on the tuning in the UV/vis population changes in the reactant ground or excited

states or in the product state.

quantity in observation of these processes. In time-resolved experiments one can trigger a chemical reaction at a

well-defined point in time, after which one can follow the conformational changes from reactant – via possible

intermediates – to the product, and identify any possible transition states along the reaction pathways.

During the last century the technological advances in time-resolved spectroscopy have prompted significant

breakthroughs in the study of chemical reaction dynamics. Initially the temporal resolution was given by the

duration of flashes from light bulbs (“flash” spectroscopy) or by rapid mixing of reaction partners (stopped-flow

technique), as developed by Nobel Laureates Eigen, Norrish and Porter,[68] and many others. These techniques

allow a temporal resolution of at best milliseconds. Already up to the early 1960s it was understood that

elementary reaction dynamics occurs on much faster time scales.[69-72] Fourier spectroscopy may improve time

resolution down to microseconds or slightly less. With the advent of pulsed laser sources with ever increasing

temporal resolution, a wide arena of time-resolved spectroscopic techniques has emerged. Nanosecond laser

systems are known since the early 1960s,[73] picosecond pulses can be generated since the 1970s,[74, 75] the first

femtosecond laser system was reported in 1981,[76] and currently laser engineering groups are breaking into the

attosecond domain. [77-79] The majority of pulsed laser systems operates at visible and near-infrared wavelengths.

This means that usually chemical events in ultrafast time-resolved spectroscopy are initiated by electronic

excitation (“photochemistry”). Subsequent molecular rearrangements have then to be followed by probing the

electronic states through electronic resonances (UV/vis-pump – UV/vis-probe or UV/vis four-wave mixing

6 NIBBERING

UV-VISPump

Mid-IRProbe

Mid-IRProbe

Mid-IRProbe

UV-VISPump

Mid-IRProbe

Mid-IRProbe

Mid-IRProbe

n

n

1

1

0

n

10

n

1

Figure 3 Ultrafast time-resolved infrared spectroscopy. A UV/vis-pump pulse excites an electronic resonance, after

which the populations are followed with an IR-probe pulse by inspection of marker modes in the different electronic

states.

spectroscopy) (Fig. 2), or via vibrational resonances (UV/vis-pump – IR/Raman-probe spectroscopy) (see Figs. 3

and 4).

LASER TECHNOLOGY Since the beginning of the 1990s a major advancement in femtosecond laser technology has emerged by the

discovery of Kerr lens mode locking in laser oscillators [80] with Ti:sapphire as lasing material.[81] Ultrastable and

ultrashort laser pulses tunable between 700 and 1000 nm can be generated with durations as short as 5 fs.[82] The

second advancement is due to the ability of Ti:sapphire as laser amplifying material using the method of chirped

pulse amplification.[83-86] Pulses with output powers up to several W with repetition rates in the kHz-regime are

now routinely generated.[87-91] These developments facilitate the efficient generation of ultrashort pulses tunable

from the UV/vis [92-101] to the mid-infrared [102, 103] and even beyond to the THz-regime [104] using schemes based

on nonlinear optics. Frequency conversion of the fundamental output of an amplified Ti:sapphire laser system is

then achieved with nonlinear processes [105] such as self-phase modulation and supercontinuum generation, nth-

order harmonic generation (n = 2,3,4..), sum and difference frequency generation, or parametric generation and

amplification. As a result one can now almost arbitrarily tune the excitation and probe wavelengths of the

applied laser pulses. Additional parameters that one may alter are pulse duration (from less than 10 fs up to

several ps) and pulse energy (in the mJ-range or less; typically one uses for condensed phase spectroscopy pulse

FEMTOSECOND STRUCTURAL DYNAMICS 7

UV-VISPump

RamanProbe

RamanProbe

RamanProbe

UV-VISPump

RamanProbe

RamanProbe Raman

Probe

n

n

1

1

0

n

10

n

1

Figure 4 Ultrafast time-resolved Raman spectroscopy. A UV/vis-pump pulse excites an electronic resonance, after

which the populations are followed with (resonance) Raman-probe pulse by inspection of marker modes in the different

electronic states.

energies in the range of nJ to µJ). More advanced pulse manipulation schemes use amplitude and phase

masks.[106] The latter aspect holds the promise of amplitude and phase control of molecular excitation, feeding

the ultrafast chemist’s dream of laser control of molecular reaction dynamics.[107-112]

ULTRAFAST ELECTRONIC SPECTROSCOPIC TECHNIQUES Pulsed laser sources enable a variety of experimental approaches to determine transient states. Already since the

development of nanosecond laser sources time-resolved pump-probe and four-wave mixing spectroscopy have

been widely used. In a first approach often one uses these forms of spectroscopy to probe in real time

populations of reactant, transient and product states as function of pulse delay.[6] With the advent of pico- and

femtosecond laser sources the dynamics of coherence properties of the material response [53, 54, 113-115] have been

investigated as well.

The following techniques have been widely used in ultrafast electronic condensed phase spectroscopy

a) UV/VIS-PUMP – UV/VIS-PROBE SPECTROSCOPY

In this technique a “pump” pulse resonant to a transition promotes the molecules to an electronic excited state,

and the time-dependent populations are determined by measurement of the transmission of a time-delayed

“probe” pulse (Fig. 5). The absorbance change ∆A(τ) = -10log[T(τ)/T(τ =0)] (with T for transmission change and

8 NIBBERING

DetectorSample

Eprobe

Epump

Eprobe

P(3)τ

Sample

Eprobe

P(3)

Monochromatorand Detector

Eprobe

Epump

τ

(a)

(b)

DetectorSample

Eprobe

Epump

Eprobe

P(3)τ

Sample

Eprobe

P(3)

Monochromatorand Detector

Eprobe

Epump

τ

(a)

(b)

Figure 5 Pump-probe technique: A pump pulse induces an absorbance change in a sample, that is subsequently

measured by a time delayed probe pulse, either spectrally integrated (a) or spectrally resolved (b).

τ the pulse delay) is a direct indication of populations of states. Here one can tune the probe pulse to the same

transition as the pump pulse, and one measures an absorbance decrease due to a bleach of ground state

population (i.e. fraction of molecules being excited), and due to molecules in the excited state stimulated back to

the ground state. One could also tune the probe pulse to transitions between the transient state and higher lying

states. For electronic spectroscopy often transitions between the ground and first excited state (S0 → S1) overlap

with excited state absorptions (S1 → Sn). One has to rely then on numerical analysis procedures [116] such as

singular value decomposition and decay associated spectra, that correlate spectral features with temporal

behaviour.

The time resolution of the experiment is given by the cross-correlation between pump and probe pulses. In

the case of extremely short pulses one has to consider the effects of group velocity dispersion between different

frequency components due to different travel times through the samples, leading to pulse temporal broadening.

For the case of pump and probe tuned to different wavelengths the time-resolution is usually dominated by group

velocity mismatch between pump and probe pulses.

When ultrashort laser pulses are used, often the applied spectral bandwidth enables simultaneous excitation

of several vibrational sub-levels in vibronic transitions. Due to the phase-relationship of the applied laser pulses

these vibrational sub-levels are prepared in states that are well-defined in temporal phase with respect to each

other. Such a coherent superposition of vibrational eigenstates is nothing less than a vibrational wavepacket, that

will evolve in time. The occurrence of vibrational wavepacket motions leads to oscillatory modulations of the

pump-probe signals.

b) TIME-RESOLVED FLUORESCENCE SPECTROSCOPY

Here again a “pump” pulse excites an electronic resonance. The transient excited states are now probed through

their spontaneous fluorescence emission, either by use of time-correlated single photon counting or by

fluorescence upconversion with a gating pulse in a nonlinear medium (Fig. 6). Single-photon counting has a time

resolution of at best a few picoseconds. Fluorescence upconversion can be performed with a time resolution of

100 – 200 fs. The advantage lies in the fact that the measured signals are more likely due to the specific excited

states only, and not corrupted by other contributions.

FEMTOSECOND STRUCTURAL DYNAMICS 9

Sample

Epump

Single Photon Counter

CollimatingLens

(a)

(b)

Detector

SampleEpump

UpconversionCrystal

Esum

P(2)

Egate

τ

CollimatingLens

Sample

Epump

Single Photon Counter

CollimatingLens

Sample

Epump

Single Photon Counter

CollimatingLens

(a)

(b)

Detector

SampleEpump

UpconversionCrystal

Esum

P(2)

Egate

τ

CollimatingLens

Detector

SampleEpump

UpconversionCrystal

Esum

P(2)

Egate

τ

CollimatingLens

Figure 6 Time-resolved emission detection: The fluorescence emission is either temporally resolved by time-

correlated single photon counting (a) or by up-conversion with a gating pulse (b).

c) ELECTRONIC FOUR WAVE MIXING SPECTROSCOPY

In time-resolved degenerate electronic four-wave mixing spectroscopy, where all the three applied laser pulses

(with wavevectors k1, k2 and k3) are tuned to the same electronic resonance, one can detect a nonlinear signal in

the phase-matched direction kS = k3 + k2 – k1 (Fig. 7). The first laser pulse generates a coherent superposition

between the electronic ground and excited states. This electronic coherence evolves in time during the period

(denoted as coherence time τ), until the second pulse converts this coherence in a population frequency grating

in the electronic ground and excited states.[113] After a second time period of free evolution (denoted as

population or waiting time T), this frequency grating is again transformed into an electronic coherence, which

after some time evolution generates a macroscopic polarization in the phase-matched directions. This

macroscopic polarization is either directly measured by a time-integrating detector (in which case one measures

the intensity signal proportional to the absolute square of the nonlinear polarization), or one uses another light

pulse that mixes as local oscillator with the nonlinear polarization (and the signal is proportional to the amplitude

of the polarization). The time resolution is determined by the third-order autocorrelation of the applied pulses.

10 NIBBERING

Sample

k2

k1P(3)

k3 + k2 - k1

Detector

k3

τΤ

Sample

k2

k1 P(3)

k3 + k2 - k1 Detector

k3

ELO

τΤ

(a)

(b)

Sample

k2

k1P(3)

k3 + k2 - k1

Detector

k3

τΤ

Sample

k2

k1P(3)

k3 + k2 - k1

Detector

k3

τΤ

Sample

k2

k1 P(3)

k3 + k2 - k1 Detector

k3

ELO

τΤ

Sample

k2

k1 P(3)

k3 + k2 - k1 Detector

k3

ELO

τΤ

(a)

(b)

Figure 7 Beam geometry for time-resolved four-wave mixing (grating/photon echo) spectroscopy: The nonlinear

signal is transmitted by the sample in a phase-matched direction and its intensity signal (a) or its amplitude by

interference with a local oscillator field (b) is detected by a time-integrating detector.

Depending on the delay between the three input pulses, and which delay one varies, different dynamics can

be measured.[64, 113, 117] When one performs an experiment with τ scanned and T set at zero, one measures the

electronic coherence decay (due to electronic dephasing) time in a two-pulse photon echo experiment. When on

the other hand τ is set to zero, and T is scanned, one measures the evolution of a population grating, and

electronic lifetimes can be deduced. In a three pulse photon echo experiment both pulse delays are scanned, and

the signals give insight into both electronic coherence and frequency grating decay (due to electronic dephasing

and spectral diffusion respectively). A novel extension of four-wave mixing technique is the three pulse echo

peak shift (3PEPS).[118-120] In a 3PEPS measurement the delay time τmax is determined, where the echo signal has

a maximum along the coherence delay τ, as a function of the population delay T. This method provides direct

insight into the frequency fluctuation correlation function that governs the linear and nonlinear signals.

Extension of the method to multi-colour four-wave mixing spectroscopy [121, 122] has been demonstrated.

Photon echo spectroscopy of electronic transitions has until now been performed using homodyne detection

geometries, where the photon echo intensity signal is measured by a time-integrating detector (Fig. 7(a)). A

recent development in phase-stable heterodyne detection schemes involving an additional local oscillator pulse

(fig. 7(b)), enables measurement of the complex amplitude of the photon echo signal.[123-125] This advancement

has set the route for multi-dimensional spectroscopy, correlating coherences between coupled two level systems.

FEMTOSECOND STRUCTURAL DYNAMICS 11

ELECTRONIC CONDENSED PHASE SPECTROSCOPY:

FEMTOCHEMISTRY AND SOLVATION DYNAMICS In the narrow sense of meaning “femtochemistry” deals with the real-time observation of nuclear motions during

chemical reactions. Quantum mechanics teaches us that this is only possible when molecular systems are

prepared in vibrational superpositions.[126] Only in the case of quantum states describing vibrational wavepackets

evolving in time the narrow-defined positions of nuclei allow for a real-time description of molecular

rearrangements, or in other words: dynamics. This has been demonstrated in the gas-phase in landmark

experiments [8] by Nobel Laureate Zewail [68] (Fig. 8).

In the condensed phase at room temperature, however, often such well-defined quantum states are harder to

prepare, since the ensembles of molecular systems exist in mixed states. Coherent laser excitation on an

ensemble will then prepare mixed states as well, and then information that one obtains from the experiments is

less explicit. In addition, room temperature liquid solutions induce extremely fast fluctuations in the states of the

molecules under study and as a consequence the electronic and vibrational coherences show ultrafast decaying

behaviour. This limits the temporal window with which one can follow the evolution of vibrational

wavepackets.[127]

Most of the ultrafast spectroscopic experimental work performed on photochemistry in the condensed phase,

being described as “femtochemistry”, only deals with the time scales of changes in electronic states (usually

indicated with reaction rates typefied with exponentially decaying or rising functions). In this regard it is better

to speak of “kinetics” rather than “dynamics”, although both terms are used in condensed phase spectroscopy.

Due to the relative broad electronic spectra of condensed phase molecules masking any structural detail, it is

hard to derive nuclear motions in real time, and as such it is difficult to make any statements about dynamics of

molecular structures during chemical reactions.

1

0

nuclear coordinate Q

v=0v=1

v=2

v=0v=1

v=21

0

nuclear coordinate Q

v=0v=1

v=2

bound potential

dissociative potential

Figure 8 Ultrafast excitation with broadband pulses creates coherent superpositions of vibrational eigenstates, i.e.

vibrational wavepackets. These wavepackets are generated in electronic excited and ground states (a), and their time-

dependence can be determined in pump-probe and four-wave mixing experiments. When level crossing occurs to

another electronic state, the vibrational wavepacket may be observed to evolve along the reaction coordinate (b).

12 NIBBERING

Electronic resonances have also been used to probe the dynamical interactions between a non-reactive solute

and the surrounding fluctuating solvent.[21, 49-54, 128] The idea behind these experiments is to achieve information

about the time scales of solvent fluctuations on (and solvent motions due to a change in) the electronic charge

distribution in a molecule (as induced by an electronic excitation). Usually the solvent motions are characterized

with a frequency fluctuation correlation function (Fig. 9). This information is highly relevant for the case of

reacting molecules, where electronic motions accompany the relocations of nuclei, even in electron transfer

reactions, where the nucleic rearrangements are modest. Solvent motions have been shown to often control

reaction rates. Solvent rearrangement as a response to a change in electronic charge distribution is known as

solvation dynamics. Experimental probes for solvent motional fluctuations are electronic coherence decay

(electronic dephasing) and population frequency grating decay (spectral diffusion) that can be measured in two

and three pulse photon echoes.[48-51, 53, 54] Solvent adaptation to new charge distributions can be followed with the

time-dependent Stokes-shift in fluorescence emission,[21, 52, 128] as well as in three pulse photon echoes [49, 53, 54]

(in so-called three pulse echo peak shift measurements). Ultimately these experiments resolve the temporal

characteristics of the frequency fluctuation correlation function. Multi-dimensional photon echo spectroscopy

does not only provide insight into the dephasing and spectral diffusion of electronic transitions, but also has the

potential to reveal the couplings between chromophores.[129-131] This information is important in the context of

transfer of electronic excitation in pigments, in particular light harvesting complexes.[23, 132, 133]

0.0

0.2

0.4

0.6

0.8

1.0

E2E1* E3

τ ΤEcho

E2E1* E3

τ ΤEcho

E2E1* E3

τ Τ

EchoE2E1* E3

τ Τ

Echo

C(t)

Time t

Popu

latio

n gr

atin

g

Frequency

Popu

latio

n gr

atin

g

Frequency

Popu

latio

n gr

atin

g

Frequency

Popu

latio

n gr

atin

g

Frequency

Figure 9 Solute-solvent interactions as characterized by the transition frequency fluctuation correlation function C(t)

= <δω(t)δω(t=0)>, with δω(t) the fluctuating part of the transition frequency. This quantity can be measured in photon

echo and time-resolved fluorescence experiments. In a three pulse photon echo experiment phase information as given

by the excited state frequency grating created with the first two interactions, is washed out due to solvent fluctuations

and Stokes-shifts to lower frequencies due to solvent rearrangement. A similar plot can be drawn for the ground state

frequency grating. (Figure adapted from Ref. [54]).

FEMTOSECOND STRUCTURAL DYNAMICS 13

ULTRAFAST VIBRATIONAL SPECTROSCOPY IN PHOTOCHEMISTRY: STRUCTURAL DYNAMICS Vibrational spectroscopy has despite a smaller signal sensitivity due to smaller transition cross sections several

advantages over electronic spectroscopy. Vibrational transitions have typically smaller bandwidths than

electronic transitions due to longer dephasing times (the exception to the rule: O-H/O-D stretching bands of

hydrogen bonded hydroxyl groups). Specific signal contributions are thus easier tractable in vibrational

spectroscopy than in electronic spectroscopy. Vibrational bands can often be correlated to specific vibrational

motions by inspection of the transition frequencies, (e.g. O-H, N-H, and C-H stretching bands can be found in

the 3000 cm-1 range, C=O and C=N stretching modes are located near 1600-1750 cm-1, O-H, N-H and C-H

bending vibrations between 1500-1650 cm-1, C-O stretching mode around 1200-1300 cm-1 etc.), and in particular

the frequency range between 1000 and 1800 cm-1 is called the fingerprint region because of this reason.

Measurement of vibrational bands thus leads to identification of particular vibrational motions, and conclusions

can be drawn on specific structural motifs in the molecules. Vibrational bands can be infrared (IR) or Raman

active (sometimes: both), and both IR and Raman techniques have been widely applied in structural

determination studies ranging from small molecules to larger biomolecular systems. In ultrafast photochemistry

one thus excites the molecules with a UV/vis-pump pulse, and an IR/Raman-probe pulse follows the outcome of

the chemical reaction by inspection of vibrational bands of reactant, transients and products.

Vibrational spectroscopy has the potential of revealing site-specific information if the marker modes are due

to nuclear motions of specific molecular side-groups.[134-138] For instance, hydrogen bonding induces marked

shifts of O-H, N-H, C=O and C=N bands.[139, 140] Observation of changes in spectral shifts reveal important

information on hydrogen bond interactions (weakening/strengthening or even hydrogen bond cleavage).[134-136,

141, 142] Infrared spectroscopy is able to probe small molecular species in solution that typically have their

electronic resonances in the far-UV (making electronic spectroscopy impossible, since normally the solvent

would absorb this radiation) (Fig. 10). For instance, in acid-base neutralization reactions,[143, 144] where the

acidity of a so-called photoacid is switched on by use of a UV pulse, a vis probe pulse would only be able to

probe the photoacid (or its conjugate photobase), thus revealing only when a proton leaves the photoacid. An

infrared pulse can, besides probing vibrational resonances of the photoacid, also probe bands of a small base (or

its conjugated acid) indicating when a proton arrives at the base.

In the case that the vibrational normal modes do not allow such a structural insight into site-specific groups,

one can make a comparison of the experimentally observed vibrational bands with predictions made by quantum

chemical calculations.[145-147] When a full correspondence between experiment and theory is possible, one can

make statements about the three-dimensional structure. With current quantum chemical calculational routines

such as density functional theory, medium-sized molecules are routinely calculated for electronic ground state

conformations. Reliable results of transient and product states in electronic excited states can be obtained with

the routine ab initio complete active space self-consistent field (CASSCF),[148] albeit for mid-size molecules of at

most on the order of about 20 atoms. New developments in numerical procedures, such as time-dependent

density functional theory (TD-DFT) [149] may prove fruitful in the calculation of larger molecular systems in

electronic excited states.

14 NIBBERING

-O3S

SO3--O3S

O--O3S

SO3--O3S

O

H

-O3S

SO3--O3S

O

HD-O3S

SO3--O3S

O-

250 psdeuteron transfer to solvent water + +D O2

+D O3

D

~ 10-100 ps

CCH

O

O 3

_ CCH

O

OD 3

deuteron scavenging by acetate + +

CCH

O

O 3

_

CCH

O

OD 3

indirect deuteron transfer to

acetate

+~ 200-500 ps

-O3S

SO3--O3S

O--O3S

SO3--O3S

O

H

-O3S

SO3--O3S

O

HD-O3S

SO3--O3S

O

HD-O3S

SO3--O3S

O-

250 psdeuteron transfer to solvent water + +D O2

+D O3

D

~ 10-100 ps

CCH

O

O 3

_ CCH

O

OD 3

deuteron scavenging by acetate + +

CCH

O

O 3

_

CCH

O

OD 3

indirect deuteron transfer to

acetate

+~ 200-500 ps

Figure 10 In excited state acid-base neutralization experiments the proton dissociation can be followed on two sides

with ultrafast infrared spectroscopy. Vibrational marker modes of photoacid and conjugated photobase indicate when

the proton leaves the acid. The marker mode of acid formed by proton pick-up by the accepting base is a direct measure

of the arrival time of the proton at the base. Depending on relative concentrations either the indirect proton transfer via

the solvent or direct scavenging of the proton by the base dominates the dynamics. (Adapted from Refs. [143, 144]), where

the experiment was performed on photoacid-base pairs in deuterated water).

During the last years femtosecond IR spectroscopy has been used in photoinduced chemical reactions

ranging from hydrogen bond rearrangements,[134-136, 141, 142] excited state hydrogen [137, 138] and proton transfer,[143,

144, 150] transformations of photochromic switches,[151-154] excited state charge transfer,[145-147, 155, 156] bond fission [12, 157-163] and cis-trans isomerizations.[164-167] Here an ultrashort UV/vis “pump” pulse promotes the molecule to

an electronic excited state, and the reaction is followed by a measurement of the absorbance change of a “probe”

pulse tuned in the mid-IR region where the vibrational marker modes sensitive to structural changes have their

resonances. Experimentally one performs spectrally-resolved transient IR spectroscopy. Femtosecond IR

parametric devices deliver pulses with bandwidths of 150 cm-1 or more. In order to be able to observe shifts as

small as the linewidths of IR-active vibrations, one usually measures the IR absorbance change with a detector

after spectral dispersion with a monochromator. As a side-effect of this spectral dispersion ground-state bleach

signals often appear to grow in at negative pulse delay with the dephasing time of the transition.[168, 169] This

effect, known as “perturbed free induction decay” is a common feature of spectrally-resolved nonlinear pump-

probe spectroscopy of bleached transitions with dephasing times much longer than the time resolution of the

experiment. The time resolution of the experiment is given by the cross-correlation between the UV/vis-pump

and IR-probe pulse (about 100-200 fs), and is typically dominated by group velocity mismatch in samples with

thicknesses of about 100 µm.

In principle the same approach can be followed by probing Raman-active vibrations.[170, 171] In this case the

spectral resolution is not only determined by the monochromator through which the spontaneous Raman

emission is dispersed, but also by the bandwidth of the gating pulse by which the Raman effect is induced. As a

result UV/vis-pump – Raman-probe spectroscopy has a temporal resolution of around 1 ps (a compromise

between spectral and time resolution). Due to the even weaker cross sections of Raman transitions one often uses

resonance enhancement by tuning the gating pulse close to or resonant with an electronic transition of the state

that is probed. The advantage is that one usually only observes Raman bands of the state under inspection. The

FEMTOSECOND STRUCTURAL DYNAMICS 15

drawback is that fluorescence resulting from the resonant electronic excitation often inhibits detection of Raman

bands in extended spectral ranges. Unwanted fluorescence can be suppressed by gating the Raman signal,

rejecting the fluorescence emitted on longer time scales,[172] or can be circumvented by applying coherent

vibrational spectroscopic techniques, e.g. coherent anti-Stokes Raman scattering (CARS).[173-175]

Raman spectroscopy offers an additional insight into how chemical reactions evolve. By comparison of the

intensities of anti-Stokes and Stokes lines of a particular vibration, it is possible to derive time-dependent

changes in the excitation level of this vibration.[176-179] One can draw conclusions on whether particular modes

initially drive a chemical reaction (“state transition promoting modes”), or only get excited after the transition is

made, by taking up the excess energy released by the reaction (“accepting modes”). In such a way insight is

obtained on the energy flow inside a molecular system (intramolecular vibrational redistribution, abbreviated as

IVR) and vibrational energy dissipation to the surrounding solvent (vibrational cooling). Infrared spectroscopy is

less powerful in revealing this, since red-shifted transient absorption of vibrational bands may either mean that

the particular mode is highly excited (the red-shift is a consequence of the diagonal anharmonicity of the

vibration), or it may mean that other modes are highly excited and these cause a red-shift of the marker mode

under inspection (the red-shift is then due to off-diagonal anharmonic coupling with other modes).

EQUILIBRIUM STRUCTURAL DYNAMICS IN THE ELECTRONIC GROUND STATE Existence of anharmonic coupling between vibrational modes means that the vibrational motions are not

decoupled from each other. These couplings always exist, since otherwise no IVR and vibrational cooling [57, 58]

(no relaxation) would occur. When the couplings are significantly large, excitation of one specific mode will

induce a significant instantaneous shift of other vibrations anharmonically coupled to this particular mode.

Estimation of the magnitude of these couplings should lead to a determination of the curvatures of potential

energy surfaces along the respective coordinates. With current state-of-the-art femtosecond infrared technology

(with time resolution of about 150 fs or less) it is possible to excite infrared-active vibrations and to probe the

same or other vibrations (either in the IR or by the Raman process), enabling insight into the anharmonic

couplings between these vibrational modes.[180]

Direct anharmonic couplings are probed in IR-pump – IR-probe and IR photon echo spectroscopy (Fig.

11).[55, 181] In the case of peptides the excitonic (Davydov) couplings between amide I vibrations of the amino

acid units have been the subject of extensive study since these lead to spatial information of the relative

orientations of the different amide I modes, and – thus – of the spatial orientation of the amino acid units inside

the peptide.[182, 183] Exploring the effects of the delay between the IR pulses on the observed signals give insight

into the fluctuations of these orientational features, or in other words: into structural dynamics of peptides.[184, 185]

In the case of hydrogen bonded O-H/O-D stretching vibrations the anharmonic couplings with underdamped

low-frequency modes modulating the hydrogen bond distance lead to marked modulations of observed pump-

probe [186-190] and echo signals [189, 191, 192] as function of pulse delay. These experiments reveal which modes

couple strongly to the hydrogen stretching oscillator.

Fluctuations in couplings between vibrational modes (in particular with those of the solvent) induce

dephasing of the vibrational coherences.[181, 193-195] The dephasing and spectral diffusion dynamics of vibrational

transitions, e.g. the O-H stretch vibration of HOD dissolved in D2O,[196-201] or the O-D stretch of HOD in

H2O,[202] have been determined by IR photon echo spectroscopy. From these studies the time scales of structural

memory decay of hydrogen bond networks can be derived.[55, 59, 203, 204] As opposed to isotopically diluted

16 NIBBERING

q

nuclear coordinate Q

v = 0

q

Qv = 1Q

v = 2Q

v = 0Qv = 1Q

v = 2Q

Q(t)

q(t)

Q(t)

q(t)

q(t) O-H stretching vibrationhigh frequency ~ 3000 cm-1

period ~10 fsdephasing ~ 100 fs

Q(t) O....O H-bond vibrationlow frequency ~ 50-350 cm-1

period ~ 100-500 fsdephasing ~ 1 ps

anharmonic coupling

q(t) O-H stretching vibrationhigh frequency ~ 3000 cm-1

period ~10 fsdephasing ~ 100 fs

Q(t) O....O H-bond vibrationlow frequency ~ 50-350 cm-1

period ~ 100-500 fsdephasing ~ 1 ps

anharmonic coupling

Figure 11 In hydrogen-bonded hydroxyl groups a strong anharmonic coupling exists between the hydroxyl stretching

and modes modulating the hydrogen-bond distance. A Born-Oppenheimer like separation of time scales of vibrational

motion between the high-frequency O-H/O-D stretching and the low-frequency hydrogen bond modes enables a

description with potential energy surfaces for the low-frequency mode as function the quantum state of the hydroxyl

stretching vibration. The formation of low-frequency vibrational wavepackets in ultrafast IR spectroscopy of O-H/O-D

stretching modes is then fully analogous to wavepacket formation in ultrafast electronic spectroscopy.

O- H/O-D stretching oscillators (HOD in D2O or HOD in H2O), the vibrational dynamics in neat liquid H2O

appears to be much faster. An accurate determination of the population relaxation, anisotropy decay, vibrational

dephasing and spectral diffusion has recently been performed in two-dimensional heterodyne detected photon

echo experiments.[205] In neat H2O the couplings of the O-H stretching and bending [206] oscillators with

librational modes (hindered rotations of the hydrogen bond network) have been assigned as the reason for these

ultrafast dynamics.

Couplings between vibrational modes ultimately lead to population relaxation through intramolecular

vibrational redistribution effects and vibrational cooling. Ultrafast two-colour IR-pump – IR-probe reveals

whether excitation of one IR-active vibration is followed by population transfer into another IR-active mode.[207,

208] With IR-pump – Raman-probe one can estimate whether Raman-active vibrations are transiently excited.[209]

In the case of hydrogen-bonded O-H/O-D stretching bands an extremely short population relaxation time T1

(below 1 ps) is observed, as well as an ultrarapid dephasing time T2 (200 fs or less).[55] These values are

definitely longer for other vibrations, from which it has been concluded that the strong anharmonicities [210, 211]

in hydrogen-bonded vibrational systems enable dephasing [198] and relaxation processes [212-214] to have such a

rapid impact.

FEMTOSECOND STRUCTURAL DYNAMICS 17

FUTURE DEVELOPMENTS Future activities in the determination of dynamically evolving molecular structures in the condensed phase may

involve the aforementioned application of amplitude and phase controlled excitation pulses in optimal control of

chemical reactions.[215] Vibrational spectroscopy may be used as a tool in determination of evolving structures

after electronic excitation by a shaped pulse tuned in the UV/vis. Infrared pulses could also be shaped in

amplitude and phase,[216] and may be used in the exploration of steering of chemical reactions in the electronic

ground state. Multi-dimensional vibrational spectroscopy is an object of extensive research.[129, 130, 217, 218]

Extension of the method to determination of structure of transient states [155, 165] will be lead to new results on

molecular rearrangements, e.g. polypeptides reaching a new equilibrium after inducing a UV-excitation induced

geometric change of a photochromic switch incorporated in the peptide structure. Much activity is currently also

put in the development of ultrafast structure resolving techniques such as electron diffraction,[219-223] X-ray

diffraction [160, 224-231] and X-ray spectroscopy.[232] While still extremely demanding at a technological level,

definitive implementation of these structure resolving techniques will open up new areas of ultrafast chemical

dynamics.

OUTLINE OF THE HABILITATION THESIS In this Habilitation Thesis a review of ultrafast vibrational spectroscopy of the hydrogen stretching oscillator as a

probe for structural dynamics of hydrogen bond equilibrium dynamics is presented in Chapter 2 (see Fig 12).

Our results, obtained with a research effort embedded within the collaborative research centre Analysis and

Control of Ultrafast Photoinduced Reactions (SFB 450) are summarized together with those of other groups

worldwide active in the research field of ultrafast hydrogen bond dynamics. In Chapter 3 a review is given on

recent efforts of ultrafast vibrational spectroscopy on elementary chemical transformations, ranging from excited

state intramolecular hydrogen transfer, intermolecular proton transfer in bimolecular acid-base neutralization

reactions, hydrogen bond dynamics in solvation dynamics studies, charge transfer, bond fission, cis/trans

isomerizations and more elaborate rearrangements. The most notable research efforts on these – more or less in

chronological order – from the Max Born Institut are collected in detailed reports in the following chapters.

Hydrogen bond and solvation dynamics of a coumarin laser dye is discussed in Chapter 4. Here an optical

excitation of the coumarin chromophore induces breaking of the hydrogen bond with hydrogen bond donors, and

rearrangements of the solvation shells. In Chapter 5 excited state intramolecular hydrogen transfer is studied in

2-(2’-hydroxyphenyl)benzothiazole. In this case the hydrogen transfer is induced along an existing

intramolecular reaction coordinate. Bimolecular proton transfer in photoacid-base pairs including a study of the

excited state characteristics of the photoacid pyranine is treated in Chapter 6, where the solution phase proton

transfer reaction dynamics is intertwined with diffusional motions of the reactants towards each other. The

competition between internal conversion and photoinduced conversion in the photochromic switch pair

spiropyran–merocyanine is discussed in Chapter 7. Finally Chapter 8 reports on polarization-sensitive infrared

spectroscopy of model compounds for photosensor proteins as well as of photoactive yellow protein describe

how structural information on cis/trans isomerization in transient states can be extracted.

18 NIBBERING

Hydrogen bond dynamics in the condensed phase

Structure: - hydrogen bond network - anharmonic coupling between vibrational modes Dynamics: - coherent hydrogen bond wave packet motion - hydrogen bond fluctuation - intramolecular vibrational redistribution - vibrational energy dissipation

Ultrafast chemistry: probing through vibrations

CO

HO

electronic spectroscopy vibrational spectroscopy Physics: - electronic state coupling - vibrational energy redistribution and dissipation Chemistry: - excited state intramolecular hydrogen transfer - bimolecular proton transfer - electron transfer - hydrogen bond rearrangements - bond fission - cis-trans isomerizations

Figure 12(a) Overview of the Habilitation Thesis: hydrogen bond dynamics in the electronic ground state (Chapter

2), and general overview of determination of ultrafast chemistry through probing of vibrations (Chapter 3).

FEMTOSECOND STRUCTURAL DYNAMICS 19

Solute-solvent hydrogen bond dynamics

coumarin 102

hydrogen bond cleavage +

phenol dimer rearrangement

H

H HH

phenoldimercoumarin 102

hydrogen bond cleavage +

phenol dimer rearrangement

H

H HH

phenoldimer

Structure: - site-specific hydrogen bond interaction - solute-solvent coupling Dynamics: - hydrogen bond rearrangement - solvation dynamics

Excited-state intramolecular hydrogen transfer

HN

S

O

HBT-enol

HN

S

O

HBT-enol

hydrogentransfer

hydrogentransfer

HBT-keto

OHN

S

Structure: - electronic state coupling - hydrogen bond reaction coordinate - anharmonic coupling between vibrational modes Dynamics: - intramolecular hydrogen transfer - intramolecular vibrational redistribution - vibrational energy dissipation

Figure 12(b) Overview of the Habilitation Thesis: hydrogen bond dynamics after electronic excitation of a laser dye

(Chapter 4), and excited state intramolecular hydrogen transfer (Chapter 5).

20 NIBBERING

Bimolecular proton transfer in aqueous solution

-O3S

SO3--O3S

O

HD

pyraninephotoacid

-O3S

SO3--O3S

O

HD-O3S

SO3--O3S

O

HD

pyraninephotoacid

CCH

O

O 3

_

acetate

CCH

O

O 3

_

acetate

-O3S

SO3--O3S

O-

pyraninephotobase

-O3S

SO3--O3S

O-

pyraninephotobase

CCH

O

OD 3

acetic acid

CCH

O

OD 3

acetic acid

++bimolecular

deuterontransfer

bimoleculardeuterontransfer

Structure: - acid-base complexation - hydrogen bond reaction coordinate - involvement of water bridges - solvent stabilisation of charge separated states Dynamics: - diffusion - on-contact reaction dynamics - von Grotthuss proton transmission

Photophysics of photochromic spiropyrans

ONCH3

CH3 CH3

NO2NCH3

CH3 CH3

O

NO2

δ−

δ+

spiropyranmerocyanine

ring opening +

cis-trans isomerization

ONCH3

CH3 CH3

NO2ONCH3

CH3 CH3

NO2NCH3

CH3 CH3

O

NO2

δ−

δ+NCH3

CH3 CH3

O

NO2

δ−

δ+

spiropyranmerocyanine

ring opening +

cis-trans isomerization

Structure: - electronic state coupling - large displacement of vibrational modes - anharmonic coupling between vibrational modes Dynamics: - internal conversion - intramolecular vibrational redistribution - vibrational energy dissipation - ring-opening - cis-trans isomerisation

Figure 12(c) Overview of the Habilitation Thesis: bimolecular proton transfer in aqueous solution (Chapter 6) and

photophysics of photochromic spiropyran-merocyanine (Chapter 7).

FEMTOSECOND STRUCTURAL DYNAMICS 21

Photosensor protein chromophore dynamics

HO

NN

CH3

O

CH3HO

N

N

O CH3

CH3

excited statetwist

E-HBDIZ-HBDI

HO

NN

CH3

O

CH3HO

N

N

O CH3

CH3

HON

N

O CH3

CH3

excited statetwist

excited statetwist

E-HBDIZ-HBDI

Structure: - chromophore-solvent or chromophore-protein interaction - site-specific hydrogen bond interaction - single twist vs. hula twist coordinate - electronic state coupling - anharmonic coupling between vibrational modes Dynamics: - hydrogen bond rearrangement - internal conversion - cis-trans isomerisation - intramolecular vibrational redistribution - vibrational energy dissipation - solvent or protein rearrangement

Figure 12(d) Overview of the Habilitation Thesis: photosensor protein chromophore dynamics (Chapter 8).

22 NIBBERING

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