phys. stat. sol. (c) 3, No. 10, 3531–3534 (2006) / DOI 10.1002/pssc.200672158

© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Ultrafast time-resolved infrared luminescence spectroscopy

in halogen-bridged Pd complexes

Youtarou Takahashi*, 1, Hiroshi Kitagawa2, and Tohru Suemoto1

1 Institute for Solid State Physics, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa,

Chiba 277-8581, Japan 2 Department of Chemistry, Faculty of Science, Kyushu University, Hakozaki, Fukuoka 812-8581, Japan

Received 19 June 2006, revised 8 July 2006, accepted 8 July 2006

Published online 2 November 2006

PACS 71.35.Aa, 78.47.+p, 78.55.Kz

The ultrafast luminescence spectroscopy on [PdBr(chxn=cyclohexanediamine)2]Br2 was performed from

near to mid infrared region. The instantaneous luminescence with a large Stokes shift indicated the ex-

istence of the large lattice relaxation in excited states. The temporal waveform of the luminescence

showed ultrafast decay and the decay time constant is smaller than 300 fs in low energy region. This ten-

dency of the waveform is different from that of self-trapped exciton and consistent with the wavepacket

motion on the potential surface with no barrier. The result suggests the appearance of a excited state that

is different from the ordinary self-trapped exciton.

© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

1 Introduction The one-dimensional halogen-bridged complexes have been attracted much attention

for years. The main structure of the one-dimensional chain consists of alternate halogen X (Cl, Br, I) and

transition metal M (Ni, Pd, Pt) ions. The platinum complexes (hereafter abbreviated as Pt-X) have a

charge density wave (CDW) state, i.e. a sequence of - Pt4+- X- - Pt2+ - X- - with displacement of halogen

ion, as a ground state because of the strong electron-lattice interaction. The photo-excited states, such as

self-trapped excitons, solitons, and polarons, are investigated intensively in Pt-X. The ultrafast transient

absorption, reflection and luminescence spectroscopies clarified the dynamics of these excitations and

the vibrational-modes coupled to them [1–4]. In contrast, nickel complexes show a Mott-insulator type

ground state ( - Ni3+- X- - Ni3+ - X- - ), that is caused by strong on-site Coulomb repulsion force on the Ni

ions. The ultrafast relaxation, photoinduced phase transitions and non-linear optical properties have been

investigated by spectroscopic approach [5–7].

In this paper, we treated [PdBr(chxn=cyclohexanediamine)2]Br2 (hereafter abbreviated as Pd-Br),

which has CDW ground state ( - Pd4+- X- - Pd2+ - X- - ). The study on the Ni-doped Pd-Br show that it is

located near the phase boundary between CDW and Mott-insulator on the phase diagram [8, 14]. The

spin soliton, which is the domain boundary of the CDW phase, is observed directly using STM on Pd-Br

[9]. Judging from these experimental results, the physical properties of Pd-Br would be expected to be

similar to that of Pt-X. In this report, we investigated the relaxation process of the Pd-Br by time-

resolved luminescence from near- to mid-infrared region.

2 Experiment The single crystals of PdBr were prepared by the electrochemical oxidation technique

[10]. The time-resolved luminescence spectroscopy was done using so-called up-conversion technique.

* Corresponding author: e-mail: youtarou@issp.u-tokyo.ac.jp, Phone: +81471363377, Fax: +81471363377

3532 Y. Takahashi et al.: Ultrafast time-resolved IR luminescence spectroscopy in halogen-bridged Pd complexes

© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.pss-c.com

The amplified fundamental pulses of the Ti:sapphire laser with a photon energy of 1.55 eV and a repeti-

tion rate of 200 kHz were used. The luminescence from the sample surface was collected and focused by

paraboloidal mirrors on a non-linear optical crystal and mixed with the gating laser pulses. The sum

frequency was focused into a double grating monochromater and detected by a photomultiplier tube

(Hamamatsu Photonics R943-02). The system has a sensitivity between 0.23 and 1.3 eV, and the overall

time resolution of this system was 100 fs and the time interval of the measurement was 40 fs. All meas-

urements were performed at room temperature. The background noise was determined by averaging the

signal between -0.3 and -0.7 ps and subtracted from the data.

3 Results and discussion The excitation en-

ergy of our experiment, 1.55 eV, corresponds to

the upper tail of the charge transfer absorption

band, whose peak energy is 0.72 eV and full

width at half maximum is 0.4 eV [8]. This ab-

sorption band is assigned to the charge transfer

(CT) transition from Pd2+ to Pd4+ as in Pt-X.

There is no other structure around 1.55 eV and

thus the laser pulses yield this CT transition.

Figure 1 shows the polarization characteristics

of the luminescence at a photon energy of 0.9 eV

under the excitation light polarized parallel to

the one-dimensional chain. There is no lumines-

cence signal at an angle of 90° relative to the

one-dimensional chain, i.e. the luminescence

from Pd-Br is polarized parallel to the one-

dimensional chain. In the case of the excitation

light polarized perpendicular to the one-

dimensional chain, the luminescence was not

observed. So that this luminescence is assigned

to the CT transition along the one-dimensional

chain.

Figure 2 shows a time evolution of the lumi-

nescence at room temperature. The luminescence intensity was normalized to unity at the peaks. The

luminescence was observed in the energy range from 1.3 to 0.23 eV. The photon energies of 1.3 and 1.0

eV correspond to the energy above the CT absorption peak. So that the luminescence at these energies

will originate from radiative relaxation of the CT exciton and the waveform will reflect the intraband

relaxation of the CT exciton. The luminescence decay curves are represented by single exponential func-

tions and the time constants are 130 and 220 fs at 1.3 and 1.0 eV, respectively. The intraband energy

relaxation will make these time constants longer. The luminescence at 0.23 eV, which is below the low-

est edge of the CT band (0.3 eV) [8], strongly suggests existence of the some relaxation mechanism in

Pd-Br. The ratio of the lowest luminescence energy (0.23 eV) to the CT absorption peak energy (0.72

eV) is (1/3.1). In Pt-X the luminescence with a large Stokes shift is observed and assigned to the self-

trapped exciton (STE). The typical ratio of the luminescence energy of the STE to the CT absorption

peak energy is (1/2.3) [11]. In the ground state, the chain-parallel displacement of the Br ion from mid-

point of the Pd ions is very small [12] and this suggests that electron-lattice interaction, which provides

the Stokes shift of the luminescence, is weak. Therefore, the measurement energy range is sufficient for

detecting the luminescence from STE, if it ever exists. In the [Pt(en)2][Pt(en)2Br2](ClO4)4 (en = ethyl-

enediamine) luminescence shows the waveform reflecting nuclear wavepacket oscillation with a lifetime

of 5 ps at room temperature [13]. In contrast, the luminescence waveform of the Pd-Br shows ultrafast

decay with a lifetime of 280 fs at 0.3 eV and no wavepacket oscillation. This ultrafast decay of lumines-

cence will be possible if a strong non-radiative relaxation cannel on the STE exists. In the case of the

Fig. 1 The closed circles, open squares and open

triangles correspond to the luminescence time evolu-

tion, whose polarization angles are 0°, 45° and 90°

relative to the one-dimensional chain, respectively. The

luminescence photon energy is 0.9 eV. The excitation

light is polarized parallel to the one-dimensional chain.

100

80

60

40

20

0Lu

min

esc

en

ce in

ten

sity

(a.u

.)

0.80.60.40.20.0-0.2-0.4Time (ps)

phys. stat. sol. (c) 3, No. 10 (2006) 3533

www.pss-c.com © 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

STE luminescence, a fast decay will be observed on

the high and low energy side of the luminescence

band within a few hundred fs as a result of the

cooling. However the decay constant is almost the

same except that of 1.3 eV. Therefore, the observed

luminescence cannot be ascribed to STE.

We can suggest a possibility of the relaxation

different from that of Pt-X. Iwano calculated the

excited states of the Pd-Br [14, 15]. The Mott-

insulator phase domain can exist as a metastable

state, because the CDW ground state of the Pd-Br is

located near the boundary between CDW and Mott-

insulator phase in the phase diagram. It is expected

that a photo-excited state relaxes to Mott-insulator

domain without a potential barrier and the Mott-

insulator phase domain appears in the CDW ground

state in this theory. We propose a relaxation model

of Pd-Br on the adiabatic potential energy surface

based on this theory as shown in Fig. 3. The solid

curve is a potential energy surface of the Pd-Br and

the dotted curve is that of ordinary STE. The hori-

zontal axis indicates the domain size of the Mott-

insulator.

The CT excitation created by photo-absorption

alters the charge on Pd site and the bromine ions

around the CT exciton start moving due to the

Coulomb force. This lattice relaxation will be simi-

lar to that of STE and this STE-like lattice distor-

tion will be observed as a luminescence with a

large Stokes shift. However the later stage would

be different in these two cases. According to the

calculation on the Pt-X, the final size of the STE corresponds to five metal sites [16]. The position of the

potential minimum on STE corresponds to this final size of the STE. In contrast, the STE-like lattice

distortion in Pd-Br would spread broader, because the potential barrier of STE is absent as shown in Fig.

3. This large domain would have the Mott-insulator type electronic state, in which all the Pd ions are

trivalent. The slide down motion of wavepacket on the potential surface corresponds to the spreading of

the Mott-insulator domain. The oscillation and cooling of the wavepacket would not be observed due to

absence of the potential barrier. Therefore, it would be reasonable to interpret the transient luminescence

observed in our measurement as this slide down motion of the wavepacket. It seems that the wavepacket

motion starts from the absorption peak energy at 0.7 eV by analogy with Pt-X. In Pt-X, the returning

motion of the wavepacket has been observed above the absorption peak energy by ultrafast luminescence

spectroscopy [17]. Therefore, the potential energy surface of Pt-X extends in the CT absorption band. In

Pd-Br the potential energy surface will extend in the absorption band similar to the situation of Pt-X. The

maximum position in the waveform shifts to later timing in low energy as shown in Fig. 2. In fact that of

0.27 eV shows a delay of 120fs compared with that of 0.5 eV. This tendency will be consistent with the

motion of the wavepacket on the potential surface. Based on the energy diagram in Ref. [15], the photon

energies of 0.5, 0.3 and 0.23 eV correspond to the domain size of 2.5, 5 and 7 metal sites, respectively.

So the final domain size would be at least 7 metal sites.

Fig. 2 The time evolution of luminescence observed

in PdBr at room temperature. The photon energies are

indicated on the right side.

8

6

4

2

0

Lum

inesc

ence

inte

nsi

ty (

norm

aliz

ed)

1.00.80.60.40.20.0-0.2 Time (ps)

1.3 eV

1.0 eV

0.7 eV

0.5 eV

0.3 eV

0.27 eV

0.25 eV

0.23 eV

3534 Y. Takahashi et al.: Ultrafast time-resolved IR luminescence spectroscopy in halogen-bridged Pd complexes

© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.pss-c.com

4 Conclusion We performed the femtosecond luminescence spectroscopy on the Pd-Br and observed a

transient luminescence from near to mid infrared region. This waveform of the luminescence showed the

ultrafast decay and it suggests non-existence of potential barrier in relaxation process. That is far differ-

ent from the waveform of Pt-X, which has a CDW type ground state as Pd-Br. We interpret this behavior

based on the theoretical calculation in Ref. [14, 15]. We attributed the luminescence waveform to the

wavepacket dynamics on this potential energy surface.

Acknowledgements This work was supported in part by the Grant-in-Aid for Scientific Researches (A) and (B)

from the Ministry of Education, Culture, Sports, Science and Technology of Japan.

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Fig. 3 The potential energy surfaces in Pd-Br and the

model of the relaxation dynamics. The horizontal axis

indicates the size of the Mott-insulator domain. The

solid curves indicate the excited state and ground state

in Pd-Br. The dashed curve indicates the potential

energy surface with a STE minimum. The thick arrow

shows the lattice relaxation process to Mott-insulator

state.