Results using atomic targetsResults using atomic targets

• Suppression of Nonsequential ionization from an atomic ion target (comparison of double ionization of Ar and Ar+).

• Determination of intensity ratio factor for comparison of linear to circular polarization.

• Linear-Circular comparisons of nonsequential ionization of Xe using the intensity selective scanning technique.

• Strong field ionization of C+ ions – observation of forbidden transition from metastable state.

ISS results for ArISS results for Ar++ ion beam ion beam….….

• Using the Intensity Selective Scan (ISS) technique (see Experimental Setup presentation) Ar2+ production from an Ar+ ion beam is measured as a function of focusing lens position.

• The groundstate peak at z = 0 therefore disappears rapidly compared to the metastable peaks at z = ±12 as the intensity decreases.

• The large peak at z=0mm results from ionization of groundstate Ar+ ions requiring the strongest field (highest intensity).

• The two peaks either side of zero position are due to metastable Ar+ ions (lower IP) displaying the expected profile resulting from the interplay of increasing interaction volume with decreasing intensity as the lens moves away from zero.

Ar2+ production from Ar+ ion beam(a) 1016 Wcm-2

(b) 5 1016 Wcm-2

Groundstate

Metastable

Suppression of Nonsequential componentSuppression of Nonsequential component….….

Ratio of Ar3+ to Ar2+ production

• Neutral target 0.038 ± 0.002

• Ionic target 0.007 ± 0.002

Comparison of double ionization from Ar+ ionic target and Ar neutral target

Multiple ionization of ionic target is strongly suppressed!

For more info, see publication:J. B. Greenwood et al Phys. Rev. Lett. 88. 233001 (2002)

TOF spectra (similar conditions) for (a) neutral Ar and (b) Ar+ beamNote: (b) (i) Ar2+ and (ii) Ar3+ (x 50 vertical scale)

For a quantitative comparison of linear-circular nonsequential contribution, need to match the sequential ionization components.

As linear polarization is an oscillating field but circular is constant in amplitude, it’s not suffice to simply match the peak E-field amplitude. We need to match the time-averaged tunnel rates over the period of a laser cycle.

From ADK tunneling theory,

Linear rate 1.3E0 Circular rate E0

For Circular field E0/ 2 (intensity I),

matching Linear field is 1.3 x (E0/ 2)

i.e. 0.81E0 (intensity 0.65I)

Circular

Intensity I

Linear

Intensity 0.65I

Comparison of Linear – Circular PolarizationComparison of Linear – Circular Polarization….….

Intensity (Wcm-2)

1014 1015 1016 1017

Ion

Yie

ld (

arbi

trar

y)

102

103

104

105

106

107

108

109

1010

Linear ICircular 1.3*I

Unshifted

ADK curves for Xe (circular intensity scaled x1.3)

= 0.5

Xe+

Xe2+

Experimental verification of intensity ratio of 0.65Experimental verification of intensity ratio of 0.65….….

We experimentally verified the linear-circular intensity ratio of 0.65 by performing an ISS on Ne gas in the neutral gas target apparatus

Ne was a good test gas to use as previous observations have found suppressed Nonsequential double ionization from this target (Becker and Faisel J. Phys. B 32 L335 (1999))

The results show excellent matching of the linear and circular yields for single and double ionization of Ne using the factor of 0.65.

ISS for single and double ionization of Ne

Ne+

Ne2+

Linear Circular

Z Position (arb.)

Tim

e o

f Flig

ht (

sec)

Signal (arb.)

132Xe2+

131Xe2+

129Xe2+

Xe+

Xe8+

Temporally resolved isotopes of Xe2+

ISS results from Xe neutral target (linear polarization)ISS results from Xe neutral target (linear polarization)….….

Inte

grat

ed Io

n Y

ield

(ar

b.)

Z Position (mm)

Circular Linear

Xe Xe+

Xe Xe2+

Xe Xe3+

Xe+ Xe3+

Linear-Circular ISS results Linear-Circular ISS results for Xe and Xefor Xe and Xe++….….

Loss of linear flux in Xe+ appears as an enhanced linear signal in higher charge states

Large enhancement of linear signal in the low intensity regions is due to the Nonsequential ionization!

Suppressed Nonsequential component from the ionic target as was the case for Ar also!

For more information see :M. Suresh et al.RAL Annual Report 2003-2004 (2004)

ISS for C2+ produced via C+ ionization

Focal Detuning (mm)

0.0 2.0 4.0 6.0 8.0 10.0

Inte

gra

ted

Io

n Y

ield

(a

rb)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

C+ 2P - C2+ 1S

C+ 4P - C2+ 1S

C2+ 1S - C3+ 2STotal FitExperiment

Strong Field Ionization of CStrong Field Ionization of C++…………

ISS results for ionization of C+ ions show ionization of groundstate and higher lying metastable level.

Can fit to ISS results by using saturated volume method (El-Zein et al). Gives a good indication of the ionization potential for a particular process.

ISS for C2+ produced via C+ ionization

Focal Detuning (mm)

0.0 2.0 4.0 6.0 8.0 10.0

Inte

gra

ted

Io

n Y

ield

(a

rb)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

C+ 2P - C2+ 1S

C+ 4P - C2+ 1S

C2+ 1S - C3+ 2STotal FitExperiment

Strong Field Ionization of CStrong Field Ionization of C++…………

The IP from the saturated volume fits suggests;

The metastable transition C+ 4P C2+ 1S however is a forbidden transition.

Groundstate: C+ 2P C2+ 1S

Metastable: C+ 4P C2+ 1S

Requires an electron spin-flip event!