Laser-assisted Autoionzation

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Laser-assisted Autoionzation

• Z. X. Zhao
• X. M. Tong and C. D. Lin

KSU AMO Seminar, 3/10/04
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Outline

• Introduction
• Autoionization
• Time-resolved measurement
• Analytical model
• Laser-assisted photoionization
• Lorentzian shape
• Fano resonance
• Numerical simulation
• Discussion of results
• Comparison of total spectra
• Deduce lifetime
• More than one resonancequantum beat

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Introduction
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Aautoionization / Fano profile
Reduced energy
Shifted resonance position
Resonance width
?
q parameter ratio of direct ionization and
autoionization. measure the strength of
interference.
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Illustration of pump-probe schemes
Linear or circular ?
Pump X-ray
Initiate atomic process
Probe laser
Time-resolved spectra

• Cross-correlation
• Probe atomic dynamics

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Time-resolved measurements previous work

• With
• attosecond soft-X-ray and fs laser pulse,
• Cross-correlation can be built for laser assisted
  photoionization to
• Measure X-ray pulse duration1,2
• Measure absolute phase of the laser pulse(?)
• Measure the lifetime of a resonance laser
  assisted auger decay 3
• Study Laser-assisted autoionization.

1. Hentschel et al, Nature 414, 509
2. Drescher et al, Science 291, 1923
3. Drescher et al, Nature 419, 803

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Example of attosecond metrology Laser-assisted
Photoionization
X-ray
AL(t)
t
Spectrum
W/o Laser
Delay0
DelayT/4
Kitzler et al, PRL88, 173904
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analytical model
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Formulation of Laser-assisted PI
Free electron Coulomb field, laser field, X-ray
field
Strong field approximation
Bound electron excitation
Assumptions
depletion of ground state
Photoionization Laser field, X-ray field
Electron amplitude
ts Saddle point
Stationary phase equation
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Kinetics
energy conservation
Linear polarization
Electron energy at observation angle ?
Or
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Laser-assisted autoionization Lorentzian shape
Time profile
Field-free
Energy domain
Laser-assisted electron spectrum under strong
field approximation (SFA)

• Virtual three-step process
• Resonance state excited by X-ray at time t1
• Decay at time t2 giving birth to continuum
  electrons
• Propagation of electrons in the laser field.

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Laser-assisted autoionization Fano shape
Profile in energy domain
Profile in time domain
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Numerical model
Two-channel TDSE to model two-e- system in a
laser field

• Split-Operator propagation method used to solve
  TDSE
• Two channel continuum constructed by applying
  scattering wave boundary condition

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Feshbach resonance two-ch potential with coupling
Xray pulse 0.5 fs, 1x1012 W/cm2, 38.1 eV Laser
10 fs, 2x1012 W/cm2. Phase0 and frequency 0.04
a.u.(1 eV).
Energy gap 27.21 eV Resonance 23 eV Ground state
-16.1 eV
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results
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Angle-Integrated spectra
Two pulses on top of each other for negative q
Fano resonance 22.9 eV (position), 0.055 eV (12
fs) (width) and -4.2 (q number).
Xray 0.5 fs, 1x1012 W/cm2, 38.1 eV Laser 10
fs, 2x1012 W/cm2. Phase0 and frequency 0.04
a.u.(1 eV).
Show agreement between analytical-model and
num-simulation
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1 resonance case
q4.2 only change
Two pulses on top of each other for positive q
(delay zero)
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Zero angle, no delay
Laser freq1eV
Total
Resonance only
interference from direct and resonance
better sideband developed
Laser freq2eV
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Time resolved spectra in forward direction
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Measuring lifetime
Laser phase pi/2
Laser phase 0
Electron counts within sideband from 0.5 a.u. to
0.7 a.u. are plotted verse time delays.
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2 resonances case
No laser field
Parameters (0, 1)6 with energy 64.96eV,
lifetime of 667 fs and (0, 1)7 with energy 65.08
eV, lifetime of 1000 fs. q-2.6 Xray pulse
duration 4 fs with intensity 1x1012 W/cm2.
Energy (a.u.)
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Laser-modified spectra for 2 res
Parameters (0, 1)6 with energy 64.96eV,
lifetime of 667 fs and (0, 1)7 with energy 65.08
eV, lifetime of 1000 fs. q-2.6 Xray pulse
duration 4 fs with intensity 1x1012 W/cm2.
Laser duration 50 fs with intensity 5x1011
W/cm2. Phasepi/2 and frequency 1.55 eV (800nm).
Energy (a.u.)
Counts in sideband is 1 of total resonance
population
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Measuring energy separation from quantum beat
Phase difference (E2-E1)tdelay ?E0.1 EV
correspondent to 34.5 fs
Fiting by
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Conclusions

• Build an analytical model for laser-assisted AI
• Justified by numerical simulation
• Deduce lifetime and
• Energy separation of two resonance
• Q parameter?,and
• other significance?

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2 resonances case
Electron spectra from decay itself at fixed time
delay with different laser pulse duration
Width of individual sideband decreases as laser
pulse duration increased. For long enough pulse,
each sideband shows two sub-peaks correspondent
to contribution from both resonances. For short
pulse, it cant be resolved, interference is
expected.
Energy (a.u.)
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2 resonances case
Only first resonance
Only second resonance
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Two resonances
(0,1)6 64.96 eV,667 fs (0,1)7 65.08 eV,
1000 fs
Energy separation 0.12 eV (34.5 fs)
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Drescher et al, Nature 419, 803
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Fano profile
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Observation angle 900
Center of gravity
Hentschel et al, Nature 414, 509
Drescher, Science 291, 1923
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Zero observation angle
Forward direction
Backward direction
Measuring laser pulse
Measuring duration of X-ray
Bandrauk et al, PRA 68, 041802
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Measuring instantaneous field of laser pulses
Measured
Real
Absolute phase measurement Shot to shot varying
phase asymmetry of electron counts from
PI,Nature 414, 182 Phase stabilized laser
structure of soft X-ray emission, Nature 421, 611
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Total photoelectron spectrum
IL2x1012, IX1012 W/cm2,wL1 eV, ?L10fs, ?X0.5
fs, delay0
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Measuring atto pulse duration
Bandrauk et al, PRA 68, 041802
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Measurement of lifetime in time-domain
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Angular distribution of photoelectron spectrum