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Acta Physica Sinica

ABOVE-THRESHOLD-IONIZATION BY MULTIPHOTON ABSORPTION OF ATOM UNDER SHORT LASER PULSE

Wenqing Zhang, TAN WEI-HAN, 中国科学院上海光学精密机械研究所,上海,201800

Published Jan 1, 1991• 0 citations
Publisher

Abstract

The Above-threshold-ionization(ATI) Spectrum of electrons produced by multiphoton absorption of atom has been obtained with spatial Gauss and temporal Sech shape of the incident laser pulse. It is found that our model can explain many phenomena such as peak suppression and switching. In addition, what is more important is that we can explain the red shift of the peaks decreases to zero gradually, as the pulse width changes from short to long pulse regime.

DOI
10.7498/aps.40.1755

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The Above-threshold-ionization(ATI) Spectrum of electrons produced by multiphoton absorption of atom has been obtained with spatial Gauss and temporal Sech shape of the incident laser pulse. It is found that our model can explain many phenomena such as peak suppression and switching. In addition, what is more important is that we can explain the red shift of the peaks decreases to zero gradually, as the pulse width changes from short to long pulse regime.
We present an atomic model to study above-threshold ionization (ATI). We show that when the laser field is sufficiently intense the usual perturbation theory is not valid. We solve the model dynamics exactly, and several interesting quantities, including the initial-state decay rate and photoelectron spectrum, are calculated. These quantities are related to recent ATI experiments, and qualitatively they agree with these experiments. We specify the strong-field condition for continuum–continuum transitions and explore its physical meaning. Finally some comments on perturbation theory are made.
Previous results for the shift of the ionisation potential of an atom in a laser field are used to investigate the kinematics of multiphoton ionisation in either a laser beam or a laser pulse. Previous results for a laser beam show essentially no shifts of the 'above-threshold ionisation' peaks in the electron energy distribution and only small widths. Results for a laser pulse show the possibility of large shifts and widths growing as the pulse becomes shorter. Electrons which absorb a larger number of photons are predicted to show less shifted and narrower peaks.
The resonantly enhanced multiphoton ionization of atomic hydrogen is studied for the case of four-photon ionization (three-photon resonant). The time-dependent probabilities, the shape of the resonance profile at different intensities, and the nonlinear index are computed for coherent light at frequencies around the resonance (365 nm). The systematic dependence of the cross section as a function of the principal quantum number of the resonant state (as this tends towards the Rydberg states) is investigated at zero detuning. The probabilities of absorption of one photon more than necessary for the four-photon ionization process (resonantly enhanced above-threshold ionization) is computed and the relevant branching ratios are examined. Then it is shown that the effect of the spatio-temporal structures of the laser pulse is to red shift the maximum of the resonance profile, whereas that of an ideal chaotic light is to blue shift and broaden the resonance peak. The shape, shift, and width of the profile resulting from both these effects are in very fair agreement with the experimental ones.
We present experimental and theoretical data on the electron energy spectra from resonance-enhanced multiphoton ionization of atoms. In experiments performed with 594-nm wavelength, linearly polarized laser pulses of 300-fs and 600-fs durations, it is found that excitation of the intermediate resonances during the rising edge of the laser pulse can dominate the electron spectrum. Good agreement is found between the experimental results and a model based on the Landau-Zener energy level crossing theory. Theoretical results obtained with direct integration of the Schrödinger equation show that intermediate resonances can be excited appreciably off resonance if the coupling between the ground state and the excited state is highly nonlinear. The excitation of the intermediate state is not localized at the intensity which makes it six-photon resonant with the ground state. A multilevel model used to simulate the experimental spectra, based on direct integration of the Schrödinger equation, shows excellent agreement with the data.