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Physical Review Letters

Dynamics of resonances rapidly shifting under short laser pulses

Xian Tang, A. Lyras, P. Lambropoulos

Published Aug 28, 1989• 19 citations
Publisher

Abstract

We present a theoretical analysis of multiphoton ionization through intermediate states shifted through resonance during a short intense pulse. The dynamical behavior is described in terms of a density matrix evolving under a realistic temporal and spatial pulse shape. Our results, on atoms and with realistic parameters, show structure similar to that observed in recent experiments and suggest further interesting phenomena.

DOI
10.1103/physrevlett.63.972

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We present a theoretical analysis of multiphoton ionization through intermediate states shifted through resonance during a short intense pulse. The dynamical behavior is described in terms of a density matrix evolving under a realistic temporal and spatial pulse shape. Our results, on atoms and with realistic parameters, show structure similar to that observed in recent experiments and suggest further interesting phenomena.
We present a theoretical analysis of multiphoton ionization through resonant intermediate states shifted through resonance during a short, intense pulse. The resonant structure in the photoelectron spectra is compared with what it would be if the peak intensity or the frequency of the laser were scanned, and the various differences are discussed in detail. The dynamical behavior is described in terms of a density matrix evolving under a realistic temporal and spatial pulse shape. Our results, on atoms with realistic parameters, show structure similar to that observed in recent experiments. Results for circularly polarized light are also presented. The limitations and advantages of the model are discussed.
It has been widely believed that short-pulse multiphoton ionization is dominated by a resonance process: excited states of the atom are shifted by the radiation field into multiphoton resonance with the ground state, are populated, and then rapidly ionized. A recent experiment, shows significant population remaining in excited states at the end of the laser pulse, and this result has been interpreted as indicating that ionization proceeds slowly out of these states. We investigate this issue in the framework of Floquet theory, supplemented by direct numerical solution of the time-dependent Schrodinger equation. In this framework, the standard picture is portrayed as an adiabatic transition between Floquet eigenstates, which occurs as the amplitude of the laser field is ramped through the resonance value (i.e. through an avoided crossing of Floquet quasienergies). Thus it follows that the same type of transition must occur as the field is turned off. This mechanism can yield significant populations of excited states that do not suffer appreciable ionization. We exhibit this effect by numerical studies of a model atom with two bound states and two continua.
We present results from a time-dependent two-level-plus-continuum model of multiphoton ionization in order to better understand the short-pulse photoelectron spectrum resulting from multiphoton ionization and the role of intermediate states. Both resonant and nonresonant population transfer to the upper state are found to play an important role in the multiphonon ionization process. The dynamic modeling reveals the potential for broadening and shifting peaks in the photoelectron spectrum. However, we conclude that despite the complex dynamics of the excitation, short-pulse photoelectron spectra can be used to measure ac Stark shifts of both ponderomotively and nonponderomotively shifted states in strong laser fields.
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.
We have studied population trapping in a one-dimensional model atom interacting with a short-pulse high-intensity laser, using Floquet analysis and direct numerical integration of the time-dependent Schrödinger equation for a range of peak pulse intensities. We find that photoelectrons are efficiently produced only by pulses whose peak intensities lie in a narrow range around the resonant intensity for a given intermediate state. Excited-state populations, however, are generated with comparable efficiencies for all pulses whose peak intensities exceed the resonant intensity. This implies that, for a realistic laser pulse with an inhomogeneous spatial distribution of intensity, excited atoms are generated throughout a larger volume than are photoelectrons. Interpretations of recent experiments are reexamined in light of this result.