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Journal of Modern Optics

Ionization Spectra Induced by Phase-varying Fields

PAUL M. RADMORE, Sam Tarzi, K.L. Tang

Published Nov 1, 1995• 7 citations
Publisher

Abstract

We consider the effect of a field with time-varying phase on the final-state spectrum produced by ionization of a single ground-state. Although the ground-state population still decays according to the Weisskopf-Wigner formula, the final-state spectrum exhibits a redistribution of population. A single Lorentzian is, in general, no longer obtained and the spectrum may be asymmetric. We concentrate on analytically soluble models with periodic phase-variation, but also briefly consider a random phase and chirp. Finally, the case of two continua is discussed.

DOI
10.1080/09500349514551921

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We consider the effect of a field with time-varying phase on the final-state spectrum produced by ionization of a single ground-state. Although the ground-state population still decays according to the Weisskopf-Wigner formula, the final-state spectrum exhibits a redistribution of population. A single Lorentzian is, in general, no longer obtained and the spectrum may be asymmetric. We concentrate on analytically soluble models with periodic phase-variation, but also briefly consider a random phase and chirp. Finally, the case of two continua is discussed.
The atomic system under discussion contains one discrete ground level and two orthogonal continua coupled by a train of ultra-short laser pulses separated by a time interval T. Recurrent relations derived for the probability amplitudes enable us to simulate the time evolution of the system. We discuss the cases of Lorentzian and Gaussian continua, and perform simulations for specified widths and shapes of the continua, the laser strength, and the time interval in their effect on the probabilities of finding electrons in the atomic states.
The photoexcitation of a structured continuum of states from a single ground state is considered. We investigate an analytically soluble model in which the dipole matrix elements between the ground state and the continuum vary sinusoidally between zero and a maximum. The time-evolution of the ground-state population still exhibits the periodic disruption observed in the Bixon-Jortner model, although the disruption is much reduced. We allow the ground state to be resonant with an arbitrary place in the continuum; population trapping is observed in the case of resonance with a zero in the continuum. We compare and contrast the results obtained with this model with those of quasi-continuum excitation. Lastly, we examine the final-state spectrum.
A theoretical analysis of the phase control of a two-channel atomic ionization scheme is given. We report results on phase-selective excitation, both with square and smooth laser pulses. We show that the phenomenon of population trapping, which is known to occur in this system, can be greatly influenced by the phase difference of the lasers involved in the excitation. As a result, the time dependences of the ionization probability, the populations in the bound states and the form of the photoelectron energy spectra can all be controlled by varying the phase difference of the lasers. We also show that for specific atomic and phase parameters population can be entirely trapped within a superposition state, totally immune to decay, with no ionization whatsoever.
A theoretical analysis of the phase control of a two-channel atomic ionization scheme is given. We report results on phase-selective excitation, both with square and smooth laser pulses. We show that the phenomenon of population trapping, which is known to occur in this system, can be greatly influenced by the phase difference of the lasers involved in the excitation. As a result, the time dependences of the ionization probability, the populations in the bound states and the form of the photoelectron energy spectra can all be controlled by varying the phase difference of the lasers. We also show that for specific atomic and phase parameters population can be entirely trapped within a superposition state, totally immune to decay, with no ionization whatsoever.
In this paper we analyze the dynamics of single-excitation states, which model the scattering of a single photon from multiple two level atoms. For short times and weak atom-field couplings we show that the atomic amplitudes are given by a sum of decaying exponentials, where the decay rates and Lamb shifts are given by the poles of a certain analytic function. This result is a refinement of the "pole approximation" appearing in the standard Wigner-Weisskopf analysis of spontaneous emission. On the other hand, at large times, the atomic field decays like O(1/t3)O(1/t^3)O(1/t3) with a known constant expressed in terms of the coupling parameter and the resonant frequency of the atoms. Moreover, we show that for stronger coupling, the solutions also feature a collection of oscillatory exponentials which dominate the behavior at long times. Finally, we extend the analysis to the continuum limit in which atoms are distributed according to a given density.