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Journal of Physics B Atomic Molecular and Optical Physics

Laser intensity effects in photoemission and photoelectron spectra produced in laser-induced autoionisation

Guanhua Yao, Zhizhan Xu

Published Jul 14, 1989• 0 citations

Abstract

Strong laser-field-induced autoionisation in the presence of both photoionising and radiative decay of the autoionising state (AS) is investigated, focusing on the laser intensity dependence of the photoemission and photoelectron spectra. In contrast to previous predictions, power broadening and increasing reduction of the doublet peak heights with field strength are found in the photoemission spectrum. Similar effects leading to considerable suppression and even complete disappearance of the lowest-order peaks in the photoelectron spectrum, together with peak switching, are also demonstrated, which are closely related to above-threshold ionisation. In addition, it is suggested that the total number of energetic photoelectrons may serve as an alternative to measuring the atomic parameters of the AS. All these effects are attributed to the presence of the strong 'probe': laser-induced decay of the AS.

DOI: 10.1088/0953-4075/22/13/015

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Strong laser-field-induced autoionisation in the presence of both photoionising and radiative decay of the autoionising state (AS) is investigated, focusing on the laser intensity dependence of the photoemission and photoelectron spectra. In contrast to previous predictions, power broadening and increasing reduction of the doublet peak heights with field strength are found in the photoemission spectrum. Similar effects leading to considerable suppression and even complete disappearance of the lowest-order peaks in the photoelectron spectrum, together with peak switching, are also demonstrated, which are closely related to above-threshold ionisation. In addition, it is suggested that the total number of energetic photoelectrons may serve as an alternative to measuring the atomic parameters of the AS. All these effects are attributed to the presence of the strong 'probe': laser-induced decay of the AS.

Photoemission spectra from autoionizing states in the presence of a strong laser field are calculated. Such spectra have very sharp features near "confluence" (at the Fano minimum) and are very similar, in structure, to photoelectron spectra. The total intensity, as a function of laser intensity or detuning, has a peak near confluence, which can be used, among other things, for the accurate determination of Fano asymmetry parameter

q

.

Atoms in an autoionisation state can be excited to higher states in the continuum as well as performing spontaneous transitions to bound states by laser-induced autoionisation. Both effects are investigated by means of the reaction matrix. Results obtained by Armstrong et al (1978) using the same method are extended. The shape of the photoelectron spectrum is different from that of the photon spectrum emitted from the autoionisation state but is similar to that of the photon spectrum emitted from the same bound state which is excited to the ionisation states in the former case. It is predicted that the shape of the fluorescence yield with the pump laser detuning is Lorentzian. The multiphoton process contributes to the photoelectron spectrum in a complicated way.

Laser-induced autoionisation is discussed in the simple model in which the autoionising level is embedded in several atomic continua. The coupling to the second continuum can drastically diminish the characteristic narrowing in the photoelectron spectra near the confluence of coherences point. The results seem to be important for possible experimental verification of this narrowing, as in the real atom such coupling is hardly avoidable. The similarity of the effects caused by the spontaneous emission and by the coupling to the second continuum is pointed out.

The fluorescence produced by an autoionizing state under strong pumping by a coherent laser field is considered. It is shown how the spectrum of emitted photons and the total number of photons are related to the two-time and single-time atomic expectation values, and explicit expressions are derived for them. Numerical results for the time development of the number of photons ejected are presented for finite times and in the long-time limit. The photon spectra are shown to exhibit ac Stark splitting of the states involved. The spectrum is a doublet for transitions to a stable final state and a triplet for transitions connected back to the initial, strongly driven state. The decay of an autoionizing state in the absence of a laser driving field is considered, and the square of the Fano

q

value is interpreted as the ratio of two decay rates.

We theoretically study the process of photoelectron emission from the helium atom using a high-intensity short-wavelength laser at a resonance condition of the residual singly charged ion. Photoionization followed by strong resonance coupling in the ion leads to a change in the photoelectron spectrum due to Rabi oscillations in the residual ion. Similarly to resonance fluorescence at high laser intensity, the photoelectron spectrum at high intensities evolves into a multipeaked structure. The number of peaks in the photoelectron spectrum is related to the number of Rabi cycles following the photoionization process. Moreover, the strong laser-induced coupling to nonresonant states of the residual ion has an imprint on the photoelectron spectrum, leading to additional, isolated peaks at the lower- or higher-energy sides. The effect should be observable at current seeded extreme ultraviolet (XUV) free-electron lasers and persists after volume integration in a realistic experimental geometry.