Energy dependence of photoelectron angular distributions from two- and four-photon ionization of Mg in the vicinity of the 3p^2 1S₀doubly excited state

Abstract

We report on measurements concerning the variation of photoelectron angular distributions (PADs) from two-and four-photon ionization of ground state Mg, as the laser wavelength is scanned across the 3p 2 1 S 0 resonance. The energy dependence of the asymmetry parameters determined from the PADs and corresponding to two-photon ionization is found to be in good agreement with prior theoretical predictions. For the four-photon PADs there are at present no theoretical results. Nevertheless, the resonant character is found considerably red-shifted in accordance with earlier experimental and theoretical studies of the single and double ionization yields. In either case, the PADs show a dominant s-wave on-resonance contribution, while the d-wave contribution apparently dominates under non-resonant excitation within the scanned wavelength range. The PADs offer detailed wavelength-and channel-dependent information on the dynamics of the autoionization resonances under pulsed laser excitation, corroborating and elucidating the conclusions reached from ionization yield studies.

Transition from correlated to single-active-electron excitation in strontium nonlinear ionization

“…The I ≈ 5 TWcm -2 5s(0) PAD additionally exhibits a weak side lobe within the 0 o <θ<90 o interval, probing a quite small 5sεg contribution. These PAD shapes are similar to the ones reported earlier for four-photon ionization via long (~ns) pulses, for either Mg [ 9 ] or Sr [ 12 ] out of their respective ground states. Based on them one can easily understand the quite asymmetric shape of the 5s(1) PAD at ~1.9 eV which, as Fig.…”

“…The nonlinear (single, double or even multiple) ionization of multi-electron atoms is a subject of high interest in photophysics as it questions the nature of electron correlation. Early investigations in intense laser-atom interaction concerned alkaline earth metal atoms [1,2,3,4,5,6,7,8,9,10,11,12] ionized by long laser pulses (~ns or ~ps) at moderate intensity (≤10 TWcm -2 ).…”

“…Thus, they appear to be suitable for studying the role of multielectron effects which are at play in nonlinear ionization. Indeed, earlier long-pulse and low-intensity investigations [ 5,7,9,10,11,12 ] already pointed out that the single-active-electron assumption is frequently invalid for these systems.…”

“…Despite this fact, "direct" two electron ejection was never evidenced in alkaline earths under these conditions. Nevertheless, the production of excited ionic level populations facilitated their sequential double ionization where atoms are ionized first and the resulting singly charged ions are further ionized within the same laser pulse [ [1][2][3][4][5][6][7][8][9][10][11][12] ].…”

“…The purpose of the present work is to shed more light on the above issue, by investigating the nonlinear single and double ionization of Sr using intense ≈800 nm and ≈25 fs laser pulses and a velocity-map imaging spectrometer (VMIS). Strontium atom is chosen because earlier long-pulse studies showed that CI-mediated above-threshold ionization is much stronger for it [ 10,11,12 ] as compared to other alkaline earths atoms [ 7,9 ] under similar excitation conditions. The recorded photoelectron energy spectra (PES) and photoelectron angular distributions (PADs) show indeed such evidence.…”