Multiphoton detachment of a negative ion by an elliptically polarized, monochromatic laser field

The quasistationary quasienergy state approach (QQES) is applied to the analysis of partial (n-photon) decay rates and angular distributions (ADs) of photoelectrons produced by an elliptically polarized laser field. The problem is formulated for a weakly bound electron with an energy E0 in the threedimensional δ-model potential (which approximates the short-range potential of a negative ion) interacting with a strong monochromatic laser field having an electric vector F (ωt). The results presented cover weak (perturbative), strong (nonperturbative), and superstrong field regimes as well as a wide interval of frequencies ω extending from the tunnelling (h̄ω |E0|) and multiphoton (h̄ω < |E0|) cases up to the high frequency domain (h̄ω > |E0|). For a weak laser field, exact equations for the normalization factor and for the Fourier coefficients of the QQES wavefunction at the origin (|r| → 0) (that are key elements of the QQES approach for a δ-model potential) as well as for the detachment amplitudes are analysed analytically using both standard Rayleigh– Schrödinger perturbation theory (PT) in the intensity, I , of the laser field and Brillouin–Wigner PT expansions involving the exact (complex) quasienergy . The lowest-order perturbative results for the n-photon ADs are presented in analytic form, and the parametrization of ADs in terms of polarizationand angular-independent atomic parameters is discussed for the general case of elliptical polarization. The major emphasis is on the analysis of an ellipticity induced distortion of three-dimensional ADs and, especially, on the elliptic dichroism (ED) effect, i.e. the dependence of the photoelectron yield in a fixed direction n on the sign of the ellipticity (or on the helicity) of a laser field. The dominant role of binding potential effects for a correct description of ED and threshold effects is demonstrated, and the intimate relationship between atomic ED factors and scattering phases of the detached electron is established for multiphoton detachment, including the above-threshold case. 3 Present address: JILA, University of Colorado and National Institute of Standards and Technology, Boulder, CO 80309-0440, USA. 4 On leave from: Institute for Space Sciences, Bucharest-Magurele 76900, Romania. 0953-4075/03/090049+76$30.00 © 2003 IOP Publishing Ltd Printed in the UK R49