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

Abstract

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 ellipticallypolarized laser field. The problem is formulated for a weakly bound electron with an energyE0 in the three-dimensionalδ-modelpotential (which approximates the short-range potential of a negative ion) interactingwith a strong monochromatic laser field having an electric vector F (ωt).The results presented cover weak (perturbative), strong (nonperturbative), andsuperstrong field regimes as well as a wide interval of frequencies ωextending from the tunnelling (ℏω ≪ |E0 |) and multiphoton(ℏω < |E0 |)cases up to the high frequency domain (ℏω > |E0 |).For a weak laser field, exact equations for the normalization factor and for theFourier coefficients of the QQES wavefunction at the origin (|r| → 0)(that are key elements of the QQES approach for a δ-modelpotential) as well as for the detachment amplitudes are analysed analytically usingboth standard Rayleigh–Schrödinger perturbation theory (PT) in the intensity,I, of the laserfield and Brillouin–Wigner PT expansions involving the exact (complex) quasienergyϵ.The lowest-order perturbative results for then-photonADs are presented in analytic form, and the parametrization of ADs in terms ofpolarization- and angular-independent atomic parameters is discussed for the generalcase of elliptical polarization. The major emphasis is on the analysis of an ellipticityinduced 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 non the sign of the ellipticity (or on the helicity) of a laser field. The dominant roleof binding potential effects for a correct description of ED and threshold effects isdemonstrated, and the intimate relationship between atomic ED factors andscattering phases of the detached electron is established for multiphotondetachment, including the above-threshold case. For a strong laser field, wepresent an accurate derivation for the QQES wavefunction and decay rates in theKeldysh approximation (KA) from exact QQES equations, including analytical,first-order (‘rescattering’) corrections to the KA results. The symmetries of ADsand the existence of ED are established using the exact analytical result for then-photondetachment amplitude. Accurate numerical results are presented for thevariation of the structure of the ADs as well as of the ED effect withincreasing laser intensity. For the high frequency case, ℏω > |E0 |, arigorous analytical treatment of higher-order PT effects is presented forone-photon detachment, taking into account corrections of higher orders inI to thewell-known photodetachment cross section for a short-range potential. Together withthe exact numerical analysis of the total and partial decay rates for ℏω > |E0 |,these results demonstrate the existence of a quasistationary stabilization regime inthe decay of a weakly bound electron for any polarization of the laser field. Moreover,this stabilization occurs only over a limited interval of intensity, up to the closureof the direct photodetachment channel. In the superstrong field regime, the totaldecay rate of a weakly bound electron may be described by cycle-averaging theresults for an instantaneous static electric field of strength |F (ωt)|(for any laser frequency and polarization). All results in this paper are presentedin scaled units and are illustrated numerically for the case of the H−negative ion.