Extracting spectroscopic molecular parameters from short pulse photo-electron angular distributions

Abstract

Using a quantum wave packet simulation including the nuclear and electronic degrees of freedom, we investigate the femtosecond and picosecond energy- and angle-resolved photoelectron spectra of the E(Σg+1) electronic state of Li. We find that the angular distributions of the emitted photoelectrons depend strongly on the pulse duration in the regime of ultrashort laser pulses. This effect is illustrated by the extraction of a time-dependent asymmetry parameter whose variation with pulse duration can be explained by an incoherent average over different ion rotational quantum numbers. We then derive for the variation of the asymmetry parameter a simple analytical formula, which can be used to extract the asymptotic CW asymmetry parameters of individual transitions from measurements performed with ultra-short pulses.

Anisotropy control in photoelectron spectra: A coherent two-pulse interference strategy

“…Considering the fact that both a photon absorption and a pelectron escape modify the angular momentum by ±1, the ion rotational channels which are ultimately reached are N + = 0, 2 for the initial state N E = 0, or N + = 0, 2, 4 for the initial state N E = 2. The corresponding rotational levels are separated by 6B for N + = 0, 2, and by 20B for N + = 0, 2, 4, B being the ion rotational constant for v + = 0, taken as B 0.5 cm −1 [19].…”

“…The ionization amplitude A N + (ε) seen in Eqs. (19), (22) and (24) is, in principle, specific to each exit channel N + and according to Eq. (18) the total photoelectron spectrum is given by an incoherent sum over all ion rotational levels N + .…”

“…For the implementation of this model we will hereafter simply use Eqs. (19), (22) and (24), replacing ε N + with the particular value obtained for N + = N E and replacing the envelop |A N + (ε)| 2 with the photoelectron spectrum P (ε) calculated for a single pulse. In summary, it is worthwhile mentioning that we are considering two models:…”

“…In a recent work, we have examined the energy and pulse duration dependence of β [19] for a single ionizing pulse. In particular, we were able to assign a well defined value of β for each individual N + channel reached by photoionization, even with an extremely short pulse of bandwidth much larger than the rotational spacing.…”

“…In particular, we were able to assign a well defined value of β for each individual N + channel reached by photoionization, even with an extremely short pulse of bandwidth much larger than the rotational spacing. If we start from the initial state N E = 0, in the cw limit the N + = 0 exit channel is characterized by a peaked anisotropy with β = 2, whereas a quasi-isotropic β = 1/5 angular distribution is reached for N + = 2 [19].…”