Rotationally Resolved Photoelectron Spectra at Near-Threshold Kinetic Energies

Abstract

Rotationally resolved photoelectron spectra and their associated photoelectron angular distributions can clearly provide significant insight into the dynamics of molecular photoionization, one of the simplest of molecular fragmentation processes. Such state-resolved spectra are an obvious signature of the exchanges of energy and angular momentum between the photoelectron and molecular ion. With the exception of H2 (Pollard et al., 1982; Anderson et al., 1984; Pratt et al., 1984, 1990; O'Halloran et al. , 1987; McCormack et al., 1990) and D2 (Peatman et al., 1983; O'Halloran et al., 1989; Pratt et al., 1990a) and of higher J levels of diatomic systems such as NO (Wilson et al., 1984; Viswanathan et al., 1986a, l986b; Allendorf et al., 1989), NH (de Beer et al., 1991; Wang et al., 1992) and OH (de Beer et al., 1991 b) with their large rotational spacing, rotational resolution in photoelectron spectra has generally been beyond the reach of the techniques of conventional photoelectron spectroscopy. With the recent development of zero kinetic energy (ZEKE) photoelectron spectroscopy based on pulsed field ionization (PFI) of very high Rydberg levels, however, it is now possible to exploit the narrow bandwidth of laser radiation to achieve sub-wavenumber resolution in ion rovibronic state distributions (Sander et al. , 1987; Reiser et al., 1988; Tonkyn et al., 1989; Grant and White, 1991; Muller-Dethlefs and Schlag, 1991). The unprecedented resolution of this technique makes it a powerful tool for investigating the dynamics of near-threshold photoionization and has led to a surge of experimental activity in molecular photoelectron spectroscopy. Furthermore, laser-induced fluorescence (LIF) studies can provide even higher resolution than energy-resolved photoelectron measurements and may allow monitoring of the populations of A components of ion rotational levels (Xie and Zare, 1989, and personal communication). Rotationally resolved measurements have revealed such interesting spectral features as the dependence of photoelectron angular distributions on rotational levels (Anderson et al., 1984; Allendorf et al., 1989), rotational propensity rules (Viswanathan et al. , 1986a, 1986b; Sander et al., 1987; Allendorf et al., 1989; Xie and Zare, 1989), parity selectivity in transitions involving electronically degenerate states (Pratt et al., 1984; Viswanathan et al. , 1986a; O'Halloran et al. , 1987; Fujii et al., 1988; Xie and Zare, 1989), autoionization (Pratt et al. , 1990a, 1990b; Wiedmann et al., 199 1), effects of alignment (McCormack et al., 1990), and the influence of Cooper minima (de Beer et al., 199la, 199lb; Wang et al., 1992) and shape resonances (Braunstein et al. , 1990) on ion distributions. The underlying dynamics of such rotationally resolved photoelectron spectra can clearly be expected to present new theoretical challenges.

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