A new time-dependent wavepacket approach has been applied to illustrate a rigorous method of calculating the transition function for bound-to-continuum transitions in translational energy spectrometric experiments. The method is numerically exact and fully quantal, and is devoid of any classical or semi-classical approximations. The validity of the hitherto-applied reflection approximation is investigated in a comparative study. The method has been applied to two different translational energy spectrometry experiments: collision-induced dissociation of CO' and collisional excitation of HZ.Electronic excitation of molecules by collisions with slow ions is a topic of much current interest in molecular dynamics. Such interest is spurred by the fact that electronic excitation plays a critical role in the fundamental understanding of many collision phenomena apart from the specific role it has in determining the abundance of certain molecules (e.g., H2) in astrophysical objects like cold interstellar clouds.' Of late there has been a renewed interest both in the experimental developments and theoretical understanding of the proces~.'.~.' On the experimental side, efforts have concentrated on improving the energy resolution, to widen the domain of application of translational energy spectrometry (currently available resolutions are of the order of 0.1 eV for =3 keV collision energy). On the theoretical side, more progress on interpretation of the experimental spectra through spectral simulation is needed. In particular, since collisions with molecular ions often lead to highly excited electronic states which are dissociative or predissociative in nature, efficient methods must be developed which allow one to easily calculate transition matrix elements between the initial bound vibrational states prior to collision and the final dissociative continuum states without resorting to severe classical or semiclassical approximations.We describe here a new time-dependent wavepacket (TDWP) method to analyse translational energy spectra in the study of two different collision phenomena involving transitions to repulsive electronic states. This method has been previously used to analyse photodissociation spectra for molecules such as HOD,' CH,,7 and H20.' In the case of fast-ion collisions, where the projectile-target interaction times are usually much smaller than molecular vibrational periods, photodissociation (that is, radiative excitation to a repulsive surface) and collisional excitation are physically analogous in the sense that both are 'driven' by the overlap between the initial vibrational and final continuum wavefunctions. Hence, we expect successful application of the TDWP method to studies of electronic excitation in collision experiments. We demonstrate here such an application by the study of the collision-induced dissociation (CID) of CO' and in energy loss spectroscopy of electronic excitation of neutral Hz.
METHODAn outline of the TDWP method is as follows. Following Heller,9 we imagine that the initial vibrati...
