Above-threshold dissociation in the long-wavelength limit

Abstract

Detailed measurements of fragment kinetic energy spectra coming from the interaction between intense laser pulses and hydrogen and deuterium molecules have been made. Both the proton and the deuteron spectra exhibit a broad continuum distribution with a finer peak structure superimposed. These fragments are associated with the dissociation stage of the interaction. The broad continuum is understood using a simple two-step model in which the first stage involves the barrier-suppression dissociation of the molecular ion followed by a second step involving the interaction between the dissociated, charged fragment and the electric field. This model is analogous to the `Simpleman theory' (van Linden van den Heuvell and Muller 1988 Multiphoton Processes, Cambridge Studies in Modern Optics vol 8 ed S J Smith and P L Knight (New York: Camridge University Press)) which has been used in the interpretation of long-wavelength above-threshold ionization results (Gallagher 1988 61 2304, Corkum et al 1989 62 1259). The fine peak structure is related to the above-threshold dissociation picture. The deuteron spectra also contain a set of higher-energy fragments which are associated with a secondary, enhanced ionization process. The absence of corresponding higher-energy protons can be understood in the context of the proposed two-step dissociation model. The experimental spectra are also compared with the results of both quantum and semiclassical calculations; these agree qualitatively well.