ABOVE-THRESHOLD DISSOCIATION IN THE LONG-WAVELENGTH LIMIT

Detailed measurements of fragment kinetic energy spectra coming from t he interaction between intense 10.6 mu m 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 struc ture superimposed. These fragments are associated with the dissociatio n stage of the interaction. The broad continuum is understood using a simple two-step model in which the first stage involves the barrier-su ppression dissociation of the molecular ion followed by a second step involving the interaction between the dissociated, charged fragment an d the electric held. This model is analogous to the 'Simpleman theory' (van Linden van den Heuvell and Muller 1988 Multiphoton Processes, Ca mbridge Studies in Modern Optics vol 8 ed S J Smith and P L Knight (Ne w York: Cambridge University Press)) which has been used in the interp retation of long-wavelength above-threshold ionization results (Gallag her 1988 Phys. Rev. Lett. 61 2304, Corkum et al 1989 Phys. Rev. Lett. 62 1259). The fine peak structure is related to the above-threshold di ssociation picture. The deuteron spectra also contain a set of higher- energy fragments which are associated with a secondary, enhanced ioniz ation process. The absence of corresponding higher-energy protons can be understood in the context of the proposed two-step dissociation mod el. The experimental spectra are also compared with the results of bot h quantum and semiclassical calculations; these agree qualitatively we ll.