Field-ionization, Coulomb explosion of diatomic molecules in intense laser fields

A refinement to the field-ionization, Coulomb explosion model of diatomic molecules in intense laser fields is described, which predicts classical appearance intensities for molecular ions as a function of internuclear separation. The ionization process is found to be optimized at the critical internuclear distance, R(c), which is virtually the same for all fragmentation channels. The appearance intensities are coupled with classical trajectory calculations to produce fragmentation energy releases that are in good agreement with experiment, in the case of I-2 at a wavelength of 750 nm and for a range of pulse lengths from 55 to 400 fs. The model also predicts that charge transfer will lead to the observation of the charge-asymmetric I + I2+ channel at pulse lengths below about 200 fs; this prediction is confirmed by experiment. At 30 ps, the observation of substantial post-dissociative ionization is explained by a reduction in the dissociation rate of the I-2(+) ions at 1064 nm. Data on N-2 using 55 and 400 fs pulses at 750 nm show the effect of competition between ionization and dissociation. An explanation is given for the contrasting ion time-of-flight (TOF) spectra at visible and ultraviolet wavelengths. Finally, in the case of H-2 in short, intense laser fields, the ion TOF spectra are explained without recourse to vibrational trapping or bond hardening in H-2(+).