Single and multiple ionization of in intense laser fields: wavelength dependence and energetics

Single and multiple ionization of carbon disulphide by intense picosecond laser fields is the subject of this paper. Mass spectra were measured at five wavelengths from the infrared to the ultraviolet. In terms of the Keldysh adiabaticity parameter, we cover both the multiphoton and the tunnelling regimes. The dynamics of the dissociative ionization process is shown to be dependent upon the regime in which the laser - molecule interaction occurs. Resonances, which may be possible and which could access electronically excited states of the molecule, appear to play little part in the dynamics. Ion abundances have been measured as a function of laser intensity in the tunnelling regime; no correlation is found between measured values of saturation intensity and zero-field molecular properties such as dissociation or ionization thresholds and ionization energies. In addition, the covariance mapping technique is applied to study the dissociation dynamics of multiply charged ions at 1064 nm. The measured values of kinetic energy release accompanying formation of fragment ion-pairs are very much less than those measured in single-photon and electron-impact experiments. It is postulated that this reduction may be a manifestation of the extent to which potential energy surfaces of ions are `flattened' by the action of the intense, linearly polarized laser radiation, akin to the bond-softening process that has been observed in the case of diatomic molecules. Our observations indicate that distortion of molecular potential energy surfaces may be the dominating feature in intense laser - molecule interactions.