The vibrational excitation of H-2(+) after multiphoton ionization of H-2

We have ionized H-2 molecules in intense fields of a 6-ns pulsed Nd:YAG laser at the fundamental, second, and third harmonic wavelengths (lambda = 1064, 532, and 355 nm) and measured the relative population of the vibrational levels of the H-2(+) molecular ions that were thus produced. In comparison with the standard Franck-Condon overlap between the ground state of the neutral molecule and the vibrational levels of the ion, the population is very much shifted towards the lower levels, the more so for shorter wavelengths. In fact, for 355 nm, the shift is so large that practically only the v' = 0 level is populated. At 1064 run, the experimental results compare well with the predictions from a model developed recently on the basis of tunneling ionization directly from the around state of H-2. At shorter wavelengths, transient excitation must be invoked, either to field-dressed ionic states, H+H-, or to short lived resonances in Rydberg, states.