Kinetic energy distributions of ionic fragments produced by subpicosecond multiphoton ionization of N2

A study of the kinetic energy distributions of ionic fragments produced by subpicosecond irradiation of N2 with 248-nm radiation at an intensity of 1/41016 W/cm2 is reported. These measurements, in comparison to other findings involving molecular excitation with charged particles and soft x rays, reveal several important features of the nonlinear coupling. Four ionic dissociative channels are identified from the data on the multiphoton process. They are N22+N++N+, N22+N+N2+, N23+N++N2+, and N24+N++N3+, three of which are charge asymmetric. The data for the energy distributions are found to be in approximate conformance with a simple picture involving ionizing transitions occurring within a time of a few cycles of the ultraviolet wave at an internuclear separation close to that of the ground-state (X g+1) molecule. The implication follows that a strong nonlinear mode of coupling is present which causes a high rate of energy transfer. A simple hypothesis is presented which unites the ability for rapid energy transfer with the observed tendency to produce charge-asymmetric dissociation. © 1989 The American Physical Society.