Kinetic energy distributions of ionic fragments produced by subpicosecond multiphoton ionization ofN2

Abstract

A study of the kinetic energy distributions of ionic fragments produced by subpicosecond irradiation of ${\mathrm{N}}_2$ with 248-nm radiation at an intensity of ∼${10}^{16}$ W/${\mathrm{cm}}^2$ 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 ${\mathrm{N}}_2^{2+}$→${\mathrm{N}}^{+}$+${\mathrm{N}}^{+}$, ${\mathrm{N}}_2^{2+}$→N+${\mathrm{N}}^{2+}$, ${\mathrm{N}}_2^{3+}$→${\mathrm{N}}^{+}$+${\mathrm{N}}^{2+}$, and ${\mathrm{N}}_2^{4+}$→${\mathrm{N}}^{+}$+${\mathrm{N}}^{3+}$, 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 ${}_{}{}^1{}_{}{}^{}\mathrm{\Sigma }_{\mathit{g}}^{+}{}_{}{}^{}$) 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.