Multiphoton dissociative ionization of molecular deuterium

The kinetic energy spectra of deuterium ions produced from ${\mathrm{D}}_2$ arising from collision-free subpicosecond irradiation at 248 nm with intensities spanning the ${10}^{13}$–${10}^{16}$-W/${\mathrm{cm}}^2$ range have been measured by time-of-flight analysis. The behaviors of the kinetic energy distributions of the fragments and the relative abundances of atomic (${\mathrm{D}}^{+}$) and molecular (${\mathrm{D}}_2^{+}$) ions reveal the presence of two mechanisms of multiphoton dissociative ionization. Calibration of the energy scale for ${\mathrm{D}}^{+}$ is facilitated by comparison with ${\mathrm{He}}^{2+}$. For intensities in the ${10}^{13}$–${10}^{15}$-W/${\mathrm{cm}}^2$ region, intermediate three-photon resonances and the optical Stark shift play important roles. At an intensity ∼${10}^{16}$ W/${\mathrm{cm}}^2$, a direct transition from the molecular ground state to the dissociative ionic level appears as a significant channel. No evidence of direct double ionization was observed.