Dissociation and ionization of H$_{2}^{+}$ in intense femtosecond laser fields studied by coincidence 3D momentum imaging*

Dissociation and ionization of H$_{2}^{+}$ in intense laser fields have been measured simultaneously using a coincidence 3D momentum imaging method. The H$_{2}^{+}$ beam is crossed by a laser beam (45-135 fs, 790 nm, 10$^{13}$-10$^{15}$ W/cm$^{2})$, and the momentum of each fragment in H$^{+}$+H and H$^{+}$+H$^{+}$ is determined. The angular and kinetic energy release spectra are obtained. At similar intensities, the dissociation mechanisms in long and short pulses are found to be quite different, dominated by bond-softening and above threshold dissociation, respectively. The ionization of H$_{2}^{+}$ becomes measurable from about 2$\times $10$^{14}$ W/cm$^{2}$, increases rapidly with laser intensity, and aligns strongly along the laser polarization with a broad kinetic energy distribution. The overall ionization to dissociation ratio is less than previously predicted by theory.