Resonant multiphoton ionization ofH2via theE,FΣg+1state: Absorption of photons in the ionization continuum

This work reports the five-photon ionization of the ${\mathrm{H}}_2$ molecule via the four-photon transitions (E,F ${\mathrm{}}^1$ ${\Sigma }_g^{+}$, $v_E$=1,2,3)←(X ${\mathrm{}}^1$ ${\Sigma }_g^{+}$, v=0), in the ${10}^{11}$-W/${\mathrm{cm}}^2$ laser-intensity range. The resulting ${\mathrm{H}}^{+}$- and ${\mathrm{H}}_2$ ${\mathrm{}}^{+}$-ion spectra exhibit essentially the Q(J) resonances, and the ratio [${\mathrm{H}}^{+}$]/[${\mathrm{H}}_2$ ${\mathrm{}}^{+}$] is found to increase drastically with the vibrational quantum number $v_E$. The photoelectron energy spectra are dominated by the peak corresponding to the five-photon ionization of ${\mathrm{H}}_2$ leading to ${\mathrm{H}}_2$ ${\mathrm{}}^{+}$ ions in the $v_{+}$=$v_E$ vibrational level. The photodissociation of ${\mathrm{H}}_2$ ${\mathrm{}}^{+}$ is found to be the only process which forms ${\mathrm{H}}^{+}$. In addition, we report—for the first time, to our knowledge—the absorption by a molecule of an extra photon above the ionization potential, when the laser intensity is increased up to 7×${10}^{11}$ W/${\mathrm{cm}}^2$.