Field ionization of high-lying states ofH2

We report the first systematic study of the field ionization threshold of molecular Rydberg states. We find that the v=0 np series of both ortho and para ${\mathrm{H}}_2$, converging to the lowest rotational state of ${\mathrm{H}}_2$ ${\mathrm{}}^{+}$ with n ranging from 18–30, ionize at fields given by 1/16${\mathrm{n}}_{\mathrm{s}}^4$ with ${\mathrm{n}}_{\mathrm{s}}$, the effective quantum number at the field at which ionization takes place given by ${\mathrm{n}}_{\mathrm{s}}$=n-0.5. Our threshold field measurements suggest that the ionization process is described classically.