Observation of a new inner-orbital molecular transition at 55.8 nm inN22+produced by multiphoton coupling

A new transition at 55.8 nm (22.2 eV) has been observed in ${\mathrm{N}}_2^{2+}$ arising from multiphoton excitation of ${\mathrm{N}}_2$ with intense subpicosecond 248-nm radiation. The band has an observed width of ∼0.3 nm full width at half maximum, exhibits vibrational splittings, has an estimated radiative lifetime of ∼100 ps, and is attributed to a ${}_{}{}^3{}_{}{}^{}\mathrm{\Pi }_{\mathrm{u}}^{}{}_{}{}^{}$ ${\to }^3$ ${\mathrm{\Pi }}_{\mathrm{g}}$ excimerlike transition arising from the direct multiphoton production of molecular levels having a 2${\sigma }_g$ vacancy. The upper level has a (2${\sigma }_g^{\mathrm{-}1}$, 1${\pi }_u^{\mathrm{-}1}$) configuration, an equilibrium internuclear separation of ∼1.2 Å, and results from an entirely new strong-field mode of electronic excitation.