Photoelectron spectra ofN2+:Rotational line profiles studied with He I–excited angle-resolved spectroscopy and with synchrotron radiation

Abstract

We have recorded angle-resolved He I photoelectron spectra of the three outermost valence states in ${\mathrm{N}}_{2}^{+},$ with high enough resolution to observe rotational line profiles. For the two $\ensuremath{\Sigma}$ states, the $X{}^{2}{\ensuremath{\Sigma}}_{g}^{+}$ and the $B{}^{2}{\ensuremath{\Sigma}}_{u}^{+},$ we found that the rotational branches corresponding to different changes in rotational quantum number can differ dramatically in $\ensuremath{\beta}$ value. The well-known difference in $\ensuremath{\beta}$ value for the $\ensuremath{\nu}=0$ and \ensuremath{\nu}=1 vibrations of the $X{}^{2}{\ensuremath{\Sigma}}_{g}^{+}$ state was found to be due to different rotational branching ratios and also different $\ensuremath{\beta}$ values of the rotational branches. For the $\ensuremath{\nu}=0--2$ vibrations of the $A{}^{2}{\ensuremath{\Pi}}_{u}$ state, the $\ensuremath{\beta}$ value difference between rotational branches is much less pronounced than in the X and B states. We have also recorded synchrotron-radiation-excited photoelectron spectra of the $\ensuremath{\nu}=0$ vibrational peaks of the $X{}^{2}{\ensuremath{\Sigma}}_{g}^{+}$ and $B{}^{2}{\ensuremath{\Sigma}}_{u}^{+}$ states where rotational line profiles are resolved. The intensities of the rotational branches were studied as function of photon energy, the X state between 23 and 65 eV, and the B state between 23 and 45 eV. The results for the X state have recently been presented in a Letter [G. \"Ohrwall, P. Baltzer, and J. Bozek, Phys. Rev. Lett. 81, 546, 1998]. The rotational branching ratios of the two states have very different behaviors as functions of photon energy. The relative intensities of the rotational branches in the X state change significantly over the studied energy range. The $3{\ensuremath{\sigma}}_{g}\ensuremath{\rightarrow}k{\ensuremath{\sigma}}_{u}$ shape resonance apparently gives rise to a non-Franck-Condon-like behavior for the rotational branching ratio of the X state. In the B state, the rotational branching ratios remain essentially constant over the studied energy range.