Ionization and Dissociation of $N_2 $ from 17.5 to 36.5 eV by Linearly and Circularly Polarized Light

We have measured the linear $\left( {P_1 } \right)$ and circular $\left( {P_3 } \right)$ polarization of the fluorescence emitted in the $\,B{ }^2\Sigma _u^+ \to \,X{ }^2\Sigma _g^+ \;\left( {{\nu }'=0,{\nu }''=0} \right)$ transition (391.4 nm) of $N_2^+ $ after photoionization of $N_2 $ by both linearly and circularly polarized VUV radiation. The value of $P_1 $ for linearly polarized excitation is in qualitative agreement with previous results [1]. Results for circularly-polarized excitation show significantly different energy dependence. In this energy range, photofragmentation into neutral atoms caused by the predissociation of doubly-excited Rydberg states via non-Rydberg doubly-excited resonances competes with photoionization [2]. We have measured the intensity and a distinct non-zero $P_3 $ of the fluorescence from the $NI\,\,3p\,{ }^4P^o\to 3s\,{ }^4P\,$ transition (818 nm) between 22.5 and 25 eV which corresponds to the initial excitation of the $N_2 $ Rydberg R(C) states. [1] J. A. Guest \textit{et al.}, Phys. Rev. A \textbf{28}, 2217 (1983) [2] P. Erman \textit{et al.}, Phys. Rev. A \textbf{60}, 426 (1999) Support provided by the NSF (Grant PHY-0354946), the DOE (LBNL/ALS) and the ANSTO (Access to Major Research Facilities Programme).