DISPERSED FLUORESCENCE SPECTROSCOPY OF THE NO3 B̃ 2E′ - X̃ 2A2′ SYSTEM: OBSERVATION OF OUT-OF-PLANE LEVELS IN THE X̃ 2A2′ STATE

Abstract

The $\tilde{B}$ $^2E'$ -- $\tilde{X}$ $^2A_2'$ system of jet cooled NO$_3$ is investigated via laser induced fluorescence (~LIF~). Dispersed fluorescence (~DF~) spectra from single vibronic levels of the upper electronic state have been measured. It has been reported that the vibronic bands observed in the red absorption spectrum of the $\tilde{B}$ -- $\tilde{X}$ system have broadened characteristics\footnote{D.~A.~Ramsay, in Proceedings of the Tenth Colloquium Spectroscopicum Internationale, edited by E.~R.~Lippencott and M.~Margoshes (Spartan Books, Washington, DC., 1963), p. 593-596.}. The vibrational structure of the DF spectrum obtained by excitation of the 0+770 cm$^{-1}$ band shows several fluorescence bands terminated to the out-of-plane vibrational levels in the lower $\tilde{X}$ $^2A_2'$ state, such as $a_2''$ and $e''$, e.g.~the fluorescence band emitting to the $e''$ level of $\nu_2$ + $\nu_4$. The present observation indicates that the fluorescent vibronic level lying 770 cm$^{-1}$ above the vibrationless level of the $\tilde{B}$ $^2E'$ state has contributions of the $\nu_2$ and $\nu_4$ modes and/or that the fluorescent level has an out-of-plane geometry. It is concluded that, because the fundamental and over-tone bands of the $\nu_2$ and $\nu_4$ modes, respectively, in the $\tilde{X}$ $^2A_2'$ state lie $\sim$ 760 cm$^{-1}$\footnote{R.~R.~Friedl and S.~P.~Sander, $J$. $Phys$. $Chem$. {\bf 91}, 2721 (1987)}\footnote{K.~Kawaguchi $et$ $al$., $J$. $Mol$. $Spectrosc$. {\bf 334}, 10 (2017)}, the 0+770 cm$^{-1}$ level is attributed the level coupled by the $\nu_2$ fundamental and $2\nu_4$ over-tone levels. This also suggests that the out-of-plane vibrational levels are important to analyze the vibrational structure of the $\tilde{X}$ $^2A_2'$ state.