JET-COOLED CAVITY RING-DOWN SPECTROSCOPY OF THE $\tilde{A} ^2E^{\prime\prime}$-$\tilde{X} ^2A_2^{\prime}$ VIBRONIC TRANSITION OF NO$_3$

Abstract

The three energetically lowest electronic states ($\tilde{X}$ $^{2}A_{2}^{\prime}$, $\tilde{A}$ $^{2}E^{\prime\prime}$, and $\tilde{B}$ $^{2}E^{\prime}$) of NO$_3$ are strongly coupled by vibronic interactions and have been treated in considerable detail theoretically., nderline{\textbf{126}}, 134309 (2007)}} Corresponding experimental characterization of the interaction is much less detailed. Previous experimental results primarily consist of IR measurements of vibrational transitions in the ground state., nderline{\textbf{93}}, 951 (1990)}}, nderline{\textbf{231}}, 193 (1998)}} In addition, the electronically forbidden $\tilde{A}$-$\tilde{X}$ transition has been observed in ambient temperature CRDS studies., nderline{\textbf{122}}, 224305 (2005)}}, nderline{\textbf{107}}, 2829 (1997)}} A slit-jet nozzle with a high voltage pulsed discharge has been applied to produce the NO$_3$ radical by dissociating the N-O bond of N$_2$O$_5$, and the jet-cooled NO$_3$ CRDS absorption spectrum has been successfully observed with a high-resolution laser source ($\Delta\nu\approx$250MHz, intrinsic resolution considering the instrumental linewidth and the residual Doppler broadening in the jet). The 4$^1_0$ band (parallel band) shows complex rotational structure which is presently being analyzed. The 2$^1_0$ band has also been measured as an example of a perpendicular band. Besides the $\nu_2$ and $\nu_4$ vibronic bands, the vibronically forbidden origin band (0$^0_0$ band) has been recorded under the same experimental conditions. The weakly observed $\tilde{A}$-$\tilde{X}$ origin band is likely either a magnetic dipole or an electric quardrupole transition.