HIGHLY EXCITED VIBRATIONAL LEVELS OF SINGLET METHYLENE: RENNERTELLER COUPLING AND HIGH $\Delta K_{a}$ ROTATIONAL TRANSITIONS

Abstract

Dispersed fluorescence spectra from the $CH_{2} b^{-1}B_{1}$ state to highly vibrationally excited levels of the $a^{-1}A_{1}$ state were recorded by time-resolved Fourier transform emission spectroscopy (FTES). Four new $a^{-1}A_{1}$ vibrational levels, (2,0,0), (0,5,0), (1,2,0), and (1,3,0), were observed in the $5,000-8,000 cm^{-1}$ region. The $a^{-1}A_{1} (0,5,0)$ and (0,6,0) levels, which are close to the a state bending barrier and the $\bar{b}$ state origin, suffer from Renner-Teller coupling with the $\bar{b}$ state levels. A dramatic effect on the $a^{-1}A_{1} (0,5,0)$ rotational constants due to Renner-Teller coupling is observed. FTES also allows accurate measurement of the $\Delta K_{a} = 3$ transition intensities and comparison with rigid rotor calculations. The enormously strong $\Delta K_{a} = 3$ transitions in the $\bar{b} \leftrightarrow \bar{a}$ bands can be completely attributed to the asymmetry in the $\bar{a}$ state of $CH_{2}$.