PREDISSOCIATIVE LEVELS OF CH $\tilde{B} {^{2}\Sigma^{-}}$ STATE OBSERVED BY 2-COLOR LASER-INDUCED GRATING TECHNIQUE

Abstract

Trace amount of CH in an acetylene flame was detected by resonance 4-wave mixing/2-color laser-induced grating spectroscopy (LIGS). Selected vibronic lines were detected either by probing the $\tilde{A} {^{2}\Delta-\tilde{X} {^{2}\Pi}$ transition in the 419-430 nm region with a grating formed from the $\tilde{B} {^{2}\Sigma^{-}} - \tilde{X}^{2}\Pi$ transition in the 387.396 nm region or by probing the $\tilde{B} -\tilde{X}$ transition with a grating formed from the $\tilde{A}- \tilde{X}$ transition. The double-resonance nature of this technique is advantageous in marking the specific state and obtaining unambiguous assignments. Furthermore, predissociative states with small yield of fluorescence (hence unobservble by the laser-induced fluorescence technique) can be readily detected with LIGS. The predissociative levels of the $\tilde{B}$ state were detected by means of the former scheme with grating formed from the $\tilde{B} - \tilde{X}$ transition; lines up to $N^{\prime} = 21 of $\nu^{\prime} = 0$ were observed. The line position and the linewidth (hence the lifetime) of these previously unobserved predissociative levels were determined. At $N^{\prime} = 19 (\nu^{\prime} = 0)$, linewidth broadening becomes apparent and the linewidth increases to $\sim 7 cm^{-1} at N^{\prime} = 21$; a more accurate predissociation threshold was consequently determined.