Highly predissociative levels of the D (2)Pi state of CH studied with the two-color resonant four-wave mixing technique

We investigated highly predissociative transitions D (2)Pi-B (2)Sigma(-)(0- 0) of CH in an oxyacetylene flame with two-color resonant four-wave mixing, in which two grating beams are in resonance with the D-B transition and th e pump (and signal) beam is resonant with a selected B-X transition. A tota l of 86 predissociative lines in six branches are observed, corresponding t o excitation of the D state up to N-'=16; in previous experiments lines up to only N-'=6 were observed. Observed rovibronic transitions of the D-B(0-0 ) band are fitted to known Hamiltonians to yield improved spectral paramete rs of the D (2)Pi (v=0) state. The parameters for centrifugal distortion (D ,H) and Lambda doubling (p,q,q(D)) of the D (2)Pi (v=0) state are deduced f or the first time. The predissociative line width is about 2 cm(-1) for the rotational level with N-'=1 and it gradually increases with N-'; for N-'=1 5 the width is about 7 cm(-1). Predissociation mechanisms of the D (2)Pi st ate are discussed. A barrier height similar to 5600 cm(-1) for the D state is estimated based on observed predissociative lifetimes. (C) 1999 American Institute of Physics. [S0021-9606(99)00835-1].