Absorption spectroscopy of singlet CH2 near 9500 cm-1

The (b) over tilde B-1(1)-(a) over tilde (1)A(1) spectrum of methylene between 9317 and 9610 cm(-1) was recorded using frequency-modulated diode laser absorption spectroscopy. This is the lowest-energy region ever observed in the CH2((b) over tilde B-1(1)) electronic state. Transitions to four upper rovibronic levels were identified using known ground-state combination differences. On the basis of comparison with published results of ab initio calculations, they were assigned to levels with both (a) over tilde and (b) over tilde primary electronic wave function character: K = 0 (b) over tilde (0,1,0), K = 4 (b) over tilde (0,0,0), K = 1 (a) over tilde (0,8,0), and K = 0 (b) over tilde (0,2,0). The first three of these levels had not previously been observed. The rotational assignments of the last, K = 0 (b) over tilde (0,2,0), observed via transitions from the (a) over tilde (0,1,0) vibrationally hot bending level, differ from the original assignments of Herzberg and Johns [Proc. R. Sec. London, Ser. A 1966, 295, 107]. A comparison with the results of the most recent high-level quantum chemical calculations suggests that further refinement of the potential energy surface of the b state near its equilibrium would be desirable.