Absorption spectroscopy of singlet CH₂ near 9500 cm^-1 1

The b̃ ¹B₁-ã ¹A₁ 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(b̃ ¹B₁) 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 ã and b̃ primary electronic wave function character: K = 0 b̃ (0,1,0), K -4 b̃ (0,0,0), K = 1 ã (0,8,0), and K = 0 b̃ (0,2,0). The first three of these levels had not previously been observed. The rotational assignments of the last, K = 0 b̃ (0,2,0), observed via transitions from the ã (0,1,0) vibrationally hot bending level, differ from the original assignments of Herzberg and Johns [Proc. R. Soc. 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 b̃ state near its equilibrium would be desirable.