The (b) over tilde B-1(1)-(a) over tilde (1)A(1) spectrum of methylene betw
een 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 dif
ferences. On the basis of comparison with published results of ab initio ca
lculations, they were assigned to levels with both (a) over tilde and (b) o
ver 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 previou
sly been observed. The rotational assignments of the last, K = 0 (b) over t
ilde (0,2,0), observed via transitions from the (a) over tilde (0,1,0) vibr
ationally hot bending level, differ from the original assignments of Herzbe
rg and Johns [Proc. R. Sec. London, Ser. A 1966, 295, 107]. A comparison wi
th the results of the most recent high-level quantum chemical calculations
suggests that further refinement of the potential energy surface of the b s
tate near its equilibrium would be desirable.