New measurements in the (A) over tilde (1)A " - (X) over tilde (1)A' band s
ystem of monobromomethylene (HCBr) at near-infrared wavelengths are reporte
d. Rotationally resolved spectra of HCBr and DCBr for both naturally occurr
ing Br isotopes are analyzed and provide accurate energies for the low-lyin
g bending vibrational levels in both the (A) over tilde and (X) over tilde
states. The experimental results are compared to extensive ab initio calcul
ations of these two states and the low-lying (a) over tilde (3)A " state. T
he ab initio T-00 value for the (A) over tilde-(X) over tilde system is cal
culated within 500 cm(-1) of the experimental results. For the triplet stat
e, T-00 (<(<alpha>)over tilde>-(X) over tilde) was calculated to be 1833 cm
(-1). The measurements of the bending intervals in the (X) over tilde (1)A'
state suggest that this is about 250 cm(-1) too low. With this shift and i
nclusion of spin-orbit coupling between the (a) over tilde and (X) over til
de states, the calculated bending vibrational levels in the (X) over tilde
state reproduce the observations, and the calculations make testable predic
tions of the position of the triplet and other singlet state vibronic level
s. Combining the experimental and computational results, we estimate the tr
iplet, T-00(<(<alpha>)over tilde>-(X) over tilde), excitation energy to be
2028 cm(-1). (C) 2001 American Institute of Physics.