Infrared laser absorption spectroscopy of the nu(19a) band of jet-cooled fluorobenzene

The diode laser absorption spectrum of the nu (19a) band of fluorobenzene m olecule near 1500 cm(1) has been measured in a pulsed supersonic jet and tr ansitions from low J, K-a, and K-c values (2-24) are reported. The band cen tre has been found at <(<nu>)over bar>(0) = 1500.018(1) cm(1) and rotationa l constants for the upper state as determined from a least-squares fitting to experimental data have been found to be A(19a) = 0.188 77(5), B-19a = 0. 085 69(5) and C-19a = 0.058 94(1) cm(1). In addition, a number of weak tran sitions assigned to DeltaK(a) = +/-2, DeltaK(c) = +/-1, +/- 3 have been obs erved, with higher intensities than those expected in the rigid rotor model . A discussion of their origin is presented.