Axis-switching and coriolis coupling inthe AÃ(010)-X^˜(000) transitions of DCCl and HCCl

The rotationally resolved Ã(010)-X̃(000) spectrum of DCCl between 12 880 and 12 964 cm^-1 was measured using frequency-modulated laser absorption spectroscopy of jet-cooled and ambient temperature samples. Transitions to levels with K_a′ = 0 and 1 were assigned, and their analysis leads to improved accuracy of both the ground state rotational constants of DCCl and, when combined with existing data for HCCl, the geometry of the radical. In addition to the expected perpendicular band structure, a number of parallel (ΔK_a = 0) subbands were observed. Their intensity derives from a combination of c-type Coriolis coupling and axis-switching (J. T. Hougen and J. K. G. Watson, 1965, Can. J. Phys. 43, 298) resulting from the change in geometry between the two states. The two contributions have been calculated for the (010)-(000) band of DCCl and previously recorded data for HCCl. Satisfactory agreement with experimental measurements was obtained. The Coriolis contributions are small for these transitions, but may add to or subtract from the axis-switching. For higher bending excitation in the upper state, Coriolis coupling is predicted to make larger contributions to the parallel subband intensities.