ROTATIONALLY RESOLVED SPECTRUM OF THE NU-1 CH STRETCH OF THE PROPARGYL RADICAL (H2CCCH)

The nu-1 acetylenic CH stretch of the propargyl radical (HCCCH2) near 3322 cm-1 has been observed by infrared laser kinetic spectroscopy at Doppler limited resolution. Propargyl is prepared by flash photolysis of propargyl bromide or propargyl chloride at 193 nm and its transient infrared absorption probed by a cw color center laser. The spectrum consists, as is expected, of only a-type (DELTA-K=0) transitions, and near the origin appears quite simple resembling that of a diatomic molecule. This means that the A and D(K) rotational constants in the upper state must differ little from the ground state so that the various K subbands overlap almost exactly. However, at N greater-than-or-equal-to 14 the spectrum no longer has this regular pattern, probably because of perturbations of the excited state by nearby vibrational levels. Ground and excited state rotational constants were determined by fitting the K = 1 lines having N" < 14,