A computer controlled infrared color center laser has been developed primarily for the purpose of studying free radicals in the gas phase and has been applied to the study of the C(,2)H radical. The laser spectrometer, based on a commercial Burleigh FCL-20 color center laser, has been shown to be a stable, single frequency, tunable, c.w. infrared source capable of continuous scans of 10 cm('-1) or more with resolution of 10 MHz or better.
A number of tests were performed to determine the sensitivity of the spectrometer. Noise limitations inherent to color center lasers have been overcome by combining the color center laser spectrometer with sensitivity enhancement techniques. These techniques include multi-pass absorption, Stark modulation, magnetic rotation, and frequency modulation. Experiments were conducted to test the combination of these techniques with the spectrometer. It has been found that magnetic rotation spectroscopy combined with multi-pass methods offer greatly increased sensitivity along with the advantage of being selective to only paramagnetic species, such as free radicals.
Using this computer controlled color center laser spectrometer, the free radical C(,2)H has been detected in a discharge of argon over polyacetylene. This is the first reported gas phase absorption spectra of the vibrational and electronic transitions of C(,2)H.
The C(,2)H radical was extensively studied in the 4200-3580 cm('-1) region. A rich spectrum has been observed, with typical line density of 1 or 2 lines per cm('-1), most of which have been assigned to argon transitions. A number of regular bands have been detected in this region, of which five have been firmly assigned to C(,2)H. These bands are centered at 3693, 3772, 3786, 4012, and 4108 cm('-1). The 3772 cm('-1) band has been assigned to the transition from the first excited bending mode of the ground state to the first excited bending mode of the A('2)(PI) electronic excited state. The 3786 cm('-1) band has been tentatively assigned to the origin band of this electronic transition while the remaining bands have been assigned to ground state vibrational transitions.
The rotational and vibrational nature of these bands has been determined. Estimates of all three vibrational frequencies have been made, with (nu)(,1) = 3625 cm('-1), (nu)(,2) = 375 cm('-1), and (nu)(,3) = 1860 cm('-1). This is the first reported estimate of the bending frequency, (nu)(,2).