Pulsed-discharge-nozzle Fourier-transform microwave spectroscopy of HC3S(2Πr) and HC4S(2Πi)

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Abstract
Rotational spectra of sulfur‐bearing carbon‐chain free radicals HC3S and HC4S have been observed for the first time by using a Fabry–Perot type Fourier‐transform microwave spectrometer combined with a pulsed discharge nozzle. Rotationally cooled (∼1–2 K) HC3S(2Π1/2) and HC4S(2Π3/2) in their ground vibronic states have been produced by a discharge in a supersonic free jet of an Ar‐diluted mixtures of C2H2 and CS2. Transitions of an isotopic species, HC334S, have also been observed in the same system, and those of DC3S and DC4S have been observed by using C2D2 instead of C2H2. The observed spectral patterns for these species have clearly indicated that HC3S and HC4S are linear free radicals in the ground electronic states with 2Π symmetry. The spectra of HC3S, HC334S, and DC3S have shown both the Λ‐type doubling and the hyperfine splittings due to the H or D nucleus, whereas those of HC4S and DC4S have shown only the hyperfine splittings. The observed transition frequencies for the radicals in the 7–25 GHz region have been fitted to the Hamiltonian appropriate to 2Π electronic states, yielding the effective molecular constants including the hyperfine parameters. The total bond length of HC3S has been determined to be 5.107 Å from the substituted‐coordinate analysis.

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