Sub-Doppler laser absorption spectroscopy of the A2Π i − X2Σ + (1, 0) band of CN: Measurement of the 14N hyperfine parameters in A2Π CN

Abstract

Measurements of multiple rotational lines in the (1, 0) band of the A 2Π i − X 2Σ + “red” system of the cyanogen radical (CN) at sub-Doppler resolution are reported. The CN radical was produced by 193 nm photodissociation of NCCN (ethane dinitrile) and detected with a Ti:sapphire ring laser operating near 10 900 cm −1. The sample was exposed to a weak, frequency-modulated probe beam and a strong, counterpropagating bleach laser beam. Demodulated probe laser signals display absorption and dispersion features derived from Doppler-free saturation of the hyperfine components as the laser scans across the central region of a Doppler-broadened rotational line spectrum. Hyperfine-resolved transition frequencies were combined with known ground-state X 2Σ hyperfine term values to determine A 2Π state hyperfine term values, which were analyzed in terms of an effective Hamiltonian for the A 2Π state. All the expected hyperfine and 14N quadrupolar parameters were determined and their values analyzed in terms of a simple molecular orbital picture of the bonding in the radical. Higher sensitivity obtained with 400 kHz amplitude modulation of the bleach laser and additional phase-sensitive detection allowed hyperfine splittings in some rotational lines of 13C 14N to be observed in natural abundance. Excited state hyperfine splittings were determined for a selection of rotational states, but not enough to determine the 13C hyperfine parameters.