Sub-Doppler laser absorption spectroscopy of the A2Πi-X2Σ+ (1,0) band of CN: Measurement of the 14N hyperfine parameters in A2Π CN

Measurements of multiple rotational lines in the (1,0) band of the A(2)Pi(i) - X-2 Sigma(+) "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, counter-propagating 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-2 Sigma hyperfine term values to determine A(2)Pi state hyperfine term values, which were analyzed in terms of an effective Hamiltonian for the A(2)Pi state. All the expected hyperfine and N-14 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 (CN)-C-13-N-14 to be observed in natural abundance. Excited state hyperfine splittings were determined for a selection of rotational states, but not enough to determine the C-13 hyperfine parameters. (C) 2008 Elsevier Inc. All rights reserved.