Transient vibrational spectroscopy by flash photolysis stimulated emission pumping: 3ν2 of singlet methylene

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Abstract
A vibration–rotation spectroscopic method based on stimulated emission pumping has been developed for transient molecules generated by flash photolysis. This method may be generally applicable to radicals with electronic excited states accessible by visible or UV laser pulses. This first demonstration on the bending v2 =3 level of ã 1A1 CH2 generated a vibrational term value of 3950.45(7) cm−1 and the rotational constants A=25.665(40), B=11.470(7), and C=6.7140(61) cm−1. These constants indicate that the 3ν2 level is well below the bending-potential barrier. Perturbations of several rotational levels by the triplet state are identified. In addition, and most interestingly, abnormally intense ΔKa =3 transitions were observed in the ã←b̃ 2163 band.