CO 2 laser-microwave double resonance spectroscopy of HCNO: Precise measurement of the dipole moment and its vibrational dependence

Abstract

The HCNO molecule was investigated by laser-microwave double resonance (LMDR) with a strong electric field. Stark Lamb-dip spectra due to the 2 ν 4[000(20) 0 ← 000(00) 0] and 2 ν 4 + ν 5 − ν 5 [000(21) 1 ← 000(01) 1] bands, where ν 4 and ν 5 denote the CNO and HCN bending modes, respectively, were observed with the CO 2 laser lines in the 9.4-μm region. More than 30 LMDR signals associated with these infrared transitions were detected, which were ascribed to rotational transitions in the 000(00) 0, 000(01) 1, 000(20) 0, and 000(21) 1 vibrational states. Dipole moments were determined for these vibrational states with an accuracy better than 10 −4 D. The dipole moments in the ground and ν 5 vibrational states are 3.099345(65) D and 2.910195(58) D, respectively, with the 2.5σ uncertainties in parentheses, the difference being δμ 5 = −0.189150(85) D. The abnormally large dipole moment change caused by the excitation of the HCN bending vibration is discussed in connection with the peculiar potential governing the bending motion. The 2 ν 4 and 2 ν 4 + ν 5 − ν 5 band origins derived from the Stark Lamb-dip spectra are 1069.92955(23) and 1067.63291(8) cm −1, respectively. Stark Lamb-dip spectra were also observed for the ν 4 + 2 ν 5 [000(12) 1 ← 000(00) 0] band, for which two subband origins at 1032.79764(10) and 1077.4378(15) cm −1 were derived, corresponding to the two possible Π ( l = 1) substates of the ν 4 + 2 ν 5 vibrational state. Dipole moments were determined for both substates.