Spectroscopy and predissociation dynamics of the à 1A′′ state of HNO

Abstract

The spectroscopy and predissociation dynamics of the à 1A′′ state of HNO have been investigated by measurement of line positions and lifetime broadened linewidths in the cavity ring-down (CRD) spectrum. CRD spectroscopy is a technique better suited to studies of molecular predissociation than methods such as laser induced fluorescence in cases where the excited state dissociation lifetime is short compared to its fluorescence lifetime. The CRD spectrum extends well beyond the dissociation limit (we have identified transitions to rotational states lying up to 2400 cm−1 above the dissociation limit of 16450 cm−1). The lifetime-dependent Lorentzian components of the line shapes of numerous rovibrational features of the à 1A′′–X̃ 1A CRD absorption spectrum have been deconvoluted from the Doppler and laser line profiles to obtain lifetimes and predissociation rates for individual |v1v2v3〉|JK〉 states. Here, the labels v1, v2, and v3 denote the number of quanta of the N–H stretch, N=O stretch, and H–N–O bending vibrations, respectively. We have measured line broadening (of up to 0.3 cm−1) in transitions to six vibronic states above the predissociation threshold (the 100 and 020 states, for which the higher K levels are above the dissociation limit, and the 101, 030, 110, and 111 states). For three substates (100 K=5, 101 K=1 and 110 K=4) strongly J-dependent transition linewidths are seen. The 100 K=5 and 101 K=1 substates show maximum transition linewidths midway through the observed spectral transitions while the linewidths for transitions involving the 110 K=4 substate increase with J. Linewidths also generally increase with K. Some lines involving the 100 K=5 state are markedly asymmetric. Linewidths for transitions to states having excitation of the bending mode (the 101 and 111 states) are larger than those for which v3=0. These observations clarify the predissociation mechanism suggested by previous absorption and LIF studies. We attribute the primary predissociation mechanism to a-axis Coriolis coupling of à state levels to discrete quasibound highly vibrationally excited levels of the ground state which in turn are coupled to the electronic ground state continuum corresponding to dissociation to H(2S)+NO(X 2Π). Predissociation of à state levels with K=0 is probably caused by b-axis Coriolis coupling to such quasibound levels of the ground state. The variation of predissociation rates with J and K for the à 110 K=4, 5, and 6 substates cannot be accounted for by this mechanism and we propose the onset of predissociation to the continuum of the ã 3A′′ state. Interpretation of our experimental data is assisted by calculations performed using the potential energy surfaces of Guadagnini et al. [J. Chem. Phys. 102, 774 (1995)].