Vibrational state-to-state resolution in the collisional relaxation of a highly vibrationally excited polyatomic: HCN

Vibrational Energy Transfer

“…Perhaps far more important, however, has been the introduction of high- and superhigh-resolution spectroscopic probe methods that employ lasers to achieve both the necessary resolution and the requisite sensitivity needed for the most sophisticated experiments. Increasingly, these experimental techniques are moving toward the complete definition of collisional energy transfer processes in which the vibrational, rotational, and translational motions of molecules can be defined before and after a collision has occurred. − ,− (Indeed on the distant horizon can be seen the possibility of studying collisions in which the spatial orientation dependence and the impact parameter distribution can be controlled or revealed. − ) Equally impressive have been theoretical developments that now provide predictive values for quantum state resolved energy transfer processes as well as a description of collisional vibrational energy transfer for small molecules with chemically significant amounts of energy. For example, the “azimuthal and vibrational close-coupling, infinite-order sudden approximation” theoretical approach has been used to make quantitative predictions of the quantum state resolved rovibrational energy transfer in electronic ground state (S 0 ) glyoxal .…”

“…Quantum state resolved experiments on highly vibrationally excited molecules have followed two important trails, blazed by the two laser-based methods: overtone and stimulated emission pumping , (SEP). The thriving field of overtone spectroscopy and dynamics, ,− which is built entirely upon harmonically forbidden transitions, is testament to the amazing times in which we live. High spectral brightness lasers in the infrared and visible force one to consider carefully the meaning of the phrase “forbidden transition”.…”

“…There are, however, a number of molecules that are clearly tractable. One of these is HCN, and there has been some work done using overtone pumping. − Eventually one must come to grips with the fact that larger molecules have complex spectra that cannot be easily assigned. It remains a fascinating challenge for the future to see what can be learned about the collisional properties of such molecules.…”

Shock-tube study of relaxation in HCN

“…[2][3][4][5][6][7][8][9][10][11] Nonetheless, the LS technique allows unambiguous resolution of the thermal process. The relaxation seen here is one that clearly takes place in two well-separated stages ͓putting aside any possible ͑001͒ excitation͔, i.e., this is at least a "double relaxation" much like those seen in ethane, 29 trifluoroethane, 30 SO 2 , 31 and CH 2 Cl 2 .…”

“…However, collisional energy-transfer rates and probabilities have been determined for several vibrational levels involving HCN ͑001͒ C-H stretch excitation ͑3312 cm −1 ͒, 2-8 ͑011͒, 7-9 ͑101͒, 7,10 ͑002͒, 6 ͑121͒, ͑003͒, and ͑301͒. 11 These studies utilized V-V transfer from laser-excited HF, DF, and H 2 with detection of infrared fluorescence, 2 laser induced excitation with infrared fluorescence, [3][4][5] and laser-induced fluorescence. 9 The only study with varied temperatures found a strong slowing, a negative dependence, for HCN +HCN͑001͒ → HCN͑mn ᐉ 0͒ + HCN over 240-390 K. 2 This behavior suggested to Arnold et al 4 that this efficient process probably involves V-V transfer facilitated in a weakly bound dimer created by hydrogen bonding, 12,13 and such strong interaction might also explain the rapidity indicated in the failure of early efforts to observe thermal relaxation.…”