Shock-tube study of relaxation in HCN

Thermal vibrational relaxation in HCN mixtures with Kr has been observed with the laser-schlieren technique. The experiments cover the temperatures 750-2900 K and a large pressure range of 13-420 Torr in 5% and 20% HCNKr mixtures. Relaxation is extremely fast but appears to occur in two well-separated stages that are assigned to the vibrational transitions (000)-->(010) and (000)-->(100) with perhaps some lesser contribution from (000)-->(001). This interpretation is strongly supported by a comparison of net density changes to thermodynamic calculations. The first and faster process shows near constant relaxation times whereas the latter slower stage has a slight decrease of these with T. Relaxation times in pure HCN obtained by neglecting the small contribution of krypton are as follows: (a) Ptau(HCN-HCN)=27 exp(1.473T(13)) ns atm (000)-->(010); (b) Ptau(HCN-HCN)=11 exp(32.6T(13)) ns atm (000)-->(100). Probabilities suggested by these results are around 0.05 for the fast step and 0.0035 to 0.005 for the slow process. These results are close to those found by laser fluorescence measurements for deactivation of levels involving excitation of the C-H stretch (001) at 3312 cm(-1). These results are also consistent with the notion of a dominance of the fast stage by T,R-V transfer (thermal relaxation) occurring in a weakly bound complex. However, the slow step most likely occurs through a V-V process (03 (1)0)-->(100), DeltaE=27.7 cm(-1), after multiple excitation of the (010) mode. These are the first thermal measurements of relaxation in HCN and the first to see energy transfer involving the low-frequency modes.