Vibrational relaxation of HCN(002)a)

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Abstract
Tuned output from an optical parametric oscillator has been used to excite HCN to its (002) level for measurements of vibrational energy transfer. By observing time‐resolved (002,001) fluorescence in the presence of different collision partners, the following rate constants (k002/cm3 molecule−1 s−1) were obtained for relaxation of HCN(002) at 298±4 K: kHCN=(6.8±0.4)×10−12; kHe=(5.6±0.25)×10−15; kNe=(3.8±0.16)×10−15; kAr= (3.0±0.4)×10−15; kKr=(3.25±0.12)×10−15; k02=(1.39±0.08)×10−14; kN2=(2.6±0.15)×10−14; kCO=(6.8±0.4)×10−14. These results are compared with the rate constants (k001) obtained for relaxation of HCN(001) and the mechanisms for relaxation are discussed. With noble gases as collision partners, the ratios (k002/k001) are less than one. Comparison of these ratios with predictions of the ‘‘breathing sphere’’ theory indicates that relaxation of HCN(00v3=1,2) occurs via transfer to HCN (120 {v3−1}), the lower rate for v3=2 being due to the increased energy gap between initial and final states.