Vibrational relaxation rate constants for HF(v=1–4) by CO, CO2, and HCN with product identification by infrared emission

Abstract

Vibrational relaxation of HF(v=1–4) by CO, CO2, and HCN has been studied by infrared emission in a flow reactor. The vibrationally excited HF molecules were generated by the reactions of F atoms with CH4, CH3Cl, H2S, and HBr in 0.8 Torr of Ar carrier gas. By monitoring the rotational distribution in the lower v levels, the fraction of the relaxation proceeding by Δv=−1 and −2, V→R transfer was determined. Numerical integration of the rate equations and modified Stern–Volmer analysis of the HF(v) distribution vs reagent concentration were used to determine the relaxation rate constants and the mechanisms. The relaxation rate constants increase with v, but the magnitude of the rate constants depend upon the collision partner; the HF(v=3) values (in cm3 s−1) are 2.5×10−12 for CO, 1.1×10−11 for CO2, and 1.0×10−10 for HCN. The HF(v) relaxation mechanism by CO is dominated by V→R transfer (≥70%), giving HF(v−1, J=10–14). Relaxation by CO2 has significant Δv=−2 and −3, V→R (≊30%) and Δv=−1, V→R (∼20%) pathways, as well as a V–V pathway. Comparing the HCN(ν3) emission intensity to the loss in HF(v) emission intensity gave the efficiency of V→V transfer from HF(v) to HCN(ν3) as 55±15%; some Δv=−1, V→R transfer also was identified. The HF(v, J≥8) rotational relaxation rate constants increase in the CO, CO2, HCN series. R–V transfer from HF(J≥16) to the CO2 bending mode was identified as the fastest rotational relaxation process.