Effect of Molecular Orientations on Vibrational-Translational Energy Transfer

An evaluation of the steric factor ${\text{1/Z}}_0$ for molecule‐atom and molecule‐molecule collisions has been made for the orientation‐dependent Morse potential over the temperature range from 300° to 4000 °K. ${\text{1/Z}}_0$ is found to be of the order of $\frac{1}{3}$ for ${\mathrm{H}}_2‐\mathrm{Ar},$ $\frac{1}{4}$ for ${\mathrm{N}}_2‐\mathrm{Ar},$ $\frac{1}{5}$ for $O_2‐\mathrm{Ar},$ 1/7 for ${\mathrm{Cl}}_2‐\mathrm{Ar},$ and $\frac{1}{9}$ for ${\mathrm{I}}_2‐\mathrm{Ar}$ at ordinary temperatures. ${\text{1/Z}}_0$ for molecule‐molecule collisions is very small compared to these values; it is of the order of 1/20 for ${\mathrm{H}}_2‐{\mathrm{H}}_2,$ 1/25 for ${\mathrm{N}}_2‐{\mathrm{N}}_2,$ 1/30 for ${\mathrm{O}}_2‐{\mathrm{O}}_2,$ 1/50 for ${\mathrm{C1}}_2‐{\mathrm{C1}}_2,$ and 1/55 for ${\mathrm{I}}_2‐{\mathrm{I}}_2.$ A moderately important temperature dependence of ${\text{1/Z}}_0$ is found. In the calculation of ${\text{1/Z}}_0$ we explicitly considered the importance of attractive forces and used “realistic” exponential range parameters. The temperature dependence of the parameter is discussed, and it is shown that the serious errors might occur in calculating vibrational transition probabilities if the best‐fit parameter to experimental data at a particular temperature is used over a wide temperature range $\text{(>1000°).}$