Collision-Induced Relaxation of CH(X²Π, υ=0) Radical by He, Ar, and N₂ under Low-Temperature Supersonic Flow Condition

Collision-induced relaxation process of CH(X²Π, v=0) radical in various bath gases He, Ar, and N₂ has been investigated experimentally under low-temperature (26– 52 K) supersonic flow conditions. The CH radicals were generated with internal excitation by multiphoton photolysis of CHBr₃ at 248 nm, and its rotational temperature was found to relax to the flow temperature in a few microseconds by colliding with bath gas. The relaxation rate coefficients for CH(X²Π, v=0) radical in He, Ar, and N₂ flow were obtained by time-resolved laser-induced fluorescence measurements, ranging from 10⁻¹² cm³·molecule⁻¹·s⁻¹ to 10⁻¹¹ cm³·molecule⁻¹·s⁻¹. The N₂ flow exhibits the highest relaxation rate for CH(X²Π) radical due to its additional rovibrational levels, which allow for more efficient energy dissipation during collisions compared to monoatomic gases. The Ar flow shows a larger relaxation rate than He flow due to its greater polarizability and stronger long-range interaction with the CH(X²Π) radical.