VIBRATIONAL-ENERGY TRANSFER BETWEEN THE ISOTOPOMERS OF CARBON-MONOXIDE AT LOW-TEMPERATURES

Rate constants have been measured for vibration to vibration energy tr ansfer from (CO)-C-12-O-16(v = 1) to the first vibrationally excited s tate of both (CO)-C-13-O-16 (<Delta(v)over bar> = 47 cm(-1)) and (CO)- C-13-O-18 (<Delta(v)over bar> = 100 cm(-1)) using the infrared laser f luorescence technique over the temperature range 50 to 270 K in the ga s phase and in liquid neon, deuterium and argon solutions. Fluorescenc e from (CO)-C-12-O-16(v = 1) was filtered out, and the time-dependence of the fluorescence intensity from the other isotopomer used to deter mine the rate constants for energy transfer. The results for the two i sotopomeric systems are markedly different. For that with the smaller energy mismatch, (CO)-C-12-O-16 - (CO)-C-13-O-16, the rate constants i ncrease with decrease in temperature from 270 to 80 K. This is shown t o be consistent with near-resonant energy transfer mediated by transit ion dipole-transition dipole couplings. Below 80 K, the temperature de pendence of the rate constants flattens. For the (CO)-C-12-O-16 - (CO) -C-13-O-18 system, the rate constants decrease with decrease in temper ature from 270 to 100 K, below which they increase with further decrea se in temperature. The temperature dependence in the range 270 to 100 K is shown to be consistent with energy transfer mediated by a combina tion of by long range and short range forces. Below 100 K, the tempera ture dependence of the rate constants is governed by attractive forces which influence the collision pair during their encounter. The rate c onstants are compared with previous studies of near-resonant vibration al energy transfer in other systems at low temperatures. (C) 1998 Amer ican Institute of Physics.