Collision-induced absorption in the ν2 fundamental band of CH4.: I.: Determination of the quadrupole transition moment

An experimental value for the quadrupole transition moment of the nu (2) fundamental band of CH4 has been determined by fitting the collision-induced enhancement spectrum of CH4 with Ar as the perturber. The observed quadrupole-induced absorption increases linearly with the Ar density, rho (Ar), and is comparable to the allowed dipole intensity due to Coriolis interaction with the nu (4) band at approximately 125 amagats. Ignoring vibration-rotation interaction and Coriolis interaction,, we equate the measured slope of the integrated intensity versus rho (Ar) to the theoretical expression for the quadrupole-induced absorption, and obtain the value parallel to <0 parallel toQ parallel to nu (2)> parallel to =0.445 ea(0)(2) for the quadrupole transition matrix element. A theoretical value <0 parallel toQ parallel to nu (2)> =0.478 ea(0)(2) has been determined by large-scale ab initio calculations and, considering both the theoretical approximations and experimental uncertainties, we regard the agreement as good, thus confirming our interpretation of the enhancement as due to the quadrupole collision-induced mechanism. (C) 2001 American Institute of Physics.