R.H. Tipping, A. Brown, Q. Ma, J.-M. Hartmann, C. Boulet, and J. Liévin
J. Chem. Phys. 115, 8852-8857.
Publication year: 2001

An experimental value for the quadrupoletransition moment of the ν2 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, ρAr,, and is comparable to the allowed dipole intensity due to Coriolis interaction with the ν4 band at approximately 125 amagats. Ignoring vibration-rotation interaction and Coriolis interaction,, we equate the measuredslope of the integrated intensity versus ρAr to the theoretical expression for the quadrupole-induced absorption, and obtain the value |〈0|Q|ν2〉|=0.445 ea2 for the quadrupole transition matrix element. A theoretical value ⟨0|Q|ν2⟩=0.478ea02 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.