Collision-induced absorption in the n₂ fundamental band of CH₄: I. Determination of the quadrupole transition moment

Abstract

An experimental value for the quadrupole transition moment of the n₂ 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 Ar density, r_Ar, and is comparable to the allowed dipole intensity due to Coriolis interaction with the n₄ band at approximately 125 amagats. Ignoring vibration-rotation interaction and Coriolis interaction, we equate the measured slope of the integrated intensity versus r_Ar to the theoretical expression for the quadrupole-induced absorption, and obtain the value |<0|Q|n₂>| = 0.445 ea₀². A theoretical value <0|Q|n₂> has been determined by large scale ab initio calculations, and considering both theoretical approximations and experimental uncertainties, we regard the agreement as good, thus confirming our interpretation of the enhancement as due to the quadrupolar collision-induced mechanism.

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