Influence of Foreign Gases at High Pressures on the Infrared Absorption Band of Methane at 3.3μ

Abstract

An infrared absorption cell for compressed gases was constructed and used at pressures up to 600 atmos. In the cell, which was of the reflection type, the length of the absorbing path could be changed from 5×10⁻⁴ cm to 4 cm by means of metal spacers placed between the window and the mirror. The influence of high pressures of helium, argon, and nitrogen on the intensity of the methane band at 3.3μ was investigated. It was found that the absolute absorption coefficient for the band increases linearly with the density of the foreign gas. With nitrogen at 600 atmos the absorption coefficient is 20 percent greater than that of the free methane molecule. The magnitudes of the coefficients of induced absorption are compatible with the assumption that the vibrating dipole of the absorbing molecule is increased by the reaction field of the polarizable foreign gas molecules in its neighborhood. The coefficient of absorption for the free molecule was found by extrapolating the measured coefficients to zero density of the foreign gas. The values derived from measurements with the three foreign gases agree closely, and give an average value of 1075×10¹⁰ sec⁻¹ cm⁻¹ at NTP.