Measurements on the absolute intensity of infra-red absorption

Abstract

Absolute intensities of infra -red absorption have been determined for the γ3 bands of CH4 and N20 from direct measurements on the infra-red energy absorbed. The energy was measured as a pressure rise in a constant-volume absorption cell having allowed a steady-state, with heat loss to the walls, to be attained. The pressure rise was simulated by introducing a known amount of heat energy from an electrically heated wire. The energy incident on the gas was measured by using within the cell a thin, blackened foil which absorbed all the radiation and in thermal equilibrium transferred this energy to heat energy of the gas. The resultant pressure rise was simulated by the heated wire as for the gas absorption. The value obtained for the incident energy was checked by measurements with a calibrated thermopile.

The results of 993x10(10) and 4,740x10(10)cm(-1)sec(-1) at N.T.P. for the γ3 bands of CH4 and N20 respectively confirmed that the normal method of determination, using pressure-broadened, spectrometer measurements, yields accurate results when sufficient care is taken to ensure optimum conditions of measurement.

Although the simple Elsasser theory was not fully descriptive of results on the energy absorbed for pure gases, it was used to estimate average line-widths of 0.18 and 0.13cm(-1)atm(-1) for the bands in CH4 and N20 respectively.

The technique used did not measure all the absorbed energy at very low total pressures. This was due to the fact that with decreasing pressure spontaneous emission became a competing process with the vibrational-translational energy transfer involved in the determination. The magnitude of this loss was used to estimate relaxation times of 245 and 43 μ sec. for the λ3 bands of CH4 and N20 respectively.