The quadrupole moment of acetylene from collision-induced absorption in a gaseous mixture with argon

Abstract

The collision-induced spectrum of a mixture of argon and acetylene (100:1) is reported in the frequency range below 240 cm−1. The spectrum is obtained from the total absorption in the mixture at pressures up to 1400 psi (1 psi = 6.89 kPa) and subtraction of the contribution from an allowed difference band of pure acetylene centred near 113 cm−1. The measurements are made with a Michelson Fourier-transform interferometer and with a laser source at 84.1 cm−1. The quadrupole moment of acetylene determined from the experimental spectrum and existing theory for the integrated absorption coefficient is 5.42 (± 0.41) B if a theoretical estimate of the hexadecapole moment is used. Alternatively, a value of 6.14 (± 0.46) B is obtained if the contribution from the hexadecapole moment is assumed to be zero. An extension of the line-shape theory, 1T6, that includes the contribution from hexadecapolar induction is also reported. The resulting theoretical line shape is compared with the experimental spectrum.