Measurements of argon broadened Lorentz width and pressure-induced line shift coefficients in the ν_4 band of ^12CH_4

Room temperature argon broadened halfwidth and pressure-induced line shift coefficients have been determined for 118 transitions in the ν_4 band of ^12CH_4 from analysis of high resolution laboratory absorption spectra recorded with the McMath Fourier transform spectrometer operated on Kitt Peak by the National Solar Observatory. Transitions up to J″ = 12 have been measured using a nonlinear least-squares spectral fitting procedure. The variation of the measured halfwidth coefficients with symmetry type and rotational quantum number is very similar to that measured previously for N_2 and air broadening, but the absolute values of the argon broadening coefficients are all smaller. On average, the ratio of the argon broadened halfwidth coefficient to the corresponding N_2 broadened halfwidth coefficient is 0.877 ± 0.017 (2σ). More than 95% of the pressure-induced shifts are negative with values ranging from −0.0081 to +0.0055 cm^−1 atm^−1. The pressure shifts in argon are nearly equal to corresponding values measured previously in N_2 and air.