| A high resolution difference frequency spectrometer was developed based on a LiIO{dollar}sb3{dollar} nonlinear mixing crystal, thereby giving a wide tuning range of 2.5-5.5 {dollar}mu{dollar}m, with a 10{dollar}sp{lcub}-4{rcub}{dollar} cm{dollar}sp{lcub}-1{rcub}{dollar} spectral resolution. The signal-to-noise ratio in infrared transmission is 2000:1 for a 4 second integration time per data point.; We have used the spectrometer as a probe of molecular lineshape models and to make high accuracy determinations of broadening and shifting coefficients. Several transitions of the 4.7 {dollar}mu{dollar}m fundamental ro-vibrational band of CO were studied in dilute mixtures of He, N{dollar}sb2,{dollar} Ar, and Xe. The pressure dependence of the lineshapes was investigated through the transition region from Doppler to collision broadening, at pressures ranging from 1 to 100 kPa.; The lineshapes exhibit Dicke (1953) narrowing of the Doppler contribution to the lineshape, thus deviating from a Voigt model. We have tested the validity of two models which encompass the Dicke narrowing effect, the "soft" (Galatry, 1961) and "hard" (Nelkin and Ghatak, 1964) collision models. These models use the mass diffusion coefficient, D{dollar}sb{lcub}rm m{rcub},{dollar} as a measure of the degree of Dicke narrowing, and we empirally fit our data for an "optical" diffusion coefficient, D{dollar}sb{lcub}rm opt{rcub}.{dollar} We find that at low densities, the measured D{dollar}sb{lcub}rm opt{rcub}{dollar} and D{dollar}sb{lcub}rm m{rcub}{dollar} agree, but they diverge {dollar}rm(Dsb{lcub}opt{rcub} < Dsb{lcub}m{rcub}){dollar} at high density. The inclusion of a lineshape based on speed dependent collision broadening, instead of a simple Lorentzian, restores agreement between {dollar}rm Dsb{lcub}opt{rcub}{dollar} and D{dollar}sb{lcub}rm m{rcub}{dollar} at high density. We also demonstrate that speed dependent broadening is sensitive to the nature of the intermolecular potential (Berman, 1972), and is most significant for a large perturber to radiator mass ratio, as expected (e.g. Robert et al., 1993). |
