Pressure broadening coefficients

As a further test of the ab initio potential, calculations of the pressure broadening coefficients of the R branch rotational lines of the v4 spectrum of C02 in ahelium bath at various temperatures were performed296. [The pressure broadening coefficients reflect... 

Figure 1-7. Theoretical (open circles) and experimental (full circles) pressure broadening coefficients of the CO2 spectral lines in the helium bath at various temperatures...

Heijmen TGA, Moszynski R, Wormer PES, Van der Avoird A, Rudert AD, Halpern JB, Martin J, Gao WB, Zacharias H (1999) Rotational state-to-state rate constants and pressure broadening coefficients for He— C2H2 collisions Theory and experiment. J Chem Phys 111 2519—2531... 

As can be seen in Table 4, agreement between the various determinations of the pressure broadening coefficients is good, with the exception of the O2 result of Neumann et Davis et note that this measurement... 

Absorption.—The studies reported here are conventional one-photon absorption studies on ground-state molecules and atoms. The use of a rapidly tuneable CW dye laser for direct absorption spectroscopy has been described.98 Transition probabilities in the spectra of Ne (I),97 and the classification of the 650.4 nm line of Xe97 have been discussed. Pressure-broadening coefficients for the atomic iodine 2P4-2Pj transition for C02, N2, He, Ne, Ar, Kr, and Xe have been measured as 7.4 0.7, 6.2 0.8, 3.6 0.3, 4.3 0.4, 5.1 0.5, 4.4 0.4, and 3.0 0.3, respectively.98 Measurements of the polarizabilities of alkali-metal atoms have been described,99 and hyperfine interactions in the excited states of sodium discussed.100 Excitation parameters in the vacuum-u.v. region... 

Table I. Pressure broadening coefficients I for self broadening and II for broadening by helium.

In the framework of the impact approximation of pressure broadening, the shape of an ordinary, allowed line is a Lorentzian. At low gas densities the profile would be sharp. With increasing pressure, the peak decreases linearly with density and the Lorentzian broadens in such a way that the area under the curve remains constant. This is more or less what we see in Fig. 3.36 at low enough density. Above a certain density, the l i(0) line shows an anomalous dispersion shape and finally turns upside down. The asymmetry of the profile increases with increasing density [258, 264, 345]. Besides the Ri(j) lines, we see of course also a purely collision-induced background, which arises from the other induced dipole components which do not interfere with the allowed lines its intensity varies as density squared in the low-density limit. In the Qi(j) lines, the intercollisional dip of absorption is clearly seen at low densities, it may be thought to arise from three-body collisional processes. The spectral moments and the integrated absorption coefficient thus show terms of a linear, quadratic and cubic density dependence,... 

In the absence of interferences, arises from the absorption spectrum of the test species. For atoms and certain molecules, the absorption spectrum consists of individual lines or small groupings of lines in which each line is described by a Voigt-type profile. For most molecules, pressure broadening causes a merging together of the lines into absorption bands. The absorption coefficient for an individual line may be written as... 

Lamb peaks (inverse Lamb dips) at the line centers of the absorbing transitions (Sect. 2.3). The line profiles of these peaks are determined by the pressure in the absorption cell, by saturation broadening, and by transit-time broadening (Vol. 1, Sect. 3.4). Center frequency coq, linewidth Aco, and line profile Pl(co) are measured as a function of the pressure p (Fig. 8.2). The slope of the straight line Aco p) yields the line-broadening coefficient [977], while the measurement of coo p) gives the collision-induced line shift. 

The pressure shift and pressure broadening of the spectral lines are primarily due to the collisions of the molecules. The study of these effects provides information on the mechanism of the molecular interactions which are mainly of dipole-dipole type. In addition, the experimental determination of the line broadening and line shift parameters is instrumental in the spectroscopic study of the atmosphere, where the absorption or emission of IR radiation takes place from regions where the pressure may change by several orders of magnitude. These effects are relatively small for example, for the P and R branches of the V3 band of C 02 at 2350 cm broadened by N2 at 296 K, broadening coefficients in the 0.066-0.089 cm" atm" range and shift coefficients of -0.0016-0.0037 cm" atm" have been reported by Devi and co-workers. 

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