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This paper focusses upon a less well-known source of deviation from the Beer-Lambert law whereby nonabsorbing components ('buffer gases') intensify the observed spectrum of an infrared absorber, an effect that we describe as 'spectral enhancement' and that the molar absorptivity of the absorber appears to increase with the polarity and partial pressure of the buffer gas. A dramatic of this effect, provided by J. Corbett, is shown in Figure 1, where the transmittance of 0.13 torr of CO is monitored as a function of gas pressure in a 40-m cell using a spectrometer resolution of 1 cm-1. (Figure 1 omitted) As the buffer gas pressure increases, the rotation-vibration lines become more intense causing the CO spectrum to "grow" apparently out of the spectrometer noise.
Direct observations of line-broadening for gaseous systems require the use of spectrometers that are capable of resolving the doppler line widths, but (as this paper demonstrates) considerable semi-quantitative information may be obtained using analytical spectrometers that operate at lower resolution where spectral lines are distorted by the spectrometer. The investigations are expanded readily into a student physical chemistry project. In this paper, student measurements of the enhancement of HC1(g) at low resolution by several foreign gases are reported and the enhancement rationalized in terms of rotational-translational (R-T) energy transfer between HC1 and the molecules of added gas. Familiarity with the origin of infrared spectra (1) and with intermolecular forces is assumed. Finally, the practical consequences of spectral enhancement to analytical measurements are briefly discussed.
EXPERIMENTAL
Infrared spectra were recorded over 64 scans at 2 cm-1 resolution at ambient temperature using a Digilab FTS-50 (TGS detector, dry air purge) with triangular apodisation. Peak areas were evaluated using the Digilab Quant 32 routine. Samples were contained in a 10-cm glass cell with KBr windows. HC1 (BDH Ltd) was used without purification. Liquids were degassed before use. Deuterated benzene and acetone were used to minimize the overlapping of bands between the buffer gas and HC1.
THE ENHANCEMENT OF HCl BANDS
Figure 2 shows the spectrum obtained after subtraction of the spectrum of 50 torr HC1(g) from that of a mixture of 50 torr HC1 and 710 torr argon. (Figure 2 omitted) It is evident that all the bands are enhanced by the argon, with...