Pressure Effect in Bands of Several Dipole Molecules

Several photographic infrared bands, of H₂O, HCN, and NH₃, have been investigated for line broadening due to increased pressure of the vapors themselves. The halfwidths at one atmosphere have been found, and the excess broadening per atmosphere calculated in each case. Comparison with simple dipole interaction theory has been made. Conformity is found for one H₂O band, compared with HCN. Lack of conformity for another H₂O band, and for two NH₃ bands, is discussed. For HCN a variation of half-width with J is found, in accord with theoretical considerations of a qualitative nature, leading to broader lines in the neighborhood of the thermal maximum of the rotational velocity distribution. Effect of change in slit width on the observed line width and rate of broadening, and the consequent precautions necessary, are discussed.