The Photographic Infrared Absorption Spectrum of Gaseous Ammonia

The photographic infrared absorption bands of ammonia at about 10,000A, 7920A, and 6470A have been measured with greater dispersion and resolving power than hitherto. Several series have been established in the first two bands by combination principle methods, but most of the band lines have not been classified. The complexity of structure occurs probably because each observed band is a composite of several parallel-type and perpendicular-type bands; the observed series, or most of them, probably constitute parallel bands. In the pure gas, the band lines are very sensitive to pressure, and do not become at all sharp until the latter is far below one atmosphere (cf. Fig. 1). In mixtures of ammonia and air, however, the air pressure produces very little broadening. The application of the combination principle to parallel, perpendicular, and Raman bands is discussed, taking into account the effects of un-resolved but not negligible fine structure in the lines due to near-coincidence of lines of differing $K$ values. Analysis of earlier data yields a rough value for a certain small rotational coupling coefficient and for the centrifugal deformation rotational coefficient, in the normal state of ammonia.