Low and room temperature photoabsorption cross sections of NH3 in the UV region

Using synchrotron radiation as a continuum light source, we have measured the absolute absorption cross sections of NH3 with a spectral bandwidth (FWHM) of 0.5 Å. The photoabsorption cross sections of NH3 have been measured from 1750 to 2250 Å under temperature conditions of 295, 195, and 175 K. Significant temperature effects in the absorption threshold region which are mainly due to the presence of hot band absorption are observed. The cross section value at peaks and valleys for the vibrational progressions of the (0,0) to (4,0) bands vary between −80% and +40% as the temperature of NH3 changes from 295 to 175 K. In contrast to this, the changes of cross section values, Pc,T, are found to vary less than 20% for the (v′, 0) vibrational progressions with v′ ⩾ 5. The measured separations between the doublet features of the (0,0), (1,0), and (2,0) bands are found to decrease as the temperature of NH3 decreases. The shifts of peak positions of Pc,T with respect to the corresponding room temperature absorption peaks show a sudden change at v′ = 3 which appears to agree with the trend observed in the homogeneous line widths of the vibrational bands of NH3 (Vaida et al., 1987; Ziegler, 1985; Ziegler, 1986). The unusual behavior of the line widths has been attributed to the à state potential surface which has a dissociation barrier.