[Study on temperature dependence of ultraviolet absorption cross sections of ammonia]

The present paper reports the study of the effects of temperature on the absorption cross-sections of ammonia in spectral region of 200-220 nm. By using a grating monochromator with high resolution, deuterium lamp, closed gas sample cell and gas compounding device, the photoabsorption cross sections of ammonia were measured at temperatures ranging from 308 to 397 K. The absorption spectrum of NH3 consists of discrete bands superimposed on a continuous base. Results indicated that the cross sections at the peaks of the discrete bands decreased with the increase in temperature, which corresponded to the decrease in the population of vibrational and rotational transitions from the base level to higher excitation levels. The absorption cross section peaks decreased when the temperature increased from 308 to 397 K, with a relative drop of 46%. Another distinctive feature of ammonia absorption spectra in the above spectral regions was the quasiperiodic structure of absorption peaks, whose equal wavelength interval was 4 nm. Besides, absorption peak positions were not changed. Continuous absorption cross section decreased with the increase in temperature, and the variation gradient gradually decreased with wavelength red shift. A compensation calculation for temperature should be carried out in on-line measurement of concentration because of the greater variation rate of absorption cross sections of ammonia.