The ammonia absorption spectrum between 3900 and 6350 cm-1: 15NH3 contribution and a recommended list for natural ammonia

Abstract

Following previous contributions dedicated to the main isotopologue, ¹⁴NH₃, in the present work, we examine the ¹⁵NH₃ absorption spectrum between 3950 and 6350 cm^-1 in order to elaborate an absorption line list for ammonia in natural isotopic abundance. For this purpose, several room temperature Fourier Transform absorption spectra of ammonia highly enriched in ¹⁵NH₃ measured in the 90′s and available from the Kitt Peak data centre have been analysed. In addition, a more recent high-resolution spectrum recorded in Brussels between 4150 and 4600 cm^-1 is also considered. The spectra were divided in three separate intervals (3900–4706, 4706–5650 and 5650–6350 cm^-1) for which experimental lists including 3506, 3788, and 2317 lines, respectively, were retrieved and treated independently. By comparison with an ab initio line list (the BYTe list) and systematically validating the assignments by using lower state combination difference relations (LSCD), 2013, 1771 and 570 transitions could be assigned in the three regions, respectively. They belong to 16, 22, and 10 bands and correspond to 98.5, 92.6 and 78.6 % of the experimental intensities in the region, respectively. For comparison, only four ¹⁵NH₃ bands were previously assigned in the literature in the entire studied region.

The obtained ¹⁵NH₃ assigned line lists were gathered with the previously elaborated line list of the main isotopologue to produce a recommended line list for natural ammonia between 3900 and 6350 cm^-1.