Measurements of 18O-Enriched Ozone Isotopomer Abundances Using High-Resolution Fourier Transform Far-IR Spectroscopy

Abstract

The distribution of ozone isotopomers in ozone mixtures produced by electric discharge in mixtures of 16O2 and 18O2 at 77 K was measured by high-resolution FTIR spectroscopy. It was of key importance to assess not only the total amount of isotopomers of a certain mass but also the relative amounts of corresponding asymmetric and symmetric ozone species of the same mass given as the ratios [16O16O18O]/[16O18O16O] and [16O18O18O]/[18O16O18O]. For many purposes both ratios have been assumed to have the statistical value 2.00. Pure rotational spectra in the far-IR region (30–100 cm−1) were recorded for three different 18O-enriched ozone mixtures, all at 0.00185 cm−1 resolution. All the spectra were corrected for thermal emission. Linestrengths for individual lines in a particular spectrum were measured by means of a fitting technique taking into account contributions from all other lines in the spectrum. For this purpose theoretical linestrengths for all six ozone species containing 16O and 18O obtained from a quantum-number-dependent dipole operator were used. The ratios between observed and theoretical linestrengths were used to determine the abundances of individual isotopomers in a particular ozone mixture. For one of the ozone samples the abundances of all six ozone species were determined within 1% relative uncertainty. For the three ozone mixtures studied, the ratio between asymmetric and symmetric species of mono-18O ozone were determined to 1.99(2), 2.01(2), and 2.10(6). The ratio between asymmetric and symmetric species of di-18O ozone were determined to 2.51(4), 2.42(10), and 2.46(3).