Wavelength dependence of isotope fractionation in N2O photolysis

[摘要] In previous reports on isotopic fractionation in the ultraviolet photolysis of nitrous oxide(N₂O) only enrichments of heavy isotopes in the remaining N₂Ofraction have been found. However, most direct photolysis experiments have been performed at wavelengths far fromthe absorption maximum at 182 nm. Here we present high-precision measurements of the¹⁵N and ¹⁸O fractionation constants (e) in photolysis at185 nm. Small, but statistically robust depletions of heavy isotopes for the terminal atoms in the linearN₂O molecule are found. This means that the absorption cross sectionss(¹⁵N¹⁴N ¹⁶O) and s(¹⁴N₂¹⁸O) are larger thans(¹⁴N₂¹⁶O) at this specific wavelength. In contrast, the central N atom becomes enriched in¹⁵N. The corresponding fractionation constants (±1 standard deviation) are

¹⁵e₁ =s(¹⁵N¹⁴N¹⁶O)/s(¹⁴N₂¹⁶O) - 1 =(3.7±0.2)%o
¹⁸e =s(\¹⁴N₂¹⁸O)/s(¹⁴N₂¹⁶O) - 1 =(4.5±0.2) %o and
¹⁵e₂ =s(¹⁴N¹⁵N ¹⁶O)/s(¹⁴N₂¹⁶O)- 1 = (-18.6±0.5) %o

To our knowledge, this is the first documented case of such a heavy isotope depletion in thephotolysis of N₂O which supports theoretical models and pioneering vacuum ultravioletspectroscopic measurements of ¹⁵N substituted N₂O species that predict fluctuations ofearound zero in this spectral region (Selwyn and Johnston, 1981). Such a variability in isotopicfractionation could have consequences for atmospheric models of N₂Oisotopes since actinic flux varies also strongly over narrow wavelength regions between 175 and200 nm due to the Schumann-Runge bands of oxygen. However, the spacing between maxima and minima ofthe fractionation constants and of the actinic flux differ by two orders of magnitude in thewavelength domain. The wavelength dependence of fractionation constants inN₂O photolysis can thus be approximated by a linear fit with negligible consequences on the actual value ofthe spectrally averaged fractionation constant. In order to establish this linear fit, additionalmeasurements at wavelengths other than 185 nm were made using broadband light sources,namely D₂, Hg/Xe and Sb lamps. The latter lamp was used in conjunction with variousinterference filters to shift the peak photolysis rate to longer wavelengths. From theseexperiments and existing data in the literature, a comprehensive picture of the wavelengthdependence of N₂O photolysis near room-temperature is created.

[学科分类] 大气科学