Photo-induced isotopic fractionation

This paper presents a systematic method for the analysis of photo-induced i sotopic fractionation. The physical basis for this fractionation mechanism centers on the fact that isotopic substitution alters the energy levels, mo lecular symmetries, spin statistical weights and other fundamental molecula r properties, producing spectroscopic signatures distinguishable from that of the parent isotopomer. These mass-dependent physical properties are iden tical to those invoked by Urey to explain stable isotope fractionation in c hemical systems subject to thermodynamic equilibrium. Photo-induced isotopi c fractionation is a completely general phenomenon and should be observable in virtually all gas phase photochemical systems. Water photo-induced isot opic fractionation has been examined in detail using experimental and theor etical data. These results illustrate the salient features of this fraction ation mechanism for molecules possessing continuous UV absorption spectra a nd unit photodissociation quantum yields. Using the photo-induced isotopic fractionation methodology in conjunction with standard photochemical models , we predict substantial deuterium enrichment of water vapor in the planeta ry atmospheres of Earth and Mars.