Living Reference Work Entry

Encyclopedia of Astrobiology

pp 1-2

Date: Latest Version

Self-Shielding Effects on Isotope Fractionation

  • Ko HashizumeAffiliated withDepartment of Earth and Space Sciences, Osaka University Email author 


Photochemistry Photodissociation Mass-independent fractionation


Isotope-selective photolysis; Self-shielding


The self-shielding effect is a phenomenon expected to occur in a gas medium illuminated by a light source, when a molecule in the gas absorbs light with a particular wavelength that may result in its dissociation. The light of this wavelength is gradually attenuated as it travels through the gas medium, leading to a decreased photodissociation rate of the target molecule. The attenuation rate of the light depends on the abundance of the molecule. When molecules with different combinations of isotopes are photodissociated by different wavelengths, this effect may cause an abundance-dependent isotope fractionation, as opposed to the common mass-dependent fractionation.


The self-shielding effect may occur when the following conditions are met. (1) Line absorption: The absorption of the photodissociating light occurs only at a narrow range of particular wavelengths. (2) The light with the wavelength of interest reaches to ...

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