On the strong and selective isotope effect in the UV excitation of N2 with implications toward the nebula and Martian atmosphere

B. H. Muskatel, F. Remacle, Mark H. Thiemens, R. D. Levine

Research output: Contribution to journalArticlepeer-review

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Abstract

Isotopic effects associated with molecular absorption are discussed with reference to natural phenomena including early solar system processes, Titan and terrestrial atmospheric chemistry, and Martian atmospheric evolution. Quantification of the physicochemical aspects of the excitation and dissociation processes may lead to enhanced understanding of these environments. Here we examine a physical basis for an additional isotope effect during photolysis of molecular nitrogen due to the coupling of valence and Rydberg excited states. The origin of this isotope effect is shown to be the coupling of diabatic electronic states of different bonding nature that occurs after the excitation of these states. This coupling is characteristic of energy regimes where two or more excited states are nearly crossing or osculating. A signature of the resultant isotope effect is a window of rapid variation in the otherwise smooth distribution of oscillator strengths vs. frequency. The reference for the discussion is the numerical solution of the time dependent Schrödinger equation for both the electronic and nuclear modes with the light field included as part of the Hamiltonian. Pumping is to all extreme UV dipole-allowed, valence and Rydberg, excited states of N2. The computed absorption spectra are convoluted with the solar spectrum to demonstrate the importance of including this isotope effect in planetary, interstellar molecular cloud, and nebular photochemical models. It is suggested that accidental resonance with strong discrete lines in the solar spectrum such as the CIII line at 97.703 nm can also have a marked effect.

Original languageEnglish
Pages (from-to)6020-6025
Number of pages6
JournalProceedings of the National Academy of Sciences of the United States of America
Volume108
Issue number15
DOIs
StatePublished - 12 Apr 2011

Keywords

  • Isotopic fractionation
  • Photodissociation
  • UV photodissociation

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