Massive isotopic effect in vacuum UV photodissociation of N2 and implications for meteorite data.

Nitrogen isotopic distributions in the solar system extend across an enormous range, from -400‰, in the solar wind and Jovian atmosphere, to about 5,000‰ in organic matter in carbonaceous chondrites. Distributions such as these require complex processing of nitrogen reservoirs and extraordinary isotope effects. While theoretical models invoke ion-neutral exchange reactions outside the protoplanetary disk and photochemical self-shielding on the disk surface to explain the variations, there are no experiments to substantiate these models. Experimental results of N2 photolysis at vacuum UV wavelengths in the presence of hydrogen are presented here, which show a wide range of enriched δ(15)N values from 648‰ to 13,412‰ in product NH3, depending upon photodissociation wavelength. The measured enrichment range in photodissociation of N2, plausibly explains the range of δ(15)N in extraterrestrial materials. This study suggests the importance of photochemical processing of the nitrogen reservoirs within the solar nebula.

Ultrafast predissociation mechanism of the 1Πu states of 14N2 and its isotopomers upon attosecond excitation from the ground state.

The computed time evolution of excited electronic and nuclear states of dinitrogen following a broad laser pulse excitation of the dipole allowed singlet Π states is discussed. The computations use two complementary methods to solve the time-dependent Schrödinger equation of the molecule. The electronic evolution is described as spanning seven states, the three dipole-allowed singlet states (b,c,o(1)Π(u)) and four triplet states (C,C',F,G(3)Π(u)). Spin-orbit coupling mixes states of the two manifolds. The computed dynamics includes the attosecond pulse single photon pumping from the electronic ground state. The ultrafast exit to the continuum from the bound states that are optically excited and the large isotope effect on this process are used as a probe of the electron dynamics as coupled to the onset of the nuclear motion. For (14)N(2), prompt predissociation to the continuum of the repulsive C'(3)Π(u) state is facilitated primarily by the b(1)Π(u)(v = 3)-C(3)Π(u)(v = 9) coupling whereas for (15)N(2) it is the b(1)Π(u)(v = 4)-C(3)Π(u)(v = 10) coupling term. Predissociation from the F(3)Π(u) and G(3)Π(u) states is important at the higher energies because of their strong coupling to the continuum.

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

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 N(2). 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.