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.photodissociation | isotopic fractionation | UV photodissociation P hotochemical processes are particularly pervasive in nature. Measurements of isotopic compositions provide insights into a range of such processes, both terrestrial and extraterrestrial. To adequately interpret such measurements, characterization of relevant isotopically selective physicochemical processes associated with gas phase processes is desirable. In this paper we quantify an isotopic selectivity due to electronic reorganization following ligfht absorption. The results are compared to the solar spectrum for applications in nature.There are a number of photodissociation processes that have been suggested where a more basic understanding of isotope effects could aid in the development of models. A striking example of observed, but presently not fully understood oxygen isotopic distribution occurs in the high temperature calcium aluminum rich inclusions of the Allende meteorite (1) and most oxygen bearing meteorites (see, for example, review in ref.2). Although originally thought to be nucleosynthetic in origin based upon laboratory experiments, it was later suggested that the observed isotopic composition might arise from photochemical self-shielding (3-6). It has also been proposed that given the special properties of oxygen, symmetry related properties might produce the same fraction in the actual mineral formation process (7-11). A...

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 N 2 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 NH 3 , depending upon photodissociation wavelength. The measured enrichment range in photodissociation of N 2 , 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.nitrogen isotopes | organic molecules | perturbation

Dynamics of electronic motion when the nuclei are clamped is discussed and shown to be always described as a superposition of adiabatic electronic states. These states are stationary when the nuclei are clamped but their superposition leads to multiply periodic motion where the natural frequencies are the differences in the energies of the adiabatic electronic states. When one or more of the frequencies are low and the atoms are allowed to move, the electronic rearrangement is commensurate with the motion of the nuclei. This is the usual breakdown of the Born-Oppenheimer approximation. But when the electronic frequencies are higher there is an electronic motion before the nuclei move. The motion can be demonstrated through expectation values such as the multipole moments of the charge distribution. Such superposition states will be excited when the laser pulse width in energy exceeds the spacings of the states. For low-lying valence excited or low Rydberg states this requires a femtosecond or shorter laser pulse. Since the carrier frequency has to be comparable to the excitation energy, the required laser pulses must span only a few cycles.

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.

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