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Separate coupled-channel Schrödinger-equation (CSE) models of the interacting (1)Pi(u) (b,c,o) and (3)Pi(u) (C,C(')) states of N(2) are combined, through the inclusion of spin-orbit interactions, to produce a five-channel CSE model of the N(2) predissociation. Comparison of the model calculations with an experimental database, consisting principally of detailed new measurements of the vibrational and isotopic dependence of the (1)Pi(u) linewidths and lifetimes, provides convincing evidence that the predissociation of the lowest (1)Pi(u) levels in N(2) is primarily an indirect process, involving spin-orbit coupling between the b (1)Pi(u)- and C (3)Pi(u)-state levels, the latter levels themselves heavily predissociated electrostatically by the C(') (3)Pi(u) continuum. The well-known large width of the b(v=3) level in (14)N(2) is caused by an accidental degeneracy with C(v=9). This CSE model provides the first quantitative explanation of the predissociation mechanism for the dipole-accessible (1)Pi(u) states of N(2), and is thus likely to prove useful in the construction of realistic radiative-transfer and photochemical models for nitrogen-rich planetary atmospheres.

[1] The Cassini Ultraviolet Imaging Spectrograph (UVIS) observed an occultation of the Sun by the water vapor plume at the south polar region of Saturn's moon Enceladus. The Extreme Ultraviolet (EUV) spectrum is dominated by the spectral signature of H 2 O gas, with a nominal line-of-sight column density of 0.90 ± 0.23 × 10 16 cm −2 (upper limit of 1.0 × 10 16 cm −2 ). The upper limit for N 2 is 5 × 10 13 cm −2 , or <0.5% in the plume; the lack of N 2 has significant implications for models of the geochemistry in Enceladus' interior. The inferred rate of water vapor injection into Saturn's magnetosphere is ∼200 kg/s. The calculated values of H 2 O flux from three occultations observed by UVIS have a standard deviation of 30 kg/s (15%), providing no evidence for substantial short-term variability. Collimated gas jets are detected in the plume with Mach numbers of 5-8, implying vertical gas velocities that exceed 1000 m/sec. Observations at higher altitudes with the Cassini Ion Neutral Mass Spectrometer indicate correlated structure in the plume. Our results support the subsurface liquid model, with gas escaping and being accelerated through nozzle-like channels to the surface, and are consistent with recent particle composition results from the Cassini Cosmic Dust Analyzer.

Context. Molecular nitrogen is one of the key species in the chemistry of interstellar clouds and protoplanetary disks, but its photodissociation under interstellar conditions has never been properly studied. The partitioning of nitrogen between N and N 2 controls the formation of more complex prebiotic nitrogen-containing species. Aims. The aim of this work is to gain a better understanding of the interstellar N 2 photodissociation processes based on recent detailed theoretical and experimental work and to provide accurate rates for use in chemical models. Methods. We used an approach similar to that adopted for CO in which we simulated the full high-resolution line-by-line absorption + dissociation spectrum of N 2 over the relevant 912-1000 Å wavelength range, by using a quantum-mechanical model which solves the coupled-channels Schrödinger equation. The simulated N 2 spectra were compared with the absorption spectra of H 2 , H, CO, and dust to compute photodissociation rates in various radiation fields and shielding functions. The effects of the new rates in interstellar cloud models were illustrated for diffuse and translucent clouds, a dense photon dominated region and a protoplanetary disk. Results. The unattenuated photodissociation rate in the Draine (1978, ApJS, 36, 595) radiation field assuming an N 2 excitation temperature of 50 K is 1.65 × 10 −10 s −1 , with an uncertainty of only 10%. Most of the photodissociation occurs through bands in the 957-980 Å range. The N 2 rate depends slightly on the temperature through the variation of predissociation probabilities with rotational quantum number for some bands. Shielding functions are provided for a range of H 2 and H column densities, with H 2 being much more effective than H in reducing the N 2 rate inside a cloud. Shielding by CO is not effective. The new rates are 28% lower than the previously recommended values. Nevertheless, diffuse cloud models still fail to reproduce the possible detection of interstellar N 2 except for unusually high densities and/or low incident UV radiation fields. The transition of N → N 2 occurs at nearly the same depth into a cloud as that of C + → C → CO. The orders-of-magnitude lower N 2 photodissociation rates in clouds exposed to black-body radiation fields of only 4000 K can qualitatively explain the lack of active nitrogen chemistry observed in the inner disks around cool stars. Conclusions. Accurate photodissociation rates for N 2 as a function of depth into a cloud are now available that can be applied to a wide variety of astrophysical environments.

Context. Photodissociation of 14 N 2 and 14 N 15 N occurs in interstellar clouds, circumstellar envelopes, protoplanetary discs, and other environments due to ultraviolet radiation originating from stellar sources and the presence of cosmic rays. This source of N atoms initiates the formation of more complex N-bearing species and may influence their isotopic composition. Aims. We study the photodissociation rates of 14 N 15 N by ultraviolet continuum radiation and both isotopologues in a field of cosmic ray induced photons. To determine the effect of these on the isotopic composition of more complex molecules. Methods. High-resolution theoretical photodissociation cross sections of N 2 are used from an accurate and comprehensive quantummechanical model of the molecule based on laboratory experiments. A similarly high-resolution spectrum of H 2 emission following interactions with cosmic rays has been constructed. The spectroscopic data are used to calculate photodissociation rates which are then input into isotopically differentiated chemical models, describing an interstellar cloud and a protoplanetary disc. N 2 , H 2 , and H are presented. Incorporating these into an interstellar cloud model, an enhancement of the atomic 15 N/ 14 N ratio over the elemental value is obtained due to the self-shielding of external radiation at an extinction of about 1.5 mag. This effect is larger where assumed grain growth has reduced the opacity of dust to ultraviolet radiation. The transfer of photolytic isotopic fractionation of N and N 2 to other molecules is demonstrated to be significant in a protoplanetary disc model with grain growth, and is species dependent with 15 N enhancement approaching a factor of 10 for HCN. The cosmic ray induced dissociation of CO is revisited employing a more recent photodissociation cross section, leading to a rate that is ∼40% lower than previously calculated.