Photodissociation of interstellar N2

Molecular nitrogen is one of the key species in the chemistry of interstellar clouds and protoplanetary disks and the partitioning of nitrogen between N and N2 controls the formation of more complex prebiotic nitrogen-containing species. The aim of this work is to gain a better understanding of the interstellar N2 photodissociation processes based on recent detailed theoretical and experimental work and to provide accurate rates for use in chemical models. We simulated the full high-resolution line-by-line absorption + dissociation spectrum of N2 over the relevant 912-1000 \AA\ wavelength range, by using a quantum-mechanical model which solves the coupled-channels Schr\"odinger equation. The simulated N2 spectra were compared with the absorption spectra of H2, 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.Comment: Online database: http://home.strw.leidenuniv.nl/~ewine/phot

Source of Nitrogen Isotope Anomaly in HCN in the Atmosphere of Titan

The ^(14)N/^(15)N ratio for N_2 in the atmosphere of Titan was recently measured to be a factor of 2 higher than the corresponding ratio for HCN. Using a one-dimensional photochemical model with transport, we incorporate new isotopic photoabsorption and photodissociation cross sections of N_2, computed quantum-mechanically, and show that the difference in the ratio of ^(14)N/^(15)N between N_2 and HCN can be explained primarily by the photolytic fractionation of ^(14)N^(14)N and ^(14)N ^(15)N. The [HC^(14)N]/[HC^(15)N] ratio produced by N_2 photolysis alone is 23. This value, together with the observed ratio, constrains the flux of atomic nitrogen input from the top of the atmosphere to be in the range (1-2) × 10^9 atoms cm^(-2) s^(-1)

Observation of a new electronic state of CO perturbing W¹Π1(v=1)

We observe photoabsorption of the W(1) ← X(0) band in five carbon monoxide isotopologues with a vacuum-ultraviolet Fourier-transform spectrometer and a synchrotron radiation source. We deduce transition energies, integrated cross sections, and natural linewidths of the observed rotational transitions and find a perturbation affecting these. Following a deperturbation analysis of all five isotopologues, the perturbing state is assigned to the v = 0 level of a previously unobserved (1)Π state predicted by ab initio calculations to occur with the correct symmetry and equilibrium internuclear distance. We label this new state E″ (1)Π. Both of the interacting levels W(1) and E″(0) are predissociated, leading to dramatic interference effects in their corresponding linewidths.A.H. was supported by Grant No. 648.000.002 from the Netherlands Organisation for Scientific Research (NWO) via the Dutch Astrochemistry Network. This research was supported by funds from National Aeronautics and Space Administration (NASA) (Grant Nos. NNX09AC5GG to Wellesley College and NNG 06- GG70G and NNX10AD80G to the University of Toledo), CNRS (France), and Programme National Physico-Chimie du Milieu Interstellaire (PCMI). L.G. and J.L.L. acknowledge the financial support of the European Community 7th Framework Programme (FP7/2007-2013) Marie Curie ITN under Grant Agreement # 238258

Photodissociation and chemistry of N 2 in the circumstellar envelope of carbon-rich AGB stars

Context. The envelopes of asymptotic giant branch (AGB) stars are irradiated externally by ultraviolet photons; hence, the chemistry is sensitive to the photodissociation of N2 and CO, which are major reservoirs of nitrogen and carbon, respectively. The photodissociation of N2 has recently been quantified by laboratory and theoretical studies. Improvements have also been made for CO photodissociation. Aims. For the first time, we use accurate N2 and CO photodissociation rates and shielding functions in a model of the circumstellar envelope of the carbon-rich AGB star, IRC +10216. Methods. We use a state-of-the-art chemical model of an AGB envelope, the latest CO and N2 photodissociation data, and a new method for implementing molecular shielding functions in full spherical geometry with isotropic incident radiation. We compare computed column densities and radial distributions of molecules with observations. Results. The transition of N2→ N (also, CO → C → C+) is shifted towards the outer envelope relative to previous models. This leads to different column densities and radial distributions of N-bearing species, especially those species whose formation/destruction processes largely depend on the availability of atomic or molecular nitrogen, for example, CnN (n = 1, 3, 5), CnN− (n = 1, 3, 5), HCnN (n = 1, 3, 5, 7, 9), H2CN and CH2CN. Conclusions. The chemistry of many species is directly or indirectly affected by the photodissociation of N2 and CO, especially in the outer shell of AGB stars where photodissociation is important. Thus, it is important to include N2 and CO shielding in astrochemical models of AGB envelopes and other irradiated environments. In general, while differences remain between our model of IRC +10216 and the observed molecular column densities, better agreement is found between the calculated and observed radii of peak abundance

Indirect predissociation of highly excited singlet states of N₂

Indirect predissociation of the b′¹Σu⁺(v = 20) level of N₂ is studied experimentally by vacuum-ultraviolet photoabsorption employing synchrotron radiation and a Fourier-transform spectrometer, and interpreted with the aid of a quantitative model of interacting ¹∏u and ¹Σu⁺, bound and unbound states which solves the coupled Schrödinger equation. An observed rotationally-localised peak in the b′(20) predissociation linewidths is identified by the model as arising from an interaction with a strongly predissociated and unobserved bound level of the mixed c₃ ¹∏u and o₃ ¹∏u Rydberg states. This leads to the dissociation of b′(20) into the continuum of the b ¹∏u valence state. The residual observed predissociation of b′ ¹Σu⁺(v = 20) apart from the rotationally-localised peak cannot be explained by a mechanism of ¹∏u and ¹Σu⁺ interaction, and must involve states of higher multiplicity.A. H. was supported by grant number 648.000.002 from the Netherlands Organisation for Scientific Research (NWO) via the Dutch Astrochemistry Network. Calculations of the N2 photodissociation cross sections were supported by the Australian Research Council Discovery Program, through Grant Nos. DP0558962 and DP0773050. G. S. was supported by NASA grant NNX08AE78G to Wellesley College

VUV–VIS FT spectroscopy of the rare 13C18O isotopologue of carbon monoxide: Analysis of the A1Π(v = 1) multiply-perturbed level

Ro-vibronic spectra of the 13C18O carbon monoxide isotopologue were obtained with (i) emission spectroscopy in the visible region using a Bruker IFS 125HR spectrometer (University of Rzeszów) and (ii) vacuum-ultraviolet absorption spectroscopy using the wave-front-division spectrometer on the DESIRS beamline of the SOLEIL synchrotron. A deperturbation analysis of the 13C18O A1Π(v = 1) level was conducted from 598 observed transitions from the B1Σ+ - A1Π(0, 1), C1Σ+ - A1Π(0, 1), A1Π - X1Σ+(1, 0), B1Σ+ - X1Σ+(0, 0), C1Σ+ - X1Σ+(0, 0), I1Σ– - X1Σ+(2, 0) bands and five further nominally forbidden bands. An effective Hamiltonian and term-value fitting analysis was implemented. Consequently, 135 parameters were floated: 23 molecular parameters, including molecular constants for A1Π(v = 1), I1Σ–(v = 2), d3Δ(v = 6), e3Σ–(v = 3) and D1Δ(v = 1); rotation-electronic (L-uncoupling) mixing of A1Π(v = 1) ∼ [D1Δ(v = 1), I1Σ–(v = 1), I1Σ–(v = 2)] and spin–orbit interaction parameters for A1Π(v = 1) ∼ [d3Δ(v = 6), e3Σ–(v = 3), aʹ3Σ+(v = 11)]; the spin–orbit/spin-electronic/L-uncoupling a3Π(v = 12) ∼ d3Δ(v = 5) and spin–orbit a3Π(v = 12) ∼ [D1Δ(v = 1), I1Σ–(v = 2)] perturbation parameters; as well as 112 ro-vibronic term values of B1Σ+(v = 0) up to J = 50 and C1Σ+(v = 0) up to J = 60. The significant, indirect a3Π(v = 12) ∼ [e3Σ–(v = 2, 3), d3Δ(v = 5, 6)] ∼ A1Π(v = 1) spin–orbit/spin-electronic/L-uncoupling interaction and a3Π(v = 12) ∼ [I1Σ–(v = 2), D1Δ(v = 1)] ∼ A1Π(v = 1) spin–orbit/L-uncoupling interaction were detected and analysed. Thus, this study, using modern experimental methods and deperturbation analysis, leads to a much improved description in terms of molecular constants and interaction parameters, compared to previous studies of the A1Π(v = 1) energy region in the 13C18O isotopologue. This research is a continuation of the studies on the A1Π state and its numerous perturbers in the CO isotopologues made by our team

High-resolution study of oscillator strengths and predissociation rates for ¹³C¹⁸O

International audienceWe carried out experiments at the SOLEIL synchrotron facility to acquire data for modelling CO photochemistry in the vacuum ultraviolet. We report oscillator strengths and predissociation rates for four vibrational bands associated with transitions from the v = 0 level of the X1 Σ+ ground state to the v = 0–3 vibrational levels of the core excited W1Π Rydberg state, and for three overlapping bands associated with the 4pπ, 5pπ, and 5pσ Rydberg states between 92.9 and 93.4 nm in 13C18O. These results complete those obtained in the same conditions for 12C16O, 13C16O, and 12C18O recently published by us, and extend the development of a comprehensive database of line positions, oscillator strengths, and linewidths of photodissociating transitions for CO isotopologues. Absorption spectra were recorded using the Vacuum UltraViolet Fourier Transform Spectrometer (VUV-FTS) installed on the Dichroïsme Et Spectroscopie par Interaction avec le Rayonnement Synchrotron (DESIRS) beamline at SOLEIL. The resolving power of the measurements, R = 300 000 to 400 000, allows the analysis of individual line strengths and widths within the bands. Gas column densities in the differentially pumped system were calibrated using the B-X (0–0) band at 115.1 nm in 13C18O

Experimental and Coupled-channels Investigation of the Radiative Properties of the N2 c4 (sup 1)Sigma+(sub u) - X (sup 1)Sigma+(sub g) Band System

The emission properties of the N2 c(sup prime)(sub 4) (sup 1)Sigma+(sub u) - Chi (sup 1)Sigma+(sub g) band system have been investigated in a joint experimental and coupled-channels theoretical study. Relative intensities of the c(sup prime)(sub 4) (sup 1)Sigma+(sub u)(0) - Chi (sup 1)Sigma+(sub g)(v(sub i)) transitions, measured via electron-impact-induced emission spectroscopy, are combined with a coupled-channel Schroedinger equation (CSE) model of the N2 molecule, enabling determination of the diabatic electronic transition moment for the c(sup prime)(sub 4) (sup 1)Sigma+(sub u) - Chi (sup 1)Sigma+(sub g) system as a function of internuclear distance. The CSE probabilities are further verified by comparison with a high-resolution experimental spectrum. Spontaneous transition probabilities of the c(sup prime)(sub 4) (sup 1)Sigma+(sub u) - Chi (sup 1)Sigma+(sub g) modeling atmospheric emission, can now be calculated reliably