Photoabsorption and photodissociation in molecular nitrogen

The photoabsorption and photodissociation of molecular nitrogen at extreme-ultraviolet wavelengths has been precisely modelled by solution of the coupled Schr¨odinger equation, for the purposes of elucidating the spectroscopy and predissociation dynamics of N2 and for practical application to its photochemistry. The physically realistic model is capable of reliable extrapolation beyond the database of spectroscopic information necessary for its construction, to energies in the range of 100 000 to 118 000 cm−1 and, with some caveats, beyond this; and for any temperature, rotational state, and isotopomer of N2. The model simulated spectra have an effectively-infinite resolution, and reproduce the rotational level energies of all electric-dipole-allowed 1 u and 1 + u states, and their absorption f -values, to spectroscopic accuracy over its entire range. The predissociated lineshapes of calculated transitions are accurately reproduced, as is the background continuum, even where a dissociation limit is crossed. The highly perturbed spectrum is shown to arise from multi-channel effects which can only be reproduced by a coupled treatment which includes the effects of homogeneous and heterogeneous interactions. Unbound dissociative states are permitted in the model formulation and a complex of 3 u and 3 + u states is shown to be responsible for the predissociation of singlet levels via spin-orbit interaction. The spectroscopic parameters of triplet states, and the variation of 1 u predissociation rates over multiple orders of magnitude has been accurately modelled for all isotopomers and energies up to 111 000 cm−1. The transformation of the calculated dissociative wavefunctions into a basis of asymptotically well-defined atomic states allows for the calculation of their predissociation branching ratios. New and accurate determinations have been made of potential-energy curves for the coupled states, the off-diagonal matrix elements that mix them, and the electronic transition moments responsible for their optical excitation, including, in some cases, their dependence on internuclear distance. New supporting laboratory measurements of rotationally-resolved absolute f -values and predissociation linewidths have been made for many transitions, some of which have not been previously observed. These various experiments employ synchrotron and laser radiation as well as electron-impact excitation, and make detections by means of a grating spectrometer, Fourier-transform spectrometer, and the detection of photofragments. Analysis of these studies is facilitated by the coupled-channels modelling, which is then informed by the resulting new data. The characteristics of the coupled-channels model are an ideal match to the immediate need for N2 spectra by photochemical modellers of planetary atmospheres, because of its realistic extensibility to variable real-world conditions. An application of the model to the atmosphere of Titan is presented here, which explains the unusual observed ratio of nitrogen isotopes in terms of the details of N2 photoabsorption. Another use of modelled spectra is presented here, for the analysis of the terrestrial dayglow. The applicability of the coupled-channels model is quite broad and further investigations which utilise it are underway

ULTRAVIOLET PHOTOABSORPTION OF SO ISOTOPOLOGUES AND THE B 3Σ− AND C 3Π STATES

The sulphur-monoxide $B\,{}^3\Sigma^-(v\ge4)$ levels are known to be strongly affected by vibrationally-dependent predissociation and local energy perturbations (Liu et al. 2006 JMS 238:213). The isotope-dependence of this predissociation and the SO photodissociation cross section is a candidate atmospheric-source for explaining the anomalous ${}^{32}$S/${}^{33}$S/${}^{34}$S/${}^{36}$S isotopic fractionation found in 2.5Ga old sedimentary material (Ono 2017 Annu. Rev. Earth Pl. Sc. 45:301). We have recorded new photoabsorption spectra between 195 and 230 nm to determine spectroscopic constants, predissociation linewidths, and transition strengths for the excited $B\,{}^3\Sigma^-(v=4-17)$ levels of ${}^{32}$S${}^{16}$O, ${}^{33}$S${}^{16}$O, and ${}^{34}$S${}^{16}$O. The $C\,{}^3\Pi$ state is also observed and perturbs $B\,{}^3\Sigma^-(v=4-17)$ through spin-orbit interaction. $B\,{}^3\Sigma^-$ and $C\,{}^3\Pi$ potential-energy curves, electronic transition moments, and a global spin-orbit interaction are deduced from the new data so that it may be extrapolated to the rare ${}^{36}$S${}^{16}$O isotopologue. We use the new cross sections to explore the potential for isotope-dependent photodissociation of SO in the ancient-Earth atmosphere due to structured solar UV radiation and atmospheric opacity

COMPLETE PHOTOABSORPTION LINELIST FOR CO AND ITS ISOTOPOLOGUES BETWEEN 101 AND 115 NM

The photoabsorbing bands of CO and its isotopologues appearing between 101 and 115 nm provide more than half of its photodissociative potential in the interstellar medium and planetary atmospheres, and are responsible for the well-known fractionation of C and O isotopes due to self-shielding. \medskip An experimental study of this region over several years using the undulator radiation source and vacuum-ultraviolet Fourier-transform spectroscopy facilities at the SOLEIL synchrotron [1] is complete. Line frequencies [2] and oscillator strengths [3], and widths [in prep.] are deduced, and in some cases extrapolated, to provide updated and reliable cross sections over a range of temperatures, including for the rare ${}^{17}$O isotopologues. \begin{itemize} \item [1] N. de Oliveira et al. (2016). The high-resolution absorption spectroscopy branch on the VUV beamline DESIRS at SOLEIL. J. Synchrotron Radiat. 23:887. \item [2] J.L. Lemaire et al. (2018). Atlas of new and revised high-resolution spectroscopy of six CO isotopologues in the 101-115 nm range. Astron. Astrophys. (accepted) \item [3] G. Stark et al. (2014). High-resolution oscillator strength measurements of the v=0,1 bands of the B-X, C-X, and E-X systems in five isotopologues of carbon monoxide. Astrophys. J. 788:68 \end{itemize

FORBIDDEN TRANSITIONS IN THE VUV SPECTRUM OF N2

The predissociation of chem{N_2} excited levels is enabled by the presence of optically-inaccessible triplet states. We have recorded vacuum ultraviolet (VUV) spectra at the SOLEIL synchrotron which reveal these states through their perturbation of allowed transitions or their direct appearance due to intensity borrowing. Some of these measurements were recorded at 900,K in order to access high-rotational levels, other measurements investigated weak forbidden transitions at high column density. Following careful analysis, significant new information has been obtained elucidating the states responsible for the astrochemically and atmospherically signficant chem{N_2} predissociation mechanism, and allowing for improvements in its quantitiative modelling

Oscillator strengths and line widths of dipole-allowed transitions in ¹⁴N₂ between 89.7 and 93.5 nm

Line oscillator strengths in the 20 electric dipole-allowed bands of ¹⁴N₂ in the 89.7–93.5nm (111480–106950cm⁻¹) region are reported from photoabsorptionmeasurements at an instrumental resolution of ∼6mÅ (0.7cm⁻¹) full width at half maximum. The absorptionspectrum comprises transitions to vibrational levels of the 3pσᵤc′₄¹Σᵤ⁺, 3pπᵤc³Πᵤ, and 3sσgo₃¹ΠᵤRydberg states and of the b′¹Σᵤ⁺ and b¹Πᵤ valence states. The J dependences of band f values derived from the experimental line f values are reported as polynomials in J′(J′+1) and are extrapolated to J′=0 in order to facilitate comparisons with results of coupled Schrödinger-equation calculations. Most bands in this study are characterized by a strong J dependence of the band f values and display anomalous P-, Q-, and R-branch intensity patterns. Predissociation line widths, which are reported for 11 bands, also exhibit strong J dependences. The f value and line width patterns can inform current efforts to develop comprehensive spectroscopic models that incorporate rotational effects and predissociation mechanisms, and they are critical for the construction of realistic atmospheric radiative-transfer models.This work was supported in part by NASA Grant No. NNG05GA03G to Wellesley College and Australian Research Council Discovery Program Grant No. DP0558962

The High-Resolution Extreme-Ultraviolet Spectrum of N2 By Electron Impact

We have analyzed high-resolution (FWHM = 0.2 Å) extreme-ultraviolet (EUV, 800-1350 Å) laboratory emission spectra of molecular nitrogen excited by an electron impact at 20 and 100 eV under (mostly) optically thin, single-scattering experimental conditi

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

Analysis of terrestrial thermospheric N2c′41Σ+u(0) ~ b′1Σ+u(1)- X1Σ+g dayglow emission observed by the Far Ultraviolet Spectroscopic Explorer

Terrestrial thermospheric dayglow emission from the coupled and overlapping c′41Σ+u(0) and b′1Σ+u(1) levels of molecular nitrogen, observed by the Far Ultraviolet Spectroscopic Explorer, is analyzed with the aid of a coupled channels quantum mechanical model of N2 spectroscopy and predissociation dynamics. Model emission spectra for the mixed c′41Σ+u(0) ~ b′1Σ+u(1) − X1Σ+g(vi = 2, 6–9) transitions, calculated for the case of excitation by photoelectron impact, are in excellent agreement with the observations. While the principal excitation mechanism for N2 in the thermosphere is photoelectron impact, evidence is also found in other transitions of resonant fluorescence, induced by lines in the solar atomic hydrogen Lyman series, atomic oxygen transitions, and other N2 bands. The observed emission rate of the c′41Σ+u(0)~ b′1Σ+u(1)− X1Σ+(0) band is ~1% of that inferred from the emission rates to X1Σ+g(vi > 2) levels. A qualitative explanation is given for the drastically reduced intensity and band shape distortion observed in the c′41Σ+u(0)− X1Σ+g(0) emission band. Estimates of the total electron excitation rates for the nominal b′1Σ+u(1) and c′41Σ+u(0) levels are determined from the spectrum by extrapolating the model through regions containing unmeasured and/or resonantly absorbed band

Tuning out vibrational levels in molecular electron energy-loss spectra

The phenomenon whereby features associated with certain vibrational levels in molecular states of mixed electronic character disappear under specific scattering conditions in electron energy-loss spectra is investigated. In particular, using a combination of experimental measurements and coupled-channel calculations, anomalous vibrational intensities in the mixed valence-Rydberg 1Π u←X1Σg+ transition of N 2 are explained. A single parameter, i.e., the ratio of the generalized electronic transition moments to the diabatic valence and Rydberg components of the mixed states, dependent on the experimental scattering conditions, is found to be essentially capable of describing all observed relative vibrational intensities, including the near disappearance of the b1Π u(v=5) feature for momentum-transfer-squared values K2 ≈ 0.3 a.u. This result highlights the interesting possibility of experimental control of molecular quantum-interference effects in electron energy-loss spectra, something that is not possible in optical spectra