B. R. Lewis

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).

Temperature dependence of the carbon dioxide photoabsorption cross section between 1200 and 1970 Å

Abstract Photoabsorption cross sections have been measured for carbon dioxide at 0.5A intervals between 1200 and 1970A. The instrumental resolution was about 0.05A and measurements were performed using relatively low pressures at temperatures near 200, 300, and 370 K. The room temperature cross sections are found to be in good agreement with previous measurements and a temperature effect is found which is small at the shorter wavelengths, passing through a minimum near 1400A. At longer wavelengths the temperature effect increases steadily until at some fixed wavelengths above 1900 A the cross section increases by a factor of about 20 as the temperature is increased from 200 to 370 K. This behaviour is of particular significance to calculations of carbon dioxide photodissociation rates at low altitudes in the Martian atmosphere.

Rotational variation of predissociation linewidth in the Schumann-Runge bands of 16O2

Abstract Extensive high resolution photoabsorption measurements have been performed on most of the experimentally accessible rotational lines in the Schumann-Runge band system of 16O2. Predissociation linewidths are inferred from these measurements for as many lines as possible from the (1-0) to (19-0) bands. A model of the predissociation is developed, which includes the interactions of the B3Σ-u state with repulsive 5Πu, 3Πu, 1Πu and 3Σ+u states, and molecular parameters for these interactions are determined by least-squares fitting the model to the experimental vibrational widths for the (1-0) to (18-0) bands. These parameters are then used, in conjunction with the model, to predict the variation of predissociation linewidth with rotation for each Schumann-Runge band. The experimental linewidths are found to exhibit systematic variation with rotation for most of the bands studied, and agreement with the model predictions of rotational variation is excellent. Polynomial fits to the model rotational linewidths are also presented in order to facilitate atmospheric modelling applications.

Predissociation mechanism for the lowest Πu1 states of N2

Separate coupled-channel Schrodinger-equation (CSE) models of the interacting Πu1 (b,c,o) and Πu3 (C,C′) states of N2 are combined, through the inclusion of spin-orbit interactions, to produce a five-channel CSE model of the N2 predissociation. Comparison of the model calculations with an experimental database, consisting principally of detailed new measurements of the vibrational and isotopic dependence of the Πu1 linewidths and lifetimes, provides convincing evidence that the predissociation of the lowest Πu1 levels in N2 is primarily an indirect process, involving spin-orbit coupling between the bΠu1- and CΠu3-state levels, the latter levels themselves heavily predissociated electrostatically by the C′Πu3 continuum. The well-known large width of the b(v=3) level in N214 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 Πu1 states of N2, and is thus likely to prove useful in the constru...

Angular distributions for photodissociation of O2 in the Herzberg continuum

Photodissociation in the Herzberg continuum of molecular oxygen has been studied at 236, 226 and 204 nm. Using ion-imaging and monitoring of O(3Pj), j=0, 1, and 2 product-atom angular distributions, the amount of parallel character of the transition was measured. In order to interpret these data, analyses of the photoabsorption oscillator strengths and the parallel-perpendicular nature of the Herzberg I, II and III bands, and extrapolation of these properties into the Herzberg-continuum region have been performed. Our measured fine-structure-averaged angular distributions are found to be consistent with this photoabsorption model. In addition, the dynamics of the dissociation process is discussed, based on the O-atom fine-structure distributions.

Oscillator strengths for the Schumann-Runge bands of 16O18O

Abstract Oscillator strenghts were measured for the (2-0)–(15-0) Schumann-Runge bands of 16O18O. Individual rotational lines were studied at a resolution of ≈0.05A. Band oscillator strenghts decrease with increasing rotation at a rate intermediate between those for 16O2 and 18O2. Our measurements are in good agreement with oscillator strenghts calculated using potential curves and dipole moments derived from 16O2 measurements, and support conclusions that 16O18O plays only a minor role in the photodissociation of atmospheric O2. Dipole moments deduced from our oscillator strenghts agree well with those obtained for 16O2, 18O2, as well as with recent ab initio and semi-empirical determinations.

Extreme ultraviolet laser excitation of isotopic molecular nitrogen: The dipole-allowed spectrum of 15N2 and 14N15N

The work was supported by the European Community, under the Access to Research Infrastructures initiative of the Improving Human Potential Program, Contract No. HPRI-CT-1999-00064. K.G.H.B. was supported by the Scientific Visits to Europe Program of the Australian Academy of Science.

Asymmetric line shapes in the indirect predissociation of the f 1Σu+ Rydberg state of O2

Rotationally‐resolved Beutler–Fano line shapes observed in the photoabsorption spectrum of the (2,0) band of the 3pπuf 1Σu+←X 3Σg− Rydberg system of O2 are interpreted using a coupled‐channel Schrodinger equations model. It is found that the f 1Σu+ state is indirectly predissociated by the B 3Σu− continuum, and that the f←X transition borrows oscillator strength primarily from dipole‐allowed transitions into the mixed Rydberg‐valence states of 3Σu− symmetry. Both the predissociation linewidth and oscillator strength of the (2,0) resonance are controlled by the spin‐orbit interaction between the 1Σu+ and 3Σu− components of the 3p‐complex. There is some evidence for a destructive quantum interference between the transition amplitude borrowed from the 3pπuE 3Σu−←X 3Σg− transition and that borrowed weakly from the f 1Σu+←b 1Σg+ transition through spin‐orbit mixing between the b 1Σg+ and X 3Σg− states.

Photodissociation of interstellar N2

Aims. Following the recent detection of 36 ArH + in the Crab nebula spectrum, we have computed the photodissociation rate of ArH + in order to constrain the physical processes at work in this environment. Methods. Photodissociation cross-sections of ArH + were computed in an ab initio approach that includes an explicit account of spin‐ orbit coupling. Results. We report the photodissociation cross-section of ArH + as a function of wavelength. Photodissociation probabilities are derived for di erent impinging radiation fields.The photodissociation probability for a very small unshielded cloud surrounded on all sides by the standard unshielded InterStellar Radiation Field (ISRF) model is equal to 9:9 10 12 s 1 and 1:9 10 9 s 1 in the Crab nebula conditions. The dependence on the visual extinction is obtained by using the Meudon photon-dominated region (PDR) code, and corresponding analytical fits are provided. Conclusions. These data will help to produce a realistic chemical network to interpret the observations. Photodissociation of ArH + is found to be moderate, and the presence of this molecular ion mainly depends on the molecular fraction.

Rotational effects in the band oscillator strengths and predissociation linewidths for the lowest Πu1–XΣg+1 transitions of N2

A coupled-channel Schrodinger equation (CSE) model of N2 photodissociation, which includes the effects of all interactions between the b, c, and oΠu1 and the C and C′Πu3 states, is employed to study the effects of rotation on the lowest-υΠu1–XΣg+1(υ,0) band oscillator strengths and Πu1 predissociation linewidths. Significant rotational dependences are found which are in excellent agreement with recent experimental results, where comparisons are possible. New extreme-ultraviolet (EUV) photoabsorption spectra of the key bΠu1←XΣg+1(3,0) transition of N2 are also presented and analyzed, revealing a b(υ=3) predissociation linewidth peaking near J=11. This behavior can be reproduced only if the triplet structure of the C state is included explicitly in the CSE-model calculations, with a spin-orbit constant A≈15cm−1 for the diffuse C(υ=9) level which accidentally predissociates b(υ=3). The complex rotational behavior of the b–X(3,0) and other bands may be an important component in the modeling of EUV transmissio...