Glenn Stark

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...

High-resolution absorption cross sections of carbon monoxide bands at 295 K between 91. 7 and 100. 4 nanometers

Theoretical descriptions of the abundance and excitation of carbon monoxide in interstellar clouds require accurate data on the vacuum-ultraviolet absorption spectrum of the molecule. The 6.65 m spectrometer at the Photon Factory synchrotron light source has been used to measure photoabsorption cross sections of CO features between 91.2 and 100.4 nm. These data were recorded at a resolving power of 1.7 × 10 5 , more than 20 times greater than that used in previous work.

High‐resolution, VUV (147–201 nm) photoabsorption cross sections for C2H2 at 195 and 295 K

Acetylene, C2H2, plays an important role in the photochemistry of the atmospheres of the giant planets, in part because of its strong absorption features in the 145–200 nm region where there is significant solar flux. Accurate photoabsorption cross section data at the temperatures of planetary atmospheres are required for interpretation of observations and for reliable photochemical models. We have measured the photoabsorption cross sections of acetylene at 195 and 295 K in the wavelength range 147–201 nm. (The 295 K measurements extend to 137 nm.) The high-resolution spectrometers employed had instrumental bandwidths smaller than those used in previous measurements and therefore produced cross sections that are less influenced by instrumental effects. Our results indicate that hot bands make small but significant contributions to the room temperature absorption spectrum of C2H2. Photochemical models of planetary atmospheres should employ cross sections measured at the temperatures of these atmospheres.

High‐resolution photoabsorption cross‐section measurements of SO2 at 160 K between 199 and 220 nm

Photoabsorption cross sections of sulfur dioxide over a range of temperatures are required to interpret observations and to support models of the atmospheres of Io and Venus. We report high-resolution (lambda/Delta lambda approximate to 450,000) photoabsorption cross-section measurements by Fourier transform spectrometry of SO2 at 160 K in the wavelength region 199 to 220 nm, which encompasses the strongest features in the prominent (C) over tilde (1) B-2 -(X) over tilde (1)A(1) system. Our results are compared with literature values obtained at lower resolutions and with 295 K cross sections recorded earlier with the same instrument.

Absorption band oscillator strengths of N2 transitions between 95.8 and 99.4 nm

Photoabsorption cross sections for seven electric dipole‐allowed N2 bands, at wavelengths between 95.8 and 99.4 nm, have been measured with an instrumental resolution of 6.2×10−4 nm. Integrated cross sections and band oscillator strengths are presented for the c4’ 1Σu+(v’=0)–X  1Σg’(v ‘=0) and c3 1Πu(v’=0)–X  1Σg+(v‘=0) transitions, as well as for five bands (v’=0,1,2,3,4) in the b  1Πu(v’)–X  1Σg+(0) progression. The results are compared with earlier, lower resolution absorption measurements, electron scattering measurements, and calculations based on a deperturbation analysis of the excited states.

The f Rydberg series in the absorption spectrum of N2

The nf Rydberg levels of 14N2 converging to the X 2Σ+g ground state of N+2 have been studied from n=4–9 in the high‐resolution absorption spectra of supersonically expanding nitrogen at wavelengths ranging from 843.2 to 802.6 A (118 600–124 600 cm−1). The best experimental results, achieving rotational temperatures on the order of 20 to 40 K and a resolution of 0.5 cm−1, were obtained by photographing the jet absorption against the background continuum from a synchrotron radiation source. Complementary data for 14N2 and 15N2 come from the analyses of spectra recorded under equilibrium conditions at 70 K with a resolution of 1 cm−1, using the He continuum as background source. The observations are interpreted with the help of multichannel quantum defect calculations and lead to the conclusion that, to varying degrees and with the exception of 4f(v=0), all the complexes studied here show the effects of interactions with core excited d and s Rydberg levels built on the A 2Πu first excited state of N+2. Also,...

The near-threshold absorption spectrum of N2

A new comprehensive multichannel quantum defect study of the near-threshold absorption of 14N2 has been carried out over the energy range 118 720–125 425 cm−1. A nearly complete understanding of the rotationally cold spectra reported earlier [K. P. Huber and Ch. Jungen, J. Chem. Phys. 92, 850 (1990); K. P. Huber et al., ibid. 100, 7957 (1994)] has been achieved in the region where core-excited s and d Rydberg levels built on the A 2Πu state of the ion interact with the series of p and f complexes converging to the lowest vibrational levels of X 2Σg+. The interactions reduce to a purely electronic quantum defect matrix which, after suitable transformations, accounts for the observed perturbed structures and intensities arising from vibronic coupling, rotational l uncoupling, and the different geometries of the X and A ion cores. The final calculations converged with 42 nonzero quantum defect parameters reproducing the 597 upper-state rovibronic levels with a standard deviation of 1.12 cm−1. The results hav...

High-resolution photoabsorption cross-section measurements of SO2 at 198 K from 213 to 325 nm

SO2 plays an important role in the atmospheric chemistry of the Earth, Venus, and Io. This paper presents photoabsorption cross sections of SO2 from 213 to 325 nm at 198 K, encompassing the (C) over tilde B-1(2) - (X) over tilde (1)A(1) and (B) over tilde B-1(1) - (X) over tilde (1)A(1) electronic bands. These measurements are part of a series of measurements over the 160 to 300 K temperature range between 190 and 325 nm. The cross sections have been measured at high resolution (lambda/Delta lambda approximate to 450,000) using Fourier transform spectrometry and are compared to other high-resolution measurements in the literature.

High-resolution photoabsorption cross sections of E1Pi - X1Sigma(+) vibrational bands of CO-12 and CO-13

Photodissociation following absorption of extreme-ultraviolet photons is an important factor in determining the abundance and isotropic fractionation of CO in diffuse and translucent interstellar clouds. The principal channel for destruction of CO-13 in such clouds begins with absorption in the (1,0) vibrational band of the E1Pi - X1Sigma(+) system; similarly, absorption in the (0,0) band begins a significant destruction channel for CO-12. Reliable modeling of the CO fractionation process depends critically upon the accuracy of the photoabsorption cross section for these bands. We have measured the cross sections for the relevant isotropic species and for the (1,0) band of CO-12. Our results, which are uncertain by about 10 percent, are for the most part larger than previous measurements.

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

We report oscillator strengths for six strong vibrational bands between 105.0 and 115.2 nm, associated with transitions from the v = 0 level of the X 1Σ+ ground state to the v = 0 and 1 levels of the B 1Σ+, C 1Σ+, and E 1Π states, in 12C16O, 12C17O, 12C18O, 13C16O, and 13C18O. These measurements extend the development of a comprehensive database of line positions, oscillator strengths, and linewidths of photodissociating transitions for all astrophysically relevant CO isotopologues. The E-X bands, in particular, play central roles in CO photodissociation and fractionation models of interstellar clouds and circumstellar disks including the early solar nebula. The resolving powers of the room-temperature measurements, R = 300,000-400,000, allow for the analysis of individual line strengths within bands; the measurements reveal J-dependences in the branch intensities of the C(v = 0,1)-X(0) and E(v = 0,1)-X(0) bands in all isotopologues. Minimal or no isotopologue dependence was found in the f-values of the C(v = 0,1)-X(0) and E(v = 0,1)-X(0) bands at a ~5% uncertainty level. Revised dissociation branching ratios for the C(v = 0,1) and E(v = 0,1) levels are computed based on these f-values. The weak isotopologue dependence of the f-values presented here eliminates this mechanism as an explanation for the large 17O enrichments seen in recent laboratory photolysis experiments on CO at wavelengths from 105 to 108 nm.