J.H. Carver

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.

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.

Decomposition of the photoabsorption continuum underlying the Schumann-Runge bands of 16O2—I. Role of the B 3Σu- state: A new dissociation limit

Abstract Extensive measurements are presented of O2 photoabsorption cross-sections taken at selected minima between rotational lines of the Schumann-Runge band system. Both room temperature and liquid nitrogen temperature results are presented from 1750 to 1800 A and corrections are applied for the effect of the wings of the rotational lines. Step-like structure in the underlying continuum, due to absorption from rotationally excited levels of the ground state into the B 3 Σ - u state, is verified experimentally for the first time, and the observation is used to deduce a definitive dissociation limit of 57136.0 ± 0.5 cm-1 for the B state.

Resonances in the photodissociation of isotopic molecular oxygen—I. The longest band

Abstract We present detailed photoabsorption cross section measurements of the (0-0) and (0–1) bands of the E 3 ∑ - u - X 3 ∑ - g transition of molecular oxygen. The isotopes 16 O 2 , 16 O 18 O and 18 O 2 were studied at 79 and 295 K, with an instrumental resolution of 0.04–0.06 A FWHM. Our results show clearly that these bands provide one of the rarely observed examples of Beutler-Fano resonances in molecular photodissociation. With the aid of empirical modelling, we show that the widths and asymmetries of individual lines vary significantly with rotation and isotopic mass, but the oscillator strengths show no isotope effect.

Decomposition of the photoabsorption continuum underlying the Schumann-Runge bands of 16O2—II. Role of the 1 3Πg state and collision-induced absorption

Measurements are presented of molecular oxygen photoabsorption cross-sections and pressure coefficients taken at selected minima between rotational lines of the Schumann-Runge band system. Both room-temperature and liquid-nitrogen-temperature results are presented from 1760–1980 A, and corrections are applied for the effect of the wings of the rotational lines. Absorption into the B3∑−u and A3∑+u+ and is found to be insufficient to account for the total observed cross-section, and it is proposed that transitions to the 13Πg valence state account for the remainder. The pressure dependence of the cross-sections is consistent with collision-induced enhancement of the intensities of the forbidden transitions X3∑−g → A3∑+g and X3∑−g → 1 3Πg, while the temperature dependence of the pressure coefficients is not consistent with absorption due to stable (O2)2 dimers.

Pressure-broadening in the Schumann-Runge bands of molecular oxygen

Abstract The pressure-broadened widths for selected rotational lines from the (2-0) to (15-0) Schumann-Runge bands of O 2 have been measured at pressures up to 60 atm. The self-broadening coefficient exhibits no dependence on rotation and a small dependence on vibration, in agreement with theoretical expectations, and may be taken as 0.20 cm -1 /atm FWHM, implying an optical collision diameter of 6.2 A. The broadened absorption lines are shifted towards longer wavelengths and are asymmetric with a steeper blue wing, both of these effects increasing with pressure.

Rotational features in the fluorescence excitation spectrum of O(1D2) from vacuum ultraviolet laser photodissociation of O2

Seventh‐order anti‐Stokes Raman‐shifted ultraviolet laser radiation is used to dissociate O2 in the 175–177 nm region of the Schumann–Runge band system, B 3Σ−u–X 3Σ−g. A cross section for the production of O(1D2) is deduced from the 762 nm fluorescence of O2(b 1Σ+g), a collisional de‐excitation product of O(1D2) and O2(X 3Σ−g). Step structure observed in the spectrum is attributed to rotational thresholds for absorption from X 3Σ−g to energies above the B 3Σ−u dissociation limit. The threshold energies define a limiting rotationless dissociation energy of 57 136.4±0.9 cm−1. Shape resonances, quasibound by the rotational barrier of B 3Σ−u, are observed for the first time in O2. A theoretical calculation of the cross section is in agreement with the measured cross section when the near‐dissociation outer limb of the B 3Σ−u potential has an R−5 long range form. The shape of the adopted potential is consistent with an avoided crossing with another 3Σ−u state near R=4.6 A.

Resonances in the photodissociation of isotopic molecular oxygen—II. The second and third bands

Abstract We present detailed photoabsorption cross-section measurements for the (1-0) and (2-0) bands of the E3σ-u−X3σ-g transition of molecular oxygen. The isotopic molecules 16O2, 16O18O and 18O2 were studied at 79 and 295 K, with an instrumental resolution of 0.06 A FWHM. We find that the bands are further examples of Beutler-Fano resonances in molecular photodissociation.

Rotational variation of predissociation linewidths for the Schumann-Runge bands of molecular oxygen

Abstract Predissociation linewidths are presented as a function of rotation for the (3-0)-(14-0) Schumann- Runge bands of molecular oxygen. While there may be a slight tendency overall for the linewidths to increase with rotation, it is shown that previous assumptions of linewidths constant with rotation are generally valid within the experimental error for the range of rotation studied, Nn ⩽ 21. There is no evidence for the sudden increase in linewidth with rotation reported elsewhere.