D.G. McCoy

Temperature dependence in the Schumann-Runge photoabsorption continuum of oxygen

Abstract The photoabsorption cross section in the Schumann-Runge continuum of oxygen has been measured with high precision over the wavelength region 140–174 nm at temperatures in the range 295–575 K. Models for the upper state potential and the electronic transition moment were used in the calculation of the cross section and its temperature dependence. By comparing this theoretical cross section with measured values, curves for the upper state potential and the transition moment in the continuum region have been obtained independently for the first time.

Effects of the close approach of potential curves in photoabsorption by diatomic molecules—II. Temperature dependence of the O2 cross section in the region 130–160 nm

Abstract The photoabsorption cross section of oxygen has been measured at temperatures of 295 and 575 K over the wavelength range 130–160 nm. The temperature coefficient shows strong structure in the part of the Schumann-Runge continuum below 136 nm. The observed data have been fitted with theoretical calculations that include a 3Πu continuum and take account of the coupling between the valence and Rydberg 3Σ-u states. Potential curves and transition moments for these states obtained from the fitting procedure are given. The diabatic electronic coupling constant for the 3Σ-u states is found to be 0.485 eV.

The pressure dependence of the Herzberg photoabsorption continuum of oxygen

Abstract The origin of pressure dependence in the dipole forbidden Herzberg continuum of oxygen is discussed in terms of the formation of oxygen dimers and the collision of free molecules. The small temperature dependence of the pressure coefficient indicates that the collisions of free molecules have the dominant influence. Consideration of the selection rules applying to the Herzberg systems and the strengths of the associated band systems leads to the conclusion that the pressure dependence results from enforced dipole transitions in the Herzberg III ( A ′ 3 Δ u − X 3 Σ g - ) system.

Effects of the close approach of potential curves in photoabsorption by diatomic molecules—I. Theory and computational procedures

Abstract A procedure is given for the calculation of continuum photoabsorption cross sections of diatomic molecules in a spectral region corresponding to the avoided crossing of molecular potentials. The technique involves the simultaneous solution of coupled Schrodinger equations and the numerical procedures adopted for their solution are outlined. Results of calculations using simple analytic potentials similar to those of the B 3 Σ - u and E 3 Σ - u states of oxygen are presented. It was found that non-adiabatic effects are significant as much as 1 eV below the crossing energy.

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.

Effects of the close approach of potential curves in photoabsorption by diatomic molecules—III. Structure and isotope shift of the 124.4 nm band of oxygen

Abstract The photoabsorption spectrum of oxygen in the region of the band that occurs at 124.4 nm in 16O2 exhibits complex isotope effects, including dramatic variations in the widths of rotational lines and an anomalous isotope shift. The cross section in this region has been analysed by solving coupled Schrodinger equations. The potential curves and transition moments for which the calculations reproduce the observed form of the cross section and isotope effects in detail represent an extension to higher energy of those published earlier. The result is the first comprehensive model of the isotopic behaviour of this part of the spectrum.

Quantum interference in the Schumann-Runge bands of molecular oxygen

The photoabsorption cross section of O2 in the Schumann-Runge bands is investigated theoretically using the coupled-channel Schrodinger-equations method. It is found that quantum interference between vibrational bands leads to significant asymmetry in the far line wings, a result supported by experimental evidence. Thus, Schumann-Runge cross sections calculated with the usual band models based on Voigt line shapes may be in error in regions of weak absorption between vibrational band heads, suggesting the need to re-evaluate aspects of atmospheric photochemistry sensitive to those regions.

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.

Analytical potential curves for the X 1Σ+ and 0+ states of NaI

Abstract The potentials for the X 1 Σ + and 0 + states of NaI are determined in analytical form by a numerical optimization procedure. The accuracy of the optimum potentials is demonstrated by direct comparison of the calculated term energies and rotational constants with the measured values, as well as by assessment of the consistency with other experimental information about the 0 + state.

Effects of the close approach of potential curves in photoabsorption by diatomic molecules—IV. The near-diabatic and near-adiabatic cases

Abstract The suitability of the Fano-Mies type theory for the case of the close approach of two weakly-coupled diatomic potential curves, one of which supports bound vibrational states and the other continuum dissociation states, is explored. For both the near-diabatic and near-adiabatic limits, the isolated resonance approximation to the Fano theory agrees accurately with solutions of associated coupled equations only for the case of very small deviations from the corresponding limit. Calculations with the full Fano-Mies theory are so involved and time consuming as to be computationally unattractive. We conclude that the Fano-Mies approach is valuable for qualitative physical insight but not for quantitative analysis of high accuracy data.