A.J. Blake

Photo-Absorption cross sections of molecular oxygen between 1250 Å and 2350 Å

Abstract Photoelectric methods employing a variety of detectors have been used to determine the photo- absorption cross sections of molecular oxygen in the range 1250–2350 A. Significant departures from Beers Law have been observed in a number of wavelength regions. In the Schumann-Runge bands apparent cross sections determined using Beers Law are found to vary inversely as the square root of the layer thickness of the absorbing gas.

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.

Photoelectron spectra and partial photoionization cross sections for NO, N2O, CO, CO2 and NH3☆

Abstract Photoelectron spectra have been obtained for NO, N2O, CO, CO2 and NH3 over a wide range of photon wavelengths using a dispersed light source. The observations cover the wavelength range 584 to 890 A and photoelectron spectra have, in general, been recorded at 5 A intervals in order to survey broad variations in the different photodisintegration processes. Partial photoionization cross sections have been measured for all these gases for processes involving transitions to the ground states and the various excited electronic states of the residual ions; in the case of ammonia, cross sections for dissociative photoionization have also been determined.

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.

Photoelectron spectra and partial photoionization cross-sections for carbon dioxide

Abstract A photoelectron spectrometer has been used to determine partial photoionization cross-sections for carbon dioxide for monochromatic incident radiation throughout the wavelength range 584–720 A. Transitions to particular states of the CO + 2 ion were identified by measuring the photoelectron energy. Transitions to the 2 π g , 2 π u , 2 Σ + u and 2 Σ + g states of CO + 2 were observed at all photon energies for which they were energetically allowed. Partial cross-sections for the production of each of these ionic states have been determined as a function of photon wavelength.

Autoionization in diatomic molecules studied by photoelectron spectroscopy

Abstract Photoelectron spectra have been recorded at a number of well-defined autoionization resonances in nitric oxide (941·6, 897·3, 832·5, 825·0, 810·6 and 783·6 and 771·3A), molecular nitrogen (775·7, 771·6, 765·2, 761·2, 754·3 and 74sd5 A) and carbon monoxide (838·5, 827·8, 807·1 and 796·8 A). Vibrational structure is clearly resolved and considerable differences are observed between the resonance spectra and those recorded at nearby off-resonance wavelengths. The photoelectron spectra are of the general form expected on Franck-Condon considerations. At resonances with a non-zero vibrational quantum number, the photoelectron spectra are extended towards lower electron energies to include a wider range of vibrational transitions than are observed off-resonance. Comparison of the resonance spectra with computed Franck-Condon distributions allows an estimate to be made of the equilibrium internuclear separation for some excited electronic states responsible for autoionization resonances.

Photoelectron spectroscopy for some autoionized states of molecular oxygen

Abstract Photoelectron spectra for molecular oxygen have been recorded at a number of wavelengths in the range 957.0 to 832.5 A corresponding to autoionizing resonances belonging to the M(v = 0, 1, 2, 3) and I(v = 0, 1, 2, 3) vibrational progressions of the oxygen molecule. These photoelectron spectra differ significantly from spectra obtained off-resonance and show considerable extension of the ionic vibrational structure towards lower photoelectron energies particularly for resonances with v ≠ 0. The photoelectron spectra are found to be of the form predicted by Franck-Condon type calculations and the predicted modulation producing (v + 1) broad maxima in the ionic vibrational structure is confirmed by the observations. The internaclear spacing re for the autoionizing state is used as a parameter in the Franck-Condon calculation and is adjusted to obtain the best fit with the observations. Values determined in this way are r e = 1.281 ± 0.003 A for the M progression, re = 1.32 ± 0.03 A for the J progression and r e = 1.35 ± 0.02 A for the I progression.

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.