Terry N. Olney

Absolute scale determination for photoabsorption spectra and the calculation of molecular properties using dipole sum-rules

An assessment of the absolute scales of photoabsorption differential oscillator strength (df/dE) spectra which were originally normalized using valence shell TRK (i.e. S(O)) sum-rule normalization is presented for a series of dipole (e,e) measurements for 5 noble gases and 52 small molecules. This comprehensive data set involves previously published absolute high resolution dipole (e,e) spectra of the valence shell discrete region combined with very wide range low resolution measurements in the ionization continuum for each atom or molecule. The absolute scales established for the dipole (e,e) spectra using the S(O) sum-rule in the originally published works (data available on the world wide web at ftp://chem.ubc.ca/pub/cooper or by anonymous ftp - see end of present paper for more details) are assessed by deriving static dipole polarizabilities for each atom and molecule using the S(−2) sum-rule. These values are found to be highly consistent with experimental and theoretical literature values of the static dipole polarizability and in almost all cases well within the estimated ±5% accuracy of the originally published TRK sum-rule normalized absolute photoabsorption differential oscillator strength spectra. Significant errors of 8% and 19% in the previously published absolute oscillator strength scales for CCl4 (G.R. Burton, W.F. Chan, G. Cooper, C.E. Brion, Chem. Phys. 181 (1994) 147) and SiF4 (X. Guo, G. Cooper, W.F. Chan, G.R. Burton, C.E. Brion, Chem. Phys. 161 (1992) 453, 471) respectively are revealed by the dipole polarizability analysis and approproate corrections are recommended. Alternative methods of absolute scale determination for photoabsorption spectra using static or dynamic dipole polarizabilities from refractive index or dielectric constant measurements are also considered. These are found to be particularly useful especially where data are restricted to lower photon energies (< 60 eV) or where phenomena such as low lying inner shells, continuum shape resonances or Cooper minima preclude the use of the valence shell TRK sum-rule. As a result of the assessment of the absolute scales for the published dipole (e,e) spectra and since polarizabilities and refractive index data can typically be obtained with even higher precision (+-1%), these data have been used to further refine the measured differential oscillator strength scales. On this basis, dipole oscillator strength sums S(u) (u = −1, −2, −3, −4, −5, −6, −7, −10) and L(u) (u = −1, −2, −3, −4, −5, −6) are obtained from the df/dE spectra. For many of the systems considered, the presently reported dipole sums are derived molecular properties are more accurate than previously reported values. In fact, for 28 of the 52 small molecules these sum-rule values represent the only available values. The dipole sums can be used to calculate normal Verdet constants which are involved in the Faraday effect. It is also shown that accurate values of the rotationally averaged C6(A,B) dispersion coefficients for the long-range interaction of all possible pairs of atomic and/or molecular species can be obtained from the measured differential oscillator strength spectra. Alternatively, an approximation to C6(A,B), requiring only the S(−2) and L(−2) sums, is shown to provide very reliable estimates of C6(A,B).

The valence shell photoabsorption of the linear alkanes, CnH2n+2 (n=1–8): absolute oscillator strengths (7–220 eV)

Abstract The absolute valence shell photoabsorption spectra of the first eight members of the homologous linear (normal) alkane series, CH 4 to C 8 H 18 , have been obtained from electron energy loss measurements using dipole (e, e) spectroscopy at a resolution of 1 eV fwhm in the equivalent photon energy range 7–220 eV. Absolute oscillator strengths (transition probabilities) for discrete and continuum photoabsorption transitions have been determined throughout the entire energy range. The lower energy regions (7–50 eV) of the spectra have also been studied at high resolution (0.048 eV fwhm), and essentially all the features observed in the discrete excitation region of CH 4 , C 2 H 6 , C 3 H 8 , and C 4 H 10 can be reasonably assigned to Rydberg transitions by using quantum defects and the transferability of term values. The quite good agreement between the present results and the constrained dipole oscillator strength distribution (DOSD) values computed by Jhanwar, Meath, and MacDonald [Can. J. Phys. 59 (1981) 185] shows that the techniques utilized in constructing the DOSDs provide a reasonably satisfactory estimate of the absolute photoabsorption oscillator strengths for the linear alkanes.

Absolute UV and Soft X-ray photoabsorption of ethylene by high resolution dipole (e, e) spectroscopy

Abstract Absolute photoabsorption oscillator strengths (cross sections) for the valence shell discrete region of ethylene from 6 to 10.7 eV have been measured using high resolution (∼ 0.05 eV fwhm) dipole (e, e) spectroscopy. A long-range spectrum was also obtained from 6 to 200 eV at low resolution ( ∼ 1 eV fwhm), from which the absolute oscillator strength scale was obtained via valence-shell TRK sum-rule normalization. The presently determined absolute oscillator strengths were found to be more consistent with the (exact) theoretical value of the total S (0) TRK sum-rule than previously published data. In addition, the electric dipole polarizability of ethylene determined from the total S (−2) sum-rule incorporating the presently reported oscillator strengths is in very good agreement with experimentally measured polarizability values.

Gas dynamics of the ICP-MS interface: impact pressure probe measurements of gas flow profiles

A versatile ICP-vacuum interface was constructed to investigate the gas dynamics and gas flow profiles of the ICP-MS interface. This new interface combines a low interface pressure, ≥0.24 Torr, with the ability to accommodate a variety of skimmer designs at sampler to skimmer spacings up to 80 mm. Plasma beams were extracted by placing a 0.9 or 2.0 mm diameter skimming orifice at distances of 6.7-47 mm behind the sampling orifice. Gas flow profiles of each resulting plasma beam were measured using an impact pressure probe placed at distances of 35-105 mm behind the skimmer. The centreline flux and width of the gas beam were compared with those calculated for an ideally skimmed beam. The results for the 0.9 mm diameter skimmer orifice showed that placing the skimmer at 17.0 and 27.3 mm downstream of the sampler formed the highest intensity and narrowest beams. In contrast, by placing the skimmer closer to the sampler, as in common interface designs, the beam profile is less intense on the centreline and much wider. Using a larger 2.0 mm diameter skimming orifice at 16.5-26.8 mm downstream of the sampler produced a more intense beam than any arrangement using the 0.9 mm skimming orifice. However with the 2.0 mm diameter skimmer, centreline intensities were still about half those of an ideally skimmed beam.

Inductively coupled plasma mass spectrometry with a quadrupole mass filter operated in the third stability region

An inductively coupled plasma mass spectrometer system is described in which the quadrupole mass filter is operated in the third stability region with Mathieu parameters (a, q)=(3, 3). Operation at the upper tip of this region is found to give generally better sensitivity and resolution than operation at the lower tip. A limiting resolution at half height of 4000 at m/z 59 is possible with operation at the upper tip. This resolution is insufficient to separate atomic ions from molecular ions of the same nominal mass except in favorable cases. However, operation at moderate resolution (R1/2=500 to 1000) is found to give very high abundance sensitivity (>107). The results suggest that the third stability region may be advantageous for specialized applications of inductively coupled plasma mass spectrometry.

Absolute photoabsorption (6–350 eV) and photoionization (11–80 eV) of methyl chloride using dipole electron scattering and synchrotron radiation spectroscopies

Abstract Absolute oscillator strenghts (cross sections) have been measured for the photoabsorption of CH 3 Cl at low resolution (1 eV fwhm) in the equivalent photon energy range from 6 to 350 eV and at high resolution (0.05 eV fwhm) from 6 to 50 eV using dipole (e,e) spectroscopy. In the valence shell region of CH 3 Cl, from the first ionization threshold to 80 eV, the photoionization efficiency, ionic photofragmentation branching ratios and absolute partial oscillator strengths for molecular and dissociative photoionization have been determined by dipole (e,e+ion) spectroscopy. The major ionic pathways of the dipole induced breakdown scheme of CH 3 Cl are predicted from these results and the partial photoionization oscillator strengths for production of the valence shell electronic states of CH 3 Cl + are estimated. The photoabsorption data have been combined with photoelectron branching ratios for Ch 3 Cl, measured using monochromated synchrotron radiation at photon energies from 21 to 72 eV, to obtain partial photoionization oscillator strengths for production of the electronic states of CH 3 Cl + , which are compared with the partial oscillator strength estimates derived from the (e,e+ion) data.

QUANTITATIVE STUDIES OF THE PHOTOABSORPTION, PHOTOIONIZATION, AND IONIC PHOTOFRAGMENTATION OF METHYL FLUORIDE AT VUV AND SOFT X-RAY ENERGIES (7-250 EV ) USING DIPOLE ELECTRON SCATTERING AND SYNCHROTRON RADIATION

Abstract Absolute oscillator strengths (cross sections) for the valence shell photoabsorption of CH 3 F have been measured using low resolution (1 eV fwhm) dipole (e, e) spectroscopy in the equivalent photon energy range from 7 to 250 eV. In addition, high resolution (0.048 eV fwhm) dipole (e, e) spectroscopy has been used to study the valence shell region from 8 to 50 eV in more detail. The photoionization efficiency, ionic photofragmentation branching ratios and absolute partial oscillator strengths for molecular and dissociative photoionization have been determined for CH 3 F by dipole (e, e+ion) spectroscopy from the first ionization threshold to 100 eV. The major pathways of the ionic dipole induced breakdown scheme are predicted from these results. The electronic ion state branching ratios for the photoionization of CH 3 F were determined from photoelectron spectra recorded using monochromated synchrotron radiation at photon energies from 21 to 72 eV. The partial photoionization oscillator strengths for production of the electronic states of CH 3 F + derived from the PES electronic ion state branching ratios and the present low resolution photoabsorption measurements are compared with estimates derived from the dipole induced breakdown scheme and the molecular and dissociative photoionization partial oscillator strengths. The two estimates of electronic state partial photoionization oscillator strengths are in good agreement only at higher photon energies. The differences at lower photon energies arise from errors in the photoelectron branching ratios due to the difficulties of assessing the steeply rising non-spectral background in the photoelectron spectra, particularly in the (1a 1 ) −1 many-body region. Suggestions are made for improving the quantitative accuracy of PES measurements.

Quantitative studies of the photoabsorption (4.5–488 eV) and photoionization (9–59.5 eV) of methyl iodide using dipole electron impact techniques

Abstract Absolute oscillator strengths (cross sections) for the valence shell, I 4d, 4p, 4s and C 1s inner shell photoabsorption of CH 3 I have been measured using low resolution (1 eV fwhm) dipole (e,e) spectroscopy in the equivalent photon energy range from 4.5 to 488 eV. In addition, high resolution (0.05–0.1 eV fwhm) dipole (e,e) spectroscopy has been used to study the absolute valence shell and I 4d inner shell photoabsorption spectra of CH 3 I from 4 to 65 eV in more detail. The photoionization efficiency, ionic photofragmentation branching ratios and absolute partial oscillator strengths for molecular and dissociative photoionization have been determined for CH 3 I by dipole (e,e+ion) spectroscopy from the first ionization threshold to 59.5 eV. The major pathways of the ionic dipole induced breakdown scheme have been predicted from these results. Estimates of the absolute electronic state partial oscillator strengths of CH 3 I + from photoionization of CH 3 I were derived from the dipole induced breakdown scheme and the molecular and dissociative photoionization partial oscillator strengths.

Absolute photoabsorption and photoionization studies of methyl bromide using dipole electron impact and synchrotron radiation PES techniques

Abstract Absolute oscillator strengths (cross sections) have been measured for the photoabsorption of CH3Br at low resolution (1 eV fwhm) in the equivalent photon energy range from 6 to 450 eV and at high resolution (0.05 eV fwhm) from 5 to 100 eV using dipole (e,e) spectroscopy. In the valence shell and Br 3d inner shell regions of CH3Br, from the first ionization threshold to 79 eV, the photoionization efficiency, ionic photofragmentation branching ratios and absolute partial oscillator strengths for molecular and dissociative photoionization have been determined by dipole (e,e+ion) spectroscopy. The major ionic pathways of the dipole induced breakdown scheme of CH3Br have been predicted from these results and the partial photoionization oscillator strengths for production of the valence shell electronic states of CH3Br+ have been estimated. Photoelectron branching ratios for CH3Br have been measured using monochromated synchrotron radiation at photon energies from 21 to 68 eV. These have been combined with the dipole (e,e) photoabsorption data to obtain partial photoionization oscillator strengths for production of the electronic states of CH3Br+, which have been compared with the partial oscillator strength estimates derived from the (e,e+ion) data.

The complete valence shell photoelectron spectra of CH3F, CH3Cl, and CH3Br using synchrotron radiation at hv = 72 eV: a comparison with many-body Greens function calculations☆

Abstract The valence shell binding energy spectra of CH 3 F, CH 3 Cl, and CH 3 Br have been measured in both the outer and inner valence regions using monochromated synchrotron radiation at a photon energy of 72 eV. The spectrum of CH 3 Br was also recorded at photon energies of 68 and 70 eV in order to avoid the resonance Auger processes from Br3d excitation which occur in addition to direct ionization in CH 3 Br at 72 eV. In the inner valence region the spectra exhibit several peaks and broad structures indicating a breakdown of the independent particle picture due to the role of electron correlation effects in the photoionization process. The spectra are compared with previously published many-body Greens function calculations, and with binding energy spectra from electron momentum spectroscopy, in order to assign the photoelectron bands and pole strength distributions in the inner valence regions. The photoelectron spectrum of CH 3 Br at 72 eV shows clear evidence of resonance Auger processes in which the Br3d → σ* excited electron acts as a spectator in the electronic decay. Evidence for participator autoionization processes is also observed. The spectrum also shows weak but sharp peaks, some of which are at energies corresponding to autoionization of 3d excited Br atoms formed from the rapid dissociation of excited CH 3 Br in accord with the earlier suggestions of Nenner et al. (J. Electron Spectrosc. Relat. Phenom., 52 (1990) 623).