Gordon R. Burton

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.

The electronic absorption spectrum of NH3 in the valence shell discrete and continuum regions. Absolute oscillator strengths for photoabsorption (5–200 eV)

Abstract The electronic spectrum and the absolute photoabsorption oscillator strengths (cross sections) for the valence shell of NH 3 have been measured using high- (0.048 eV fwhm) and low-resolution (≈ 1 eV fwhm) dipole (e, e) spectroscopy in the photon energy ranges 5.0–31 and 5.5–200 eV, respectively. The high-resolution data have been obtained at vibrational resolution and have been used to determine the integrated oscillator strengths of several vibronic transitions in the discrete absorption region below the first ionization potential. The oscillator strength data are compared with previously published absolute measurements obtained using direct optical techniques and electron impact spectroscopy. Criteria are discussed for estimating the corrections for Pauli-excluded transitions required for the normalization of relative photoabsorption spectra when using the partial Thomas-Reiche-Kuhn sum rule.

Absolute oscillator strengths for the photoabsorption, photoionization and ionic photofragmentation of silicon tetrafluoride. I. The valence shell

Abstract Absolute photoabsorption oscillator strengths (cross sections) for the valence shell of silicon tetrafluoride have been measured using dipole (e, e) spectroscopy in the equivalent photon energy range 10–100 eV at an energy resolution of ≈ 1 eV fwhm. A high-resolution (0.048 eV fwhm) photoabsorption oscillator strength spectrum of silicon tetrafluoride has also been determined using a high-resolution dipole (e, e) spectrometer in the equivalent photon energy range 10–50 eV. Absolute oscillator strengths for the discrete features in the pre-ionizatin edge region of the high resolution spectrum have been obtained and their spectral assignments are discussed. Photoionization time-of-flight mass spectra have been collected using dipole (e, e + ion) coincidence spectroscopy from the first ionization potential up to 100 eV. Photoion branching ratios and photoionization efficiencies have been determined from the TOF mass spectra, and these have been used along with the measured absolute photoabsorption oscillator strengths to obtain the absolute partial photoionization oscillator strengths for production of the molecular and dissociative fragment ions. The ionic photofragmentation branching ratios differ substantially from previously published results (Lablanquie et al., J. Chem. Phys. 90 (1989) 7078; Imamura et al., J. Chem. Phys. 94 (1991) 4936). Absolute electronic state partial photoionization oscillator strengths have also been derived using the measured absolute photoabsorption oscillator strengths and photoionization efficiencies along with photoelectron branching ratios for the electronic states of silicon tetrafluoride reported in an earlier PES study (Yates et al., J. Chem. Phys. 83 (1985) 4906). The results are compared with MS-Xα calculations. The dipole induced breakdown for silicon tetrafluoride is also discussed.

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

Abstract Absolute photoabsorption oscillator strengths (cross sections) for the discrete electronic transitions of acetylene from 5.5 to 11.2 eV have been measured using high resolution ≈0.05 eV FWHM) dipole (e,e) spectroscopy. This experimental resolution enabled absolute oscillator strengths for most of the individual vibronic transitions to be obtained. A broader range spectrum, obtained from 5.5 to 200 eV at a resolution of ≈1 eV FWHM, was used to establish to absolute oscillator strength scale via TRK sum-rule normalization. The absolute oscillator strengths for the Rydberg transitions are in many cases significantly higher than previously reported values obtained using direct optical photoabsorption methods.

Quantitative spectroscopic studies of the valence-shell electronic excitation of freons (CFCl3, CF2Cl2, CF3Cl, and CF4) in the VUV and soft X-ray regions

Abstract Absolute photoabsorption spectra of the single carbon Freon molecules, CFCl 3 , CF 2 Cl 2 , CF 3 Cl, and CF 4 , have been measured from 5 to 60 eV at 0.05 eV resolution by high-resolution dipole ( e , e ) spectroscopy. Long-range valence-shell data (5–200 eV) obtained at lower resolution (1 eV fwhm) have been used to determine the absolute oscillator strength scales via valence-shell Thomas-Reiche-Kuhn (VTRK) sum rule normalization. Comparisons have been made with previously reported absolute photoabsorption data obtained by optical and electron impact based measurements where such data are available. The static electric-dipole polarizabilities of CFCl 3 , CF 2 Cl 2 , CF 3 Cl and CF 4 derived from the S (−2) sum-rule using the present photoabsortion oscillator strength (cross-section) data are found to be in excellent agreement (within 1–2%) with experimentally measured polarizability values in the literature.

Absolute oscillator strengths for the photoabsorption of silane in the valence and Si 2p and 2s regions (7.5–350 eV)

Abstract Absolute photoabsorption oscillator strengths (cross sections) for the valence and Si 2p discrete regions of silane have been measured using high resolution (∼ 0.05–0.1 eV fwhm) dipole (e, e) spectroscopy. Long range (7.5–350 eV) lower resolution data (1 eV fwhm) have also been obtained, from which the absolute oscillator strength scale has been determined using TRK (i.e. S (0)) sum-rule normalization. The accuracy of the presently reported measurements has been tested using the S (−2) sum-rule: Evaluation of the S (−2) sum using the presently reported data gives a dipole polarizability for silane within 0.2% of the experimental value. The present valence shell measurements were undertaken in order to investigate the differences between earlier low resolution dipole (e, e) work (Cooper et al., Chem. Phys. 140 (1990) 133) and subsequent valence shell synchrotron radiation photoabsorption measurements reported by Kameta et al. (J. Chem. Phys. 95 (1991) 1456). The presently reported high and low resolution absolute phtoabsorption oscillator strengths are much more consistent with the direct photoabsorption measurements than the earlier dipole (e, e) work (within ∼ 15%). The resolution of the present high resolution data is sufficient to allow absolute photoabsorption oscillator strengths to be determined for many of the individual electronic transitions in the Si 2p discrete spectrum.

Absolute oscillator strengths for photoabsorption (6–360 eV) and ionic photofragmentation (10–80 eV) of methanol

Abstract Absolute photoabsorption oscillator strengths (cross sections) for the valence and carbon K shells of CH 3 OH have been measured using low resolution (1 eV fwhm) dipole (e, e) spectroscopy in the equivalent photon energy range 6–360 eV. The present results are compared with previously published optical measurements obtained using conventional light sources in those limited energy regions where such data are available. In addition, high resolution (0.048 eV fwhm) dipole (e, e) spectroscopy has been used to study the valence shell photoabsorption region in more detail from 5–30 eV. The photoionization efficiency, ionic photofragmentation branching ratios and absolute partial oscillator strengths for molecular and dissociative photoionization have been determined for CH 3 OH by dipole (e, e+ion) spectroscopy from the first ionization threshold to 80 eV. A consideration of the photoabsorption and photoionization data together with thermochemical data and results from previously published coincidence experiments provides information on the dipole induced breakdown pathways of CH 3 OH in the photon energy range below 80 eV.

Absolute photoabsorption oscillator strengths by electron energy loss methods: the valence and S 2p and 2s inner shells of sulphur dioxide in the discrete and continuum regions (3.5–260 eV)

Abstract Absolute photoabsorption oscillator strengths (cross-sections) for the valence shell discrete and continuum regions of sulphur dioxide from 3.5 to 51 eV have been measured using high resolution (∼0.05 eV FWHM) dipole ( e , e ) spectroscopy. A wide-range spectrum, covering both the valence shell and the S 2p and 2s inner shells, has also been obtained from 5 to 260 eV at low resolution (∼1 eV FWHM), and this has been used to determine the absolute oscillator strength scale using valence shell TRK (i.e., S(0)) sum-rule normalization. The present measurements have been undertaken in order to investigate the recently discovered significant quantitative errors in our previously published low resolution dipole ( e , e ) work on sulphur dioxide (Cooper et al., Chem. Phys. 150 (1991) 237; 150 (1991) 251). These earlier measurements were also in poor agreement with other previously published direct photoabsorption measurements. We now report new absolute photoabsorption oscillator strengths using both high and low resolution dipole ( e , e ) spectroscopies. These new measurements cover a wider energy range and are much more consistent with the previously published direct photoabsorption measurements. The accuracy of our new measurements is confirmed by an S(−2) dipole sum-rule analysis which gives a static dipole polarizability for sulphur dioxide in excellent agreement (within 3.5%) with previously reported polarizability values. Other dipole sums S( u ) ( u =−1,−3 to −6,−8,−10) and logarithmic dipole sums L( u ) ( u =−1 to −6) are also determined from the presently reported absolute oscillator strength distributions.

Determination of ion multiplier sensitivities as a function of ionic mass for use in quantitative mass spectrometry

An experimental method has been developed for the determination of ion multiplier sensitivity to ions of different mass to charge ratio using the electron‐ion coincidence technique of dipole (e,e+ion) spectroscopy. Time‐of‐flight mass spectra of each of the noble gas atoms He, Ne, Ar, Kr, and Xe are collected using the ion multiplier under investigation. In the present work we have determined relative efficiencies, at 7.5–8 keV ion impact energy, over the mass range 1–140 amu for two different large active area multipliers, a Johnston focused mesh multiplier and a microchannel‐plate detector, both operated in the saturated pulse counting mode. Significant variations in detector sensitivity occur at low m/e values for both multipliers. The microchannel‐plate detector shows a level response function above m/e=∼30, while the Johnston multiplier exhibits a slow decrease in sensitivity in the same mass range. The applicability of the measured response functions to the correction of molecular mass spectra and e...

Absolute oscillator strengths for the photoabsorption, photoionization and ionic photofragmentation of silicon tetrafluride. II. The Si 2p and 2s inner shells

Abstract Using dipole (e, e) spectroscopy, absolute photoabsorption oscillator strengths (cross sections) for the Si 2p and 2s inner shells of silicon tetrafluoride have been obtained in the equivalent photon energy range 100–350 eV. The contribution of the valence shell to the total photoabsorption oscillator strength in the Si 2p and 2s inner shell regions has been estimated. Comparison of the present TRK sum-rule normalized photoabsorption data for SiF 4 with summed (Si+4F) atomic cross sections shows that normalization procedures based on atomic data are likely to be seriously in error in molecules where molecular (resonance) effects are strong. The high-resolution photoabsorption oscillator strength spectrum of silicon tetrafluoride has also been measured using a high-resolution dipole (e, e) spectrometer in the equivalent photon energy range 105–170 eV. Absolute oscillator strengths for the discrete transitions in the Si 2p pre-ede region have been obtained. Using dipole (e, e+ion) coincidence spectroscopy, photoionization time-of-flight mass spectra have been measured at ten selected energies corresponding to the valence ionization continuum, discrete transitions from the Si 2p orbitals to virtual valence and Rydberg orbitals, and at resonant and non-resonant energies in the Si 2p continuum. Absolute photoionization oscillator strengths for production of the molecular and dissociative fragment ions of silicon tetrafluride have been obtained at these energies using the the total absolute photoabsorption oscillator strengths, the photoionization efficiency and the ionic photofragmentation branching ratios obtained from the mass spectra. The ionic photogragmentation branching ratios differ substantialy from previously published results (Lablanqui et al., J. Chem. Phys. 90 (1989) 7078; Imamura et al., J. Chem. Phys. 94 (1991) 4936).