C.E. Brion

K-shell excitation spectra of CO, N2 and O2

Electron energy loss spectra of CO, N2 and O2 have been recorded in the regions of carbon, nitrogen and oxygen K-shell excitation and ionisation. These results are compared to previous energy loss, photoabsorption and theoretical studies of the same spectral regions. Several inconsistencies in the published spectra are clarified in the present work. Comparisons with recent calculations of the K-shell continua of these molecules are presented. Vibrational structure in the K → π * transitions of CO (C 1s) and N2 (N 1s) has been resolved in high-resolution studies (< 0.1 eV FWHM) of these species.

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).

Absolute oscillator strengths (10–60 eV) for the photoabsorption, photoionisation and fragmentation of H20s

Abstract Dipole oscillator strengths (cross sections) for the photoabsorption, partial and total photoionisation and photofragmentation of H 2 O have been obtained, over a range up to an equivalent photon energy of 60 eV, using electron-impact coincidence simulation techniques. The results are quantitatively equivalent to those that would be obtainable by photoelectron spectroscopy and photoionisation mass spectrometry with continuously tuneable light sources. The photoionisation efficiency is also reported. The results of the two experiments are combined to provide a quantitative picture of the dipole induced breakdown of H 2 O in the region up to 60 eV.

Carbon K-shell excitation of C2H2, C2H4, C2H6 and C6H6 by 2.5 keV electron impact

Abstract Energy loss spectra of 2.5 keV electrons in the region of the carbon K -edge in C 2 H 2 , C 2 H 4 , C 2 H 6 and C 6 H 6 are report

Electron momentum spectroscopy of the valence orbitals of H2O and D2O: Quantitative comparisons using Hartree—Fock limit and correlated wavefunctions

Abstract The large discrepancies found earlier between experimental measurements and calculations based on near Hartree—Fock wavefunctions for the valence orbital electron momentum distributions of H 2 O are reinvestigated. New and improved electron momentum spectroscopy measurements for the valence orbitals of H 2 O and D 2 O, together with existing experimental data, have been placed on a common intensity scale using the binding energy spectra. Investigation of possible vibrational effects by means of new measurements of the momentum distributions of D 2 O indicates no detectable differences with the H 2 O results, within experimental error. A quantitative comparison of these experimental results with both the shapes and magnitudes of momentum distributions calculated in the PWIA and THFA approximations using new, very precise Hartree—Fock (single-configuration) wavefunctions is made. These wavefunctions, which include considerable polarization and which are effectively converged at the HF limit for total energy, dipole moment and momentum distribution permit establishment of basis set independence. The significant discrepancies between theory and experiment which still remain for the momentum distributions of the 1b 1 , 3a 1 and 2a 1 orbitals at the THFA level are largely removed by CI calculations of the full ion—neutral overlap amplitude. These CI wavefunctions for the final ion and neutral ground states, generated from the accurate HF limit basis sets, recover up to 88% of the correlation energy. The present work clearly shows the need for adequate consideration of electron correlation effects in describing the low-momentum parts of the 1b 1 , 3a 1 and 2a 1 electron distributions, a region which is of crucial importance in problems related to chemical bonding and reactivity. The high level of quantitative agreement obtained between experiment and calculations using sufficiently sophisticated wavefunctions provides support for the essential validity of the plane wave impulse approximation as used in the interpretation of EMS experiments on small molecules.

Reference energies for inner shell electron energy-loss spectroscopy

Abstract The energies of a number of selected inner shell atomic and molecular electronic transitions in the energy range 100–1000eV have been carefully remeasured for energy scale calibration in electron energy loss Spectroscopy. The measurements have been made using an inner shell electron energy loss spectrometer in conjunction with a digital voltmeter of high accuracy.

Absolute optical oscillator strengths for discrete and continuum photoabsorption of molecular nitrogen (11–200 eV)

Abstract Absolute optical oscillator strengths for the photoabsorption of molecular nitrogen in the energy region 11–200 eV have been determined using low resolution (1 eV fwhm) dipole (e, e) spectroscopy. The absolute scale was obtained by TRK sum-rule normalization. High resolution (0.048 eV fwhm) dipole (e, e) spectroscopy has been used to measure the absolute optical oscillator strengths for the excitation of molecular nitrogen in the valence shell discrete region. The measurements are free of the “line saturation” (bandwidth) effects which complicate direct photoabsorption studies using the Beer-Lambert law. Absolute optical oscillator strengths for excitation to individual vibronic bands of the b 1 Π u and b′ 1 Σ + u valence excited states, the c 1 Π u , c′ 1 Σ + u and o 1 Π u Rydberg states and also the e 1 Π u and e′ 1 Σ + u states have been obtained. The present results are compared with previously published experimental data and theoretical calculations.

Experimental and theoretical investigation of the complete valence shell ionization spectra of CO2and N2O

Abstract The valence electron ionization spectra of CO 2 , and N 2 O are studied by dipole (e—2e) spectroscopy and 2ph-TDA many-body Green function calculations. Intense satellite structure in the (e—2e) spectra between ≈ 20 eV and ≈ 30 eV binding energy is assigned with the help of the calculations. While for CO 2 , only satellite lines of 2 Π u symmetry appear with significant intensity, intense satellite lines of both 2 Π and 2 γ symmetry are found for N 2 O in the 20–30 eV energy range. The theory predicts a complete breakdown of the molecular orbital picture of ionization to occur for the two innermost valence electrons of CO 2 and N 2 O. The inner valence part of the ionization spectra of CO 2 and N 2 0 is found to be considerably more complex than has hitherto been assumed. The experimental spectra confirm the main features of the theoretical results.

K-shell excitation of CH4, NH3, H2O, CH3OH, CH3OCH3 and CH3NH2 by 2.5 keV electron impact

Abstract The regions around the respective carbon, nitrogen and oxygen K-edges of CH4, NH3, H2O, CH3OH, CH3OCH3 and CH3NH2 have been investigated by electron energy loss spectroscopy using a beam of 2.5 keV electrons. All spectra show a number of discrete peaks just below the K-shell ionization threshold. These discrete structures have been interpreted as being associated with the promotion of a K-shell electron to Rydberg orbitals which converge to the K-shell ionization threshold.

Partial oscillator strengths for the photoionization of N2O and CO2 (20–60 eV)

Abstract The photoelectron branching ratios and the partial oscillator strengths (cross sections) for photoionisation of the valence orbitals of N2O and CO2 have been obtained in the energy range 20–60 eV using the magic angle dipole (e, 2e) method. In addition to single electron ionization processes there is a large contribution from multiple electron transitions at higher energies in agreement with recent theoretical predictions. The photoionization efficiency and the dipole oscillator strenght for total photoabsorption have also been measured.