A.O. Bawagan

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.

An investigation of the valence orbitals of water by high momentum resolution electron momentum spectroscopy

Abstract The momentum distributions of the valence orbitals for water well as the binding energy spectra in the region 10–45 eV have been reinvestigated with a high momentum resolution (≈0.1 a 0 −1 fwhm) binary (e.2e) spectrometer. The binding energy spectra show considerable satellite structure in the region > 25 eV which is consistent with theoretical predictions of final state configuration interaction (many-body effects) involving the (2a 1 ) −1 hole state. An investigation of the momentum distribution in the satellite region confirms this assignment. This is in accord with a variety of recent theoretical studies and also consistent with earlier experiments. Differences suggested in earlier comparisons between theory and low momentum resolution experiments for the momentum distributions of the 1b 1 and 3a 1 orbitals have been verified. Several possible theoretical studies are suggested to investigate further this discrepancy between experiment and theory. Bonding effects and thenature of the molecular orbitals of H 2 O in momentum space are also discussed.

The valence orbitals of NH3 by electron momentum spectroscopy: Quantitative comparisons using Hartree-Fock limit and correlated wavefunctions

Abstract The binding energies and momentum distributions of the four valence orbitals of H 2 S have been measured by high-resolution electron momentum spectroscopy. The momentum distributions are compared on a quantitative basis with target Hartree-Fock approximation calculations using SCF wavefunctions to the Hartree-Fock limit. The measured momentum distributions are also compared with full ion-neutral overlap calculations carried out using correlated wavefunctions by the method of configuration interaction. Good agreement is found with theory at the Hartree-Fock limit and correlation is found to have minimal effect on the momentum distributions. A detailed experimental and theoretical study of the ionization of the inner valence region shows that the extensive many-body states observed arise predominantly from the 4a 1 −1 hole state.

Orbital imaging of the “lone pair” electrons in N(CH3)3, NH3and NF3 by electron momentum spectroscopy

Abstract The electron density in each of the outermost MOs of N(CH 3 ) 3 and NF 3 , as measured by electron momentum spectroscopy, is found to exhibit a very much higher degree of s character than the corresponding orbital in NH 3 . This behaviour is clearly predicted by MO calculations which indicate appreciable delocalization of electron density away from the nitrogen in N(CH 3 ) 3 and NF 3 . The observed results for N(CH 3 ) 3 are contrary to predictions based on commonly used intuitive arguments involving lone pairs, molecular geometry and hybridized orbitals. The present work is supportive of the view that CH 3 groups are intrinsically more electron attracting than H when bonded to N.

Studies of the electron density in the outermost orbitals of NH2CH3, NH(CH3)2, N(CH3)3 and NF3 by electron momentum spectroscopy: Evidence for charge delocalization

Abstract The electron density in each of the outermost molecular orbitals of NH2CH3, NH (CH3)2, N(CH3)3 and NF3, as probed by electron momentum spectroscopy, is found to exhibit a very much higher degree of s character than the corresponding orbital in NH3. Increasing methyl substitution in NH3 results in increasing amounts of s character in the experimental momentum profiles. This trend is qualitatively predicted by MO calculations which indicate appreciable delocalization of electron density away from the nitrogen in the methyl amines and NF3. The relevance of the present results to current understanding of the methyl inductive effect is discussed.

The valence orbital momentum distributions and binding energy spectra of H2CO: A comparison of electron momentum spectroscopy and quantum chemical calculations using near-Hartree-Fock quality and correlated wavefunctions

Abstract The high momentum resolution experimental momentum profiles (XMPs) of the valence orbitals of H2CO have been measured by electron momentum spectroscopy (EMS). Good quantitative agreement is obtained between the measured XMPs and the momentum distributions calculated from a near-Hartree-Fock wavefunction except for the outermost 2b2 orbital. A configuration interaction study suggests that the difference between theory and experiment observed for the outermost 2b2 orbital cannot be accounted for by inclusion of electron correlation and electronic relaxation effects. The measured 5a1 and 1b2 XMPs confirm earlier orbital assignments made by Hood et al. Extensive many-body structures are observed in the inner valence binding energy region and these are assigned predominantly to the (3a1)−1 process.

The valence orbital momentum distributions and binding energy spectra of dimethyl ether by electron momentum spectroscopy: An investigation of the methyl inductive effect

Abstract The high momentum resolution experimental momentum profiles of some of the valence orbitals of (CH 3 ) 2 O have been measured by electron momentum spectroscopy. Good agreement is obtained between the measurements and the momentum distributions calculated from relatively simple wavefunctions except in the case of the outermost 2b 1 orbital. The effects of diffuse and polarization functions in the basis sets and also the influence of molecular geometry have been investigated. The observed features of the binding energy spectra including the inner valence region are quite well reproduced by earlier published Green function calculations. Comparison of the XMPs of the outermost (HOMO and NHOMO) orbitals of H 2 O, CH 3 OH, and (CH 3 ) 2 O shows an increase in low momentum components with increasing methyl substitution. The results are discussed in relation to commonly held views of the methyl inductive effect.

A new method for incorporating momentum resolution effects and defining the momentum scale in electron momentum spectroscopy

Abstract Two existing methods for incorporating the effects of finite momentum resolution in comparing the results of electron momentum spectroscopy (EMS) experiments with quantum mechanical calculations of the electron momentum distributions are described. Neither of these existing methods is considered to be particularly satisfactory. However, a new procedure, the momentum averaged Gaussian weighted (MAGW) method, is found to be more satisfactory in the sense that it attempts to account for the acceptance angles and transmission characteristics of the ejected and scattered electrons in the EMS spectrometer in a physically more realistic manner than previous methods. A systematic error in the usual definition of the momentum scale in EMS experiments is also noted, and the MAGW method is used to correct for this error, which is appreciable at momenta below 0.4 au. Use of the new MAGW procedure, both for momentum resolution folding and defining the momentum scale, results in excellent agreement between Hartree-Fock limit theory and the measured momentum distribution for argon 3p electrons. In addition, an existing discrepancy between even the CI overlap treatment and experiment for the 1b1 orbital of H2O is effectively eliminated when the same MAGW method is used to fold the calculations and establish the momentum scale.

Orbital momentum distributions and binding energies for the complete valence shell of molecular chlorine by electron momentum spectroscopy

Abstract The complete valence shall binding energy spectrum (10–50 eV) of Cl 2 has been determined using electron momentum (binary (e,2e)) spectroscopy. The inner valence region, corresponding to 4σ u and 4σ g ionization, has been measured for the first time and shows extensive splitting of the ionization strength due to electron correlation effects. These measurements are compared with the results of many-body calculations using Green function and CI methods employing unpolarised as well as polarised wavefunctions. Momentum distributions, measured in both the outer and inner valence regions, are compared with calculations using a range of unpolarised and polarised wavefunctions. Computed orbital density maps in momentum and position space for oriented Cl 2 molecules are discussed in comparison with the measured and calculated spherically averaged momentum distributions.

Momentum distributions for the π-electrons of para-dichlorobenzene - an investigation of inductive and resonance effects by electron momentum spectroscopy

Abstract Significant differences have been observed in the orbital momentum distributions corresponding to the π 3 and π 2 levels of para-dichlorobenzene measured by electron momentum spectroscopy. In particular the less tightly bound π 3 orbital shows a momentum distribution shifted to higher momentum than does the π 2 orbital. These trends are predicted by quantum mechanical calculations using 4-31G basis functions and further understood by consideration of orbital amplitude and phase diagrams. Through the direct probing of the fine details of the electron distribution this work provides experimental confirmation of conventional arguments based on inductive and resonance effects.