Valence Shell Ionization Potentials Research Articles

Theoretical calculation of valence shell ionization potentials of XF3(X=N, P, As) using the equation-of-motion coupled cluster method

The equation-of-motion coupled cluster method with full inclusion of singles and doubles (EOM-CCSD) is applied to evaluate the vertical ionization potential (VIP) of XF3(X=N,P,As). At the same time, symmetry adapted cluster configuration interaction (SAC-CI), the outer valence shell Green function (OVGF) and partial third order approximation (P3) are also employed. Comparison with results of some experiments show average errors of 0.2 eV in the whole valence shell ionization region and are better than OVGF and P3 values and similar to the SAC-CI values. With the increase of basis set, the difference of the first VIP value between the EOM-CCSD and experimental becomes smaller and the different is about 0.03 eV. According to results from SAC-CI, OVGF, P3 and EOM-CCSD of NF3 and PF3, we can conclude that the first IP of AsF3 is not 12.3 eV but about 12.8 eV, the valence shell ionization potentials are 12.64,15.23,16.30,17.37,18.05 and 21.98 eV.

Valence shell photoionization energies and cross-sections of NF 3 and PF 3

Relative outer valence shell ionization potentials and cross-sections were determined for the isostructural, Group 15, trifluorides NF 3 and PF 3 in the gas phase using synchrotron radiation. Excitation photon energies ranged from 70 to 160 eV. The experimental spectra were assigned and cross-sections analyzed with the aid of both MS-Xα and ab initio calculations. Spectral differences in peak energies and relative intensities are related to structural and electronic differences between these two fluoride molecules. Valence shell ionization potentials were compared to calculated values obtained by several different methods. The partial photoionization cross-sections for each orbital were obtained as a function of excitation energy and compared to theoretical results obtained with the Xα method.

2h-1p Correlation effects and virtual model spaces for the description of photoelectron spectra of complex molecules

2h-1p CI calculations of the complete valence shell ionization potentials and spectral intensities have been performed for acrolein, propynal, s-tetrazine, pyrazine, bis (π-allyl)nickel and nickel chloride, with a DZ basis set. The reduction of the virtual space to the valence or the π space by means of a projection scheme has been tested and found very satisfactory. Because of the strong reduction of the computational effort, the truncated schemes appear to be promising for the extension of ab-initio treatments to larger systems. A complete reproduction of the strong manybody effects present in these systems is observed, in good agreement with the experimental data and the results of previous 2ph-TDA calculations for all molecules except bis (π-allyl) nickel. A new correlation effect appears to be present in the latter molecule, which causes disagreement with the 2ph-TDA results and an unsatisfactory reproduction of the experimental spectrum.

Semiempirical calculations of molecular valence-shell ionization potentials by linear response theory based on coupled clusters

The recently proposed linear response theory based on coupled clusters (CCLRT) has been applied within the framework of different semiempirical approximations for the calculation of both inner and outer valence ionization potentials of a number of molecules. CCLRT with CNDO/S parametrization leads to the best results. The breakdown of Koopmans' theorem is generally observed in the inner valence region. The shake-up structure arising due to configuration mixing remains more or less unchanged in different parametrizations. The approximate ionization strength and also the important shakeup configurations have been reported.

XαMT-SW Calculation of X-Ray Excitation, Ionization, and Auger Electron Energies in SiH4, SiF4 and SiCl4

The multiple scattering XαMT model has been applied to the molecules SiH4, SiF4 and SiCl4 to calculate core and valence shell ionization potentials, Auger electron energies, Kα transition energies and X-ray excitation energies. The transition state calculations of valence electron binding energies show satisfactory agreement with measured values. The calculated shifts in core electron binding energies and Auger electron energies indicate, that theoretical values are slightly higher than expected. Experimental and calculated transition state 2p excitation energies are in good agreement. From an analysis of charge fractions of the spheres assignments of Rydberg and valence-type terminating orbitals are given.

The calculation of valence shell ionization potentials by the ΔSCF method

Valence shell ionization potentials for several small molecules (HF, H2O, CH4, CO, H2S, PH3, SiH4, SO2 and F2CO) are calculated by means of ab initio ΔSCF calculations on all the valence hole states of each molecule. The results are compared with the experimental low energy photoelectron spectra. In the case of the higher energy ion states, particular attention is paid to finding practical means for obtaining convergence of the ΔSCF iterations, and numerical evidence is presented, indicating that the ΔSCF procedure provides upper bounds to the energy of such states, at least for the cases studied herein. A new procedure, the ‘ combined ΔSCF ’ method, for the determination of a common set of molecular orbitals for use in the construction of wavefunctions for a number of ion states is described.

Valence-Shell Ionization Potentials of Halomethanes by Photoelectron Spectroscopy. I CH3Cl, CH3Br, CH3I. Vibrational Frequencies and Vibronic Interaction in CH3Br+ and CH3Cl+

Photoelectron spectra of methyl chloride, methyl bromide, and methyl iodide obtained with 584-Å radiation yield in each case four of the five valence-shell ionization potentials. Values of 11.29, 10.53, and 9.50 eV are obtained for the first ionization potentials, respectively, corresponding to ionization from halogen “lone-pair” orbitals. Resolved vibrational structure allows determination of frequencies of the corresponding normal vibrations of the positive ions in several cases. Relative intensities of vibrational components in the first ionization potential are dictated by vibronic interaction. Approximate values of the Jahn–Teller parameters D for ν4, ν5, ν6 in CH3Br+ may be obtained from these intensities. Small values of D (D ≤ 0.1) suffice to produce sizeable modifications of intensity for the intermediate spin–orbit splittings observed in these ions.