Paul E. Cade

The electronic structure and positron annihilation characteristics of positronium halides, [X−; e+]. I. Hartree–Fock calculations and stability

The electronic structure of the atom/positron systems [F−;e+], [Cl−;e+], [Br−;e+], and [I−;e+] are examined by means of restricted Hartree–Fock calculations on the ...ns2np6(nl)+, 2L and...ns2np5 (n+1) sls+, 2P,4P states. A modified version of the numerical MCHF72 program of Froese–Fischer was used in these calculations. Our theoretical investigations suggest that...ns2np61s+, 2S is the ground state of the system. It is shown that X− ions have a substantial positron affinity (e.g. ∼5 eV for F− to 3.3 eV for I−) and arguments based on pair‐correlation and many‐electron perturbation theory are used to obtain hard lower limit estimates of the binding energy of Ps to the X atom. We thus find that the ground state of [X−:e+] is stable with respect to dissociation into X and Ps with binding energies of at least 1.66, 0.80, 0.19, and −0.45 eV for [F−:e+], [Cl−:e+], [Br−:e+], and [I−:e+], respectively. The details of the positronic orbitals are presented and discussed.

The electronic structure and positron annihilation characteristics of positronium halides [X−;e+]. II. Two‐photon annihilation

The basic characteristics of two‐photon positron annihilation from the ground‐ and first‐excited states of [F−;e+], [Cl−;e+], [Br−;e+], and [I−;e+] are calculated. The properties of interest are the annihilation rate Γ, the orbital components to the annihilation rate Γnl, and the angular correlation N (ϑ) for two‐photon annihilation. The two states considered are the ...ns2np61s+, 2S and the ...ns2np62p+, 2P states of [X−;e+] and all computations presented are based on the Restricted Hartree–Fock wavefunctions for the [X−;e+] states published elsewhere. These results are compared with experiment from various sources and an assessment of the potential role of the [X−;e+] 2S and 2P states is attempted. These results serve to limit the speculation on the role of [X−;e+] bound‐state positron systems in the two‐gamma annihilation from alkali halide crystals. The calculated lifetimes for the 2S states are 1.7, 4.0, 5.0, and 6.8 nsec for [F−;e+], [Cl−;e+], [Br−;e+], and [I−;e+], respectively. These RHF results o...

Compton Scattering of X Rays from Ne, N2, and O2: A Comparison of Theory and Experiment

Experimental and theoretical Compton profiles are presented for gaseous Ne, N2, and O2. A comparison between experiment and theory indicates the ability of the Compton scattering technique to accurately probe many‐electron systems if impulse approximation corrections are made. The need to achieve Hartree— Fock accuracy in molecular wavefunctions is emphasized.

The electronic/positronic structure of positron/pseudohalide systems: [OH−; e+], [SH−; e+], [CN−; e+], and [N−3; e+]

A theoretical examination of the [OH−;e+], [SH−;e+], [CN−;e+], and [N−3;e+] at the approximate Hartree–Fock level is presented. Large Gaussian basis sets are used to represent both the electronic and positronic orbitals in conventional Hartree–Fock–Roothaan theory. Quality studies are given for [F−;e+] and [OH−;e+] and some consideration of dependence with molecular geometry is considered. The basic energy results are presented and a cycle argument is followed to suggest that [CN−;e+] and [N−3;e+] are stable to Ps detachment while [OH−;e+] and [SH−;e+] are not stable. The corresponding angular correlation curves N(θ) are given and compared. Finally, interpretative aspects are presented and discussed in terms of positron localization and contrast to proton behavior.

Restricted Hartree–Fock wave functions for positron/anion [A−;e+] and positron/atom [A;e+] systems

Numerical restricted Hartree–Fock wave functions are reported for positron/anion systems [A−;e+] with A = Li, Be, B, C, N, O, and F for positrons in 1s+, 2s+, 2p+, and 3d+ orbitals. The nature of the positronic orbitals, the modification in the electronic orbitals, and implications are considered. Positron affinities of the A− systems and the stability of [A−;e+] relative to A+Ps are examined. The corresponding positron/atom systems [A;e+] are also considered in an iterative fashion. It is found that RHF calculations suggest that neutral atoms do not bind a positron.

The electronic structure of alkali bihalide molecules MX2. I. Ground states of LiF2, NaF2, LiCl2, and NaCl2 systems

The ground electronic states of the triangular alkali bihalide molecules LiFad2, NaFad2, LiClad2, and NaClad2 are investigated theoretically at the restricted−Hartree−Fock level using large Gaussian basis sets. It is argued that these MXad2 systems are stable species on the M+X+X potential energy surface and LiFad2, NaFad2, and NaClad2 are stable entities with respect to dissociation into the MX+X systems. Potential energy curves are obtained for the (X−−−X)− and M+−−−(x2)− motion in the MXad2 systems and comparison with experiment is given in terms of calculated spectroscopic properties. The vibrational assignments of Howard and Andrews for thee MX2 systems are supported by these calculations and a study is made of the influence of the M+ cation on the spectra of the constituent X2−(X 2Σu+) molecular anions.

Positron annihilation characteristics of positronium oxides, sulfides, etc. [X=e+]: Implications for oxide and sulfide crystals☆

Abstract Theoretical Hartree-Fock calculation are summarized for the [ X = e + ] systems with X = O, S, Se, and Te which establish the stability of bound states for the system. The two-photon annihilation lifetime, τ, and angular correlation, N (θ), are calculated and positron annihilation from alkaline earth oxides is discussed.

The bond additivity of Compton profiles and momentum densities for simple molecules. I. Compton profilesa)

The bond additivity of Compton profiles is theoretically explored for a sequence of linear polyatomic molecules. The Compton profiles of HCN, HNC, FCN, ClCN, NCCN, HCCCN; HCCH, FCCH, ClCCH, LiCCH; OCO, SCO, OCN−, SCN−, and NNO, using near Hartree–Fock wave functions of McLean and Yoshimine, forms the theoretical basis for this investigation. Two approaches are employed to examine bond additivity of Compton profiles. The first approach constructs the Compton profile of molecules in our repertory from others also in the list, checking for internal consistency and against the explicitly calculated result. The second approach defines bond Compton profiles and emphasizes internal consistency of bond profiles, JAB(q) calculated in various ways. The results are critically compared to previous theoretical and/or experimental bond Compton profiles.

Theoretical condiserations regarding the existence of PsO and PsS

Abstract At has been proposed from experimental studies and in analogy with hydrogen compounds that PsO may be an entity of some importance, or an intermediate, in the reaction of positronium, Ps, with aqueous oxyacid species such as H 2 PO − 4 , HSO − 4 , ClO − 4 , and NO − 3 . This communication explores the stability of PsO and PsS, or [0 − : e + ] and [S − :e + ], respectively, relative to dissociation into Ps and O( 3 P) or S( 3 P) on the basis of restricted Hartree-Fock calculations for the PsO and PsS systems and certain correlation correction arguments. We obtain a reasonable lower estimate of the dissociation energy to Y + Ps of ⪢ −0.47 eV for PsO and ⪢ −0.70 eV for PsS. It is suggested that a modest correlation correction to the positron affinity (PA) of O − would very probably lead to a bound state system for PsO.

The existence of [NO−2;e+] and [NO−3;e+] systems

Abstract The [NO − 2 ;e + ] and [NO − 3 e + ] systems are examined via Hartree-Fock-Roothaan calculations. The positron affinities obtained are 3.43 eV (NO − 2 ) and 3.19 eV (NO − 3 ). A “cycle” argument suggests that [NO − 3 ; e + ] is stable with respect to NO 3 + Ps, but [NO − e + ] appears unstable.