Interaction between antiprotonic helium ion and He atom: Potential Energy Surface

TL;DR

This paper calculates the potential energy surface for the antiprotonic helium ion interacting with a helium atom using advanced quantum chemistry methods, providing more accurate data for collision processes.

Contribution

The study introduces a detailed potential energy surface for the antiprotonic helium ion-He atom system, improving upon previous models with quantum chemical calculations and multipole expansion.

Findings

Total cross sections for Stark transitions are 15-20% higher than previous model predictions at low energies.

The potential energy surface includes detailed multipole components and compares well with earlier models.

Results enhance understanding of collisional processes involving antiprotonic helium.

Abstract

Potential Energy Surface for the ($\mathrm{\bar{p}} - \mathrm{He}^{2+} - \mathrm{He}$) system is calculated in the framework of the restricted (singlet spin state) Hartree-Fock method with subsequent account of the electronic correlations within the second order perturbation method (MP2). The geometry of heavy particles is described in variables of distance $r$ from nucleus $a$ to antiproton, distance $R$ from the center of mass of the $(\bar{p} - a)$ pair to $\mathrm{He}$ atom containing nucleus $b$, and an angle $\theta$ between $\mathbf{r}$ and $\mathbf{R}$. The potential $V(R,r,\cos\theta)$ of the interaction between $\mathrm{He}$ atom and a $\bar{p} - a$ subsystem involves a total energy of two electrons in the field of three heavy particles and Coulomb interactions of the $b$ nucleus with antiproton and the $a$ nucleus. The expansion of this potential in terms of Legendre polynomials $\mathrm{P}_k(\cos\theta)$ is obtained. Matrices of the multipole terms $V^k(r,R)$ ($k=0,1,2$) are obtained in the basis of antiprotonic helium ion states. The results are compared with the model potential that was used earlier in the calculations of collisional Stark transitions of $(\mathrm{\bar{p}He}^{2+})$ ion.Total cross sections of collisional Stark transitions obtained with the PES potentials exceed the model results by 15 - 20\% at $E \leq 12$ K.