Collisional quenching of the pionic helium 4He long-lived states

Abstract

Collisions of metastable pionic helium atoms with helium atoms of the medium lead to the destruction of these states, as well as to the shifts and broadening of E1 spectral lines of transitions between the pionic helium states. In the paper, in order to obtain the interaction potential matrix (π −He+)−He, calculations of the potential energy surface (PES) in the unrestricted Hartree-Fock method are performed taking into account electron correlations within the second-order perturbation theory (MP2). With this potential, the system of equations of strong channel coupling is solved numerically. Various techniques are used in the calculations that eliminate degeneracy of the solution matrix in the vicinity of small distances between colliding subsystems, which arises due to strong coupling of channels in this region, owing to which the numerical solutions “forget” boundary conditions. Cross sections and rates of collisional transitions are calculated (Nσv). It is found that the collisional transition rate (n, l) = (17, 16) → (17, 15) (n, l — the principal quantum number and the angular momentum respectively) for the density of the medium N = 0.2 × 1023 cm−3 is lower than 103s−1, which indicates that it is possible to ignore the effect of collisional destruction of pionic helium long-lived states in precision laser spectroscopic experiments.