Collisional shift and broadening of the transition lines in pionic helium

Abstract

We calculate the density shift and broadening of selected dipole transition lines of pionic helium in gaseous helium at low temperatures up to T=12 K and pressure up to a few bar. In the approximation of binary collisions the shift and broadening depend linearly on the density; we evaluate the slope of this linear dependence for a few spectral lines of known experimental interest, and also investigate its temperature dependence. We find a blue shift of the resonance frequencies of the $(n,l)=(16,15) \rightarrow (16,14)$, $(17,16) \rightarrow (17,15)$, and $(16,15)\rightarrow(17,14)$ unfavored transitions, and a red shift for the favored one $(17,16) \rightarrow (16,15)$. The results are intended to significantly increase the efficiency of the laser spectroscopy investigations of pionic helium and help the interpretation of the experimental data.