ИСТИНА

Recently laser-induced transition in pionic helium was observed for the first time. A possibility of the further precision laser spectroscopy of this system depends, in particular, on the stability of the states against the quenching by collisions with the medium atoms. We consider Stark transitions between the highly excited states in the collisions \begin{equation} (\pi^- \mathrm{He}^+)^*_{nL} + \mathrm{He} \rightarrow (\pi^- \mathrm{He}^+)^*_{nL'} + \mathrm{He} \quad (L'\neq L), \end{equation} which can be expected as the most probable due to a small difference of the inner initial and final energies. The cross sections and transition rates are calculated by solving the close coupling equations involving the different $L$ at a fixed $n$. In order to obtain an interaction between the colliding systems we calculate the Potential Energy Surface (PES) of the three electrons in the field of three heavy particles (two $\alpha$-particles and $\pi^-$). Dependencies of the Stark transition cross sections on the quantum numbers of the states and on the initial kinetic energy will be presented in the talk.