  Metastable pionic helium ($\pi{\rm He}^+$) is a three-body atom composed of a
helium nucleus, an electron occupying the $1s$ ground state, and a negatively
charged pion $\pi^-$ in a Rydberg state with principal- and orbital angular
momentum quantum numbers of $n\sim \ell+1\sim 16$. We calculate the
spin-independent energies of the $\pi{\rm ^3He}^+$ and $\pi{\rm ^4He}^+$
isotopes in the region $n=15$--19. These include relativistic and quantum
electrodynamics corrections of orders $R_{\infty}\alpha^2$ and
$R_{\infty}\alpha^3$ in atomic units, where $R_{\infty}$ and $\alpha$ denote
the Rydberg and fine structure constants. The fine-structure splitting due to
the coupling between the electron spin and the orbital angular momentum of the
$\pi^-$, and the radiative and Auger decay rates of the states are also
calculated. Some states $(n,\ell)=(16,15)$ and $(17,16)$ retain
nanosecond-scale lifetimes against $\pi^-$ absorption into the helium nucleus.
We propose to use laser pulses to induce $\pi^-$ transitions from these
metastable states, to states with large ($\sim 10^{11}$ s$^{-1}$) Auger rates.
The $\pi{\rm He}^{2+}$ ion that remains after Auger emission of the $1s$
electron undergoes Stark mixing with the $s$, $p$, and $d$ states during
collisions with the helium atoms in the experimental target. This leads to
immediate nuclear absorption of the $\pi^-$. The resonance condition between
the laser beam and the atom is thus revealed as a sharp spike in the rates of
neutrons, protons, deuterons, and tritons that emerge....(continued)
