The pressure dependence of the metastable-state lifetimes in antiprotonic ${}^3\mathrm{He}$ atoms $\overline{p}{-}^3{\mathrm{He}}^{+}$ was studied using the recently observed laser resonance transition $(n,\mathcal{l})=(36,33)\mathrm{}\to \mathrm{}(35,32)\mathrm{}$ in the $\upsilon =n-\mathcal{l}-1=2$ cascade. To this end antiprotons from the Low Energy Antiproton Ring (LEAR) at CERN were extracted in approximately 200-ns-long bunches with about ${10}^8\hspace{0.5em}\overline{p}$ per bunch and stopped in a ${}^3\mathrm{He}$ gas target. Time spectra of delayed annihilation products were taken with the help of a Čerenkov counter with a gated photomultiplier. Under our experimental conditions, $p=136\mathrm{}-690\mathrm{mbars}$ and $T=5.8\mathrm{}\mathrm{K},$ the lowest metastable level $\mathrm{}(36,33)\mathrm{}$ was found to be much shorter lived than the corresponding state (37,34) in the $\upsilon =2\mathrm{}$ cascade of $\overline{p}{-}^4{\mathrm{He}}^{+}.$ It was strongly quenched at pressures above 500 mbars, whereas the higher states in the cascade remained nearly unaffected. This enabled us to investigate the $\mathrm{}(37,34)\mathrm{}\to \mathrm{}(36,33)\mathrm{}$ transition by resonant laser deexcitation. Its wavelength was found to be $\lambda =524.155\mathrm{}\pm 0.004\mathrm{nm},$ only 6 ppm lower than the theoretical value given by Korobov and Bakalov [Phys. Rev. Lett. 79, 3379 1997)] after including relativistic corrections.

• Received 17 June 1998

DOI:https://doi.org/10.1103/PhysRevA.58.3604