SENSITIVE DETECTION OF DOPPLER-FREE 2-PHOTON-EXCITED 2S POSITRONIUM BY SPATIALLY SEPARATED PHOTOIONIZATION

Using a spatially filtered pulsed laser at 486 nm we have photoexcited positronium to the 2S state and detected it via a delayed and spatially separated 532-nm photoionization pulse. We have extended our observations down to low 486-nm power to explore the possibility of a cw measurements of the positronium 1S-2S interval. A model calculation taking into account the velocity distribution of the 235-K positronium from an oxygenated cold Al(111) target, the parameters of the laser beams, and the nonlinear excitation probability explains our data. The same model indicates that a cw measurement is presently feasible.