Producing long-lived 23S positronium via 33P laser excitation in magnetic and electric

Abstract

Producing positronium (Ps) in the metastable 2 3 S state is of interest for various applications in fundamental physics. We report here on an experiment in which Ps atoms are produced in this long-lived state by spontaneous radiative decay of Ps excited to the 3 3 P level manifold. The Ps cloud excitation is obtained with a UV laser pulse in an experimental vacuum chamber in presence of guiding magnetic field of 25 mT and an average electric field of 300 V cm −1 . The evidence of the 2 3 S state production is obtained to the 3.6σ level of statistical significance using a novel analysis technique of the single-shot positronium annihilation lifetime spectra. The dynamic of the Ps population on the involved levels has been studied with a rate equation model.

Efficient 2S3 positronium production by stimulated decay from the 3PAntonello1, Belov2, Bonomi3 et al. 2019Phys. Rev. ASelf Cite10070View full textAdd to dashboardCite

“…2 3 S Ps sources have been demonstrated via RF transition from laser-excited 2 3 P Ps in a weak magnetic field [14], via two-photon Doppler-free 1 3 S-2 3 S laser excitation [15,16] and more recently via single-photon excitation of 1 3 S to 2 3 P in a rapidly switching electric field [17] and via single-photon excitation of 1 3 S to 3 3 P with radiative decay to 2 3 S in an electric field [18] and in absence of electric field [19]. This last method in particular showed that it is possible to build an almost-monochromatic 2 3 S Ps source with a selected and tunable velocity distribution in the 10 4 m s −1 range, with an overall efficiency between 0.7 − 1.4% according to the selected velocity [19].…”

“…In the present work, following this experimental line, we investigate the possibility of stimulating the 3 3 P-2 3 S transition to increase the overall 2 3 S production efficiency. Indeed, laser-excited 3 3 P Ps can spontaneously decay radiatively to 2 3 S via the dipole-allowed 3 3 P-2 3 S transition (rate A 23 = 2π×1.1·10 7 s −1 ) with ∼10% measured branching efficiency [18,19], limited by the competition with the more efficient spontaneous decay channel 3 3 P-1 3 S (A 13 = 2π × 8.4 · 10 7 s −1 ). Increasing the 3 3 P-2 3 S transition rate, and thus the branching efficiency of the 2 3 S decay, is possible through stimulated emission.…”

“…A direct way to observe an enhancement in the 2 3 S signal due to the action of the stimulating laser is to compare the Ps annihilation time distribution with the UV laser only to that measured with both UV and IR pulses. The experimental methodology was the same used in previous works [18][19][20]. Bursts of 10 7 e + , 7 ns in time length [23], were guided by a 25 mT magnetic field, focused by an electric field of about 300 V cm −1 and implanted at 3.3 keV in a circular spot of ∼ 3 mm full-width at half maximum (FWHM) into a e + /Ps converter held at room temperature.…”

“…A fraction of the emitted cloud was subsequently conveyed to 2 3 S either by the 205 nm UV beam alone (i.e. through spontaneous decay from 3 3 P, as in [18,19]) or by a combination of the UV and IR laser pulses (i.e. through stimulated decay from 3 3 P).…”

“…An alternating measurement scheme between the two families was used to minimize the effect of time drifts of the experimental conditions. Contributions of eventual residual long-time drifts in the S calculation were further reduced by normalizing the shots in each family to a second-order polynomial fit of their value versus time (detrending technique, see [18] App.). First, a set of reference spontaneous 2 3 S Ps production measurements was acquired on the detuned OPG set-point (T 2 ) to measure the S(t) = (A off (t) − A UV (t))/A off (t) parameter in the absence of stimulated emission in the conditions selected for the following experiments.…”