Laser spectroscopic studies of state-dependent collisional quenching of the lifetimes of metastable antiprotonic helium atoms

Abstract

A laser spectroscopic method was developed to directly observe the lifetimes of individual states in metastable antiprotonic helium atoms ( p 4 He ϩ ). Collisional effects with surrounding helium atoms were studied in cryogenic gas targets at pressures between 0.1 and 9.0 bars and at temperatures between 5.5 and 7.0 K. The metastable state (n,l )ϭ(39,35) showed a lifetime of 1.5 s, regardless of density, whereas the lifetime of the energetically lower-lying state (37,34) was found to shorten from 1.3 to 0.1 s with increasing density. The lifetimes of the states (38,34) and (36,33) were determined to be 10 ns and 5 ns, respectively, showing the effects of the internal Auger process. When extrapolated to zero density, the measured lifetimes agreed with the results of calculations for the radiative and Auger widths. Using the fact that the state (37,34) becomes short lived at high density, the resonance (38,35)→(37,34) at 529.62 nm was detected.

Populations and lifetimes in thev=n−ℓ−1=2and 3 metastable cascades ofp¯Hori1, Eades2, Widmann3 et al. 2004Phys. Rev. ASelf Cite352230View full textAdd to dashboardCite

“…The lifetime of state ͑39, 35͒ was fixed to the experimental value ͑39,35͒ = 1.5 s [17]. The result of the best fit are shown with solid lines in Figs.…”

“…In a previous paper [17], the shortening of the state lifetimes caused by collisions between pHe + and helium atoms was studied. State ͑39, 35͒ had a lifetime ͑39,35͒ = 1.5± 0.1 s, which was relatively unchanged between atomic densities of the helium target =2ϫ 10 20 and 1.9ϫ 10 22 cm −3 .…”

“…The intensity, time structure, and XY profile of these pulses were measured by a parallel plate ionization chamber [37] placed in the beam upstream of the target. The target [17] was a 150-mm-long, 70-mm-diam stainless-steel cylinder, into which the antiprotons entered through a 0.5-mm-thick copper-beryllium window. A 20-mm-diam quartz window was affixed to the opposite end of the chamber to admit the laser beam.…”

“…Laser spectroscopic studies of the primary populations P ͑n,ᐉ͒ ͑t =0͒ [i.e., the number of antiprotons populating a state ͑n , ᐉ͒ at atomic formation, normalized to the total number of antiprotons stopped in the target] and lifetimes of pHe + were started at LEAR [13][14][15][16][17], which delivered antiprotons as either a single high-intensity pulse containing 3 ϫ 10 7 -1ϫ 10 9 antiprotons and lasting ⌬t ϳ 200-300 ns, or as a continuous, low-intensity beam at rates ϳ10 4 p / s. Metastable pHe + atoms were produced by stopping either variety of beam in a helium target. They were irradiated with a resonant laser pulse, which induced antiproton transitions between adjacent pairs of metastable and short-lived pHe + states (Fig.…”

“…Experimental [12][13][14][15][16][17][18][19] and theoretical studies have shown that five phases can be discerned in the life history of metastable pHe + . (i) Capture: when an antiproton collides with a helium atom at electron-volt energies, it may replace one of the electrons orbiting the helium nucleus, and become captured into a pHe + state.…”