Threshold Photodetachment Spectroscopy of the Positronium Negative Ion

Threshold photodetachment spectroscopy of the positronium negative ion has been accomplished for the first time employing an efficient source of the ions and photodetachment techniques combined with a tunable optical parametric oscillator and amplifier laser. The photodetachment threshold, corresponding to the electron affinity of positronium ($1{{}^{3}S}_1$), was determined to be $326.88\pm 0.09(\mathrm{stat})\pm 0.10(\mathrm{syst})\mathrm{meV}$ by laser photodetachment threshold measurements. This result is consistent with a variational calculation corrected for leading relativistic and quantum electrodynamical effects.

Figure 1

(a) Schematic of the experimental setup for the threshold spectroscopy of the ${\mathrm{Ps}}^{-}$ ion (not to scale). A focused positron pulsed beam with a spot size smaller than 1 mm (FWHM) is converted to ${\mathrm{Ps}}^{-}$ ions through a W(100) film coated with submonolayer Na. The ${\mathrm{Ps}}^{-}$ ions are accelerated by the potential gap between the film and the grids, biased at $V_b$, and then illuminated by an IR laser pulse with a wavelength range of 3700–3850 nm in a cross-beam arrangement. The resultant Ps atoms formed by the photodetachment are detected by an assembly made up of a double-stack MCP and a delay-line anode. (b) Energy level diagram of ${\mathrm{Ps}}^{-}$ and Ps in their ground states. Only the transition from ${\mathrm{Ps}}^{-}$ to the long-lived $o$-$\mathrm{Ps}$ ($1{{}^{3}S}_1$) $+$ photoelectron (arrow) is observed in the present experiment, because the short-lived $p$-$\mathrm{Ps}$ ($1{{}^{1}S}_0$) is undetected due to its short self-annihilation lifetime. $E_{\mathrm{HFS}}$ is the hyperfine-splitting energy of the ground level of Ps.

Figure 2

Photodetachment fractions of the positronium negative ions as a function of the photon energy, at various acceleration voltages of the ions, $V_b$. The error bars on the data represent one standard deviation of the measured photodetachment fraction. The solid lines are the fits using Wigner’s $p$-wave threshold law (${\chi }^2/\nu$: $95/79$, $p$-value: 0.11), where all data sets were simultaneously fitted by sharing ${\chi }^2$ and the photodetachment threshold energy in the center-of-mass frame, $E_{\mathrm{th}}^{\prime }$. The dashed lines indicate the determined energies of the threshold including the second-order Doppler shifts.