Controlling Positronium Annihilation with Electric Fields

Abstract

We show that the annihilation dynamics of excited positronium (Ps) atoms can be controlled using parallel electric and magnetic fields. To achieve this, Ps atoms were optically excited to n ¼ 2 sublevels in fields that were adjusted to control the amount of short-lived and long-lived character of the resulting mixed states. Inclusion of the former offers a practical approach to detection via annihilation radiation, whereas the increased lifetimes due to the latter can be exploited to optimize resonance-enhanced two-photon excitation processes (e.g., 1 3 S → 2 3 P → nS=nD), either by minimizing losses through intermediate state decay, or by making it possible to separate the excitation laser pulses in time. In addition, photoexcitation of mixed states with a 2 3 S 1 component represents an efficient route to producing long-lived pure 2 3 S 1 atoms via single-photon excitation. DOI: 10.1103/PhysRevLett.115.183401 PACS numbers: 36.10.Dr, 78.70.Bj Positronium (Ps) is an atomic system composed of an electron bound to a positron and has several unique properties, the most striking of which are its low mass (M Ps ¼ 2m e ) and the fact that it can decay through various annihilation pathways [1]. Electron-positron annihilation requires overlap of the particle wave functions [2] and in Ps is therefore dependent on the square of the radial wave function at the origin. This is zero for states with l > 0 [3]. Higher order annihilation processes can occur for states with l > 0 but are strongly suppressed [4] and in practice direct Ps annihilation occurs only from the ground 1 1 S 0 and 1 3 S 1 , and metastable 2 1 S 0 and 2 3 S 1 levels. The inhibition of Ps annihilation in states with l > 0 is the basis for several proposed schemes to manipulate annihilation rates using resonant and nonresonant laser fields [5][6][7][8][9][10][11]. More complex strategies have also been suggested [12,13], although these have also not yet been experimentally demonstrated.The radiative decay and annihilation time scales in the n ¼ 2 manifold in Ps span an enormous range: The 2 1 S 0 level is radiatively metastable, fluorescing on a time scale of ≃0.24 s [3], but annihilating in 1 ns; conversely the 2 3 P 0;1;2 levels decay by fluorescence to the 1 3 S 1 state in 3.19 ns, but their annihilation lifetimes exceed 100 μs [4]. The 2 3 S 1 level is also metastable and fluoresces in ≃0.24 s, but has an annihilation lifetime of ∼1.14 μs [1]. It is the disparate properties of these states that make the approach described here to manipulating decay rates particularly effective.We report the results of experiments in which Ps annihilation rates in a weak magnetic field are manipulated using parallel electric fields. This combination of fields permits control over the spin multiplicity and orbital angular momentum character of the n ¼ 2 sublevels accessible by single-photon excitation from the 1 3 S 1 ground state. By increasing the 2 1 P character of mixed states in the fields, radiative decay to the short-lived singlet ground state (1 1 S 0 ) can be...

Experimental progress in positronium laser physics

“…5). Figure 9 shows the results of experiments in which a UV laser (λ = 243.01 nm) was used to excite Ps atoms in a 13 mT magnetic field B with applied parallel electric fields F with strengths up to ±3 kV cm −1 [129,130] (see Fig. 8).…”

“…For Rydberg production an infra-red (IR) laser (≈8 mJ, ∆ν = 5 GHz, λ ≈ 750 nm) is used to drive 2 3 P → n 3 S/n 3 D transitions. This excitation process couples the ground state atoms to the S or D character of excited Stark states, whose azimuthal quantum number can be controlled via the UV and IR laser light polarizations [125,129].…”

“…2.1.2 and Tab. 2), the application of external fields can have a dramatic effect on Ps decay rates [129,130]. Processes in which Ps decay rates are increased via singlettriplet (Zeeman) mixing are known generally as magnetic quenching (MQ).…”

“…To quantify the effects of electric and magnetic fields on Ps decay rates in some recent experiments [129,130], the combined Stark and Zeeman effects for states with n = 2 have been calculated. These calculations include all singlet and triplet terms, and their associated fine structure, and were performed by determining the eigenvalues and eigenvectors of the complete Hamiltonian matrix in an |nS JM J basis, following the convention of Bethe and Salpeter [148].…”

P− state positronium for precision physics: An ultrafine splitting at α6

“…Having shown that existing measurements are too imprecise resolve this splitting, one can ask about the future prospects. Upcoming experiments at University College London plan to remeasure the n = 2 fine-structure with a reduced uncertainty yielding ∆ 2,P ∼ 100 kHz [36][37][38]. A measurement of ∆ 2,P would require only a factor of two improvement.…”