Radiative and collisional processes in translationally cold samples of hydrogen Rydberg atoms studied in an electrostatic trap

Abstract

Supersonic beams of hydrogen atoms, prepared selectively in Rydberg-Stark states of principal quantum number n in the range between 25 and 35, have been deflected by 90 ○ , decelerated and loaded into off-axis electric traps at initial densities of ≈ 10 6 atoms/cm −3 and translational temperatures of 150 mK. The ability to confine the atoms spatially was exploited to study their decay by radiative and collisional processes. The evolution of the population of trapped atoms was measured for several milliseconds in dependence of the principal quantum number of the initially prepared states, the initial Rydberg-atom density in the trap, and the temperature of the environment of the trap, which could be varied between 7.5 K and 300 K using a cryorefrigerator. At room temperature, the population of trapped Rydberg atoms was found to decay faster than expected on the basis of their natural lifetimes, primarily because of absorption and emission stimulated by the thermal radiation field. At the lowest temperatures investigated experimentally, the decay was found to be multiexponential, with an initial rate scaling as n −4 and corresponding closely to the natural lifetimes of the initially prepared Rydberg-Stark states. The decay rate was found to continually decrease over time and to reach an almost n-independent rate of more than (1 ms) −1 after 3 ms. To analyze the experimentally observed decay of the populations of trapped atoms, numerical simulations were performed which included all radiative processes, i.e., spontaneous emission as well as absorption and emission stimulated by the thermal radiation. These simulations, however, systematically underestimated the population of trapped atoms observed after several milliseconds by almost two orders of magnitude, although they reliably predicted the decay rates of the remaining atoms in the trap. The calculations revealed that the atoms that remain in the trap for the longest times have larger absolute values of the magnetic quantum number m than the optically prepared RydbergStark states, and this observation led to the conclusion that a much more efficient mechanism than a purely radiative one must exist to induce transitions to Rydberg-Stark states of higher m values. While searching for such a mechanism, we discovered that resonant dipole-dipole collisions between Rydberg atoms in the trap represent an extremely efficient way of inducing transitions to states of higher m values. The efficiency of the mechanism is a consequence of the almost perfectly linear nature of the Stark effect at the moderate field strengths used to trap the atoms, which permits cascades of transitions between entire networks of near-degenerate Rydberg-atom-pair states. To include such cascades of resonant dipole-dipole transitions in the numerical simulations, we have generalized the two-state Förster-type collision model used to describe resonant collisions in ultracold Rydberg gases to a multi-state situation. It is only when considering the combined effects of collisional and radiative pr...

Experimental progress in positronium laser physics

“…This work allowed control over the translational motion of hydrogenic [457] and non-hydrogenic [458] atoms, as well as the realization of a wide range of atom optics elements, including mirrors [459], lenses [457], deflectors [460,461], decelerators and traps [436,[462][463][464][465][466]. These advances have been employed for a various studies, including the effects of blackbody induced transitions and photoionization of Rydberg states [463,467], m-changing dipole-dipole interactions in gases of polar Rydberg atoms and their effects on Rydberg state lifetimes [468], the preparation of long-lived high-|m| (i.e., |m| ≥ 3) Rydberg states of H 2 [469,470], and new methods to study ion-molecule reactions at low temperatures [471,472].…”

Excitation and characterization of long-lived hydrogenic Rydberg states of nitric oxide

“…In these devices the molecules are confined in continuously moving electric traps. If these traps are decelerated a) Present address: Max-Planck-Institut für Plasmaphysik, Boltzmannstraße 2, 85748 Garching bei München, Germany b) Electronic mail: s.hogan@ucl.ac.uk and brought to rest in the laboratory-fixed frame of reference the resulting samples can be used for studies of slow excitedstate decay processes that occur on timescales between 10 µs and 1 ms. [12][13][14][15] Rydberg-Stark deceleration and trapping experiments have been performed with H, D, He and H 2 . 13,[15][16][17][18][19] However, it is of interest to extend these studies to heavier and more complex molecules.…”

Velocity selection of Rydberg positronium using a curved electrostatic guide

“…The subsequent introduction of time-dependent electric fields [34] by Vliegen, Merkt, and coworkers permitted efficient control over the translational motion of hydrogenic [35] and nonhydrogenic [36] atoms in pulsed supersonic beams and the realization of a wide range of Rydberg atom and molecule optics elements, including mirrors [37], lenses [35], deflectors [38,39], decelerators, and traps based upon three-dimensional [40][41][42] and chip-based two-dimensional [43][44][45] electrode structures. These advances have allowed studies of the effects of blackbody-induced transitions and photoionization of Rydberg states [42,46], studies of m-changing dipole-dipole interactions in gases of polar Rydberg atoms and their effects on Rydberg state lifetimes [47], the preparation of long-lived high-|m| (i.e., |m| 3) Rydberg states of H 2 [48,49], and new methods to study ion-molecule reactions at low temperatures [50,51]. It is only recently, however, that these methods have been applied to the Ps system [52].…”