Stephen D. Hogan

Exploring the WEP with a pulsed cold beam of antihydrogen

The AEGIS experiment, currently being set up at the Antiproton Decelerator at CERN, has the objective of studying the free fall of antimatter in the Earth?s gravitational field by means of a pulsed cold atomic beam of antihydrogen atoms. Both duration of free fall and vertical displacement of the horizontally emitted atoms will be measured, allowing a first test of the WEP with antimatter.

Driving Rydberg-Rydberg Transitions from a Coplanar Microwave Waveguide

The coherent interaction between ensembles of helium Rydberg atoms and microwave fields in the vicinity of a solid-state coplanar waveguide is reported. Rydberg-Rydberg transitions, at frequencies between 25 and 38 GHz, have been studied for states with principal quantum numbers in the range 30-35 by selective electric-field ionization. An experimental apparatus cooled to 100 K was used to reduce effects of blackbody radiation. Inhomogeneous, stray electric fields emanating from the surface of the waveguide have been characterized in frequency- and time-resolved measurements and coherence times of the Rydberg atoms on the order of 250 ns have been determined. These results represent a key element in the development of an experimental architecture to interface Rydberg atoms with solid-state devices.

Trapping cold molecular hydrogen

Translationally cold H(2) molecules excited to non-penetrating |M(J)| = 3 Rydberg states of principal quantum number in the range 21-37 have been decelerated and trapped using time-dependent inhomogeneous electric fields. The |M(J)| = 3 Rydberg states were prepared from the X (1)Σ(+)(u)(v = 0, J = 0) ground state using a resonant three-photon excitation sequence via the B (1)Σ(+)(u)(v = 3, J = 1) and I (1)Π(g) (v = 0, J = 2) intermediate states and circularly polarized laser radiation. The circular polarization of the vacuum ultraviolet radiation used for the B ← X transition was generated by resonance-enhanced four-wave mixing in xenon and the degree of circular polarization was determined to be 96%. To analyse the deceleration and trapping experiments, the Stark effect in Rydberg states of molecular hydrogen was calculated using a matrix diagonalization procedure similar to that presented by Yamakita et al., J. Chem. Phys., 2004, 121, 1419. Particular attention was given to the prediction of zero-field positions of low-l states and of avoided crossings between Rydberg-Stark states with different values of |M(J)|. The calculated Stark maps and probabilities for diabatic traversal of the avoided crossings were used as input to Monte-Carlo particle-trajectory simulations. These simulations provide a quantitatively satisfactory description of the experimental data and demonstrate that particle loss caused by adiabatic traversals of avoided crossings between adjacent |M(J)| = 3 Stark states of H(2) is small at principal quantum numbers beyond n = 25. The main source of trap losses was found to be from collisional processes. Predissociation following the absorption of blackbody radiation is estimated to be the second most important trap-loss mechanism at room temperature, and trap loss by spontaneous emission is negligible under our experimental conditions.

Slow beams of atomic hydrogen by multistage Zeeman deceleration

Hydrogen atoms seeded in a supersonic expansion of Kr have been decelerated from an initial velocity of 435 m s−1 to 107 m s−1 in a multistage Zeeman decelerator. The operation of the decelerator has been fully quantified by numerical particle trajectory simulations and independent measurements of the velocity distribution by ion time-of-flight mass spectrometry following photoionization of the decelerated atoms. The velocity distributions of the decelerated atom clouds have a half-width at half-maximum of 25 ± 12 m s−1 corresponding to a temperature of ~30 mK. These velocities and temperatures are sufficiently low that magnetic trapping of the atoms can be envisaged.

Zeeman deceleration of H and D

Hydrogen and deuterium atoms in supersonic jet expansions have been decelerated using a multistage Zeeman decelerator. The properties of the decelerator have been completely characterized in a series of experiments in which (i) the initial longitudinal velocities of the decelerated atoms (ii) the maximum magnetic field strength, and (iii) the duration of zero-field intervals between successive field pulses in neighboring deceleration stages were systematically varied. Experiments using Ar and Kr as carrier gases have clearly revealed that the H atoms are located at the surface of the jet expansion cone in each case. Comparison of the results of these experiments with numerical simulations of the atom trajectories through the decelerator provides a full description of the phase-space distribution of the decelerated atoms. Evidence is presented of transverse guiding of the beam and of a partial redistribution of the H atom population among the M{sub F} components of the F=1 manifold at times when the magnetic field strength approaches zero.

Multistage Zeeman deceleration of metastable neon

A supersonic beam of metastable neon atoms has been decelerated by exploiting the interaction between the magnetic moment of the atoms and time-dependent inhomogeneous magnetic fields in a multistage Zeeman decelerator. Using 91 deceleration solenoids, the atoms were decelerated from an initial velocity of 580 m/s to final velocities as low as 105 m/s, corresponding to a removal of more than 95% of their initial kinetic energy. The phase-space distribution of the cold, decelerated atoms was characterized by time-of-flight and imaging measurements, from which a temperature of 10 mK was obtained in the moving frame of the decelerated sample. In combination with particle-trajectory simulations, these measurements allowed the phase-space acceptance of the decelerator to be quantified. The degree of isotope separation that can be achieved by multistage Zeeman deceleration was also studied by performing experiments with pulse sequences generated for (20)Ne and (22)Ne.

Dynamical processes in Rydberg-Stark deceleration and trapping of atoms and molecules.

The interaction between inhomogeneous electric fields and the large electric dipole moments of atoms and molecules in Rydberg states of high principal quantum number can be used to efficiently accelerate and decelerate atoms and molecules in the gas phase. We describe here how hydrogen atoms and molecules initially moving with velocities of ∼600 m/s in supersonic beams can be decelerated to zero velocity and loaded into electric traps. The long observation times that are made possible by the electrostatic trapping enables one to study slow relaxation processes. Experiments are presented in which we have observed photoionization processes and transitions between Rydberg states induced by blackbody radiation at temperatures between 10 K and 300 K on a time scale of several milliseconds. Comparison of these processes in Rydberg states of H and H(2) suggests the importance, in H(2), of collisional processes and of the process of blackbody-radiation-induced predissociation.