The authors report the observation of the 1S-2S two-photon transition in atomic hydrogen in an atomic beam, excited by light from a narrow-band continuous-wave laser. The excitation takes place in a collision-free environment and detection of the resonance is carried out by Stark-quenching the metastable atoms some distance from the interaction region. This approach removes the limitations of cell excitation employed in previous experiments with continuous-wave lasers, and should allow the full precision of modern tunable laser techniques to be applied to the 1S-2S transition. It also gives a new method for producing a collimated beam of metastable atoms which has particular applications to a variety of future experiments with cold hydrogen.
Achieving a high population of antihydrogen/hydrogen atoms in the 2S level is essential for spectroscopy measurements testing similarities between matter and antimatter. We propose and examine the efficiency of applying the STIRAP (Stimulated Raman Adiabatic Passage) process in achieving high population transfer from the 1S to the 2S levels. We utilized a circularly polarized Lyman alpha, Ly-
α, pulse to couple the 1S state to the 2P state and a microwave pulse to couple the 2P state and the 2S state. We calculate the efficiency of the STIRAP process for transferring the population between the stretched states
(1Sd,2Sd) as a function of experimental parameters such as Rabi frequencies and pulse durations. We find that a Ly-
α pulse with an energy of a few nanojoules could produce nearly perfect transfer at zero detunings for atoms on the laser beam axis. We extended the analysis to a thermal ensemble of atoms, where Doppler detuning affects the velocity distribution of the hydrogen atoms produced in the 2S level. We found that the width of such velocity distribution is controlled by the Rabi frequency. We show that the peak velocity of the hydrogen atoms in the 2S level after STIRAP can be controlled by the Ly-
α pulse detuning. The efficiency of STIRAP in transferring population increases at low temperature (T$\sim$1~mK). Finally, we show that a background magnetic field improves the transfer rates between the other trappable states
(1Sc,2Sc).
Scattering lengths for the collisions of ground and metastable state hydrogen atoms interacting via the B′1∑u +, EF 1∑g +, e3∑u + and h3∑g + molecular potentials are evaluated from published ab initio Born-Oppenheimer potentials with adiabatic corrections, matched to multipole expansions at large interatomic separations. It is demonstrated that the scattering lengths are very sensitive to the connections between the ab initio potentials and their asymptotic forms and the need is shown for accurate interatomic potential calculations at greater separations than are currently available. The density-dependent shift and broadening of the 1S–2S transition frequency in atomic hydrogen caused by two-body elastic collisions are presented.