Experiments with Supersonic Beams as a Source of Cold Atoms

scattered photon to extract information and entropy from the trapped ensemble. This
method should enable cooling to the recoil limit, increasing the phase space density of
the trapped sample. The principle of operation of single-photon cooling is described,
as is the proposed implementation in hydrogen. Cooling below the recoil limit would
require evaporative cooling, and exploration of a degenerate gas of atomic tritium is
a possible research avenue which is also explored.
5.4.1 Laser Detection and Spectroscopy of Hydrogen Isotopes
Detecting trapped hydrogen using laser excitiation should allow for a back-
ground free detection method by eliminating the signal from molecular hydrogen. The
process which will be used is to excite the trapped hydrogen to the 2S metastable
state, and then to eject the atoms onto an MCP. Since the atoms start in the 1S
state, the transition does not change the orbital angular momentum and is forbidden
by parity conservation for a single photon. This leads to a long lifetime in the excited
state, though it makes the excitation process more complicated.
While one photon cannot drive the 1S − 2S transition at 121 nm, two photons
can excite the atom through a virtual P state. This is illustrated in a simplified
energy level diagram shown in figure 5.22. In this case, the first photon excites the
atom from the ground state to a virtual state which is a linear combination of all nP
states as well as the continuum, and the second photon excites the atom from there
to the 2S state. This process is discussed in detail elsewhere [106, 107], and so only
a brief discussion is given here.
5.4.1.1 Two-Photon Excitation of Hydrogen
While the two photons which drive the 1S −2S transition do not need to be of
the same wavelength, for reasons which will become clear shortly, it is advantageous
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