Atoms trapped, energy state

In this section we discuss the bound states of the hydrogen atom. These are states where the electron stays with the nucleus. In contrast, an electron with lots of energy could simply speed past the nucleus without getting trapped. Such an unbound electron does not stop long enough form a coherent atom hence in our study of the atom, it makes sense to study only the bound states. 

The PAES mechanism, first demonstrated in 1987 [1], can be outlined as follows (1). A positron implanted at low energy diffuses to and gets trapped at the surface. (2). A few percent of the trapped positrons annihilate with core electrons leaving atom in excited state. (3). The atom relaxes via emission of an Auger electron. The PAES mechanism is contrasted with that of electron induced Auger Spectroscopy (EAES) in Figurel2.1. 

For further cooling of the atoms in the trap the different dependence of the atomic energy levels in the lower and upper state from the distance to the surface is used. For the lower state the force on between atom and surface is repulsive, in the upper state it is attractive. If the laser is red shifted against the atomic resonance it excites atoms close to the surface, which emit fluorescence on the average farther away from the surface. There the atom loses energy for each absorption-emission cycle. 

The question is, which role collision play in a BEC. The atoms are exposed to a laser field which excites them into higher electronic states. In collisions between excited atoms and ground state atoms the excitation energy can be transferred either into excitation energy of the other atom or into translational energy. The latter case will increase the velocity of atoms which then cannot be kept by the trapping potential and leave the trap. Another possibility is the associative recombination of a collision pair (see Fig. 9.17 and Sect. 9.1.6), where a stable dimer molecule is formed which, however, cannot be kept in the magnetic trap because its spin is different from that of the atoms. 

Within the past decade, an exciting new form of matter, the Bose-Einstein (BE) condensate, has been produced (7). In a typical experiment, a gaseous sample of pure sodium containing several million atoms is cooled to about a microkelvin and trapped in the F = 1, mp = -1 hyperfine state by a combination of stationary magnetic fields and laser fields. Under these conditions, the sodium atoms are bosons and essentially all of them enter the lowest energy state of the trapping potential, which is similar to the familiar three-dimensional particle-in-a-box system. When two such boxes of sodium atoms are prepared, and the atoms are subsequently released fi"om these traps, the... 

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