Spin-flip collisions

The BEC can be kept only for a limited time. There are several loss mechanisms that result in a decay of the trapped particle density. These are spin-flip collisions, three-body recombination where molecules are formed, colUsions with excited atoms where the excitation energy may be converted into kinetic energy, and collisions with rest gas atoms or molecules. The background pressure therefore has to be as low as possible (typical pressures are 10 to 10 mbar). 

On the basis of the experimental data presented above, one cjin exclude on the onset several models for the "blue peak" emission. Firstly, spin-flip collision induced population inversion on the Di transition is not involved - due to both the off-resonant character of the emission and its independence on the buffer gas pressure. Similarly, pressure induced extra resonances are rejected. Stimulated electronic Raman and three photon scattering effects, both by a two or three level system, are dependent on the laser detuning and neither their frequencies are to the blue in the vicinity of the Di line (figure 2) thus, these processes are also excluded. 

Let us examine the case where the incident beam is polarized so that all the neutron spins point in the same direction (OX for instance). Let us consider collisions with spin-flip, the initial spin state... 

Spin-flipping, which can occur only through collision with another molecule (of solvent usually), is relatively slow on the time-scale of molecular rotations and, by the time the electrons are in a fit state to pair up, the stereochemistry of the starting material has been scrambled by free rotation in the intermediate. 

Kre Krems, R.V., Dalgarno, A., Balakrishnan, N., Groenenboon, G.C. Spin- flipping transitions in molecules induced by collisions with structureless atoms, Phys. Rev. A 67 (2003) 060703, p. 1-4. 

Collision-induced spin-flip transitions in molecules transfer the optically excited molecule from a singlet into a triplet state, which may drastically change its chemical reactivity [1028]. Such processes are important in dye lasers and have therefore been intensively studied [1029]. Because of the long lifetime of the lowest triplet state To, its population N(t) can be determined by time-resolved measurements of the absorption of a dye laser tuned to the transition T. ... 

The cooling process must be slow enough to maintain thermal equilibrium, but should be sufficiently fast in order not to lose particles from the trap by collision-induced spin flips [1178]. 

All atoms for which BEC has achieved, have a magnetic moment which is due to the vector sum of electron spin and nuclear spin. Collisions between such atoms can flip the spin and therefore the direction of the magnetic moment. This will lead to an escape of the atoms from the trap and will decrease the density of the trapped atoms. Spin flips during a collision are due either to the direct dipole-dipole interaction between the collision partners or to an indirect interaction between the nuclear spins which is mediated by the electron spins, since they are coupled through the hyperfine-interaction with the nuclear spins. 

A new method has been recently proposed for spin dynamic studies in conducting polymers [33]. It basically relies on the following. Collisions between moving spins (species A) and unlike spins (species B), instead of giving rise to a motional narrowing, can result in a broadening of the line. Such an effect can be observed in two cases. First, in the slow exchange case, i.e., when the spin flip-flop frequency Larmor frequencies of the two species < Soab = weA Wcb. Second, in the fast... 

A further interesting result concerns spin component conservation. Each rotational level of this 2S state has two spin components, one (termed F ) with the spin and rotational angular momentum vectors parallel, and the other (F2) having them antiparallel. Excitation of an F component results in transfer primarily to F- components of other rotational levels, and similarly for F2 excitation. In a collision, the OH would as soon exchange several hundred cm" of energy as flip its spin around at no energy cost. Similar results were also observed in the fluorescence scans of the NH molecule (16). 

T 1 /Aco. At f = 0, that is, just after the first 7r /2 pulse, which flips all nuclear spins into the xy plane, all spins are in phase, and they retain their phase memory until f = r, when the collision changes the direction of the local field. Since t is sufficiently long to dephase the spins, they lose all phase memory before the next collision. Therefore, l/r2 = 1/t. In contrast, in the weak collision Kmit, t 1 /Aco. Since r is relatively short, insufficient dephasing builds up between two consecutive jumps. When the spin jumps randomly, many jumps are needed in order to accumulate sufficient dephasing, and one has 1/T2 = (Aco) r. In summary then, we have for these two limiting cases ... 

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