Quasi electrostatic trap

Note, finally, that the basic distinction between the far-ofT-resonance dipole trap and the quasi-electrostatic trap consists in the value of the diffusion heating. Since the population of the excited atomic state in the quasi-electrostatic trap is extremely small, the diffusion-associated heating of atoms in this trap is much smaller than in the far-off-resonance dipole trap. 

The quasi-electrostatic trap can also be used for trapping neutral molecules. The main problem here is the production of cold molecules. At present, the process of photoassociation of laser-cooled atoms is the only known way to produce a sample of cold molecules. Takekoshi et al. (1998) have reported the first observation of optical trapping of cold Cs2 molecules in a quasi-electrostatic trap (see Chapter 8). 

In a quasi-electrostatical optical trap (Letokhov 1968), the detuning of the laser beam is comparable to the optical frequencies of the atom. In this case, the trapping potential can be foimd from the general expression for the high-frequency Stark shift in an off-resonance fight field. For the atoiruc groimd state, the energy shift is... 

Recent work has examined the question of whether A OH is exclusively anode-sorbed or whether there is a continuum of species between the extremes of physisorbed and bulk hydroxyl radicals. Vatistas [13] has considered the dissociation of hydroxyl radicals from an adsorption layer ( true A OH) into a three-dimensional reactive layer close to the anode surface. A OH is thus a combination of surface and reactive layer species, both of which experience the anode electrostatically and, in principle, have a reactivity different from that of bulk OH(aq). Kapalka et al. [1] have estimated the profile of hydroxyl species adjacent to a BDD anode concurrent with the evolution of O2 in the absence or presence of an organic substrate. Their model describes the hydroxyl species as quasi-free. In the absence of substrate, the hydroxyl radicals form H2O2 within a stagnant layer of solution close to the anode. It is concluded that their concentration falls to <10 % of the value at the anode surface within 0.2 pm and almost to zero by 1 pm almost no hydroxyl radicals escape the anode surface completely and become bulk OH (aq). When a reactive organic substrate is also present, the hydroxyl radicals are trapped much closer to the anode because of the higher rate constant for reaction of OH with a substrate as compared with that for dimerization, and their concentration falls to almost zero within tens of nm. No data have yet appeared in which the reactivities of anode-sorbed and bulk hydroxyl radicals have been compared. 

Fig. 7.1 Model of a p-n junction incorporating assumptions concerning quasi-Fermi levels and electrostatic potential. Forward voltage implies V >0 and will be developed by a solar cell under illumination. When the forward voltage is due to an external voltage, the left-hand side is connected to the positive terminal, and the right-hand side to the negative, terminal of a battery. denotes a trapping level.

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