Photoionization cavity

Laser spectroscopy of the 1S-2S transition has been performed by Mills and coworkers at Bell Laboratories (Chu, Mills and Hall, 1984 Fee et al, 1993a, b) following the first excitation of this transition by Chu and Mills (1982). Apart from various technicalities, the main difference between the 1984 and 1993 measurements was that in the latter a pulse created from a tuned 486 nm continuous-wave laser with a Fabry-Perot power build-up cavity, was used to excite the transition by two-photon Doppler-free absorption, followed by photoionization from the 2S level using an intense pulsed YAG laser doubled to 532 nm. Chu, Mills and Hall (1984), however, employed an intense pulsed 486 nm laser to photoionize the positronium directly by three-photon absorption from the ground state in tuning through the resonance. For reasons outlined by Fee et al. (1993b), it was hoped that the use of a continuous-wave laser to excite the transition would lead to a more accurate determination of the frequency interval than the value 1233 607 218.9 10.7 MHz obtained in the pulsed 486 nm laser experiment (after correction by Danzmann, Fee and Chu, 1989, and adjustment consequent on a recalibration of the Te2 reference line by McIntyre and Hansch, 1986). 

Another MPE procedure, well implemented for the ab initio description of the solute, has been elaborated by Mikkelsen et al. (1988). The method is limited to spherical cavities and it is used not for chemical reactions, but for the study of solvent effects on a number of properties of solutes at fixed geometry (electronic spectra, photoionization, hyperpolarizabilities) with results of remarkable interest (Agren et al., 1993 Mikkelsen et al., 1994). 

Sulfamethoxazole failed to produce any trappable radicals with an array of different spin traps, but naproxen afforded the EPR spectrum shown in Figure 2.11 when irradiated with 330 nm UV-R in the instrument cavity in the presence of 2-methyl-2-nitroso-propane (MNP). The spectrum contains contributions from di-t-butyl nitroxide, a known photoproduct of MNP. The H-atom adduct MNP-H also evident can arise by several different mechanisms, including the trapping of an H atom by MNP the reaction of MNP with an electron followed by protonation and the direct reduction of MNP by an excited state species. In view of the flash photolysis results, it was concluded that photoionization was the major precursor of MNP-H. The third radical corresponded to a C-centered radical carrying a single H atom, leading to the postulate of a decarboxylation reaction as the primary photochemical step. Confirmation of the participation of free radical intermediates came from the initiation of the free radical polymerization of acrylamide with rates as shown in Table 2.1. 

Resonance multistep photoionization has been successfully used to detect rare trace atoms, rare actinides, and, particularly, radioactive isotopes. This method has an important specific feature the production of photoions that can be isolated and sorted according to their mass. This makes the method very versatile and compatible with nuclear-physics experiments. Figure 9.7 illustrates various ways to realize photoionization detection with (a) a thermal atomic beam obtained by heating and vaporizing a sample containing the rare species of interest, (b) rare atoms in a buffer gas, (c) atoms trapped in a hot cavity, and (d) an accelerated atomic beam under collinear irradiation conditions. Each of these techniques is effective for certain rare species under certain experimental conditions. Some examples are presented below. 

Fig. 9.7 Various resonant photoionization techniques for ultrasensitive spectroscopy of very rare atoms and isotopes (a) ionization of a transverse thermal atomic beam (b) ionization of atoms in a buffer gas (c) ionization of atoms trapped in a hot cavity (d) ionization of...

Successful experiments on the photoionization detection and separation of radioactive isotopes were conducted by Zherikhin et al. (1984). Later on it became clear that atoms could be ionized most effectively not in a beam by the scheme presented in Fig. 9.7(a), but in a hot cavity in accordance with the scheme shown in Fig. 9.7(c),... 

Alkhazov, G. D., Letokhov, V. S., Mishin, V. I., Panteleyev, V. N., Romanov, V. I., Sekatskii, S. K., and Fedoseyev, V. N. (1989). Highly effective Z-selective photoionization of atoms in hot metal cavity, followed by electrostatic confinement of ions. Journal of Technical Physics Letters, 15, 63-67. 

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