Pumping, optical

If the electronic shell has no angular momentum (J=0), direct nuclear orientation is obtained by pumping. The nuclear moments can then be determined directly, as in NMR experiments (Sect.7.1.6), but with a much higher sensitivity. 

Since extremely narrow resonance lines can be obtained in optical pumping experiments, frequency standards of comparatively simple design can be achieved. The hyperfine transitions used in the atomic-beam clock are also used in the optically pumped frequency standards. However, the resonance frequency is comparatively strongly dependent on the pressure of the buffer gas [7.18]. It is also dependent on the intensity of the pumping light ( light shifts ) [7.16]. Thus, it would seem that an absolute frequency standard of maximal precision cannot be achieved. On the other hand, optically pumped systems have proven to be very suitable for relative measurements and as secondary standards. By observing sharp AF = 0, AMp = 1 

The hydrogen maser provides another way of obtaining an atomic clock [7.24]. The principle of this device is shown in Fig. 7.9. Atomic hydrogen is 

In this chapter we shall treat the most important laser double-resonance techniques by illustration with several examples. While the pump transition is always induced by a pulsed or cw laser, the probe field may be provided by any coherent source in the spectral range between the RF region and the ultraviolet. 

There are several different aspects of optical pumping that are related to a number of spectroscopic techniques based on optical pumping. The r.y aspect concerns the increase or decrease of the population in selected levels. At sufficiently high laser intensities the molecular transition can be saturated. This means that a maximum change AN = Nis — Nio of the population densities can be achieved, where A A is negative for the lower level and positive for the upper level of the transition (Sect. 2.1). In case of molecular transitions, where only a small fraction of all excited molecules returns back into the initial level /) by fluorescence, this level may be depleted rather completely. 

Since the fluorescent transitions must obey certain selection rules, it is often possible to populate a selected level m) by fluorescence from the laser-pumped upper level (Fig. 5.1b). Even with a weak pumping intensity large population densities in the level m) may be achieved. In the pre-laser era, the term optical pumping was used for this special case because this scheme was the only way to achieve an appreciable population change with incoherent pumping sources. 

Excited molecular levels k) with Ek kT are barely populated at thermal equilibrium. With lasers as pumping sources large population densities Nk can be 

The selectivity of optical pumping depends on the laser bandwidth and on the line density of the absorption spectrum. If several absorption lines overlap within their Doppler width with the spectral profile of the laser, more than one transition is simultaneously pumped, which means that more than one upper level is populated (Fig. 5.2). In such cases of dense absorption spectra, optical pumping with narrow-band lasers in collimated cold molecular beams can be utilized to achieve the wanted selectivity for populating a single upper level (Sects. 4.3, 5.5). 

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A. Since tire applied field is red detuned, all A have negative values. Now in order for tire cooling mechanism to be effective tire optical pumping time tp should be comparable to tire time required for tire atom with velocity v to travel from tire bottom to tire top of a potential hill,... 

A wide variety of metliods has been used to pump laser systems. Altliough optical pumping has been implied, tliere is an array of collisionally or electron impact pumped systems, as well as electrically pumped metliods. The efficiency of tire pumping cycle in many ways defines tire utility and applications of each scheme. The first... 

Fig. 1. Pumping methods for lasers where is the pump light frequency and is the laser frequency, wavy lines represent radiationless transitions, and the dashed line collisions (a) optical pumping in three-level systems (b) optical pumping in four-level systems (c) pumping by electron impact and...

Figure lb shows a four-level system. The terminal level, level 2, is ordinarily empty. Atoms are optically pumped to level 4. From level 4, the atoms make a rapid radiationless transition to level 3. The first few atoms to arrive begin to contribute to the population inversion. Therefore, laser operation can begin with much less intense pumping light. After the laser transition, the atoms return to the ground state (level 1) by a radiationless transition. 

V.G. Kozlov, V. Bulovic, P.E. Burrows, M. Baldo, V.B. Khalfin, G. Pailhasarathy, S.R. Forrest, Y. You, M. E. Thompson, Study of lasing action based on Forster energy transfer in optically pumped otganic semiconductor thin films, J. Appl. Phys. 1998, 4, 4096. 

G.J. Denton, N. Tessler, M.A. Slevens, R. H. Friend, Spectral narrowing in optically pumped polyO -phenylencvinylene) films, Adv. Mater. 1997, 9, 547. 

Fig. 5.3.8 Photograph of the detection region of the NMR probe with radiofrequency coil. A methane—air mixture was ignited above the zeolite pellets. The mixture also contained xenon for NMR detection. Hp-129Xe NMR spectra with 30% xenon (from high-density xenon optical pumping) in 70% methane is depicted. (1) The spectrum in the absence of combustion and (2) the spectrum during combustion. Adapted from Ref. [2],...

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