Atomic beam lifetimes

For a typical sodium atom, the initial velocity in the atomic beam is about 1000 m s1 and the velocity change per photon absorbed is 3 crn-s. This means that the sodium atom must absorb and spontaneously emit over 3 x 104 photons to be stopped. It can be shown that the maximum rate of velocity change for an atom of mass m with a photon of frequency u is equal to hu/lmcr where h and c are Planck s constant and the speed of light, and r is the lifetime for spontaneous emission from the excited state. For sodium, this corresponds to a deceleration of about 106 m s"2. This should be sufficient to stop the motion of 1000 m-s 1 sodium atoms in a time of approximately 1 ms over a distance of 0.5 m, a condition that can be realized in the laboratory. 

Atomic beam/IJF used to measure lifetimes of Ne 2p fine- 45 structure states... 

The incorporation of POSS nanostructures into polyimides has been shown to significantly extend the lifetime of these materials in LEO. Studies on the effect of a hyperthermal O-atom beam on POSS-PIs have shown the improved oxidation resistance imparted to polyimides by the addition of POSS. XPS data of both the AO-exposed and space-flown POSS-PI materials indicated that the improved oxidation resistance of these materials is due to a rapidly formed silica layer upon exposure of POSS-polymers to high incident fluxes of atomic oxygen. 

The experimental apparatus for lifetime measurements are similar to those shown in fig. 6. Recently, sputtering techniques have been employed to produce the atomic beam specially with metals with high evaporation temperatures, Rudolph and Helbig (1982). Photon counting and averaging techniques are also... 

Lifetimes x of 51 odd levels (taken from [17]) have been measured with an accuracy of 5% by the technique of time-resolved laser-induced fluorescence on an Ru atomic beam [11,12] and compared to earlier measurements [13], see Table 2/2. 

Lifetimes of 4d 5p levels (see Table 2/11, p. 170) have been measured by the levelcrossing technique [21, 22, 28 to 30] and by laser excitation of an atomic beam and time-resolved observation of the reemitted fluorescence [31] and compared to values calculated from the experimental transition probabilities of Corliss and Bozman [32]. 

In the period between 1926 and 1939 the development of our understanding of radiation processes parallels the development of the theory of atomic and molecular structure. Many of the results which had been derived by the use of the correspondence principle and the old quantum theory were re-derived in a more rigorous and satisfactory manner. The quantum-mechanical expression for the refractive index of a gas or vapour is an example of this type of progress. In hydrogenic systems the rates of radiative transitions were calculated and the theoretical lifetimes of the different excited levels were derived. In other atoms only approximate estimates of the transition probabilities were possible, but the lifetime measurements made by canal rays and fluorescence from atomic beams were not sufficiently accurate to demand more refined calculations. 

Several similar methods were developed by the early workers in this field and these are described in Mitchell and Zemansky (1961). One technique, developed by Koenig and Ellett (1932), overcame the problem of repopulation by using optical excitation of a thermal atomic beam. However, this method was restricted to lifetimes greater than 10 s owing to the low velocity, v = 10 cm s of the atomic beam. In all of these experiments the calibration of the beam velocity led to considerable uncertainties in the final results. For this reason these techniques were little used after 1932. 

This uniform distribution is achieved by having the exciting electron beam coaxial with the atomic beam. Early experiments with transverse electron beam excitation led to atom recoil and serious systematic effects due to non-uniform velocity distributions over the beam cross-section. The mean radiative lifetime is determined by taking the ratio of the number of metastable atoms in the same velocity interval at the two spatially separated detectors since... 

Theoretical level populations. Sinee there are population variations on time seale shorter than some level lifetimes, a complete description of the excitation has been modeled solving optical Bloch equations Beacon model, Bellenger, 2002) at CEA. The model has been compared with a laboratory experiment set up at CEA/Saclay (Eig. 21). The reasonable discrepancy when both beams at 589 and 569 nm are phase modulated is very likely to spectral jitter, which is not modeled velocity classes of Na atoms excited at the intermediate level cannot be excited to the uppermost level because the spectral profile of the 569 nm beam does not match the peaks of that of the 589 nm beam. 

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