Depopulating beam

The most easily observed process with ionic, as with neutral, collisions partners is mixing. When the Na nd states are exposed to an ion beam, the field ionization signal changes from one which is predominantly adiabatic to one which is predominantly diabatic. By measuring the fraction R of signal transferred from the adiabatic to the diabatic peak of the field ionization signal MacAdam et al. measured the depopulation cross section of the Na nd states by He+ ions.1 In the limit of small values of R the depopulation cross section is given by1... 

Fig. 3.20. Signals of fluorescence kinetics representing fly-through relaxation of an optically depopulated initial level (a) rectangular profile of the beam (b) limited Gaussian profile (c) unlimited Gaussian profile (d) experimentally registered signal. Values of the non-linearity parameter Bwpvp/ro are shown in brackets.

Magnetic quantum beats in the transient process after pulsed depopulation of the ground state may be observed not only in fluorescence, but also in a more direct way, namely in absorption. In connection with what was discussed in Section 3.5, one must expect maximum sensitivity if the experiment is conducted according to the laser interrogated dichroism method see Fig. 3.17. To this end it is convenient to direct the external magnetic field B along the 2-axis as shown in Fig. 4.21 where the probe beam E-vector can be either in the xy plane (Em) or in the yz plane (Epr2). 

Fig. 9. a Double-logarithmic power dependence of the population densities, Ni and N2, of levels 1 and 2 in Fig. 5 calculated from Eq. (10) using the same parameters as in Fig. 6 c, where power is taken as G. The dashed lines are the limiting slopes from Eqs. (14) and (15). b Double-logarithmic power dependence of the level 1 depopulation rate ratio between ETU (rate = 2 Wetu i) and linear downconversion (rate = kiNi). The horizontal dashed line indicates equal rates for the two. c Time-evolution of N2 following termination of a cw beam at f = 0 for the high, medium, and low powers indicated in (b), plotted on linear axes. The dashed lines are the limiting behaviors from Eqs. (22) and (24)... 

It is seen that the ratio (120) depends crucially on the spontaneous emission rate Teg, which depopulates the state e). Maximum inversion, with P, = 1 and Pg = 0, is obtained for I = 0, when the population is said to be shelved (trapped) in the state e) from which it cannot decay to the ground state. Thus, in the case of maximum inversion one could expect maximum amplification of a probe beam on the e) —> g) transition. However, the absorption rate W((Qp) of a probe beam of amplitude E ) and frequency cop monitoring the e) —> g) transition, as defined by Mollow [38], is... 

The third chamber is used to detect the metastable atoms two electrodes produce an electric field which quenches the 2S metastable states they are mixed to 2P state which radiates to ground state emitting the Lyman a fluorescence. A photomultiplier measures the L3nnan a fluorescence which is proportional to the number of metastable atoms in the beam. The metastable beam intensity can thus be estimated to be about 10 atoms per second. The detection of the two-photon transition is done by monitoring the population of the metastable level. It uses the fact that when the Rydberg atoms fall down in cascade, about 90 % of them come back to the ground state. So the metastable state is depopulated by the two-photon transition. 

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