Level crossing experiments

This method is equally applicable to atoms 26) and to molecules 22). In molecules the Zeeman splitting depends on the quantum number / of the total angular momentum and therefore the fluorescence from a single rotational level (v, f) need be observed. Because of this necessarily selective excitation, these molecular level-crossing experiments can be performed much more easily with lasers than with conventional light sources and have been sucessfully performed with Naj 2 > and NaK 29). 

Fig. 24. Schematic representation of the fluorescence yields in a level-crossing experiment, (a) A Lorentzian yield curve, (b) A dispersion yield curve.

It was pointed out by Bixon, Jortner, and Dothan9 that the treatment presented above of the level-crossing experiment is applicable only to the... 

Population of the CO d A, v =5 level by absorption of Fourth Positive radiation from a CO discharge has been attributed (30) to rotational perturbations between the d A and A" n states. The existence of these perturbations has been demonstrated by Slanger and Black (33) who investigated the emissions from the three sublevels of the d A state to the a II state following absorption by CO of d A -> X E" " radiation from a CO discharge. This work has been extended by these authors (34) to perturbations between A II and e E . Elastic cross-relcixation was demonstrated between A II and d3A and e j ". Perturbations coupling the a 3r+ state to v = 1 of A II have been observed by Burnham et al. (35) in a level-crossing experiment. 

H. Figger, H. Walther, Optical resolution beyond the natural linewidth a level crossing experiment on the 3 Py2 level of sodium using a tunable dye laser. Z. Phys. 267,1 (1974)... 

Calculated assuming gj=. Level-crossing experiment. Resonance scattering. 

Optical Double-Resonance and Level-Crossing Experiments with Laser Excitation... 

Level-crossing experiments with time-resolved detection following pulsed excitation may even allow a spectral resolution within the natural linewidth. If only those fluorescence photons are detected which have been emitted at times t > ax after the excitation process (a l) the spectral profile of the signal is narrowed [10.92]. This technique allows one to reach a spectral resolution beyond the natural linewidth (see Sect.13.5). 

A number of stimulated level-crossing experiments have been performed on the active medium of gas lasers where the gain of the laser transition is changed when sublevels of the upper or lower laser level cross each other. 

The hfs constants of five 4d 5p levels have been determined in level crossing experiments by observing the field dependence of the polarization of the fluorescence radiation in a magnetic field [21 to 23]. The hfs constants are presented in Table 2/9. 

Kelly, R.L (1966) Theory of quantum-beat and level-crossing experiments utilizing electronic excitation. Phys. Rev., 147, 376. 

These experiments are in fact entirely analagous to the Hanle effect or zero-field level-crossing experiments involving excited atoms discussed in Chapter 15. The coherent polarization of the pumping light referred to the quantization axis Oz in Fig.17.12 prepares the atoms in a coherent superposition of ground-state Zeeman sub-levels. The ensemble density matrix now has finite off-diagonal elements... 

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