Level crossing signals

Level-crossing spectroscopy with lasers has some definite experimental advantages. Compared with other Doppler-free techniques it demands a relatively simple experimental arrangement. Neither single-mode lasers and frequency-stabilization techniques nor collimated molecular beams are required. The experiments can be performed in simple vapor cells, and the experimental expenditure is modest. In many cases no monochromator is needed since sufficient selectivity in the excitation process can be achieved to avoid simultaneous excitation of different molecular levels with a resulting overlap of several level-crossing signals. 

The level-crossing signal may only be a few percent of the field-independent background intensity. In order to improve the signal-to-noise ratio, either the field is modulated or the polarizer in front of the detector is made to rotate and the signal is recovered by lock-in detection. 

There is one important point to note. The width A5i/2 of the level-crossing signal reflects the average width y = y2) of the two crossing levels. If these lev-... 

Fig. 9.80 Comparison of calculated (a) and measured (b) line profiles of level-crossing signals under observation with different gate-delay times (in ns). The different curves have been renormalized to equal centered peak intensities [1306]...

H.H. Stroke, G. Fulop, S. Klepner, Level crossing signal line shapes and ordering of energy levels. Phys. Rev. Lett. 21, 61 (1968)... 

Level-crossing signals at certain magnetic fields can be used for determining hyperfine structure constants of excited states. Why is the number of detectable signals quite limited Describe in general terms how hyperfine-structure information can be deduced from the experimental data. 

J. Larsson, L. Sturesson, S. Svanberg Manipulation of level-crossing signals using narrow-band or pulsed laser excitation. Phys. Scr. 40,165 (1989)... 

Instead of magnetic fields electric fields can also be used to achieve crossings of Stark sublevels. From the width of the electric level-crossing signal DALBY et al. [10.87] derived the anisotropic polarizability of the iodine molecule which had been excited by an argon laser into a (v, J ) level of the B Hq state. 

Although saturation effects may influence the line shape of the level-crossing signal, for small saturation it may still be essentially Lorentzian. Stimulated level-crossing spectroscopy has been used to measure Lande factors of atomic laser levels with high precision. One example is the determination of gi P ) = 1.3005 0.1% in neon by HERMANN et al. [10.97]. 

The method of line narrowing by selective detection of spontaneous photons from those excited atoms which have survived for times t T x has been used to increase the spectral resolution of Doppler-free level-crossing signals. With T = lOx a reduction of the natural linewidth up to a factor of 6... 

Fig.15.3. Hanle effect or zero-field level-crossing signals...

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