Level-Crossing Spectroscopy with Lasers

In this section we briefly summarize the fundamentals of level-crossing spectroscopy and illustrate its relevance for the investigation of angular momentum coupling schemes in excited molecular states. A more detailed presentation of the theory can be found in [8.53] and a survey on the prelaser work in [8.52]. 

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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. 

Experimental Realization and Examples of Level-Crossing Spectroscopy with Lasers... 

A large number of atoms and molecules have been meanwhile investigated by level-crossing spectroscopy using laser excitation. Because of the available high laser intensity highly excited states can be studied, which have been populated by stepwise excitation (see Sect.8.8). Often resonance lamps are used to excite the first resonance level and a dye laser pumps the next step. Many experiments have been performed with two different dye lasers, either in a pulsed or a cw mode [10.91]. These techniques allow measurement of... 

Level-crossing spectroscopy was used in atomic physics even before the invention of lasers [831, 842-844]. These investigations were, however, restricted to atomic resonance transitions that could be excited with intense hollow-cathode or microwave atomic-resonance lamps. Only a very few molecules have been studied, where accidental coincidences between atomic resonance lines and molecular transitions were utilized [836]. 

Optical pumping with tunable lasers or even with one of the various lines of fixed-frequency lasers has largely increased the application possibilities of level-crossing spectroscopy to the investigation of molecules and complex atoms. Because of the... 

There are, of course, also some disadvantages. One major problem is the change of the absorption profile with the magnetic field. The laser bandwidth must be sufficiently large in order to assure that all Zeeman components can absorb the radiation independent of the field strength B. On the other hand, the laser bandwidth should not be too large, to avoid simultaneous excitation of different, closely-spaced transitions. This problem arises particularly in molecular level-crossing spectroscopy, where several molecular lines often overlap within their Doppler widths. In such... 

These coherence effects allow Doppler-free spectroscopy of ground states and excited states in atoms or molecules. While level crossing spectroscopy can be performed with both cw and pulsed lasers, the quantum beat technique... 

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]. 

Figure 1.50 illustrates the obtainable sensitivity by a AM = 0 Stark spectrum of the ammonia isotope NH2D composed of measurements with several laser lines [146]. An electric resonance signal is observed at every crossing point of the sloped energy levels with a fixed laser frequency. Since the absolute frequency of many laser lines was measured accurately within 20 0 kHz (Sect. 9.7), the absolute frequency of the Stark components at resonance with the laser line can be measured with the same accuracy. The total accuracy in the determination of the molecular parameters is therefore mainly limited by the accuracy of 10 for the electric field measurements. To date numerous molecules have been measured with laser Stark spectroscopy [146-149]. The number of molecules accessible to this technique can be vastly enlarged if tunable lasers in the relevant spectral regions... 

Either two or more molecular levels of a molecule are excited coherently by a spectrally broad, short laser pulse (level-crossing and quantum-beat spectroscopy) or a whole ensemble of many atoms or molecules is coherently excited simultaneously into identical levels (photon-echo spectroscopy). This coherent excitation alters the spatial distribution or the time dependence of the total, emitted, or absorbed radiation amplitude, when compared with incoherent excitation. Whereas methods of incoherent spectroscopy measure only the total intensity, which is proportional to the population density and therefore to the square ir of the wave function iff, the coherent techniques, on the other hand, yield additional information on the amplitudes and phases of ir. 

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