Spectroscopy in Molecular Beams

For many years molecular beams were mainly employed for scattering experiments. The combination of new spectroscopic methods with molecular beam techniques has brought about a wealth of new information on the structure of atoms and molecules, on details of collision processes, and on fundamentals of quantum optics and the interaction of light with matter. 

There are several aspects of laser spectroscopy performed with molecular beams that have contributed to the success of these combined techniques. First, the spectral resolution of absorption and fluorescence spectra can be increased by using collimated molecular beams with reduced transverse velocity components (Sect. 9.1). Second, the internal cooling of molecules during the adiabatic expansion of supersonic beams compresses their population distribution into the lowest vibrational-rotational levels. This greatly reduces the number of absorbing levels and results in a drastic simplification of the absorption spectrum (Sect. 9.2). 

The low translational temperature achieved in supersonic beams allows the generation and observation of loosely bound van der Waals complexes and clusters (Sect. 9.3). The collision-free conditions in molecular beams after their expansion into a vacuum chamber facilitates saturation of absorbing levels, since no collisions refill a level depleted by optical pumping. This makes Doppler-free saturation spectroscopy feasible even at low cw laser intensities (Sect. 9.4). 

New techniques of high-resolution laser spectroscopy in beams of positive or negative ions have been developed. These techniques are discussed in Sects. 9.5 and 9.6. 

Several examples illustrate the advantages of molecular beams for spectroscopic investigation. The wide, new field of laser spectroscopy of collision processes in crossed molecular beams is discussed in Chap. 13. 

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One important feature of the spectroscopy in molecular beams is nicely illustrated in this example, namely the very high resolution of absorption and emission spectra. This results from the low effective temperatures which can be reached through supersonic expansion. 

For infrared spectroscopy in molecular beams optothermal spectroscopy is a very good choice (Sect. 1.3.3). 

Additionally, several experiments on saturation spectroscopy of molecules and radicals in molecular beams have been reported [454, 455] where finer details of congested moleeular speetra, sueh as hyperfine structure or A-doubling can be resolved. Another alternative is Doppler-free two-photon spectroscopy in molecular beams, where high-lying molecular levels with the same parity as the absorbing ground state levels are aeeessible [456]. 

Laser Spectroscopy in Molecular Beams turbomolecular pump laser beam... 

Laser-RF Double-Resonance Spectroscopy in Molecular Beams... 

The problem of transit-time broadening was recognized many years ago in electric or magnetic resonance spectroscopy in molecular beams [1253]. In these Rabi experiments [1254], the natural linewidth of the radio frequency or microwave transitions is extremely small because the spontaneous transition probability is, according to Vol. 1, (2.22), proportional to co. The spectral widths of the microwave or RF lines are therefore determined mainly by the transit time AT = d/v of molecules with the mean velocity v through the interaction zone in the C field (Fig. 5.10a) with length d. 

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