Nonlinear Spectroscopy in Molecular Beams

They can be studied by laser spectroscopy, thus giving access to states that are difficult to produce in normal gas phase spectroscopy [9.57]. 

Van der Waals diatomics formed through the weak interaction between a noble gas atom and an alkali atom have been studied by noble gas beams seeded with alkali vapor [9.58]. 

The residual Doppler width from the finite collimation ratio 6 of the molecular beam can be completely eliminated when nonlinear Doppler-free techniques are applied. Since collisions can generally be neglected at the crossing point of the molecular and laser beam, the lower molecular level /) depleted by absorption of laser photons can be only refilled by diffusion of new, unpumped molecules into the interaction zone and by the small fraction of the fluorescence terminating on the initial level /). The saturation intensity / is therefore lower in molecular beams than in gas cells (Example 7.2). 

A possible arrangement for saturation spectroscopy in a molecular beam is depicted in Fig. 9.18. The laser beam crosses the molecular beam perpendicularly and is reflected by the mirror Ml. The incident and the reflected beam can only be absorbed by the same molecules within the transverse velocity group = 0ibyA if the laser frequency (o = o)o y matches the molecular absorption frequency coo within the homogeneous linewidth y. When tuning the laser frequency col one observes narrow Lamb dips (Fig. 9.19) with a saturation-broadened width y at the center of broader profiles with a reduced Doppler-width cAcod, from the collimation ratio c 1 of the molecular beam (Sect. 9.1). 

It is essential that the two laser beams cross the molecular beam exactly perpendicularly otherwise, the opposite Doppler shifts observed for the 

This technique provides access to the spectroscopy of high-spin states which are not readily formed in the gas phase, and furthermore it allows these clusters with the helium droplets to be investigated by observing their level shifts. 

A possible arrangement for saturation spectroscopy in a molecular beam is depicted in Fig. 4.24. The laser beam crosses the molecular beam perpendicularly and is reflected by the mirror Ml. The incident and the reflected beam can only be ab- 

It is essential that the two laser beams eross the molecular beam exactly perpendicularly otherwise, the opposite Doppler shifts Scod observed for the two beams result in a broadening of the Lamb dips for 2So d y, while no Lamb dips can be observed for 25 ud V  

If the width Tfg of the Lamb dips is very narrow, the demands for exactly perpendicular crossing becomes very stringent. In this case an arrangement is experimentally more convenient, where the mirror Ml is removed and replaced by the retroreflectors M2, M3. The two laser beams intersect the molecular beam at two closely spaced locations and Z2. This arrangement eliminates the reflection of the laser beam back into the laser. 

The Doppler-broadened background with the residual Doppler width due to the divergence of the molecular beam can completely be eliminated by chopping the two laser beams at two different frequencies fj, 2, and monitoring the signal at the sum frequency fj+f2 (intermodulated fluorescence, Sect.7.3.4). This is demonstrated by the insert in Fig.9.19. The linewidth of the Lamb dips in Fig.9.19 is below 1 MHz and mainly limited by frequency fluctuations of the CW single-mode dye laser [9.49]. 

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PM Felker, BF Henson, VA Venturo, GV Hartland. Applications of nonlinear Raman spectroscopy to molecular beam studies. In W Kiefer, M Cardona, G Schaack, FW Schneider, HW Schrotter, eds. Proceedings of the Xlllth International Conference on Raman Spectroscopy. Chichester Wiley, 1992, pp 230-231. 

The recent femtosecond experimental data on electron solvation in water (Gaudel et al., 1984, 1987) and observations in a mass spectroscopy on the formation of electrons stabilized in molecular beam clusters (Arbruster et al., 1984) has rekindled extensive interest on the microscopic details of the dynamics and structure of e in particular. Since the appearance of the visible spectrum of e has now been observed from the two photon photoionization of pure water (e.g., no dopant molecules or ions were present) we must focus on what responses can be Induced from the medium on this timescale. From our present databank, it is evident that following the instantaneous electronic polarizability (linear and nonlinear) response to the moving charge and/or field, it is the librational responses that must be the key motion. We assume, for the moment, that the lifetime of the autoionizing level in H2O is not a significant factor. As we have discussed elsewhere (Kenney-Wallace,... 

Dr. Rohlfing s research interests include the experimental characterization of transient molecules relevant to combustion processes, linear and nonlinear laser spectroscopies, trace detection of pollutants, molecular beam and mass spectrometric studies of carbon and metal clusters, and vibrational relaxation dynamics. He is the author of approximately 50 peer-reviewed articles, holds membership in the American Chemical Society and the American Physical Society, and is a fellow of the American Association for the Advancement of Science. 

Various techniques of sensitive absorption spectroscopy, including nonlinear techniques, which allow a spectral resolution below the Doppler width are described first. These techniques are termed sub-Doppler-spectroscopy and include linear spectroscopy in collimated molecular beams, nonlinear saturation and polarization spectroscopy, and Doppler-free two-photon spectroscopy. Emission spectroscopy, which covers laser-induced fluorescence as well as stimulated emission methods, is described next. The assignment of complex molecular... 

Sum-frequency generation (SFG) at second-order and the nonlinear Raman spectroscopy BioCARS at fourth-order can also probe chiral molecules. They have no analog in linear optics. We show that both are only symmetry allowed in a fluid, if the fluid is chiral. However, in contrast to optical activity phenomena, these processes arise entirely from induced electric-dipoles (without magnetic or quadrupolar transitions) and they are not circular differential. All laser beams can be linearly polarized and no polarization modulation is required as the detection of a sum-frequency (yiz. five-wave mixing) photon is in itself a measure of the solution s chirality. Since an achiral solvent can not contribute to the signal, these techniques are sensitive, background-free probes of molecular chirality. The SFG... 

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