Molecular beam Doppler spectroscopy

Beam Spectroscopy. Both specificity and sensitivity can be gready enhanced by suppressing coUisional and Doppler broadening. This is accompHshed in supersonic atomic and molecular beams (296) by probing the beam transversely to its direction of dow in a near-coUisionless regime. 

Landolt-Bornstein Numerical Data and Functional Relationships in Science and Technology, New Series Madelung, O., Ed.-in-Chief Group II Atomic and Molecular Physics Vol. 19 Molecular Constants Mostly from Microwave, Molecular Beam, and Sub-Doppler Laser Spectroscopy Subvol. a, b, and c Hiittner, W Ed. Springer Berlin, 1992. 

This chapter is concerned with experimental investigations of the dynamics of the dissociation of polyatomic neutral molecules carried out by the technique of laser Doppler spectroscopy, in bulk and under crossed-beam condition. Photodissociation is a basic process in the interaction of light with molecules, of interest in itself as an elementary molecular process and also with respect to a variety of applications in different fields. The interest has increased considerably in recent years, first, because the experimental investigation of photodissociation is rapidly advancing by the use of the laser, and second, because the laser makes possible to achieve photodissociation, state, and isotope selectively, by new excitation mechanisms. These are, aside from the common one-photon absorption, stepwise... 

Mostly from Microwave, Molecular Beam, and Sub-Doppler Laser Spectroscopy... 

This is subvolume D (appearing in three parts, Dl, D2, and D3) of the Landolt-Bomstein Volume 11/29 Molecular Constants Mostly from Microwave, Molecular Beam, and Sub-Doppler Laser Spectroscopy , which is planned to appear as a series A, B, C, Dl, D2, D3 for the diamagnetic, and E for the paramagnetic diatomic and paramagnetic polyatomic species, respectively. 

Along with the other sub-Doppler techniques, it has the advantage of high spectral resolution, which is mainly limited by the residual Doppler width due to the finite angle between the pump beam and the probe beam. This limitation corresponds to that imposed to linear spectroscopy in collimated molecular beams by... 

The low translational temperature achieved in supersonic beams allows the generation and observation of loosely bound van der Waals complexes and clusters (Sect. 4.3). 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. 4.4). 

Fig. 4.1 Laser excitation spectroscopy with reduced Doppler width in a collimated molecular beam (a) schematic experimental arrangement (b) collimation ratio (c) density profile n x) in a collimated beam effusing from a point source A...

A typical laser spectrometer for sub-Doppler excitation spectroscopy in a collimated molecular beam is shown in Fig. 4.2. The laser wavelength Xl is controlled by a computer, which also records the laser-induced fluorescence /fK l). Spectral regions in the UV can be covered by frequency-doubling the visible laser frequency... 

Fig. 4.5 Experimental setup for sub-Doppler spectroscopy in a collimated molecular beam. Photomultiplier PMl monitors the total undispersed fluorescence, while PM2 behind a monochromator measures the dispersed fluorescence spectrum. The mass-specific absorption can be monitored by resonant two-color two-photon ionization in the ion source of a mass spectrometer...

For molecules the line densities are much higher, and often the rotational structure can only be resolved by sub-Doppler spectroscopy. Limiting the collimation angle of the molecular beam below 2 x 10 rad, the residual Doppler width can be reduced to values below 500 kHz. Such high-resolution spectra with linewidths of less than 150 kHz could be, for instance, achieved in a molecular iodine beam since the residual Doppler width of the heavy I2 molecules, which is proportional to is already below this value for a collimation ratio 6 < 4 x 10 [400]. At... 

More examples of sub-Doppler spectroscopy in atomic or molecular beams can be found in reviews on this field by Jacquinot [396], and Lange et al. [401], in the two-volume edition on molecular beams by Scoles [402], as well as in [403 06],... 

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]. 

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. 

Ezekiel and coworkers [923] used the reduction of the Doppler width in a collimated molecular beam (Sect. 4.1) for accurate heterodyne spectroscopy. The beams of two argon lasers intersect the collimated beam of I2 molecules perpendicularly. The laser-induced fluorescence is utilized to stabilize the laser onto the centers of two hfs components of a visible rotational transition. The difference frequency of the two lasers then yields the hfs splittings. 

T.E. Gough, R.E. Miller, G. Scoles, Sub-Doppler resolution infrared molecular beam spectroscopy. Faraday Disc. 71,6 (1981)... 

J.R. Bonilla, W. Demtroder, Level crossing spectroscopy of NO2 using Doppler-reduced laser excitation in molecular beams. Chem. Phys. Lett. 53, 223 (1978)... 

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