Doppler tuning method

Perhaps even more than other fields of application in laser spectroscopy, ion physics is likely to benefit rapidly from progress in the U.V. range. Frequency doubled CW gas and dye lasers are now able to provide on a routine basis the few milliwatts of monomode laser power which have been necessary for the N20" photodissociation experiment. Further in the U.V., the Doppler tuning method could take advantage with much benefit of the high spectral brightness of injected excimer amplifiers operated at high repetition rate. 

An alternative method for overcoming the Doppler tuning problem in laser cooling has been demonstrated by Ertmer et al. 52] Instead of using a spatially varying magnetic field these authors controlled the frequency of the laser by means of an electro-optic modulator, driven by a radiofrequency source. Varying the frequency of the r.f. field applied produces sidebands to the laser frequency and these can be swept in step with the Doppler shift. 

Another strong case is in the nucleus of the spiral NGC 4258 where a number of H2O masers are observed. The radial (Doppler) velocities can be fit by rotation of a circumnuclear disk of pc scale with a Keplerian rotation velocity profile reaching up to 1080 km s-1. This means there is a point mass at the center to the tune of 4 x 107 M . Once again we would be hard put to squeeze this massive a cluster of brown dwarfs or collapsed objects inside the disk s inner radius at 0.13 pc [18]. It must thus be a BH. The Seyfert nuclei of NGC 1068 and the Scd NGC 4945 also have (more modest) SMBHs discovered in them by the maser method. 

For the second method, where the laser frequency col is kept constant, the atomic absorption frequency must be altered during the deceleration of the atoms. This can be realized by Zeeman tuning (Fig. 9.7). In order to match the Zeeman shift to the changing Doppler shift Aco(z), the longitudinal magnetic field must have the z-dependence... 

A third method, which comes more and more into use, is based on various optical double resonance techniques and requires generally at least two tunable lasers. The essential point of this method is the labelling of a selected molecular level by optical pumping with a "pump laser". A second "probe laser" is tuned through the spectral range of interest, but only those molecular transitions are monitored, which start from the "labelled" level, selected by the pump transition. The method works with pulsed and with cw lasers. Combined with one of the sub-Doppler techniques a drastic reduction of the line density as well as a reduction of the line width can be achieved. This makes double resonance techniques very attractive. 

Measuring Doppler widths of rotational lines by laser-probe techniques gives velocity distributions in just the same way as measuring Doppler widths of atomic lines by conventional means. In this method a laser beam with a very narrow band width is tuned over the spectral line to determine the profile of the Doppler broadened line. The line shape can be interpreted to give the average velocity of the product. As yet, this method has been applied only to rotational energy transfer studies however, with the availability of mode-locked lasers providing narrow band widths, this procedure may become more widely used. 

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