Laser Zeeman spectroscopy

Finally, there is a way to use fixed frequency lasers for spectroscopy if one can achieve the tuning on the side of the molecules. Species with a permanent magnetic or electric dipole moment can be tuned into resonance by the Zeeman - or Stark-effect respectively. Tunability is very limited and therefore a densely distributed series of fixed frequency laser transitions is necessary for complete coverage of the spectrum. 

This chapter is concerned with the following techniques in molecular laser spectroscopy (i) laser-Stark spectroscopy and electric field spectroscopy (ii) laser-Zeeman, or laser-magnetic-resonance spectroscopy (LMR) (iii) dispersed laser-induced fluorescence and (iv) double resonance spectroscopy. 

Laser-microwave spectroscopy based on nonlinear phenomena developed from the type of experiments on molecules already discussed in Section 3.2 which make use of optical pumping or double resonance. Occasionally, the laser and the rf power were high enough to create the nonlinear phenomena mentioned above, i.e., to saturate the transitions involved and/or to induce multiphoton transitions. The intermediate level in, e.g., two-photon transitions did not have to be a real state but could be virtual as well. Therefore, a drawback often encountered in earlier infared laser-microwave experiments could be avoided if the laser transition frequency did not exactly coincide with the molecular absorption line the Stark or Zeeman effect had to be used for tuning. This results in an undesired line splitting. With laser-microwave multiphoton processes, however, the laser can be operated at its inherent transition frequency. Exact resonance with molecular lines is then achieved by using a nonlinear effect, i.e., a radiofrequency quantum is added to or subtracted from the laser frequency (see Figure 28). 

There are several recent experimental studies on the CeO diatomic molecule. Schall et al. (1986) have studied CeO using the sub-doppler Zeeman spectroscopy. Again, the ligand-field model is so successful in explaining the observed spectra due to the ionic nature of the diatomic lanthanide oxide. Linton et al. (1979, 1981, I983a,b) as well as Linton and Dulick (1981) have studied the electronic spectrum of CeO using absorption, emission as well as laser spectroscopic method. There are many 0-0 bands for... 

A demonstration of the efficacy of MBER spectroscopy is the recent experiments on HF carried out by Bass, DeLeon, and Muenter [14]. In an effort to obtain Stark, Zeeman, and hyperfine properties, measurements were made that gave accurate values for both the ground and first excited vibrational levels of HF. Conventional resonance experiments can be done if the D = 1 state can be sufficiently populated. Using a color center IR laser to excite HF to u = 1, J = 1 levels, all the properties measured for the u = 0 and V = 1 states had essentially identical precision. The results included dipole moments, magnetic shielding anisotropies, rotational magnetic moments, magnetic susceptibilities, transition moments, and first and second derivatives with respect to internuclear separation of the properties. 

The earliest pulsed laser quantum beat experiments were performed with nanosecond pulses (Haroche, et al., 1973 Wallenstein, et al., 1974 see review by Hack and Huber, 1991). Since the coherence width of a temporally smooth Gaussian 5 ns pulse is only 0.003 cm-1, (121/s <-> 121 cm"1 for a Gaussian pulse) nanosecond quantum beat experiments could only be used to measure very small level splittings [e.g. Stark (Vaccaro, et al., 1989) and Zeeman effects (Dupre, et al., 1991), hyperfine, and extremely weak perturbations between accidentally near degenerate levels (Abramson, et al., 1982 Wallenstein, et al., 1974)]. The advent of sub-picosecond lasers has expanded profoundly the scope of quantum beat spectroscopy. In fact, most pump/probe wavepacket dynamics experiments are actually quantum beat experiments cloaked in a different, more pictorial, interpretive framework,... 

Tunable diode laser spectroscopy has been employed in order to observe the Zeeman effect in the i.r. absorption of molecules with no electromagnetic moment, due to differences between the excited- and ground-state g-factors. Doppler-limited resolution was obtained for and CHjDI in the region 820—... 

The medium infrared spectral region contains typically vibrational transitions of molecules and their rotational substructure. Therefore it is obvious, that one can use vibration rotation transitions in a laser medium itself, provided there is an inversion mechanism available. However, in the gas phase such transitions are fairly narrow and therefore will not be the ideal source for spectroscopy, where one would like to have a continuously tunable laser source in order to scan across a series of vibration-rotation transitions of the molecular gas to be investigated. Although we can make use of it for very special situations e.g.for the spectroscopy of paramagnetic molecules, where Zeeman-tuning of the molecular transition can be achieved, we must use other types of gain media for a tunable infrared laser. 

We have already discussed the use of electric field modulation as a means of providing the selective detection of those molecular absorption lines having the strongest Stark effects, using tunable lasers as sources ( 2.4). As in microwave spectroscopy, where field modulation is routine, magnetic fields may also be used for this purpose, as was demonstrated by Urban and Herrmann (1978). The spectrum of NO was recorded with extremely high sensitivity using Zeeman modulation with a spin-flip Raman tunable infrared laser. 

Even this double-resonance spectroscopy has already been applied to the study of atomic transitions before lasers were available. In these pre-laser experiments incoherent atomic resonance lamps served as pump sources and a radio frequency field provided probe transitions between Zeeman levels of optically excited atomic states [509]. However, with tunable lasers as pump sources, these techniques are no longer restricted to some special favorable cases, and the achievable signal-to-noise ratio of the double-resonance signals may be increased by several orders of magnitude [510]. 

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... 

W. Urban, W. Herrmann, Zeeman modulation spectroscopy with spin-flip Raman laser. Appl. Phys. 17, 325 (1978)... 

G. Herzberg, Molecular Spectra and Molecular Structure (van Nostrand, New York, 1950) N. Ochi, H. Watanabe, S. Tsuchiya, RotationaUy resolved laser-induced fluorescence and Zeeman quantum beat spectroscopy of the state of jet-cooled CS2. Chem. Phys. 113,... 

The deaths of both Racah and Dieke in 1965, immediately prior to the Zeeman Centennial Conference in Amsterdam, were severe blows to free-ion and crystal spectroscopy. The pall cast on the conference itself can well be imagined. But 1965 saw the continued development of two devices that have revolutionized optical spectroscopy computers and lasers. The technical details of the working of either device have no place in a review article devoted to atomic theory, but this omission should not be interpreted as a lack of appreciation for their importance. The roles they have played are implicit in much of what follows. 

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