Laser Magnetic Resonance and Stark Spectroscopy

A typical experimental arrangement [6.105] is depicted in Fig. 6.37. With specially designed electron-switching circuits, polarity modulation frequencies up to 50 kHz can be realized for gas discharges of 300 V and 3 A [6.106]. The attainable signal-to-noise ratio is illustrated by Fig. 6.38, which shows the band head of a vibrational band of the A n,/2 transition of the 

This technique was first applied to the infrared region where many vibrational-rotational transitions of ions were measured with color-center lasers or diode lasers [6.105,6.109]. Meanwhile, electronic transitions have also been studied with dye lasers [6.110]. 

A modification of this velocity-modulation technique in fast ion beams is discussed in Sect. 9.5. 

In all methods discussed in the Sects. 6.1-6.6, the laser frequency col was tuned across the constant frequencies coik of molecular absorption lines. For molecules with permanent magnetic or electric dipole moments, it is often preferable to tune the absorption lines by means of external magnetic or electric fields across a fixed-frequency laser line. This is particularly advantageous if intense lines of fixed-frequency lasers exist in the spectral region of interest 

Doppler-Limited Absorption and Fluorescence Spectroscopy with Lasers 

Another spectral range of interest is the far infrared, where the rotational lines of polar molecules are found. Here a large number of lines from H2O or D2O lasers (125 pm) and from HCN lasers (330 pm) provide intense sources. The successful development of numerous optically pumped molecular lasers [140] has considerably increased the number of FIR lines. 

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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. 

See also EPR, Methods Fluorescence Microscopy, Applications Fluorescent Molecular Probes Hole Burning Spectroscopy, Methods Laser Magnetic Resonance Laser Applications in Electronic Spectroscopy Laser Spectroscopy Theory Light Sources and Optics Luminescence Theory Near-IR Spectrometers Raman Optical Activity, Applications Symmetry in Spectroscopy, Effects of UV-Visible Absorption and Fluorescence Spectrometers Zeeman and Stark Methods in Spectroscopy, Applications. 

In the absorption method described above, the frequency of laser radiation was tuned in such a way to coincide with the center of the absorption line of the detected particle. If the frequency of laser radiation is unchanged and close to the frequency of the absorption line, to obtain resonance, one can tune the absorption line by the action of the electric or magnetic field on detected particles. The variant of absorption spectroscopy with the electric field is named laser Stark spectroscopy, and the variant using the magnetic field is called laser magnetic resonance. Laser Staik spectroscopy can be applied for the detection of stable molecules. Such paramagnetic par-... 

In this section we have described in considerable detail just one aspect of the spectroscopy of OH, namely, the measurement of zl-doubling frequencies and their nuclear hyperfine structure. This has led us to develop the theory of the fine and hyperfine levels in zero field as well as a brief discussion of the Stark effect. We should note at this point, however, that OH was the first transient gas phase free radical to be studied by pure microwave spectroscopy [121], We will describe these experiments in chapter 10. We note also that magnetic resonance investigations using microwave or far-infrared laser frequencies have also provided much of the most important and accurate information these studies are described in chapter 9, where we are also able to compare OH with the equally important radical, CH, a species which, until very recently, had not been detected and studied by either electric resonance techniques or pure microwave spectroscopy. 

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. 

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. 

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