Laser magnetic resonance

The molecular level Eq with the total angular momentum J splits in an external magnetic field B into (2/ +1) Zeeman components. The sublevel with the magnetic quantum number M shifts from the energy Eq at zero field to 

The sensitivity of this intracavity technique (Sect. 1.2.3) can even be enhanced by modulating the magnetic field, which yields the first derivative of the spectrum (Sect. 1.2.2). When a tunable laser is used it can be tuned to the center vo of a molecular line at zero field = 0. If the magnetic field is now modulated around zero, the phase of the zero-field LMR resonances for AM = +1 transitions is opposite to that for AM = — 1 transitions. The advantages of this zero-field LMR spectroscopy have been proved for the NO molecule by Urban et al. [143] using a spin-flip Raman laser. 

Because of its high sensitivity LMR spectroscopy is an excellent method to detect radicals in very low concentrations and to measure their spectra with high precision. 

1 Doppler-Limited Absorption and Fluorescence Spectroscopy with Lasers 

If a sufficient number of resonances with laser lines can be found, the rotational constant, the fine structure parameters, and the magnetic moments can be determined very accurately. The identification of the spectra and the assignment of the lines are often possible even if the molecular constants are not known beforehand [144]. Most radicals observed in interstellar space by radio astronomy [141a] have been found and measured in the laboratory with LMR spectroscopy [141b]. 

a combination of LMR spectroscopy with a fixed-frequency laser and absorption spectroscopy at zero magnetic field with a tunable laser is helpful for the identification of spectra. 

Instead of inside the laser cavity, the sample can also be placed outside between two crossed polarizers (Fig. 6.40). In a longitudinal magnetic field the plane of polarization of the transmitted light is turned due to the Faraday 

Another spectral range of interest is the far infrared of the rotational lines of polar molecules. Here a large number of lines from H2O or D2O lasers (125/zm) and from HCN lasers (330/im) provide intense sources. The successful development of numerous optically pumped molecular lasers [6.94] has increased the number of FIR lines considerably. 

Laser magnetic resonance spectroscopy, (a) Energy level scheme, (b) experimental arrangement, (c) LMR spectrum of CH(X2n) with several OH-lines in a low pressure oxygenacetylen flame, obtained with the 

The sample is placed inside the laser cavity and the laser output is 

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The OF radical has also been detected by CO2 laser magnetic resonance (30). The O—F bond length is 0.135789 nm. 

Laser magnetic resonance, which has already been used to detect the free hydroxyl, methynyl (CH), hydroperoxy, formyl (HCO), and amino radicals in low-pressure gases and could be used to determine rate constants for the reactions of the smaller free radicals. 

The formation of H02 in high yield, 95 + 15% of the OH reacted, has also been observed using laser magnetic resonance (Lovejoy et al., 1990), indicating the importance of channels such as (85d) and (85e). 

Laser Magnetic Resonance and Electron Paramagnetic Resonance. 

Laser magnetic resonance minutes 2 x 10" L — large magnet... 

A final technique which has been used by Panock et al.24 25 is laser magnetic resonance. They excited He atoms in a 20-140 kG magnetic field by electron impact. They used a line tunable C02 laser to drive atoms in the 7s 1Sq state to the... 

By using lasers of suitable frequency, fluorescence can be extended into the infrared and microwave. Offshoots of laser technology include resonance fluorescence for detecting atoms, and laser magnetic resonance for radicals. 

Experimental Techniques A absorption CIMS = chemical ionization mass spectroscopy CK = competitive kinetics DF discharge flow EPR = electron paramagnetic resonance FP = flash photolysis FT = flow tube FTIR Fourier transform intra-red GC = gas chromatography, UF = laser induced fluorescence LMR = laser magnetic resonance MS = mass spectroscopy PLP = pulsed laser photolysis SC = smog chamber SP = steady (continuous) photolysis UVF = ultraviolet flourescence spectroscopy... 

Oxidation rate constant k, for gas-phase second order rate constants, kOH for reaction with OH radical, kNQ3 with N03 radical and k03 with 03 or as indicated, data at other temperatures see reference kOH < 0.3 x io 15 cm3 molecule-1 s 1 at 296 K (discharge flow-laser magnetic resonance, Howard Enenson 1976b)... 

More recently, laser magnetic resonance (LMR) has been employed in kinetic studies to monitor concentrations of radicals such as OH [107]... 

Nitric oxide, NO, is a chemically stable molecule and not surprisingly has been studied extensively by a range of techniques. Its microwave and far-infrared laser magnetic resonance spectra are discussed in chapter 9. These involve an understanding of both the zero-field levels and also the interactions with an external magnetic field. The pure microwave and millimetre wave spectra are described in chapter 10, but they provide information, which we will use, relevant to the radiofrequency electric resonance spectrum described in this section. 

The hydroxyl radical, OH, occupies an extremely important position in spectroscopy, in free radical laboratory chemistry, and in atmospheric, cometary and interstellar chemistry. Its ultraviolet electronic spectrum has been described in many papers published over the past seventy years. It was the first short lived gaseous free radical to be studied by microwave spectroscopy, described in a classic paper by Dousmanis, Sanders and Townes [121] in 1955. The details of this work are presented in chapter 10. It was the first free radical to be studied by microwave magnetic resonance, in pioneering work by Radford [141] the microwave and far-infrared laser magnetic resonance studies are... 

The a3 n state of CO was first identified through its ultraviolet emission spectrum to the ground state, producing what are now known as the Cameron bands [160, 161, 162], Its radioffequency spectrum was then described by Klemperer and his colleagues in a classic series of molecular beam electric resonance experiments. Its microwave rotational spectrum was measured by Saykally, Dixon, Anderson, Szanto and Woods [163], and the far-infrared laser magnetic resonance spectrum was recorded by Saykally, Evenson, Comben and Brown [164], In the infrared region both electronic... 

Figure 9.4. Schematic diagram of a far-infrared laser magnetic resonance spectrometer, designed by Evenson [5] and constructed at N.B.S. Boulder.

Since the laser magnetic resonance experiment relies on a chance near-coincidence between a laser line and a molecular transition frequency, and the range over which spectroscopic transitions can be magnetically tuned is often quite small, it is desirable to have a large number of FIR laser lines available. This is now seldom a major problem, and table 9.1 lists a restricted sample of FIR laser lines that have been used for magnetic resonance studies. It is, of course, necessary to be able to measure the FIR frequency accurately and this is accomplished in Evenson s laboratory by measuring the beat... 

The initial FIR laser magnetic resonance studies [6] were performed using a sample cell which was external to the laser cavity. All experiments now use an intracavity arrangement as shown in figure 9.4 which is estimated to be 103 times more sensitive than the extracavity arrangement. It is the very high sensitivity which continues to make... 

Figure 9.10. Laser magnetic resonance spectrum of HBr+, associated with the J = 7/2 — 5/2 transition and recorded using the 180.7 pm line of CD3OH, with a polarisation (AMj = 1) [30].

Far-infrared laser magnetic resonance of CH in theX2n ground state... 

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