Far-infrared laser magnetic resonance

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 

There are no nnclear spin magnetic moments in the predominant isotopic form of SO (i.e. so that hyperfine interactions are absent. The orbital angular momen- 

In a theoretical analysis of the Zeeman effect for a diatomic molecule in a singlet state, Brown and Uehara [10] show that the effective Hamiltonian may be written. 

In these equations gi is the orbital g-factor corrected for quantum electrodynamic, relativistic and diamagnetic effects [11], Agi is a small correction to the orbital g factor arising from non-adiabatic mixing of excited electronic states, whilst and gf are the nuclear and electronic contributions to the rotational g-factor, gr. 

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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.

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

These, then, are the reasons why magnetic resonance methods, microwave or far-infrared laser, have had limited success with 2A diatomic radicals. Similar considerations apply to nonlinear polyatomic radicals in doublet states success in far-infrared laser magnetic resonance depends upon the magnitude of the spin-rotation coupling, and the size of the energy mismatch between the transition frequency and the laser frequency, since the mismatch has to be magnetically tuned. This becomes less of a limitation as more laser frequencies become available, except that one then needs to know in advance which laser frequency to choose. It becomes part of the search problem ... 

Several molecules with 3 A. ground states have been studied by both microwave and far-infrared laser magnetic resonance they include O2, SO and SeO. In O2 the observed transitions are necessarily magnetic dipole, and they are frequently used to calibrate the sensitivity of a FIR laser magnetic resonance spectrometer. The other species have electric dipole transitions, and we shall illustrate the situation by describing the studies of SO carried out by Carrington, Levy and Miller [56], SO was also one of the first free radicals to be studied by pure microwave methods, which we will describe in chapter 10. The analysis of the magnetic resonance spectrum actually made use of the parameters determined earlier by pure microwave studies. SO is an easy radical to study experimentally since it is relatively unreactive and has a lifetime of several... 

Figure 9.28. Zeeman levels for N = 0 and 1 of NH 3E (v = 0) and the observed far-infrared laser magnetic resonance transitions [58]. These were recorded using four different FIR lines at 31.7615, 32.1466, 33.0822 and 33.1922 cm-1.

The CH radical is the simplest hydrocarbon and its rotational or /I-doublet spectrum has been sought by many. The first detection of rotational transitions was a triumph for far-infrared laser magnetic resonance the experiments carried out by Evenson, Radford and Moran [173] were described in detail in chapter 9. The A-doublet transition in the lowest rotational level was first observed through radioastronomy by Rydbeck, Ellder, Irvine, Sume and Hjalmarson [174]. It was almost a further ten years before laboratory observations of the field-free spectrum were reported. 

Figure 10.68. The low-energy rotational levels of CH, with the size of the Tl-doubling exaggerated for clarity. Transitions marked with an asterisk have been observed by far-infrared laser magnetic resonance. In addition many A J = 0, Tl-doublet transitions have been observed field-free, as listed in table 10.17.

Table 10.18. Molecular parameters (in MHz) for CH in thev = 0 level of the X2Il ground state, determined from a combination of the far-infrared laser magnetic resonance, field-free microwave measurements [181], andfield-free far-infrared measurements [185]...

The far-infrared laser magnetic resonance spectrum of NiH has been studied by Nelis, Beaton, Evenson and Brown [75] and was discussed in detail in chapter 9. The electronic... 

In our discussion of the far-infrared laser magnetic resonance spectrum of NiH in chapter 9, a fairly general effective Hamiltonian was presented. This Hamiltonian included terms which would produce A-doubling in a A state, an unusual situation because one requires electronic orbital angular momentum operators to connect A = + 2 and A = - 2 components [77], The effective Hamiltonian used to analyse the mi-crowave/optical double resonance spectrum of NiH was as follows ... 

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