Nuclear spin effects

Nuclear Spin Effects on Rotation. There is an interesting effect on the rotational partition function, even for the hydrogen molecule, due to nuclear spin statistics. The Fermi postulate mandates that the overall wavefunction (including all sources of spin) be antisymmetric to all two-particle interchanges. A simple molecule like (1H1)2, made of two electrons (S = 1/2) and two protons (spin 7=1/2), will have two kinds of molecule ... 

We have now completed our derivation of the electronic Hamiltonian when external fields and nuclear spin effects are absent. In summary, the Hamiltonian is as follows ... 

In conclusion we summarise the total Hamiltonian (excluding nuclear spin effects), written in a molecule-fixed rotating coordinate system with origin at the nuclear centre of mass, for a diatomic molecule with electron spin quantised in the molecular axis system. We number the terms sequentially, and then describe their physical significance. The Hamiltonian is as follows ... 

One final result is required to analyse the weak field spectrum of CsF. We derived an expression for the matrix elements of the electric field perturbation earlier, without the inclusion of nuclear spin effects, in equation (8.278). We now repeat this derivation using the basis set employed above. Taking the direction of the electric field to define the p = 0 (Z) direction, the results are as follows ... 

The first radiofrequency/optical double resonance studies of molecules were published almost simultaneously. Observations of OH and OD were described by German and Zare [10] late in 1969, and will be discussed in detail in the next subsection. A few months later studies of the CS molecule in its excited A 1 n were reported by Silvers, Bergeman and Klemperer [11], with more detailed results described later by Field and Bergeman [12], We now describe these investigations, which are in some ways simpler than those of OH because of the absence of electron and nuclear spin effects in the CS 1n state. 

More recently, Epov et al. [65] reviewed what is known about mechanisms of fractionation not due to mass-dependent differences in bond vibrational frequencies. These mechanisms include the nuclear field shift effect, but also the nuclear spin effect, which results from interaction between the magnetic field associated with a nucleus with nonzero spin (such as 2 U) and the magnetic fields associated with electron spin angular momentum. They pointed out that so far, no data unambiguously reflect this effect, but it is possible that fractionation of U in the contexts described here results from the nuclear spin effect, rather than the nuclear field shift effect. 

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