Translational Stark effect

The first term of (3.289) represents a translational Stark effect. A molecule with a permanent dipole moment experiences a moving magnetic field as an electric field and hence shows an interaction the term could equally well be interpreted as a Zeeman effect. The second term represents the nuclear rotation and vibration Zeeman interactions we shall deal with this more fully below. The fourth term gives the interaction of the field with the orbital motion of the electrons and its small polarisation correction. The other terms are probably not important but are retained to preserve the gauge invariance of the Hamiltonian. For an ionic species (q 0) we have the additional translational term... 

At this point we have to note a restriction. The above selection rules are only vahd as long as the translational Stark-effect due to the Lorentz forces may be neglected which is not always true for symmetric top molecules as is discussed in detail later in this Chapter. 

In order to show that the translational Zeeman effect is negligible, we take an average translational velocity, Fq =yA TjM, which at T = — 60 °C leads to Fo=201 m/sec for ethylene oxide. At 25 kGauss this corresponds to a cross field Fts of 5 V/cm. With b = l-88 Debye for ethylene oxide, tlus leads to an estimate of the Stark effect energy, < ts > = lyMb TsI =4.7 MHz. We now take the most critical case, the Stark effect perturbation of the loi and lio rotational levels respectively, where the rotational energy difference in the denominator is smallest (11.4 GHz), and we get a second order perturbation in the order of 2 kHz which may come into the range of the experimental accuracy of... 

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