Transition intensities space selection

Muns ENDOR mvolves observation of the stimulated echo intensity as a fimction of the frequency of an RE Ti-pulse applied between tlie second and third MW pulse. In contrast to the Davies ENDOR experiment, the Mims-ENDOR sequence does not require selective MW pulses. For a detailed description of the polarization transfer in a Mims-type experiment the reader is referred to the literature [43]. Just as with three-pulse ESEEM, blind spots can occur in ENDOR spectra measured using Muns method. To avoid the possibility of missing lines it is therefore essential to repeat the experiment with different values of the pulse spacing Detection of the echo intensity as a fimction of the RE frequency and x yields a real two-dimensional experiment. An FT of the x-domain will yield cross-peaks in the 2D-FT-ENDOR spectrum which correlate different ENDOR transitions belonging to the same nucleus. One advantage of Mims ENDOR over Davies ENDOR is its larger echo intensity because more spins due to the nonselective excitation are involved in the fomiation of the echo. 

The wide variety of interstellar molecules detected so far in our Galaxy (see Table 6) are composed of the most abundant chemically reactive elements, i.e. H, C, N, 0, Si and S. The selection of detected molecules is influenced by molecular and observational considerations i) the molecules must be polar ii) they must have sufficient vapor pressure for their laboratory spectra to be known, iii) of the known spectra, only the most intense transitions can be expected to be observable in interstellar space, and iv) the frequencies of these transitions have to be located within the Earth s atmospheric windows Only molecules which satisfy these conditions are amenable to radio techniques. 

A harmonic oscillator potential with vibrational levels is illustrated in figure 6.22. The levels are equally spaced and, as we show later, the main vibrational transitions occur between adjacent levels, with Au = l. The v = 1 0 transition is called the fundamental and occurs at the harmonic tfequency v. However transitions which obey the selection rule Au = 2, known as overtone transitions, also have finite intensity and are often observed. 

We should not leave this discussion of the intensity of rotational transitions without some mention of the parity selection rule. Electric dipole transitions involve the interaction between the oscillating electric field and the oscillating electric dipole moment of the molecule. The latter is represented in quantum mechanics by the transition moment fjLx b,a) given in equation (6.300). For this transition moment to be non-zero, the integrand ijry i ust be totally symmetric with respect to all appropriate symmetry operations, which includes the space-fixed inversion operator E. Now the electric dipole moment operator,... 

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