Oscillator electric-dipole-allowed

Figure 2.14. The coupling scheme by which the spin-forbidden a 3A2(rc,7t ) <— X Ai(n2) transition borrows oscillator strength from the B 1A1(jt,jt ) <— X iAi(n2) electric dipole allowed transition.

The rotational transition TV = 2 1 is electric dipole allowed, and if the oscillating... 

The rotational transition AT = 2 <- 1 is electric dipole allowed, and if the oscillating electric field is applied in the p direction, the relative intensities of the spin components are obtained by evaluating file expression... 

The electric dipole selection rule for a hannonic oscillator is Av = 1. Because real molecules are not hannonic, transitions with Av > 1 are weakly allowed, with Av = 2 being more allowed than Av = 3 and so on. There are other selection niles for quadnipole and magnetic dipole transitions, but those transitions are six to eight orders of magnitude weaker than electric dipole transitions, and we will therefore not concern ourselves with them. 

However, although f f transitions are, in principle, forbidden by the Laporte parity rule, most of the transitions in (RE) + ions occur at the electric dipole (ED) order. As we have already mentioned, this is an ED allowance due to the admixture of the 4f" states with opposite parity excited states 4f" 5d, as a result of the lack of inversion symmetry (ED forced transitions). The oscillator strength, /, for a / f absorption band can be estimated using expression (5.19). We now rewrite this expression as follows ... 

The characteristic absorption and emission spectra of lanthanide compounds in the visible, near ultra-violet and infra-red is attributed to transitions between 4/ levels due to the fact that they present a sharp line with oscillators strengths typically of the order of 10 . These transitions are electric dipole forbidden but became allowed as forced electric dipole transitions. 

Second, after an appropriate time interval to allow the gas pulse to reach an optimum position between the cavity mirrors, a 1 qs pulse of monochromatic microwave radiation is introduced into the cavity, which is itself tuned to the correct matching resonant frequency. The pulse carries with it a band of frequencies Av 1 MHz, centred at the resonant frequency v of the cavity. The cavity has a bandwidth of approximately 1 MHz, so that the microwave radiation density is high. If the molecular species under investigation has one or more resonant frequencies within this bandwidth, an appreciable macroscopic polarisation is induced, corresponding classically to a phase-coherent oscillation of the molecular electric dipole moments. The microwave pulse must arrive at the correct time interval after the gas pulse. 

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