Photonic excitation rules

A further technique exists for the determination of triplet energy levels. This technique, called electron impact spectroscopy, involves the use of inelastic scattering of low-energy electrons by collision with molecules. The inelastic collisions of the electrons with the molecules result in transfer of the electron energy to the molecule and the consequent excitation of the latter. Unlike electronic excitation by photons, excitation by electron impact is subject to no spin selection rule. Thus transitions that are spin and/or orbitally forbidden for photon excitation are totally allowed for electron impact excitation. 

Spectroscopic techniques look at the way photons of light are absorbed quantum mechanically. X-ray photons excite inner-shell electrons, ultra-violet and visible-light photons excite outer-shell (valence) electrons. Infrared photons are less energetic, and induce bond vibrations. Microwaves are less energetic still, and induce molecular rotation. Spectroscopic selection rules are analysed from within the context of optical transitions, including charge-transfer interactions The absorbed photon may be subsequently emitted through one of several different pathways, such as fluorescence or phosphorescence. Other photon emission processes, such as incandescence, are also discussed. 

Thus it was not observed until lasers were invented. In principal, one-photon and two-photon excitation follow different selection rules. For example, the inner shell one-photon transitions in transition metal, rare earth, and actinide ions are formally forbidden by the parity selection rule. These ions have d- or/-shells and transitions within them are either even to even (d d) or odd to odd (f /). The electric dipole transition operator is equal to zero. 

The two-photon excitation spectrum of 22h taken as a neat film shows a broad peak with an onset of 1.32 eV and a maximum at 1.63 eV [391]. The larger TP excitation energy (1.32 eV 2hco = 2.65 eV) compared to OP excitation (2.34 eV) implies that the TP transition has a different selection rule than the OP transition. This is similar to behavior obtained in solution [384], A similar excitation pattern as disclosed in Figure 3.2a was obtained for such polymeric materials that is, the TP excited state possesses a higher excitation energy in comparison with the lowest OP excited state. Thus, the results obtained for diphenylpolyenes with more than two ethylene moieties, in which the Sj state is assigned to the TP excited state, are considered as an exception for chromo-phores with no donor substitution pattern [29]. 

In the cooperative case, the two molecular transitions are separately allowed under well-known two-photon selection rules, since each molecule absorbs one laser photon and either emits or aosorbs a virtual photon. In the same way, the distributive case provides for excitation through three-and one-photon allowed transitions, and may thus lead to excitation of states that are formally two-photon forbidden. (In general, it is sufficient to stipulate that both transitions involved in the distributive mechanism are one-photon allowed since, with the rare exception of icosahedrally symmetric molecules, all transitions which are one-photon allowed are of necessity also three-photon allowed (Andrews and Wilkes 1985).)... 

Since the electronic selection rule is AA = 0, 1, 2, 3, three-photon excitations from, for example, a + state, can terminate in 1 An and 1 u states as well as the + and 1n,1 states accessible via one-photon transitions. 

Selection rules are different from one-photon selection rules, leading to facile selective excitation of one component in a mixture of compounds with overlapping one-photon absorption spectra. 

Advantages of the two-photon laser spectroscopy are as follows high resolution, tolerance of infrared light by objects under investigation, different selection rules, and vibronic coupling. The last feature allows simultaneous accomplishment of two-photon and one-photon excitations. 

Due to the selection rule f =0, l, 2, 3 for three photon dipole transitions the excitation of a state is possible. As shown in the simplified potential diagram in fig. 3 two almost resonant intermediate steps (b ttCOJ) and ZTg ) are involved in the three photon excitation of the Rydberg states. This explains the comparatively strong appearance of the triplett bands and leads to the assumption that one of... 

Fig. 19. Composite two-photon excitation spectrum of the 4f ->5d transition in 0.003% in CaF, at 6K. The transition is studied by monitoring the 5d— 4f, no phonon transition occurring at 313.1 nm. As noted in text this transition is normally two-photon forbidden because of parity selection rules, however, odd crystal-fields components admix parity to make the transitions partially allowed. The pure electronic transition of the state is labeled as 0 other excitations, 1 to 12, are identified as phonon or normal mode excitations of the lattice which couple to the pure transition. Selection rules for assisted transitions follow selection rules which differ from the one-photon case. After Gayen and Hamilton (1982).

In a perfect molecule, electronic transitions would go like this Absorption of a photon excites a molecule from initial (usually ground) state to excited state excited state emits a photon having the same energy/frequency/wavelength and molecule goes from excited state to previous initial ground state. The first process, excitation, would be followed by the exact opposite process, called deexcitation or decay. Such processes would follow quantum-mechanical selection rules strictly. 

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