Photoexcitation, selection rules

Ms level (selection rule AMs = 1). EPR is applicable to solid (powder, single crystal, and frozen solutions) as well as liquid samples, and to electronic ground-state systems as well as to photoexcited and carrier injection-induced systems (i.e., device structures). The information contained in an EPR spectrum also reflects hyperfine interactions and magnetic anisotropy (zero-field) effects. 

The above results indicate that the selection rules are relaxed when the geometry modifications taking place upon photoexcitation are considered. Although the transition dipole moment between the ground state and the lowest excited state remains small, the luminescence is no longer entirely quenched by the interchain interac-... 

In the present chapter, we consider line strengths of transitions between bound states, i.e. of lines whose only form of natural broadening is radiative, and which lie below the first ionisation potential. The simplest situation is encountered in the photoabsorption or photoexcitation of an atom, initially in its ground state >, in which case one transition is observed to each excited final state / >. The price one pays for this simplicity is that all excited states cannot be reached in this way because of selection rules. 

There are many variants of REMPI. In most REMPI experiments one or more of the photoexcitation steps is a multiphoton transition. The rotation and electronic selection rules and relative intensity factors are quite different for a one-photon... 

This expression shows that there is no coupling between the states with different in parallel fields. This is similar to the selection rule AMy = 0 for the photoexcitation of the molecule with linearly polarized laser light [36]. 

When, as is nearly always the case, the initial state is Sq, photoexcitation to a triplet is assumed to be so highly forbidden as to be negligible. The selection rules for excitation to the various higher singlets will not be restated [3] let us merely note three qualitative points ... 

There are selection rules which determine whether the transition moment is nonzero.In most photochemical applications three features of the wave functions are of major importance. The first is spin multiplicity. As long as the molecule to be photoexcited is composed of lighter atoms (atomic number < 36) spin conservation in a transition is a good approximation. Only singlet singlet or triplet triplet transitions are intense enough to be accessible photochemically excitation is between states in... 

According to the dipole selection rules, photoexcitation of a core electron with angular momentum I probes the I 1 components of the final state. In the Lm absorption processes I — 1 transitions (2p - 6s) can be neglected. Their probability is less than 3% of the / -I-1 (2p - 5d) transition probability (cf., e.g., Barinski and Nefedow 1969). The contribution of quadrupolar transitions to the absorption coefficient can also be neglected. Quadrupolar transitions turn out to be important only in the higher Z elements (in Lnm absorption for Z > 70). Calculated ratios of quadrupolar to dipole transitions for K and L absorption are given in table 2 (Muller and Wilkins 1984). 

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