Transition to excited state

Electron-impact energy-loss spectroscopy (EELS) differs from other electron spectroscopies in that it is possible to observe transitions to states below the first ionization edge electronic transitions to excited states of the neutral, vibrational and even rotational transitions can be observed. This is a consequence of the detected electrons not originating in the sample. Conversely, there is a problem when electron impact induces an ionizing transition. For each such event there are two outgoing electrons. To precisely account for the energy deposited in the target, the two electrons must be measured in coincidence. 

However, in polyatomic molecules, transitions to excited states involving two vibrational modes at once (combination bands) are also weakly allowed, and are also affected by the anharmonicity of the potential. The role of combination bands in the NIR can be significant. As has been noted, the only functional groups likely to contribute to the NIR spectrum directly as overtone absorptions are those containing C-H, N-H, O-H or similar functionalities. However, in combination with these hydride bond overtone vibrations, contributions from other, lower frequency fundamental bands such as C=0 and C=C can be involved as overtone-combination bands. The effect may not be dramatic in the rather broad and overcrowded NIR absorption spectrum, but it can still be evident and useful in quantitative analysis. 

I and T2 taking into account that the corresponding correlation functions of electronic magnetization are to contain the probabilities of transitions to excited states, i.e. Boltzmann factors eKp Alk T). The nuclear relaxation will be considered in more detail in the following subsection. 

Fig. 1.20 UV/Vis absorption spectrum of anthracene in cyclohexane for the So -> Sj transition. The vibrational progression in the absorption spectrum corresponds to transitions to excited-state vibrations. The hatched area under the peak corresponds to the integrated absorption coefficient (lAC) as defined in Eq. 1.26...

As is well known from electronic spectroscopy [1], the probability of light absorption (and thus the intensity of the corresponding absorption band) is related to the characteristics of the states involved and particularly to their spin quantum number. Transitions from the ground state to excited states having the same spin value are allowed and give rise to intense bands, whereas transitions to excited states of different spin value are forbidden and... 

An early semiempirical MO calculation at r = 2.64A (adopted) predicted a closed-shell S ground state and transitions to excited states A in the near IR and in the visible [6]. 

Since tbe inversion operation alters die sign of the two vectors, according to q. (8.65), a remains unaffected. It, therefore, belongs to an even symmetry species. It is evident diat the excited state wave function must possess the same symmetry properties as a if the transition is to be Raman active. Thus, for molecules with center of symmetry fundamental transitions to excited states belonging to even symmetry species (Ag, Bg, etc.) are only active. 

---