Types of Electronic Spectra

excited state 3. excited state 2. excited state 

The dominating importance of single excitations is based on physical reasons. They dominate the most common one-photon processes, since they usually have low excitation energies and involve large transition moments (intensities). This is different for two-photon spectra for a recent DFT study on this topic, see Ref. 18. 

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The simplest type of electronic spectra result from transitions within the simplest atom, hydrogen. The emission spectrum of the hydrogen atom at ultraviolet wavelengths consists of a series of emission peaks (or in a photographic emulsion, dark lines) called the Lyman series. 

Unfortunately, none of the methods currently in use simultaneously fulfill all these requirements. The user s choice is thus determined by a consideration of the assets and drawbacks of a particular method which, in relation to the type of electronic spectra/states, may change from one problem to the other. Figure 8 provides the reader with a comprehensive overview of existing quantum chemical methods for excited states. 

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