Excited-State Levels

It was shown above that the normal two-level system (ground to excited state) will not produce lasing but that a three-level system (ground to excited state to second excited state) can enable lasing. Some laser systems utilize four- or even five-level systems, but all need at least one of the excited-state energy levels to have a relatively long lifetime to build up an inverted population. 

---

Two typical dye molecules. The europium complex (a) transfers absorbed light to excited-state levels of the complexed Eu , from which lasing occurs. The perylene molecule (b) converts incident radiation into a triplet state, which decays slowly and so allows lasing to occur. 

Figure 1. Schematic of the radial cuts of the ground- and excited-state potential energy surfaces at the linear and T-shaped orientations. Transitions of the ground-state, T-shaped complexes access the lowest lying, bound intermolecular level in the excited-state potential also with a rigid T-shaped geometry. Transitions of the linear conformer were previously believed to access the purely repulsive region of the excited-state potential and would thus give rise to a continuum signal. The results reviewed here indicate that transitions of the linear conformer can access bound excited-state levels with intermolecular vibrational excitation.

He2 ICl complex will access excited-state levels that have one He atom locahzed in the T-shaped well and the other in bending or hindered-rotor levels. Our group is currently undertaking dynamical studies on He2 ICl. As mentioned above, we have already measured the binding energy of the... 

He2 ICl conformer using action spectroscopy to find the bound-free continuum associated with the He + He IC1(B, V = 3) dissociation limit. It would also be insightful to perform time-resolved experiments on the different conformers of these systems to directly monitor the kinetics for forming the different products and intermediates as a function of the different excited-state levels prepared. 

The wave function /lo constructed from SCF orbitals is "so good that it cannot be improved by the inclusion of singly excited determinants. The main effect of increasing the number of singly excited determinants in the Cl problem will be a better description of the excited state levels. 

Fig. 9. Diagram of the excited states levels of Rufll) polypyridyl complexes with different pathways for the photodechelation [adapted from Durham B, Casper JJ, Nagle JK, Meyer TJ (1982) J. Am. Chem. Soc. 104 4803]...

Emission selection rules constrain the emission from 1 So primarily to excited state levels, rather than directly to the 3Hj ground multiplets, allowing the potential for a second photon to be emitted to the 3Hj states. 

Figure 11.5. Ground and excited state levels involved in the radiofrequency/optical double resonance study of CS in its A 1 n state [12].

Here, we will mainly focus on two other important aspects of photosensitization the fundamental role of deeply colored compounds as light-harvesting antenna chromophores for solar energy conversion and the possibility of reaching spectroscopically hidden, but photochemically active excited state levels by means of spectral sensitization. 

As already mentioned above, spectral sensitization may also become indispensable when the light absorption properties of a potentially photoreactive compound do not permit direct excitation in the desired wavelength region. By application of sensitizers with adjusted excited state properties, it is, for example, possible to induce photochemical reactions of otherwise colorless compounds with visible light. Another important application in photochemistry is the sensitized population of excited state levels, which are not easily reached by direct absorption of light due to the limitations of quantum chemical selection rules. This phenomenon has been extensively exploited in mechanistic and synthetic organic photochemistry, where enhanced yields of triplet state population could be achieved in various dye-photosensitized processes (98). 

In photochemical reactions, the population of excited states of different orbital origins can result in quite different reactivity patterns. Therefore, reaction products may occur, which are not accessible at all in thermochemical pathways. Especially in organometallic and coordination compounds, the primary photoproducts obtained are not always resulting from the lowest-lying excited state levels. Wavelength-selective excitation may then be exploited to channel the product formation process and to control a possible branching between different reactivity patterns. 

---