Upper excited state

This figure shows that there are many touching points between the lower and upper excited states. The shown structures are all on the ground-state surface. At... 

We describe the vibrational levels in the lower (ground-state) Morse curve with the quantum number v", the lowermost vibrational level being v" = 0. The vibrational states in the upper (excited-state) Morse curve are described by the quantum number v. ... 

The first excited state of cyclobutene (o27t ) is correlated with the upper excited state ( /J /2 /3) of butadiene making it a high energy symmetry forbidden process. 

Similarly the first excited state of butadiene V1V2V3 is correlated with a high energy upper excited state G27tc of cyclobutene. Thus a photochemical conrotatory process in either direction would be a symmetry forbidden reaction. 

The much larger energy difference between Si and S0 than between any successive excited states means that, generally speaking, internal conversion between Si and S0 occurs more slowly than that between excited states. Therefore, irrespective of which upper excited state is initially produced by photon absorption, rapid internal conversion and vibrational relaxation processes mean that the excited-state molecule quickly relaxes to the Si(v0) state from which fluorescence and intersystem crossing compete effectively with internal conversion from Si. This is the basis of Kasha s rule, which states that because of the very rapid rate of deactivation to the lowest vibrational level of Si (or Td, luminescence emission and chemical reaction by excited molecules will always originate from the lowest vibrational level of Si or T ... 

The rate of internal conversion between electronic states is determined by the magnitude of the energy gap between these states. The energy gaps between upper excited states (S4, S3, S2) are relatively small compared to the gap between the lowest excited state and the ground state, and so the internal conversion between them will be rapid. Thus fluorescence is unable to compete with internal conversion from upper excited states. The electronic energy gap between Si and S0 is much larger and so fluorescence (Si —> S0) is able to compete with Si(v = 0) So(v = n) internal conversion. 

D. Lynch, J. F. Endicott. A Pulsed Photoacoustic Microcalorimeter for the Detection of Upper Excited-State Processes and Intersystem Crossing Yields. Appl. Spectroscopy 1989, 43, 826-833. 

Sorokin and Lankard illuminated cesium and rubidium vapors with light pulses from a dye laser pumped by a ruby giant-pulse laser, and obtained two-step excitation of Csj and Rbj molecules (which are always present in about 1 % concentration at atomic vapor pressures of 10" - 1 torr) jhe upper excited state is a repulsive one and dissociates into one excited atom and one ground-state atom. The resulting population inversion in the Ip level of Cs and the 6p level of Rb enables laser imission at 3.095 jum in helium-buffered cesium vapor and at 2.254 pm and 2.293 /zm in rubidium vapor. Measurements of line shape and frequency shift of the atomic... 

In this paper, we will present our recent observations on the upper excited-state emission of a variety of metalloporphyrins. 

S2 emission—See Upper excited-state emission spectroscopy Self-consistent field molecular orbital and configuration interaction (SCF-MOCI) calculations, 23 Solvents, magnesium... 

Upper excited-state emission spectroscopy deuterium isotope effect, 227-229 diacid porphyrins, 110-112 diamagnetic... 

As we have seen above (Section IV.C), in the polynuclear complexes dealt with in this review it is possible to identify components which can undergo photoexcitation independently from one another. The excited component can then give rise to intercomponent energy transfer processes, in competition with intracomponent decay. For most of the components which constitute the examined systems, the lifetime of the lowest excited state is long enough to allow the occurrence of energy transfer to nearby components when suitable energetic and electronic conditions are satisfied. This is not usually the case for upper excited states, which usually decay very rapidly (picosecond time scale) to the lowest excited state within each component. 

Bimolecular Reactions from Upper Triplet States. Other cases of sensitization by second excited triplet states have not yet come to light however, several bimolecular reactions of this sort have been reported. Since an upper excited state that lives long enough to undergo a bimolecular reaction should also be capable of transferring energy, these reactions will be discussed briefly. 

By using high intensity flash lamps and laser sources, photophysical and photochemical properties of the triplet states can be studied. These sources also help to study emission from upper excited state. 

The analysis of the transient fluorescence spectra of polar molecules in polar solvents that was outlined in Section I.A assumes that the specific probe molecule has certain ideal properties. The probe should not be strongly polarizable. Probe/solvent interactions involving specific effects, such as hydrogen-bonding should be avoided because specific solute/solvent effects may lead to photophysically discrete probe/solvent complexes. Discrete probe/solvent interactions are inconsistent with the continuum picture inherent in the theoretical formalism. Probes should not possess low lying, upper excited states which could interact with the first-excited state during the solvation processes. In addition, the probe should not possess more than one thermally accessible isomer of the excited state. 

Upper excited states are extremely short-lived. When the molecule is promoted to an excited singlet state beyond S1 the non-radiative deactivation by internal conversion is much faster than the spin-forbidden intersystem crossing to any triplet state. Therefore, the first excited singlet state is formed with near unit quantum yield. If an upper triplet state could be reached, it would also deactivate very rapidly to T1 and no singlet excited state would be formed. The extremely short lifetime of all upper excited states Sb(m>1) and Tb(w>1) means that luminescence emission and chemical reaction are, as a rule, not observed from such states. There are some exceptions to this rule, but there are many more mistaken reports of chemical reactions from short-lived upper excited states. Any such report... 

The photodissociation of aromatic molecules does not always take place at the weakest bond. It has been reported that in a chlorobenzene, substituted with an aliphatic chain which holds a far-away Br atom, dissociation occurs at the aromatic C-Cl bond rather than at the much weaker aliphatic C-Br bond (Figure 4.30). This is not easily understood on the basis of a simple picture of the crossing to a dissociative state, and it is probable that the reaction takes place in the tt-tt Si excited state which is localized on the aromatic system. There are indeed cases in which the dissociation is so fast (< 10-12 s) that it competes efficiently with internal conversion. 1-Chloromethyl-Np provides a clear example of this behaviour, its fluorescence quantum yield being much smaller when excitation populates S2 than when it reaches Figure 4.31 shows a comparison of the fluorescence excitation spectrum and the absorption spectrum of this compound. This is one of the few well-documented examples of an upper excited state reaction of an organic molecule which has a normal pattern of energy levels (e.g. unlike azulene or thioketones). This unusual behaviour is related of course to the extremely fast dissociation, within a single vibration very probably. We must now... 

Chemical and Photophysical Processes of Transients Derived from Multiphoton Excitation Upper Excited States and Excited Radicals... 

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