Dimer, excited

Excimer lasers use gases, but because of their special properties are usually considered as a class of their own. Excimer is short for excited dimer, which consists of two elements, such as argon and fluorine, that can be chemically combined in an excited state only. These lasers typically emit radiation with veiy small wavelengths, in the ultraviolet region of the electromagnetic spectrum. This shorter wavelength is an enormous advantage for many applications. 

When excited, the molecules of organic dyes tend to form complexes with unexcited molecules like themselves. These excited dimeric complexes are called the excimers. The excimer emission spectrum is easy to observe because it is very different from that of a monomer. It is usually broad and strongly shifted to longer wavelengths, and it does not contain vibrational structure. If the excimer is not formed, we observe emission of the monomer in the fluorescence spectra, and upon its formation there appears a characteristic emission of the excimer. 

If Q is identical to M (self-quenching without formation of products), (MM) is called an excimer (excited dimer) with its own fluorescence spectrum and lifetime (Section 4.4.1). 

Excimers are dimers in the excited state (the term excimer results from the contraction of excited dimer ). They are formed by collision between an excited molecule and an identical unexcited molecule ... 

Light is generated upon relaxation from the excited state to a ground state that is repulsive, e.g., Xe+Cl. This condition is extremely favorable for population inversion, i.e., the number of excited dimer molecules is greater than the number of dimer molecules in the ground state, a condition necessary for light amplification... 

Excimers are excited dimers (D ) produced by collisionai interaction between excited (M ) and unexcited monomers (M), that is, M + M - D. ... 

Photolysis of the dimer, reaction (44), proceeds primarily via generation of Cl + ClOO (Cox and Hayrnan, 1988 Molina et al., 1990). For example, Molina et al. (1990) reported the quantum yield for this channel at 308 nm to be unity, with an uncertainty of 30%. Okumura and co-workers (Moore et al., 1999) and Schindler and co-workers (Schmidt et al., 1998) have reported that the quantum yield is less than 1.0. For example, Schmidt et al. (1998) used resonance-enhanced multiphoton ionization (REMPI) with time-of-flight (TOF) mass spectrometry to follow the production of oxygen and chlorine atoms as well as CIO in vibrational levels up to v" = 5 in the photolysis of the dimer. At a photolysis wavelength of 250 nm, the quantum yield for chlorine atom production was measured to be 0.65 + 0.15, but CIO was not observed. Assuming that all of the excited dimer dissociates, this suggests that the production of CIO in vibrational... 

Excimer lamps, a relatively recent new development, represent a new class of monochromatic UV sources. Excimers (excited dimers, trimers) are weakly bound excited states of molecules that do not possess a stable molecular state. ... 

Excimer formation can serve as a sensitive probe of group proximities Excimers make evident the interaction of an excited molecule M, (typically an aromatic hydrocarbon), with a molecule in the ground state M producing an excited dimer Mf (or D ). The dimer must be formed within the lifetime of the excited species (e.g., for pyrene derivatives, about 100 nsec). For molecules such as pyrene, excimer formation and fluorescence are contingent on attainment of a well-defined steric arrangement in the dimer.41... 

E. Possibility of Excited Dimer Formation in Dyestuff Solutions. 340... 

The normal violet fluorescence band of pyrene solutions shows concentration-quenching which is accompanied by the appearance of a blue structureless emission band. Forster and Kasper40 showed that the blue band is due to emission from an excited dimer formed by the combination of an excited singlet molecule with a molecule in the ground state. Most of the light in both spectral bands has a relatively short lifetime but Stevens and Hutton87 observed a long-lived component of the dimer... 

Since the intensities of delayed fluorescence of both monomer and dimer bands were found to be proportional to the square of the rate of absorption of exciting light under nearly all conditions (see Table IX), the delayed fluorescence is assumed to be produced by the triplet quenching mechanism. Thus the sequence of reactions giving rise to singlet-excited monomer and excited dimer as observed by normal fluorescence are as follows ... 

The normal (short-lived) fluorescence spectrum of 3 X 10 2M naphthalene at —105 °C. [Fig. 21, curve (a) ] shows not only the band due to the singlet excited monomer but also the broad dimer emission band, with maximum at 400 m which is similar to that observed by Doller and Forster46 in toluene solutions. The spectrum of the delayed emission at the same temperature [Fig. 21, curve (b)] also shows both bands, but the intensity of the dimer band is relatively much greater. When the concentration is reduced to 3 X 10 W, the intensity of the dimer band at —105 °C. is very small in normal fluorescence but is still quite large in delayed fluorescence.45 The behavior of naphthalene solutions at —105° C. is thus qualitatively similar to that of pyrene at room temperature. At temperatures greater than — 67 °C. (Table XII) the proportion of dimer observed in delayed fluorescence is almost the same as that observed in normal fluorescence, and presumably at these temperatures, establishment of equilibrium between the excited dimer and excited monomer is substantially complete before fluorescence occurs to an appreciable extent. The higher the temperature, the lower is the proportion of dimer observed in either normal or delayed fluorescence because the position of equilibrium shifts in favor of the excited monomer. 

The mixed triplet quenching mechanism just described implies the formation of mixed dimers AD and AD. This is not an unreasonable proposition because Birks and Christophorou48 have already demonstrated the formation of mixed singlet excited dimers by the Forster— Kasper mechanism ... 

Photodecomposition accompanying directly excited delayed fluorescence will clearly be less obvious. For example, at the same rate of light absorption in a 5 X 10-SM solution of anthracene, and with the same quantum efficiency of photodecomposition, a period 1000 times as long would be required to produce the same percentage decomposition. It was in fact found that after prolonged irradiation, a deaerated solution of 5 X 10-BM anthracene was almost completely decomposed and a crystalline precipitate was formed which had the appearance of dianthracene. It seems not unlikely therefore that one or other of the excited dimers formed by triplet-triplet quenching may be partly converted into a stable dimer. If this is so, mixed triplet quenching should result in the formation of stable mixed dimers. Experiments are in hand to test these hypotheses. 

The third main type includes directly excited, and sensitized delayed fluorescence. It may show only the band corresponding to the singlet-excited monomer, or it may show also a band due to an excited dimer. 

In the pyrene system, the intermediate X has been identified with an excited dimer or excimer, since characteristic excimer emission is also observed. The process of triplet-triplet annihilation then consists of transfer of energy from one triplet to another to form excited dimeric species (S), which dissociates into an excited and a ground state singlets ... 

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