Excimer, definition

The phenomenon of fluorescence has been synonymous with ultraviolet (UV) and visible spectroscopy rather than near-infrared (near-IR) spectroscopy from the beginning of the subject. This fact is evidenced in definitive texts which also provide useful background information for this volume (see, e.g., Refs. 1-6). Consequently, our understanding of the many molecular phenomena which can be studied with fluorescence techniques, e.g., excimer formation, energy transfer, diffusion, and rotation, is based on measurements made in the UV/visible. Historically, this emphasis was undoubtedly due to the spectral response of the eye and the availability of suitable sources and detectors for the UV/visible in contrast to the lack of equivalent instrumentation for the IR. Nevertheless, there are a few notable exceptions to the prevalence of UV/visible techniques in fluorescence such as the near-IR study of chlorophyll(7) and singlet oxygen,<8) which have been ongoing for some years. 

With an interplanar separation of 3.73 A, 4,4 -paracyclophane is the lowest member of the series to exhibit an alkylbenzene absorption spectrum and the broad structureless fluorescence spectrum of this molecule with a peak intensity at 3400 A is by definition an excimer band further separation of the aromatic rings in 4,5 and 6,6 -paracyclophanes restores the fluorescence spectrum to that of the alkylbenzenes. These observations by Rice et al.115 illustrate the critical nature of the interplanar separation in determining the extent of interaction between -electron systems in the ground and excited configurations. 

There have been several experimental developments in the last few years that have resulted in more definitive experiments on photodissociation dynamics. First and foremost is the greater availability of intense laser sources, particular in the region below 300 nm (8,9). Excimer laser sources provide large numbers of photons at 157, 193, 222, 249, and 308... 

The investigation of the ion beam interaction with polystyrene by means of ion beam pulse radiolysis has the advantage that the reactive intermediates can be directly detected. Time profiles of the excimer fluroescence from ion irradiated polystyrene were measured using the polystyrene thin films. Thus, the transient phenomenon excited by the ion with a definite kinetic energy was observed. 

Changes in the electronic and molecular structures after CT excitation can also result in chemical bond formation between the excited complex [AB[ and another molecule (Q) of the system, yielding an encounter complex AB-Q. The complex formed by interaction of an excited molecular entity with a ground state partner of the same structure is called an excimer, whereas an electronically excited dimer of definite stoichiometry, formed by interaction of an excited molecular entity with a ground state partner of a different structure, is named an exciplex [29], Both excimers and exciplexes have their own properties and can thus be regarded as new chemical species. Their generation is well documented by the concentration effect on the fluorescence of some solutes or by flash photolysis measurements [11,24],... 

Professor Oppenlander is well qualified to write about AOPs/AOTs, since he has contributed to this literature in a very significant manner. It is thus particularly valuable to see the application of incoherent excimer lamps covered so well, a subject that has been a central part of his research. Finally, it is good to see careful attention being made to consistent terms, definitions and units in accord with lUPAC recommendations. 

One of the methods under development at AT T Bell Laboratories for submicron lithography is deep ultraviolet projection photolithography. (O Fine line definition is obtained by use of 248 nm light and a lens of large numerical aperture. Because of the large chromatic aberration of the quartz lens a spectrally line-narrowed krypton fluoride excimer laser is used as a light source. 

The theoretical implications of (a) are fascinating since single step transfer as part of extensive e.e.m. would occur on a time-scale much faster than would be predicted by exchange of dipole transfer and would be of the magnitude typical of crystals which exhibit ground-state interaction. Furthermore, if (a) were the case then all excimer traps would be populated on this very fast time basis. However the observed behaviour shows a definite rise-time of the order of a nanosecond for the excimer state. 

The fierce dispute over the suggested detection of triplet excimers of naphthalene derivatives continues. Locke and Lin report that in solutions of naphthalene and 1,n-di-a-naphthylalkane the measured emission is definitely not due to biacetyl-like impurities as suggested by other workers in a reinterpretation and re-examination of earlier data. The final decision on the matter is still awaited. 

We provide some definitions used in the subsequent text. An excimer is a simple and excited molecule. In... 

The fact that the molecule only exhibits a stable existence in its excited state benefits the laser because the repulsive, dissociative ground state does not allow population to accumulate it is by definition empty. This means that every excited molecule contributes to the population inversion. Furthermore, no absorption of generated laser light by molecules in the ground state can occur, as is the case for most other types of laser there is only spontaneous and stimulated emission. As a consequence, excimer lasers exhibit very high laser gain but, because of the nature of the chemical production process of the excited molecule, only pulse operation is realized in commercial systems. 

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