Showing papers on "Excimer published in 1976"

Acid-Base Properties of Electronically Excited States of Organic Molecules

Abstract: Publisher Summary This chapter discusses acid–base properties of electronically excited states of organic molecules. Excited state pK-values are most easily accessible through the use of the Forster cycle. To perform this calculation for a particular molecule, it is necessary to know the ground state equilibrium constant for the reaction in question and to have some measure of the energy difference between the lowest vibrational level of the ground and the excited state in both the B and BH + forms. The effects of solvation on 0–0 energies are discussed. The changes in molecular fluorescence with acidity give information about the protolytic behavior of the excited singlet state of a compound. Two techniques, phase and pulse fluorometry, are used for the direct measurement of fluorescence decay rates. The excited state acid-base behavior of molecules has direct implications in the field of analytical fluorimetry and phosphorimetry.

Quenching rate constants for metastable argon, krypton, and xenon atoms by fluorine containing molecules and branching ratios for XeF* and KrF* formation

Abstract: The flowing afterglow technique was used to study the reactions of Xe(3P2), Kr(3P2), and Ar(3P2) metastable atoms with small fluorine containing molecules. Fluorides from Groups III through VIII (XeF2) were examined. Although all the fluorides have large quenching rate constants, only F2, XeF2, some interhalogen fluorides, and small molecules with the OF or NF bond have high branching ratios for XeF* or KrF* excimer formation. The branching ratio measurements were made via comparison to the XeCl* and KrCl* emission intensities from reaction of Xe(3P2) and Kr(3P2) with Cl2, which were adopted as reference reactions. Within experimental error, the branching ratios are unity for Kr(3P2) and Xe(3P2) with Cl2, F2, and OF2. Increasing the argon pressure from 1–40 torr gives extensive vibrational relaxation but no electronic quenching of the lowest energy excimer state of XeF*, XeCl*, KrCl*, or KrF*. Increasing pressure also reduces the intensity of the secondary emission system of KrCl* and XeCl* which implies collisional transfer from the (2Π3/2) to the 2Σ+ excimer state. The occurrence of vibrational relaxation suggests a spontaneous radiative lifetime of ∼50 nsec. A higher energy excimer state (2Π1/2) was identified for XeF*, XeCl*, XeBr*, XeI*, KrF*, and KrCl*. Discussion is presented regarding the chemical dynamics and chemiluminescence of these reaction systems.

Reactions of electronically excited-state atoms

Abstract: Gas-phase electronic energy transfer processes are reviewed for excited atoms with sufficient energy to yield electronically excited products in encounters with other atoms or molecules. Attention is focused on excited states that result from orbital excitation rather than low-energy spin-orbit states that have the same electronic configuration as the ground state. 281 references (GHT)

A potential high‐efficiency Cl2 ultraviolet laser

Abstract: An emission band extending from 2200 to 3000 A has been observed following proton beam excitation of Ar‐Cl2 mixtures which is much more intense than the emission from Ar‐N2. The emission band was assigned to a bound‐free transition in the chlorine molecule. The energy precursor of excited Cl2 is the excimer Ar2* which emits a 1300‐A continuum. Ion‐ion recombination can also contribute to this excited chlorine molecule. The emission continuum near the 1750 A associated with ArCl* was also investigated.

Optical absorption and fluorescence studies in high pressure cesium-xenon mixtures

Abstract: The pressure and temperature dependent absorption and fluorescence spectra of the cesium-xenon (CsXe) molecule have been examined. In contrast to previous investigations of the alkali-rare gas molecules, cesium atomic states that have weakly allowed optical transitions have been studied and have been shown to form excimer levels that are attractive for application as potential dissociation lasers. In particular, the (Cs[ 7^{2}S ]Xe)* excimer appears promising as a source of high-energy laser radiation due to 1) its large dissociation energy (0.132 eV), 2) its stimulated emission cross section of \simeq10^{-17} cm2, and 3) its small population threshold inversion densities ( \simeq10^{13} cm-3).

Geometrical requirements for fluorescent intramolecular excimer formation and fluorescence quenching

Abstract: From a study of fluorescence quantum yields for several α,ω-di-naphthylalkanes and related compounds it is concluded that the geometrical requirements for intramolecular fluorescence quenching are similar to those for fluorescent excimer formation.

Noble gas-halogen transfer laser method and means

Abstract: Halogen transfer laser method and means are disclosed employing a dilute mixture of molecular halogen vapor in high pressure noble gas. Noble gas atoms and molecules are excited by use of electrons to high energy metastable and/or excimer states. Collisional and/or radiative transfer of electronic excitation from the excited noble gas atoms and molecules to the lasing halogen molecules takes place for excitation of halogen molecules to upper ionic states in sufficient number to establish a population inversion in the electronic energy levels thereof for lasing. In addition to halogens, halogen-bearing compounds which dissociate and appropriately recombine to provide diatomic halogen for lasing also may be used. The laser is tunable over an electromagnetic energy range which includes ultraviolet radiation. Operation at high peak power levels with high efficiency is possible.

Triplet Excimer of Phenanthrene in Fluid Solution. Dynamic Analysis of Time-Resolved Phosphorescence

Abstract: The excimer phosphorescence of phenanthrene in isooctane at temperatures over a range of 300–77 K has been studied by means of a photon-counting technique. At temperatures above the melting point of isooctane a broad emission spectrum with its intensity maximum at 525 nm was obtained, which is essentially different from the structured phosphorescence spectrum obtained in rigid media at 77 K. The time dependence of the spectral shape and intensity of the phosphorescence emission affords strong evidence that the broad emission spectrum originates from the triplet excimer. A scheme of the triplet excimer formation involving a self-quenching pathway of the triplet monomer is proposed to interpret the time and concentration dependence of the monomer and excimer phosphorescence.

The thallium mercury excimer and its potential as a laser

Abstract: Normalized fluorescence measurements of the thallium mercury band emission at 4590 and 6560 A show that these bands have great promise for excimer laser action. The use of mercury as a broadening gas results in potential wells approximately four times as deep as those of the corresponding thallium xenon states. This effect, which is probably due to mercury’s high polarizability, leads to excimer band emission which is about a factor of 100 times more intense than that for thallium xenon at a given perturber gas density. Using the density dependence of the molecular fluorescence intensity, the molecular formation rate constant of the thallium mercury B 2Σ1/2 well is estimated to be 2.0×10−31 cm6/sec. It appears that the ideal laser operating conditions should be about 1019 Hg/cm3, 1016 Tl/cm3, and 6×1019 He or Ar/cm3. Under these conditions, at 720°C, an excited state Tl density of 3×1014/cm3 will result in a stimulated emission coefficient of about 1% per centimeter at either 4590 or 6560 A. If successful...

Excimer emission in the thermoluminescence of gamma-irradiated hydrocarbon glasses: some new spectra

Abstract: Excimers are formed by neutralisation of dimer cations of aromatic hydrocarbons when glassy solutions in methyl cyclohexane + isopentane mixtures, in cumene, decalin or squalane are allowed to warm up after radiolysis at 77 K. Squalane was found to be a useful solvent for the larger molecules. Excimer fluorescence of biphenyl, p-terphenyl, trans-stilbene, diphenylacetylene and perfluoronaphthalene has been observed : these emissions cannot be produced by photo-excitation. Naphthalene, pyrene, anthracene, 9-methylanthracene and perylene also gave excimers; in the last three cases, the emission was at shorter wavelengths than usual. cis-Stilbene, 1,4-diphenylbutadiene, 1,1-binaphthyl, phenanthrene, chrysene, triphenylene, biphenylene, acenaphthylene and fluoranthene gave negative or doubtful results. No excimer phosphorescence was detected despite the favourable conditions, but self-quenching of the phenanthrene triplet was observed. Methods of producing excimers are compared and the role of constraint by the matrix is discussed.

Excited complexes in oxidation-reduction photoreactions of pigments. I. Spectral detection of triplet state exciplex in the chlorophyll oxidation reaction.

Abstract: Photooxidation of chlorophyll "a" with p-benzoquinone in toluene and dioxane is studied by the method of flash photolysis. It is shown that in nonpolar medium the triplet exiplex is the product of this reaction. The experiments on competitive quenching of chlorophyll triplet state with naphthacene prove the diffusion mechanism of exiplex formation. The spectrum of triplet-triplet absorption of the exiplex is presented and the rate constant of intercombination degradation equalling 1.103 sec(-1) is measured. Possible structure of the exiplex is discussed.

Fluorescence and dissociative energy transfer in high pressure Ar–HCN mixtures excited by relativistic electrons

Abstract: Studies of the radiation from Ar–HCN and Ar–H2O mixtures, excited by relativistic electrons from a Febetron 706 electron gun, are presented. The energy deposited by the electrons channels rapidly into the Ar* and Ar2* states; then by dissociative collisions with HCN it is transferred partly into the CN(B) state and partly into a reservoir state [most probably the CN(A) state] with near unit efficiency. Fluorescence emission from the electronic states A (2Πi) and B (2Σ+) of the CN radical, as well as the emission from the Ar2* excimer state, were measured for an Ar pressure range of 750–9000 torr [15–180 psi (absolute)], and an HCN pressure range of 0.2–13 torr. The quenching rates of Ar* metastable and the Ar2* excimer states, by collision with HCN molecules, were measured to be 6×10−10 and 1×10−9 cm3 sec−1, respectively. Our results suggest that population inversion can be achieved by collisional dissociation of simple molecules with excited rare gas atoms and excimers. Indeed the peak CN(B) population ...

Organic transfer laser method and means

Abstract: Organic transfer laser method and means are disclosed employing a gas mixture which includes an acceptor gas comprising an organic molecular vapor, and a donor gas comprising a noble gas or a group II B transition metal, such as mercury, in the vapor state. The donor gas atoms are excited to high energy metastable and/or excimer states by electron beam excitation or electron beam initiated-sustainer discharge type exciting means. Collisional transfer of electronic excitation from the excited donor gas to the lasing organic molecules takes place for excitation of organic molecules to excited singlet states in sufficient number to establish a population inversion in the electronic energy levels thereof for lasing. Depending upon the mixture employed, the laser is tunable over the visible and ultraviolet regions of the electromagnetic spectrum. Operation at high peak power levels with high efficiency is possible.

Excimer emission band at 235.5 nm in the XeCl molecule

Abstract: Band emission at 235.5 nm is observed in fast discharge in a Xe/Cl2 mixture at low pressure. This emission has the same spectral width as that of the excimer 2Σ+1/2→ 2Σ+1/2 transition at 307 nm. It is suggested that this band is also due to bound‐free transitions, but belongs to the spin‐orbit split level 2Π state of the ionic XeCl molecule that correlates with Xe+(2P1/2) and Cl−(1S). The molecular energy gap between 2Σ+1/2 and 2Π1/2 is almost equal to the atomic spin‐orbit splitting and their potential curves are very similar. This bound‐free 2Π1/2→2Σ+1/2 emission may be an excimer laser candidate.