Showing papers on "Excimer published in 1979"

New lines in the UV: SRS of excimer laser wavelengths

Abstract: The UV excimer lasers ArF, KrCl, KrF, and XeCl were utilized to create new families of UV lines by multiple orders of broad-band nonresonant stimulated Raman scattering (SRS) in the media H 2 , D 2 , CH 4 , and LN 2 . Mixed-media and excited-state SRS were also studied.

A new blue‐green excimer laser in XeF

Abstract: A new blue‐green excimer laser has been demonstrated on the C‐A transition in XeF, with an output energy of greater than 1 mJ Xe*2 excimer fluorescence at 172 nm was used to photodissociate XeF2, producing XeF[B (1/2)] and XeF[C (3/2)] The B (1/2) state was collisionally relaxed to the C (3/2) state with an Ar buffer Lasing then occurs sequentially on the B (1/2) ‐X (1/2) and C (3/2) ‐A (3/2) transitions Measurements of the C‐A laser spectrum showed a peak wavelength of 483 nm with a bandwidth of 12 nm This new laser is potentially highly efficient and scalable and should be tunable over a bandwidth greater than 40 nm

Potential energy curves of several excited states of the Ne2* excimer: Assignment of the transient absorption spectra of the excimer

Abstract: Most of the excited states of Ne2, which are correlated with the Rydberg state transitions 2p → 3s, 3p, and 4s of Ne, are studied by ab initio CI calculations. Two transient absorption spectra from the lowest excimer state Σu+ recently observed by Arai et al., are discussed on the basis of calculated potential energy curves. Possible assignments are presented. The calculated transition energies are in good agreement with the observed ones.

Excimers in solid non-crystalline anthracene

Abstract: By employing time-resolved fluorescence spectroscopy it is shown that the emission spectrum of non-crystalline layers of anthracene, prepared by vacuum sublimation onto a 80 K substrate, consists of three distinct excimer bands. The band maxima are at 21 900 cm-1, 21 500 cm-1 and 19 600 cm-1, the corresponding lifetimes are 10 ns, 35 ns and 170 ns, respectively. The underlying molecular pair conformations, formed during sample preparation, must be metastable in the groundstate.

Selected aspects of photochemistry in polymer media

Abstract: A number of selected aspects of the reciprocal interactions of polymers with excited solutes or polymer-bound chromophores are given. The role of free volume, glass-transition temperature (Tg), microscopic viscosity, and polarity or environments to which excited molecules are exposed is emphasized in order to illustrate possible manners in which the photophysical, photochemical, and subsequent chemistries can be influenced. Many of these factors can affect several monomolecular and bimolecular processes encountered in the sensitization of photocrosslinkable polymers: the photophysics of the sensitizer, energy transfer to the reactive sites on the polymer, and the formation of crosslinks. In fact, the triplet yields of aroylnaphthothiazole derivatives, a widely used class of sensitizers, are considerably higher in the more viscous polymeric matrices. These 0. in the polymer approach, however, a value of only ca. 0.7. As a result a search for more efficient triplet sensitizers led to a new class of compounds: 5- and 7-substituted 3-ketocoumarins. As a model for study of the bimolecular processes in polymers, we chose exciplex and excimer probes. The following conclusions were drawn: 1. Several exciplex and excimer emissions in polymeric media are considerably shifted to shorter wavelengths as compared with the maxima measured in fluid media, indicating that interactions are impaired in polymeric matrices. 2. Emissions from the polymer matrix above the glass-transition temperature are similar in wavelength and temperature dependence thereof to those observed in fluid solutions. 3. Improper orientation of the reactants in polymers is responsible for the shift observed in excimer emission and for a part or all of the shift in exciplex emissions. 4. Little if any difference is observed between exciplex emissions in polymers of low and moderate macroscopic polarities. This may be attributed to two causes: a) Due to improper orientation the dipole moment of the exciplex in the polymer is expected to be smaller and, therefore, less solvation energy can be gained. b) The segmental motion of the polymer required to properly solvate the complex is probably too slow at room temperature compared with the lifetime of the exciplex. 5. Bichromophoric molecules, which form exciplexes in fluid media, fail to reach an exciplex configuration when dissolved in polymers.

Excimers in Crystals

Abstract: The concept of the reaction cavity is used to indicate what types of reaction are allowed in the crystal state. Application is made to the formation of excimers in the crystal. The lattice determines whether or not the excimer is formed. There two types of excimer-forming structures, stack and pair ones; it appears that the former forms cis-excimers, the latter trans- ones. In the anthracenes, and probably generally, the excimer is an intermediate on the route to photodimer. The crystal constraints which determine the geometry of the excimer determine the stereochemistry of the product. However, the geometry of the product and of the precursor excimer may differ between bulk phase and structural defects. Reactions in chiral crystals may yield optically active products. In an example of photodimerization the chirality of the dominant product appears to be determined by the chirality of the dominant excimer precursor.

Steric effects on excimer formation in isotactic and atactic polystyrene and poly(ar‐methylstyrene)s in solution

Abstract: Effects of tacticity and steric hindrance on excimer formation were investigated in isotactic and atactic polystyrene, poly(o-methylstyrene), poly(m-methylstyrene), and poly(p-methylstyrene) in the presence and absence of a quencher (CCl4). The calculated rate constants for excimer formation in the isotactic polymers except for poly(o-methylstyrene) were almost the same and larger than those in the corresponding atactic polymers. These results indicate that excimer formation was due to not only rotational sampling but also energy migration to trapping sites. It was found that steric hindrance on excimer formation was intimately related to the excition diffusion length in the polymer chain.

Evidence for a change in the fluidity of erythrocyte membranes following X irradiation by measurement of pyrene excimer fluorescence.

Abstract: The changes in the fluidity of erythrocyte membranes following x irradiation were investigated with the aid of a fluorescent aromatic hydrocarbon, pyrene. The method used in the present experiments, pyrene excimer fluorescence, is based upon the microviscosity dependence of the lateral diffusion rate of the probe in the membrane plane. The lateral diffusion rate obtained by this fluorescence technique can provide valuable information about the fluidity of the membrane environment of the probe. It was found that the excimer/monomer ratio, a parameter of the lateral diffusion rate, of pyrene bound to erythrocyte membranes irradiated with x rays was higher than that in unirradiated membranes. This effect was observable when 100 R were given to the membranes. In addition, the fluorescence polarization of membrane-bound pyrene decreased in x-irradiated membranes. From these results it was concluded that x irradiation can induce an increase in the fluidity of the hydrocarbon regions of the membranes.

Picosecond relaxation processes in excited state quinoxaline

Abstract: Excited state absorption spectra of quinoxaline in benzene, isooctane, and cis‐1,3‐pentadiene at 7 and 130 ps after excitation are reported. Picosecond absorption kinetics at 422 and 533 nm are also reported. In the nonaromatic solvents, bands at ∼425 nm (Sn←S1 and Tn←T1) and ∼ 530 nm (mostly Sn←S1) are observed in the transient spectra. Decay of the ∼530‐nm band gives k−1isc?23 ps in the nonaromatic solvents. In benzene solvent, the excited state spectra are more complex, the decay kinetics are longer, and quinoxaline–benzene exciplex formation is suggested.

Study of Nonconjugated Bichromophores Linked by a Polyether Chain. I. Photochemical Synthesis of Crown-Ethers from bis-(9-Anthryloxy) Polyoxaalkanes

Abstract: A series of bisanthracenes (In) linked by a polyoxyethylene chain: 9-anthryl-O–CH2–CH2–O–CH2–(CH2–O–CH2)n–CH2–O-9-anthryl, where n = 1–4, was synthesized in order to investigate the bichromophoric interactions of the two aromatic rings in connection with the special properties of the chain. UV absorption spectra at room temperature in benzene and ethanol exhibit, in general, a weak hypochromicity compared with other 9–9′ bisanthracenes. Under the same experimental conditions, the fluorescence of In is twofold: (i) an emission from the locally excited state (“monomer fluorescence”) with quantum yield (ΦFM) ca. 0.10–0.17 (benzene) and 0.03–0.08 (ethanol); (ii) an excimer-type fluorescence centered at ca. 470–515 nm, of which quantum yields (ΦFD) are in the range 0.01–0.025 (benzene) and 0.01–0.012 (ethanol). Of note is the observation of an intramolecular excimer formed from bischromophores linked by long chains (10–19 atoms). Photocyclomerization quantum yields were found to be (365 nm) 0.18–0.26 (benzene and ethanol); closure effiency is almost as high as for bisanthracenes linked by short chains. The photocyclomers, thermally unstable, are readily dissociated to In at room temperature. Several closure and reopening cycles could be observed in degassed diluted solutions. Under our conditions, addition of inorganic salts such as LiClO4 does not distinctly alter closure quantum yields of In. Rather, the complexes formed between the photocyclomers IIn (crown-ethers) and the cations are thermally stable up to high temperatures and were not observed to revert to In either thermally or by irradiation with a low pressure mercury lamp. But the compounds In were quantitatively recovered with an excess of polar solvents which compete with the crown-ethers to solvate the cation.

Quenching of the lowest‐lying metastable excimer of argon by N2 and effects of ’’three‐body’’ Ar(1s) quenching on the yield of Ar2(3Σ+u)

Abstract: Fast absorption spectrophotometry has been used to monitor the growth and decay of the Ar2(3Σ+u) excimer species at 992 nm in argon–nitrogen mixtures following passage of single pulses of a high current, high energy electron beam. The rate constant for electronic energy transfer to N2 is equal to 2.24 (±0.08)10−12 cm3 molecule−1 s−1 at 294 K. The yield of Ar2(3Σ+u) is found to be markedly enhanced relative to predictions based on the total quenching efficiencies of N2 for the Ar(1s) atoms. This enhancement is attributed to ’’three‐body’’ quenching by N2, i.e., vibrational and rotational relaxation of dissociative excimer molecules formed by two‐body Ar(1s)+Ar(1S0) combinations.

Polarized Fluorescence Spectrum of Naphthalene Excimer in a Stretched Polymer Film

Abstract: The polarized fluorescence spectrum of the naphthalene excimer was measured with the aid of a stretched polymer film technique, and the polarization of the excimer fluorescence was determined to be isotropic. The PPP method extended to a three dimensional way has been applied to calculate the electronic structure of the naphthalene excimer, in which various kinds of thermal distortion (vibration) modes are considered. The isotropic nature of the excimer fluorescence polarization has been explained assuming that two kinds of thermal distortion modes are evenly contributed to the excimer emission. Mit Hilfe der Methode der gereckten Polymerfolien wurde festgestellt, das das polarisierte Fluoreszenzspektrum des Naphthalin-Excimeren isotrop ist. Zur Berechnung der Struktur des Excimeren wurde eine dreidimensionale PPP-Methode benutzt. Dabei wurden verschiedene mogliche Geometrien fur das Excimere berucksichtigt. Die isotrope Natur der Excimer-Fluoreszenzpolarisation wurde dadurch erklart, das zwei verschiedene, im thermischen Gleichgewicht miteinander stehende, Excimere zur Emission beitragen.

Kinetics of excimer formation in lasers utilizing rare gas–fluorine mixtures

Abstract: An investigation was made of the intensity of excimer emission bands in Ar–F2, Ar–Kr–F2, and Kr–Xe–F2 gas mixtures as a function of pressure and temperature. The observed increase in the spontaneous and stimulated emission intensities at the 248 nm wavelength with rising temperature in an Ar–Kr–F2 mixture is associated with energy redistribution between KrF*, ArKrF*, and Kr2F* excimers.

Selective optical excitation and inversions via the excimer channel: Superradiance at the thallium green line

Abstract: Inversion and superradiance at the Tl green line (5351 A) has been observed using Tl‐Hg and Tl‐Cd‐Ar systems pumped optically via the excimer channel with a pulsed nitrogen dye laser. Superradiant power outputs up to 200 W averaged over the pulse (without any mirrors) have been obtained with a 90‐cm‐long Tl‐Hg tube. The general applicability of this method to other excimer systems should provide a means of generating coherent radiation at new wavelengths and in understanding collision processes.