Showing papers on "Excimer published in 1980"

Source of anomalous fluorescence from solutions of 4-N,N-dimethylaminobenzonitrile in polar solvents

Abstract: The interactions of the first excited singlet state of 4-N,N-dimethylaminobenzonitrile (DMABN) and of 3,5-N,N-tetramethyl-4-aminobenzonitrile (3,5-TMABN) with polar molecules have been studied. Mixtures of cyclohexane and a small amount of polar liquid have been used as solvents. The anomalous fluorescence of DMABN originates from a 1:1 complex between excited solute and a polar solvent molecule. The fluorescence of 3,5-TMABN in polar solvents arises only from exciplexes. A model is presented which describes the binding in the exciplexes in terms of a localized interaction between lone pair electrons of the polar solvent molecule with lone pair electrons of the dimethylamino group of the excited solute. The solvent-induced wavelength shift of any particular exciplex fluorescence of the two compounds can be correlated with the dielectric properties of the medium. If the conformation of the amino group is fixed as in 1-methyl-5-cyano-indoline (1-M-5-CI) and in 1-methyl-6-cyano-1,2,3,4-tetrahydroquinoline (1-M-6-CTHQ) no formation of exciplexes with polar solvents is observed. The formation of exciplexes does not only lead to a new fluorescence, but also enhances the intersystem crossing in the two compounds, DMABN and 3,5-TMABN. There is no enhancement of the triplet quantum yield in polar solvents in the case of 1-M-6-CTHQ. The quantum yield of the normal fluorescence of both DMABN and 3,5-TMABN in polar solvents depends on the wavelength of excitation. The quantum yield of exciplex emission does not show this behaviour.These two observations are explained by taking the existence of weakly bound complexes of ground state solutes as well as the high rate of exciplex formation into account.

Influence of starting conformations on intramolecular exciplex formation in .omega.-phenyl-.alpha.-N,N-dimethylaminoalkanes

Abstract: The intramolecular complex formation of w-phenyl-a-N,N-dimethylaminoalkanes in isopentane was investigated using stationary and nonstationary techniques. Analysis of the decay curves indicates that ground-state starting conformations influence the rate of complex formation. In deviation of the classical kinetic scheme for interand intramolecular exciplex and excimer formation, the emission of the locally excited state contains a slow-decaying component whose lifetime is longer than that of the exciplex. This slow-decaying emission is attributed to molecules in which the C-N bond has a conformation unfa­ vorable for exciplex formation. Introduction the intermolecular processes.5 The adopted scheme could be Up to now it has been assumed that intramolecular exciplex presented as shown in Scheme I. The ratio of the quantunl and excimer formation 24 follows the same kinetic scheme as yields for emission from the exciplex, cPE, and from the locally

Luminescence of Free and Self Trapped Excitons in Pyrene

Abstract: The resonance luminescence due to radiative annihilation of free excitons in Pyrene has been observed. The luminescence band is located at 376 nm (F) at room temperature and is quite distinctive from so-called excimer luminescence band located at 450 nm. There is a potential barrier between the free exciton state and the excimer state which is located 540 cm -1 (66 meV) lower in energy with respect to the free exciton state. The luminescence decays exponentially with a life time of 180 nsec for both free excitons and excimers at 282 K. Discussion is given to explain free exciton creation and luminescence processes in terms of thermal equilibrium established between free excitons and excimers. The excimer is recognized as the self trapped exciton.

Nanosecond laser photolysis of the benzene monomer and eximer

Abstract: A dilute solution of benzene was photolyzed with a KrF excimer laser. The Sn←S1 and Tn←T1 absorption spectra are measured with time‐resolved spectroscopy. The Sn←S1 absorptions in the energy range between 850 and 220 nm were analyzed. The transient absorption band at 500 nm, which was observed previously by several workers disappeared completely upon dilution of the sample, and thus was assigned to be the excimer band. We assigned the other states as follows: 620 nm (1E1u, f∼0.003), 540–325 nm (11E2g, f∼0.04), and 270 nm (21E2g, f∼0.12). The observed energies from the ground state to the 11E2g and 21E2g states 7.8 and 9.2 eV, respectively. The Tn←T1 spectrum showed a single peak at 235 nm (f∼0.35) and a shoulder around 310 nm (f∼0.12).

One host–two guests complexation between γ-cyclodextrin and sodium α-naphthylacetate as shown by excimer fluorescence

Abstract: The effect of increasing cyclodextrin (CD) concentration on the fluorescence spectra of sodium α-naphthylacetate is negligible for α-CD, large only in the normal fluorescence band for β-CD, and large for both the normal and the excimer fluorescence bands for γ-CD, showing the ability of γ-CD to form one host–two guests complexes.

Excimers and excimer lasers

Abstract: The development of excimers and excimer lasers are reviewed The excimers of the noble gases (Xe 2 * , Kr 2 * , Ar 2 * ) and of the noble gas halides (eg KrF, XeCl) which, respectively, radiate in the vacuum ultra-violet and ultra-violet regions of the spectrum are described in terms of their structure, spectroscopy and formation kinetics and the methods of pumping, operational characteristics and applications of the lasers are discussed

EXCITED STATES OF DNA AND ITS COMPONENTS AT ROOM TEMPERATURE—III. SPECTRA, POLARISATION AND QUANTUM YIELDS OF EMISSIONS FROM ApA AND poly rA*

Abstract: — Corrected normalized emission spectra are reported for poly rC, CpC and CMP in neutral aqueous solution at room temperature. The emissions from CpC and poly rC show progressive and extensive depolarization across the emission band compared with CMP. Self-consistent analysis of the polarization and spectral data allows the resolution of red-shifted component emissions from the overall spectra and the evaluation of component polarizations. By comparison with experimental reference spectra three long wavelength components are identified in the poly rC emission and assigned as excimer fluorescence, monomer phosphorescence and excimer phosphorescence, whereas for CpC only two components are observed. The quantum yields are interpreted quantitatively in terms of the known stacking behavior of CpC and poly rC. with excimer emissions originating from the stacked fraction. Quantitative differences between CpC and poly rC are due to their different hypochromism and degree of stacking. Differences of polarization of excimer fluorescence are a consequence of different stacking geometries, and the directional properties of the transition moment suggest the excimer fluorescence is largely charge-resonance in nature. The behavior of the CpC/poly rC system is compared with ApA/ poly rA.

Li2 and Na23Σg+–3Σu+ excimer emission

Abstract: Ab initio calculations show the 3Σg+–3Σu+ in Li2 and Na2 dimers to be primarily a near‐infrared continuum with respective v′=0 lifetimes of 62 and 15 nsec. The peak stimulated emission cross section for v′=0 is 4.5×10−−6 at 1.3μ for Li2 and 1.8×10−15 cm2 at 0.83μ for Na2. These calculations suggest a tunable high gain, near‐infrared laser excimer if the 3Σg+ state can be populated sufficiently rapidly.

Intramolecular excimer fluorescence: a new probe of phase transitions in synthetic phospholipid membranes

Abstract: — The molecule (1,1'-dipyrenyl)-methyl ether [dipyme] shows intramolecular excimer fluorescence in competition with fluorescence from the locally excited pyrene chromophore. This intensity ratio Ic/Im is sensitive to solvent viscosity. The molecule is soluble in synthetic phospholipid membranes. 1H NMR studies suggest that the molecule is localized in the hydrocarbon region of the membrane. Fluorescence measurements at various temperatures of dipyme dissolved in these membranes show that Ic/Im is exceedingly sensitive to fluidity changes accompanying both the pretransitions and the gel to liquid crystalline transitions of the membrane. These studies can be carried out at a mole ratio of probe to lipid 102-103 smaller than that necessary to observe bimolecular pyrene excimer formation.

Intramolecular excimer formation in macromolecules. II. Energy migration and excimer formation in acenaphthylene-methyl methacrylate copolymers

Abstract: Excimer formation has been studied in a series of ancenaphthylene—methyl methacrylate copolymers of content varying over the total composition range. In accord with a previous model [Reid and Soutar, J. Polym. Sci. Polym. Lett. Ed., 15, 153 (1977)], the excimer formation has been described by a composite function representative of energy migration and excimer site concentration. In tramolecular energy migration in glassy solutions of the polymers was characterized by use of fluorescence depolarization measurements. The extent of energy migration in the acenaphthylene copolymers is determined by the mole fraction of aromatic chromophore fa. A function Σ descriptive of the excimer site concentration has been generated on the assumption that excimer formation results predominantly from next- to nearest-neighbor interactions. Σ is based upon the pentad distribution of aromatic chromophores in the chain and contains pentad fractions appropriately weighted according to the frequency and relative importance of their potential excimer sites. Reactivity ratios of acenaphthylene and methyl methacrylate in copolymerization at 70°C were ra = 0.80 and rm = 0.57.

Photoionization mass-spectrometry of benzene and benzaldehyde molecules with an excimer KrF laser

Abstract: A laser photoionization time-of-flight mass-spectrometer has been developed and used to investigate the photoionization and photofragmentation of benzene and benzaldehyde molecules by an excimer KrF laser radiation at the wavelength of 249 nm in the intensity range from 5·104 to 5·109 W/cm2. It has been found that at low laser intensity ions formed by two-step photoionization are most abundant in mass spectra. By increasing laser intensity an extensive fragmentation of molecules, up to C+ ions, was observed. The maximum ionization yield of benzaldehyde comes to 0.1% of the number of molecules in the photoionization volume and, according to calculations, to 10% for benzene molecules, when the radiation intensity is 5·109 W/cm2.

Vacuum ultraviolet excimer lasers

Abstract: The operation and characteristics of VUV noble gas excimer lasers are discussed. Some applications to nonlinear optics, photoionization, and photodissociation studies are described.

Models of high-power discharges for metal-Xe excimer lasers

Abstract: High‐power (∼108 W/l) discharges in metal‐doped Xe are modeled for typical metal atom densities of 1015–1017 cm−3 and Xe densities of ∼1020 cm−3, and electron densities of 1014–1017 cm−3 as appropriate for proposed excimer lasers. Na is used as a prototype species, while its properties are varied to indicate some of the changes that could result from the use of different metals. The model includes sixteen excited levels of Na, three ionic species, the excimer levels of NaXe, and Na2. The degree of ionization is determined by collisional multistep excitation and ionization of excited atoms versus dissociative recombination of electrons with Na+2. Steady‐state conditions in the positive column are calculated for typical gas temperatures of ∼0.06 eV and electron temperatures Te of 0.3–0.5 eV. The Na population distribution is largely Boltzmann at the electron temperature and the electron density is close to the Saha equilibrium value except at low electron temperatures and very high extracted laser powers. U...

Fluorescence and energy transfer in disordered solid tetracene

Abstract: Spectrum and decay time of the fluorescence of vacuum sublimed tetracene layers were measured as a function of temperature. The substrate temperature (Tf) during vapor deposition was varied between 80 K and 220 K. The emission spectrum can be fitted by superposition of two gaussians centered near 18.500 cm−1 (band I) and 17.850 cm−1 (band II), respectively, and a broad excimer band with a maximum near 16.400 cm−1. The decay time of band I and II is 6.2 ns, that of the excimer band is 21.3 ns, independent of temperature. Band I appears only with films prepared at Tf ⪢ 140 K and is associated with emission from crystalline areas of submicroscopic size, band II is attributed to (monomeric) defects present in a concentration of about 1.5% as evidenced by absorption studies. Since in low temperature films (Tf < 140 K) the originally excited singlet state does not fluorescence rapid energy transfer to both monomeric defects and incipient dimers capable of excimer formation must occur. The average hopping frequency of the S1-state must be at least 5 × 1010 s−1. Fluorescence excitation spectra support defect and dimer excitation by energy transfer from a common absorbing state.

Direct observation of intramolecular anthracene excimer in 1,3‐dianthrylpropane

Abstract: The observation of intramolecular anthracene excimer in 1,3,‐dianthrylpropane by coincidental line constant of the excimer rise with fluorescence decay of anthryl moiety is reported. (AIP)