Temperature dependence of the dual phosphorescence from xanthone in n-hexane matrices

The phosphoresence spectrum and lifetime of xanthone embedded in an n-hexane matrix has been investigated as a function of temperature (1·6-100 K). Vibrational analyses of the spectra reveal that emission occurs from three sites, two of which are dominant. Emission from one site (B) occurs from the ³ nπ* state of a planar xanthone molecule and is characterized by strong totally symmetric carbonyl stretching vibrations and a short lifetime (2·4 ms). Emission from the other site (C) is shown to arise from the ³ππ* state of an out-of-plane distorted xanthone molecule. It displays a vibrational structure rich in modes of a ₁, b ₁, and b ₂ (C _2v notation) symmetry and a long lifetime (115 ms). Both direct spin orbit coupling via configurational mixing of the nπ* and ππ* states due to the non-planarity of the molecule in its ³ππ* state and spin-orbit vibronic interaction involving ³ A ₁(ππ*)-¹ A ₂(nπ*) spin-orbit and ¹ A ₂(nπ*)-¹ B ₂(ππ*) vibronic interaction via out-of-plane b ₁ vibrations are shown to be responsible for the C site emission intensity. Vibronic mixing between the ³ππ* and ³ nπ* states is not important. With increasing temperature, the phosphorescence intensity from the B site (³ nπ*) emitters increases at first, reaches a maximum at ～20 K and then decreases. The C site (³ππ*) intensity simply decreases with rising temperature. At a given temperature, the phosphorescence lifetimes are identical and exponential for all emission bands regardless of site origin. These observations indicate an equilibrium between emitters in the two sites throughout the lattice. A phonon-assisted energy transfer mechanism is proposed to account for these observations.     

Modellrechnungen zur Isotopentrennung durch Multiphotonen-Photodissoziation von Molekülen

Für stoßfreie SF₆-Moleküle werden Mulltiphotonen-Photofragmentierungswahrscheinlichkeiten im intensiven IR-LASER-Feld berechnet einschließlich der Isotopenselektivität derartiger Prozesse in Abhängigkeit von der eingestrahlten LASER-Linie, der Leistungsdichte, dem Impulsprofil und der Temperatur. Über ein quantenmechanisches Modell werden die Rotationsschtwingungsniveaus des SF₆-Moleküls, die Verstimmung zwischen LASER- und Molekülschwin-gungsfrequenz und die intramolekulare Schwingungsrelaxation berücksichtigt.     

Reviews

Ion Implantation, Proceedings from the European Conference on Ion Implanatation, Reading, September 7–9 1970 Peter Peregrinus Limited, Southgate House, Stevenage Herts, England. England 1970, 241 pages. Price: $6.00

Lectures in Theoretical Physics Volume XI-C: Atomic Collision Processes Editors: S. Geltman, K. T. Mahanthappa and W. E. Brittin. Gordon and Breach 1969, pp xiii + 337.     

Electrodynamic Dust Shields on the International Space Station: Exposure to the space environment

Electrodynamic Dust Shields (EDS) have been in development at NASA as a dust mitigation method for lunar and Martian missions. An active dust mitigation strategy, such as that provided by the EDS, that can remove dust from surfaces, is of crucial importance to the planetary exploration program. We report on the development of a flight experiment to fully expose four EDS panels to the space environment. This flight experiment is part of the Materials International Space Station experiment X (MISSE-X), an external platform on the International Space Station that will expose materials to the space environment.     

Temperature dependent photoluminescence down to 4.2 K in EuTFC

The temperature dependence of photoluminescence in Europium tris[3-(trifluoro-methylhydroxymethylene)-(+)-camphorate] (EuTFC) embedded in polymer films has been examined from 40 K down to 4.2 K with the goal of preparing sensor films for low-temperature thermal imaging. The behavior of EuTFC showed significant difference when based on polystyrene compared to poly(n-alkyl methacrylate)s. In poly(n-alkyl methacrylate)s prepared by standard methods for imaging applications, the photoluminescence is fully saturated below 30 K, whereas in polystyrene films there is a strong temperature dependence even down to 4.2 K. By optimizing the preparation procedure for films made of poly(butyl methacrylate) (PBMA) and poly(methyl methacrylate), also these polymers became very sensitive down to liquid helium temperature. The maximum temperature sensitivity of EuTFC in PBMA is found to be 1.0%/K at 4.2 K. The problem of delamination and cracking of the polymer film at cryogenic temperature is also avoided by the special preparation method.     

Search for beta-delayed proton emission from $$^{11}$$Be

The European Physical Journal A - A deterministic treatment of sequential neutron emission, based on recursive equations of the residual temperatures, was applied to numerous fission cases (i.e. 49...     

Form factor π0 → γ* + γ* at different photon virtualities

The π⁰ γγ vertex for virtual photons of squared masses q ₁ ² and q ₂ ² plays a vital rôle in several physical processes; for example for q ₁ ² < 0, q ₂ ² < 0, in the two-photon physics reaction e ⁺ e ^? → e ⁺ e ^?π⁰, and for q ₁ ² > 0, q ₂ ² > 0, in the annihilation process e ⁺ e ^? → π⁰ l ⁺l^?. It is also of interest because of its link to the axial anomaly. We suggest a new approach to this problem. We have obtained a closed analytic expression for the vertex in the limit in which at least one of ¦q ₁ ² ¦ and ¦q ₂ ² ¦ is large for arbitrary fixed values of the ratio q ₁ ² /q ₂ ² . We compare our results with those obtained previously by Brodsky and Lepage. It should be straightforward to test our predictions experimentally.     

Cleavage of the ether bond by electron impact: differences between linear ethers and tetrahydrofuran

Dissociative electron attachment (DEA) to diethyl ether yielded primarily the C(2)H(5)O(-) ion, with a strong Feshbach resonance band at 9.1 eV and a weaker shape resonance band at 3.89 eV. Very similar spectra were obtained for dibutyl ether, with C(4)H(9)O(-) bands at 8.0 and 3.6 eV. Some of these primary ions subsequently lost H(2) and yielded weaker signals of the C(2)H(3)O(-) and C(4)H(7)O(-) ions. In contrast, DEA to the cyclic ether tetrahydrofuran (THF) yielded mainly a fragment of mass 41, presumably deprotonated ketene, at 7.65 eV. The low-energy band was missing in THF. H(-) with two bands at 6.88 and 8.61 eV, and an ion of mass 43 (presumably deprotonated acetaldehyde) with two bands at 6.7 and 8.50 eV were also observed. We propose that in the primary DEA step the C-O bond is cleaved in both the open-chain and the cyclic ethers. In the open-chain ethers the excess energy is partitioned between the (internal and kinetic) energies of two fragments, resulting in an RO(-) ion cool enough to be observed. The CH(2)(CH(2))(3)O(-) ion resulting from cleavage of the C-O bond in THF contains the entire excess energy (more than 6 eV at an electron energy of 7.65 eV) and is too short-lived with respect to further dissociation and thermal autodetachment to be detected in a mass spectrometer. These findings imply that there could be a substantial difference between the fragmentation in the gas phase described here and fragmentation in the condensed phase where the initially formed fragments can be rapidly cooled by the environment.     

A dramatic difference between the electron-driven dissociation of alcohols and ethers and its relation to Rydberg states

A difference was observed in the reactivity of alcohols and ethers toward free electrons. Whereas the lowest core-excited state of the negative ion-a (2)(n,3s(2)) Feshbach resonance-of the alcohols readily dissociates by losing a hydrogen atom, ethers show no observable signal from this resonance. This difference in reactivity has a parallel in the anomalous shapes and energies of the parent states of the Feshbach resonances, the (1)(n,3s) Rydberg states of the neutral alcohols. We explained this anomaly using potential surfaces of the alcohols and ethers calculated using the TD-DFT method as a function of the dissociation coordinate. The lowest excited state of alcohols was found to be repulsive, whereas a barrier to dissociation was found in the ethers. Rydberg-valence mixing and avoided crossings are decisive in determining the shapes of the potential surfaces. It is concluded that the reactivities of alcohols and ethers toward free electrons are rationalized by assuming that the potential surfaces of the daughter Feshbach resonances closely follow those of the parent Rydberg states, i.e., the lowest Feshbach resonance is repulsive, but a barrier occurs in ethers. The potential surfaces of both the Rydberg states and the Feshbach resonances thus differ dramatically from the non-dissociative surface of the grandparent (2)(n(-1)) positive ions, despite the nominally non-bonding character of the Rydberg electrons.