M.C. Castex

Experimental determination of the lowest excited Xe2 molecular states from VUV absorption spectra

A quantitative analysis of absorption by Xe2 pairs in the wings of the two first resonant lines (1469.6 and 1295.6 A) is presented as a function of pressure (P<760 Torr) and temperature (130 K<T<300 K). Using a quasistatic theory, absorption measurements provide valuable information on the lowest 0u+ and 1u excited states, the 0g+ ground state potential being known from literature. In the near wing region (‖Δν‖<100 cm−1) the shape of the line wings is used as a probe of the asymptotic −2C3/R3−C60/R6 (0u+) and +C3/R3−C61/R6 (1u) potentials assuming at large R a constant oscillator strength, C3 and C6 excited constants are determined. Furthermore the dependence of absorption with temperature is used to derive additional information about the upper states and also to put in evidence some variation of transition moments, at shorter internuclear distances (3.7 A <R<4.6 A). The potential curves experimentally derived are compared with ab initio calculated curves.

White organic light-emitting diodes with fine chromaticity tuning via ultrathin layer position shifting

Nondoped white organic light-emitting diodes using an ultrathin yellow-emitting layer of rubrene (5,6,11,12-tetraphenylnaphtacene) inserted on either side of the interface between a hole-transporting 4,4′-bis[N-(1-naphtyl)-N-phenylamino]biphenyl (α-NPB) layer and a blue-emitting 4,4′-bis(2,2′-diphenylvinyl)-1,1′-biphenyl (DPVBi) layer are described. Both the thickness and the position of the rubrene layer allow fine chromaticity tuning from deep blue to pure yellow via bright white with CIE coordinates (x=0.33, y=0.32), an ηext of 1.9%, and a color rendering index of 70. Such a structure also provides an accurate sensing tool to measure the exciton diffusion length in both DPVBi and NPB (8.7 and 4.9nm, respectively).

Spectroscopic investigation of the electronic structure of the chlorine molecule in the VUV

Abstract Vacuum ultraviolet absorption spectroscopy and fluorescence analysis under selective optical excitation have been combined to deduce the electronic structure of Cl2. Between 73000 and 81000 cm−1 five bound electronic states could be analysed. Some of them are affected by pronounced Rydberg—valence mixing. Especially the 1 1∑u+ state clearly yields a double-well structure which results from an avoided crossing between an ionic valence and a Rydberg state. The double-well structure is responsible for an irregular vibrational sequence between 73000 and 74500 cm−1 which either prevented an analysis in earlier investigations or lead to wrong assignments. The FC factors of the 1 1∑u+ fluorescence with its bound—bound and bound—free contributions between ≈ 73000 and 50000 cm−1 are also severely affected by the double-well structure. The results are compared with the first ab initio calculations of the electronic structure of Cl2. In general, excellent agreement is found.

Solar blind chemically vapor deposited diamond detectors for vacuum ultraviolet pulsed light-source characterization

A major difficulty in characterizing vacuum ultraviolet (VUV) radiation produced by harmonic generation or four-wave sum frequency mixing arises in differentiating between the desired VUV signal and the remaining fundamental pump laser beam. To overcome this problem, visible and near UV blind VUV detectors, made from natural and synthetic diamond, have been developed. Such detectors have been used to characterize coherent VUV pulses (λ=125 nm, pulse duration at full width half maximum (FWHM) τFWHM∼7 ns) generated by resonance-enhanced four-wave sum mixing in mercury vapor. They allow full characterization of the intensity profile of the VUV pulses, without any significant parasitic signal from simultaneous stray light irradiation at λ=313 nm. Detectors were fabricated exhibiting response times of less than 70 ps at FWHM, corresponding to the lowest response time obtainable with a 7 GHz bandwidth single-shot oscilloscope.

Theoretical and experimental determination of the lowest excited states of the Kr*2 excimer

The potential energy curves of the Kr*2 excimer dissociating into Kr (4s24p6 1S0)+Kr*(4s24p55s) are determined (i) theoretically from ab initio CI calculation and semiempirical SO coupling and (ii) experimentally from the temperature dependence of absorption profiles. The results are carefully examined with the help of theoretical simulation of line profiles using semiclassical or quantal models.

Formation of KrCl* and ArCl* molecules and radiative lifetimes of their B states investigated with selective synchrotron radiation excitation

Abstract Monochromatic pulsed VUV excitation of Cl2 or inert gas atoms in Cl2 doped Kr and Ar leads to formation of KrCl* and ArCl*. The radiative lifetimes of the B states (KrCl* 19 ns, ArCl* 9 ns) and rate constants for excimer formation, quenching and collisional mixing of B and C states are given. The radiative lifetime and the quenching rate increase with vibrational excitation of the B state (KrCl*).

Diamond UV detectors for future solar physics missions

Despite their steady improvement over the last decades, the present UV detectors exhibit some limitations inherent to their silicon technology. Yet, the utmost spatial resolution, temporal cadence, sensitivity and photometric accuracy will be decisive for the forthcoming space solar missions. The advent of novel diamond or nitride imagers would surmount many current weaknesses, thus opening up new prospects and making the instruments cheaper. As for projects like the Solar Probe of NASA, or the Solar Orbiter of ESA, the aspiration for diamond UV detectors is still more sensible since these spacecrafts will approach very near to the Sun where the heat and the radiation fluxes are tremendously high. This triggered the initiative of an original R and T programme entitled BOLD described in this paper. We depict motivations and intentions and report on dedicated experiments with several devices under EUV synchrotron light, NUV laser and micro-Raman spectroscopy.

Laser operation in nondoped thin films made of a small-molecule organic red-emitter

Stimulated emission in small-molecule organic films at a high dye concentration is generally hindered by fluorescence quenching, especially in the red region of the spectrum. Here we demonstrate the achievement of high net gains (up to 50 cm−1) around 640 nm in thermally evaporated nondoped films of 4-di(4′-tert-butylbiphenyl-4-yl)amino-4′-dicyanovinylbenzene, which makes this material suitable for green-light pumped single mode organic lasers with low threshold and superior stability. Lasing effect is demonstrated in a distributed Bragg resonator configuration, as well as under the form of random lasing at high pump intensities.

Theoretical study of the electronic structure of the ArKr* exciplex

Abstract The lowest energy potential curves of the ArKr* exciplex are computed using non-empirical pseudopotentials and multireference configuration interaction. The {σL S Sσ} states of ArKr* are obtained from relativistic averaged pseudopotentials, while the fine structure {σW} states are derived using ab initio spin-orbit pseudopotentials and achieving a diagonalisation of the total Hamiltonian(electrostatic+spin-orbit) in the basis of {σL S σS} states. The electronic structure of the exciplex is discussed and the ionic core nature of the Rydberg states is analysed.

Photoluminescence of donor–acceptor carbazole-based molecules in amorphous and powder forms

We present absorption and photoluminescence features of four samples of carbazole molecules substituted with various electron–acceptor groups. These molecules named 1-(N-ethylcarbazolyl)-2-substituted-2-cyanovinylene contain in their structure the electron–donor carbazole nucleus and cyanovinylene bearing either another nitrile function, an ethylester, a phenyl, or a para-nitrophenyl groups. It is shown that depending on the strength of the donor–acceptor internal charge transfer, both the absorption and emission spectra are more or less redshifted. It is found that the ethyl-ester derivative displays the best relative photoluminescence efficiency among all the samples and its peak is measured at 490 nm when taking amorphous thin film. The microcrystalline powder form of the same material exhibits spectral narrowing and shift of the peak emission. We obtain further narrowing of the emission band and further redshifting of the emission when we illuminate, transversely, a glass capillary containing the crystalline sample by an ultraviolet light-emitting diode.