C. E. Fellows

The long‐range potential of the K2 X 1Σ+g ground electronic state up to 15 Å

Titanium sapphire laser induced fluorescence spectra of the A 1Σ+u→X 1Σ+g electronic system have been recorded at high resolution by Fourier transform spectroscopy. Ground state vibrational levels are observed up to v=81 corresponding to an internuclear distance of 15.4 A and to 99.96% of the potential well depth. A long range study of the potential energy curves (RKR and IPA) allowed the determination of the coefficients of the dispersion energy (multipolar expansion representation) and of the exchange energy (exponential representation). The dissociation energy is found to be 4450.75±0.15 cm−1.

Photolysis of methane revisited at 121.6 nm and at 118.2 nm: quantum yields of the primary products, measured by mass spectrometry.

Methane photolysis has been performed at the two Vacuum UltraViolet (VUV) wavelengths, 121.6 nm and 118.2 nm, via a spectrally pure laser pump-probe technique. The first photon is used to dissociate methane (either at 121.6 nm or at 118.2 nm) and the second one is used to ionise the CH(2) and CH(3) fragments. The radical products, CH(3)(X), CH(2)(X), CH(2)(a) and C((1)D), have been selectively probed by mass spectrometry. In order to quantify the fragment quantum yields from the mass spectra, the photoionisation cross sections have been carefully evaluated for the CH(2) and CH(3) radicals, in two steps: first, theoretical ab initio approaches have been used in order to determine the pure electronic photoionisation cross sections of CH(2)(X) and CH(2)(a), and have been rescaled with respect to the measured absolute photoionisation cross section of the CH(3)(X) radical. In a second step, in order to take into account the substantial vibrational energy deposited in the CH(3)(X) and CH(2)(a) radicals, the variation of their cross sections near threshold has been simulated by introducing the pertinent Franck-Condon overlaps between neutral and cation species. By adding the interpolated values of CH quantum yields measured by Rebbert and Ausloos [J. Photochem., 1972, 1, 171-176], a complete set of fragment quantum yields has been derived for the methane photodissociation at 121.6 nm, with carefully evaluated 1σ uncertainties: Φ[CH(3)(X)] = 0.42 ± 0.05, Φ[CH(2)(a)] = 0.48 ± 0.05, Φ[CH(2)(X)] = 0.03 ± 0.08, Φ[CH(X)] = 0.07 ± 0.01. These new data have been measured independently of the H atom fragment quantum yield, subject to many controversies in the literature. From our results, we evaluate Φ(H) = 0.55 ± 0.17 at 121.6 nm. The quantum yields for the photolysis at 118.2 nm differ notably from those measured at 121.6 nm, with a substantial production of the CH(2)(X) fragment: Φ[CH(3)(X)] = 0.26 ± 0.04, Φ[CH(2)(a)] = 0.17 ± 0.05, Φ[CH(2)(X)] = 0.48 ± 0.06, Φ[CH(X)] = 0.09 ± 0.01, Φ(H) = 1.31 ± 0.13. These new data should bring reliable and essential inputs for the photochemical models of the Titan atmosphere.

A new genetic algorithm to be used in the direct fit of potential energy curves to ab initio and spectroscopic data

We propose a two-step genetic algorithm (GA) to fit potential energy curves to both ab initio and spectroscopic data. In the first step, the GA is applied to fit only the ab initio points; the parameters of the potential so obtained are then used in the second-step GA optimization, where both ab initio and spectroscopic data are included in the fitting procedure. We have tested this methodology for the extended-Rydberg function, but it can be applied to other functions providing they are sufficiently flexible to fit the data. The results for NaLi and Ar2 diatomic molecules show that the present method provides an efficient way to obtain diatomic potentials with spectroscopic accuracy.

The diffusion kinetics of a nanofilled and a midifilled resin composite immersed in distilled water, artificial saliva, and lactic acid

This study investigated the diffusion kinetics of a nanofilled (Filtek Z350) and a midifill (Filtek P60) resin composite immersed in distilled water, artificial saliva and lactic acid. Resin composite specimens were desiccated, immersed in the media, weighed at suitable time intervals until they reached sorption equilibrium and were then desiccated again. Sorption and solubility (µg/mm3) were calculated based on ISO 4049:2000(E). The diffusion coefficient (m2.s−1) was determined according to Flick’s second law. The degree of conversion (DC%) was evaluated by FT-IR and the action of the media on the surfaces of the resin composite was evaluated by SEM. Z350 immersed in lactic acid presented the highest sorption (25.9 ± 1.3). The highest solubility was presented by Z350 immersed in lactic acid (5.6 ± 0.9), followed by P60 immersed in lactic acid (4.4 ± 0.5). The other groups presented no significant difference among them. The diffusion coefficients of both resin composites immersed in lactic acid and that of Z350 immersed in artificial saliva were significantly higher. The lowest diffusion coefficient was presented by P60 immersed in distilled water. The DC% was not significant, (p > 0.05). The SEM analysis showed that the effect of lactic acid on the resin composites was more deleterious than those of water and artificial saliva.

GENERALIZED SIMULATED ANNEALING METHOD IN THE ANALYSIS OF ATOM-ATOM INTERACTION

Abstract The techniques of generalized simulated annealing are here presented in a way to fit the external turning points of a diatomic molecule potential energy curve. In order to evaluate the advantages of this method the long-range part of the potential energy curve for the A 1 Σ + u electronic state of the 6 Li 2 molecule is treated using the multipolar expansion and the asymptotic exchange energy considering the gradual change from a Hunds case a towards Hunds case c. The results are carefully compared with previous determinations using conventional fitting methods of molecular spectroscopy.

Amplified spontaneous emission of proton transfer dyes in polymers

We compare the Amplified Spontaneous Emission (ASE) of the proton transfer dyes 2-(2’-hydroxyphenyl) benzimidazole (HPBI) and 2-(2’-hydroxy-5’-chlorphenyl)benzimidazole (Cl-HPBI) in three different polymeric hosts under UV pulsed laser excitation. Pulseform and degradation of the dyes in the solid polymers were analysed. Spectroscopic data were used in a simple four-level kinetic rate equation system, in the attempt to explain the observed results, i.e. first order exponential decay of maximum output energy and pulse shortening. The model includes transitions for the normal and tautomer form of the molecule as well as intersystem crossing and triplet-triplet transitions.

A 3% efficiency N2 laser

A 3% efficiency, crossed-field type, N2 laser (337.1 nm) has been developed, based on very low inductance circuits (coaxial configuration) and a careful selection of their resonance frequencies.

Direct fit of spectroscopic data of diatomic molecules by using genetic algorithms: II. The ground state of RbCs

We extend our previous methodology based on genetic algorithms (Marques et al 2008 J. Phys. B: At. Mol. Opt. Phys. 41 085103) to carry out the challenging fit of the RbCs potential curve to spectroscopic data. Specifically, we have fitted an analytic functional form to line positions of the high-resolution Fourier transform spectrum of RbCs obtained by a laser-induced fluorescence technique. The results for the ground electronic state of RbCs show that the present method provides an efficient way to obtain diatomic potentials with great accuracy.

Preionization effects of iso-C4H10 in N2 TE UV lasers

In this paper the ionizing role of isobutane (iso-C4H10) in the operation of a N2TE UV laser is analyzed. Laser pulse width value modifications, different laser wavelengths generated in the 2+ system and voltage pulse period modifications are analysed in order to show that isobutane, in very reduced quantities, generates a preionizing effect that can be higher than that produced by a conventional wire preionization system.

Additional data on the (4)Σ1+ state of RbCs and potential determination

We report measurements of nine additional term values of the (4)Σ1+ state of RbCs. These data plus previously published data on band origins and on term values from higher rovibrational levels yield a more accurate determination of the potential of this state, which confirms the form obtained in ab initio relativistic calculations.