G. Baldacchino

A femtosecond fluorescence up-conversion study of the dynamic Stokes shift of the DCM dye molecule in polar and non-polar solvents

Abstract Time-resolved fluorescence spectra of the DCM dye molecule in methanol and chloroform have been studied using the fluorescence up-conversion technique with femtosecond time resolution. There is only one fluorescent excited state for all times and the dynamic Stokes shift observed is only due to solvent relaxation. In methanol, the mean position of the fluorescence spectrum shifts towards the red bi-exponentially (175 fs and 3.2 ps) while in chloroform it remains practically unchanged for all times. A spectral narrowing with a 10 ps time constant in methanol (7 ps in chloroform), due to vibrational energy dissipation from the excited solute to the surrounding solvent, is also observed.

Direct time-resolved measurement of radical species formed in water by heavy ions irradiation

Abstract Time-resolved absorption spectroscopy has been used to detect radical species in water ionised by swift heavy ions. These experiments have been performed at the GANIL cyclotron (Caen–France) with 12C6+ (75 MeV/A) and 36S16+ (77 MeV/A) particles whose linear energy transfer (LET=−(dE/dx)elec is respectively two and three orders of magnitude greater than electron or gamma rays. The chemistry in water is thus considerably modified at the first moments after the ionisation. In particular the radiolytic yields of the radicals like hydrated electron decrease when the LET is increasing. The temporal structure of the beam is used to perform time-resolved spectroscopy of the transient radicals formed by water radiolysis within the microsecond time scale. It is possible to show the inhomogeneous chemistry around the ion tracks and to compare these results with the Monte Carlo simulations.

A nanosecond pulse radiolysis study of the hydrated electron with high energy carbon ions

The radiolysis yields in pure water were determined using nanosecond pulse radiolysis using high linear energy transfer (LET) particles (1 ns pulses of high energy 1 GeV carbon ion beam). The main characteristics of this experiment were the nanosecond time resolution with heavy ion beam and the single value of LET along the ion track: 27 eV/nm. The kHz repetition rate of the pulsed beam used to reach a good signal-to-noise ratio in these experiments required the development of an acquisition method and a mathematical treatment of a DPO screen. The measured radiolytic yield for the hydrated electron at the nanosecond scale is quite high at 4.5×10−7 mol/J. A comparison of the experimental results obtained with a Monte Carlo simulation shows a rather good agreement at this LET value. To a certain extent this will allow a better adjustment of the Monte Carlo code.

Femtosecond absorption and emission spectroscopy of the DCM laser dye

Abstract The results of a femtosecond laser investigation of the photophysical and photochemical properties of the DCM styryl laser dye molecule are reported. Femtosecond pump-probe experiments (absorption/gain spectroscopy using a white light continuum and up-conversion fluorescence spectroscopy) show a red shift of the gain and emission spectra and a blue shift of the S 1 → S N absorption band only due to a fast reorganization of the solvent cage around the highly polar fluorescent first singlet excited state of DCM. This first complete study (spontaneous emission, gain and absorption) of DCM with a pump-probe intercorrelation function around 300 fs (FWHM) demonstrates that the hypothesis of a transition from a locally excited (LE) to a twisted intramolecular charge transfer (TICT) state is not valid for DCM.

Mechanism of radiation-induced reactions in aqueous solution of coumarin-3-carboxylic acid: Effects of concentration, gas and additive on fluorescent product yield

Abstract The radiation-induced reactions of a water-soluble coumarin derivative, coumarin-3-carboxyl acid (C3CA), have been investigated in aqueous solutions by pulse radiolysis with a 35 MeV electron beam, final product analysis following 60Co γ-irradiations and deterministic model simulations. Pulse radiolysis revealed that C3CA reacted with both hydroxyl radicals (•OH) and hydrated electrons (e− aq) with near diffusion-controlled rate constants of 6.8 × 109 and 2.1 × 1010 M−1 s−1, respectively. The reactivity of C3CA towards O2• − was not confirmed by pulse radiolysis. Production of the fluorescent molecule, 7-hydroxy-coumarin-3-carboxylic acid (7OH-C3CA), was confirmed by final product analysis with a fluorescence spectrometer coupled to a high performance liquid chromatography (HPLC) system. Production yields of 7OH-C3CA following 60Co γ-irradiations depended on the irradiation conditions and ranged from 0.025 to 0.18 (100 eV) −1. Yield varied with saturating gas, additive and C3CA concentration, implying the presence of at least two pathways capable of providing 7OH-C3CA as a stable product following the scavenging reaction of C3CA with •OH, including a peroxidation/elimination sequence and a disproportionation pathway. A reaction mechanism for the two pathways was proposed and incorporated into a deterministic simulation, showing that the mechanism can explain experimentally measured 7OH-C3CA yields with a constant conversion factor of 4.7% from •OH scavenging to 7OH-C3CA production, unless t-BuOH was added.

Radical mechanisms in the radiosterilization of metoprolol tartrate solutions.

AbstractPurpose. Study of the radical mechanisms in the radiosterilization of metoprolol tartrate aqueous solutions in order to determine the parameters governing its radiostability. Methods. Pulse radiolysis with pseudo-first-order kinetics to measure the reaction rate constants of hydrated electrons and hydroxyl radicals with metoprolol tartrate. Chemsimul® was used to solve the decay kinetics of transients and to simulate the radiolysis of metoprolol tartrate solutions. Results. Hydrated electrons react with metoprolol and the tartrate ion with rate constants of 6.8 × 107 M−1 s−1 and 1.7 × 107 M−1 s−1, respectively. Hydroxyl radicals react with metoprolol and the tartrate ion with rate constants of 5.2 × 109 M−1 s−1 and 5.5 × 108 M−1 s−1, respectively. The hydroxyl-metoprolol transients are found to scavenge the superoxide anion (5.5 × 1010 M−1 s−1), react with oxygen (1.0 × 108 M−1 s−1), and follow a biradical decay (2.0 × 108 M−1 s−1). A simplified radical mechanism is used to simulate the loss of potency of metoprolol tartrate aqueous solutions during radiosterilization. Conclusions. To decrease the loss of potency of metoprolol tartrate, the sterilization dose must be lowered and very high dose rates used.

Sub-picosecond fluorescence study of the LDS 751 dye molecule in ethanol

Abstract A time-dependent fluorescence Stokes shift study of the LDS 751 (styryl 8 ) dye molecule in ethanol solution is presented. Fluorescence intensity curves were recorded with sub-picosecond time resolution at various wavelengths using the fluorescence upconversion method. From the experimental curves, time-resolved fluorescence spectra have been reconstructed, allowing the calculation of the spectral shift correlation function c(t)=[ ν (t)- ν (∞)]/[ ν (0)- ν (∞)]. We find c(t) of LDS 751 in ethanol at room temperature to be well described by a monoexponential function with a 1/e time of 5±1 ps.

Femtosecond laser studies of the DCM push-pull molecule

We report the results of our femtosecond laser study of the photophysical and photochemical properties of the DCM styrenic dye. Our femtosecond pump-probe experiments using a white light continuum show a red shift of the gain spectrum and a blue shift of the S1 yields Sn absorption band due to a fast reorganization of the solvent cage around the highly polar fluorescent first singlet excited state. There is no evidence of any locally excited (LE) to twisted intramolecular charge transfer (TICT) state transition.

Revisited water radiolysis at elevated pH by accounting O3˙− kinetics at low and high LET

Ozonide radical, O3˙−, is used in this study for probing radiolytic species formed in the radiolysis of liquid water at elevated pH. Its formation allows the scavenging of O˙− whereas its disappearing is due to its reactions with HO2− and O2˙−. This article focuses on the formation and the reaction of O3˙− at pH 13.2 by using pulse radiolysis technique, with 10 MeV electrons and 1 GeV C6+ ions beams for the first time. This allowed us to work with two different Linear Energy Transfer (LET) values (0.27 and 33 eV nm−1 respectively), which means with two different primary distributions of radiolytic species. Consequently the rates constant of reactions O3˙− + HO2− and O3˙− + O2˙− could be revised with acceptable accuracy thanks to deterministic simulations: (1.1 ± 0.2) × 106 M−1 s−1 and (1.5 ± 0.5) × 107 M−1 s−1 respectively. Furthermore, the primary radiolytic yields of HO˙/O˙− and H2O2/HO2− at high LET and pH 13.2 were estimated at 6.5 and 9.6 × 10−8 mol J−1 respectively which corresponds to the literature values.

Understanding of Corrosion Mechanisms after Irradiation: Effect of Ion Irradiation of the Oxide Layers on the Corrosion Rate of M5

Irradiation damage in fuel cladding material is mainly caused by the neutron flux that results from fission reactions occurring in the fuel. To avoid the constraints inherent in handling radioactive material, the irradiation effects on the corrosion resistance of zirconium alloys can be studied by irradiating the materials with ions. We performed an original experiment using ion irradiation to more specifically study the influence of irradiation damage in the oxide on the corrosion rate of M5®. It has been established that irradiation with a 1.3-MeV helium ion at a fluence of 1017 cm−2 results in significant modifications of oxide properties, oxygen diffusion flux, and oxidation kinetics, as evidenced by Raman spectroscopy, secondary ion mass spectrometry (SIMS) analyses, and measurements of mass gains. A newly identified Raman vibration band at 712 cm−1 was linked to the presence of irradiation defects and allowed the evolution of their concentrations to be followed. The oxygen diffusion flux was significantly reduced after irradiation partly due to a surface concentration decrease of oxygen. The defects remained present in the oxide after 100 days of annealing in pressurized water reactor (PWR) conditions and were thus very stable in PWR conditions, which probably means that these defects would be stable in the reactor. According to the kinetics and in agreement with the results obtained by SIMS analyses, the oxidation rate was significantly reduced after ion irradiation, and this effect remained beyond 100 days in agreement with the high stability of irradiation defects in PWR conditions. An original model described quite well the oxidation kinetic results.