Propriétés optiques de petits agrégats de métaux nobles en matrice de néon

Sylvain Lecoultre
EPFL, 2009
Thèse EPFL

Résumé

The optical properties of small noble metal clusters have been studied by absorption and fluorescence for sizes between 1 and 10 atoms. The experimental technique consists in depositing size-selected clusters in a neon matrix and analyzing with a spectrometer the transmitted and fluorescing light through the sample in an energy range between 1.8 and 5.5 eV. A new experimental setup has been built. It allows refrigeration of an Aluminium support down to 6 K and condensation of the neon matrix. The design of the sample holder allows a precise measurement of the cluster current during deposition. In comparison to measurements in argon matrices, the optical transmission has been increased by two orders of magnitude on the entire UV-Visible range. Consequently, the signal to noise ratio is greatly improved. Absorption spectroscopy on silver, copper and gold clusters with sizes between 1 and 9 atoms have revealed for each case a large amount of optical transitions with peakwidth as narrow as 50 meV, typical for a molecular-like behavior. Fluorescence measurements by exciting the sample with a tunable laser source have been achieved. For the atomic and dimer gold systems, the experimental results show several new emission lines. The fluorescence spectra obtained for Ag8 and Ag9 in neon confirm the results obtained in argon and demonstrate that the interaction with the environment is relatively weak for larger clusters. A study on the efficient trapping of positively charged clusters inside argon and neon matrices has allowed the measurement of the absorption spectra of the silver trimer and pentamer in their cationic form. TD-DFT calculations have been performed in order to find the lowest energy structures and the excitation spectra for each system. The excellent agreement between theory and experiment allows us to assign with more certainty the measured data to given isomers for all the silver clusters and for about 50% of the copper and gold clusters. A Mulliken population analysis has allowed to find the s, p and d contributions for each calculated spectrum. This analysis shows that we have more or less pure s transitions up to 5 eV for silver clusters while the d electrons participate actively to the transitions even at low energies between 2 and 3 eV in copper and gold. A detailed discussion on the pentamers of silver, copper and gold is presented on the basis of the experimental and theoretical results.

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Sylvain Lecoultre
EPFL, 2009
Thèse EPFL

Résumé

Official source

https://infoscience.epfl.ch/record/133632?ln=fr

À propos de ce résultat

Concepts associés (24)

Concepts associés

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Publications associées (26)

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We report for the first time the absorption and fluorescence spectra of gold dimers in a neon matrix. The dimer absorption spectra show the A←X transition predicted from measurements in the gas phase and not observed so far in a matrix, as well as the so-called B←X and C←X transitions. Fluorescence measurements on the atom reveal new emission lines at 1.97, 3.59 and 4.09 eV that can be assigned to the 2P1/2→2D3/2, 2P1/2→2D5/2 and the 2P3/2→2D5/2 transitions. For the dimer, excitation of both A and B state results in distinct emission spectra with vibrational structure.

2009

Chargement

Influence of the protein environment on spectroscopy and ultrafast dynamics of retinal in bacteriorhodopsin

Selma Schenkl

The environment is important for the exact course of a chemical reaction. As an example of the strong influence of the environment on the reaction, this work studies the isomerization reaction of the chromophore retinal in the binding pocket of the protein bacteriorhodopsin. Three selected distance scales with reference to the chromophore are addressed: The distant protein surface at a length scale of 3-5 nm, the fuzzy environment of the binding pocket (1-2 nm) and the immediate neighborhood represented by the amino acid tryptophan Trp86 (5 Å). First, static absorption and fluorescence spectroscopies are applied to three dimensionally crystallized bacteriorhodopsin. An especially adapted set-up was developed for each task. The analysis of data is carried out in comparison with the measured spectra of the native protein membrane in solution. While the absorption spectrum shows a broadening on the high-energy side, the fluorescence spectrum deviates in the general shape of that measured in solution. The synoptic analysis of all spectra reveals the existence of three spectroscopically distinct species in the crystal: BR490 (absorption maximum at 490 nm) with a relative contribution of 28%, BR570 (native spectrum of solution, 68%), and BR610 ("blue membrane", 4%). BR490 appears particularly in the additional fluorescence band at 550 nm and is assigned to the dehydrated species of bacteriorhodopsin. The presence of that particular species indicates the suppression of rehydration in a partially dehydrated crystal and shows that the three dimensional arrangement hinders water diffusion. A simple model, close domains are assumed to form, eventually suppressing free water diffusion in a crystal. The lack of water on the protein surface modifies the protonation states of the amino acids in the binding pocket and therefore the optical properties of the retinal. The chromophore actually reacts on the alteration of its distant environment upon incorporation in the protein crystal. The second distance scale - the fuzzy environment - is addressed by a time-integrated fluorescence spectroscopy on native bacteriorhodopsin as function of the excitation wavelength (430 nm to 650 nm). The optimized experimental set-up guaranteed that the fluorescence of only the excited state I460 contributed. The measured spectra show maxima at 740 nm, and exhibit a Stokes shift of 5000 cm-1, independent of the excitation wavelength. The similarity of the fluorescence excitation spectrum with the spectrum of absorbed photons excludes energy-dependent loss channels. However, the spectra show a growing asymmetric broadening on the high-energy side with decreasing excitation wavelength. The additional fluorescence originates from vibrational hot states, also supported by the decomposition by Gaussian functions. A comparison with the fluorescence spectrum of blue membranes shows that the low-energy part also arises from vibrational hot states. Consequently, the first fast intramolecular vibrational energy relaxation does not entirely distribute the access-energy to other modes. The remaining energy is found in a quasi-static occupation of vibrational modes. The subsequent second vibrational relaxation — probably not only involving the retinal but also the protein — takes place on a time scale similar to that of the isomerization. The fast isomerization reaction prevents dissipation of energy relaxation of the excited state of the retinal into the protein environment. On a third distance scale, femtosecond spectroscopy in the UV offers a direct view into the electrostatic interaction of the chromophore with its immediate neighborhood. For the first time, the retinal isomerization was studied within the spectral window of the tryptophan absorption from 265 nm to 310 nm. For this, a set-up adapted to UV wavelengths was realized, basing on non-linear optics. The developed single-shot detection system yields a sensitivity of 10-5 in optical density. In the transient absorption measurements, a time resolution of 90 fs was achieved. At short probe wavelengths (265 nm - 294 nm), the measured transients show first a fast reduction of the absorption which subsequently recovers on three time scales (380 fs, 3 ps, > 16 ps). With increasing probe wavelength an induced transient absorption becomes dominant (282 nm - 294 nm). It has a rise time of 280 fs, and decays with a time constant of 4.3 ps. With further increasing probe wavelength the absorption signal decays faster. In response to the complex signature of the transients, an iterative and model-independent transient-decomposition is introduced. A set of three basis elements were identified: one bleach, and two absorption transients. The two induced absorption transients are assigned to retinal. The dynamics of the excited state (I460) in the first retinal-transient is described by two time constants (280 fs and 1 ps). The fast time constant leads to the photoproduct, while the slower one leads back to the ground state, presumably by internal conversion. The 250 fs delayed onset of the second transient (photo-intermediate J) is connected to a wave packet motion in the excited state. This delayed onset of the transient was observed for the first time. It demands a rise time of the J photo-intermediate of only 280 fs. This contrasts common estimates of 500 fs given in literature. For the first time, the temporal behavior of the permanent dipole moment of retinal is observed experimentally. Its evolution is imprinted in the dominant bleach feature (265 nm - 294 nm), originating from tryptophan Trp86. In the excited state, the permanent dipole moment increases strongly within the first 250 fs, and decays exponentially afterwards, following the formation of the photoproduct. However, the related time constant of 380 fs is slightly greater than that of 280 fs of the photoproduct J. This discrepancy may indicate a remaining long-lived polarization of the binding pocket. In brief, by using the intrinsic tryptophan as a probe, the local evolution of the electric field during the isomerization reaction was investigated. Indeed the isomerization reaction is reflected in the immediate environment. The variety of optical spectroscopy provides methods especially adapted for studying the influences of the environment on a chemical reaction. The results achieved here highlight the importance of these influences.

EPFL2004

Chargement

Low Pressure Plasma Spraying (LPPS) processes use a DC plasma jet expanding at low pressure for fast deposition of dense coatings in a controlled atmosphere. The LPPS technology is widely used industrially in particular in the aeronautics and medical industries among others. Unlike atmospheric pressure plasma jets, which have been extensively studied experimentally and theoretically, the interest in low pressure DC plasma jets only occurred recently. However, the process development has been mainly based on empirical methods and the basics of the physical mechanisms that govern them still remain to be investigated. Further improvement of the processes requires, in particular, the knowledge of physical properties of the plasma jet such as the temperature, flow velocity and plasma density. Low pressure plasma jets present unconventional properties such as low collisionality, large dimensions and supersonic flow. Therefore specific diagnostics have to be adapted to these conditions. In this study, argon plasma jets at pressures between 2 and 100 mbar are investigated. Imaging has been used to allow a qualitative description of the plasma jet topology for different pressures and torch parameters. Low pressure plasma jets are most of the time supersonic, compressible and in an aerodynamic non-equilibrium, which results in visible successive compression and expansion zones corresponding to a variation of the local pressure, temperature and density. Imaging, combined with pressure measurements inside the plasma torch, has evidenced three different types of flow regimes with respect to the chamber pressure. For chamber pressures below 45 mbar, the flow is under-expanded and is characterized by an exit pressure higher than the chamber pressure. For pressures above 45 mbar, the plasma jet is over-expanded, in this case the exit pressure is lower than the chamber pressure. When the exit pressure is equal to the chamber pressure, the plasma jet is in the so-called design pressure regime. A diagnostic tool, extensively applied on atmospheric plasma jets, the enthalpy probe system, has been modified in order to allow gas sampling from the plasma jet at low pressures. A shock wave appears in front of the probe when it is immersed in a supersonic plasma jet, making the interpretation of enthalpy measurements more difficult.The free-stream properties, like the Mach number, temperature and free-stream enthalpy have to be inferred from stagnation measurements. Two interpretations of enthalpy probe measurements are described in this study. The first method uses the energy conservation equation and LTE assumptions with calorically perfect gas and neglecting the aerodynamic non-equilibrium, whereas the second method, uses a complementary measurement of the static pressure just after the shock using a specially developed tool: the Post Shock Static Pressure Probe (PSSPP). This allows the use of the conservation equations to determine the free-stream properties of the plasma jet without the assumption of calorically perfect gas and aerodynamic non-equilibrium. Determination of the free-stream enthalpy, Mach number and temperature were possible on over-expanded jets for pressures higher than 40 mbar. At 100 mbar with torch parameters of 400 A and 40 SLPM argon flow, the temperature of the plasma jet reaches 10000 K and the velocity is about 3000 m/s on axis. Measurement of plasma jet properties such as the Mach number, electron density and temperature, were performed using double Langmuir probes and Mach probes. In particular, under-expanded jets are studied in detail by performing complete mappings of plasma jet properties at 10 and 2 mbar chamber pressure. These results show that the measured physical properties are consistent with the jet flow phenomenology such as the presence of periodic expansion and compression zones, the effect of the pressure and the location of the shocks. It is shown in particular that for highly under-expanded jets at 2 mbar, the Mach number reaches 2.8 in the first expansion zone followed by a strong drop to subsonic flow revealing the presence of a Mach reflection. The flow is accelerated further and a periodic structure of compression/expansion cells is observed until the local static pressure is in equilibrium with the surrounding pressure. Another diagnostic often used in plasma spraying is optical emission spectroscopy (OES) which is non-intrusive and gives information about the plasma excited species.However, the determination of the excitation temperature, obtained by the Boltzmann plot method, relies on the assumption of local thermodynamic equilibrium (LTE), which is no longer satisfied at low working pressures. The result of the deviation from LTE is that the heavy particle, electron and excitation temperatures are different. In this study, Boltzmann plots have been used to evaluate the deviation from LTE as a function of the working pressure and the location in the plasma jet. It has been shown that the plasma jet is closer to LTE in the compression zones and close to the axis. Measurements of spectral line broadening due to the Stark effect allowed to determine the electron density for under-expanded jets and give results similar than with electrostatic probe measurements.On the other hand, excitation temperatures are systematically lower than the electron temperature for the same plasma conditions. For a 10 mbar plasma jet, the excitation temperature of argon is between 0.73 and 0.78 eV whereas the electron temperature is between 0.7 and 1.2 eV. This shows that at low pressure the plasma jets are not in LTE.These results contribute to the understanding of the supersonic plasma jet behavior at low pressure and can be used to quantify the deviation from local thermodynamic equilibrium (LTE). The extensive mapping of the measured physical properties of the jet will also serve as input for modeling.

EPFL2004

Chargement

Influence of the protein environment on spectroscopy and ultrafast dynamics of retinal in bacteriorhodopsin

Selma Schenkl

EPFL2004