Optical properties of small metal clusters

This thesis is devoted to the study of the optical properties of small metal clusters and metal cluster organic compounds. For this purpose a ultra-high vacuum (UHV) experimental apparatus has been built which allows for the deposition of mass selected neutral clusters in a size range up to 16,000 amu which corresponds to Ag150 or Au80. Cluster cations are produced in a gas aggregation cluster source, steered in an ion optics device and co-deposited with electrons and excess rare gas on a cryogenic deposition head. Neutral mono dispersed clusters are analyzed by optical absorption, fluorescence and excitation spectroscopy in the UV-visible energy range. A novel custom-made conical octupole combined to an organic molecule injector has been built, serving as a generator of metal cluster-organic compounds. These are atomically precise model systems which permit to study surface-enhanced Raman scattering (SERS) in order to test the theoretical models and better understand the mechanism of SERS. Optical absorption on size-selected silver clusters of Agn (n= 20 â 120) embedded in neon solid matrix was investigated. The spectra show plasmon-like absorption profiles around a photon energy of 4.0 eV which match in energy molecular like absorption spectra for the smallest Ag clusters but are sensitively higher than what would be expected for the dipolar absorption for bulk silver of about 3.3 eV. The central absorption peak, i.e. the plasmon energy as a function of cluster size is presented. The data fall in a transition between a blueshift of the plasmon resonance with decreasing size to a superimposed red shift for the very smallest sizes. Superimposed, we observe a distinct structure where clusters with atom number of 8,18,34,58,92 show a localized maximum value of the plasmon energy. These numbers correspond perfectly to the fully filled states of 1p, 1d, 1f, 1g and 1h according to the electronic shell model, manifesting a clear sub-shell effect. Optical absorption, fluorescence and excitation spectroscopy on size-selected small gold clusters of Aum (m= 19,20,21) embedded in solid neon were measured. These are the first optical data on ligand free neutral atomically precise Au clusters. Au20, the famous tetraedic structure, shows a rich structured absorption spectrum, superimposed on a continuous background increasing to the UV part of the spectrum. Au21 and Au19 show the same tendency, however without any superimposed structure. Very interestingly, all three Au clusters show strong fluorescence centered at around 1.85 eV, here again Au20 being the prominent species. Comparison to ligand protected Au clusters clearly proves that the fluorescence comes from the Au cluster itself and is only weakly influenced by the ligands. For the first time, size-selected small metal-organic compounds of silver-pyridine AgaPyb (a= 1,3; b= 1,2) which are prototypes for surface enhanced Raman scattering (SERS) have been produced and isolated in a solid Ne matrix. Optical absorption and fluorescence spectra have been measured. Raman scattering cross sections with an enhancement factor of 103 have been found in fair agreement with recent high level time-dependent density functional theory (TDDFT) calculations.

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

Study of the Optical UV-Visible Electron Induced Luminescence of Small Size Selected Metal Clusters in Rare Gas Solids

Alexandre Rydlo

The optical properties of small size selected metal clusters have been studied by fluorescence spectroscopy. Cun (n = 1-5, 8), Agn (n = 1-5, 8, 9, 20), Aun (n = 1-5, 8) and Nin (n = 1-3) clusters embedded in condensed neon are photoionized by VUV photons in the solid, and the fluorescence originating from the reneutralization of the transient cluster ions is recorded. The VUV photons result from the radiative decay of excitons which are generated by the impact of electrons of variable energy on the solid. A new experimental setup has been developed, which permits the excitation of the rare gas solid by electrons with an energy between 0 and 200 eV. A field emission electron source based on carbon nanotubes provides an intense electron beam without any light generation. A custom built cryopumped gas purification system allows the production of rare gas solids with a purity in the order of up to 10-5. The characteristics and a physical descriptive model of electron induced luminescence is presented. The measurement of electron currents through the solid are thoroughly analyzed. Based on localized exciton production and radiative deexcitation of the host rare gas atoms near the surface of the solid, the excitation can be spread over distances as large as 20-30 µm by high energy photons which excite the species contained in the solid. Quenching and amplification effects on the luminescence intensities are observed depending on the material and the size of the dopants contained in the rare gas solids. These effects are believed to originate from the local enhanced electric field induced by the metal clusters which act as antennas. The analysis of the luminescence spectra reveals rich spectroscopic signatures from the UV to the near IR. A certain amount of fluorescent transitions could be identified from conventional excitation fluorescence spectroscopy. This proves that this new method is sensitive and reliable. The so far reported new transitions await interpretation on the basis of sophisticated quantum calculations.

EPFL2010

Ultraviolet-visible absorption of small silver clusters in neon: Ag-n (n=1-9)

Wolfgang Harbich, Sylvain Lecoultre, Alexandre Rydlo

We present optical absorption and fluorescence spectra in the UV-visible range of size selected neutral Ag-n clusters (n = 1-9) in solid neon. Rich and detailed optical spectra are found with linewidths as small as 50 meV. These spectra are compared to time dependent density functional theory implemented in the TURBOMOLE package. Excellent agreement between theory and experiment is achieved in particular for the dominant spectroscopic features at photon energies below 4.5 eV. This allows a clear attribution of the observed electronic transitions to specific isomers. Optical transitions associated to the s-electrons are concentrated in the energy range between 3 and 4 eV and well separated from transitions of the d-electrons. This is in contrast to the other coinage metals (Au and Cu) which show a strong coupling of the d-electrons. (C) 2011 American Institute of Physics. [doi:10.1063/1.3589357]