Acquiring quantitative information on charge transfer is performed on a single intramolecular swi... more Acquiring quantitative information on charge transfer is performed on a single intramolecular switch. Using local electronic excitations allow to trigger (CT) process and tune its switching efficiency. Our method can be applied to investigate various molecular systems.
Journal of Physics: Condensed Matter, Dec 19, 2014
In this article, we study the origin of the corrugating and anticorrugating states through the el... more In this article, we study the origin of the corrugating and anticorrugating states through the electronic properties of the Si(1 0 0) surface via a low-temperature (9 K) scanning tunneling microscope (STM). Our study is based on the analysis of the STM topographies corrugation variations when related to the shift of the local density of states (LDOS) maximum in the [Formula: see text] direction. Our experimental results are correlated with numerical simulations using the density-functional theory with hybrid Heyd-Scuseria-Ernzerhof (HSE06) functional to simulate the STM topographies, the projected density of states variations at different depths in the silicon surface as well as the three dimensional partial charge density distributions in real-space. This work reveals that the Si(1 0 0) surface exhibits two anticorrugating states at +0.8 and +2.8 V that are associated with a phase shift of the LDOS maximum in the unoccupied states STM topographies. By comparing the calculated data with our experimental results, we have been able to identify the link between the variations of the STM topographies corrugation and the shift of the LDOS maximum observed experimentally. Each surface voltage at which the STM topographies corrugation drops is defined as anticorrugating states. In addition, we have evidenced a sharp jump in the tunnel current when the second LDOS maximum shift is probed, whose origin is discussed and associated with the presence of Van Hove singularities.
Page 165. Chapter 6 Electronic Control of Single-Molecule Nanomachines AJ Mayne, D. Riedel, G. Co... more Page 165. Chapter 6 Electronic Control of Single-Molecule Nanomachines AJ Mayne, D. Riedel, G. Comtet, and G. Dujardin Institut des Sciences Moléculaires d'Orsay, CNRS, Bat. 210, Univ Paris-Sud, 91405 Orsay, France gerald. dujardin@ u-psud. ...
We report a surface photochemistry study using ultrafast pulses of vacuum ultraviolet (vuv) light... more We report a surface photochemistry study using ultrafast pulses of vacuum ultraviolet (vuv) light for excitation. Tunable vuv radiation (10 38 eV) is produced via high harmonic generation. We have studied the model system of physisorbed O2 on graphite. The only observed desorption product is O+, the yield of which is measured as a function of the excitation photon energy.
Journal of physics. Condensed matter : an Institute of Physics journal, Jan 11, 2015
Ultra-thin CaF2 layers are grown on the Si(1 0 0) surface by using a Knudsen cell evaporator. The... more Ultra-thin CaF2 layers are grown on the Si(1 0 0) surface by using a Knudsen cell evaporator. These epitaxial structures are studied with a low temperature (9 K) scanning tunneling microscope and used to electronically decouple hexaphenyl molecules from the Si surface. We show that the ultra-thin CaF2 layers exhibit stripe structures oriented perpendicularly to the silicon dimer rows and have a surface gap of 3.8 eV. The ultra-thin semi-insulating layers are also shown to be functionalized, since 80% of the hexaphenyl molecules adsorbed on these structures self-orients along the stripes. Numerical simulations using time-dependent density functional theory allow comparison of computed orbitals of the hexaphenyl molecule with experimental data. Finally, we show that the hexaphenyl molecules can be manipulated along or across the stripes, enabling the molecules to be arranged precisely on the insulating surface.
ABSTRACT In this work, we show that the hopping directivity of individual hexaphenyl-benzene (HPB... more ABSTRACT In this work, we show that the hopping directivity of individual hexaphenyl-benzene (HPB) molecules physisorbed along the SA step edge of a bare Si(100)-2×1 surface can be reversibly controlled with a periodic hopping length. This is achieved by using the tunnel electrons of a low temperature (9 K) scanning tunneling microscope (STM). A statistical analysis of the electronic excitations applied at various positions on the HPB molecule reveals that the hopping process is related to a strong decrease of the tunnel junction conductance. This process is associated with a charge transfer from the silicon surface to the HPB molecule leading to a hopping mechanism that occurs in two sequential steps. The first step of the hopping process involves the formation of an HPB– anion that triggers the molecular motion into a metastable state. The second step is related to the neutralization of the HPB– anion which provokes the manipulation of the molecule to its final steady position. Our experimental data are supported by the calculations of the relaxed molecule using the density functional theory on the Si(100) surface that takes the van der Waals forces interactions into account. Additional calculations of the HPB– anion orbitals depict the spatial localization of the extra charge inside the HPB molecule and the relative energies of the HPB– molecular orbitals. Finally, our study shows that the hopping direction can be optimized by positioning the STM tip at specific locations along the hopping pathway.
ABSTRACT Powering and controlling the operation of a single molecule adsorbed on a surface can be... more ABSTRACT Powering and controlling the operation of a single molecule adsorbed on a surface can be achieved by using the tip of a scanning tunnelling microscope (STM) as an atomic-size source of electrons. We review the various electronic excitation processes induced by the electrons from the STM tip which are able to activate the functions of a molecular nanomachine. In particular, we review recent results illustrating the electronic control of molecular dynamics at the level of a single molecule.
We report a study of the vacuum ultraviolet (VUV) surface photochemistry of H2O (D2O) films on a ... more We report a study of the vacuum ultraviolet (VUV) surface photochemistry of H2O (D2O) films on a graphite substrate at 80 K. Experiments utilized a He discharge lamp to generate VUV photons. For 21 eV excitation, H+ (D+) is the only ionic fragment observed in desorption. When 41-eV photons are used, H+ (D+) is again observed: however, photochemically produced H3O+ (D3O+) is also detected. The ratio of D+ to D3O+ depends upon the water coverage. Furthermore, the D3O+ signal is larger than D+, whereas the H3O+ signal is smaller than H+ for irradiation of adsorbed D2O and H2O, respectively. At low coverages where the average coordination of water is also lower, D3O+ production is enhanced compared with higher coverages. The formation of D3O+ is attributed to the reaction of the photodissociation product D+ with adsorbed D2O. We suggest that the opening of the D3O+ product channel at 41 eV and its preponderance over D+ is related to the higher kinetic energy of, and consequently greater...
Damien Riedel received his B.Sc. in physics from the University of Besanc ¸on. His M.Sc. work in ... more Damien Riedel received his B.Sc. in physics from the University of Besanc ¸on. His M.Sc. work in optics and electronics was on speckle interferometry using a scanning near-field optical microscope in the Laboratoire d'Optique P. M. Duffieux. He obtained his Ph.D. in physics in 1998 under the supervision of Prof. M. C. Castex from the University of Paris XIII, where his postgraduate work in laser physics and surface science concerned the vacuum ultraviolet laser interaction with polymer surfaces and fluorescence spectroscopy. He then joined the group of Prof. R. E. Palmer in Birmingham, U.K., on a postdoctoral Marie-Curie Fellowship. In the Nanoscale Physics Laboratory, he worked on an ultrashort-pulse laser created by high-order harmonic generation in rare gases. He developed applications on surface photodesorption experiments and room temperature STM combined with a laser. In 2001, he joined the CNRS to work with Dr. Ge ´rald Dujardin, and his research deals with molecular electronics and molecular manipulation via highly localized electronic excitation with a low-temperature STM on semiconductors. He is developing projects that combine a laser and an STM and involve collection of fluorescence on the low-temperature STM.
Applied Physics A: Materials Science & Processing, 1999
. First measurements of effective absorption coefficient and penetration depth are given here f... more . First measurements of effective absorption coefficient and penetration depth are given here from the ablation of poly-methylmethacrylate (PMMA) and poly-tetrafluoroethylene (PTFE) samples at 125 nm (≈10 eV). The coherent VUV source used which provides smooth, efficient and clean etched areas, is briefly described. Experimental curves of etch depth as a function of the number of laser shots and etch rate as
The low temperature electron transport on semiconductor surfaces has been studied using an ultra ... more The low temperature electron transport on semiconductor surfaces has been studied using an ultra high vacuum, variable temperature Scanning Tunneling Microscope (STM). The STM I(V) spectroscopy recorded at various temperatures has enabled to investigate the temperature dependence (300 K to 35 K) of the surface conductivity of three different semiconductor surfaces: highly doped n-type Si(100), p-type Si(100), and hydrogenated C(100). Low temperature freezing of specific surface electronic channels on the higly doped n-type Si(100) and moderately doped p-type Si(100) surfaces could be achieved whereas the total surface conductivity on the hydrogenated C(100) surface can be frozen below only 180 K.
Controlling the properties of quantum dots at the atomic scale, such as dangling bonds, is a gene... more Controlling the properties of quantum dots at the atomic scale, such as dangling bonds, is a general motivation as they allow studying various nanoscale processes including atomic switches, charge storage, or low binding energy state interactions. Adjusting the coupling of individual silicon dangling bonds to form a 2D device having a defined function remains a challenge. Here, we exploit the anisotropic interactions between silicon dangling bonds on ntype doped Si(100):H surface to tune their hybridization. This process arises from interactions between the subsurface silicon network and dangling bonds inducing a combination of Jahn-Teller distortions and local charge ordering. A three-pointed star-shaped device prototype is designed. By changing the charge state of this device, its electronic properties are shown to switch reversibly from an ON to an OFF state via local change of its central gap. Our results provide a playground for the study of quantum information at the nanoscale.
Le but de ce travail est multiple : il correspond tout d'abord a la modelisation d'une so... more Le but de ce travail est multiple : il correspond tout d'abord a la modelisation d'une source coherente vuv a 10 ev realisee par melange non lineaire a quatre ondes dans une vapeur de mercure a temperature ambiante dans le cas de faisceaux pompes non paraxiaux. Les processus somme 2w#1 + w#2 amplifies par resonance a deux photons (6s#1s#0 7s#1s#0) ou trois photons (6s#1s#0 9p#1p#1) sont etudies, lors de simulations numeriques, pour differents angles de croisements entre les faisceaux pompes et differentes polarisations, ainsi que pour le cas particulier de la resonance a deux photons (parametre d'accord de phase). Cette transition etant rapidement saturee dans nos conditions experimentales, nous avons montre qu'il est important de prendre en compte la proportion d'atomes dans l'etat 7s#1s#0 afin d'obtenir des valeurs negatives de ce parametre (conditions d'accord de phase). Les qualites de la source vuv obtenues a l'aide d'un laser accordable a colorant ou d'un opo (oscillateur parametrique optique) sont comparees. A la suite d'une caracterisation rigoureuse de cette source (energie par impulsion, duree d'impulsion, taille du spot, transmission des optiques utilisees, etalonnage des appareils de mesure d'intensite), nous avons mesure les constantes d'ablation du pmma ou poly(methylemethacrylate) (plexiglas) et du ptfe ou poly(tetrafluoroethylene) (teflon). Outre une tres grande qualite de gravure, elles revelent un fort couplage laser matiere et un seuil de densite d'energie faible traduisant des processus photochimiques preponderants. La creation d'un montage experimental ameliorant considerablement la separation des faisceaux pompes (uv et visible) du faisceau harmonique vuv nous a permis de realiser l'etude de la luminescence du pmma lors d'excitation uv et vuv : la mise en evidence du chromophore excite (groupement carbonyle -o-c=o) lors d'irradiations a faible densite d'energie, et la presence de fragments (ch et c#2) pour des irradiations a plus forte densite d'energie sont autant d'informations nous permettant de conclure et de discuter un schema general traduisant les processus de degradation de ce materiau a 10 ev.
The polarization of the VUV light generated by four-wave sum frequency mixing process ω 4 = 2ω 1 ... more The polarization of the VUV light generated by four-wave sum frequency mixing process ω 4 = 2ω 1 + ω 2 in mercury vapor at room temperature is analyzed in detail. Due to the specific two-photon transition used to enhance the nonlinear process, the polarization of the VUV wave is shown to be identical to the polarization of the wave at the frequency ω 2. In particular, circularly polarized VUV is observed with degree of circular polarization exceeding 0.99.
The adsorption of Co atoms on the bare Si(100)-2 × 1 surface at low temperature (12 K) is perform... more The adsorption of Co atoms on the bare Si(100)-2 × 1 surface at low temperature (12 K) is performed in the low coverage regime (b 0.1 monolayer). The ensuing surface is studied by means of a low temperature (9 K) scanning tunneling microscope (STM). Several adsorption sites are described via STM topographies and differential conductance spectroscopy. Our study reveals that at low temperature (12 K), a significant fraction of the Co atoms diffuse into the surface and form the first stage reaction sites that are relevant for the silicidation mechanism of the Si(100). Furthermore, the low temperature conditions allow to describe surface Co adatoms conformations that are stabilized on the Si(100)-c(4 × 2) surface. Interestingly, we have observed the irreversible transformation of the symmetric pedestal site (P s) to the under dimer site (U D) via specific STM scanning conditions. Finally, the presence of dimer vacancy lines is discussed and reveals that the activated etching process of the silicon dimer with metals can be induced at very low temperature.
Figure S1: (a) STM topography (Vs =-2.2 V, 5 pA) of a Fe-TPP molecule in a CR conformation. (b) a... more Figure S1: (a) STM topography (Vs =-2.2 V, 5 pA) of a Fe-TPP molecule in a CR conformation. (b) and (d) apparent height profiles of the Fe-TPP molecule along two different lines reported in (a) in red and purple.
The reversible hopping of a bistable atom on the Sið100Þ-ð2 Â 1Þ:H surface is activated nonlocall... more The reversible hopping of a bistable atom on the Sið100Þ-ð2 Â 1Þ:H surface is activated nonlocally by hole injection into Si-Si bond surface states with a low temperature (5 K) scanning tunneling microscope. In the contact region, at short distances (<1:5 nm) between the hole injection site and the bistable atom, the hopping yield of the bistable atom exhibits remarkable variations as a function of the hole injection site. It is explained by the density of state distribution along the silicon bond network that shows chargetransfer pathways between the injection sites and the bistable atom.
Nous presentons ici une etude experimentale detaillee sur la polarisation d'un rayonnement VU... more Nous presentons ici une etude experimentale detaillee sur la polarisation d'un rayonnement VUV a 10 eV engendre par melange a quatre ondes resonnant ω VUV = 2ω UV + ω VIS dans une vapeur de mercure a temperature ambiante. Nous montrons, en particulier, qu' en raison de la specificite de la transition a deux photons choisie pour augmenter l'efficacite du processus non lineaire, la polarisation de l'onde VUV est identique a celle de l'onde visible a la frequence ω VIS . Nous avons pu ainsi obtenir du rayonnement VUV polarise circulairement avec un degre de polarisation excedant 0,99.
Acquiring quantitative information on charge transfer is performed on a single intramolecular swi... more Acquiring quantitative information on charge transfer is performed on a single intramolecular switch. Using local electronic excitations allow to trigger (CT) process and tune its switching efficiency. Our method can be applied to investigate various molecular systems.
Journal of Physics: Condensed Matter, Dec 19, 2014
In this article, we study the origin of the corrugating and anticorrugating states through the el... more In this article, we study the origin of the corrugating and anticorrugating states through the electronic properties of the Si(1 0 0) surface via a low-temperature (9 K) scanning tunneling microscope (STM). Our study is based on the analysis of the STM topographies corrugation variations when related to the shift of the local density of states (LDOS) maximum in the [Formula: see text] direction. Our experimental results are correlated with numerical simulations using the density-functional theory with hybrid Heyd-Scuseria-Ernzerhof (HSE06) functional to simulate the STM topographies, the projected density of states variations at different depths in the silicon surface as well as the three dimensional partial charge density distributions in real-space. This work reveals that the Si(1 0 0) surface exhibits two anticorrugating states at +0.8 and +2.8 V that are associated with a phase shift of the LDOS maximum in the unoccupied states STM topographies. By comparing the calculated data with our experimental results, we have been able to identify the link between the variations of the STM topographies corrugation and the shift of the LDOS maximum observed experimentally. Each surface voltage at which the STM topographies corrugation drops is defined as anticorrugating states. In addition, we have evidenced a sharp jump in the tunnel current when the second LDOS maximum shift is probed, whose origin is discussed and associated with the presence of Van Hove singularities.
Page 165. Chapter 6 Electronic Control of Single-Molecule Nanomachines AJ Mayne, D. Riedel, G. Co... more Page 165. Chapter 6 Electronic Control of Single-Molecule Nanomachines AJ Mayne, D. Riedel, G. Comtet, and G. Dujardin Institut des Sciences Moléculaires d&#x27;Orsay, CNRS, Bat. 210, Univ Paris-Sud, 91405 Orsay, France gerald. dujardin@ u-psud. ...
We report a surface photochemistry study using ultrafast pulses of vacuum ultraviolet (vuv) light... more We report a surface photochemistry study using ultrafast pulses of vacuum ultraviolet (vuv) light for excitation. Tunable vuv radiation (10 38 eV) is produced via high harmonic generation. We have studied the model system of physisorbed O2 on graphite. The only observed desorption product is O+, the yield of which is measured as a function of the excitation photon energy.
Journal of physics. Condensed matter : an Institute of Physics journal, Jan 11, 2015
Ultra-thin CaF2 layers are grown on the Si(1 0 0) surface by using a Knudsen cell evaporator. The... more Ultra-thin CaF2 layers are grown on the Si(1 0 0) surface by using a Knudsen cell evaporator. These epitaxial structures are studied with a low temperature (9 K) scanning tunneling microscope and used to electronically decouple hexaphenyl molecules from the Si surface. We show that the ultra-thin CaF2 layers exhibit stripe structures oriented perpendicularly to the silicon dimer rows and have a surface gap of 3.8 eV. The ultra-thin semi-insulating layers are also shown to be functionalized, since 80% of the hexaphenyl molecules adsorbed on these structures self-orients along the stripes. Numerical simulations using time-dependent density functional theory allow comparison of computed orbitals of the hexaphenyl molecule with experimental data. Finally, we show that the hexaphenyl molecules can be manipulated along or across the stripes, enabling the molecules to be arranged precisely on the insulating surface.
ABSTRACT In this work, we show that the hopping directivity of individual hexaphenyl-benzene (HPB... more ABSTRACT In this work, we show that the hopping directivity of individual hexaphenyl-benzene (HPB) molecules physisorbed along the SA step edge of a bare Si(100)-2×1 surface can be reversibly controlled with a periodic hopping length. This is achieved by using the tunnel electrons of a low temperature (9 K) scanning tunneling microscope (STM). A statistical analysis of the electronic excitations applied at various positions on the HPB molecule reveals that the hopping process is related to a strong decrease of the tunnel junction conductance. This process is associated with a charge transfer from the silicon surface to the HPB molecule leading to a hopping mechanism that occurs in two sequential steps. The first step of the hopping process involves the formation of an HPB– anion that triggers the molecular motion into a metastable state. The second step is related to the neutralization of the HPB– anion which provokes the manipulation of the molecule to its final steady position. Our experimental data are supported by the calculations of the relaxed molecule using the density functional theory on the Si(100) surface that takes the van der Waals forces interactions into account. Additional calculations of the HPB– anion orbitals depict the spatial localization of the extra charge inside the HPB molecule and the relative energies of the HPB– molecular orbitals. Finally, our study shows that the hopping direction can be optimized by positioning the STM tip at specific locations along the hopping pathway.
ABSTRACT Powering and controlling the operation of a single molecule adsorbed on a surface can be... more ABSTRACT Powering and controlling the operation of a single molecule adsorbed on a surface can be achieved by using the tip of a scanning tunnelling microscope (STM) as an atomic-size source of electrons. We review the various electronic excitation processes induced by the electrons from the STM tip which are able to activate the functions of a molecular nanomachine. In particular, we review recent results illustrating the electronic control of molecular dynamics at the level of a single molecule.
We report a study of the vacuum ultraviolet (VUV) surface photochemistry of H2O (D2O) films on a ... more We report a study of the vacuum ultraviolet (VUV) surface photochemistry of H2O (D2O) films on a graphite substrate at 80 K. Experiments utilized a He discharge lamp to generate VUV photons. For 21 eV excitation, H+ (D+) is the only ionic fragment observed in desorption. When 41-eV photons are used, H+ (D+) is again observed: however, photochemically produced H3O+ (D3O+) is also detected. The ratio of D+ to D3O+ depends upon the water coverage. Furthermore, the D3O+ signal is larger than D+, whereas the H3O+ signal is smaller than H+ for irradiation of adsorbed D2O and H2O, respectively. At low coverages where the average coordination of water is also lower, D3O+ production is enhanced compared with higher coverages. The formation of D3O+ is attributed to the reaction of the photodissociation product D+ with adsorbed D2O. We suggest that the opening of the D3O+ product channel at 41 eV and its preponderance over D+ is related to the higher kinetic energy of, and consequently greater...
Damien Riedel received his B.Sc. in physics from the University of Besanc ¸on. His M.Sc. work in ... more Damien Riedel received his B.Sc. in physics from the University of Besanc ¸on. His M.Sc. work in optics and electronics was on speckle interferometry using a scanning near-field optical microscope in the Laboratoire d'Optique P. M. Duffieux. He obtained his Ph.D. in physics in 1998 under the supervision of Prof. M. C. Castex from the University of Paris XIII, where his postgraduate work in laser physics and surface science concerned the vacuum ultraviolet laser interaction with polymer surfaces and fluorescence spectroscopy. He then joined the group of Prof. R. E. Palmer in Birmingham, U.K., on a postdoctoral Marie-Curie Fellowship. In the Nanoscale Physics Laboratory, he worked on an ultrashort-pulse laser created by high-order harmonic generation in rare gases. He developed applications on surface photodesorption experiments and room temperature STM combined with a laser. In 2001, he joined the CNRS to work with Dr. Ge ´rald Dujardin, and his research deals with molecular electronics and molecular manipulation via highly localized electronic excitation with a low-temperature STM on semiconductors. He is developing projects that combine a laser and an STM and involve collection of fluorescence on the low-temperature STM.
Applied Physics A: Materials Science & Processing, 1999
. First measurements of effective absorption coefficient and penetration depth are given here f... more . First measurements of effective absorption coefficient and penetration depth are given here from the ablation of poly-methylmethacrylate (PMMA) and poly-tetrafluoroethylene (PTFE) samples at 125 nm (≈10 eV). The coherent VUV source used which provides smooth, efficient and clean etched areas, is briefly described. Experimental curves of etch depth as a function of the number of laser shots and etch rate as
The low temperature electron transport on semiconductor surfaces has been studied using an ultra ... more The low temperature electron transport on semiconductor surfaces has been studied using an ultra high vacuum, variable temperature Scanning Tunneling Microscope (STM). The STM I(V) spectroscopy recorded at various temperatures has enabled to investigate the temperature dependence (300 K to 35 K) of the surface conductivity of three different semiconductor surfaces: highly doped n-type Si(100), p-type Si(100), and hydrogenated C(100). Low temperature freezing of specific surface electronic channels on the higly doped n-type Si(100) and moderately doped p-type Si(100) surfaces could be achieved whereas the total surface conductivity on the hydrogenated C(100) surface can be frozen below only 180 K.
Controlling the properties of quantum dots at the atomic scale, such as dangling bonds, is a gene... more Controlling the properties of quantum dots at the atomic scale, such as dangling bonds, is a general motivation as they allow studying various nanoscale processes including atomic switches, charge storage, or low binding energy state interactions. Adjusting the coupling of individual silicon dangling bonds to form a 2D device having a defined function remains a challenge. Here, we exploit the anisotropic interactions between silicon dangling bonds on ntype doped Si(100):H surface to tune their hybridization. This process arises from interactions between the subsurface silicon network and dangling bonds inducing a combination of Jahn-Teller distortions and local charge ordering. A three-pointed star-shaped device prototype is designed. By changing the charge state of this device, its electronic properties are shown to switch reversibly from an ON to an OFF state via local change of its central gap. Our results provide a playground for the study of quantum information at the nanoscale.
Le but de ce travail est multiple : il correspond tout d'abord a la modelisation d'une so... more Le but de ce travail est multiple : il correspond tout d'abord a la modelisation d'une source coherente vuv a 10 ev realisee par melange non lineaire a quatre ondes dans une vapeur de mercure a temperature ambiante dans le cas de faisceaux pompes non paraxiaux. Les processus somme 2w#1 + w#2 amplifies par resonance a deux photons (6s#1s#0 7s#1s#0) ou trois photons (6s#1s#0 9p#1p#1) sont etudies, lors de simulations numeriques, pour differents angles de croisements entre les faisceaux pompes et differentes polarisations, ainsi que pour le cas particulier de la resonance a deux photons (parametre d'accord de phase). Cette transition etant rapidement saturee dans nos conditions experimentales, nous avons montre qu'il est important de prendre en compte la proportion d'atomes dans l'etat 7s#1s#0 afin d'obtenir des valeurs negatives de ce parametre (conditions d'accord de phase). Les qualites de la source vuv obtenues a l'aide d'un laser accordable a colorant ou d'un opo (oscillateur parametrique optique) sont comparees. A la suite d'une caracterisation rigoureuse de cette source (energie par impulsion, duree d'impulsion, taille du spot, transmission des optiques utilisees, etalonnage des appareils de mesure d'intensite), nous avons mesure les constantes d'ablation du pmma ou poly(methylemethacrylate) (plexiglas) et du ptfe ou poly(tetrafluoroethylene) (teflon). Outre une tres grande qualite de gravure, elles revelent un fort couplage laser matiere et un seuil de densite d'energie faible traduisant des processus photochimiques preponderants. La creation d'un montage experimental ameliorant considerablement la separation des faisceaux pompes (uv et visible) du faisceau harmonique vuv nous a permis de realiser l'etude de la luminescence du pmma lors d'excitation uv et vuv : la mise en evidence du chromophore excite (groupement carbonyle -o-c=o) lors d'irradiations a faible densite d'energie, et la presence de fragments (ch et c#2) pour des irradiations a plus forte densite d'energie sont autant d'informations nous permettant de conclure et de discuter un schema general traduisant les processus de degradation de ce materiau a 10 ev.
The polarization of the VUV light generated by four-wave sum frequency mixing process ω 4 = 2ω 1 ... more The polarization of the VUV light generated by four-wave sum frequency mixing process ω 4 = 2ω 1 + ω 2 in mercury vapor at room temperature is analyzed in detail. Due to the specific two-photon transition used to enhance the nonlinear process, the polarization of the VUV wave is shown to be identical to the polarization of the wave at the frequency ω 2. In particular, circularly polarized VUV is observed with degree of circular polarization exceeding 0.99.
The adsorption of Co atoms on the bare Si(100)-2 × 1 surface at low temperature (12 K) is perform... more The adsorption of Co atoms on the bare Si(100)-2 × 1 surface at low temperature (12 K) is performed in the low coverage regime (b 0.1 monolayer). The ensuing surface is studied by means of a low temperature (9 K) scanning tunneling microscope (STM). Several adsorption sites are described via STM topographies and differential conductance spectroscopy. Our study reveals that at low temperature (12 K), a significant fraction of the Co atoms diffuse into the surface and form the first stage reaction sites that are relevant for the silicidation mechanism of the Si(100). Furthermore, the low temperature conditions allow to describe surface Co adatoms conformations that are stabilized on the Si(100)-c(4 × 2) surface. Interestingly, we have observed the irreversible transformation of the symmetric pedestal site (P s) to the under dimer site (U D) via specific STM scanning conditions. Finally, the presence of dimer vacancy lines is discussed and reveals that the activated etching process of the silicon dimer with metals can be induced at very low temperature.
Figure S1: (a) STM topography (Vs =-2.2 V, 5 pA) of a Fe-TPP molecule in a CR conformation. (b) a... more Figure S1: (a) STM topography (Vs =-2.2 V, 5 pA) of a Fe-TPP molecule in a CR conformation. (b) and (d) apparent height profiles of the Fe-TPP molecule along two different lines reported in (a) in red and purple.
The reversible hopping of a bistable atom on the Sið100Þ-ð2 Â 1Þ:H surface is activated nonlocall... more The reversible hopping of a bistable atom on the Sið100Þ-ð2 Â 1Þ:H surface is activated nonlocally by hole injection into Si-Si bond surface states with a low temperature (5 K) scanning tunneling microscope. In the contact region, at short distances (<1:5 nm) between the hole injection site and the bistable atom, the hopping yield of the bistable atom exhibits remarkable variations as a function of the hole injection site. It is explained by the density of state distribution along the silicon bond network that shows chargetransfer pathways between the injection sites and the bistable atom.
Nous presentons ici une etude experimentale detaillee sur la polarisation d'un rayonnement VU... more Nous presentons ici une etude experimentale detaillee sur la polarisation d'un rayonnement VUV a 10 eV engendre par melange a quatre ondes resonnant ω VUV = 2ω UV + ω VIS dans une vapeur de mercure a temperature ambiante. Nous montrons, en particulier, qu' en raison de la specificite de la transition a deux photons choisie pour augmenter l'efficacite du processus non lineaire, la polarisation de l'onde VUV est identique a celle de l'onde visible a la frequence ω VIS . Nous avons pu ainsi obtenir du rayonnement VUV polarise circulairement avec un degre de polarisation excedant 0,99.
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