Jean-Marie Gilles

Vertical two‐dimensional piezoelectric inertial slider for scanning tunneling microscope

To achieve both high stability during scanning tunneling microscopy (STM) measurements at atomic resolution and long‐range imaging of nonhomogeneous samples, we have developed a fully 2D vertical inertial nanopositioner which allows tunneling tip displacements in the 0.01–1000 μm range while keeping high‐resolution STM conditions. The inertial sliding of the tip is obtained by overriding the static friction criterion between the tip and its support. By controlling the expansion of the piezotubes and the direction of the applied acceleration, one can obtain displacements as small as 100 A in the vertical direction. The same piezotubes are also used to scan the sample during conventional STM measurements. Increasing the resonance frequency of the scanner above 10 kHz was essential to ensure good operation. The complete device has not altered the mechanical stability of the microscope; it works in air as well as in ultra‐high vacuum conditions. Its reliability is demonstrated by the possibility of obtaining ...

An IRAS/TDS study of the adsorption of CH3NH2 on Cu(110)

Abstract The adsorption and condensation of monomethylamine on Cu(110) at 90K is studied by TDS and IRAS spectroscopies. TDS shows that the adsorption is molecular and that a first chemisorbed layer is fairly stable at room temperature. The chemisorbed layer is completely filled for an exposure of 1.5 Langmuir. The density in this chemisorbed layer is estimated to be equal to 1 molecule per 15(±2)A 2 . IRAS results are in agreement with bonding of methylamine with the substrate through the nitrogen lone pair. Effects of annealing on a multilayer film are also discussed.

Infrared laser stimulated desorption of adsorbates on metal surfaces

Abstract An adsorbate is irradiated by a pulsed infrared laser beam tuned in resonance with an internal vibrational mode of the adsorbed molecules. The adsorbate vibrational energy damps into the metal by electron-hole pair creation. This energy transfer from the infrared laser beam to the substrate competes with the direct optical absorption by the metal. Both mechanisms lead to a transient increase of the substrate surface temperature and to a partial desorption of the adsorbate. The resonant infrared laser stimulated desorption of NH 3 from a copper single crystal surface is analyzed quantitatively within the framework of this model. The adsorption energy of the photodesorbed molecules and the infrared absorbance of the adsorbate can be determined from the experimental data.

A 2600‐K tantalum cavity as an interesting infrared source for absorbance measurements

A design is proposed for a stable infrared source suited to absorbance measurements. The source is a parallelepipedic cavity of tantalum, heated to 2600 K, with an apparent emissivity in excess of 0.7. It is observed to be three times brighter at 3000 cm−1 than the 1440‐K SiC bar. A model is constructed in order to estimate the temperature distribution. For this kind of source, the temperature and emissivity are found to be quite uniform. A comparison between estimated and experimental values of temperature and emissivity is discussed.

Photoelastic modulated infrared spectra of methanol adsorbed on metallic surfaces

We have set up an infrared reflection absorption grating spectrometer working under high vacuum conditions. The infrared source is made of a tantalum cavity heated to 2500 K. Its brightness at 3000 cm-1 (emissivity > 0.7) is four times higher than that of a common silicon carbide source at 1500 K. The adsorbate absorption detection technique is based on the use of a photoelastic modulator. All those features permit a high absorbance sensitivity while avoiding the measured infrared spectrum to be affected by the absorption from residual gases in the spectrometer. The measurement of infrared spectra between 2.6 and 5 μm allowed us to specially follow the evolution of the methyl and hydroxyl stretching bands of CH3OH and CD3OD versus exposure at 80 K and versus annealing temperature for a polycrystailine platinum and a Cu(110) substrates. The strong attenuation of the OH band at low coverage indicates that, in the submonolayer coverage range, the methanol molecules are adsorbed with their OH group axes quasi parallel to both metallic surfaces. Because both the symmetric and asymmetric CH3 (or CD3) stretching modes are detected, the methyl group axis must be inclined versus the surface normals. In the multilayer coverage range, the methanol forms a thin amorphous phase which can be crystallized upon annealing. The simultaneous detection of the in-phase and out of phase components of the crystallized layer hydroxyl stretching indicates that this layer is constituted by arbitrarily oriented small crystallites.

Resonant photodesorption of CH3OH from Cu(110) using an IR beam generated by an optical parametric oscillator

Abstract In this work, we use an optical parametric oscillator, furnishing a pulsed infrared laser beam continuously tunable from 2.9 to 3.7 μm, in order to excite the asymmetric methyl (CT 3a ) and the hydroxyl (OH) stretchings of methanol condensed on a Cu(110) surface at 90 K. The resonant character of the photodesorption process is demonstrated. The relative intensity and the width of the resonant structures of the photodesorption yield spectra strongly depend on the laser fluence. This is a direct consequence of the nonlinear dependence of the desorption yield versus the latter. The resonant structure corresponding to the OH group excitation is blue shifted of 100 cm −1 versus the OH band in the IRAS spectrum of the amorphous methanol film. This testifies that the liquid phase plays an important role in the photodesorption mechanism.

Comment on linear infrared spectra and laserinduced resonant desorption in the coadsorbate 12C16O/13C16O-NaCl

Abstract Herein, we evaluate the importance of vibrational energy exchange between two different coadsorbed isotopic molecules due to their direct dipole coupling, when one of them is resonantly excited by a pulsed infrared laser beam. It is shown that, for the system 12 C 16 O- 13 C 16 O/NaCl film investigated by J. Heidberg, K.-W. Stahmer, H. Stein and H. Weiss [J. Electron. Spectrosc. Relat. Phenom., 45 (1987) 87], the dipole coupling is one order of magnitude too small to be responsible for the lack of isotopic selectivity when performing resonant infrared laser stimulated desorption.