Internal energy distribution of recombinatively desorbing D2 from sulfur covered poly-Pd

Rotational and vibrational population distributions have been determined for D₂ molecules recombinatively desorbing from polycrystalline Pd surfaces by tunable vacuum-ultraviolet laser-induced fluorescence. In the temperature range 550 KT _s 1050 K studied in this work a rotational temperature ofT _rot 450 K was found, nearly independent of the surface temperature. Similarly, the vibrational temperature could be described by a value ofT _vib1100 K, being always higher than the surface temperature.     

Infrared absorption of H2 adsorbed on porous glass,silica gel,and MgO

Infrared absorption of the vibrational band of H₂ in the bulk is known to arise from overlap and quadrupolar interactions. For adsorbed H₂, in addition to intermolecular interactions, an interfacial mechanism of inducing absorption has been identified in porous Vycor glass. A large enhancement of the fundamental vibrational line (by a factor of 60) from the bulk H₂ (liquid) has been observed. The vibrational overtone absorption is similarly enhanced by 150. We discuss these results as well as infrared absorption measurements for H₂ on MgO and silica gels.     

Raman spectroscopy of infrared multiple-photon excited molecules

The method of spectroscopy of spontaneous Raman scattering (RS) with time resolution has been applied for the first time to diagnose the process of multiphoton ir molecular excitation (MPE). Some aspects of RS diagnostics of MPE processes are being analyzed. It has been shown experimentally on SF₆ and CF₃I molecules that it is possible to study such important characteristics of excitation process as the fraction of molecules involved in the process of excitation, vibrational energy distribution of molecules, stochastization of inner molecular energy.     

Laser FM spectroscopy with photochemical modulation

We have combined the techniques of frequency-modulation spectroscopy (FMS) and photochemical-modulation spectroscopy to carry out high-resolution, high-sensitivity absorption measurements on the formyl and amino radicals. Using the (0,9⁰, 0)-(0,0¹, 0) band of theÃ,² A- ,² A transition of HCO at 614 nm, we obtained a sensitivity limit for absorption of 1.5×10^–6. Reconstructed spectra of several HCO lines are presented.     

Dynamical parameters of desorbing molecules

The aim to uncover fundamental details of the desorption process as well as the needs of more applied research, like fusion, led recently to a growing interest in the study of the parameters of desorbing molecules. The main experimental methods and the most significant experimental results obtained so far are reviewed and discussed. The investigation of the parameters of desorbing molecules being one of the oldest fields of surface science, a number of pre- and misconceptions still hamper the mutual understanding and hereby impede progress. Accordingly, a relatively large pace is dedicated to the historical development, to definitions, and even to semantics.     

Raman diagnostics of heterogeneous chemical processes

The suitability of cw Raman spectroscopy for characterization of mass transport phenomena in connection with heterogeneous chemical processes is demonstrated for the example of the catalytic hydrogenation of acetylene. Temperature and concentration profiles in a model reactor were derived simultaneously from H₂ pure rotational lines and from the most intenseQ branches of C₂H₂ and of C₂H₄. The temperature dependence of the band contours has been considered; by careful calibration the systematic error of the derived concentration values is limited to less than 2%. The measurements allow the separation of the effects of thermal diffusion and of the reaction.     

CARS spectroscopy of molecules and clusters in supersonic jets

Supersonic molecular beams of D₂, CH₄, NH₃, and C₂H₄ are investigated in the expansion region employing collinear coherent anti-Stokes Raman spectroscopy (CARS). The analysis of rotationally resolved CARS spectra allows the determination of temperatures in the beam. The rotational relaxation as a function of stagnation pressure and separation from the nozzle is studied by recording theQ branch for D₂ and the ₃ R andS branches for CH₄. Rotational temperatures for NH₃ are determined by investigating the complete ₃ band. At strong stagnation conditions broad structures arise which can be attributed to the formation of NH₃ clusters. For C₂H₄ the ₅ band with resolved rotational structure is reported. Again, at larger distances from the nozzle, broad structures are observed. They are assigned to the ₁ and ₅ vibrations in the C₂H₄ cluster.     

Application of molecular (static) secondary ion mass spectroscopy to detection of organic molecules in amorphous titanium carbide

Molecular, or static, secondary ion mass Spectroscopy (SIMS) is applied to the detection of organic molecules in amorphous titanium carbide films. The presence of such organic clusters is thought to stabilize the amorphous phase to higher temperatures (>1000°C) and greater thicknesses. The high corrosion resistance properties of the TiC deposits are also attributed to the inclusion of such molecular entities. The processes whereby these molecular entities in the films are transformed into secondary ions during SIMS analysis are also investigated. It is shown that the dominant ionization mechanisms in this case are electron and momentum transfer.     

Desorption rates and mechanisms for dissociatively chemisorbed molecules

The desorption kinetics of dissociatively chemisorbed diatomic molecules are examined from a kinetic-modeling point of view. A comparison is made between a one-step process, resulting in the usual second-order kinetics, and a two-step process which takes into account explicity recombination of atoms and subsequent desorption of molecules. The kinetics from the two-step mechanism are found to be equivalent to second-order desorption with a coverage-dependent activation energy which, in many cases, is a linear function of coverage. The two-step process leads to second-order kinetics with a constant activation energy only for special values of the model rate parameters, or if chemisorption is activated. The steady onate approximation for the intermediate in the two-step process is often adequate, but the transient period during which a steady state is reached also contains important kinetic information. The implications of these results for desorption kinetics and molecular beam reaction experiments are discussed. Work performed under the auspices of the Office of Basic Energy Sciences of the Department of Energy     

Fast deuterium molecules desorbing from metals

The velocity distribution of D₂ desorbing from Pd(100) reveals the existence of two distinct groups of molecules, fast and slow ones. A new mechanism is proposed for the desorption of the molecules belonging to the fast group: after diffusing to the surface, absorbed D atoms recombine directly, without equilibrating in the chemisorption well. The fast molecules observed in the D₂ desorption spectra from Cu and Ni seem to desorb through the same mechanism. In the case of Pd this mechanism is controlled by adsorbed sulphur.     

Laser generated microstructures

Deposition and etching processes based on the interaction of laser light with a substrate surface and molecules of the surrounding ambient are discussed in this tutorial review. This laser writing approach is based on photolytic, pyrolytic, or photoelectrochemical microreactions. The fundamental properties of such reactions and corresponding processing parameters (e.g. deposition or etch rate, resolution) are discussed. Important published results for deposition by photolysis, pyrolysis, and etching are summarized in the form of tables. A special list of potential applications for such techniques and a list of all materials used thus far for laser deposition and etching are included.     

Laser kinetic spectroscopy of solvated dye molecules

Journal of Russian Laser Research -     

Thermal desorption spectroscopy of condensed lead films on {100} GaAs surfaces

The desorption of Pb from MBE-grown {100} GaAs surfaces has been investigated using modulated-beam mass spectrometry, isothermal desorption rate measurements and temperature programmed thermal desorption spectroscopy. The results are in general agreement with a model in which desorption occurs from a disperse phase which is in equilibrium with adsorbate islands. From the independence of the activation energy of desorption on coverage from <10^–3 s monolayers to several tens of monolayers, single atom desorption is the probable rate limiting step. An important conclusion is that it is not possible to perform a unique analysis of temperature programmed desorption data from condensed films without prior knowledge of the dependence of the desorption activation energy on coverage.     

Laser photochemical etching of silicon

Argon laser induced chemical etching of single crystal silicon with chlorine is studied. Etch rates are determined as a function of gas pressure, crystal orientation, laser power and wavelength. Analysis of gas phase and surface products by Fourier transform IR and X-ray photoelectron spectroscopy are used to probe the reaction mechanism. Contrary to previous reports, no thermally enhanced etch rate is observed for Si (111) and the presence of oxide on the surface is found to inhibit etching even at high laser power.     

Sublevel spectroscopy of alkali atoms in superfluid helium

We report the results of optical pumping and optical detection of magnetic resonance of alkali atoms (Cs and Rb) in superfluid helium. The magnetic resonances between the ground-state Zeeman sublevels and hyperfine levels are observed through monitoring theD ₁ fluorescence by means of the optical-rf double resonance technique. Although the ground stateg values in superfluid helium are the same as in vacuum within the experimental error, the hyperfine constant of the ground state of the Cs atom in superfluid helium is found to be slightly larger than in vacuum. Coherent transient spectroscopy is also performed.     

Adsorption behavior of binary mixed alkanethiol molecules on Au: Scanning tunneling microscope and linear-scan voltammetry investigation

Adsorption behavior of binary mixed alkanethiol molecules of octanethiol (OT) and dodecanethiol (DDT) on Au substrate has been investigated with a variation of molar fraction in solution at room temperature. Linear-scan voltammograms (LSV) taken from the binary mixed self-assembled monolayer (SAM) on Au/Si showed a single reductive desorption peak, continuously shifting with a variation of mixing ratio in solution. Molecule-resolved scanning tunneling microscope (STM) images obtained from the binary SAM on Au/mica showed the phase separation with a few nanometer sized domains. The difference in the chain length between OT and DDT was not large enough to induce the phase separation with much larger domains, which could be well detected by LSV. There was no preferential adsorption of DDT to OT.     

Resonant soft X-ray fluorescence spectra of molecules

Accepted: 6 March 1997     

Laser optogalvanic spectrum of pure bromine discharge

Doppler limited laser optogalvanic (LOG) spectra are obtained by irradiating a bromine discharge with a cw dye laser. The discrete bands of the Br₂ B-X system appear superimposed on a strong continuum. The LOG spectrum is closely identical with the absorption/emission spectrum of Br₂. Some extra bands and assigned in theB-X system are also observed and their vibrational quantum number assignment is given.