F. Pradère

Evaporation of small fragments during the scattering of argon clusters at thermal kinetic energies from a graphite surface

We present a thermokinetic model together with new experimental results for the scattering of large argon clusters off a graphite surface. Both angular and time‐of‐flight distributions are shown for a large range of surface temperatures, incidence angles, and incident cluster sizes. A quantitative comparison between the proposed thermokinetic model and our measurements allows one to interpret most of the experimental results as due to thermal evaporation of very small fragments from their parent clusters gliding along the surface. The coefficient of tangential velocity conservation cF and the local temperature Tlocal of the evaporating fragments have been determined quantitatively. Although the investigated parameters were varied over a large range, Tlocal remains essentially constant around (140 ± 20) K. The coefficient cF turns out to be approximately (0.80 ± 0.05) independent of surface temperature and incident cluster size for all incidence angles larger than 40°. It increases, however, rapidly to 1.4...

Powerful tunable infrared generation by stimulated Raman scattering

Abstract We report the generation of powerful infrared radiation continuously tunable between 14 000 and 2 300 cm -1 . The beam is either the output of a dye laser (with a linewidth of 0.07 cm -1 and a full angle divergence of 0.2 mrd) or its Raman shifted components. We have obtained 20 to 60 MW peak power between 14 000 and 9 700 cm -1 , 5 to 20 MW between 9 700 and 5 500 cm -1 and 0.5 to 2 MW between 5 500 and 2 300 cm -1 . As an application we measured the hyperpolarizability of some organic molecules.

Argon cluster scattering from a graphite surface

Abstract Experimental results from the scattering of argon clusters off a graphite surface are presented. Angular distributions of cluster fragments have been recorded at monomer, dimer and trimer masses. While some of our findings are similar to previous observations and can be interpreted in the framework of existing theories, two distinctly new features have been seen: first, the angular distributions of the scattered fragments are clearly correlated with their masses; and second, a new grazing exit angle component appears for sufficiently large incident angles or backing pressures, which is interpreted as evidence of large cluster fragments surviving the surface collision.

Dynamic zone structure model for the surface scattering of large van der Waals clusters at thermal kinetic energies

A semiempirical dynamic zone structure model is presented to explain the behavior of the grazing exit angle component previously observed in experiment and simulation during the scattering of large van der Waals clusters from surfaces. The proposed model that is based on a simple energy balance and the Leidenfrost phenomenon does not only qualitatively reproduce the measured importance of the grazing exit angle component as a function of incident cluster size, incident velocity, angle of incidence, and surface temperature, but it also gives an order‐of‐magnitude estimate for the size of the large fragments scattered in this component and for the involved picosecond interaction times. Recent results obtained from both trajectory calculations and experiments show very good agreement with the predictions concluded from the proposed model.

Experimental evidence of enhanced diffuse monomer scattering in cluster-surface collisions. Arn on graphite

Abstract We report the first experimental observation of diffuse scattering of monomers exhibiting significant velocity thermalization in large argon clusters collisions with a pyrolytic surface. Angular distributions of the outcoming argon atoms show a diffuse component for which time-of-flight spectra are independent of incidence angle and incoming cluster size, while they vary significantly with surface temperature. The amplitude of the diffuse scattering component is enhanced by up to a factor of three for incident clusters over its value for incident monomers. This enhancement is discussed by using a simple model of atom trapping-desorption on flat surfaces.

Tunable far infrared generation in hydrogen fluoride

Abstract The use of resonantly enhanced stimulated Raman scattering for Q( J ) transitions in hydrogen flouride permits a broad coverage of the far infrared spectrum. The tunability was about 5 cm -1 around several transitions ( J = 0 to 5). The output power was measured up to 300 kW corresponding to a photon efficiency of 18%.

Average cluster size determination in supersonic beams from angular distribution measurements after scattering by a buffer gas

A method for the determination of average cluster size in supersonic beams is presented. Based on angular distribution broadening of the beams caused by passing through a buffer gas, this method is well suited for in situ determination of the mean cluster size when the apparatus contains a movable detector with sufficient spatial resolution. The shape and width of the beam profile after scattering by a buffer gas are evaluated theoretically as functions of buffer gas pressure and atom-cluster collision cross-section. Experimental results are presented for an argon beam, yielding average cluster sizes between 300 and 7000 atoms depending on the stagnation pressure. Simple criteria to assess the applicability of the method to a given experimental situation are discussed. The average cluster sizes determined in this work agree quite satisfactorily with previously published values for similar beam generation conditions.

Ultrahigh‐vacuum multichamber apparatus for molecule‐surface interaction studies

We present a complete apparatus for molecule‐surface interaction studies. Three UHV chambers are connected together allowing preparation, characterization, transfer, and experimental investigations with well‐defined surface samples under ultrahigh‐vacuum conditions. A chopped, supersonic molecular beam with well‐controlled profile enters the main UHV chamber where it is scattered by the sample under study. Detection systems have been designed to measure angular and time‐of‐flight distributions and rotational populations of beam particles scattered off the surface sample giving access to the energy exchange between the internal degrees of freedom of the scattered molecules and the surface. The characteristics of our apparatus are reported and experimental tests for nitrogen and argon molecular beams scattered off a graphite surface are shown.

Tunable, millijoule radiation extending to the 16 μm region

Abstract Efficient Stimulated Raman Scattering (SRS) in liquid nitrogen between 15 and 18 μm was obtained. Infrared radiation output was continuously tunable and narrow-band with energies up to 1.7 mJ.

Observation of the ν3 → (2ν3, A1) band of methane

Abstract Double-resonance spectra were recorded for several rotational sublevels of the (2ν 3 , A 1 ) vibrational level of methane. The observed vibrational energy is 5968.1 cm −1 , in good agreement with local-mode theory. The effective rotational constant for this level was found to be 5.31 cm −1 . This unexpectedly high value could involve interactions with lower vibrational levels.