H. R. Wendt

Laser desorption jet-cooling of organic molecules

Laser desorption followed by jet-cooling allows wavelength-selective as well as mass-selective detection of molecules desorbed from a surface without fragmentation. The cooling characteristics and detection sensitivity of laser desorption jet-cooling of organic molecules are investigated. From the rotational contour of the electronic origin of the S1 ← S0 transition of laser-desorbed anthracene, rotational cooling to 5–10 K is demonstrated. Vibrational cooling is studied for laser-desorbed diphenylamine, a molecule with low-energy vibrations, and a vibrational temperature below 15 K is found. The absolute detection sensitivity is determined for the perylene molecule. Using two-color (1+1) resonance enhanced multi-photon ionization (with a measured ionization efficiency of 0.25) for detection, it is found that one ion is produced in the detection region for every 2×105 perylene molecules evaporated from the desorption laser spot. A two-color (1+1) REMPI spectrum (400 points) of perylene is recorded using only 30 picogram of material.

A SEARCH FOR C60 IN CARBONACEOUS CHONDRITES

Analysis of interior samples of the Murchison meteorite by two routes yielded an upper limit of 2 ppb for its C60 content, as compared to parts per million levels for individual polycyclic aromatic hydrocarbons (PAHs). Provided the samples contain an interstellar component, which is probable since Murchison hydrocarbons contain excess deuterium, this result argues against the ubiquitous presence of C60 in the interstellar medium. A possible explanation for the absence of C60 was found in experiments showing how PAHs replace fullerenes as stable end products when hydrogen is present during carbon condensation. As a secondary result we found high molecular weight PAHs in the Murchison and Allende meteorites. Coronene and its methyl derivatives are especially interesting since features in the coronene spectrum have been shown to match some of the unidentified interstellar infrared emission bands.

Characterization of fullerenes and doped fullerenes

Abstract The results of a variety of experiments used to characterize fullerenes, metallofullerenes and alkali-metal intercalated C 60 are reported. It is shown that differential scanning calorimetry and NMR characterization of the orientational phase transition in C 60 provide sensitive means to assess the purity and crystallinity of fullerene samples. The results of a mass-spectrometric investigation of metallofullerene samples produced by co-vaporization of carbon and metals are described. An account is given of some electron paramagnetic resonance results obtained for LaC 82 and solid-state NMR results obtained on an alkali-intercalated fullerite, Rb 3 C 60 , which show for the first time the presence of magnetically inequivalent carbons in underivatized C 60 . Together these experiments yield a great deal of information about the phase purity, molecular dynamics and structure of a variety of fullerene materials.

Pulsed laser desorption near a jet orifice: Concentration profiles of entrained perylene vapor

We have investigated the entrainment in a jet expansion of material vaporized with a laser pulse from a surface below but closely adjacent to the jet orifice. Jets of He and Ar were used as carriers, and perylene was the test substance. Its distribution in space and time far from the nozzle was measured by laser-induced fluorescence and one-dimensional imaging with an optical multichannel analyzer. The width of the perylene concentration profile was found to be much narrower than the width of the gas expansion, especially in He. This makes it possible to extract a useful portion of the entrained material through a skimmer. Depending on the values of experimental parameters, the center of the profile first appears above, on, or below the jet axis and then moves lower with time. Effects of several parameters on shape and time-dependent position of the profile were investigated. The results can be used to optimize the overlap of the concentration pulse with a skimmer or other probe.

Laser FM spectroscopy with photochemical modulation

AbstractWe 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,2A″-

Laser desorption in front of a free jet nozzle: Distribution of desorbed material in the gas expansion

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Imaging C60 clusters on a surface using a scanning tunnelling microscope

,2A′ 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.

Laser Desorption Jet-Cooling of Organic Molecules Cooling Characteristics and Detection Sensitivity

Abstract We report angle-resolved photoemission data from single crystals of C60 cleaved in UHV. Unlike the other forms of pure carbon, the valence band spectrum of C60 consists of many sharp features that can be essentially accounted for by the quantum chemical calculations describing individual molecules. This suggests that the electronic structure of solid C60 is mainly determined by the bonding interactions within the individual molecules. We also observe remarkable intensity modulations of the photoemission features as a function of photon energy, suggesting strong final state effects. Finally, we address the issue of the band width of the HOMO state of C60. We assert that the width of the photoemission peak of C60 does not reflect the intrinsic band width because it is broadened by the non 0-0 transitions via the Franck-Condon principle. Our view point provides a possible reconciliation between these photoemission data and those measured by other techniques.