ALBERT

Notes: Low energy helium diffraction has been used to study the packing and thermal motion of the terminal CH3 groups of monolayers of n-alkane thiols self-assembled on Au(111)/mica films and a Au(111) single crystal surface. At low temperatures (〈100 K), the terminal CH3 groups are arranged in domains containing a hexagonal lattice with a lattice constant of 5.01 A(ring). As the length of the carbon chain is shortened, an abrupt decrease in the diffraction peak intensities is observed for CH3(CH2)9SH/Au(111)/mica, and no diffraction is observed for CH3(CH2)5SH/Au(111)/mica. This is indicative of a sudden decrease in surface order at around ten carbon atoms per chain. A semi-quantitative estimation of the average domain size of each monolayer surface shows a maximum of 46 A(ring) at intermediate chain length [CH3(CH2)13SH/Au(111)/mica], decreasing to 26 A(ring) at longer [CH3(CH2)21SH/Au(111)/mica] and 41 A(ring) at shorter [CH3(CH2)9SH/Au(111)/mica] chain lengths. No phase transitions could be detected at the surfaces of these monolayers from 35 K to 100 K, but as expected for a soft material, the thermal motion of the n-alkane thiol molecules increases with increasing surface temperature and reduces the diffraction intensities to zero at around 100 K. The relative mean square displacements of the surface CH3 groups along the directions perpendicular and parallel to the surface have been calculated from the temperature dependence of the diffraction peak intensities using the standard Debye–Waller formalism. The measured values are in good agreement with the results from a recent molecular dynamics simulation. [J. Hautman and M. Klein, J. Chem. Phys. 93, 7483 (1990).]

Notes: We demonstrate that the surface structure of organic monolayers can be determined by low energy helium diffraction at low surface temperatures. This uniquely surface-sensitive and nondestructive technique shows that the CH3-terminated surface of a monolayer of docosane thiol (CH3(CH2)21SH) on Au(111) is composed of small, ordered domains (lattice constant 5.01±0.02 A(ring)), a large fraction of which share a common orientation. The helium diffraction intensities decrease monotonically with increasing temperature and vanish around 100 K, due to thermal motion of the CH3 groups. Surface order is observed for chains as short as ten carbons (CH3(CH2)9SH) but a shorter chain, (CH3(CH2)5SH), gave no diffraction.

Notes: In this article, recent developments in helium nanodroplet isolation (HENDI) spectroscopy are reviewed, with an emphasis on the infrared region of the spectrum. We discuss how molecular beam spectroscopy and matrix isolation spectroscopy can be usefully combined into a method that provides a unique tool to tackle physical and chemical problems which had been outside our experimental possibilities. Next, in reviewing the experimental methodology, we present design criteria for droplet beam formation and its seeding with the chromophore(s) of interest, followed by a discussion of the merits and shortcomings of radiation sources currently used in this type of spectroscopy. In a second, more conceptual part of the review, we discuss several HENDI issues which are understood by the community to a varied level of depth and precision. In this context, we show first how a superfluid helium cluster adopts the symmetry of the molecule or complex seeded in it and discuss the nature of the potential well (and its anisotropy) that acts on a solute inside a droplet, and of the energy levels that arise because of this confinement. Second, we treat the question of the homogeneous versus inhomogeneous broadening of the spectral profiles, moving after this to a discussion of the rotational dynamics of the molecules and of the surrounding superfluid medium. The change in rotational constants from their gas phase values, and their dependence on the angular velocity and vibrational quantum number are discussed. Finally, the spectral shifts generated by this very gentle matrix are analyzed and shown to be small because of a cancellation between the opposing action of the attractive and repulsive parts of the potential of interaction between molecules and their solvent. The review concludes with a discussion of three recent applications to (a) the synthesis of far-from-equilibrium molecular aggregates that could hardly be prepared in any other way, (b) the study of the influence of a simple and rather homogeneous solvent on large amplitude molecular motions, and (c) the study of mixed 3He/4He and other highly quantum clusters (e.g., H2 clusters) prepared inside helium droplets and interrogated by measuring the IR spectra of molecules embedded in them. In spite of the many open questions, we hope to convince the reader that HENDI has a great potential for the solution of several problems in modern chemistry and condensed matter physics, and that, even more interestingly, this unusual environment has the potential to generate new sets of issues which were not in our minds before its introduction. © 2001 American Institute of Physics.

Notes: It is demonstrated that matrix-like spectroscopy may be carried out in the gas phase using molecular beams of clusters and the conditions under which bulk-matrix-like behavior is achieved are illustrated. At the same time, we obtain information on the structural evolution of noble gas clusters as a function of their size. Infrared spectra for SF6 attached to noble gas clusters of argon, krypton, and xenon were recorded using a free jet cluster source and a laser photofragmentation detection technique. When a dilute mixture of the chromophore in Ar and Kr is expanded at relatively low pressures, the clusters spectra show a feature characteristic of the SF6 solvated in a defective, unannealed matrix. This feature disappears at higher source pressures (larger sizes) at which the chromophore prefers to reside on the surface of the cluster. This can be demonstrated by producing neat clusters and depositing the chromophore on them. However, on producing still larger clusters, a different absorption appears which is accurately located at the same position as the main absorption in a well-annealed matrix of Ar or Kr. This behavior is related to the transition of clusters from a Mackay icosahedral structure, shown to be the most stable for smaller clusters, to the face-centered-cubic (fcc) structure which is observed in the bulk phase. This structural transition occurs at a nozzle stagnation pressure which corresponds to an average cluster size of about 2000 atoms for both Ar and Kr. Scattering studies performed on argon clusters suggest that the fcc-type clusters correspond to the largest sizes in the cluster size distribution present in the beams. A similar structural transition for xenon cluster was not established as the SF6 appears to solvate only slightly in Xe in the size range studied here. © 1995 American Institute of Physics.

Notes: The eigenstate-resolved 2ν1 (acetylenic CH stretch) absorption spectrum of propane has been observed for J'=0–11 and K=0–3 in a skimmed supersonic molecular beam using optothermal detection. Radiation near 1.5 μm was generated by a color center laser allowing spectra to be obtained with a full-width at half-maximum resolution of 6×10−4 cm−1 (18 MHz). Three distinct characteristics are observed for the perturbations suffered by the optically active (bright) acetylenic CH stretch vibrational state due to vibrational coupling to the nonoptically active (dark) vibrational bath states. (1) The K=0 states are observed to be unperturbed. (2) Approximately 2/3 of the observed K=1–3 transitions are split into 0.02–0.25 cm−1 wide multiplets of two to five lines. These splittings are due to intramolecular coupling of 2ν1 to the near resonant bath states with an average matrix element of 〈V2〉1/2=0.002 cm−1 that appears to grow approximately linearly with K. (3) The K subband origins are observed to be displaced from the positions predicted for a parallel band, symmetric top spectrum.The first two features suggest that the coupling of the bright state to the bath states is dominated by parallel (z-axis) Coriolis coupling. The third suggests a nonresonant coupling (Coriolis or anharmonic) to a perturber, not directly observed in the spectrum, that itself tunes rapidly with K; the latter being the signature of diagonal z-axis Coriolis interactions affecting the perturber. A natural interpretation of these facts is that the coupling between the bright state and the dark states is mediated by a doorway state that is anharmonically coupled to the bright state and z-axis Coriolis coupled to the dark states. Z-axis Coriolis coupling of the doorway state to the bright state can be ruled out since the ν1 normal mode cannot couple to any of the other normal modes by a parallel Coriolis interaction. Based on the range of measured matrix elements and the distribution of the number of perturbations observed we find that the bath levels that couple to 2ν1 do not exhibit Gaussian orthogonal ensemble type statistics but instead show statistics consistent with a Poisson spectrum, suggesting regular, not chaotic, classical dynamics.

Notes: Sequential infrared/infrared double resonance excitation of an optothermally detected molecular beam has been used to obtain the eigenstate resolved spectrum of the second C–H stretch overtone in propyne near 9700 cm−1. The high resolution and sensitivity of this technique allows for extraction of detailed information about the dynamics of intramolecular vibrational energy redistribution from this highly fractionated spectrum. The analysis suggests a coupling mechanism consisting of anharmonic coupling out of the bright state through a doorway state or first tier, followed by subsequent coupling to a strongly Coriolis mixed bath. The lifetime of the bright state, which is determined by the first step and is not dependent on the total angular momentum, is measured to be about 320 ps for the K=0 clumps and about 210 ps for the K=1 clumps. The root mean square coupling matrix element determined for the J'=0 clump is 0.008 cm−1 and decreases with increasing J'. According to the level spacing and Heller's F statistics, the spectrum shows evidence indicating that the underlying dynamic behavior is chaotic.

Notes: We report direct evidence of a unit mesh containing more than one hydrocarbon chain at the surface of a self-assembled monolayer of long-chain n-alkanethiols. Our helium diffraction measurements for a monolayer of n-octadecanethiol on Au(111) are consistent with a rectangular primitive unit mesh of dimensions 8.68×10.02 A(ring) containing four crystallographically distinct hydrocarbon chains. This packing arrangement can also be described as a c(4×2) superlattice with respect to the fundamental simple hexagonal [((square root of)3×(square root of)3)R30°] array of lattice parameter 5.01 A(ring) previously observed for monolayers of other n-alkanethiols on gold. No temperature-dependent phase behavior is observed in the temperature range where surface diffraction is measurable (30–100 K) and cycling up to temperatures as high as 50 °C caused no observable change in the diffraction. It is proposed that this larger unit mesh is the result of a patterned arrangement of rotations of the hydrocarbon chains about their molecular axes. This patterned arrangement must be different than the herringbone structure expected by simple analogy to bulk n-alkanes.