Collisionally aligned molecular beams: a tool for stereodynamical studies in the gas phase and at surfaces (original) (raw)
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Effect of the molecular structure on the gas-surface scattering studied by supersonic molecular beam
The European Physical Journal D, 2006
The experimental apparatus for investigating the gas-surface interaction has been newly developed. The coherent length of the helium, the energy resolution and the angular spread of the beam in the apparatus were established as ω = 16 nm, ∆E/E = 2.4% and ∆θ = 0.5 • , respectively, through the measurements of the time-of-flight of He beam and of the angular intensity distributions of He scattered from LiF(001). The angular intensity distributions of Ar, N2 and CO scattered from the LiF surface along the [100] azimuthal direction were then measured as a function of incident translational energy. The effects of the molecular structural anisotropy and center-of-mass position on the gas-surface inelastic collision at the corrugated surface are discussed with predictions based on a recently developed simple classical theory of the ellipsoid-washboard model. PACS. 68.49.Bc Atom scattering from surfaces (diffraction and energy transfer) -68.49.Df Molecule scattering from surfaces (energy transfer, resonances, trapping) -34.50.Dy Interactions of atoms and molecules with surfaces; photon and electron emission; neutralization of ions -68.49.-h Surface characterization by particle-surface scattering 130 The European Physical Journal D
Interaction of rotationally aligned and of oriented molecules in gas phase and at surfaces
Progress in Surface Science, 2010
Recent developments concerning the generation of molecular beams containing oriented/aligned molecules will be reviewed and applications of such tools to the study of elementary processes occurring both in homogeneous and heterogeneous phases will be presented. First we will discuss the case of symmetric top molecules oriented by hexapoles. Here the molecular polarization is obtained by the use of an external field and allows to control which end of the molecular projectile is going to collide with the target. Then we will review the so-called collisional alignment, a molecular polarization phenomenon occurring in supersonic expansions of gaseous mixtures. The key feature, in this case, is the velocity dependence of the alignment degree, which allows the use of mechanical devices to filter out of the beam the molecules having either a random (statistical) or a preferential (non-statistical) spatial distribution of their rotational angular momentum J with respect to the molecular beam axis. The physical mechanisms underlying the collisional alignment will be resumed and some relevant gas-phase experiments demonstrating its occurrence will be illustrated. Application of such methodologies to the investigation of the stereodynamics of elementary 0079-6816/$ -see front matter Ó
Journal of Physical Chemistry A, 1997
Measurements of total integral cross sections for scattering of nitrogen molecules by Xe atoms in the glory collision energy range (40-600 meV) are reported under two different experimental conditions, using either a rotationally "hot" (most probable levels J ) 8, 9) effusive beam of nitrogen (obtaining information on the isotropic component of the interaction potential) or rotationally "cold" N 2 seeded beams emerging from supersonic expansions (obtaining quantitative information on rotational alignment of molecular nitrogen). The scattering results presented in this paper allow us to establish that the recently reported phenomenon of the strong correlation between the degree of collision-induced alignment and the speed of the molecules within a supersonic seeded velocity distribution, as previously observed for the first time in molecular oxygen, also occurs for the case of nitrogen molecules. Alignment parameters are reported for both the ortho and para forms of nitrogen, which are cooled down in the seeded supersonic expansion process to their lowest rotational levels J ) 0, 1, 2.
Physical Review Letters, 2001
This work represents the first experimental demonstration that planar molecules tend to travel as a "frisbee" when a gaseous mixture with lighter carriers expands into a vacuum, the orientation being due to collisions. The molecule is benzene, the prototype of aromatic chemistry. The demonstration is via two complementary experiments: interrogating benzene by IR-laser light and controlling its orientation by selective scattering on rare gas targets. The results cast new light on the microscopic mechanisms of collisional alignment and suggest a useful way to produce intense beams of aligned molecules, permitting studies of steric effects in gas-phase processes and in surface catalysis.
Review of Scientific Instruments, 2007
We describe an advanced and highly sensitive instrument for quantum state-resolved molecule-surface energy transfer studies under ultrahigh vacuum ͑UHV͒ conditions. The apparatus includes a beam source chamber, two differential pumping chambers, and a UHV chamber for surface preparation, surface characterization, and molecular beam scattering. Pulsed and collimated supersonic molecular beams are generated by expanding target molecule mixtures through a home-built pulsed nozzle, and excited quantum state-selected molecules were prepared via tunable, narrow-band laser overtone pumping. Detection systems have been designed to measure specific vibrational-rotational state, time-of-flight, angular and velocity distributions of molecular beams coming to and scattered off the surface. Facilities are provided to clean and characterize the surface under UHV conditions. Initial experiments on the scattering of HCl͑v =0͒ from Au͑111͒ show many advantages of this new instrument for fundamental studies of the energy transfer at the gas-surface interface.
Review of Scientific Instruments, 2004
A light and compact mechanical velocity selector, of novel design, for applications in supersonic molecular-beam studies has been developed. It represents a simplified version of the traditional, 50 year old, slotted disks velocity selector. Taking advantage of new materials and improved machining techniques, the new version has been realized with only two rotating slotted disks, driven by an electrical motor with adjustable frequency of rotation, and thus has a much smaller weight and size with respect to the original design, which may allow easier implementation in most of the available molecular-beam apparatuses. This new type of selector, which maintains a sufficiently high velocity resolution, has been developed for sampling molecules with different degrees of rotational alignment, like those emerging from a seeded supersonic expansion. This sampling is the crucial step to realize new molecular-beam experiments to study the effect of molecular alignment in collisional processes.
Physical Review Letters, 1995
Measurements are reported for the scattering of molecular oxygen with both a hot rotational energy distribution and cooled at its ground rotational state with a controlled alignment of the rotational angular momentum. The cooling and control of the alignment are achieved using supersonic expansion in beams of molecular oxygen seeded in various mixtures of Ne, He, and H 2 and monitored by a Stern-Gerlach magnetic deflection technique. It is shown by the example of integral scattering cross section with Xe in the glory energy range that information can be obtained on the isotropic and anisotropic components of intermolecular van der Waals interactions. PACS numbers: 34.20.Gj, This Letter reports the first application to molecular scattering of a natural and facile technique for alignment of molecular rotations by seeded supersonic expansions of gaseous mixtures . The seeding phenomenon provides a general way not only to molecular acceleration and cooling of the heavier components of the mixtures but also to polarization of their rotational angular momentum . This alignment has been found [2] to be drastically dependent on final speed: In the prototypical case of oxygen molecules diluted in mixtures with lighter gases such as H 2 , He, and Ne, cooling occurs down to the rotational ground state K 1 and strongly nonstatistical distribution is observed for the rotational angular momentum projections along the propagation direction (helicities) M 0 and 1 (either 11 or 21).