Interaction of rotationally aligned and of oriented molecules in gas phase and at surfaces (original) (raw)
Related papers
Physica Scripta, 2006
Production, characterization and control of alignment degree of molecules are of importance for investigating in detail the stereodynamics of elementary processes involving elastic, inelastic and reactive events and also to prepare gas-phase species for selective surface scattering investigations. The focus here is on collisional alignment in supersonic seeded molecular beams, a technique which shows perspectives on the applications, offering appealing features for 'duty cycle' and intensity characteristics. Attention will be addressed to recent stereodynamical studies carried out on hydrocarbon molecules in the gas phase and on applications of such aligned beams to surface scattering studies.
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.
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).
Physical Chemistry Chemical Physics, 2014
Selection and alignment of rotamers and, more in general, of conformers in the gas phase is a challenge that we tackle experimentally by supersonic expansion of seeded molecular beams and hexapolar electrostatic fields with quadrupole mass detection. The studied system involves the nine conformers of the asymmetric-top molecule 2-butanol, which coexist because of nearly free rotations around a CC and a CO bond. From the measured time-of-flight of a 2-butanol supersonic molecular beam seeded in either He or Ar, the corresponding velocity distributions are obtained. The different nature and masses of the seeding gas decrease selectively the vibrational temperature and determine the population of the conformers, which is assessed on the basis of their statistical distribution, derived from high level accompanying quantum mechanical calculations. The use of a hexapolar electrostatic field permits us to induce a variation of the population distribution as a function of the applied voltage and of the selective focusing and alignment of the conformers. A technique, recently developed for treating asymmetric tops and involving extensive trajectory simulations, is applied to obtain the link between the focusing curves, i.e. the dependence of the beam intensity on the hexapole voltage, and the conformers' populations and alignment. Perspectives are provided for photo-and stereo-dynamics experiments, particularly appealing also on account that 2-butanol is the simplest chiral alcohol.
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.
Alignment and orientation of rotationally cool molecules
The Journal of Physical Chemistry, 1991
energies as sums over discrete rotational levels, then we obtain where the limits on the summations within each bin are defined in a manner analogous to that in eq A.7. Next we convertq1 the
Aligned molecular collisions and a stereodynamical mechanism for selective chirality
Rendiconti Lincei, 2011
Investigations of the stereodynamics of elementary processes provide the background for perspective demonstration of the manifestation of chiral effects in molecular collisions. Advances in experimental vacuum technology allow the generation of molecular beams containing oriented molecules. Here, we focus on collisional alignment, a molecular polarization phenomenon occurring in supersonic expansions of gaseous mixtures. The underlying physical mechanisms, relevant gas-phase experiments and molecular dynamics simulations are illustrated, with reference to applications of these tools to the study of elementary processes occurring both in homogeneous and in heterogeneous phases, and are discussed in view of their fundamental relevance and also of possible interest in an astrochemical, protobiological context.