Scattering of Rotationally Aligned Oxygen Molecules and the Measurement of Anisotropies of van der Waals Forces

Abstract

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 H2 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, 34.50.s 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 [1,2]. 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 [1). 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 H2, 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 +1 or -1).We denote the relative number of molecules with M = 0 as wp. For them the rotational angular momentum is perpendicular to the propagation direction. The relative number of molecules pertaining to the other possibility, i.e. , corresponding to helicity M = 1, will be denoted as wlthis number is the same in both the +1 and -1 cases, which only differ by the sense of rotation, irrelevant under the axial symmetry of the molecular beam expansion. Alignment is conveniently measured by the ratio wo/wI, which is 1 in the statistical case (no alignment).It had been found [2] that, within a seeded supersonic expansion, fast molecules are much more highly aligned than slower ones and, specifically, states of zero helicity are strongly favored for those molecules that travel faster (wp/wi~1): Thus by changing the gas mixture and its composition we can prepare molecules with well-defined velocities and alignment ratios. Pictorially, zero helicity molecules can be considered as traveling edge-on and those with helicity M = 1 as traveling broadside towards a scatterer, and therefore an alignment control opens up the possibility of carrying out experiments with the aim of assessing the role of spatial aspects in the dynamics of atomic and molecular collisions.A demonstration of the potentiality of this novel technique is presented here for the case of experimental measurements for 02-Xe scattering with a controlled alignment ratio wo/wI, an outline is also given of the theoretical analysis for the extraction of information on the interaction from d...

Quantum Interference Scattering of Aligned Molecules: Bonding inO4and Role of Spin Coupling

“…The alignment degree, obtained by paramagnetism measurements [20] and confirmed by molecular scattering experiments [19], can be varied in a controlled way [19]. The two extreme cases are reported in the lower panel of Fig.…”

“…This phenomenon can be exploited to produce beams of projectile molecules slowly rotating with a controlled alignment degree [20]. Collisional experiments with "hot" effusive beams mainly probe the isotropic component of the interaction potential while those performed with aligned oxygen molecules uniquely probe the anisotropy of the surfaces [19].…”

“…Curves in the lower panel of the figure are the results of quantum mechanical close coupling scattering calculations on our best-fit potential energy surfaces using as before [19] the MOLSCAT [27] package. The second virial coefficients are reproduced within experimental errors over the entire temperature range.…”

“…1 where each point is the average of several measurements. The dependence on velocity is obtained from 20 different mixtures of 2.5% O 2 in carrier gases such as H 2 , He, and Ne in varying compositions [19].…”

“…Following the guidelines of our previous determination of potential energy surfaces interaction of O 2 with rare gases [19], we have measured total integral cross sections in the thermal energy range using beams of oxygen molecules in different internal states, as produced under both effusive and supersonic conditions. Hot effusive beams contain fast rotating and randomly oriented molecules, while the "seeding" phenomenon associated with the supersonic expansion naturally leads to a rotational cooling and to a strong alignment of the rotational angular momentum.…”