Orientation of benzene in supersonic expansions

F. Pirani, D. Cappelletti, M. Bartolomei and V. Aquilanti,

INFM and Dipartimento di Chimica and Dipartimento di Ingegneria Civile ed Ambientale

Universita’ di Perugia, 060123-Perugia, Italy

M. Scotoni, M. Vescovi, D. Ascenzi and D. Bassi

INFM and Dipartimento di Fisica, Universita’ di Trento, 38050-Povo, Trento, Italy

Evidence that molecules naturally align, assuming an anisotropic orientation of their plane of rotation when diluted gaseous mixtures in lighter carriers expand into a vacuum, first came from measurements of polarization effects in spectra of some simple molecules (1). In 1994 the first evidence of the strong dependence of the alignment on the final speed v for rotationally relaxed O₂ molecules in a seeded supersonic beam was reported by some of the authors (2). The most probable molecular velocity v_m can be varied changing the gas carrier: a correlation was found between the alignment degree and the v/v_m ratio. Later, similar effects were observed by UV spectroscopy on CO in beams seeded in He (3).

These findings were ascribed to the sequence of state-to-state elastic and inelastic events, associated to the large number of collisions in the expansion zone, and the selective dependence on the angular momentum of the collision complex (4).

Further cross section measurements, performed downstream for scattering of velocity selected O₂ and N₂ seeded beams by rare gas targets (5), confirmed the correlation between molecular alignment and molecular velocity and allowed both an accurate determination of the involved interaction potential energy surfaces and the characterization of the collisional dynamics of aligned molecules. These experiments suggested that the measurements of anisotropy effects in the scattering cross sections, combined with a proper velocity selection of the beams, is an alternative source of information on the molecular alignment degree when the topography of the potential energy surface and of the details of the involved collisional dynamics.

This paper (6) reports the first evidence of a related phenomenon in benzene beams with new and interesting findings. These can be crucial to model alignment mechanisms at a microscopic level. The study has been carried out through two complementary IR direct laser absorption (7) and molecular beam scattering experiments (5). The two experimental arrangements are identical in the molecular beam source but use different diagnostic methods and different angular resolutions. The optical absorption analyzes the molecules perpendicularly to the expansion direction and in a defined angular cone (~10^-4 steradians) around the molecular beam axis. The scattering exploits the same cylindrical symmetry of the beam expansion but probes the molecular alignment directly along the beam axis and in a narrower angular cone (~10^-6 steradians): this extreme angular resolution conditions is required to properly measure the quantum cross section.

The main findings of the present experiments can be summarized saying that benzene molecules in seeded beams are rotationally relaxed and preferentially aligned in the ''edge-on'' mode. For molecules flying at a speed close to the most probable value v_m this mode prevails on the ''broad-side'' one by a factor 2.5 ± 0.1, according to the IR laser absorption experiments, and 4.0 _-1.0^+1.7 as probed by molecular beam scattering measurements. Both experimental values are significantly higher than the factor two, expected for a statistical distribution in a non--aligned beam. These results represent also a proof of the strong angular dependence of the induced molecular alignment degree. Indeed the scattering probes a much narrower cone along the beam axis with respect to the IR experiment and the small angular deviation from the forward beam direction is crucial in exalting this important feature. This interpretation has been supported by further IR absorption measurements performed inserting an additional molecular beam collimator, leading to a higher angular resolution (2.0´10^-5 steradians): an increase of the alignment to 2.7 ± 0.1 has been observed at v~v_m, confirming the angular dependence of the phenomenon. Scattering experiments also provide information on the velocity dependence of the alignment on the final molecular speed. Specifically, total cross section measurements suggest that the alignment is negligible for the slowest molecules: it increases for velocities v~v_m and does not vary significantly for velocities higher than the peak value (a behavior at variance with that previously observed for O₂ and N₂. These findings regarding the angular and velocity dependence of collisional alignment reflect important features of the microscopic mechanisms responsible for the phenomenon. Benzene is an oblate symmetric top molecule and its rotational states involve in-plane and out-of-plane motions, and therefore these mechanisms of rotational relaxation and bending of the molecular plane are expected to differ from the linear molecule case.

References

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(2) V. Aquilanti, D. Ascenzi, D. Cappelletti, F. Pirani, Nature, 371, 399 (1994); V. Aquilanti, D. Ascenzi, D. Cappelletti, F. Pirani, J. Phys. Chem., 99, 13620 (1995)

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(6) F. Pirani, D. Cappelletti, M. Bartolomei, V. Aquilani, M. Scotoni, M. Vescovi, D. Ascenzi and D. Bassi, submitted for publication (2001);

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