Electrostatic hexapole state-selection of the asymmetric-top molecule propylene oxide: Rotational and orientational distributions

This theoretical study is complementary to previous experimental work (see D.-C. Che, F. Palazzetti, Y. Okuno, V. Aquilanti, T. Kasai, J. Phys. Chem. A 114 (2010) 3280) on the orientation and rotational state-selection of supersonic molecular beams of the asymmetric-top molecule propylene oxide, both pure and seeded (in He and in Ar) by using an 85-cm length hexapole state-selector. One objective is to obtain an accurate distribution of the rotational states after hexapole selection for the three molecular beams, the most relevant feature consisting in the evaluation of the variation in energy of the manifold of rotational states when an electric field is applied (the Stark forces). Previously, the Stark effect on the effective dipole moment was considered through second order for all rotational states, while in this work the energy derivatives of the rotational states with respect to the applied electric field, are obtained accurately by diagonalizing the very large Stark matrices, whose elements depend on the dipole moment components of the molecule. The Stark energies and the corresponding forces were calculated for values of the electric field between 0 and 80 kV cm (1) in steps of 0.5 kV cm (1) and then linearly interpolated, covering the whole experimental range of the hexapole. A treatment is given for the intricate pattern of avoided crossings among derivatives of the rotational levels and two limiting cases were considered, corresponding to transitions occurring either adiabatically or diabatically. The two treatments lead to slightly different distributions of the rotational states for the pure and Ar seeded molecular beams, while for the He seeded molecular beam the two distributions are substantially the same. This experimental arrangement is a perspective tool for experiments of photochemistry and scattering of oriented molecules and clusters, and therefore we calculated the orientational distributions in a configuration where a uniform electric field is placed between the hexapole field and the detector. (C) 2011 Elsevier B.V. All rights reserved.