Electrostatic Hexapole State-Selection of the Asymmetric-Top Molecule Propylene Oxide

Abstract

Rotational state-selection of the asymmetric-top molecule propylene oxide was carried out using an electrostatic hexapole field of 85-cm length. Molecular beam intensities were monitored by a quadrupole mass spectrometer. It was found that beam intensities of molecular beams for pure propylene oxide and those seeded in He and in Ar increased with increasing hexapole voltages. The hexapole voltage dependence of the beam intensity, which is called the focusing curve, was interpreted by computer simulation of the trajectories of molecules in the hexapolar field due to the Stark effect, as a function of rotational temperatures of molecular beams. The calculated best fit focusing curves, when compared with the experimental results, demonstrated that the rotational temperatures, associated with the distribution of states of a given rotational angular momentum J, are similar to the translational temperatures. It was found that the M = 0 states (where M is the projection of J along the direction of the electrostatic field) and negative values of the pseudoquantum number tau of propylene oxide can be selected using our experimental setup. These results suggest that the hexapole electric field is a tool even for the selection of rotational and orientation states of asymmetric-top molecules.

Rotational state-selection and alignment of chiral molecules by electrostatic hexapoles

“…Propylene oxide is an asymmetric-top molecule with a Ray's parameter of -0.87 and a permanent dipole moment of 2.2 D. Its focusing curve was characterized as a pure beam, seeded in He (20 % in propylene oxide) and seeded in Ar (20 % in propylene oxide. Details of the experimental apparatus and of trajectory simulations are given in references [9,10]. In Figure 2 (left panel) we report, as an example, the experimental focusing curve of pure propylene oxide and the simulated ones by assuming adiabatic and non-adiabatic transitions in case of avoided curve crossing.…”

“…The picture in the left shows the permanent dipole moment parallel to the molecular axis, while in the picture in the right we illustrate of the perpendicular arrangement. (Adapted from references [9,14]). …”

“…Figure 2c shows the change of intensity of detected Br photofragment with (red line) and without (black line) application of the orienting field. (Adapted from references [9,12,14])…”

“…They were initially applied to linear and symmetric-top molecules, while more recently they have been successfully applied to molecules of higher complexity such as the chiral molecules propylene oxide 9,10 (for a characterization of the reaction pathways of propylene oxide see reference [11]) and 2-bromobutane 12 (see also reference [13]). Hexapolar technique has been also employed for the first time in the selection of conformers of 2-butanol 14 .…”

Aligned molecular collisions and a stereodynamical mechanism for selective chirality

“…As detailed in the next section, these systems are being extensively studied through advanced quantum chemical techniques. In addition, beams of laboratory available chiral molecules, such as propylene oxide and 2-butanol have been already successful generated and tested (Che et al 2010).…”