Control of conformers combining cooling by supersonic expansion of seeded molecular beams with hexapole selection and alignment: experiment and theory on 2-butanol.

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.