Oriented molecule beams: Focusing and orientation of <i>t</i>-butyl iodide with analysis by polarized laser photofragmentation

Abstract

The tert-butyl iodide molecule is readily focused with the electrostatic hexapole, via its first-order Stark effect as a pseudo-symmetric top. The pulsed, seeded supersonic focused beam, characterized by 〈cos θ〉=Vth/ V0 (where θ is the angle between the molecular dipole axis μ and the electric field E; ±V0 the hexapole ‘‘rod voltage,’’ and Vth the so-called threshold voltage), passes into a small homogeneous electric field in which it is oriented. The degree of laboratory orientation achieved is measured using the method of linearly polarized laser-induced photofragmentation [S. R. Gandhi, T. J. Curtiss, and R. B. Bernstein, Phys. Rev. Lett. 59, 2951 (1987)], operating (at three laser wavelengths) on the I(2P3/2) and I(2P1/2) as well as the t-C4H9 radical photofragments. The results show that the oriented beam molecules of t-butyl iodide (at a rotational temperature near 15 K) have a higher degree of orientation than the prototype CH3I molecules (JKM state-selected and focused similarly), explainable by the greater importance of the so-called hyperfine disorientation effect for the prolate symmetric top (CH3I) than for the t-C4H9I. For the latter, orientations with photofragment up–down asymmetry ratios as large as a factor of 10 can be achieved, suggesting that t-C4H9I is an excellent candidate reagent for reactive asymmetry studies.