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

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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. 5 9, 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.

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