Molecular Rotor in Cold Reactions

Physical and Analytical Chemistry Seminar

Lecturer: Prof. Ed Narevicius

15 Nov 2015 @ 12:30

Location: Faculty Seminar Room

The role of the internal molecular degrees of freedom, such as rotation, in low energy reactions has been unexplored experimentally despite their significance to cold and ultracold chemistry. Particularly important to astrochemistry is the case of the most abundant molecule in interstellar space, hydrogen, where two spin isomers with rotationally ground and excited levels have been detected. Here we demonstrate that quantization of molecular rotation plays a key role in cold reaction dynamics, where rotationally excited ortho-hydrogen reacts faster due to a stronger long-range attraction. We observe rotational state dependent non-Arrhenius universal scaling laws in chemi-ionization reactions of para-H2 and ortho-H2 by He(23P2), spanning three orders-of-magnitude in temperature. Different scaling laws serve as a sensitive gauge enabling us to directly determine the exact nature of the long-range intermolecular interactions. Our results show that the quantum state of the molecular rotor determines whether or not anisotropic long-range interactions dominate cold collisions [1]. We will also discuss the effect of molecular rotation on orbiting resonances that we have observed in our earlier work [2,3] in the case of normal H2. We will demonstrate that orbiting resonance structure is highly dependent on the rotational state of a molecule.
[1] Y. Shagam, A. Klein, W. Skomorowski, R. Yun, V. Averbukh, C. Koch and E. Narevicius, “Molecular hydrogen interacts more strongly when rotationally excited at low temperatures leading to faster reactions”, Nature Chemistry, 7 (11) 921, (2015)
[2] A. B. Henson, S. Gersten, Y. Shagam, J. Narevicius, E. Narevicius, “Observation of Resonances in Penning Ionization Reactions at Sub-Kelvin Temperatures in Merged Beams” Science 338, 234–238 (2012).
[3] E. Lavert-Ofir, Y. Shagam, A. B. Henson, S. Gersten, J. Klos, P. S. Zuchowski, J. Narevicius and E. Narevicius, “Observation of the isotope effect in sub-kelvin reactions” Nature Chemistry 6, 332–335 (2014).