Comments page for Ed Narevicius's talk "Chemistry of the Quantum Kind" 01/29/13

Link to recorded talk
abstract:
There has been a long-standing quest to observe chemical reactions at
low temperatures where reaction rates and pathways are governed by
quantum mechanical effects. So far this field of Quantum Chemistry has
been dominated by theory. The difficulty has been to realize in the
laboratory low enough collisional velocities between neutral
reactants, so that the quantum wave nature could be observed. We will
discuss our merged neutral supersonic beams method that enabled the
observation of clear quantum effects in low temperature reactions. We
observed orbiting resonances in the Penning ionization reaction of
argon and molecular hydrogen with metastable helium leading to a sharp
increase in the absolute reaction rate in the energy range
corresponding to a few degrees kelvin down to 10 mK. Our method is
widely applicable to many canonical chemical reactions, and will
enable experimental studies of Quantum Chemistry.

COMMENTS:

Paul J. comment: In a new preprint arXiv:1301.5857, Jachymski, et al implement a general quantum defect theory (QDT) for collisions in a -1/R^n potential, and apply it to explain Ed's experiments in his Science paper on merged beam He* + Ar ionizing collisions. This atomic system is simpler in some ways than the He* + H_2 one. QDT replaces the short range complex potential V(R) - i Gamma/2 by two dimensionless QDT parameters s and y, which respectively represent a short-range phase and the ionization probability P=4y/(1+y)^2. We find that the measured cross section between around 40 mK and 1.4 K can be reproduced by a sum over partial waves L using only 2 energy-independent s and y QDT parameters, assumed independent of L, in accordance with angular-momentum-insensitive QDT. The prominent observed cross section peak is due to an L=5 shape resonance.