ImPhys Colloquium | Alec Wodtke

Title: Condensed Phase Tunneling from Enzyme Kinetics to Astrochemistry

Abstract:

Superconducting  nanowire single-photon detectors (SNSPDs) provide sufficient sensi-tivity to enable laser induced fluorescence (LIF) experiments in the mid-infrared, an ex-citing technical development for studying molecule-surface interactions. In this talk, I will present results of experiments on the vibrational dynamics of monolayers and multi-layers of solid CO adsorbed at the surface of a NaCl crystal that provide observations of quantum state resolved dynamics. When, for example, a pulsed ns laser excites CO to its v=1 state, a monochromator equipped with an SNSPD detects wavelength- and time-re-solved mid-infrared emission from CO vibrational states up to v=27 that are produced by vibration-vibration (V-V) energy transfer. Kinetic Monte Carlo (kMC) simulations show that vibrational energy collects in a few CO molecules at the expense of those up to eight lattice sites away. The excited CO molecules relax by a mechanism resembling Sommer-feld’s theory of ground waves important to radio wave propagation, losing their energy to NaCl lattice-vibrations via the electromagnetic near-field. This is a weak coupling limit, where the potential energy surface is not needed to describe the relaxation process.

At high resolution, we observe new lines appearing in the in-frared emission spectra, show-ing that CO vibrational energy converts “the right-side up” where CO is bound by its C-atom to the surface to an “up-side down” metastable iso-mer. Flipping back involves thermally activated tunneling, exhibiting a large isotope ef-fect, where the lightest isotope is not the fastest tunneller. This is explained by a quan-tum rate theory of isomeriza-tion involving tunneling gate-ways. Near resonant states, lo-calized on opposite sides of the isomerization barrier are coupled by collisions with a phonon bath. This represents gateways.
An alternative to traditional tunneling pictures like Instanton and WKB, which are based on a continuum scattering picture that is not valid in condensed phases.