2018-10-17

'Watching' Atoms Move on Atomic Length and Time Scales with Helium Spin Echo

by John Ellis from SMF

Existence and Imaging of Atoms

• 60BC, Lucretius, brownian motion

• Inferred by chemists, Dalton

• Strongly rejected by physics establishment till Perrin

Imaging atoms: STEM

• Imaging COlumns of atoms, TEM

• Watching atoms move? - a Holy Grail

quantify using pair correlation funciton

• Spin Echo Technique

• Hexapole Polarizer for a beam

why surfaces

• use in chemical engineering
• thermally activated motion

Analytical Diffusion Signitures

• Modelling Motion at Surfaces: Na adatoms on a Cu(100) Surface

• Modelling with 2d Langevin Equation
  use the idea of that random impulse

MOdelling: validity?

• But Langevin assumes 'white' noise, and real noise has a frequency cut-off, simulation suggests that the region of interest is white

Light Molecule Quantum Ballistic Motion

• UNactivated (Brownian) benzene

Benzene/graphite: simple model of nano motor

Rate theory

• The rate of a thermally activated process?
• work out the pre-exponential factor.

'Fiction' and Transition State Theory

• Problems:

1. low frictions: rollercoasters from distant wells

• at high frictions atoms may recross barrier

Band structures

• energy

Use bandstructures for transition state calculation

• band width -> group velocity -> transition rate

How you work out the quantum mechanics connected to a heat bath???

We need data to move forward -> need to build better measurement devices