Gas-Surface Dynamics

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Introduction

Why and how do molecules break up on surfaces ?

This apparently simple question lies at the heart of much of the chemical industry, for example synthesis of ammonia and methanol or the reduction of noxious emissions from car exhausts first require molecules to dissociate on (metal) surfaces.

To find an answer we need to perform experiments which are as precisely controlled as possible, for instance (near perfect) single crystals of metal are used since we can find out where the atoms are on these (real catalysts are very complicated mixtures of different materials giving different required properties), and the experiments are performed in an ultra-high vacuum chamber, so that there are almost no atmospheric gases around to confuse things.

Theoretical models aren't any easier, they basically involve two distinct branches of study; we need to follow the motion of a molecule (i.e. solve the dynamics) to find out what happens to it, but in order to do this we must compute the energy it has at each position.

Here are some pointers to pages of people who compute energies of molecules at surfaces:-

We need the energies because ultimately it is these which determine how the molecule moves, for instance the reaction will tend to go from high potential energy to low potential energy.

The dynamics then comes into play when we give the molecule some energy to begin with, say by making it move fast. It doesn't just slide gently downhill then, but can go into regions of high potential, or swap translational energy for internal energy in the collision.

The following pages contain a guide through some history to the up-to-date stuff. Mostly it's about the theory, since that's what I do and know a bit about.

It is roughly based on a review article The Dissociation of Diatomic Molecules at Surfaces, in Reports on Progress in Physics Vol. 58 (1995) pp 1595-1672.

Some key points are:

A (very) little bit of history
(4 black and white GIFs - 10, 9.6, 9.3 and 10.3 kbytes)

Electronic energies and Potential Energy Surfaces

How to follow the atomic motion

Wavepackets - a special bit of its own.

What happens if the surface atoms move?

Excited electrons

here are some more specific examples

Direct or Precursor Mediated Dissociation
(2 colour GIFs - 552 bytes)

Angular effects in dissociation

Vibrationally enhanced dissociation

What does molecular rotation do ?

What if the molecule gets trapped before dissociating ?
The very latest stuff