Temperature Programmed Desorption (TPD)
Sticking probability and TPD measurements help to explain if adsorbates:
-
chemisorb or physisorb?
-
dissociate on adsorption?
-
desorb easily from the ice surfaces, or remain H-bonded to the surface
until the ice itself sublimes?
-
migrate into the bulk?
In this experiment we dose in simple molecules of astronomical relevance,
e.g. H2O, HCN, CO, CO2, NH3, CH4
at low concentrations for short time-periods. All these adsorbates are
likely to freeze-out onto the ice surface at 10K, but not all the molecules
that are dosed into the chamber will stick. Some are pumped away directly
and others are scattered from the ice surface.
Sticking Probabilities
The sticking probability is a measure of the likelyhood of the adsorbate
bonding to ice the ice surface. It depends on:
-
the ice substrate
-
the adsorbate
-
the existing surface coverage of the adsorbate
-
other adsorbates either already present on the surface or competing for
surface bonding sites
Once we know how things stick onto the ice surface we then want to know
how they are liberated again...
TPD
Using line-of-sight (LOS) methods developed here in Nottingham by Jones
et.al. we can specifically observe the molecular desorbtion originating
from a well defined patch on the surface in a direct line of sight to the
Quadropole Mass Spectrometer.
TPD is a controlled heating of the ice substrate to sublimation and
evapouration. During the heating process the QMS is used to monitor the
molecular species that are desorbing. The TPD data is used to calculate
the enthalpy of desorption - the bond energy between the adsorbate and
the ice. This gives us information on bond strengths and the nature of
the bonding.
The Surface Astrophysics experiment is part of 
Nottingham Astronomy Research Forum
and is based in the Surface
Science Group in the School
of Chemistry at the University
of Nottingham
These pages were last updated 14/10/01