Thermal desorption spectroscopy
Encyclopedia
Thermal desorption spectroscopy (TDS), also known as temperature programmed desorption (TPD) is a Buzzword
Buzzword
A buzzword is a term of art, salesmanship, politics, or technical jargon that is used in the media and wider society outside of its originally narrow technical context....

 for the method of observing desorbed
Desorption
Desorption is a phenomenon whereby a substance is released from or through a surface. The process is the opposite of sorption . This occurs in a system being in the state of sorption equilibrium between bulk phase and an adsorbing surface...

 molecule
Molecule
A molecule is an electrically neutral group of at least two atoms held together by covalent chemical bonds. Molecules are distinguished from ions by their electrical charge...

s from a surface
Surface
In mathematics, specifically in topology, a surface is a two-dimensional topological manifold. The most familiar examples are those that arise as the boundaries of solid objects in ordinary three-dimensional Euclidean space R3 — for example, the surface of a ball...

 when the surface temperature is increased. Many researchers prefer the name TPD because it is not a spectroscopic
Spectroscopy
Spectroscopy is the study of the interaction between matter and radiated energy. Historically, spectroscopy originated through the study of visible light dispersed according to its wavelength, e.g., by a prism. Later the concept was expanded greatly to comprise any interaction with radiative...

 method.

Desorption

When molecules come in contact with a surface, they adsorb
Adsorption
Adsorption is the adhesion of atoms, ions, biomolecules or molecules of gas, liquid, or dissolved solids to a surface. This process creates a film of the adsorbate on the surface of the adsorbent. It differs from absorption, in which a fluid permeates or is dissolved by a liquid or solid...

 onto it, minimizing their energy by forming a chemical bond with the surface. The binding energy varies with the combination of the adsorbate and surface. If the surface is heated, at one point, the energy transferred to the adsorbed species will cause it to desorb. The temperature at which this happens is known as the desorption temperature. Thus TDS shows information on the binding energy.

Measurement

Since TDS observes the mass of desorbed molecules, it shows what molecules are adsorbed on the surface. Moreover, TDS recognizes the different adsorption conditions of the same molecule from the differences between the desorption temperatures of molecules desorbing different sites at the surface, e.g. terraces vs steps. TDS also obtains the amounts of adsorbed molecules on the surface from the intensity of the peaks of the TDS spectrum, and the total amount of adsorbed species is shown by the integral of the spectrum.

To measure TDS, one needs a mass spectrometer, such as a quadrupole mass spectrometer
Quadrupole mass analyzer
The quadrupole mass analyzer is one type of mass analyzer used in mass spectrometry. As the name implies, it consists of 4 circular rods, set parallel to each other. In a quadrupole mass spectrometer the quadrupole is the component of the instrument responsible for filtering sample ions, based on...

 or a time-of-flight (TOF) mass spectrometer, under ultrahigh vacuum (UHV) conditions. The amount of adsorbed molecules is measured by increasing the temperature at a heating rate of typically 2 K/s to 10 K/s. Several masses may be simultaneously measured by the mass spectrometer, and the intensity of each mass as a function of temperature is obtained as a TDS spectrum.

The heating procedure is often controlled by the PID control
PID controller
A proportional–integral–derivative controller is a generic control loop feedback mechanism widely used in industrial control systems – a PID is the most commonly used feedback controller. A PID controller calculates an "error" value as the difference between a measured process variable and a...

 algorithm, with the controller being either a computer or specialised equipment such as a Eurotherm
Eurotherm
Eurotherm is a worldwide supplier of Control and Measurement Instrumentation to Industrial and Process Markets. They are part of Invensys plc, a leading global automation and controls company...

.

Quantitative Interpretation of TDS data

This is usually done using the "Redhead" method, assuming the exponential prefactor and the desorption energy to be independent of the surface coverage.

External links

The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.
 
x
OK