Thermal ionization

Key takeaways

Thermal ionization belongs to engineering; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Thermal ionization to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Thermal ionization from memory before moving on to harder problems.

Reference excerpt

Thermal ionization, also known as surface ionization or contact ionization, is a physical process whereby the atoms are desorbed from a hot surface, and in the process are ionized. Thermal ionization is used to make simple ion sources, for mass spectrometry and for generating ion beams. Thermal ionization has seen extensive use in determining atomic weights, in addition to being used in many geological/nuclear applications.

Physics

The likelihood of ionization is a function of the filament temperature, the work function of the filament substrate and the ionization energy of the element. This is summarised in the Saha–Langmuir equation:

n + n 0 = g + g 0 exp ⁡ ( W − Δ E I k T ) {\displaystyle {\frac {n_{+}}{n_{0}}}={\frac {g_{+}}{g_{0}}}\exp {\Bigg (}{\frac {W-\Delta E_{\text{I}}}{kT}}{\Bigg )}}

where

n + n 0 {\displaystyle {\frac {n_{+}}{n_{0}}}} = ratio of ion number density to neutral number density

g + g 0 {\displaystyle {\frac {g_{+}}{g_{0}}}} = ratio of statistical weights (degeneracy) of ionic (g+) and neutral (g0) states

W {\displaystyle W} = work function of surface

Δ E I {\displaystyle \Delta E_{\text{I}}} = ionization energy of desorbed element

k {\displaystyle k} = Boltzmann constant

T {\displaystyle T} = surface temperature Negative ionization can also occur for elements with a large electron affinity Δ E A {\displaystyle \Delta E_{\text{A}}} against a surface of low work function.

Thermal ionization mass spectrometry

One application of thermal ionization is thermal ionization mass spectrometry (TIMS). In thermal ionization mass spectrometry, a chemically purified material is placed onto a filament which is then heated to high temperatures to cause some of the material to be ionized as it is thermally desorbed (boiled off) the hot filament. Filaments are generally flat pieces of metal around 1–2 mm (0.039–0.079 in) wide, 0.1 mm (0.0039 in) thick, bent into an upside-down U shape and attached to two contacts that supply a current. This method is widely used in radiometric dating, where the sample is ionized under vacuum. The ions being produced at the filament are focused into an ion beam and then passed through a magnetic field to separate them by mass. The relative abundances of different isotopes can then be measured, yielding isotope ratios. When these isotope ratios are measured by TIMS, mass-dependent fractionation occurs as species are emitted by the hot filament. Fractionation occurs due to the excitation of the sample and therefore must be corrected for accurate measurement of the isotope ratio. There are several advantages of the TIMS method. It has a simple design, is less expensive than other mass spectrometers, and produces stable ion emissions. It requires a stable power supply, and is suitable for species with a low ionization energy, such as strontium and lead. The disadvantages of this method stem from the maximum temperature achieved in thermal ionization. The hot filament reaches a temperature of less than 2,500 °C (2,770 K; 4,530 °F), leading to the inability to create atomic ions of species with a high ionization energy, such as osmium and tungsten. Although the TIMS method can create molecular ions instead in this case, species with high ionization energy can be analyzed more effectively with MC-ICP-MS.

See also Langmuir–Taylor detector Irving Langmuir Meghnad Saha

References

Worked examples

Example 1 — a first encounter with Thermal ionization

Start with the simplest possible case. Write down what Thermal ionization claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In engineering, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Thermal ionization before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Thermal ionization ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Thermal ionization

In research: Thermal ionization appears in engineering research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Thermal ionization in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Thermal ionization is common in secondary-school and first-year university syllabi. It links to neighbouring topics Ion source, Ionization, so understanding it makes those chapters shorter.
In everyday life: Look for Thermal ionization outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

How to study Thermal ionization in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Thermal ionization means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Thermal ionization out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Thermal ionization in simple terms?

Thermal ionization, also known as surface ionization or contact ionization, is a physical process whereby the atoms are desorbed from a hot surface, and in the process are ionized. Thermal ionization is used to make simple ion sources, for mass spectrometry and for generating ion beams.

Why does Thermal ionization matter?

Because it connects several engineering ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Thermal ionization?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Thermal ionization.