Single-photon ionisation vs multiphoton ionisation: ‘consuming’ one photon at a time, or saving up a few before ionising

Photoionisation is the process by which a photon interacts with an atom or molecule, causing the latter to lose an electron and form an ion (IUPAC Gold Book: photoionisation, P04620). Depending on ‘how many photons a molecule absorbs before it is ionised’, it can be classified into two categories: single-photon ionisation and multi-photon ionisation.

Single-photon ionisation (SPI)

In single-photon ionisation, a photon with energy not less than the ionisation energy (IE) of the substance is absorbed, directly ejecting an electron. The IUPAC states that the photoionisation cross-section (the probability of ionisation occurring) depends on the photon energy; when the photon energy is below the ionisation threshold, the photoionisation cross-section approaches zero. Consequently, SPI imposes a strict requirement on photon energy: the photon energy must be ≥ the ionisation energy.

Common SPI light sources are vacuum ultraviolet (VUV) lamps: krypton (Kr) lamps at approximately 10.6 eV, argon (Ar) lamps at approximately 10.0 eV, and xenon (Xe) lamps at approximately 8.4 eV. The ionisation energies of most organic small molecules lie between 7 and 11 eV; consequently, VUV photons are sufficiently ‘universal’ to ionise them. SPI is a form of soft ionisation, tending to produce molecular ions with few fragments; molecules can be ionised provided their ionisation energy is lower than the photon energy, with selectivity primarily determined by the ionisation energy threshold.

Multiphoton ionisation (MPI)

In multiphoton ionisation, multiple photons, each with energy below the ionisation threshold, ‘combine’ to provide sufficient energy to ionise an atom or molecule (Wikipedia: Photoionisation). This process typically requires a high-intensity pulsed laser to generate extremely high transient light intensities. In the perturbation region (at optical frequencies, below approximately 10¹⁴ W/cm²), the probability of absorbing N photons is proportional to the Nth power of the light intensity I (Iᴺ); the higher the light intensity, the more pronounced the multiphoton process.

Building upon MPI, resonance-enhanced multiphoton ionisation (REMPI) utilises a real intermediate resonance state to act as a ‘bridge’; tunable lasers can be used to achieve wavelength selectivity and highly selective ionisation of specific species, and it is commonly employed in the spectral analysis of atoms and small molecules.

Key differences between the two

Relationship with Neo Source products

Neo Source INSTRUMENT’s MSI DPI (DESI+PI) and MSI LDPI (LDI+PI) imaging ion sources utilise vacuum ultraviolet photoionisation to perform ‘post-photoionisation’ on neutral molecules following primary desorption (DESI or LDI); this photoionisation step follows a single-photon ionisation pathway: Provided that the molecular ionisation energy is lower than the energy of the VUV photons, the molecule will be ionised; this enables coverage of molecules of all polarities and recovers signals missed by the primary ionisation. Both sources can be fitted to mass spectrometers from Agilent, AB SCIEX and Thermo.

References

1. IUPAC Gold Book: photoionisation. https://goldbook.iupac.org/terms/view/P04620

2. Wikipedia: Photoionisation. https://en.wikipedia.org/wiki/Photoionization

3. University of Science and Technology of China: New progress in the development of a compact photoionisation mass spectrometry imaging source. https://kyb.ustc.edu.cn/_t1002/2024/0401/c6076a634490/page.htm

4. Qi K, et al. Development of Transmission Ambient Pressure Laser Desorption Ionisation/Post-photoionisation Mass Spectrometry Imaging. Analytical Chemistry. 2024. DOI:10.1021/acs.analchem.3c05605