# Pulse radiolysis

Pulse radiolysis is a time-resolved spectroscopy technique in which a short pulse of high-energy electrons generates reactive intermediates in a sample, and their spectra and reaction kinetics are followed from femtoseconds in specialized systems, and from picoseconds to nanoseconds in other setups.<sup>[1](https://pubs.rsc.org/en/content/articlehtml/2026/ra/d6ra00147e)</sup> Together with laser flash photolysis, it is one of the two main methods for creating and monitoring short-lived species on the sub-microsecond timescale, and the two methods complement one another.<sup>[2](https://www.osti.gov/biblio/1395945)</sup> Because ionization affects predominantly solvent molecules when the solute concentration is below about 1 M, the technique generates both reducing species, such as the hydrated electron, and strongly oxidizing species, such as the hydroxyl radical, which makes it a standard tool in radiation chemistry and in mechanistic studies of electron transfer and catalysis.<sup>[3](https://www.osti.gov/servlets/purl/2525829)</sup><sup> • </sup><sup>[2](https://www.osti.gov/biblio/1395945)</sup>

| Key fact | Value |
|---|---|
| Excitation source | Pulses of MeV-range electrons from linacs, FEBETRON-type accelerators, or photocathode RF guns<sup>[3](https://www.osti.gov/servlets/purl/2525829)</sup><sup> • </sup><sup>[4](https://www.intechopen.com/chapters/71392)</sup> |
| Primary species in water | \( e_{\mathrm{aq}} \)⁻, •OH, H•, \( H_{2} \), and \( H_{2} \)\( O_{2} \)<sup>[3](https://www.osti.gov/servlets/purl/2525829)</sup> |
| Initial yield of hydrated electrons | G-value of 4.15 to 4.90 per 100 eV at 1 ps (reported range); 4.05 per 100 eV predicted by dynamic Monte Carlo simulation at 10 keV<sup>[1](https://pubs.rsc.org/en/content/articlehtml/2026/ra/d6ra00147e)</sup> |
| Time resolution | From microseconds in the first systems, to 23 ps in the first picosecond system, to 240 fs in femtosecond systems<sup>[5](https://pubs.aip.org/aip/rsi/article/41/3/333/304937/Design-and-Performance-of-a-Pulse-Radiolysis)</sup><sup> • </sup><sup>[4](https://www.intechopen.com/chapters/71392)</sup> |
| Spectral range of analyzing light | Typically 300 to 1500 nm, depending on the probe source<sup>[6](http://ibic12.kek.jp/mirror/www.pasj.jp/web_publish/sast1997/FS4.pdf)</sup> |
| Hydrated electron absorption peak | 720 nm in water<sup>[7](https://proceedings.jacow.org/PAC2009/papers/tu6pfp027.pdf)</sup> |
| Main resolution limits | Electron pulse duration, probe pulse duration, timing jitter, group velocity mismatch, and space charge<sup>[4](https://www.intechopen.com/chapters/71392)</sup> |

## How it works

A pulse of high-energy electrons, typically in the MeV range, passes through the sample and deposits energy by ionization. When the solute concentration is below about 1 M, ionization affects predominantly solvent molecules, producing within roughly \( 10^{-16} \) s in water three primary energetic species: the solvent radical cation, a pre-solvated electron, and an excited state of the solvent molecule.<sup>[3](https://www.osti.gov/servlets/purl/2525829)</sup> In water these relax into the well-known radiolysis products \( e_{\mathrm{aq}}^{-} \), \( \cdot \mathrm{OH} \), \( \mathrm{H} \cdot \), \( H_{2} \), and \( H_{2}O_{2} \).<sup>[3](https://www.osti.gov/servlets/purl/2525829)</sup>

The chemistry unfolds on distinct timescales. Electron hydration is substantially complete on a sub-picosecond-to-few-picosecond timescale, after which isolated spurs containing \( e_{\mathrm{aq}}^{-} \), \( \cdot \mathrm{OH} \), and \( \mathrm{H_{3}O^{+}} \) are distributed along the electron track; intra-spur reactions begin after about 100 ps and inter-spur reactions after a few hundred nanoseconds.<sup>[1](https://pubs.rsc.org/en/content/articlehtml/2026/ra/d6ra00147e)</sup> The hydrated electron forms by decay of a pre-hydrated precursor on the several-hundred-femtosecond scale; in one femtosecond-resolved measurement the precursor appeared within \( \tau_{1} = 110 \sim 200 \) fs and hydration proceeded with \( \tau_{2} = 550 \pm 50 \) fs.<sup>[4](https://www.intechopen.com/chapters/71392)</sup><sup> • </sup><sup>[8](http://beam-physics.kek.jp/mirror/www.pasj.jp/web_publish/sast1999/9BS3.pdf)</sup>

The hydrated electron is itself the classic probe species: it absorbs strongly near 720 nm in water.<sup>[7](https://proceedings.jacow.org/PAC2009/papers/tu6pfp027.pdf)</sup> Solutes added to the water scavenge specific primary species; for example, \( N_{2}O \) converts \( e_{\mathrm{aq}}^{-} \) to the oxidizing hydroxyl radical via \( N_{2} \) and \( O^{\bullet-} \), letting the experimenter choose whether reducing or oxidizing chemistry dominates.<sup>[3](https://www.osti.gov/servlets/purl/2525829)</sup>

## How it is done

A practitioner selects an accelerator: experiments have used linear accelerators (linacs) and FEBETRON-type machines, while photocathode RF guns driven by a mode-locked laser deliver picosecond and sub-picosecond bunches synchronized to the probe light.<sup>[4](https://www.intechopen.com/chapters/71392)</sup><sup> • </sup><sup>[9](https://www.sciencedirect.com/science/article/abs/pii/S0168900202007969)</sup><sup> • </sup><sup>[10](https://pubs.aip.org/aip/rsi/article/58/3/363/1030994/Pulse-radiolysis-equipment-A-setup-for)</sup> The electron beam is focused onto a sample cell, often with [quadrupole magnets](https://www.edgepedia.org/quadrupole-magnet).<sup>[7](https://proceedings.jacow.org/PAC2009/papers/tu6pfp027.pdf)</sup>

A probe light source crosses the cell at a controlled delay after the electron pulse. Early and nanosecond systems use xenon flash lamps; picosecond systems use Čerenkov light generated by the electron beam itself, or a femtosecond laser, or a white-light continuum generated from one, covering for example 360 to 1000 nm or, with Ti:sapphire-based probes, 300 to 1500 nm.<sup>[10](https://pubs.aip.org/aip/rsi/article/58/3/363/1030994/Pulse-radiolysis-equipment-A-setup-for)</sup><sup> • </sup><sup>[5](https://pubs.aip.org/aip/rsi/article/41/3/333/304937/Design-and-Performance-of-a-Pulse-Radiolysis)</sup><sup> • </sup><sup>[6](http://ibic12.kek.jp/mirror/www.pasj.jp/web_publish/sast1997/FS4.pdf)</sup> Transmitted light is dispersed and detected, classically by an array of photomultipliers on a spectrograph.<sup>[10](https://pubs.aip.org/aip/rsi/article/58/3/363/1030994/Pulse-radiolysis-equipment-A-setup-for)</sup>

The overall time resolution combines the contributions of the electron bunch, the probe pulse, and geometry as

\[ \sigma = \left( \sigma_{\mathrm{Bd}}^{2} + \sigma_{\mathrm{Ls}}^{2} + n^{2} \sigma_{\mathrm{Bs}}^{2} + \sigma_{\mathrm{Ld}}^{2} \right)^{1/2} \]

where \( \sigma_{\mathrm{Bd}} \) and \( \sigma_{\mathrm{Ld}} \) are the electron bunch duration and the probe pulse duration, \( \sigma_{\mathrm{Bs}} \) and \( \sigma_{\mathrm{Ls}} \) the beam size at the sample and the probe spot size expressed as equivalent time contributions via the relevant propagation velocities and geometry, and n the sample refractive index.<sup>[7](https://proceedings.jacow.org/PAC2009/papers/tu6pfp027.pdf)</sup>

## Origin

The technique grew out of [George Porter](https://www.edgepedia.org/george-porter)'s flash photolysis, a new method for the study of free radical reactions published in 1950, which used a light flash instead of radiation to generate transients.<sup>[11](https://doi.org/10.1098/rspa.1950.0018)</sup> The idea of detecting short-lived transients in radiation chemistry was proposed by Max S. Matheson and Leon M. Dorfman in a 1960 paper in The Journal of Chemical Physics.<sup>[12](https://doi.org/10.1063/1.1731035)</sup> Pulse radiolysis was then reported by Edwin J. Hart and J. W. Boag, who described the absorption spectrum of the hydrated electron in water and aqueous solutions in the [Journal of the American Chemical Society](https://www.edgepedia.org/journal-of-the-american-chemical-society) in 1962, using 1.8-MeV, 3-μs electron pulses and giving the first direct spectroscopic observation of solvated electrons in aqueous solution.<sup>[13](https://doi.org/10.1021/ja00880a025)</sup><sup> • </sup><sup>[4](https://www.intechopen.com/chapters/71392)</sup> In 1964 a multi-institution collaboration led by Baxendale and colleagues published a consolidated pulse radiolysis paper in Nature.<sup>[14](https://doi.org/10.1038/201468a0)</sup>

## Variants

Nanosecond single-pulse systems, with a xenon flash lamp probe and photomultiplier detection, measure hydrated-electron time profiles and spectra, for example over 520 to 920 nm.<sup>[7](https://proceedings.jacow.org/PAC2009/papers/tu6pfp027.pdf)</sup><sup> • </sup><sup>[10](https://pubs.aip.org/aip/rsi/article/58/3/363/1030994/Pulse-radiolysis-equipment-A-setup-for)</sup> The first picosecond capability came from a stroboscopic pump-and-probe design by M. J. Bronskill, W. B. Taylor, R. K. Wolff, and J. W. Hunt, described in the Review of Scientific Instruments in 1970, which used the sub-10-ps Čerenkov light flashes from the fine-structure pulses of a 40 MeV linac as analyzing light and achieved a measured time resolution of 23 ps.<sup>[5](https://pubs.aip.org/aip/rsi/article/41/3/333/304937/Design-and-Performance-of-a-Pulse-Radiolysis)</sup> Later arrangements include the twin-linac geometry, in which one linac produces the radiolytic pulse and a second produces the probe light, and laser-synchronized systems in which a photocathode RF gun is locked to a femtosecond laser through a common RF clock.<sup>[9](https://www.sciencedirect.com/science/article/abs/pii/S0168900202007969)</sup><sup> • </sup><sup>[15](https://epaper.kek.jp/e06/PAPERS/WEPCH188.PDF)</sup> [Femtosecond](https://www.edgepedia.org/femtosecond) systems compress the electron pulse with a magnetic pulse compressor and use femtosecond laser probes; a time resolution of 240 fs has been reported.<sup>[4](https://www.intechopen.com/chapters/71392)</sup><sup> • </sup><sup>[16](https://epaper.kek.jp/e04/PAPERS/WEPCH188.PDF)</sup>

## Applications

The core application is the radiation chemistry of water itself: measuring the time evolution and yields of \( e_{\mathrm{aq}}^{-} \), •OH, and other primary species, quantities expressed as G-values (species per 100 eV absorbed).<sup>[1](https://pubs.rsc.org/en/content/articlehtml/2026/ra/d6ra00147e)</sup> Because water radiolysis generates the hydroxyl radical, whose standard reduction potential is about 1.90 V versus NHE for the \(\mathrm{OH^{\bullet}/OH^{-}}\) couple \(\mathrm{OH^{\bullet}+e^-\to OH^-}\), and about 2.72 V at pH 0 for the \(\mathrm{OH^{\bullet}/H_{2}O}\) couple \(\mathrm{H_{3}O^{+}+OH^{\bullet}+e^-\to 2\,H_{2}O}\), the method is a convenient source of strongly oxidizing radicals for studying electron transfer.<sup>[3](https://www.osti.gov/servlets/purl/2525829)</sup> Transient redox equilibrium methods initiated by pulse radiolysis can initially produce radical cations with potentials as positive as about 4 V versus Fc⁺/0.<sup>[3](https://www.osti.gov/servlets/purl/2525829)</sup>

In non-polar liquids, the technique directly observes the geminate recombination of cation radical–electron pairs created by the ionizing pulse; in n-dodecane the decay of the cation radical follows diffusion theory after about 50 ps but deviates at earlier times.<sup>[8](http://beam-physics.kek.jp/mirror/www.pasj.jp/web_publish/sast1999/9BS3.pdf)</sup><sup> • </sup><sup>[16](https://epaper.kek.jp/e04/PAPERS/WEPCH188.PDF)</sup> In catalysis research aimed at artificial photosynthesis, pulse radiolysis generates and characterizes short-lived catalytic intermediates, and is preferred when a photosensitizer's absorption overlaps spectroscopically with the species of interest.<sup>[2](https://www.osti.gov/biblio/1395945)</sup>

## Limitations and alternatives

At the femtosecond frontier, resolution is limited by group velocity mismatch (GVM): the electron pulse and the analyzing light travel at different speeds through the sample, so for water (n = 1.33) and a 2 mm cell the 10–90% rise time is limited to more than 1.8 ps. Thinner cells reduce GVM but proportionally reduce the absorption signal and degrade the signal-to-noise ratio.<sup>[8](http://beam-physics.kek.jp/mirror/www.pasj.jp/web_publish/sast1999/9BS3.pdf)</sup><sup> • </sup><sup>[4](https://www.intechopen.com/chapters/71392)</sup> [Space charge](https://www.edgepedia.org/space-charge) is the second constraint: ultrashort electron pulses are realized only at low charge, with sub-20-fs pulses reported at 2.1 pC, so improving time resolution and absorption signal simultaneously is difficult.<sup>[4](https://www.intechopen.com/chapters/71392)</sup> Timing jitter between the electron beam and the laser probe adds directly to the resolution budget.<sup>[15](https://epaper.kek.jp/e06/PAPERS/WEPCH188.PDF)</sup>

Compared with laser flash photolysis, pulse radiolysis ionizes the solvent rather than exciting a chosen chromophore, so it is non-selective in what it ionizes but avoids the spectroscopic-overlap problem: where a photosensitizer's absorption interferes with the species under study, pulse radiolysis is the better choice, and it can access more positive oxidation potentials than photochemical initiation.<sup>[2](https://www.osti.gov/biblio/1395945)</sup><sup> • </sup><sup>[3](https://www.osti.gov/servlets/purl/2525829)</sup>

## References

1. [An initial G value of hydrated electrons updated by a dynamic Monte Carlo simulation (RSC Advances, 2026)](https://pubs.rsc.org/en/content/articlehtml/2026/ra/d6ra00147e)
2. [Application of Pulse Radiolysis to Mechanistic Investigations of Catalysis Relevant to Artificial Photosynthesis (review)](https://www.osti.gov/biblio/1395945)
3. [Transient methods for understanding the properties of strongly oxidizing radicals (review)](https://www.osti.gov/servlets/purl/2525829)
4. [Femtosecond Pulse Radiolysis (book chapter, IntechOpen)](https://www.intechopen.com/chapters/71392)
5. [Design and Performance of a Pulse Radiolysis System Capable of Picosecond Time Resolution (Rev. Sci. Instrum. 41, 333, 1970)](https://pubs.aip.org/aip/rsi/article/41/3/333/304937/Design-and-Performance-of-a-Pulse-Radiolysis)
6. [Development of a Pulse Radiolysis System by Using Picosecond Electron Pulses and Femtosecond Laser Pulses (Particle Accelerator Society of Japan)](http://ibic12.kek.jp/mirror/www.pasj.jp/web_publish/sast1997/FS4.pdf)
7. [Improvement of Compact Pico-Second and Nano-Second Pulse Radiolysis Systems at Waseda University (PAC2009 proceedings)](https://proceedings.jacow.org/PAC2009/papers/tu6pfp027.pdf)
8. [Development and Application of Pulse Radiolysis Systems in Time Region From Femtosecond To Picosecond (Particle Accelerator Society of Japan, ISIR Osaka)](http://beam-physics.kek.jp/mirror/www.pasj.jp/web_publish/sast1999/9BS3.pdf)
9. [Ultra-fast pulse radiolysis system combined with a laser photocathode RF gun and a femtosecond laser (Nucl. Instrum. Methods A)](https://www.sciencedirect.com/science/article/abs/pii/S0168900202007969)
10. [Pulse radiolysis equipment: A setup for simultaneous multiwavelength kinetic spectroscopy (Rev. Sci. Instrum. 58, 363, 1987)](https://pubs.aip.org/aip/rsi/article/58/3/363/1030994/Pulse-radiolysis-equipment-A-setup-for)
11. [George - Na3240 Porter (1950). Flash photolysis and spectroscopy. A new method for the study of free radical reactions. Proceedings of the Royal Society of London A Mathematical and Physical Sciences.](https://doi.org/10.1098/rspa.1950.0018)
12. [Max S. Matheson, Leon M. Dorfman (1960). Detection of Short-Lived Transients in Radiation Chemistry. The Journal of Chemical Physics.](https://doi.org/10.1063/1.1731035)
13. [Edwin J. Hart, J. W. Boag (1962). Absorption Spectrum of the Hydrated Electron in Water and in Aqueous Solutions. Journal of the American Chemical Society.](https://doi.org/10.1021/ja00880a025)
14. [J. H. BAXENDALE and colleagues (1964). Pulse Radiolysis. Nature.](https://doi.org/10.1038/201468a0)
15. [Compact Picosecond Pulse Radiolysis System Using Photo-cathode RF Gun (Waseda University, EPAC 2006)](https://epaper.kek.jp/e06/PAPERS/WEPCH188.PDF)
16. [Development of a Femtosecond Pulse Radiolysis for Reaction Analysis in Nano-space (Osaka, EPAC 2004)](https://epaper.kek.jp/e04/PAPERS/WEPCH188.PDF)

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*Topic: Encyclopedia › Physical world and mathematics › Chemistry › Chemical principles and methods › Reaction rates, mechanisms, and engineering › Chemical kinetics and reaction engineering*

*Initially written Sep 29, 2026 · Edited: Oct 15, 2026*

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