Actinometry and the ability to measure light absorption
In photochemistry, light is a reagent. And no chemist performs a reaction without knowing the reagents’ stoichiometry. Accordingly, it is important for chemists to know the number of photons from the light source required for any given experiment. That one detail (the amount of light penetrating the sample) provides the requisite information to properly study, optimize, and scale any experiment. Unfortunately, too few publications document the photochemistry experiment’s light source. Even fewer publications specify the required number of photons for the reaction.1
In this post, we look at the most important considerations when evaluating photochemistry light sources. We also discuss Actinometry and how to measure your light’s chemical energy in your reactions.
Electrical power vs light energy
Scientific publications typically describe photochemistry light sources in terms of color and power, usually wattage. But a light source’s electrical power rating (wattage) is only an indication of that light’s energy. However, LED and CFL light sources (as an example) do not have the same luminous efficacy. They don’t deliver the same amount of light to the reaction. Nearly all commercial light bulbs are rated in lumens. The lumen is “a measure of the total quantity of visible light emitted by a source per unit of time.” In other words, lumens represent visible light generated by the bulb. And while a luxmeter measures the light’s intensity (bright intensity) at a specific position (lux, measured in lumen/m2), monochromatic light sources (those used in photochemistry experiments) make these measurements irrelevant.
We must also consider the geometry of the light as it disperses around the sample. Regular bulbs (like those found in homes) diffuse light in every direction while focused light sources direct the light in one direction.
A radiospectrometer measures a light source’s power (radiant flux, in watt) and light intensity (irradiance in watt/cm2). However, irradiance is different from lux and is not based on human eye sensitivity and visible light. Irradiance is measured at a specific position along a light’s source (coming from one direction). Additionally, it effectively compares different light sources in a standardized setup. Therefore, it is a better measurement for non-visible light sources (like near UV). But keep in mind that the light source’s sensor position effects the irradiance measurement. Comparisons are difficult if you don’t know the sensor’s exact location.
If you place your sample in the same position you can estimate the amount of light (number of photons) that irradiates your sample using the exposed surface area of your sample. You need to take into consideration the light coming from other directions as well as the reflection of the light on the surface of the vial.
Photon flux and actinometry
Actinometry is a standard method to measure the actual amount light penetrating your sample. “Actinometers” are the method’s reagents and Ferrioxalate is the most widely used.2 This iron (III) complex produces iron (II) in known photochemical yields. Thus, the specific number of photons penetrating the sample can be determined based on how much iron (II) is produced from the ferrioxalate complex.
The photon flux (or irradiance) is specific to the vial, volume of the reaction and the light source. Using this method, you can calibrate your setup and know how much light penetrates your sample.
1. Bonfield, H.E., Knauber, T., Lévesque, F. et al. Photons as a 21st century reagent. Nat Commun 11, 804 (2020) https://doi.org/10.1038/s41467-019-13988-4
2. Hatchard C.G.; Parker C.A. A new sensitive chemical actinometer. 2. Potassium ferrioxalate as a standard chemical actinometer. Proc. R. Soc. London, Ser. A. 1956, 235, 518-536.