Real-time in situ monitoring of dust particle growth in a low-pressure nanodusty plasma based on laser-induced photodetachment

Particulate matter air pollution in the form of ultrafine dust is a growing global concern. In this Letter, we will use a nanodusty Ar/HMDSO plasma as a model system for a heavily contaminated gas and we present the development of a technique for real-time in situ measurements of the dust particle size. The method is based on laser-induced photodetachment of bound electrons from the surface of dust particles. These photo-released electrons are measured as an increase in the free electron density of the plasma using microwave cavity resonance spectroscopy. We show that instead of reconstructing the entire resonance profile, the temporal response of a single microwave frequency was enough to perform the measurements. More specifically, the decay timescale of the cavity response can be interpreted as the re-charging timescale of the dust particles. Then, using a stochastic model, this timescale can be modeled, which eventually retrieves the dust particle size. We found good agreement between the predicted dust particle size and the average dust particle size obtained from ex situ scanning electron microscopy measurements. This method allows for the real-time monitoring of the dust particle size and a controlled production of nanometer-scale dust particles, which gives opportunities both for fundamental dusty plasma physics and models, as well as for applications in monitoring ultrafine dust air pollution.