Charge measurement of silica nanoparticles in an RF-plasma by IR absorption

The particle charge is an essential parameter in dusty plasmas. To measure the dust charge on submicron particles, we have performed measurements of the IR absorption of SiO₂ nanoparticles confined in an argon radio-frequency plasma discharge using an FTIR spectrometer. By varying the gas pressure of the discharge and duty cycle of the applied radio-frequency voltage we observed a shift of the absorption peak of SiO₂. We attribute this shift to charge-dependent absorption features of SiO₂. The charge-dependent shift has been calculated for SiO2 particles and from comparisons with the experiment the particle charge has been retrieved. With the two different approaches of changing the gas pressure and altering the duty cycle we are able to deduce a relative change of the particle charge with pressure variations and an absolute estimation of the charge with the duty cycle.