Charge measurement of SiO 2 nanoparticles in an rf plasma by ir absorption

We have performed measurements of the ir absorption of ${\text{SiO}}_{2}$ nanoparticles confined in an argon radiofrequency plasma discharge using a Fourier transform infrared spectrometer. By varying the gas pressure of the discharge and duty cycle of the applied radiofrequency voltage, we observed a shift of the absorption peak of ${\text{SiO}}_{2}$. We attributed this shift to charge-dependent absorption features of ${\text{SiO}}_{2}$. The charge-dependent shift has been calculated for ${\text{SiO}}_{2}$ particles, and from comparisons with the experiment the particle charge has been retrieved using our infrared phonon resonance shift method. 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 estimate of the charge with the duty cycle.