  We have performed measurements of the IR absorption of $\text{SiO}_2$
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 $\text{SiO}_2$. 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 IRPRS (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.
