An important characterization parameter associated with ionized physical vapor deposition systems is the amount of metal vapor that is actually ionized. In such systems, metal ions, in this case copper, formed by passing sputtered metal vapor through a high density plasma are used for ion plating applications. Here, a Langmuir probe technique is utilized which takes advantage of changes in the ratio of the ion saturation current to the electron saturation current to determine the variations in both copper and argon ion concentrations as a function of sputter power. The data indicate that at fixed rf power, the copper ion concentration increases rapidly with increasing sputter rate, eventually overtaking argon as the dominant ion species in the discharge. These findings also suggest that the fraction of electrons with sufficient energy to ionize argon neutrals decreases as a function of magnetron power. Optical emission spectra collected as a function of magnetron sputter power provide supporting evidence that the electron population in the tail of the electron energy distribution function is depleted by the introduction of the copper vapor. This quenching is attributed to inelastic collisions with copper associated with the low threshold energies and large excitation and ionization cross sections as compared to argon, and the resulting reduction in the measured plasma potential.