State-specific mobility of excited cadmium and calcium ions in a discharge plasma measured by a tunable diode laser

The state-specific drift velocity of ${\mathrm{Ca}}^{+}$ ions in the 3 ${}_{}{}^2{}_{}{}^{}\mathrm{D}_{3/2}^{}{}_{}{}^{}$ metastable state and of ${\mathrm{Cd}}^{+}$ in the 6 ${}_{}{}^2{}_{}{}^{}$ ${\mathit{P}}_{3/2}$ highly excited state in a helium glow-discharge plasma have been precisely measured by using a tunable single-mode diode laser and the state-specific mobility of excited ions has been discussed. The results show that the mobility of ${\mathrm{Ca}}^{+}$ ions in the metastable state is determined by momentum-loss collisions and is almost identical with the value for ${\mathrm{Ca}}^{+}$ in the ground state. In the experiment with a thin cylindrical tube, the apparent value of the mobility is dependent on the diffusion in the radial direction. On the other hand, the apparent mobility of ${\mathrm{Cd}}^{+}$ ions in the 6 ${}_{}{}^2{}_{}{}^{}$ ${\mathit{P}}_{3/2}$ state is much smaller than the calculated result for ${\mathrm{Cd}}^{+}$ in the ground state, but the value increases significantly with Cd vapor density. This is explained by the fact that ${\mathrm{Cd}}^{+}$ ions in the 6 ${}_{}{}^2{}_{}{}^{}$ ${\mathit{P}}_{3/2}$ highly excited state decay quickly by the radiative process and the apparent mobility of the ${\mathrm{Cd}}^{+}$ ions in this state in the plasma is dependent not only on the radiative lifetime but also on the excitation mechanisms for the state.