Translational motion of SiO+ produced by laser ablation

We have measured the Doppler width of laser-induced fluorescence of SiO+ ions produced by laser ablation of a Si target with a pulsed Nd:YAG laser in oxygen gas. The technique of pulsed amplification of a narrow-band cw laser was used to obtain a probe pulse whose bandwidth was much narrower than the Doppler width. The measured Doppler width was converted into an effective temperature representing the distribution of the velocity component of the SiO+ ions along the path of the probe laser pulse. By changing the delay time between the ablation and probe laser pulses, we have found that the effective temperature does not decrease monotonically to room temperature but forms humps. This implies that the ablation cloud may pulsate while diffusing.