Reactive crossed beam scattering of a Ti plasma and a N2 pulse in a novel laser ablation method

Abstract

The interaction and energy transfer of a laser ablation plasma of Ti with a pulsed N2 supersonic expansion are investigated using time-of-flight quadrupole mass spectroscopy and Langmuir probe techniques. The Ti ablation target and the exit nozzle of the pulsed gas source are positioned so that the plasma plume and gas pulse interact near to their respective origins, where the number density is still high, which hence results in strong coupling of the nascent plasma with the gas pulse. The timing between the gas pulse and ablation plume is shown to be critical in determining the scattering processes and the chemical nature of the films grown by this method, an example of which is presented. The degree of ionization of the plasma when crossed with the gas pulse compared to that for expansion into vacuum increases from less than 10−3 to 0.28±0.11, which is attributed to collision-induced ionization of Ti atoms. Further increasing the N2 number density quenches the ion signal. The effective bimolecular cross section for scattering of Ti with the high density N2 pulse is about 4 times larger than that with a static background of low pressure N2, while the fractional depletion of the N2 pulse by the Ti plume depends on the N2 number density in the pulse, indicating that at these high local pressures, collective effects prevail. We propose a simple model for the resulting evolution of the plasma based on electrostatic considerations.