Influence of ion acceleration on LIBS in reduced-pressure and vacuum environments

With the important application of LIBS for deep-space exploration, characterizing laser-plasma in reduced-pressure environment, such as on Mars, and in vacuum, such as on Moon and in tokamak, is of vital significance for the optimization of LIBS operation. In these environments, the well-known ion acceleration effect within laser-plasma leads to distinct dynamic characteristics of the plasma as well as the associated LIBS signal. Although the physical process of the ion acceleration effect has been extensively studied in the area of Pulsed Laser Deposition (PLD), its influence on LIBS has not been fully understood. In this work, we investigated the impact of ion acceleration on the characteristics of laser-induced titanium plasma in these environments by using fast imaging and spatiotemporally resolved spectroscopy. It can be found that in a reduced-pressure environment, a diminished resistance of the ambient gas enables ions in different charge states to dominate among different components of a plasma with a split two-component structure during its early stage of expansion. As these components expand and interact, a plasma with a homogeneous spatial distribution of Ti I and Ti II forms at a delay of 1000 ns, characterized by an emission with high signal-to-noise ratio (SNR) and signal-to-background ratio (SBR). This indicates an optimal time window for LIBS measurements in a reduced-pressure environment at this time stage. In the entire absence of ambient gas as in vacuum environment, the lack of the background gas resistance allows the ions in highly charge state to be accelerated freely in the ambipolar electric field, resulting in the severe shortening of the plasma duration. On the other hand, a stronger ion acceleration facilitates highly charged ions, such as Ti IV, to expand and predominate at the plume front during the early stage of plasma expansion, when continuum background is rather lower. As a result, the SBR of Ti IV is several times higher than that of the other lines. Considering the species segregation caused by ion acceleration, the local thermodynamic equilibrium (LTE) for titanium atoms and ions in different charge states are evaluated based on temperature and electron density of the plasmas induced in different environments. The results indicate that the transient nature of Ti III and Ti IV conducts them to deviate from LTE during the early stage of plasma expansion, while Ti I and Ti II remain in LTE throughout the investigated delays. These results offer insights into the characteristics of laser-induced plasma in reduced-pressure and vacuum environments, helping therefore the application of LIBS in deep-space exploration.