# Propagation Regimes and Signal Enhancement Mechanisms of Collinear Double-Pulse Plasma with Varying Inter-Pulse Delays

## TL;DR

This paper studies how laser pulses affect plasma signals in LIBS, showing that adjusting pulse delays can improve detection of trace elements.

## Contribution

The study reveals how inter-pulse delays influence plasma propagation regimes and signal enhancement in DP-LIBS.

## Key findings

- Short inter-pulse delays in DP-LIBS produce laser-supported detonation waves and enhance signals via reheating.
- Longer delays result in laser-supported combustion waves, with signal improvement due to increased ablation.
- Trace element detection is enhanced by selecting appropriate delays to excite difficult spectral lines.

## Abstract

Laser-induced breakdown spectroscopy (LIBS) is an in situ analytical technique. Compared to traditional single-pulse LIBS (SP-LIBS), collinear double-pulse LIBS (DP-LIBS) is a promising technique due to its lower limit of detection for trace elements. In this paper, we analyze the spectral and image information obtained from the emissions emitted by single/double pulse (SP/DP) laser-induced plasmas. The types of laser-supported absorption (LSA) waves of the plasmas were determined according to the interactions among the ablation vapor, the ambient gas, and the laser. Furthermore, the influence mechanisms of plasma shielding on DP-LIBS signal intensity enhancement with different inter-pulse delay were investigated. In our experimental conditions, the propagation regime of SP plasma is a laser-supported combustion (LSC) wave. The DP plasmas with short inter-pulse delays show the characteristics of a laser-supported detonation (LSD) wave, and the enhancement mechanism is mainly reheating for pre-plasma. On the contrary, the DP plasmas with longer inter-pulse delays show the characteristics of a LSC wave, and the increase in laser ablation is a major contributing factor to the signal improvement. In addition, the spectral lines, which are difficult to excite by SP-LIBS, can be obtained by selecting an appropriate inter-pulse delay and setting a short delay, which provides a new idea for the measurement of trace elements.