Results and Discussions The experimental results show that the spectral intensity of the characteristic line Cu Ⅰ 521.8 nm does not show a monotonic relationship with the ambient pressure, and there is an obvious interval effect, as shown in Fig.4. The spectral intensity decreases with the ambient pressure in the range of 10^-2-1 Pa, increases with the ambient pressure in the range of 1-10⁴ Pa, and decreases again with the increase in ambient pressure in the range of 10⁴-10⁵ Pa. The radiation lifetime of the characteristic line Cu Ⅰ 521.8 nm is analyzed, and the results show that the radiation lifetime of the characteristic line Cu Ⅰ 521.8 nm increases with the ambient pressure. As shown in Fig.8, the spectral intensities of the characteristic spectra of the gaseous elements N, H, and O are monotonically related to the ambient pressure. The spectral intensities of the characteristic spectra of the gas elements increase monotonically with the ambient pressure owing to the increase in the particle number density of the ambient gas and the enhancement of the restriction effect of the ambient gas. However, the characteristic spectral intensities of the gas elements N, H, and O have different attenuation rates with delay time owing to the different molecular bond energies of different gas elements. Among them, the characteristic spectral intensities of N have the fastest attenuation with the delay time, followed by H, and O has the slowest attenuation. The results show that the radiation lifetime of the characteristic spectra of gaseous elements increases with an increase in atmospheric pressure. Furthermore, it is found that the lowest pressure at which the characteristic spectra of gaseous elements can be detected is not the same, in which the characteristic spectra of N can be detected only when the ambient pressure is greater than 10 Pa, while the characteristic spectra of H and O can be detected at a pressure of 10^-2 Pa. The signal-to-noise rate (SNR) analysis of the gas element LIBS signal shows that the SNR of the N, H, and O gas element LIBS signal changes with the delay time in the same way, which increases first and then decreases with the delay time. However, the time required to obtain the maximum SNR of the gas-element LIBS signals is not completely consistent but is roughly between 500 ns and 1000 ns. The maximum SNR of the LIBS signals of each gas element is inconsistent at different ambient pressures, and the maximum SNR of the O element reached 19 when the air pressure was 10⁵ Pa and the delay time was 1000 ns.

Laser-induced breakdown spectroscopy (LIBS) technology has attracted the attention of numerous researchers since its inception. It has been widely used in material analysis owing to its advantages such as high detection speed, small sample damage, no sample preparation, and online analysis. It has applications in fields such as material detection, element analysis, deep space exploration, and various other fields. However, current studies on LIBS mainly focus on the LIBS signals of target material elements, and there is little analysis of the LIBS signals of environmental gas elements. In the laser-induced plasma process, the high-energy pulse laser will not only excite the target material elements but the ambient gas elements near the target surface will also be excited by the high-energy pulse laser, resulting in a unique LIBS signal of the ambient gas elements. Therefore, the laser-induced plasma process is related to not only the target material elements but also environmental gas elements. The laser plasma expands rapidly after it is generated and interacts with the ambient gas during the expansion process. The plasma expansion process is significantly influenced by the types of gases and air pressure environments. High-pressure ambient gases restrict the expansion of the plasma and affect its spectral radiation. Both the laser-induced plasma and the plasma expansion processes can be significantly impacted by ambient gas. The variation in the target element LIBS and the gas element LIBS signal with air pressure and delay time is of major significance for understanding the influence of ambient gas on laser-induced plasma.

Nd∶YAG laser with wavelength of 1064 nm and pulse width of 10 ns was used to irradiate Cu target without O and with Cr, Fe, Cr elements to generate plasma. The pulse laser energy and repetition frequency were set to 50 mJ and 1 Hz, respectively. Pulsed laser was reflected by two mirrors and then focused by a focal lens with a focal length of 100 nm on Cu target in the vacuum cavity. Mechanical and molecular pumps were used to realize the experimental vacuum degree of 5.5×10^-5-1.0×10⁵ Pa. The characteristic spectra of the target elements Cu, N, H, and O in laser plasma were analyzed by collecting plasma spectra from the normal direction of the laser incident plane. The variation in the characteristic spectra of the target element Cu and gas elements N, H, and O in the laser plasma with ambient pressure and delay time was studied.

This work can help understand the influence of ambient gas elements on laser plasma and on the optimization of the experimental parameters of LIBS technology.