Dear Qing Xing Peng, To identify the plasma emission spectrum of unknown gases in Laser-Induced Breakdown Spectroscopy (LIBS), compare the observed spectrum to standard reference spectra of known gases. Databases like NIST’s Atomic Spectra Database provide emission lines and intensities for elements in various states. By matching prominent emission peaks with these reference spectra, the gas can be identified. For increased accuracy, use software tools or spectral libraries specifically tailored to LIBS, which often include algorithms for spectrum matching and gas identification.