Hi Qing Xing Peng

If you want to identify specific CH₄ characteristics from your recorded LIBS spectra, from what I can see, there are two main approaches:

Using atomic emission lines:

• For C, the 247.88 nm emission line is commonly used
• For H, consider the Balmer series, particularly: Hα at 656.3 nm (frequently used), Hβ at 486.1 nm, and Hγ at 434.0 nm

Hence, by analyzing the C:H intensity ratio from the LIBS spectra, you can determine whether it is consistent with methane's molecular structure (1:4). Note that, due to various experimental factors, the LIBS intensity ratio would likely deviate from the exact 1:4 ratio. Therefore, it is necessary to establish baseline ratios by developing a calibration curve from known methane concentrations. From this calibration, a correction factor can be derived as follows: (C:H = 1:4) / (IC:IH).

Using diatomic molecular bands:

• C2 Swan Bands (possibly); Δv = 0 band around 516.5 nm, Δv = +1 band around 473.7 nm, and Δv = -1 band around 563.5 nm.
• CH Bands; A²Δ → X²Π around 431.4 nm, B²Σ- → X²Π around 387.1 nm, and C²Σ+ → X²Π around 314.5 nm

The intensity ratios of these molecular bands can also provide insight into methane presence and concentration.

Additionally, consider the detector time delay at which these spectra are recorded: diatomic molecules recombine later (typically, of the order of a few tens of microseconds) than atomic species. Thus, optimizing the gate delay of your LIBS detection system is crucial to achieving reliable results.