Investigation of signal enhancement in nanoparticle enhanced molecular LIBS of graphite

In the present study, we have investigated the time evolution of molecular LIBS (MLIBS) and nanoparticle enhanced molecular LIBS (NEMLIBS) of graphite samples using 10 nm gold nanoparticles for delays up to 100 μs. A systematic study of signal enhancement observed in the CN violet band (B²Σ⁺ → X²Σ⁺) and C₂ Swan band (d³Π_g → a³Π_u) was carried out. For acquisition delays of a few μs to tens μs, we have observed typically 1–2 orders of optical signal enhancement in these molecular bands of the graphite target using Au NPs. We have studied the mechanism of signal enhancement in the context of lifetime and plasma parameters viz., rotational and vibrational temperature of molecular bands as well as electron number density. The relative enhancement observed in C₂ and CN bands, which are produced through different routes/mechanisms, has been explained through our analysis. The enhancements observed in NEMLIBS at higher delays are due to the increase of atomic species in the plasma which are antecedent of the molecules formed. This is evident through higher electron number density and slightly lower vibrational temperature in the case of NEMLIBS as observed in our analysis.