CH(A) Radical Formation in Coulomb Explosion from Butane Seeded Plasma Generated with Chirp-Controlled Ultrashort Laser Pulses

We experimentally studied the formation of CH(A) radicals in butane seeded plasma generated with chirp-controlled ultrashort laser pulses ($\sim 760\mu \text{J}\mathrm{/}\text{pulse}$, 890 nm, 1 kHz, 8 fs). The focused beam with high peak intensity ($\sim {10}^{14}-{10}^{16}{\text{W/cm}}^2$) caused Coulomb explosion (CE). The time dependent emission spectra were observed with the Fourier-transform Visible spectroscopy (FTVis) step-scan method. The average signal intensity decreased with the chirp in the Ar${}^{+}$ > C${}_2$ > H-$\alpha$ $\sim$ CH(A) order with a plateau for CH(A) in the $-200$ to $-100{\text{fs}}^2$ range. The short rise time of the CH(A) emission signal, the monoexponential emission decay, and the nearly constant rotational and vibrational temperatures of the CH(A) radicals ($\sim 3000\text{K}$ and $\sim 3800\text{K}$) all support their formation as a primary product. Our TDDFT calculations predict that CH and many other fragments can be formed beyond CE at $\sim 7\times {10}^{14}{\text{W/cm}}^2$ intensity. The average charge of CH (+0.6) and its relative abundance (0.5%) support the formation of detectable CH(A) within 120 fs.