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

We experimentally studied the formation of CH-(A) radicals in butane seeded plasma generated with chirp-controlled ultrashort laser pulses (∼760 μJ/pulse, 890 nm, 1 kHz, 8 fs). The focused beam with high peak intensity (∼10¹⁴-10¹⁶ W/cm²) 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⁺ > C₂ > H-α ∼ CH-(A) order, with a plateau for CH-(A) in the -200 to -100 fs² 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 (∼3000 and ∼3800 K) all support their potential formation as a primary product (<120 fs) or in other photodissociation, neutralization processes before collisions of the fragments (<2 ns). Our TDDFT calculations predict that CH and many other fragments can be formed beyond CE at ∼7 × 10¹⁴ W/cm² intensity. The average charge of CH (+0.6) and its relative abundance (0.5%) support the formation of detectable CH-(A) within 120 fs. Suitable optical gating techniques and measurement of the temporal evolution of the electron density in plasma could elucidate the relative importance of the different formation pathways of the CH-(A) radicals in the first nanosecond after CE.