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We experimentally studied the formation of CH(A) radicals in butane seeded plasma generated with chirp-controlled ultrashort laser pulses (\(\sim 760 \, \mu \text{J}/\text{pulse}\), 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.

Ion acceleration resulting from the interaction of 11 fs laser pulses of ~35 mJ energy with ultrahigh contrast (<10^-10), and 10^19 W/cm^2 peak intensity with foil targets made of various materials and thicknesses at normal (0-degree) and 45-degree laser incidence is investigated. The maximum energy of the protons accelerated from both the rear and front sides of the target was above 1 MeV. A conversion efficiency from laser pulse energy to proton beam is estimated to be as high as ~1.4 % at 45-degree laser incidence using a 51 nm-thick Al target. The excellent laser contrast indicates the predominance of vacuum heating via the Brunels effect as an absorption mechanism involving a tiny pre-plasma of natural origin due to the Gaussian temporal laser pulse shape. Experimental results are in reasonable agreement with theoretical estimates where proton acceleration from the target rear into the forward direction is well explained by a TNSA-like mechanism, while proton acceleration from the target front into the backward direction can be explained by the formation of a charged cavity in a tiny pre-plasma. The exploding Coulomb field from the charged cavity also serves as a source for forward-accelerated ions at thick targets., Comment: 12 pages, 7 figures