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

TL;DR

This study investigates the formation of CH(A) radicals in laser-induced Coulomb explosion of butane, revealing their primary formation, dependence on laser chirp, and supporting theoretical predictions of fragment formation at high intensities.

Contribution

It provides experimental evidence of primary CH(A) radical formation in Coulomb explosion with chirp-controlled ultrashort laser pulses and supports this with TDDFT calculations predicting fragment formation.

Findings

CH(A) radicals are formed as primary products in Coulomb explosion.

The emission intensity of CH(A) depends on laser chirp, with a plateau observed.

Theoretical calculations predict formation of CH and other fragments at high laser intensities.

Abstract

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.