Strong-Field Coulomb Explosion of Ethane, Propane, and Butane in Circularly Polarized Laser Fields

Abstract

We investigate the Coulomb explosion of ethane (C$_2$H$_6$), propane (C$_3$H$_8$), and \textit{n}-butane (C$_4$H$_{10}$) driven by intense circularly polarized laser pulses using real-time time-dependent density functional theory (RT-TDDFT). The ionization dynamics are benchmarked against those obtained with linearly polarized fields oriented along the $x$, $y$, and $z$ axes at the same peak intensity. Under the laser conditions considered here, circular polarization produces greater ionization than any of the linearly polarized configurations for all three molecules, indicating that the rotating electric field enhances the initial electron-removal stage that triggers Coulomb explosion. Using circularly polarized excitation, we systematically characterize fragmentation thresholds, product distributions, channel branching ratios, and bond-breaking dynamics across the alkane series. Atomic hydrogen is the most abundant fragment in all three systems, demonstrating that hydrogen loss is the dominant fragmentation pathway. Ethane primarily retains its two-carbon backbone through partial dehydrogenation, propane exhibits the broadest range of fragmentation channels and the strongest competition between C--H and C--C bond cleavage within the present ensemble, and butane favors backbone cleavage into relatively stable two-carbon fragments, most notably through the $2\mathrm{C_2H_4} + 2\mathrm{H}$ channel. Analysis of the earliest bond-breaking events further shows that C--H dissociation is the preferred initial fragmentation step throughout the series, although the degree of competition with C--C cleavage depends on molecular size.