Resonant versus nonresonant impulsive molecular alignment in acetylene via vibrational excitation

We investigate the impulsive alignment of acetylene (${\mathrm{C}}_{2}{\mathrm{H}}_{2}$) molecules via resonant vibrational excitation using ultrashort infrared laser pulses. Focusing on two vibrational bands, the fundamental mode ${\ensuremath{\nu}}_{3}$ at 3 \textmu{}m and the combination band ${\ensuremath{\nu}}_{1}+{\ensuremath{\nu}}_{3}$ at 1.5 \textmu{}m, we compare theoretical predictions with experimental observations. Numerical simulations reveal that excitation of the ${\ensuremath{\nu}}_{3}$ band induces a pronounced and persistent planar alignment characteristic of resonant processes, akin to phenomena recently observed in iodine molecules. In contrast, alignment via the ${\ensuremath{\nu}}_{1}+{\ensuremath{\nu}}_{3}$ band is predominantly governed by nonresonant interactions due to the weaker transition dipole moment, resulting in a negligible resonant contribution. Experimental measurements at 1.5 \textmu{}m confirm the dominance of nonresonant dynamics. These findings highlight the importance of dipole strength in resonant alignment and support the broader applicability of vibrationally driven alignment strategies in molecular control.