Direct imaging of coherent bending vibrations in a triatomic molecule

Light-induced vibrational wave packets play a fundamental role in many types of molecular dynamics and have been studied extensively. However, most studies using conventional mass spectrometry provide only indirect information on evolving molecular structures. In this work, we show that one can directly map coherent bending vibrations in a laser-ionized small molecule with excellent temporal resolution using Coulomb explosion imaging (CEI). We use a strong 28-fs near-infrared (NIR) laser pulse to ionize SO₂ and induce vibrational wave packets in its cationic states. A second NIR pulse of higher intensity is then used to further ionize and/or dissociate the molecules, and the ionic products are detected in coincidence using a COLTRIMS apparatus. We focus on mapping the O-S-O bending vibrations and find that this motion influences many observables in the multi-dimensional coincidence data. We also show that the bending-mode vibration can be observed directly in the delay-dependent KER spectra and angular distributions of high-charge-states channels. Our observations are supported by classical Coulomb explosion simulations.