arXiv subjects

Vinod Kumarappan

Publications and source records attributed to Vinod Kumarappan.

5 recordsLinked to original sources

arXiv2025

Rotational coherences in O$_2^+$ following strong-field ionization

We investigate the wave packet that remains bound in the ground and excited cationic states of oxygen after strong-field ionization by an intense 800-nm pulse. Much weaker probe pulses (800 or 264 nm) are used to dissociate these still-bound cations. The momentum distribution of O$^+$ is measured as a function of pump-probe delay and Fourier-transformed to obtain kinetic-energy-dependent and rotational-state-resolved quantum beat spectra. The sub-cm$^{-1}$ resolution of the Fourier transform allows unambiguous identification of the electronic, vibrational, and rotational states populated by the pump and then dissociated by the probe. Although strong-field ionization is expected to populate the lower-lying $X^2\Pi_g$ and $a^4\Pi_u$ states more effectively than the $b^4\Sigma^{-}_g$ state, a wave packet in the $X^2\Pi_g$ state is seen only with the 264-nm probe and only weak signatures of the $a^4\Pi_u$ states are found with either probe. The experiment confirms the role of the resonant coupling between the $b^4\Sigma^{-}_g$ and $a^4\Pi_u$ states by the 800 nm pulses [Xue \textit{et al.}, Phys. Rev. A 97, 043409 (2018)] and reveals the importance of rovibrational excitation in determining the momentum distribution of the O$^+$ fragments. The strong $X^2\Pi_g$ state contribution observed with the 264-nm probe also shows the importance of resonant coupling in the probe pulse. The sub-cm$^{-1}$ resolution also resolves spin-orbit splitting in both the $X^2\Pi_g$ and $a^4\Pi_u$ state wave packets.

physics.atom-ph↗

arXiv2024

Simultaneous imaging of vibrational, rotational, and electronic wave packet dynamics in a triatomic molecule

Light-induced molecular dynamics often involve the excitation of several electronic, vibrational, and rotational states. Since the ensuing electronic and nuclear motion determines the pathways and outcomes of photoinduced reactions, our ability to monitor and understand these dynamics is crucial for molecular physics, physical chemistry, and photobiology. However, characterizing this complex motion represents a significant challenge when different degrees of freedom are strongly coupled. In this Letter, we demonstrate how the interplay between vibrational, rotational, and electronic degrees of freedom governs the evolution of molecular wave packets in the low-lying states of strong-field-ionized sulfur dioxide. Using time-resolved Coulomb explosion imaging (CEI) and quantum mechanical wave packet simulations, we directly map the bending vibrations of the molecule, show how the vibrational wave packet is influenced by molecular alignment, and elucidate the consequences of nuclear motion for the coupling between the two lowest electronic states of the cation. Our results demonstrate that multi-coincident CEI can be an efficient experimental tool for characterizing coupled electronic and nuclear motion in polyatomic molecules.

physics.chem-ph↗

arXiv2024

Differentiating Three-Dimensional Molecular Structures using Laser-induced Coulomb Explosion Imaging

Coulomb explosion imaging (CEI) with x-ray free electron lasers has recently been shown to be a powerful method for obtaining detailed structural information of gas-phase planar ring molecules [R. Boll et al. Nat. Phys. 18, 423-428 (2022)]. In this Letter, we investigate the potential of CEI driven by a tabletop laser and extend this approach to differentiating three-dimensional (3D) structures. We study the static CEI patterns of planar and nonplanar organic molecules that resemble the structures of typical products formed in ring-opening reactions. Our results reveal that each molecule exhibits a well-localized and distinctive pattern in 3D fragment-ion momentum space. We find that these patterns yield direct information about the molecular structures and can be qualitatively reproduced using a classical Coulomb explosion simulation. Our findings suggest that laser-induced CEI can serve as a robust method for differentiating molecular structures of organic ring and chain molecules. As such, it holds great promise as a method for following ultrafast structural changes, e.g., during ring-opening reactions, by tracking the motion of individual atoms in pump-probe experiments.

physics.atm-clus↗

arXiv2017

Isomer-Dependent Fragmentation Dynamics of Inner-Shell Photoionized Difluoroiodobenzene

The fragmentation dynamics of 2,6- and 3,5-difluoroiodobenzene after iodine 4$d$ inner-shell photoionization with soft X-rays are studied using coincident electron and ion momentum imaging. By analyzing the momentum correlation between iodine and fluorine cations in three-fold ion coincidence events, we can distinguish the two isomers experimentally. Classical Coulomb explosion simulations are in overall agreement with the experimentally determined fragment ion kinetic energies and momentum correlations and point toward different fragmentation mechanisms and time scales. While most three-body fragmentation channels show clear evidence for sequential fragmentation on a time scale larger than the rotational period of the fragments, the breakup into iodine and fluorine cations and a third charged co-fragment appears to occur within a few hundred femtoseconds -- a time scale larger than the rotational period of the fragments, the breakup in other channels appears to occur within a few hundred femtoseconds.

physics.chem-ph↗

arXiv2016

Orientation Resolution through Rotational Coherence Spectroscopy

The rich information content of measurements in the molecular frame rather than the laboratory frame has motivated the development of several methods for aligning gas phase molecules in space. Even so, for asymmetric tops the problem of making molecular frame measurements remains challenging due to its inherently multi-dimensional nature. In this Letter we present a method, based on the analysis of observables measured from rotational wavepackets, that does not require either 3D alignment or coincident momentum measurements to access the molecular frame. As an application we describe the first fully-orientation-resolved measurements of strong-field ionization and dissociation of an asymmetric top (ethylene).

physics.atom-ph↗