arXiv subjects

Ali Belkacem

Publications and source records attributed to Ali Belkacem.

5 recordsLinked to original sources

arXiv2025

Interplay between ultrafast electronic and librational dynamics in liquid nitrobenzene probed with two-color four-wave mixing

We present an experimental and theoretical study of the interplay between ultrafast electron dynamics and librational dynamics in liquid nitrobenzene. A femtosecond ultraviolet pulse and two femtosecond near infrared pulses interact with nitrobenzene molecules, generating a four-wave mixing nonlinear signal that is measured in the Optical Kerr Effect geometry. The near infrared nonlinear signal is measured to be non-zero only at negative time delays, corresponding to the near infrared pulses arriving earlier than the ultraviolet pulse. We perform time-dependent Quantum Master Equation calculations, which include a classical libration model, to simulate the experiment. The simulations support the conclusion that the near infrared pulses launch librational motion, while simultaneously creating electronic coherences that result in a libration-modulated electronic nonlinear response. Furthermore, we conclude that the measured nonlinear optical signal corresponds to a non-parametric process that leaves the molecules in an excited electronic state. This work provides new insight into ultrafast nonlinear optical interactions in liquids and is an important step towards probing ultrafast electronic coherences in large molecules in the liquid phase.

physics.chem-ph↗

arXiv2020

Time-Resolved Ultrafast Transient Polarization Spectroscopy to Investigate Nonlinear Processes and Dynamics in Electronically Excited Molecules on the Femtosecond Time Scale

We report a novel experimental technique to investigate ultrafast dynamics in photoexcited molecules by probing the third-order nonlinear optical susceptibility. A non-colinear 3-pulse scheme is developed to probe the ultrafast dynamics of excited electronic states using the optical Kerr effect by time-resolved polarization spectroscopy. Optical heterodyne and optical homodyne detection are demonstrated to measure the third-order nonlinear optical response for the S1 excited state of liquid nitrobenzene, which is populated by 2-photon absorption of a 780 nm 35 fs excitation pulse.

physics.chem-ph↗

arXiv2019

Ultrafast dynamics of excited electronic states in nitrobenzene measured by ultrafast transient polarization spectroscopy

We investigate ultrafast dynamics of the lowest singlet excited electronic state in liquid nitrobenzene using Ultrafast Transient Polarization Spectroscopy (UTPS), extending the well-known technique of Optical-Kerr Effect (OKE) spectroscopy to excited electronic states. The third-order non-linear response of the excited molecular ensemble is highly sensitive to details of excited state character and geometries and is measured using two femtosecond pulses following a third femtosecond pulse that populates the S1 excited state. By measuring this response as a function of time delays between the three pulses involved, we extract the dephasing time of the wave-packet on the excited state. The dephasing time measured as a function of time-delay after pump excitation shows oscillations indicating oscillatory wave-packet dynamics on the excited state. From the experimental measurements and supporting theoretical calculations, we deduce that the wave-packet completely leaves the S1 state surface after three traversals of the inter-system crossing between the singlet S1 and triplet T2 states.

physics.chem-ph↗

arXiv2018

Ultrafast Photodissociation Dynamics and Nonadiabatic Coupling Between Excited Electronic States of Methanol Probed by Time-Resolved Photoelectron Spectroscopy

The electronic and nuclear dynamics in methanol, following 156~nm photoexcitation, are investigated by combining a detailed analysis of time-resolved photoelectron spectroscopy experiments with electronic structure calculations. The photoexcitation pump pulse is followed by a delayed 260~nm photoionization probe pulse, to produce photoelectrons that are analyzed by velocity map imaging. The yield of mass-resolved ions, measured with similar experimental conditions, are found to exhibit the same time-dependence as specific photoelectron spectral features. Energy-resolved signal onset and decay times are extracted from the measured photoelectron spectra to achieve high temporal resolution, beyond the 20~fs pump and probe pulse durations. When combined with {\it ab initio} calculations of selected cuts through the excited state potential energy surfaces, this information allows the dynamics of the transient excited molecule, which exhibits multiple nuclear and electronic degrees of freedom, to be tracked on its intrinsic few-femtosecond timescale. Within 15~fs of photoexcitation, we observe nuclear motion on the initially bound photoexcited 2$^{1}$A$''$ (S$_2$) electronic state, through a conical intersection with the 1$^{1}$A$'$ (S$_3$) state, which reveals paths to photodissociation following C--O stretch and C--O--H angle opening.

physics.chem-ph↗

arXiv2016

Focal overlap gating in velocity map imaging to achieve high signal-to-noise ratio in photo-ion pump-probe experiments

We demonstrate a new technique in velocity map imaging (VMI) that allows spatial gating of the laser focal overlap region in time resolved pump-probe experiments. This significantly enhances signal-to-noise ratio by eliminating background signal arising outside the region of spatial overlap of pump and probe beams. This enhancement is achieved by tilting the laser beams with respect to the surface of the VMI electrodes which creates a gradient in flight time for particles born at different points along the beam. By suitably pulsing our microchannel plate detector, we can select particles born only where the laser beams overlap. This spatial gating in velocity map imaging can benefit nearly all photoion pump-probe VMI experiments especially when extreme-ultraviolet (XUV) light or X-rays are involved which produce large background signals on their own.

physics.chem-ph↗