arXiv subjects

David Picconi

Publications and source records attributed to David Picconi.

6 recordsLinked to original sources

arXiv2025

Roadmap for Molecular Benchmarks in Nonadiabatic Dynamics

Simulating the coupled electronic and nuclear response of a molecule to light excitation requires the application of nonadiabatic molecular dynamics. However, when faced with a specific photophysical or photochemical problem, selecting the most suitable theoretical approach from the wide array of available techniques is not a trivial task. The challenge is further complicated by the lack of systematic method comparisons and rigorous testing on realistic molecular systems. This absence of comprehensive molecular benchmarks remains a major obstacle to advances within the field of nonadiabatic molecular dynamics. A CECAM workshop, Standardizing Nonadiabatic Dynamics: Towards Common Benchmarks, was held in May 2024 to address this issue. This Perspective highlights the key challenges identified during the workshop in defining molecular benchmarks for nonadiabatic dynamics. Specifically, this work outlines some preliminary observations on essential components needed for simulations and proposes a roadmap aiming to establish, as an ultimate goal, a community-driven, standardized molecular benchmark set.

physics.chem-ph↗

arXiv2021

Following excited-state chemical shifts in molecular ultrafast x-ray photoelectron spectroscopy

The conversion of photon energy into other energetic forms in molecules is accompanied by charge moving on ultrafast timescales. We directly observe the charge motion at a specific site in an electronically excited molecule using time-resolved x-ray photoelectron spectroscopy (TR-XPS). We extend the concept of static chemical shift from conventional XPS by the excited-state chemical shift (ESCS), which is connected to the charge in the framework of a potential model. This allows us to invert TR-XPS spectra to the dynamic charge at a specific atom. We demonstrate the power of TR-XPS by using sulphur 2p-core-electron-emission probing to study the UV-excited dynamics of 2-thiouracil. The new method allows us to discover that a major part of the population relaxes to the molecular ground state within 220-250 fs. In addition, a 250-fs oscillation, visible in the kinetic energy of the TR-XPS, reveals a coherent exchange of population among electronic states.

physics.chem-ph↗

arXiv2019

Quantum dynamics and spectroscopy of dihalogens in solid matrices. I. Efficient simulation of the photodynamics of the embedded $\mathrm{I_2 Kr_{18}}$ cluster using the G-MCTDH method

The molecular dynamics following the electronic $B\ ^3Π_u\left(0^+\right) \longleftarrow X\ ^1Σ_g^+$ photoexcitation of the iodine molecule embedded in solid krypton are studied quantum mechanically using the Gaussian variant of the multiconfigurational time-dependent Hartree method (G-MCTDH). The accuracy of the Gaussian wave packet approximation is validated against numerically exact MCTDH simulations for a fully anharmonic seven-dimensional model of the $\mathrm{I_2 Kr_{18}}$ cluster in a crystal Kr cage. The linear absorption spectrum, time-evolving vibrational probability densities, and $\mathrm{I_2}$ energy expectation value are accurately reproduced by the numerically efficient G-MCTDH approach. The reduced density matrix of the chromophore is analyzed in the coordinate, Wigner and energy representations, so as to obtain a multifaceted dynamical view of the guest-host interactions. Vibrational coherences extending over the bond distance range $ 2.7\,\mboxÅ < R_\mathrm{I-I} < 4.0\,\mboxÅ$ are found to survive for several vibrational periods, despite extensive dissipation. The present results prepare the ground for the simulation of time-resolved coherent Raman spectroscopy of the $\mathrm{I_2}$-krypton system addressed in a companion paper.

physics.chem-ph↗

arXiv2019

Quantum dynamics and spectroscopy of dihalogens in solid matrices. II. Theoretical aspects and G-MCTDH simulations of time-resolved coherent Raman spectra of Schrödinger cat states of the embedded $\mathrm{I_2 Kr_{18}}$ cluster

This companion paper to [D. Picconi et al., J. Chem. Phys. 150 (2019)] presents quantum dynamical simulations, using the Gaussian-based multiconfigurational time-dependent Hartree (G-MCTDH) method, of time-resolved coherent Raman four-wave-mixing spectroscopic experiments for the iodine molecule embedded in a cryogenic crystal krypton matrix. These experiments monitor the time-evolving vibrational coherence between two wave packets created in a quantum superposition (i. e. a 'Schrödinger cat state') by a pair of pump pulses which induce electronic $B\ ^3Π_u\left(0^+\right) \longleftarrow X\ ^1Σ_g^+$ transitions. A theoretical description of the spectroscopic measurement is developed, which elucidates the connection between the nonlinear signals and the wave packet coherence. The analysis provides an effective means to simulate the spectra for several different optical conditions with a minimum number of quantum dynamical propagations. The G-MCTDH method is used to calculate and interpret the time-resolved coherent Raman spectra of two selected initial superpositions for a $\mathrm{I_2 Kr_{18}}$ cluster embedded in a frozen Kr cage. The time- and frequency-dependent signals carry information about the molecular mechanisms of dissipation and decoherence, which involve vibrational energy transfer to the stretching mode of the four 'belt' Kr atoms. The details of these processes and the number of active solvent modes depend in a non-trivial way on the specific initial superposition.

physics.chem-ph↗

arXiv2017

Photodissociation dynamics in the first absorption band of pyrrole: II. Photofragment distributions for the $^1\!A_2(πσ^*) \leftarrow \tilde{X}^1\!A_1(ππ)$ transition

The analysis of the total kinetic energy release (TKER) of the photofragments pyrrolyl + H-atom formed in the photodissociation of pyrrole in the low-lying state $^1\!A_2(πσ^*)$ is presented. The TKER distributions contain complementary and often more precise information on the fragmentation process than the broad diffuse absorption spectra. The distributions are calculated quantum mechanically for the diabatic state $^1\!A_2(πσ^*)$ either isolated or coupled to the ground electronic state at an exit channel conical intersection. The calculations use the novel ab initio quasi-diabatic potential energy matrix constructed in paper I. The approximate overlap integral-based adiabatic mapping approach is introduced with which the quantum mechanical TKER distributions can be efficiently and accurately reproduced. Finally, the calculated TKERs are compared with the experimental results. The main features of the measured vibrationally resolved distributions are reproduced, and the spectral peaks are assigned and interpreted in detail.

physics.chem-ph↗

arXiv2017

Photodissociation dynamics in the first absorption band of pyrrole: I. Molecular Hamiltonian and the Herzberg-Teller absorption spectrum for the $1^1A_2(πσ^*) \leftarrow \tilde{X}^1A_1(ππ)$ transition

This paper opens a series in which the photochemistry of the two lowest $πσ^*$ states of pyrrole and their interaction with each other and with the ground electronic state $\tilde{X}$ are studied using ab initio quantum mechanics. New 24-dimensional potential energy surfaces for the photodissociation of the N-H bond and the formation of the pyrrolyl radical are calculated using the CASPT2 method for the electronic states $\tilde{X}(ππ)$, $1^1A_2(πσ^*)$ and $1^1B_1(πσ^*)$ and locally diabatized. In the first paper, the ab initio calculations are described and the photodissociation in the state $1^1A_2(πσ^*)$ is analyzed. The excitation $1^1A_2 \leftarrow \tilde{X}$ is mediated by the coordinate dependent transition dipole moment functions constructed using the Herzberg-Teller expansion. Nuclear dynamics, including 6, 11, and 15 active degrees of freedom, is studied using the multi-configurational time-dependent Hartree method. The focus is on the frequency resolved absorption spectrum, as well as on the dissociation time scales and the resonance lifetimes. Calculations are compared with available experimental data. An approximate convolution method is developed and validated, with which absorption spectra can be calculated and assigned in terms of vibrational quantum numbers. The method represents the total absorption spectrum as a convolution of the diffuse spectrum of the detaching H-atom and the Franck-Condon spectrum of the heteroaromatic ring. Convolution calculation requires a minimal quantum chemical input and is a promising tool for studying the $πσ^*$ photodissociation in model biochromophores.

physics.chem-ph↗