Periodic Reporting for period 1 - ATTOP (ATTOsecond Photochemistry: controlling chemical reactions with electrons)
Periodo di rendicontazione: 2022-10-01 al 2025-03-31
ATTOP is a theoretical chemistry project that explores the synergy between the two fields of attoscience and photochemistry. Chemical processes initiated by light are extremely widespread and their applications cover vital molecular research fields from medicine to computer science and energy conversion. However, photochemical reactions are limited by the nature and finite number of molecular electronic excited states. To overcome this fundamental limitation, one can bring the recent technological progress in extreme ultrashort light pulses - attosecond science - to the field of photochemistry, launching the field of “atto-photochemistry”. Light pulses of such short duration have a large spectral bandwidth and excite multiple electronic excited states in a simultaneous and coherent manner. This superposition, called an “electronic wavepacket”, has a new electronic distribution and is expected to lead to a new chemical reactivity. Using theoretical approaches, ATTOP aims to describe accurately chemical reactions induced by electronic wavepackets via attosecond domain pulses. The principal objectives of the project are the following: (i) to investigate to what extent the manipulation of electronic wavepackets can be used to control the outcome of a photochemical reaction; (ii) to develop a computational protocol for simulating fully quantum mechanically the coupled electron-nuclear dynamics of larger molecules in reduced dimensionality but with controlled accuracy; (iii) to define a pump-probe scheme for performing a given atto-photochemical reaction, and to work with experimentalist collaborators to implement such schemes leading to atto-photochemical experiments; (iv) to develop chemical descriptors of electronic wavepackets and build up general intuitive rules for the new field of atto-photochemistry.
We investigated theoretically the relaxation dynamics of ethylene upon ionisation. Photo-ionised and electronically excited ethylene C2H4+ can undergo H-loss, H2-loss, and ethylene−ethylidene isomerisation, where the latter entails a hydrogen migration. Recent pioneering experiments with few-femtosecond extreme ultraviolet pulses and complementary theoretical studies have shed light on the photodynamics of this prototypical organic cation. However, no theoretical investigation based on dynamics simulations reported to date had described the mechanisms and time scales of dissociation and isomerization. We simulated the coupled electron−nuclear dynamics of ethylene following vertical ionisation and electronic excitation to its four lowest-lying cationic states. The electronic structure was treated at the CASSCF level with an active space large enough to describe bond breaking and formation. The simulations indicated that dissociation and isomerization take place mainly on the cationic ground state and allow the probing of previous hypotheses concerning the correlation between the photochemical outcome and the traversed conical intersections. The results, moreover, support the long-standing view that H2-loss may occur from the ethylidene form. However, the ethylene−ethylidene isomerisation time predicted by the simulations is considerably longer than those previously inferred from indirect experimental measurements.
We also performed a methodological project regarding the simulation of attochemical processes in molecules. Such molecular processes can, in principle, be simulated with various nonadiabatic dynamics methods, yet the impact of the approximations underlying the methods was rarely assessed. We evaluated the performances of widely used mixed quantum-classical approaches, Tully surface hopping and classical Ehrenfest methods, against the high-accuracy DD-vMCG quantum dynamics. This comparison was conducted for the valence ionisation of fluorobenzene. Analysing the nuclear motion induced in the branching space of the nearby conical intersection, the results showed that the mixed quantum-classical methods reproduce quantitatively the average motion of a quantum wavepacket when initiated on a single electronic state. However, they fail to properly capture the nuclear motion induced by an electronic wavepacket along the derivative coupling, the latter originating from the quantum electronic coherence property, key to attochemistry. Our simulations further validate the control over the molecular motion in the branching space achieved by tuning the initial electronic wave packet composition. They also unravel both interstate and intrastate quantum interferences that leave clear signatures of attochemistry and charge-directed dynamics in the shape of the autocorrelation function. The latter is accessible experimentally via high-harmonic spectroscopy (HHS). We collaborated with the experimental team of Prof. Jon Marangos (Imperial College London). The predicted autocorrelation functions are in very good agreement with experimentally measured dynamics via HHS in both benzenes and fluoro-benzene, highlighting the sensitivity of autocorrelation functions to the composition of the initial electronic wavepacket. These results are direct evidence of attochemical control.
We also performed a methodological project regarding the simulation of attochemical processes in molecules. Such molecular processes can, in principle, be simulated with various nonadiabatic dynamics methods, yet the impact of the approximations underlying the methods was rarely assessed. We evaluated the performances of widely used mixed quantum-classical approaches, Tully surface hopping and classical Ehrenfest methods, against the high-accuracy DD-vMCG quantum dynamics. This comparison was conducted for the valence ionisation of fluorobenzene. Analysing the nuclear motion induced in the branching space of the nearby conical intersection, the results showed that the mixed quantum-classical methods reproduce quantitatively the average motion of a quantum wavepacket when initiated on a single electronic state. However, they fail to properly capture the nuclear motion induced by an electronic wavepacket along the derivative coupling, the latter originating from the quantum electronic coherence property, key to attochemistry. Our simulations further validate the control over the molecular motion in the branching space achieved by tuning the initial electronic wave packet composition. They also unravel both interstate and intrastate quantum interferences that leave clear signatures of attochemistry and charge-directed dynamics in the shape of the autocorrelation function. The latter is accessible experimentally via high-harmonic spectroscopy (HHS). We collaborated with the experimental team of Prof. Jon Marangos (Imperial College London). The predicted autocorrelation functions are in very good agreement with experimentally measured dynamics via HHS in both benzenes and fluoro-benzene, highlighting the sensitivity of autocorrelation functions to the composition of the initial electronic wavepacket. These results are direct evidence of attochemical control.
The following achievements are the most significant ones of the ATTOP project so far:
- Non-adiabatic dynamics simulations shedding light on the timescale, yield and mechanisms of H-loss, H2-loss and ethylene-ethylidene isomerisation upon ionisation and excitation of ethylene. This goes well beyond pre-existing state-of-the-art dynamics simulations on this system. This is thanks to a higher electronic structure level used (CASSCF with a large active space), allowing for the description for bond breaking and bond formation.
- Assessment of non-adiabatic dynamics methods for the simulation of attochemical processes in molecules and demonstration that fully quantum dynamics methods are required. This provides essential guidelines, for the whole community, regarding the simulations of attochemical processes in molecules.
- Clear signature of not only the ability to influence the induced molecular dynamics through electronic wavepackets, but also of the coherent behaviour and interference effects that may influence chemical dynamics on ultrafast timescales, through a combined theoretical-experimental work on fluoro-benzene. It is a direct theoretical and experimental evidence of attochemical control, but also of the coherent behaviour and quantum interference effects that drive it.
- Non-adiabatic dynamics simulations shedding light on the timescale, yield and mechanisms of H-loss, H2-loss and ethylene-ethylidene isomerisation upon ionisation and excitation of ethylene. This goes well beyond pre-existing state-of-the-art dynamics simulations on this system. This is thanks to a higher electronic structure level used (CASSCF with a large active space), allowing for the description for bond breaking and bond formation.
- Assessment of non-adiabatic dynamics methods for the simulation of attochemical processes in molecules and demonstration that fully quantum dynamics methods are required. This provides essential guidelines, for the whole community, regarding the simulations of attochemical processes in molecules.
- Clear signature of not only the ability to influence the induced molecular dynamics through electronic wavepackets, but also of the coherent behaviour and interference effects that may influence chemical dynamics on ultrafast timescales, through a combined theoretical-experimental work on fluoro-benzene. It is a direct theoretical and experimental evidence of attochemical control, but also of the coherent behaviour and quantum interference effects that drive it.