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Ultrafast quantum dynamics of a triatomic molecule induced by short strong optical pulses
van den Wildenberg, Stéphan; Mignolet, Benoît; Remacle, Françoise
2016Journée scientifique de la Société Royale de Chimie
 

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Abstract :
[en] The development of methods for synthesizing ultrashort optical pulses makes it possible to probe, to follow and to control charge transfer and formation and breaking of chemical bonds on different timescales. Nuclear motions that leads to bond rearrangements typically take place on the femto (fs)- to picosecond (ps) timescale, which is the domain of femtochemistry. The engineering of attosecond (1as =10-18 s) optical pulses by high harmonic generation1-3 paves the way for attochemistry4. The excitation of molecules by ultrashort intense optical pulses leads to coherent non-stationary electronic density which can be tailored by the time profile of the electric field of the pulse. The coupling between this controlled electronic density and the nuclei opens the way to new avenues for selectively inducing chemical reactions5. In order to understand this new mechanism of control, it is necessary to compute the coupled quantum dynamics of electrons and nuclei when molecules are excited by attopulses. Until now, this kind of computation has mostly been performed on diatomic molecules, which include only a single vibrational degree of freedom6-10. In this work we computed the coupled electron-nuclei dynamics of the HCN molecule, induced by intense fs and as optical pulses. Nuclear dynamics was constrained to two nuclear degrees of freedom, on the two lowest electronic states, and neglecting non-adiabatic coupling.Adiabatic potential energy surfaces of the two lowest electronic states and dipole moment surfaces of HCN have been computed at the CASSCF level. Quantum dynamics have been computed by numerically integrating the nuclear time dependent Schrödinger equation on a grid on two electronic states. We studied the dissociation of HCN on its first excited electronic state by exciting the molecule using a UV femtopulse that is resonant with the electronic transition and builds a coherent superposition of electronic states (Fig. 1c). The electronic coherence is observable through the oscillations over time of the electronic dipole induced by the pulse. Furthermore, we have computed the dynamics of HCN induced by single-cycle UV attopulses. We have shown that the control of the Carrier Envelope Phase (CEP) of the pulse, that is the phase between the envelope of the pulse and the carrier frequency, allows controlling the oscillations of the electron density and therefore the amplitude and the phase of the induced dipole. We have also shown that the electronic coherence is modulated and is not quenched by the nuclear motion in two coupled nuclear degrees of freedom, even after several vibrational periods. Future plans include computing the coupled electronic-nuclear dynamics of di- tri- and tetra-atomic molecules on several electronic states, including non-adiabatic coupling between those states. In this way, we would move toward the control of nuclear motion by controlling the motion of the electronic density in increasingly complex molecules.
Disciplines :
Chemistry
Author, co-author :
van den Wildenberg, Stéphan ;  Université de Liège - ULiège > Département de chimie (sciences) > Laboratoire de chimie physique théorique
Mignolet, Benoît ;  Université de Liège - ULiège > Département de chimie (sciences) > Laboratoire de chimie physique théorique
Remacle, Françoise  ;  Université de Liège - ULiège > Département de chimie (sciences) > Laboratoire de chimie physique théorique
Language :
English
Title :
Ultrafast quantum dynamics of a triatomic molecule induced by short strong optical pulses
Publication date :
2016
Event name :
Journée scientifique de la Société Royale de Chimie
Event date :
13-10-2016
Available on ORBi :
since 20 January 2020

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