Executive Summary : | The force induced by an ultrashort intense laser field on the electrons in an atom or a molecule originates many non-linear, non-perturbative phenomena, viz., above-threshold ionization (ATI), above-threshold dissociation (ATD), high-order harmonic generation (HHG), tunnel ionization, charge-resonance enhanced ionization, etc. These facts result in coupling between the electronic and the nuclear motions, although their characteristic time scales are much different, which in turn, lead to the failure of the Born-Oppenheimer approximation. In recent past, the experiments investigating the influence of ultrashort intense laser fields on bigger molecules reported many important findings like: (i) bond softening and hardening in polyatomic molecules; (ii) dependency of ionization and dissociation of polyatomic molecules on the carrier envelope phase (CEP) of such pulses. Few theoretical approaches dealt with this problem in an ``exact'' way, but only for one/two electron systems. Moreover, various chemical properties, viz., the acidic and basic character, redox properties, protein activation, photosynthesis, photo-induced DNA repair, etc., depend on the electron dynamics. However, the typical time scale of electronic motion in a molecule is in the attosecond domain (10^-¹⁸ sec), as for example, the period of an electron in 1s orbit of an H-atom is ~150 attosecond. Laser pulses with ultrashort (10 femtosecond or less) pulse-width provide an opportunity to observe and control such fast dynamics. The first real-time observation of electrons was experimentally achieved for the D₂ molecule in 2006. For a deeper insight into such intra-molecular motion of the electron, theoretical simulations were successfully performed by using the quantum and classical dynamical methods. On the other hand, the first experiment on the attosecond dynamics of a relatively larger molecule was performed on phenylalanine in 2014. The theoretical study based on this experiment showed that the electronic charge fluctuates rapidly all over the molecule without any preference of functional groups, indicating the invalidity of the concept of electron affinity at the ultrafast time scale. Those above facts are enough encouraging to have a re-look at our chemical intuitions through the field of atto-chemistry. Thus, development of an ``accurate'' method is needed to elucidate the coupled electron-nuclear dynamics in many electron molecules.
In this project, the attosecond charge fluctuations and charge transfer dynamics in the multi-electron molecules will be studied by using the ultrashort pulses with sub-femtosecond pulse-width. Also, some crucial phenomena, viz., sub-cycle ionization phenomena, HHG process, attosecond ultrafast magnetic field generation under molecular resonant excitation, etc. will be investigated. To simulate the ultrafast dynamics of such multi-electron larger molecular systems, both quantum and classical based methods will be employed. |