Final Report Summary - ELYCHE (Electron-scale dynamics in chemistry)
The target of the project is the investigation and control of ultrafast electron motion in many-particle systems, ranging from simple molecules to complex biomolecules, with temporal resolution down to the attosecond regime. Such electron motion precedes and determines the subsequent nuclear rearrangement, which ultimately leads to the chemical change. The application of attosecond pulses and the development of attochemistry techniques for the investigation of the primary electronic steps of chemical processes, is a completely new and challenging research field, with tremendous prospects for both fundamental research and technology. The investigation of attosecond electron dynamics in biomolecules has the potential to open new avenues in the field of biophysics, since the electron migration process in biomolecules forms the basis of fundamental reactions in biology.
Sub-5-fs pulses, with stable carrier-envelope phase and energy up to ~2.5 mJ, have been used for the production of attosecond pulses, by employing high-order harmonic generation in noble gases. We have implemented various techniques to confine the harmonic generation process to a single event. In particular we have experimentally demonstrated a temporal gating scheme for the generation of isolated attosecond pulses, based on the use of few-optical cycle driving pulses with peak intensity above the saturation intensity of the gas medium used to produce the XUV radiation. By using such ionization gating technique, sub-160-as pulses with ~10-nJ energy have been generated in xenon, corresponding to ~ 2-nJ pulse energy on target.
So far only a few applications of attosecond pulses to the investigation of ultrafast electron dynamics in molecules have been reported. During the project, attosecond pulses have been used for the investigation of ultrafast processes in small molecules and in complex biomolecules. As a first important example, we have studied the dissociative ionization of nitrogen molecules initiated by attosecond pulses, by using pump-probe measurements. We have evidenced a sub-cycle process of quantum interference between electron wave-packets generated by an attosecond pump pulse. Moreover, we have investigated the ultrafast dynamics of autoionization, an important many-body process related to electron correlations, which has been demonstrated to contribute to radiation damage in materials and biological systems.
For the first time, at the best of our knowledge, attosecond science has been extended towards complex biomolecules. The amino acid phenylalanine has been investigated by using the pump-probe technique. Charge dynamics has been initiated by isolated XUV sub-300-as pulses and it has been probed by 4-fs, waveform-controlled visible/near infrared (VIS/NIR) pulses. We have measured the yield for the production of doubly-charged immonium ions as a function of the time delay between the attosecond pump pulse and the VIS/NIR probe pulse (the immonium ion is formed by loss of the -COOH group). Charge migration has been clearly evidenced by oscillations in the yield of doubly-charged immonium ion as a function of the pump-probe delay. Two main frequencies were measured: 0.24 PHz (corresponding to a period of 4.2 fs) and 0.36 PHz (period of 2.8 fs), thus confirming the electronic origin of the measured dynamics. Numerical simulations of the temporal evolution of the electronic wave packet created by the attosecond pulse strongly support the interpretation of the experimental data in terms of charge migration.
The ability to initiate and observe purely electronic dynamics in an amino acid represents a crucial step forward in attosecond science and can be considered as a first contribution towards attobiology. The experimental observation of charge migration on an electron temporal scale can open new perspectives in the understanding of the physical processes governing electron transport mechanisms of biological signals. For instance, how the ultrafast motion of a hole in DNA created by a high-energy particle might initiate cell necrosis or mutation. Ultrafast charge migration is also the first step in many biological processes following a sudden energy deposition, either from light or electron and ion bombardment, like in photo- and hadron-therapy.
Sub-5-fs pulses, with stable carrier-envelope phase and energy up to ~2.5 mJ, have been used for the production of attosecond pulses, by employing high-order harmonic generation in noble gases. We have implemented various techniques to confine the harmonic generation process to a single event. In particular we have experimentally demonstrated a temporal gating scheme for the generation of isolated attosecond pulses, based on the use of few-optical cycle driving pulses with peak intensity above the saturation intensity of the gas medium used to produce the XUV radiation. By using such ionization gating technique, sub-160-as pulses with ~10-nJ energy have been generated in xenon, corresponding to ~ 2-nJ pulse energy on target.
So far only a few applications of attosecond pulses to the investigation of ultrafast electron dynamics in molecules have been reported. During the project, attosecond pulses have been used for the investigation of ultrafast processes in small molecules and in complex biomolecules. As a first important example, we have studied the dissociative ionization of nitrogen molecules initiated by attosecond pulses, by using pump-probe measurements. We have evidenced a sub-cycle process of quantum interference between electron wave-packets generated by an attosecond pump pulse. Moreover, we have investigated the ultrafast dynamics of autoionization, an important many-body process related to electron correlations, which has been demonstrated to contribute to radiation damage in materials and biological systems.
For the first time, at the best of our knowledge, attosecond science has been extended towards complex biomolecules. The amino acid phenylalanine has been investigated by using the pump-probe technique. Charge dynamics has been initiated by isolated XUV sub-300-as pulses and it has been probed by 4-fs, waveform-controlled visible/near infrared (VIS/NIR) pulses. We have measured the yield for the production of doubly-charged immonium ions as a function of the time delay between the attosecond pump pulse and the VIS/NIR probe pulse (the immonium ion is formed by loss of the -COOH group). Charge migration has been clearly evidenced by oscillations in the yield of doubly-charged immonium ion as a function of the pump-probe delay. Two main frequencies were measured: 0.24 PHz (corresponding to a period of 4.2 fs) and 0.36 PHz (period of 2.8 fs), thus confirming the electronic origin of the measured dynamics. Numerical simulations of the temporal evolution of the electronic wave packet created by the attosecond pulse strongly support the interpretation of the experimental data in terms of charge migration.
The ability to initiate and observe purely electronic dynamics in an amino acid represents a crucial step forward in attosecond science and can be considered as a first contribution towards attobiology. The experimental observation of charge migration on an electron temporal scale can open new perspectives in the understanding of the physical processes governing electron transport mechanisms of biological signals. For instance, how the ultrafast motion of a hole in DNA created by a high-energy particle might initiate cell necrosis or mutation. Ultrafast charge migration is also the first step in many biological processes following a sudden energy deposition, either from light or electron and ion bombardment, like in photo- and hadron-therapy.