Periodic Reporting for period 1 - CHARISMA (CHARge transport in Intermediate-Sized Molecules on Attosecond time scales)
Periodo di rendicontazione: 2018-09-01 al 2020-08-31
Attosecond science has been a very active field of research for almost 20 years now and ultrafast electron dynamics in atoms and small benchmark molecules have drawn a lot of attention. Typically, pump-probe experiments are performed, in which an attosecond pulse in the extreme ultraviolet (XUV) generated by high harmonic generation induces ultrafast photoionization dynamics, which are probed in a delayed femtosecond infrared (IR) field coherent with the pump pulse, since XUV+XUV pump/probe schemes are not yet feasible for a wide range of applications. The choice of observables, in which the ultrafast dynamics are encoded, is however not straightforward especially for complex targets. In addition, the full theoretical description of electron dynamics on the attosecond time scale and the coupling to nuclear motion requires high computational costs and are thus not yet applicable for large target systems. However, there is a huge interest in the physics and chemistry community to push the boundaries in attosecond science towards more complex molecules to understand the underlying processes and ultimately gain control over the electron dynamics, which control the outcome of a photoionization reaction. This might make it possible to steer the latter with an unprecedented precision.
Upon photoionization with an attosecond pulse in the XUV, a molecule is ionized into a coherent superposition of ionic states, which can lead to ultrafast oscillations of the charge density (charge migration) along the molecular backbone before nuclear dynamics set in after few femtoseconds. These coupled electron-nuclear dynamics can then lead to charge localization and finally induce a fragmentation. Only few experimental studies report on the observation of charge migration in molecules. CHARISMA aimed at gaining deeper insights how to study these ultrafast dynamics experimentally at the well-equipped attosecond laboratory at Politecnico di Milano, Italy. The focus was on intermediate-sized molecules of 10-20 atoms with structures found similarly in more complex molecules of biological-relevance, such as amino acids and proteins, and DNA bases.
Upon photoionization with an attosecond pulse in the XUV, a molecule is ionized into a coherent superposition of ionic states, which can lead to ultrafast oscillations of the charge density (charge migration) along the molecular backbone before nuclear dynamics set in after few femtoseconds. These coupled electron-nuclear dynamics can then lead to charge localization and finally induce a fragmentation. Only few experimental studies report on the observation of charge migration in molecules. CHARISMA aimed at gaining deeper insights how to study these ultrafast dynamics experimentally at the well-equipped attosecond laboratory at Politecnico di Milano, Italy. The focus was on intermediate-sized molecules of 10-20 atoms with structures found similarly in more complex molecules of biological-relevance, such as amino acids and proteins, and DNA bases.
For studying the gas phase photoionization dynamics in molecules which are solid at room temperature, oven sources are commonly employed. Upon heating, however, the molecules can take different conformations and be excited in many different vibrational and rotational states. Therefore, it is advantageous to pick up the hot gas phase molecules by a supersonic cold jet of a rare gas, e.g. He or Ar. Collisions between the molecules and the rare gas atoms lead to an efficient cooling and the jet transports the molecules into the interaction region with the ionizing light. Two oven source designs were adapted to the setup of the ELYCHE high harmonic generation (HHG) beamline. One embedded the molecular powder in a graphite matrix for reducing the risk of self-condensation, in the second one the molecules were heated directly. The oven source was coupled to a pulsed solenoid valve, synchronized with the laser, through which the supersonic jet of the carrier gas was produced. A data acquisition program for recording time-of-flight mass spectra was furthermore developed and both oven source designs were tested successfully yielding similar results. They proved to be perfectly suited for transferring solid intermediate-sized molecules intact into the gas phase. It was taken care that the experiment can be switched easily between electron detection, helpful for time delay calibration, and ion detection and a highly versatile setup for gas phase attosecond spectroscopy was therefore built.
While attosecond experiments offer naturally an unprecedented time-resolution, the bandwidth of ultrafast pulses is intrinsically large. That is why in CHARISMA stationary high-resolution synchrotron experiments were performed to gain insights into the photoionization and fragmentation of potential target molecules. Threshold photoelectron-photoion coincidence (TPEPICO) experiments were performed on the VUVbeamline of the Swiss Light Source. Here, for each photoionization event producing a photoelectron the corresponding ion fragment can be assigned. By scanning the photon energy, high-resolution threshold photoelectron spectra revealing the ionization energies of different electronic states can be recorded, and the appearance energies of fragment ions can be determined state-selectively. This helps understanding which state contributes to the formation of an ionic fragment and if it is suited as an observable in an ultrafast experiment. Due to their similar structure to aromatic amino acids and proteins, respectively, the photoionization of the three cresol (hydroxytoluene) isomers and N-methyl acetamide were investigated. High quality experimental data were obtained, and the threshold photoelectron spectra and breakdown curves were interpreted with the help of quantum chemistry and statistical rate theory. A publication on these data will follow soon after the end of the project.
It has been shown previously that charge migration in a molecule can be tracked by ultrafast oscillations when recording the yield of a certain ionic fragment as a function of the pump-probe delay. In CHARISMA the new oven source and the knowledge on the steady state synchrotron photoionization of the potential target molecules were the pillars for conducting such experiments. The basis, however, is a well characterized attosecond light source. This characterization was done by performing angle-resolved attosecond spectroscopy in rare gases employing RABBITT (reconstruction of attosecond beatings by two-photon transitions) and quantum beat spectroscopy. These experiments served on one hand for benchmarking the beamline but introduced some original aspects due to the angular resolution, too. The rare gas experiments revealed several issues with the performance of the laser, which were solved during the project. This led to some delay in the work plan, but the performance could ultimately be improved significantly by re-designing some key elements of the beamline. The results for the rare gas experiments were finally of good quality and a publication will follow. The focus was then shifted to molecules and promising preliminary results were obtained for several target species. However, the Covid-19 lockdown in Italy reduced the effective experimental time of the CHARISMA project significantly and impacted the stability of the laser system, which reacted very sensible to such an unusually long downtime. The goal of CHARISMA to resolve charge migration in molecules has not yet been achieved. The researcher will continue to reach those in a future collaboration with the host and the improved experimental setup will guarantee high-quality results.
While attosecond experiments offer naturally an unprecedented time-resolution, the bandwidth of ultrafast pulses is intrinsically large. That is why in CHARISMA stationary high-resolution synchrotron experiments were performed to gain insights into the photoionization and fragmentation of potential target molecules. Threshold photoelectron-photoion coincidence (TPEPICO) experiments were performed on the VUVbeamline of the Swiss Light Source. Here, for each photoionization event producing a photoelectron the corresponding ion fragment can be assigned. By scanning the photon energy, high-resolution threshold photoelectron spectra revealing the ionization energies of different electronic states can be recorded, and the appearance energies of fragment ions can be determined state-selectively. This helps understanding which state contributes to the formation of an ionic fragment and if it is suited as an observable in an ultrafast experiment. Due to their similar structure to aromatic amino acids and proteins, respectively, the photoionization of the three cresol (hydroxytoluene) isomers and N-methyl acetamide were investigated. High quality experimental data were obtained, and the threshold photoelectron spectra and breakdown curves were interpreted with the help of quantum chemistry and statistical rate theory. A publication on these data will follow soon after the end of the project.
It has been shown previously that charge migration in a molecule can be tracked by ultrafast oscillations when recording the yield of a certain ionic fragment as a function of the pump-probe delay. In CHARISMA the new oven source and the knowledge on the steady state synchrotron photoionization of the potential target molecules were the pillars for conducting such experiments. The basis, however, is a well characterized attosecond light source. This characterization was done by performing angle-resolved attosecond spectroscopy in rare gases employing RABBITT (reconstruction of attosecond beatings by two-photon transitions) and quantum beat spectroscopy. These experiments served on one hand for benchmarking the beamline but introduced some original aspects due to the angular resolution, too. The rare gas experiments revealed several issues with the performance of the laser, which were solved during the project. This led to some delay in the work plan, but the performance could ultimately be improved significantly by re-designing some key elements of the beamline. The results for the rare gas experiments were finally of good quality and a publication will follow. The focus was then shifted to molecules and promising preliminary results were obtained for several target species. However, the Covid-19 lockdown in Italy reduced the effective experimental time of the CHARISMA project significantly and impacted the stability of the laser system, which reacted very sensible to such an unusually long downtime. The goal of CHARISMA to resolve charge migration in molecules has not yet been achieved. The researcher will continue to reach those in a future collaboration with the host and the improved experimental setup will guarantee high-quality results.
CHARISMA exploited the complementarity of advanced light sources in the XUV, which offer high spectral or temporal resolution, respectively, to get a broad understanding of the photoionization in intermediate sized molecules. This will ignite further activities in this promising field of research and push the boundaries of attosecond science towards attochemistry and in the future to applications in fields like medicine and energy conversion.