Periodic Reporting for period 1 - TD-PICO-MF (Time-Delay PhotoIonization of Core-Orbitals in the Molecular Frame)
Reporting period: 2023-03-15 to 2025-03-14
Summary of the context and overall objectives of the project
The recent development of broadband coherent extreme ultraviolet sources makes it possible to measure dynamics with attosecond resolution (1 attosecond = 10^-18 s). This time scale is associated with the electron dynamics and more particularly to the electronic correlation in various systems, from atoms and molecules to solids.
The investigation of the electronic dynamics of atomic and molecular systems requires remarkably precise tools in order to follow the motion of the electronic wave packets. The corresponding timescale is often related, in Bohr’atomic theory, to the time for a ground state electron in a hydrogen atom to accomplish a full revolution around the nucleus, namely 150 attoseconds. This time scale was experimentally reached at the beginning of the 21st century when light pulses with durations of few hundreds of attosecond and photon energies ranging from few eV’s to hundreds of eV’s were generated through a physical process termed high-harmonic generation (HHG). These novel pulses triggered the emergence of the attosecond research field, which aimed at unravelling the physical processes that are occurring within this time and energy scale. In addition to the fundamental interest to study ultrafast dynamics triggered/probed by the interaction of photons with matter, the attosecond domain has also opened up the possibility to directly control the rapid electronic motion inside a molecule, as well as the subsequent nuclear dynamics such as bond breaking and/or formation giving promising opportunities for future applications in chemistry and biology.
These ultrashort light pulses are described as a series of XUV attosecond pulses, and are referred to as Attosecond Pulses Train (APT). They are generated by focusing an intense laser field (10^14W/cm2) on an atomic gas target. The non-linear electronic response of the medium causes the generation of the APT, which is described in the frequency domain as a comb of odd-order harmonics of the generating field. When the generating laser field duration is sufficiently short (few cycles), only an isolated attosecond pulse is created.
By photoionizing a target gas with attosecond pulses combined with a phase locked infrared (IR) field, it is possible to retrieve fundamental quantities such as the delay of photoemission of photoelectrons from different atomic orbitals. In fact, the delay of photoemission is an extremely valuable observable in the identification and the study of autoionizing states, Cooper minima and shape resonances.
One of the major techniques to access the delay of photoemission is named RABBIT. In this case, the dressing IR field used is at low intensity (10^11W/cm2) and has the same frequency as the field used to generate the harmonics. Scanning the delay between the IR pulse and the XUV APT shows the formation and disappearance of energy channels named sidebands that oscillates at twice the frequency of the IR pulse (Figure 1 (a)). These sidebands arise from two-photon transitions (XUV±IR) and their oscillations are theoretically described as the interference between two quantum paths: the first with the absorption of an XUV photon and the emission of an IR photon and the second with the absorption of one XUV and one IR photon. Each sideband thus gives access to the spectral variation of the scattering phase, directly related to the delay of photoemission (given by the spectral derivative of the scattering phase).
The vast majority of RABBIT studies have focused on atomic systems, which hold spherical symmetry with respect to the probing process. It is only over the past few years that studies were directed toward molecular systems expanding the field toward physical chemistry. To that regard, the HO initiated this trend toward molecular systems with the N2 molecule. Molecular photoionization in general presents much richer phenomena compared to atomic systems as the probing process depends on the relative alignment of the molecule with respect to the XUV polarization axis. Yet, most RABBIT studies have been performed in randomly aligned molecules. In this arrangement, the measured quantities only represent an average over all possible orientations, thus losing most of the structural information of the molecule under study. This restriction can be lifted in some special cases involving dissociative ionization using coincidence detection in a COLTRIMS to retrieve the molecular frame. However, the latter can only be retrieved for fast dissociative ionization process such that the initial structure can be correctly inferred. Another way to access the molecular frame is achieved with the prealignment of the molecular system prior to the ionization step. It then becomes possible to investigate the ionization process directly in the molecular frame from any accessible cationic states. Molecular alignment can be achieved through impulsive alignment by a weak non-resonant light pulse and is now a recognized technique to access the molecular frame. The electric field of the laser pulse interacts with the induced dipole of the molecule producing a torque on the molecule described as the formation of a rotational wave packet. In the case of linear and symmetric top molecules, it leads to revivals of the alignment after the laser pulse at time delays related to the moment of inertia of the system. By implementing this methodology, 3D-angularly resolved photodetection scheme experiments within the molecular frame in a field-free environment can be achieved.
The TD-PICO-MF project aims at developing a detailed understanding of electronic correlation effects in photoemission by mapping the molecular cationic potential experienced by the electron during its scattering. The approach relies on the combination of attosecond photoionization of an atomic core-orbital with laser-induced control of molecular systems in order to resolve the photoemission process both spatially and temporally. By merging these two domains, both the electron’s initial localization and final momentum are known with respect to the molecular axis with an attosecond time resolution. Therefore, a global picture of the evolution of the cationic molecular potential during a chemical reaction becomes accessible.
In particular, the research project proposes to study, as a prototype, the photoemission from the 4d core-orbitals of the iodine atom in iodine monochloride (ICl) molecules in gas phase. The measurements will be realized during different laser controlled configurations of the molecular system e.g. during its alignment, orientation and dissociation. The expected impact is to gain a global picture of the influence of the molecular cationic potential during a chemical reaction and to evidence the role of electronic correlation through the measurement of 3D-scattering phase maps in the molecular frame.
The investigation of the electronic dynamics of atomic and molecular systems requires remarkably precise tools in order to follow the motion of the electronic wave packets. The corresponding timescale is often related, in Bohr’atomic theory, to the time for a ground state electron in a hydrogen atom to accomplish a full revolution around the nucleus, namely 150 attoseconds. This time scale was experimentally reached at the beginning of the 21st century when light pulses with durations of few hundreds of attosecond and photon energies ranging from few eV’s to hundreds of eV’s were generated through a physical process termed high-harmonic generation (HHG). These novel pulses triggered the emergence of the attosecond research field, which aimed at unravelling the physical processes that are occurring within this time and energy scale. In addition to the fundamental interest to study ultrafast dynamics triggered/probed by the interaction of photons with matter, the attosecond domain has also opened up the possibility to directly control the rapid electronic motion inside a molecule, as well as the subsequent nuclear dynamics such as bond breaking and/or formation giving promising opportunities for future applications in chemistry and biology.
These ultrashort light pulses are described as a series of XUV attosecond pulses, and are referred to as Attosecond Pulses Train (APT). They are generated by focusing an intense laser field (10^14W/cm2) on an atomic gas target. The non-linear electronic response of the medium causes the generation of the APT, which is described in the frequency domain as a comb of odd-order harmonics of the generating field. When the generating laser field duration is sufficiently short (few cycles), only an isolated attosecond pulse is created.
By photoionizing a target gas with attosecond pulses combined with a phase locked infrared (IR) field, it is possible to retrieve fundamental quantities such as the delay of photoemission of photoelectrons from different atomic orbitals. In fact, the delay of photoemission is an extremely valuable observable in the identification and the study of autoionizing states, Cooper minima and shape resonances.
One of the major techniques to access the delay of photoemission is named RABBIT. In this case, the dressing IR field used is at low intensity (10^11W/cm2) and has the same frequency as the field used to generate the harmonics. Scanning the delay between the IR pulse and the XUV APT shows the formation and disappearance of energy channels named sidebands that oscillates at twice the frequency of the IR pulse (Figure 1 (a)). These sidebands arise from two-photon transitions (XUV±IR) and their oscillations are theoretically described as the interference between two quantum paths: the first with the absorption of an XUV photon and the emission of an IR photon and the second with the absorption of one XUV and one IR photon. Each sideband thus gives access to the spectral variation of the scattering phase, directly related to the delay of photoemission (given by the spectral derivative of the scattering phase).
The vast majority of RABBIT studies have focused on atomic systems, which hold spherical symmetry with respect to the probing process. It is only over the past few years that studies were directed toward molecular systems expanding the field toward physical chemistry. To that regard, the HO initiated this trend toward molecular systems with the N2 molecule. Molecular photoionization in general presents much richer phenomena compared to atomic systems as the probing process depends on the relative alignment of the molecule with respect to the XUV polarization axis. Yet, most RABBIT studies have been performed in randomly aligned molecules. In this arrangement, the measured quantities only represent an average over all possible orientations, thus losing most of the structural information of the molecule under study. This restriction can be lifted in some special cases involving dissociative ionization using coincidence detection in a COLTRIMS to retrieve the molecular frame. However, the latter can only be retrieved for fast dissociative ionization process such that the initial structure can be correctly inferred. Another way to access the molecular frame is achieved with the prealignment of the molecular system prior to the ionization step. It then becomes possible to investigate the ionization process directly in the molecular frame from any accessible cationic states. Molecular alignment can be achieved through impulsive alignment by a weak non-resonant light pulse and is now a recognized technique to access the molecular frame. The electric field of the laser pulse interacts with the induced dipole of the molecule producing a torque on the molecule described as the formation of a rotational wave packet. In the case of linear and symmetric top molecules, it leads to revivals of the alignment after the laser pulse at time delays related to the moment of inertia of the system. By implementing this methodology, 3D-angularly resolved photodetection scheme experiments within the molecular frame in a field-free environment can be achieved.
The TD-PICO-MF project aims at developing a detailed understanding of electronic correlation effects in photoemission by mapping the molecular cationic potential experienced by the electron during its scattering. The approach relies on the combination of attosecond photoionization of an atomic core-orbital with laser-induced control of molecular systems in order to resolve the photoemission process both spatially and temporally. By merging these two domains, both the electron’s initial localization and final momentum are known with respect to the molecular axis with an attosecond time resolution. Therefore, a global picture of the evolution of the cationic molecular potential during a chemical reaction becomes accessible.
In particular, the research project proposes to study, as a prototype, the photoemission from the 4d core-orbitals of the iodine atom in iodine monochloride (ICl) molecules in gas phase. The measurements will be realized during different laser controlled configurations of the molecular system e.g. during its alignment, orientation and dissociation. The expected impact is to gain a global picture of the influence of the molecular cationic potential during a chemical reaction and to evidence the role of electronic correlation through the measurement of 3D-scattering phase maps in the molecular frame.
Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far
To achieve the best conditions to control the molecule with laser pulses, we improved the experimental setup. This was done through two aspects, experimental and software development.
On the experimental side, we first designed and installed a new vacuum chamber to separate the injection of gas from the detection region. This was done to reduce the background signal and to properly select the coldest part of the molecular beam. We then added a pulsed valve in it to achieve an efficient adiabatic gas expansion, a required condition for a proper cooling of the molecule. To reduce the disturbance of the magnetic field on the photoelectron trajectories from the other devices in the room and the earth, we added a double magnetic shielding around the velocity map imaging spectrometer.
In addition, the RABBIT measurements can last for hours as the signal level is fairly low. For this reason, it is necessary to make the experimental setup as stable as possible. For this purpose, we added an active stabilization scheme to control with an accuracy <100as the relative delay between our laser pulses.
Moreover, the signal of interest can be hidden behind background. To circumvent this issue and properly isolate the signal of interest, we implemented a chopper in the acquisition scheme to alternatively block and unblock one of the laser pulses. By comparing the measurements with and without the presence of the laser pulse, we can effectively remove the undisturbed contributions and properly select the signal originating from the conjoint action of the two laser pulses.
We also worked on software development for parallel acquisition in multiple spectrometers. This approach permits to measure synchronously in different gases and thus serves as a ultimate reference tool. The possibility to measure in two magnetic bottle electron spectrometers is now possible and has recently been tested.
In the mean time, the activities performed focused on the application of the RABBIT methodology on 4d core orbital ionization of iodine compounds. These activities have been performed in a magnetic bottle electron spectrometer which provides an angularly integrated information. For this purpose, an APT with photon energy ranging from 60 to 100 eV has been generated by focusing an intense infrared laser pulse in a cell filled with Neon gas and spectrally filtered with a Zr filter. By spatially and temporally overlapping a weak additional infrared laser pulse in the spectrometer with the attosecond pulses , RABBIT measurements were performed with an active stabilisation scheme. The chopper was then used to altenatively block the weak infrared laser pulse, providing very clean transient RABBIT spectrograms.
On the experimental side, we first designed and installed a new vacuum chamber to separate the injection of gas from the detection region. This was done to reduce the background signal and to properly select the coldest part of the molecular beam. We then added a pulsed valve in it to achieve an efficient adiabatic gas expansion, a required condition for a proper cooling of the molecule. To reduce the disturbance of the magnetic field on the photoelectron trajectories from the other devices in the room and the earth, we added a double magnetic shielding around the velocity map imaging spectrometer.
In addition, the RABBIT measurements can last for hours as the signal level is fairly low. For this reason, it is necessary to make the experimental setup as stable as possible. For this purpose, we added an active stabilization scheme to control with an accuracy <100as the relative delay between our laser pulses.
Moreover, the signal of interest can be hidden behind background. To circumvent this issue and properly isolate the signal of interest, we implemented a chopper in the acquisition scheme to alternatively block and unblock one of the laser pulses. By comparing the measurements with and without the presence of the laser pulse, we can effectively remove the undisturbed contributions and properly select the signal originating from the conjoint action of the two laser pulses.
We also worked on software development for parallel acquisition in multiple spectrometers. This approach permits to measure synchronously in different gases and thus serves as a ultimate reference tool. The possibility to measure in two magnetic bottle electron spectrometers is now possible and has recently been tested.
In the mean time, the activities performed focused on the application of the RABBIT methodology on 4d core orbital ionization of iodine compounds. These activities have been performed in a magnetic bottle electron spectrometer which provides an angularly integrated information. For this purpose, an APT with photon energy ranging from 60 to 100 eV has been generated by focusing an intense infrared laser pulse in a cell filled with Neon gas and spectrally filtered with a Zr filter. By spatially and temporally overlapping a weak additional infrared laser pulse in the spectrometer with the attosecond pulses , RABBIT measurements were performed with an active stabilisation scheme. The chopper was then used to altenatively block the weak infrared laser pulse, providing very clean transient RABBIT spectrograms.
Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)
RABBIT measurements have been achieved for the first time in 4d ionization of iodine compounds using an APT with photon energy ranging from 60 to 100 eV . These measurements were first done on Neon gas to serve as a reference and repeated in two iodine compounds, CH3I and C3H7I. The photoelectron spectrum from the interaction between the XUV pulses and the molecules is shown in Photoelectron spectrum.png. We are able to identify the two spin-orbit components (3/2 and 5/2) from the ionization of 4d level of iodine in the region from 20 eV to 45 eV. However, the infrared laser pulse leads to a spectral overlap of the two contributions and only the 5/2 component could effectively be resolved. Nevertheless, thanks to the implementation of the chopper, we can effectively observe the RABBIT oscillations both for photoelectrons coming from the valence and from the 4d shells. As an example, the RABBIT trace for C3H7I is displayed in RABBIT.png with the transient signal is highlighted in red and blue for the sideband and harmonic lines, respectively.
Additional experiments are currently ongoing to respectively measure Neon and iodine compounds in two different spectrometers coincidentally. This will permit to obtain an absolute reference on the relative phases/delays of the electron wave packets during ionization.
Additional experiments are currently ongoing to respectively measure Neon and iodine compounds in two different spectrometers coincidentally. This will permit to obtain an absolute reference on the relative phases/delays of the electron wave packets during ionization.