The target of this laboratory is the generation and application of tunable extreme ultraviolet (XUV) pulses for the study of the electron-nuclear coupled dynamics in different types of molecules. The high temporal resolution of the beamline (down to sub-10 fs) allows us to explore a plethora of processes (internal conversion, charge transfer…) taking place inmediately after light-matter interaction that determine the response of the molecule in longer timescales.
The aim of this laboratory is the experimental investigation, with femtosecond temporal resolution, of ultrafast processes in atoms and molecules excited by pulses in extreme ultraviolet spectral range (XUV) generated through the High-order Harmonic Generation process. A single harmonic is selected with a monochromator compensated in time (that allows for preserving the temporal duration of the given harmonic) characterized by a high photon flux and good spectral tunability. Such pulses are used to study ultrafast processes in atoms and molecules with pump-probe techniques.
LASER SOURCE
The driving source is a Ti:sapphire amplified laser system (Coherent Astrella) which generates 35 fs pulses with an energy of 7 mJ and repetition rate of 1 kHz. A fraction of the amplified femtosecond laser (1.4 mJ energy per pulse) is then sent to a hollow fiber compressor for the generation of sub-10-fs fs pulses with an energy of 0.9 mJ. The remaining beam is used for the generation of ultrashort pulses in the ultraviolet spectral region by means of non-linear processes.
UV PUMP – XUV PROBE BEAMLINE
The UV pump – monchromatized XUV probe beamline.
The TDCM allows to generate tuneable monochromatic XUV light with pulse duration as short as 5 fs as demonstrated by XUV-IR sideband experiments.
Extreme ultraviolet (XUV) pulses, generated through the High-order Harmonic Generation process in noble gases, are spectrally selected with a time-delay compensated monochromator (TDCM). The TDCM employs conical diffraction, where wavevectors of incident and diffracted waves are almost parallel to the grating stripes. Such conical diffraction allows for a considerable improvement in terms of transmission efficiency with respect to traditional methods. With this method it is possible to get tunable pulses in a wide spectral region, with duration down to 5 fs and high photon flux.
The UV spectral tuneability. Sub-20-fs pulses are generated across the entire 255-285 nm bandwidth, as demonstrated by DFG-XFROG experiments.
UV pulses with 𝜇J energy level are generated by frequency up-conversion between the ultrashort visible/infrared pulses and narrow-band pulses in the visible, obtained directly from the frequency-doubled Ti:sapphire system. Both beams are focused noncollinearly on a 20 𝜇m Type-I BBO crystal for SFG. The resulting UV pulses have energy up to 7 𝜇J, spectral tunability between 255 nm and 285 nm and a retrieved pulse duration below 20 fs.
TIME-RESOLVED PHOTOELECTRON SPECTROSCOPY
The laboratory is equipped with several instruments for gas-phase spectroscopy, such as a Time of Flight spectrometer (ToF) for electrons and ions and a Velocity-Map Imaging spectrometer (VMI).
UV-XUV sideband experiments in noble gases allow to assess the overall temporal resolution of the experiment.
In-situ characterization of the XUV and UV pulses is performed by a two-color photoelectron cross-correlation in argon: the sideband (SB) signal resulting from XUV-UV photoionization contains information on the spectral phase of both pulses and allows for an unambiguous characterization of the instrument response function. We obtain a final temporal resolution of 20 fs.
As an example of the capability of the experimental setup, here we show the tr-PES trace of 1,3-cyclohexadiene. The ultrafast dynamics are induced by the UV pump pulse and probed by the XUV pulse. The resulting experimental photoelectron spectrogram shows the details of the relaxation of the system and enables to follow the different stages of the photochemical process.
TR-PES in 1,3-cyclohexadiene.
TIME- AND ANGLE- RESOLVED PHOTOEMISSION SPECTROSCOPY
The ARPES station delivered by SPECS.
The laboratory is also equipped with a new setup (from SPECS) for Angular Resolved Photoelectron Spectroscopy (ARPES), which will enable time- and angle- resolved measurements of solid-state samples and molecules deposited on substrates. The ARPES station features a PHOIBOS 150 WAL analyzer. It also includes a preparation chamber for the evaporation and annealing of solid-state samples and for the characterization of their crystalline quality through a Low Energy Electron Diffraction (LEED) apparatus. UV and X-ray sources for performing static ARPES for characterization purposes are also available.
As an example of the capabilities of the apparatus, we show a LEED image obtained for a graphene sample, which shows its hexagonal crystal structure, as well as a measurement of the band structure in proximity of its K point, which exhibits a distinctive Dirac cone feature.
Graphene LEED measured with the SPCS apparatus.
Graphene Dirac cone measured with the SPECS apparatus.
Recent publications:
- A. Crego, S. Severino, L. Mai, F. Medeghini, F. Vismarra, F. Frassetto, L. Poletto, M. Lucchini, M. Reduzzi, M. Nisoli, R. Borrego-Varillas “Sub-20-fs UV-XUV beamline for ultrafast molecular spectroscopy“, Scientific Reports 14, 1-9 (2024)