Ultrafast relaxation processes of molecular highly excited states investigated with extreme-ultraviolet pulses

The understanding of ultrafast relaxation dynamics in highly excited states of molecules can shed light on many chemical reactions. These processes are difficult to observe because it is necessary to create an electronic wavepacket on few selected excited states and map its evolution on a femtosecond time scale. A possible approach is based on multiphoton absorption, but this scheme has the disadvantage of perturbing the system modifying the structure of the molecule. Another scheme is based on the use of an extreme ultra-violet (XUV) light source. Exploiting high harmonic generation in a noble gas, it is possible to obtain an attosecond pulse train composed by odd harmonics of the generating laser pulse (central energy = 1.55 eV) with corresponding photon energies in the XUV range. An attosecond pulse train will excite tens of electronic states making it impossible to identify the role played by each state [8]. At the same time to capture their dynamics are necessary XUV pulses with a comparable or shorter temporal duration. Previous studies [6,9,10] demonstrated that the relaxation processes of highly excited states are in the femtosecond temporal scale. To have a higher energetic resolution without degrading the temporal characteristics of the laser pulse a time-delay compensated monochromator (TDCM) able to select a single harmonic has been employed. The characterization of the XUV pulses has been performed by applying the FROG-CRAB technique, commonly used to characterize XUV attosecond pulses. We have employed for the first time extended ptychographic iterative engine to retrieve from a photoelectron spectrogram the temporal characteristics of the originating pulses. At the output of the TDCM, the XUV pulses have a temporal duration of few femtoseconds, the shortest ever achieved with this kind of setup. The first molecule we have studied is ethylene. The exact relaxation mechanism of the first excited states of ethylene cation was not clear and different theoretical interpretations have been proposed. To understand this mechanism, we have performed a series of experiments with different harmonics, respectively with the 9th, 11th and 13th harmonics, to selectively populate the ground state and different excited states of the cation. We have also performed the first observation of ultrafast relaxation and vibrational dynamics of superexcited and highly excited states of carbon dioxide. Superexcited states are neutral electronic states with internal energy higher than the ionization energy of the molecule or atom under investigation. There are very few theoretical and experimental studies on the ultrafast relaxation dynamics of superexcited states. The 11th harmonic, centered at 17 eV, populates different series of superexcited Rydberg states lying just below the first excited state of the cation. With a synchronized IR pulse, it is then possible to ionize the molecule. The same scheme was used to study the relaxation dynamics of the Rydberg states lying near to the ionization threshold. In this case, we have employed time-resolved photoelectrons spectroscopy to study the evolution of the excited electron wave packet. We also studied the relaxation processes and the dynamics of the carbon dioxide Rydberg states lying under the ionization potential using the 9th harmonic, centred at 14 eV.

Studio di processi di ultraveloci di stati molecolari altamente eccitati.