Time-resolved measurement of interatomic Coulombic decay induced by two-photon double excitation of Ne₂

On the 24th of March 2017, Tsukasa Takanashi gained his doctorate from the University of Tohoku (Japan), together with the President’s Award prize (総長賞). The prize is awarded each year to the best PhD students in recognition of their outstanding academic curriculum, and particularly for the excellent results obtained during their studies. Tsukasa carried out his studies under the supervision of Professor Kiyoshi Ueda, a leading figure on the international scene of atomic and molecular physics, and until recently, a member of the FERMI Review Panel. In his thesis, Tsukasa used the light from Free Electron Lasers (FELs) to study the dynamics of highly excited molecular systems; in his home country, he utilized the Japanese FEL SACLA, and he studied the Coulomb explosion of the molecule CH₂I₂ (diiodomethane). This process is the fragmentation by multiple ionization of a sample, and the successive repulsion of the ions by the positive charge which is generated.
An important part of his work was carried out at FERMI, currently the only FEL source in the world able to provide Tsukasa the wavelength (75.6 nm) and temporal resolution (10^-13 s) necessary to study the dynamics of his system: the Ne₂ molecule, which consists of two neon atoms bound by their weak van der Waals interaction. The apparent simplicity of this system allows the detailed study of complex phenomena, such as the exchange of energy after electronic excitation, which is basic to all photochemical processes. The experiment was carried out at the LDM (Low Density Matter) beamline of FERMI, where the expansion of neon gas into vacuum under controlled conditions produces about 1% of the gas in the form of Ne₂ dimers, which have well-defined energy states and sharp absorption resonances. In the presence of a very intense pulse of light at the required wavelength of 75.6 nm, which only FERMI is able to generate, the dimers absorb two quanta (photons) of light, and two electrons, each mainly localized on one of the two atoms, are promoted to an excited level (figure 1, above). Left to themselves, the two electrons exchange energy, and so one returns to the ground state, and the other is emitted, leaving an ionized molecule (Ne₂⁺), whose presence is detected and quantified using a mass spectrometer. The system has been carefully chosen so that of all the many channels for decay normally possible in a molecule, only the one just described (called Interatomic Coulombic Decay, ICD) is accessible, and so can be studied without interference from other processes, and compared to theoretical models. The goal of the experiment was to measure the exact time in which the ICD process occurred (a few hundred femtoseconds). For this, a probe was used consisting of a pulse of ultraviolet light (260 nm) which arrived a precise time after the FEL pulse and fragmented the excited molecule and caused a tiny decrease of the signal from Ne₂⁺. Comparing measurements repeated at different times with a theoretical model allows the determination of the lifetime of the ICD process (see figure 2 for details).