Transient evolution of quasifree electrons of plasma in liquid water revealed by optical-pump terahertz-probe spectroscopy

Abstract

The fundamental properties of laser-induced plasma in liquid water, such as the ultrafast electron migration and solvation, have not yet been clarified. We use 1650-nm femtosecond laser pulses to induce the plasma in a stable free-flowing water film under the strong field ionization mechanism. Moreover, we adopt intense terahertz (THz) pulses to probe the ultrafast temporal evolution of quasifree electrons of the laserinduced plasma in water on the subpicosecond scale. For the first time, the THz wave absorption signal with a unique two-step decay characteristic in time domain is demonstrated, indicating the significance of electron solvation in water. We employ the Drude model combined with the multilevel intermediate model and particlein-a-box model to simulate and analyze the key information of quasifree electrons, such as the frequencydomain absorption characteristics and solvation ratio. In particular, we observe that the solvation capacity of liquid water decreases with the increase of pumping energy. Up to ∼50% of quasifree electrons cannot be captured by traps associated with the bound states as the pumping energy increases to 90 μJ∕pulse. The ultrafast electron evolution in liquid water revealed by the optical-pump/THz-probe experiment provides further insights into the formation and evolution mechanisms of liquid plasma.

The birth and evolution of solvated electrons in the water

“…When the intensity of the visible electromagnetic radiation is high enough, photoionization of water molecules can occur in the liquid phase. As has been confirmed in many experimental and theoretical studies, the fully relaxed, solvated, or hydrated aqueous electrons are generated in less than ( 6 – 13 ). In previous papers, it was proposed that the electronic charge density is localized in a cavity with a radius of ( 14 – 16 ) and surrounded by water molecules ( 17 , 18 ), two of which are hydrogen bonded to the electron and the other two participating in the water–water network.…”

Hot Carrier Transfer in PtSe2/Graphene Enabled by the Hot Phonon Bottleneck

“…The DS model is given by , where ω p 2 = e 2 N /ε 0 m * is the plasma frequency, N is the charge carrier density, m * is the carrier effective mass, ε 0 is the vacuum permittivity, and τ is the electron momentum relaxation time. The Smith parameter c denotes the backscattering parameter constant (between −1 and 0), where c = −1 corresponds to full backscattering and c = 0 implies full momentum random scattering, and eq reduces to the classic Drude formula. , Panels a and b of Figure summarize the results of the DS model fitting the frequency-dependent complex conductivity of the P/G heterostructure at 0 and 2 ps delay times with 1300 nm excitation, respectively (the fitting results are listed in part 3 of the Supporting Information).…”