Melamine and melem molecules are widely used precursors for synthesizing graphitic carbon nitride (g-C3N4), the latter also a hot two-dimensional material with photocatalytic applications. The molecular structures of both are respectively identical to the repeating units of two distinct g-C3N4 phases. In this work, the adsorption and self-assembly of melamine and melem on an Au(111) surface were investigated with low-temperature scanning tunneling microscopy (STM). Particularly, the patterns of hydrogen bonds (HBs) in their assemblies were identified and compared. It was found that melamine can only form one type of HB and two kinds of assembly structures, whereas melem can form three types of HBs and six kinds of assembly structures in total. Moreover, the involved HBs can be transformed by tip manipulation. These findings may provide a new strategy for tuning the functionality of surface self-assemblies by manipulating intermolecular hydrogen bonds. This also paves a route for the in situ synthesis of g-C3N4 on metallic surfaces and subsequent investigations of their physicochemical properties.
: Femtosecond time-resolved multiphoton ionization dynamics of xenon was investigated using a homemade ion imaging detector. A comparison experiment comprising of the multiphoton ionization of Xe at 408 nm showed that the energy resolution of our homemade image detector was similar to that of a commercial detector. Under 272 nm femtosecond laser irradiation, photoelectrons with a kinetic energy of 1.57 and 0.26 eV, produced by three-photon ionization, corresponded to two different Xe + spin states, respectively. For the ionization at 408 nm, an additional first-order above-threshold ionization of Xe was also observed. In the two-color femtosecond time-resolved experiments, the photoelectron kinetic energy spectra varied with the delay time between the pump and the probe. The photoelectron intensities produced by 3+1' and 4'+1 two-color multiphoton ionization schedules became stronger with an increase in the degree of overlap between the two laser beams. The kinetic energy of the photoelectrons produced by one-color multiphoton ionization showed obvious red shifts, which were modulated by the second laser beam. Depopulation of the excited states was also observed upon application of the second laser beam. The red shifts in the photoelectron kinetic energy spectra reflect the time-dependent dynamical modulation process of the laser induced ponderomotive effect in an atomic system.
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