【演講取消】
Abstract:
The unoccupied electronic states between the Fermi and the vacuum levels play a crucial role for photoexcited and catalytical properties of solids [1]. Conventionally the experimental access to these states with energy and momentum resolution includes time- and angle- resolved photoelectron spectroscopy (tr-ARPES) [2], inverse photoemission spectroscopy [3], scanning tunneling spectroscopy [4], and electron energy loss spectroscopy [5]. Among all these surface-sensitive spectroscopic tools, tr-ARPES relies on the application of ultrafast laser pulses, thereby providing information about the lifetime and the coherence of photoexcited electronic states on the femtosecond time scale.
As one branch of the tr-ARPES, time-resolved two-photon photoemission (tr-2PPE) experiments have been established several decades ago [6]. Starting from the earlier studies on metal surfaces [7], tr-2PPE has been significantly extended to semiconductors [8], topological insulators [9], as well as strongly correlated systems such as transition metal oxides [10] and high-temperature superconductors [11]. In strong contrast to the vast literature of tr-ARPES on 2D materials [12], there are much fewer tr-2PPE studies [13]. In this talk, an overview of tr-2PPE will be provided together with our recent development of static 2PPE experiments on NiTe2.