two-photon photoemission

**2PPE** (Two-Photon Photoemission) is a **surface-sensitive technique that uses two sequential photons to eject an electron** — the first photon excites an electron to an intermediate state, and the second photon ejects it into vacuum, probing both occupied and unoccupied states as well as electron dynamics. **How Does 2PPE Work?** - **First Photon (Pump)**: Excites an electron from an occupied state to an intermediate unoccupied state. - **Second Photon (Probe)**: Ejects the electron from the intermediate state into vacuum for detection. - **Time-Resolved**: Varying the pump-probe delay measures the lifetime of intermediate states (fs-ps resolution). - **Geometry**: Angle-resolved 2PPE maps the band structure of both occupied and unoccupied states. **Why It Matters** - **Hot Carrier Dynamics**: Directly measures the lifetime and relaxation of excited electrons at surfaces. - **Image Potential States**: Probes image potential states and surface states that are inaccessible to UPS or IPES. - **Femtosecond Resolution**: Time-resolved 2PPE reveals electron dynamics on the femtosecond timescale. **2PPE** is **photoemission with a two-step ladder** — using two photons to access and time-resolve intermediate electronic states at surfaces.