inverse photoemission spectroscopy

**IPES** (Inverse Photoemission Spectroscopy) is a **technique that probes empty electronic states above the Fermi level** — by injecting electrons into the sample and detecting the emitted photons as electrons decay into unoccupied states, providing the complementary information to UPS/XPS. **How Does IPES Work?** - **Electron Source**: Low-energy electron beam (5-30 eV) directed at the sample. - **Photon Detection**: Electrons occupy empty states and emit UV/visible photons. - **Unoccupied DOS**: The photon spectrum maps the unoccupied density of states (conduction band, LUMO levels). - **Combined**: UPS (occupied) + IPES (unoccupied) gives the complete electronic structure around $E_F$. **Why It Matters** - **Band Gap**: UPS + IPES directly measures the transport band gap (HOMO-LUMO gap for organics). - **LUMO Position**: Determines the electron affinity and LUMO position for organic semiconductors. - **Interface Alignment**: Complete band alignment at heterointerfaces (both VB and CB offsets). **IPES** is **the mirror of photoemission** — probing the empty states that electrons can flow into, completing the electronic structure picture.