ultraviolet photoelectron spectroscopy

**UPS** (Ultraviolet Photoelectron Spectroscopy) is a **surface technique that uses UV light (typically He I at 21.2 eV or He II at 40.8 eV) to eject valence electrons** — mapping the valence band density of states, work function, and ionization energy with extreme surface sensitivity (~0.5-1 nm). **How Does UPS Work?** - **UV Source**: He discharge lamp (21.2 or 40.8 eV) or synchrotron. - **Valence Band**: UV photons have enough energy to eject valence electrons, not core electrons. - **Spectrum**: Photoelectron kinetic energy distribution maps the valence band density of states. - **Work Function**: The secondary electron cutoff edge gives the sample work function. **Why It Matters** - **Work Function**: The standard method for measuring work function and electron affinity. - **OLED/OPV**: Maps energy level alignment at organic semiconductor interfaces (HOMO position). - **Band Alignment**: Determines valence band offsets at semiconductor heterojunctions. **UPS** is **the work function ruler** — using UV light to measure how tightly a material holds its outermost electrons.