# Definition Angle-Resolved Photoemission Spectroscopy is an experimental technique based on the [[photoemission|photoelectric effect]] and is used to probe the [[Band Structure|electronic band structure]] of solids. There are many varieties of ARPES, including [[Physics/Spin]]-resolved ARPES (spin-ARPES), soft-X-ray ARPES, time-resolved ARPES, etc. ARPES is the key experimental technique used to study [[topological systems]] ![[42254_2019_88_Fig1_HTML.png]] In panel (a), the figure illustrates the geometry of ARPES. The light is incident with energy $h \nu$, and an electron is ejected at an angle $(\theta, \phi)$, where it is detected. By translation symmetry along the surface of the sample, we have $k^{i}_{\parallel} = k^{f}_{\parallel}$. But translation symmetry is not conserved perpendicular to the sample so that $k^{i}_{\perp} \neq k^{f}_{\perp}$. In panel (b), the figure illustrates the energetics of ARPES. via the [[density of states|density of states]] $N(E)$. Energy is conserved: $E_{\text{kin}} = h \nu - \Phi - E_{B}$ where $h \nu$ is the photon energy, $\Phi$ is the work function, $E_B$ is the binding energy of the electron. $E_{\text{vac}}$ is the vacuum level and $E_f$ is the Fermi level. $E_{\text{kin}}$ represents the kinetic energy of the photoelectron. # Resources [Nature Review](https://www.nature.com/articles/s42254-019-0088-5)