Quantum tunnelling

Quantum tunneling is a phenomenon where particles move through a barrier that they classically shouldn't be able to pass. In the context of ionization and excitation, it helps explain how electrons can escape from an atom or jump to higher energy levels despite not having enough energy to do so according to classical physics. This occurs because, at a quantum level, particles have wave-like properties, allowing them to "tunnel" through energy barriers. Essentially, there's a probability of finding an electron on the other side, enabling processes like ionization when the barriers are thin or weak.