Principal series (spectroscopy)

In the context of spectroscopy, "principal series" refers to a specific set of spectral lines that arise from the electronic transitions of atoms or ions, particularly in the hydrogen atom and hydrogen-like systems. These transitions occur between different energy levels characterized by principal quantum numbers. For hydrogen, the principal series typically involves transitions where an electron moves from higher energy levels (n ≥ 2) down to the first energy level (n = 1).