H-alpha (Hα) emission is the red visible spectral line created by a hydrogen atom when an electron falls from the third lowest to second lowest energy level. This transition corresponds to a wavelength of 656.28 nm (red light) and is the first transition in the Balmer series. The spectrum below shows the four visible hydrogen emission spectrum lines that make up the Balmer series, with the H-alpha transition in red on the far right.
The Balmer Series
The Bohr model of the atom describes electrons as existing in quantized energy levels surrounding the atom’s nucleus. Electrons can jump from the ground state (n = 1) to higher energy levels if the atom is excited (absorbs incoming radiation). When an excited electron drops back to a lower energy level, it emits light at a specific wavelength corresponding to that particular transition. The set of transitions from a n ≥ 3 energy levels to the n = 2 energy level is named the ‘Balmer Series.’
It takes almost as much energy to ionize a hydrogen atom as it does to excite the atom’s electron to the n = 3 energy state. In fact, it is very rare for an electron to be excited to this energy state without being removed from the atom. Once an atom is ionized, the nucleus and removed electron can recombine to form a new atom with the electron typically in a higher energy state. This electron will then cascade back to the ground state, a process which produces H-alpha emission about half of the time. Thus, observing H-alpha emission in a region indicates that hydrogen is being ionized there. Astronomers use observations at this wavelength as an effective probe of star formation regions, where surrounding gas is begin continually ionized by newly formed stars.