In the energy diagram for a hydrogen atom at the far right, the
vertical lines marked a, b, c,... represent transitions from the
lowest energy level, n = 1, to various higher levels.
A certain fraction of atoms at any given temperature already will
be in the first excited state (i.e. n = 2) and these atoms can absorb
radiation as indicated by vertical lines g, h, i,...and go to even
higher states.
Those few atoms already in an n = 3 state can absorb radiation as
shown by lines l, m, n,... and the progression can continue.
Each of these series of absorptions, starting with a different initial
quantum state, has an upper limit, which is the energy needed to
ionize the atom. These are shown as lines x, y, and z for absorptions
from n = 1, 2, and 3 to n = .