The fundamental series lines for rubidium appear in the near infrared. The valence electron moves from the 4d level as the 3d is contained in an inner shell. They were observed by R von Lamb.
Relevant energy levels are 4p4d j=5/2 19,355.282 cm and j=3/2 19,355.623 cm, and the first f levels at 4p4f j=5/2 26,792.185 cm and j=7/2 26,792.169 cm.