Here the wikipedia intro for him as context:

Arnold Johannes Wilhelm Sommerfeld (German: [ˈaʁnɔlt ˈzɔmɐˌfɛlt]; 5 December 1868 – 26 April 1951) was a German theoretical physicist who pioneered developments in both atomic and quantum physics, and also educated and mentored many students for the new era of theoretical physics.

Sommerfeld served as doctoral advisor and postdoctoral advisor to seven Nobel Prize winners and supervised at least 30 other famous physicists and chemists.

Sommerfeld introduced the second quantum number, azimuthal quantum number, and the third quantum number, magnetic quantum number. He also introduced the fine-structure constant and pioneered X-ray wave theory.

Never heard his name during my quantum physics courses, seems he was one generation too early to get stuff named after him. Edit: after checking, Planck was born 10 years earlier.