Two broad absorption features were discovered in the {\it Chandra}/ACIS spectrum of the isolated neutron star 1E1207.4-5209 (Sanwal et al. 2002). We will present spectral analysis from the recent 250ks XMM-Newton observations. We describe the atomic physics methodology which leads to the identification of this neutron star's atmospheric composition. The solution of Schr{\"o}dinger's equation in an intense magnetic field combined with simple atomic physics arguments leads to simultaneous determination of magnetic field strength, atmospheric elements and gravitational redshift. Atomic physics arguments alone establish that the absorption features must be due to atomic transitions from He-like Oxygen or Neon at $B\sim 10^{12}$ G, comparable to the magnetic field strength independently derived from the neutron star spin parameters. We discuss several important implications to neutron star physics.