Effects of the electron-electron interaction on the excitation spectrum of the n-inversion layer on the Si (100) surface of the metal-oxide-semiconductor structure have been calculated. The correlation energy of an electron excited to the first subband of the doubly degenerate set or to the lowest subband of the fourfold degenerate set is of the order of - 10 meV. The latter subband is found to be lower in energy for almost all inversion-layer concentrations and satisfactory agreement with magnetoconductivity measurements is obtained. A theory for infrared absorption which includes both resonant screening and exchange and correlation in the long-wavelength limit is developed and the results agree very well with experiments. The theory of a bound intersubband exciton is presented and binding energies of the order of 1 meV are calculated. © 1977 The American Physical Society.