The electron spectrum of a crystal with an adsorbed centre is considered, using the mathematical techniques of the quantum mechanical theory of scattering. A special pole approximation is developed which is substantiated by specific features of the problem. Formulas are derived for the adsorption-caused changes of the thermodynamical characteristics and compared with the experimental data. The behaviour of the density of states as an adatom recedes away from the crystal is elucidated. The dynamic effects of surface states are considered using field emission as an example. The formula obtained is compared with experiment.