Abstract
Excitation energies and eigen widths of the 1P(-) low-lying autoionising levels of the helium atom converging to the n=3 state of the He+ ion have been obtained in the diagonalisation approximation of Balashov et al., (1970). Calculations were made of all the experimentally obtained parameters which determine the photoabsorption cross section in the vicinity of the (3s3p) 1P resonance. From comparison of the calculation results with the experimental data and with other calculations, a conclusion is drawn about the applicability of the diagonalisation approximation for the calculation of the total photoabsorption cross sections in which the direct coupling between open channels is neglected.