Surface abundances of light elements for a large sample of early B-type stars - I. Spectral observations of 123 stars; measurements of hydrogen and helium lines; infrared photometry

High-resolution spectral observations of 123 B0-B5 stars in the main sequence evolutionary phase were obtained at two observatories, namely the McDonald Observatory (McDO) and the Crimean Astrophysical Observatory (CrAO). Accurate equivalent widths W of two Balmer lines, H beta and H gamma, and ten He I lines were obtained for all the stars, as well as of the He II lambda 4686 line for the hottest ones. A careful analysis of the measured equivalent widths was performed. It is shown that there is a very good agreement between the W values derived from the McDO and CrAO spectra for 14 common stars. A comparison with published data leads to the conclusion that the W values measured earlier by some authors for strong He I lines are very likely to be underestimated. Infrared photometric observations in the J, H, and K bands were performed for 70 programme stars. All these data will be used in other papers: in particular for the T-eff and log g determination and for the He, C, N and O abundance analyses.