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, Hβ and Hγ, and ten He i lines were obtained for all the stars, as well as of the He iiλ4686 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 Teff and log g determination and for the He, C, N and O abundance analyses.