  In this work the complete valence-band structure of the molybdenum
dichalcogenides MoS_2, MoSe_2, and alpha-MoTe_2 is presented and discussed in
comparison. The valence bands have been studied using both angle-resolved
photoelectron spectroscopy (ARPES) with synchrotron radiation, as well as,
ab-initio band-structure calculations. The ARPES measurements have been carried
out in the constant-final-state (CFS) mode. The results of the calculations
show in general very good agreement with the experimentally determined
valence-band structures allowing for a clear identification of the observed
features. The dispersion of the valence bands as a function of the
perpendicular component k_perp of the wave vector reveals a decreasing
three-dimensional character from MoS_2 to alpha-MoTe_2 which is attributed to
an increasing interlayer distance in the three compounds. The effect of this
k_perp dispersion on the determination of the exact dispersion of the
individual states as a function of k_parallel is discussed. By performing ARPES
in the CFS mode the k_parallel-component for off-normal emission spectra can be
determined. The corresponding k_perp-value is obtained from the symmetry of the
spectra along the GammaA, KH, and ML line, respectively.
