Normal-emission spectra from 1 T-TiS2 have been calculated within the one-step model with explicit consideration of the dielectric response to the incident p-polarized photon field, because calculations within the dipole approximation failed to explain relative intensities of experimental energy distribution curves. Following the hydrodynamic model we added to the photon vector potential a longitudinal plasmon-induced plane wave. We show that these corrections generally are of distinct importance, and, in the present case, lead to an essentially better agreement between theoretical and experimental spectra. PACS numbers: 79.60.Eq, 78.65.GbAngle-resolved photoemission is a widespread experimental tool, used very successfully to map the dispersion of bulk energy bands of solids as well as surfaces [1][2][3][4]. With respect to theory, the single-particle picture of the photoemission process [5,6] is well established, and programs for calculating the photocurrent have been applied to various substances [7-10]. In general, peak positions are described quite accurately, while the computed intensities sometimes differ drastically from experiment. As peak positions depend on the initial-and final-state band structure alone, they are by far easier to access than intensities, which incorporate transition matrix elements between the initial-and final-state electron wave functions. In the past a lot of effort has been spent in a reliable description of the electronic structure, i.e., the wave functions entering the matrix elements. However, the vector potential of the incident light wave has been given less attention, although the importance of screening of the electric field was pointed out even in the early papers on photoemission theory [5].In this Letter we show that layered structures, like the 1T titanium dichalcogenides TiSe2 and TiS2, are especially well suited for the investigation of polarization effects. This is due to their two-dimensional structure. Different maxima in the normal-emission spectra can unambiguously be assigned to direct transitions in the band structure. As a result of symmetry these energetically separated transitions are excited by different components of the electric field. Electron emission normal to the layer plane emerges from two different types of valence bands, two nondispersive bands connected with in-plane bonding orbitals, and two dispersive ones arising from vertical bonds between neighboring planes. Only the latter couple to the normal component of the electric field, which is strongly influenced by dynamical screening. To put it in terms of symmetry arguments [11], the in-plane even symmetry of the final state together with the even symmetry of the plasmon-induced longitudinal field imply transitions from even valence states only. Therefore the relative heights of the intensity maxima in normal emission spectra for different photon energies can supply information on frequency-dependent screening of the light wave at the surface. Evidence for the quantitative importance of this eff...

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