Observation of replica bands in the ARPES spectra of the single-layer FeSe on strontium titanate substrate revealed a phonon component among mechanisms behind high Tc superconductivity there. We study the interaction of the in-layer FeSe electrons with the electric potential of the longitudinal (LO) modes at surface of bulk SrTiO3. A two-dimensional system of charges at the FeSe/SrTiO3 interface includes both the itinerant and immobile electrons. The latter significantly change the interface

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... racteristics, increasing screening at the substrate surface and reducing thereby the strength of the electron-LO phonon interactions. In that follows the dielectric constant serves as a free parameter and is found from the ARPES data for the replicas. The two-dimensional Coulomb screening is accounted for in the random phase approximation. It is shown that the model is applicable within the whole range of the parameters typical for current experiments. The estimates allow concluding that the LO-phonon mediated pairing alone cannot account for the temperatures of the superconducting transition Tc in the single-layer FeSe/SrTiO3 reported in these experiments. This does not exclude that the LO-phonons mechanism can become more significant in the differently and better prepared single layer FeSe films. Available experiments are briefly discussed. Thus far no data exist on the dependence of Tc on the concentration of electrons doped into the in-layer FeSe band.