On the nature of red and blue shifts of light emission relative to the absorption spectrum

Photoluminescence spectrum of a matter is usually red-shifted relative to the spectrum of light absorption, although there are some cases of the opposite (blue) shift. These are interpreted conventionally in terms of the multiple-level models, referring to the Stokes shift, Kirchhoff law, Levshin rule, Jablonski diagrams, etc. This report is aiming to provide arguments for the red or blue shift to be the inherent quantum-mechanical property of systems with a single pair of inversely populated levels. Solutions for absorptivity and emissivity of nanometer-scale layers and dots in a matrix are presented referring to existing experimental PL and absorption data.