Thermally Assisted Fluorescence:

In thermally assisted fluorescence, an electron excited to a given level through absorption of radiation gets additional excited along with the help of thermal energy within a radiation less process. A fluorescence emission occurs from both the excited energy levels. In that case part of the emitted radiation has shorter wavelength (higher energy) as compared to the exciting radiation. It is also known as anti-Stokes fluorescence. An origin of these categories of fluorescence within terms of the energy level diagram is given in Figure (b).

Figure: Schematic energy level diagram showing the excitation and de-excitation processes involved in origin of (a) double resonance fluorescence and (b) thermally assisted fluorescence