determining ageing by luminescence spectroscopy

Luminescence is the umbrella term for phosphorescence and fluorescence.

Each bitumen fraction has a unique features regarding its spectra. The main part of saturates exhibit less luminescence. Aromatics exhibit the highest intensity, slowly decreasing forwards resins. The decreasing intensity of bitumen over ageing is likely owned to the change in the amount of aromatics. Resins and solid asphaltenes have two characteristic peaks at about 536 nm and 605 nm, which too can be found for saturates, but at a lower intensity. Asphaltenes in a solid state exhibit nearly no luminescence. In solution asphaltenes displays a spectrum similar to aromatics, but with lowest intensity and due to the phenomenon that aspahltenes form stacks and agglomerate in solution, it is assumed that asphaltenes dissolved in maltenes emit a spectrum similar to solid asphaltenes.

normalized emission spectra (excitation at 280 nm)

The figure shows the changes in the spectrum over the three ageing stages. Non-aged bitumen (B) lacks the peaks typically for resins and asphaltenes, but short-term aged (B_LRTFOT) and long-term aged (B_PAV and B_F282) exhibit these peaks. Unfortunately there is not a correlation between peak intensity and ageing stage. The different curve slopes, starting at about 340 nm, are caused partially by normalizing of the spectra. Not normalized B, B_LRTFOT and B_PAV spectra have the same starting slope, but differ in intensity maxima. Due to normalization the maxima are the equal, yet the slope changes. So the maxima or the slope can provide a hint to the ageing progress.