Non-monotonic temperature dependence in molecular referenced pressure-sensitive paint (MR-PSP)

A molecular reference for PSP showed an unexpected temperature dependence. At atmospheric pressure the slope of the temperature dependence was similar to - 0.2%/degrees C at all temperatures. However, at vacuum the temperature dependence had a negative slope at lower temperature (similar to 10 degrees C) but a positive slope at higher temperature (similar to 40 degrees C). We attribute this to delayed fluorescence. At higher temperature in the absence of oxygen, the excited singlet state can be thermally repopulated from the triplet state. This results in an increase in the apparent fluorescence as temperature rises. The fluorescence decay at vacuum and 50 degrees C is composed of two parts: a short (similar to 10 ns) decay from the prompt fluorescence and a longer-lived (similar to 40 ms) decay from the delayed fluorescence, which reflects the triplet decay time. Delayed fluorescence is quenched at 2% atmospheric pressure, so it should not be a problem in wind-tunnel research, although it can cause confusion in calibration runs at vacuum. (C) 1999 Elsevier Science B.V. All rights reserved.