FAQ: What does the temporal signal intensity distribution tell about a sample?
06:51 Wed 19.10.2022
FAQ series: This series of publications explores some of the most frequently asked questions regarding Timegated® spectrometers and measurements without having you read page after page of text.
Timegated®Raman spectrometers produce time-resolved information. This means that the produced spectral information contains a temporal dimension which can be seen in the spectra which includes intensity, Raman shift, and delay axes (see figure below). These “3D” spectra can be easily converted to more conventional Raman spectra with just intensity and delay axes.
A time-resolved spectrum with intensity, Raman shift, and time axes.
The temporal signal intensity distribution conveys information on some of the sample characteristics: Does the sample produce a lot of fluorescence and is the fluorescence fast or slow and what kind of decay properties does the photoluminescence have?
The emission/scattering pulse contains information on both the Raman and fluorescence characteristics.
The temporal intensity distribution can be easily studied when plotting intensity vs. time. Fluorescence with a long decay time can be observed for an extended time period after each excitation pulse. In the figure below, we can first see the Raman peaks at the pulse forefront and after that a long slope of long lifetime fluorescence. The fluorescence intensity and decay characteristics may be used for e.g. sample classification or for detecting impurities.
Fluorescence with a long decay time produces a slowly descending slope after the Raman signals. Please note that the slope would continue even further if the measurement delay range would be extended.
Sometimes the time-resolved spectrum does not include a distinct fluorescence decay tail. Either the photoluminescence is very weak, very fast, or extremely long-lived. In the case of very fast i.e. short-lifetime fluorescence, the temporal shape of its emission pulse coincides with that of Raman. On the other hand, very long-lifetime photoluminescence will show up as a relatively weak, time-independent signal.
A sample with a very low amount of fluorescence. The slowly descending fluorescence decay slope is not observable.
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