I used acetaminophen pill as a sample and I exposed it to the 532 nm laser at 75 mw power for about 5 to 40 minutes.During this time, photobleaching accured and the fluorescence of raman spectrum decreased significantly, but I did not see any reduction in raman signals after that time. we know that during photobleaching, molecules change chemically and do not radiate fluorescence. so there must be a reduction in their raman scattering too because of reduction of molecules. So why did not I see raman reduction?
The chemical moiety responsible for fluorescence may be different from the chemical moiety detected by Raman spectroscopy. Exposure to the laser may destroy one without destroying the other.
Another consideration: Are you sure the Raman scattering is due to the compound rather than the solvent?
The chemical moiety responsible for fluorescence may be different from the chemical moiety detected by Raman spectroscopy. Exposure to the laser may destroy one without destroying the other.
Another consideration: Are you sure the Raman scattering is due to the compound rather than the solvent?
Are you sure it is fluorescence? Acetaminophen should no absorb at 532nm. A strong background to Raman can also derive from scattering which enters the spectrometer as stray light. Rather than bleaching, some local thermal annealing may reduce the scattering
"In vitro analysis of immersed human tissues by Raman microspectroscopy", F. Bonnier, A.Mehmood, P. Knief, A. Meade, W. Hornebeck, H. Lambkin, K. Flynn, V. McDonagh, C. Healy,T.C. Lee, F.M. Lyng, H.J. Byrne, Journal of Raman Spectroscopy, 42, 888–896(2011)
“Analysis of human skin tissue by Raman microspectroscopy: Dealing with the background” F. Bonnier, S.M. Ali, P. Knief, H. Lambkin, K. Flynn, V. McDonagh, C. Healy, T.C. Lee, F.M. Lyng, H.J. Byrne Vibrational Spectroscopy, 61, 124-132 (2012)
In addition to what Adam B Shapiro said
The modifications induced via photobleaching, which is primarily induced by formation of singlet oxygen and other reactive oxygen species, simply modifies the molecule so that it cannot enter any more repetitive cycles of excitation/emission, but rather a permanent dark state is formed. This can already happen when a large conjugated pi-system looses one bond (say somewhere in the middle of the system) through H-abstraction which forms the first radical of the now no longer fluorescent molecule...the changes in the Raman spectrum might be subtle. look at it this way...if a fluorescent molecule becomes dark permanently, you can still record its absorption...for that property, nothing changes. Only after major destruction do degradation products or even polymerization occur.
Acetaminophen does not absorb at 532nm, therefore it does not fluoresce, or photobleach. The previous references I have provided explain the background to Raman as a stray light/scattering phenomenon.
Hugh J Byrne is right. Acetaminophen: Exc=320 nm/Em=425 nm (had to look it up though)
- Badges
- Science topic
- Similar topics
- Physical Sciences
- Physical Phenomena
- Luminescence
- Fluorescence