Hi everyone,
I have performed a laser induced Raman characterization of a perovskite oxide. For increasing power, the Raman spectra red shifts upto a certain power. After that, the spectra blueshifts to a higher frequency. For subsequent powers, the spectra again shows a redshift. In the case of the redshift, the main contributing factor is the thermal behaviour. However, what can be the contributing factors for the sudden blueshift?
Is the effect reversible or irreversible? If it is irreversible, there may be some kind of photochemistry going on in your crystal that may affect its vibrational properties.
Based on your description of the phenomena, this most likely is due to thermal induced phase transition comprise of heat induced "softening" of vibrational mode (i.e., red-shift) of the original phase, followed by a sudden phase transition (i.e., blue shift), which takes place at a specific temperature, followed by the heat induced mode softening of the new phase. In fact the ferroelectric phase transition of Provskite ceramics is behind their use in thermal imaging or creation of phase array anntenas. In some ceramic materials such as YBCO high temperature superconductors the redshift is assoicated with oxygen depletion in the YBCO materials (going from 7 oxygen to 6 oxygen per unit cell), and if the sample is exposed to rich oxygen environbmen using the same laser the process is reversable by thermal induced process that creates a transition back to 7 oxygen per unit cellConference Paper Raman Microprobe Investigation of Laser-Assisted Chemically ...
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Please look at the following below links which may help you in your analysis:
https://aip.scitation.org/doi/pdf/10.1063/1.4921316
https://iopscience.iop.org/article/10.1088/1361-6463/aa98fe/pdf
Thanks
Interesting. I have some ideas as below
1. As always you want to calibrate your raman system against standards, both liquids (i.e. 50:50 toluene/acetonitrile, cyclohexane) and solids (i.e. sulfur, 4-acetamidophenol (active ingredients of Tylenol), naphthalene) to see if the frequency shift effect comes back then you can conclude it is chemical related or system issue. If the whole spectrum is shifted, it could be the binning process of your spectrograph’s ccd. Your laser linewidth also plays a critical role.
2. In raman exp, the incident beam is usually tightly focused and in the case of high input power, then the thermal effect appears in many ways. 2.1. Local heat causes compressibility in material. 2.2. Increase in the population of first vibrational excitedstate. In this case, the spectrum not only could change change.
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