We tried two different temperature dependent Raman set ups for our study. Every time we do temperature dependent Raman spectroscopy for a sample, the Raman signal is lost or quenched at some temperature. The Raman operators tell us that the laser's focus on the sample is probably lost at some temperature. The readjust the focus and get the signal back. But it continues to happen throughout the measurements. Is that normal? what can be the reasons?
If you have a sample or a sample holder that expands over several micrometers during the heating, it is not surprising that the focus gets lost upon heating/cooling. The focus at Raman spectroscopy, especially if you're doing thin films, is very crucial and sensitive.
If the temperature dependence measurements are induced locally by the laser beam heating, then normally you shouldn't face focusing issues during the temperature dependence Raman measurements. However, if the temperature is being controlled such that the sample and the sample holder, etc. all are experiencing temperature changes, then any fraction of micron dimensional changes related to focal point should drastically impact the intensity of the Raman peaks. In such case, you may want to set the temperature for what the measurements will be done and wait for a few minutes till the thermal expansions related changes are stabilized, select a specific Raman peak, tweak the Raman probe focus to maximize counts for that peak, and then collect the Raman spectra.
Hi, Muhammad
I agree with the previous answers. Does your system have Auto-Focus ? If yes, you can select a peak and program the system to auto-focus and maximize this peak at each X measurements (scanning over a range in Z). The only problem is that this can increase considerably the time of the analysis, depending on the Z range chosen for the auto-focus, plus the acquisition time of your spectra, grating changes and etc..
Good luck !
João
What NA you are using for the experiment? If you are using something very high (~0.9), your depth resolution will be very high, i.e. even a µm change of focus will change your intensity.
You can try using a lower NA, if the lowered lateral and depth resolution and less signal is less crucial for you analysis.
A 0.9 objective will have a depth of focus on the order of around 1µm or so, while going to 0.55 you will have more like 4 or 5 µm of focus depth. So it will be less sensitive. Lower NA will even give you much more leverage as the depth resolution scales with 1/NA^2.
Also, are you going up or down from the room temperature? If down and you do not have vacuum cryostat, but only one purged by, say, nitrogen, then water vapor from atmosphere starts to precipitate on the surface sample typically within half an hour. The resulting 'snow' is very effective in screening the sample signal..
Dusan Hemzal I am going down, the measurements are done in very low vacuum. A very small pump is attached which is not capable of creating a very high vacuum. But its usually enough to avoid water vapors. I also thought that it was water that was interfering but I am not sure yet.
OK, in that case the issue is not the pump, but whether your system is air-tight or not. If yes, the water present in cryostat will just once condensate without much harm. If not, there is a constant draught of water from outside. In either case, you have a visual control of the surface, and the optional 'snow' is usually well visible.
If temperature keeps changing during spectral acquisition, your sample will also continue to vibrate thus causing it to undergo changes in both physical and compositional orientation, this can lead to out of focus data.
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