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Hello! I am analyzing some data from some rotational transitions between 2 electronic energy levels in a diatomic molecule. I noticed that for different runs that covered the same regions, the peaks we observe are shifted with respect to each other when the power of the laser driving the transition is changed.

I attached 2 such scans below, where the red one has a power of the laser of 2 mW while the blue one has 0.06 mW (they are not normalized for the molecular flux).

Assuming everything else, beside the laser power is the same between the 2 runs (which might not be the case, I need to look more into it, but for now assume it is), what can be the reason for this? I know that bigger power leads to broadening, but not a shift in the central value.

Also the AC Stark shift, as far as I understand, comes when you have a second laser involved i.e. the second laser shifts the transitions that the first laser sees. But I didn't think that the AC Stark shift works with only one laser (is that the case?).

Can someone help me understand what is happening and how can I account for this shift in the analysis? Thank you!

I attached 2 such scans below, where the red one has a power of the laser of 2 mW while the blue one has 0.06 mW (they are not normalized for the molecular flux).

Assuming everything else, beside the laser power is the same between the 2 runs (which might not be the case, I need to look more into it, but for now assume it is), what can be the reason for this? I know that bigger power leads to broadening, but not a shift in the central value.

Also the AC Stark shift, as far as I understand, comes when you have a second laser involved i.e. the second laser shifts the transitions that the first laser sees. But I didn't think that the AC Stark shift works with only one laser (is that the case?).

Can someone help me understand what is happening and how can I account for this shift in the analysis? Thank you!