How to determine rotational constants from CRASY data

Observing rotational coherences in time allows you to determine rotational constants of molecules with high accuracy. But what does high accuracy mean? Indeed, you can only be as accurate as the measurement you performed. If your measurement is sensitive to enviromental conditions, you have to also measure these conditions precisely.

In the case of CRASY the precision of the measurement is determined by the precision of the total delay time Δt = Δd/c, where Δd is the length of the delay stage and c is the speed of light. Here comes the problem; we are operating our delay stage outside of our vacuum chamber. Speed of light in vacuum and speed of light outside on the laser table is of course somewhat different. The value has to be corrected by the refractive index of air, which depends on temperature, humidity, pressure and carbon dioxide concentration. NIST supplies a tool to calculate the refractive index of moist air:

We can now accuratly determine the delay times by correcting them with the refractive index, or we just determine the rotational constants and correct them afterwards. The determination of rotational constants might still be simple for linear molecules, but what about more complex molecules? Rotational (Raman) Spectroscopy on molecules isn`t something we invented. Measeruments in this field have been done for almost 90 years now [1]. No need to spend time on reinventing the wheel. I found a very nice tool, that helps to assign the rotational bands in our Fourier transformed delay traces. The tool is

PGOPHER, A Program for Simulating Rotational, Vibrational and Electronic Spectra by C. M. Western.

The proper way of citing the program can be found at the linked webpage. Anyways, it isn’t that easy to fiddle your way through the software, but the documentation on the webpage is very helpful, in case you stumble across certain questions. At the moment, this program is my first choice, if I want to determine rotational frequencies by simply assigning and fitting the line positions of rotational raman transitions.


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