Are A1g modes reduced at 180 degrees polarisation in Raman spectra?

Question

I have read that A1g modes are identifiable in Raman spectra by looking at cross-polarised Raman spectra and seeing peaks that strongly reduce in intensity. So essentially, the intensity of A1g modes is at a maximum at 0 degrees (or XX polarisation where the first is the incident laser light polarisation and the second X is the polarisation of the backscattered light) and a minimum at 90 degrees (YX polarisation configuration).

I have some Raman spectra where I'm looking at A1g modes by varying the polarisation of the incident laser across 360 degrees and I saw intensity minima at 90 degrees and 270 degrees as expected but I also saw a dip in the intensity at 180 degrees (i.e. -XX configuration). My question is, should A1g peaks be reduced at 180 degrees due to being antiparallel to the polarisation of the backscattered light? Or might it be the case that the dips in intensity are due to something else as A1g modes are totally symmetric under all symmetry operations?

Answer 1

In solution the theoretical ratio of parallel to perpendicular polarised Raman intensity (measured relative to laser polarisation) is $\rho=3/4 $ if the mode is non-totally symmetric, and $0\lt \rho<3/4$ if it is totally symmetric, and closer to zero the more highly symmetric the molecule is. Also the totally sym. vib. often is the most intense. $180$ deg should be be the same as 0, so it seems that your signals may have another source. (The intensity is over the integrated band, not just peak)