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

A1g modes in Raman spectra should not be reduced at 180 degrees due to being antiparallel to the polarisation of the backscattered light.

Step-by-step explanation:

In Raman spectroscopy, A1g modes can be identified by observing the peaks in cross-polarised Raman spectra. These peaks are characterized by a strong reduction in intensity when the angle between the incident laser light's polarization and the backscattered light's polarization is at 90 degrees (YX polarisation configuration). However, it is worth noting that A1g modes are totally symmetric under all symmetry operations, which means that the peaks should not be reduced at 180 degrees (-XX configuration) due to being antiparallel to the polarisation of the backscattered light.