Distinct spectral line shifts in the Raman lines' Stokes and anti-Stokes components with temperature in the liquid crystalline system 4DBA.

An anomalous Raman phenomenon (ARP) refers to a scenario in which the Stokes and anti-Stokes frequencies of a Raman mode are different. Here we report that ARP introduces different line shifts in Stokes and anti-Stokes frequencies in 4-Decyloxy Benzoic acid with temperature across two thermally driven structural transitions: the smectic (S) to nematic (N) phase transition at 375 K and the nematic (N) to isotropic (I) transition at 390 K. Four Raman modes near 663, 773, 1128, and 1168 cmshift towards higher frequencies (blueshift) in the Stokes frequency and towards lower frequencies (redshift) in the anti-Stokes frequency with rise in temperature near S to N phase transitions. However, in the isotropic liquid phase, both the Stokes and anti-Stokes components exhibit a redshift as temperature increases, without any noticeable distinct spectral lineshifts. Our studies bring out the importance of phonon band structure in ARP.