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

Abstract

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 (4DBA) 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 cm-1 shift 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.

Keywords: 4-Decyloxy Benzoic acid (4DBA); Stokes Raman line; anomalous Raman phenomenon (ARP); anti-stoke Raman line; phase transition.