Combined experimental and computational study of the pressure dependence of the vibrational spectrum of solid picene C_{22}H_{14}

We present high-quality optical data and density functional perturbation theory calculations for the vibrational spectrum of solid picene (C22H14) under pressure up to 8 GPa. First-principles calculations reproduce with a remarkable accuracy the pressure effects on both frequency and intensities of the observed phonon peaks. We use the projection on molecular eigenmodes to unambiguously fit the experimental spectra, resolving complicated spectral structures, in a system with hundreds of phonon modes. With these projections, we can also quantify the loss of molecular character under pressure. Our results indicate that picene, despite a ∼20% compression of the unit cell, remains substantially a molecular solid up to 8 GPa, with phonon modes displaying a smooth and uniform hardening with pressure, without any evidence of structural phase transitions. The Grüneisen parameter of the 1380 cm−1 a1 Raman peak (γp=0.1) is much lower than the effective value (γd=0.8) due to K doping. Therefore, doping and pressure have very different effects and it can be argued that softening of the 1380 cm−1 mode is probably due to coupling with electronic states in K-doped solid picene.