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Crystalline As, S, (orpiment) is a layer-structure semiconductor whose optical properties, under ordinary conditions, are determined by its diperiodic layer symmetry. . We have investigated the effect of hydrostatic pressure on the optical-absorption edge and the Raman scattering spectrum of orpiment. With increasing pressure the optical band gap red-shifts rapidly, decreasing from 2.7 eV at P = 0 to 1.6 eV at 100 kbar. The initial slope, dEG IdP at P = 0, is -(14~3) meV/kbar, and the closing of the gap is interpreted in terms of pressureinduced enhancement of the interlayer-interaction broadening of intralayer bands. At high pressure, dimensionality effects (20~30) are clearly discerned in the vibrational Raman spectrum. An intralayer quadruplet at 355 cm ', near degenerate at zero pressure, disperses at high pressure as the dominance of the diperiodic (isolated layer) symmetry is broken and the admixture repulsion of modes of like crystal symmetry forces them apart. A similar effect occurs at 25 kbar in the forbidden-crossover repulsion of a pair of modes at 145 cm . Also, the rigid-layer modes roughly double in frequency by 100 kbar, corresponding to a quadrupling of the layer-layer coupling.Finally, the pressure data have been used to separate the phonon-occupation-driven (explicit) and the volumedriven {implicit) contribution to the temperature dependence of the band gap and the Raman-active phonon frequencies. We find that E~is controlled by the explicit electron-phonon contribution to dEG ldT. For the phonons, dv/dT of the low-frequency interlayer-interaction modes is dominated by the volume-driven term, while the explicit phonon-phonon effect dominates dv/d T for the intralayer modes.tinuous-random-network model" for the structure of glasses. ' Spectral comparisons between the crystalline and amorphous forms of As, S, have proved to be highly useful for gaining insight about elementary excitations in both solids, ' ' with the crystalline partner generally serving as a firm base from which to launch attacks on the less-wellunderstood amorphous phase.