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The absorption spectra in an oriented film of pentacene have been studied up to 10 GPa under hydrostatic conditions. The absorption bands of this hydrocarbon rapidly shifted to a longer wavelength region. The Davydov splitting of the 0-0 band in the oriented film abruptly increased with increasing pressure up to 1.2 GPa. However, this splitting disappeared at around 1.5 GPa. The absorption spectrum at this pressure is similar to that of the amorphous film. The molecular arrangements in the oriented film of pentacene changed to the amorphous state at around 1.5 GPa. When the pressure was increased further, the absorption peaks disappeared at pressures greater than 6.6 GPa. When the pressure is reduced from 10 GPa to the ambient pressures, we cannot find the original absorption spectra of the oriented film of pentacene. The irreversible optical property arises from the solid-phase reaction at higher pressures.We have observed in situ shear stress effects on the oriented film of pentacene in the sapphire-anvil cell under the microscope. After the pressure is increased up to 2 GPa, one sapphire-anvil is rotated by applied force to generate shear stress at this pressure. The color of the thin film changed remarkably from blue to yellow at the outer part on the anvil, but the color at the center was blue, insensitive to shear stress. The absorption intensity of the visible bands sharply decreased, and the width vastly broadened in the outer part. The amorphization of the oriented film is accelerated under shear deformation. On the other hand, the intensity of the absorption band below 400 nm increased in the outer part. After shear stress was reduced, the original spectrum of pentacene did not recover completely. The conjugated system in pentacene is partly broken by shear stress. Thus, the solid-phase reaction is also induced by shear stress for the thin film. [ABSTRACT FROM AUTHOR]