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We have investigated the effect of pressure on the lowest direct band-edge exciton of the layered semiconductor InSe by optical-absorption measurements at 10 K and for pressures up to 4 GPa. The Elliott-Toyozawa formalism is used to analyze the line shape of the exciton absorption spectra. In this way we determine the pressure dependence of the lowest direct band gap, the exciton binding energy, and the exciton linewidth. The band gap exhibits a pronounced nonlinear shift with pressure, which is a consequence of the strong anisotropy of intralayer and interlayer bonding. The exciton binding energy decreases with pressure, mainly due to the large increase of the low-frequency dielectric constant parallel to the c axis. The reversible broadening of the exciton line with pressure is related to phonon-assisted scattering of electrons into lower-lying indirect-gap states. From the pressure dependence of the exciton linewidth, we determine the magnitude of the related intervalley-deformation-potential constant.

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