We have investigated the effects of uniaxial stress up to 2.5 kbar on the first four dipole-forbidden states of the yellow exciton series in cuprous oxide. The linear and nonlinear splittings and shifts as well as the symmetries of the stress-split components were determined using the quadrupole-dipole Raman-scattering technique involving odd-parity phonons. %e have observed the triply degenerate 1S quadrupole state split into a singlet and a doublet with the singlet energy increasing and the doublet energy decreasing with stress as first seen by Gross and Kaplyanskii. For the higher quadrupole S states, however, the sense of the splitting was reversed from that of the 1S, while the magnitude of the 3S splitting exceeded that of the 1S as previously noted by Agekyan and Stepanov. The quadratic stress coefficients for all components of the 1S, 3D, and 4S states were found to be equal while those for the 3S state were markedly different. A theoretical analysis was carried out for [001] stress using the effective-Hamiltonian formalism including both deformation of the bands and electron-hole exchange. The theory accounts satisfactorily for the general features of the 1S, 3D, and 4S states only if both the magnitude and sign of the exchange constant are considered to be dependent on the exciton state. For the 3S, however, the nonlinear stress dependence cannot be reconciled with the theory for yellow excitons. These results suggest that the "3S yellow exciton" may actually belong to the green-exciton series, or that interactions between yellow and green excitons with different principal quantum numbers may be significant although not included in the theory.

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