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Excitons --- Coulomb-bound electron-hole pairs --- are the primary photo-excitations in two dimensional Ruddlesden Popper metal halides (RPMH). Spectral signatures of these quasi-particles manifest in the optical spectra as well-defined resonances separated by a characteristic energy. Here, we use these distinct excitonic resonances and their coherent dynamics as a spectroscopic probe of the unique characteristics of these excitations. We observe that the spectroscopic observables reflect the peculiar interactions of excitons with the anharmonic RPMH lattice that can be contextualized within the “exciton-polaron” framework. Unlike conventional 2D Wannier exitons, we consider that the electron-hole pairs in RPMHs intrinsically carry the lattice dressing, which we measure using impulsive vibrational spectroscopy. Moreover, we observe that there are multiple excitons that are dressed distinctly by the lattice phonons. We measure the intrinsic and density-dependent exciton dephasing rates of these multiple excitons and their dependence on temperature by means of two-dimensional coherent excitation spectroscopy. We find that diverse excitons display distinct intrinsic dephasing rates mediated by phonon scattering involving different effective phonons and contrasting rates of exciton-exciton elastic scattering. These findings establish specifically the consequence of anharmonic lattice interactions on the exciton many-body quantum dynamics, which critically define fundamental optical properties that underpin photonics and quantum optoelectronics. We further explore the origin of the plurality of excitons in RPMHs by systematic investigation of other materials variants obtained via substitution of either the organic cation or the metal cation. We find that a complex electronic structure with multiple carrier bands emerges, which may be responsible for the distinct excitons. Such an electronic structure originates from variations in coordination geometries of the metal halide octahedra induced by subtle changes in the organic-inorganic interactions, with measurable consequences on the exciton polaron characteristics.
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