  We use femtosecond optical spectroscopy to systematically measure the primary
energy relaxation rate k1 of photoexcited carriers in cuprate and pnictide
superconductors. We find that k1 increases monotonically with increased
negative strain in the crystallographic a-axis. Generally, the Bardeen-Shockley
deformation potential theorem and, specifically, pressure-induced Raman shifts
reported in the literature suggest that increased negative strain enhances
electron-phonon coupling, which implies that the observed direct correspondence
between a and k1 is consistent with the canonical assignment of k1 to the
electron-phonon interaction. The well-known non-monotonic dependence of the
superconducting critical temperature Tc on the a-axis strain is also reflected
in a systematic dependence Tc on k1, with a distinct maximum at intermediate
values (~16 ps-1 at room temperature). The empirical non-monotonic systematic
variation of Tc with the strength of the electron-phonon interaction provides
us with unique insight into the role of electron-phonon interaction in relation
to the mechanism of high-Tc superconductivity as a crossover phenomenon.
