A new technique of atom-photon coincidence laser spectroscopy has been developed and used to study the isotope shifts of $^{78\mathrm{\ensuremath{-}}84}\mathrm{Sr}$. The results show that neither the droplet model nor existing interacting boson model calculations can adequately describe the rapid onset of nuclear deformation below N=50. The odd-even staggering of the charge radius is found to be opposite to that normally encountered, indicating the possible existence of permanent octupole distortions.