Observation of Electric Quadrupole Decay inXe45+andXe44+

We have observed, for the first time in highly ionized atoms, electric quadrupole transitions between the $3p$ and $2p$ levels of fluorinelike and neonlike ${\mathrm{Xe}}^{45+}$ and ${\mathrm{Xe}}^{44+}$. We have made precision measurements ($\frac{\lambda }{\Delta \lambda }=2\mathrm{}\times {10}^4$) of the relative wavelengths of the allowed electric dipole ($E1\mathrm{}$) transitions and the forbidden electric quadrupole ($E2\mathrm{}$) transitions connecting the $n=3\mathrm{}$ and the $n=2\mathrm{}$ levels. We have also derived from these measurements several $\Delta n=0\mathrm{}$ intervals between states of opposite parity.