Observation of forbidden (E2) lines in the ultraviolet spectra of Ca II, Sr II, and Ba II by inductively coupled plasma emission spectroscopy

Forbidden (electric quadrupole, E2) transitions of the type ns 2S1/2–nd 2D3/2 and ns 2S1/2–nd 2D5/2 in the ultraviolet spectra of singly ionized Ca, Sr, and Ba (with n = 4, 5 or 6 for Ca, Sr, and Ba, respectively) have been observed in the emission spectrum of an inductively coupled argon plasma. Wavelengths and wavenumbers of the six lines are reported and the values are in good agreement with those expected from literature data for the energy levels involved.