## Abstract

Absorption spectra of europium vapour irradiated by intense, monochromatic resonance radiation at the wavelengths of the three principal resonance lines, 4f$^7$6s$^2$, $^8$S (J = $\frac{7}{2}$) $\rightarrow$ 4f$^7$6s6p, y $^8$P (J = $\frac{5}{2}, \frac{7}{2}$ and $\frac{9}{2}$) at 466.2, 462.7 and 459.4 nm respectively, have been photographed at high resolution. Pulsed resonance radiation was obtained from a tunable, narrowband dye laser pumped by a nitrogen laser: a broad-band dye laser pumped by the same nitrogen laser provided background radiation. Our spectra covered the ranges 380-400 nm, and 410-450 nm, each one showing transitions from a single resonance level to upper levels in the region of either the 4f$^7$6s, $^7$S or the 4f$^7$6s, $^9$S ionization limit of Eu II. In the shorter wavelength range the spectra consisted of weak autoionized series converging towards the $^7$S limit. In the longer wavelength range the three spectra were surprisingly dissimilar. The majority of the upper levels could be arranged into five highly-perturbed series, one corresponding to each of the J values $\frac{3}{2}, \frac{5}{2}, \frac{7}{2}, \frac{9}{2}$ and $\frac{11}{2}$. These series arose from excitation of the 6p electron to high lying d-orbitals. The absorption transitions to the series members are only prominent in regions where the series are strongly perturbed, indicating that most of the line strength is derived from the perturbing levels. Possible origins for the perturbing levels are discussed. Little evidence was found for a series arising from excitation of the 6p electron to high lying s-orbitals.