  We have measured the hyperfine structure and isotope shifts of the 402.0 nm
and 399.6 nm resonance lines in 229Th+. These transitions could provide
pathways towards the 229Th isomeric nuclear state excitation. An unexpected
negative isotope shift relative to 232Th+ is observed for the 399.6 nm line,
indicating a strong Coulomb coupling of the excited state to the nucleus. We
have developed a new all-order approach to the isotope shift calculations that
is generally applicable to heavy atoms and ions with several valence electrons.
The theoretical calculations provide an explanation for the negative isotope
shift of the 399.6 nm transition and yield a corrected classification of the
excited state. The calculated isotope shifts are in good agreement with
experimental values.
