Precision measurement of the 5 2S1/2-4 2D5/2 quadrupole transition isotope shift between 88Sr+ and 86Sr+

We present a high-precision measurement of the isotope shift of the narrow quadrupole-allowed 5 S-2(1/2) -4 D-2(5/2) transition in Sr-86(+) relative to the most abundant isotope Sr-88(+). This was accomplished using high-resolution laser spectroscopy of individual trapped ions, and the measured shift is Delta nu(88,86)(meas) = 570.281( 4) MHz. We also tested a recently developed and successful method for ab initio calculation of isotope shifts in alkali-metal-like atomic systems against this measurement, and our initial result of Delta nu(88,86)(calc) = 457( 28) MHz is also presented. While the measurement and the calculation are in broad agreement, there is a clear discrepancy between them, and we believe that the specific mass shift was underestimated in our calculation. Our measurement provides a stringent test for further refinements of theoretical isotope shift calculation methods for atomic systems with a single valence electron.