We demonstrate laser frequency stabilization with at least 6 GHz of offset
tunability using an in-phase/quadrature (IQ) modulator to generate electronic
sidebands (ESB) on a titanium sapphire laser at 714 nm and we apply this
technique to the precision spectroscopy of 226Ra, and 225Ra. By
locking the laser to a single resonance of a high finesse optical cavity and
adjusting the lock offset, we determine the frequency difference between the
magneto-optical trap (MOT) transitions in the two isotopes to be 2630.0±0.3
MHz, a factor of 29 more precise than the previously available data. Using the
known value of the hyperfine splitting of the 3P1​ level, we calculate
the isotope shift for the 1S0​ to 3P1​ transition to be
2267.0±2.2 MHz, which is a factor of 8 more precise than the best available
value. Our technique could be applied to countless other atomic systems to
provide unprecedented precision in isotope shift spectroscopy and other
relative frequency comparisons