Rubidium isotope shift measurement made simple

Experimentally determining the isotope shift in atomic spectra leads students beyond their naive understanding of atomic structure and can introduce them to the basics of optical metrology. Standard undergraduate laboratory experiments for the isotope shift on the principal D1 and D2 transitions in rubidium typically use saturated absorption spectroscopy. We show that using a different atomic state, namely the 5D52 level, significantly simplifies the data collection, analysis, and interpretation in comparison to the standard experiments. With two low-bandwidth photodiodes and a Fabry–Perot cavity, the isotope shift is measured to be 162.8 ±0.9stat±1syst MHz. This result guides students to the necessity of including an ∼ 30 MHz (many-body electronic) specific mass shift, in addition to the contributions of the nucleus's countermotion and the finite nuclear charge radius.