Two‐photon absorption yields Doppler‐free spectroscopy

In one of those events so typical of research, three groups have independently and almost simultaneously demonstrated two‐photon absorption in sodium vapor without blurring from Doppler broadening. The technique appears to open up a new way of achieving high‐resolution spectroscopy with tunable lasers. And it can be applied, for example, to induce photochemical reactions selectively, to separate isotopes, or to provide more precise values for the Rydberg constant and Lamb shifts.