Two-photon Doppler-free ultraviolet laser spectroscopy on sulphur atoms

The $3p^4$ ${}^3$P${}_J$ - $3p^{3}4p$ ${}^3$P${}_J$ transition in the sulphur atom is investigated in a precision two-photon excitation scheme under Doppler-free and collision-free circumstances yielding an absolute accuracy of 0.0009 cm${}^{-1}$, using a narrowband pulsed laser. This verifies and improves the level separations between amply studied odd parity levels with even parity levels in S I. An improved value for the ${}^3$P${}_2$ - ${}^3$P${}_1$ ground state fine structure splitting is determined at $396.0564$ (7) cm${}^{-1}$. A ${}^{34}$S - ${}^{32}$S atomic isotope shift was measured from combining time-of-flight mass spectrometry with laser spectroscopy.