Chlorine atom densities in the (3_p⁵)² P_1/2^oexcited spin-orbit state measured by two-photon absorption laser-induced fluorescence in a chlorine inductively coupled plasma

Chlorine atom densities in the(3_p⁵)² P_1/2^o spin-orbit excited state were measured by two-photon absorption laser-induced fluorescence (TALIF) in an inductively coupled plasma discharge in pure Cl₂. The atoms were excited by two photons at 235.702 nm to (4p) ⁴S^o_3/2 the state and detected by fluorescence to the (4s) ⁴P_5/2 state at 726 nm. The population of this state relative to that in the 3_p ⁵)² P_1/2^o ground state, n _P1/2/n_P3/2 was determined from the relative TALIF signal intensity from the two states, combined with new calculations of the two-photon absorption cross-sections. n_P1/2/n_P3/2 was found to increase continuously with radio-frequency power (50-500 W), whereas with Cl₂ pressure (5-90 mTorr) it passes through a maximum at 10 mTorr, reaching ∼30% at 500 W. This maximum corresponds to the maximum of electron density in the discharge. Combining this density ratio measurement with previous measurements of the absolute ground state chlorine atom density [1] allows the absolute spin-orbit excited state density to be estimated. A significant fraction of the total chlorine atom density is in this excited state which should be included in plasma chemistry models.