MICROWAVE-OPTICAL-DOUBLE-RESONANCE: STUDY OF THE $X^{2}\Sigma^{+}$ STATE OF THE SrF RADICAL

Microwave spectra of molecules in $^{2}\\Sigma$ states have historically been difficult to obtain because of the unstable nature of the radicals and the corresponding low sensitivity of absorption measurements. Our recent laser excitation spectrum of SrF, produced in a metal-oxidant flame, extended to sufficiently low J values so that Microwave-Optical-Double-Resonance (MODR) measurements on several transitions were possible. In all cases, the MODR signal was detected as an increase in photoluminescence intensity indicating strong optical pumping in the $B^{2}\\Sigma^{+} - X^{2}\\Sigma^{+}$ system. Rotational transitions in $v^{\\prime\\prime} = 0$ and $v^{\\prime\\prime} = 1$ were used to obtain accurate values of the $B_{e}, \\alpha_{e}$ and $\\gamma$ (spin-rotation) constants. The lines were broadened by unresolved hyperfine structure, stray Zeeman fields and microwave power saturation. The general technique of detecting rotational transitions as a change in optical photoluminescence intensity will be discussed. Several simplifications in the rotational spectrum of $^{2}\\Sigma$ states which arise from the resonant two photon nature of the method will also be described.