Knowledge Bank

The weak vibration-rotation emission spectrum of OH has been recorded photoelectrically in the range $\lambda 6650–8800$ {\AA} with a 1.26 m granting spectrometer. The emission was excited by the reaction of $0(3P)$ atoms with ethylene or propylene in a fast-flow system at pressures near 3 Torr. The experimental resolution in the second-order of a 1200 groove/mm grating was 0.04 {\AA}, which enabled A-doubling to be resolved at J$ \geq 3 1/2$.\marginpar{*Check} The spectra were calibrated by superposition of hollow cathode Fe-lines. Six bands have been rotationally analysed (6-2, 7-2, 8-3, 9-4, 10-4, and 10-5), and least squares fits of the measured line frequencies have been made to determine the first reliable molecular constants of OH $X2\pi$ in the range $6\le v\le 10$. The standard deviations of the fits are typically $0.03cm-1$. The v-dependences of the A-doubling constants p and q and the spin-orbit coupling constant A are well determined. A minimum value of A is observed at v = 7. The final constants to be obtained from the data will be reported together with a new RKR curve for $v\le 10$. The v = 10 level lies only $7200cm-1$ from the dissociation limit.