The microwave spectrum of the cesium monoxide CsO radical

The microwave spectrum of CsO has been observed and analyzed, not only in t he ground vibrational state, but also in the upsilon = 1-3 excited vibratio nal states. The CsO radical was generated by the reaction of N2O with Cs va por, which was produced by the reaction of Li metal with CsCl at 500-530 de grees C. The observed spectra were found to conform to those expected for a (2)Sigma diatomic molecule, thereby establishing the ground electronic sta te of CsO to be of (2)Sigma. The observed rotational and centrifugal distor tion constants yielded the equilibrium bond length and the harmonic vibrati onal frequency to be 2.300 745 (16) Angstrom and 356.78 (11) cm(-1), respec tively, based on the Born-Oppenheimer approximation. A careful examination of the observed spectral pattern definitely concluded that the spin-rotatio n interaction constant was positive, at variance with the expectation from a simple (2)Sigma/(2)Pi two-states interaction. This observation was interp reted by assuming positive contributions from higher excited electronic sta tes which superseded a negative contribution from the (2)Pi lowest excited state; the latter state was responsible for the large dependence of the spi n-rotation interaction constant on the vibrational quantum number and was e stimated from this vibrational dependence to be located at 1225 cm(-1) abov e the ground electronic state. In reverse to the spin-rotation splitting, t he hyperfine splitting was found to increase with the vibrational excitatio n; in the upsilon = 3 state the hyperfine structure was found completely re solved. However, the hyperfine coupling constants did not vary much with th e vibrational quantum number, namely the vibrational dependence of the hype rfine splitting was caused primarily by that of the spin-rotation splitting . The observed hyperfine interaction constants indicated that CsO was an io nic molecule. (C) 1999 American Institute of Physics. [S0021-9606(99)00845- 4].