Optical–optical double-resonance spectroscopy of autoionizing states of water

Double resonance excitation of vibrationally autoionizing states of the water molecule has been performed for the first time. Tunable, coherent vacuum ultraviolet light was used to excite selected rotational levels of the C̃ B1 state with one quantum of symmetric stretch vibrational energy ~100!, from which ~100! Rydberg states were excited with a second dye laser. The Rydberg character of the C̃ B1 state leads to strong vibrational selectivity for the ~100! states in the second transition; therefore, the resonances due to these states are greatly enhanced compared to the case of direct excitation from the ground state, and spectrally simplified due to the well-defined angular momentum of the intermediate state level. The spectra show unexpected peaks which might be attributable to significant 1-mixing in the C̃ B1 state as well as the presence of interloping linear states. © 1997 American Institute of Physics. @S0021-9606~97!02539-7#