Dipole Moment of Water in Highly Vibrationally Excited States: Analysis of Photofragment Quantum-Beat Spectroscopy Measurements Using a Local-Mode Hamiltonian

We present here the analysis of experimental Stark effect measurements made using photofragment quantum beat spectroscopy on the 14,0->, 15,0->, 18,0+> and 14,0-> 12 > vibrational states of H2O [Callegari, A.; et al. Science 2002, 297, 993.]. To link the measured Stark coefficients with the dipole surface, we analyze our results using a coupled anharmonic oscillator model, which takes into account the local-mode nature of higly excited OH stretching vibrations in water, and the tunneling between the two equivalent bonds. The large inertial frame tilt associated with the local-mode bond stretching results in a complex interaction between rotational-, vibrational-, and tunneling-motion, all of which become deeply entangled in the Stark coefficients. A perturbational approach makes it possible to analyze the problem at increasingly higher levels of approximation and to disentangle the different contributions, according to the different time scales involved. This simple model reproduces most experimental values to within a few percent, even for these highly vibrationally excited levels, and gives valuable insight into the complex rotational and vibrational motions that link the dipole moment surface with the Stark coefficients.