Quantitative State-Selective Spectroscopy of Water Below and Above the First Dissociation Limit

We use triple resonance vibration overtone spectroscopy to characterize quantum states of water with up to 19 quanta of stretching vibration – the last stretching state below dissociation. State-selectivity, offered by the triple resonance in conjunction with theoretical predictions allows unambiguous assignment of the observed ro-vibrational transitions. Totally 38 new vibrational states and about 360 rotation-vibration energy levels are characterized. These levels span the energy region from 35 500 cm-1 above the vibrational ground state up to the first dissociation limit of the molecule D0 = 41 145.94±0.15 cm-1. In order to increase the number of observed transitions we employ collisionally assisted double/triple-resonance overtone spectroscopy (CADROS/CATROS). In this approach we admit rotational relaxation of water molecules at intermediate levels of double/triple-resonance excitation scheme. Two CADROS-type spectra, for ortho and para water, were measured in the energy region of the vstr = 10 vibrational manifold. Totally 81 new rotation-vibration levels were obtained from these spectra. We extend the use of triple resonance excitation to access states of water above D0. From known assignment of the employed gateway states we perform strict assignments of the nuclear spin and parity of the observed resonances. By comparison of measured dissociation spectra we strictly derive the rotational quantum number J and perform tentative vibrational characterization of quasi-bound states of water. We fit asymmetric shapes of the observed resonances by Fano profiles. The assignments and Fano profile parameters stand as a benchmark for extension of accurate quantum-mechanical calculations to activated water complexes. We consider the possible implication of the observed quasi-bound states for the kinetics of water dissociation and the OH+H association reaction. We finally performed measurements of Stark coefficients for J = 2 rotational levels in vibrational states within an energy region from 27 500 to 40 984 cm-1, using a combination of triple resonance overtone excitation with photofragment detection and with Stark induced quantum beat spectroscopy. We compare these data with the Stark coefficients, calculated using the most recently developed dipole moment surface and two different potential energy surfaces.


Related material