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.

Feshbach resonances in the water molecule revealed by state-selective spectroscopy

Maxim Grechko, Pavlo Maksyutenko, Thomas Rizzo

We employ triple-resonance vibrational overtone excitation to access quasibound states of water from several fully characterized bound states of the molecule. Comparison of the measured dissociation spectra allows a rigorous assignment of rotational quantum numbers J, nuclear spin and parity and a tentative vibrational characterization of the observed resonances. Their asymmetrical shapes (Fano profiles) reflect interference of dipole moments for transitions to these resonances with that to the dissociative continuum. The assignments and Fano profile parameters of the resonances stand as a benchmark for the extension of accurate quantum-mechanical calculations to activated complexes of water. The narrow widths of some of these resonances indicate that water molecules may survive for as long as up to 60 ps in states above the dissociation threshold. We consider the possible implication of such long-lived states for the kinetics of water dissociation and the OH+H association reaction.

American Institute of Physics2010

Multiple-resonance spectroscopy of high vibrational levels in water and methanol

Pavlo Maksyutenko

We have measured the rovibrational levels in the electronic ground state of the water molecule at the previously inaccessible energies above 26000 cm-1. The use of laser double-resonance overtone excitation combined with laser-induced fluorescence (LIF) photofragment detection extends this limit to 34200 cm-1, which corresponds to 83% of the water dissociation energy. These data have allowed the theoretical group of Oleg Polyansky to generate a semiempirical potential energy surface on the basis of their ab initio surface [O. L. Polyansky, A. G. Csaszar, S. V. Shirin et al., Science 299 (5606), 539 (2003)] that now allows prediction of water levels with sub-cm-1 accuracy at any energy up to the new limit. A new ab initio potential energy surface is being constructed by Polyansky et al. that is intended to reproduce the rovibrational levels of water up to the dissociation threshold. We have calculated the electronic energy of ca. 2500 nuclear geometries at multireference configuration interaction level with 6Z basis set. We have performed a direct measurement of one of the most fundamental thermochemical values: the O-H bond energy in water. Using a triple-resonance laser excitation scheme, we excite the molecule through a series of vibrational overtone transitions to access directly the onset of the dissociative continuum. The dissociation energy obtained from our experiments, 41145.94 ± 0.15 cm-1, is ca. 30 times more accurate than the currently accepted value and has important implications for other thermochemical quantities linked to the bond energy of water. We have studied the conformational dependence of intramolecular vibrational redistribution in the 5v1 OH stretch overtone region of methanol. Previous state-selected spectra in the 5ν1 region [O. V. Boyarkin, T. R. Rizzo, and D. S. Perry, J. Chem. Phys. 110, 11346 (1999)] revealed a structure indicating an intramolecular vibrational redistribution on three time scales. Whereas in that work, methanol in the 5 v1 bright state was prepared close to the staggered conformation, methanol in the "partially eclipsed" conformation is prepared in our experiments by double resonance excitation through a torsionally excited intermediate state. We detect the excited molecules by infrared laser assisted photofragment spectroscopy (IRLAPS). In partially eclipsed methanol, the strong coupling of the v1 OH stretch to the v2 CH stretch becomes weaker, but the coupling responsible for the widths of the narrowest features becomes stronger.

EPFL2008

Multiple-resonance spectroscopy of high vibrational levels in water and methanol

Pavlo Maksyutenko

EPFL2008