Global spectroscopy of the water monomer

Given the large energy required for its electronic excitation, the most important properties of the water molecule are governed by its ground potential energy surface (PES). Novel experiments are now able to probe this surface over a very extended energy range, requiring new theoretical procedures for their interpretation. As part of this study, a new, accurate, global spectroscopic-quality PES and a new, accurate, global dipole moment surface are developed. They are used for the computation of the high-resolution spectrum of water up to the first dissociation limit and beyond as well as for the determination of Stark coefficients for high-lying states. The water PES has been determined by combined ab initio and semi-empirical studies. As a first step, a very accurate, global, ab initio PES was determined using the all-electron, internally contracted multi-reference configuration interaction technique together with a large Gaussian basis set. Scalar relativistic energy corrections are also determined in order to move the energy determinations close to the relativistic complete basis set full configuration interaction limit. The electronic energies were computed for a set of about 2500 geometries, covering carefully selected configurations from equilibrium up to dissociation. Nuclear motion computations using this PES reproduce the observed energy levels up to 39 000 cm(-1) with an accuracy of better than 10 cm(-1). Line positions and widths of resonant states above dissociation show an agreement with experiment of about 50 cm(-1). An improved semi-empirical PES is produced by fitting the ab initio PES to accurate experimental data, resulting in greatly improved accuracy, with a maximum deviation of about 1 cm(-1) for all vibrational band origins. Theoretical results based on this semi-empirical surface are compared with experimental data for energies starting at 27 000 cm(-1), going all the way up to dissociation at about 41 000 cm(-1) and a few hundred wavenumbers beyond it.

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

State-selective spectroscopy of water up to its first dissociation limit

Maxim Grechko, Pavlo Maksyutenko, Thomas Rizzo

A joint experimental and first-principles quantum chemical study of the vibration-rotation states of the water molecule up to its first dissociation limit is presented. Triple-resonance, quantum state selective spectroscopy is used to probe the entire ladder of water’s stretching vibrations up to 19 quanta of OH stretch, the last stretching state below dissociation. A new ground state potential energy surface of water is calculated using a large basis set and an all-electron, multireference configuration interaction procedure which is augmented by relativistic corrections and fitted to a flexible functional form appropriate for a dissociating system. Variational nuclear motion calculations on this surface are used to give vibrational assignments. A total of 44 new vibrational states and 366 rotation-vibration energy levels are characterized; these span the region from 35 508 to 41 126 cm-1 above the vibrational ground state.

2009

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

Pavlo Maksyutenko

EPFL2008