Potential energy surface

Summary

A potential energy surface (PES) describes the energy of a system, especially a collection of atoms, in terms of certain parameters, normally the positions of the atoms. The surface might define the energy as a function of one or more coordinates; if there is only one coordinate, the surface is called a potential energy curve or . An example is the Morse/Long-range potential. It is helpful to use the analogy of a landscape: for a system with two degrees of freedom (e.g. two bond lengths), the value of the energy (analogy: the height of the land) is a function of two bond lengths (analogy: the coordinates of the position on the ground). The PES concept finds application in fields such as chemistry and physics, especially in the theoretical sub-branches of these subjects. It can be used to theoretically explore properties of structures composed of atoms, for example, finding the minimum energy shape of a molecule or computing the rates of a chemical reaction. Mathematical defi

Official source

https://en.wikipedia.org/wiki/Potential_energy_surface

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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.

2012

A global line list for HDO between 0 and 35000 cm−1 constructed using multiphoton spectra

Multiphoton spectroscopy of monodeuterated water is employed to determine more than 210 new energy levels of HDO in the 25 000–35 000 cm region. These new empirical energy levels are used to fit a potential energy surface (PES) valid between 20 000 cm and 35 000 cm. Using this PES and an accurate lower energy PES as starting points, an energy-switching surface is constructed which is capable of reproducing the whole HDO spectrum from 0 to 35 000 cm. This PES is used to calculate a line list for HDO which covers infra red, visible and near ultraviolet transitions.

2021

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.

EPFL2010

Global spectroscopy of the water monomer

2012

EPFL2010