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The vibrational motion of highly excited molecules is analyzed in terms of exact quantum and classical mechanics calculations and a spectroscopic Hamiltonian and its semiclassical limit by employing global potential energy surfaces. The existence of saddle-node bifurcations and their influence on the spectrum are discussed.

This article discusses energetic forces involved in the interconversion of polyatomic molecules from one mode to another. The vibrational spectrum of phosphaethyne is explained in the context of its potential energy surface, Schrodinger's equation, and the resonance Hamiltonian model.

We have measured the rotationless photodissociation threshold of six isotopologues of NO2 containing 14N, 15N, 16O, and 18O isotopes using laser induced fluorescence detection and jet cooled NO2 (to avoid rotational congestion). For each isotopologue, the spectrum is very dense below the dissociation energy while fluorescence disappears abruptly above it. The six dissociation energies ranged from 25 128.56 cm-1 for 14N16O2 to 25 171.80 cm-1 for 15N18O2. The zero point energy for the NO2 isotopologues was determined from experimental vibrational energies, application of the Dunham expansion, and from canonical perturbation theory using several potential energy surfaces. Using the experimentally determined dissociation energies and the calculated zero point energies of the parent NO2 isotopologue and of the NO product(s) we determined that there is a common De=26 051.17±0.70 cm-1 using the Born-Oppenheimer approximation. The canonical perturbation theory was then used to calculate the zero point energy of all stable isotopologues of SO2, CO2, and O3, which are compared with previous determinations. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We propose an efficient method for calculating the eigenstates and adjusting the parameters of an effective Hamiltonian, which reproduces the experimentally observed energy levels of NO[sub 2] up to 11 800 cm-1 above the quantum mechanical ground state, that is a few thousands of cm-1 above the X [sup 2]A[sub 1]–A [sup 2]B[sub 2] conical intersection, with a rms error less than 4 cm-1. This method principally relies on the determination, through first-order perturbation theory, of an optimal basis for each surface, which takes into account the nonresonant energy shifts experienced by the states of this surface. As a result, the size of the matrix, which one has to build and diagonalize to converge the spectrum up to 11 800 cm-1, is of the order of 500–1000 instead of several tens of thousands. Thank to this Hamiltonian, the analysis of the experimental spectrum up to 11 800 cm-1 could be completed. A detailed description of all states located above 9500 cm-1 is proposed, those lying below 9500 cm-1 being already known and tabulated. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

In this paper we analyze the vibrational spectra of the Σ+g ground state of CS2, the experimental results of which have been described in a forth coming paper. We show that, up to 12 000 cm-1, CS2 can be described by a system of two degrees of freedom strongly coupled by a 1:2 type Fermi resonance. The corresponding vibrational spectra are refitted with the aid of only seven parameters. Analysis of the spectra by the statistical Fourier transform technique reveals stroboscopic effects between the symmetric stretching mode and the bending mode. The distinction between the ‘‘stroboscopic hole’’ due to these effects and the ‘‘correlation hole’’ due to nonintegrable terms in the Hamiltonian is discussed in detail. The study of the topology of the phase space of CS2 in the regular and chaotic cases is carried out in the basis described by a vibrational angular momentum which includes the Fermi resonance. We show the analogy between the localization of the wave packets of the eigenstates and the trajectories. We also show the destabilization of the trajectories due to a term in the Hamiltonian which couples neighboring polyads and which is a second Fermi resonance. We show that only two resonances are enough to induce a chaotic situation. [ABSTRACT FROM AUTHOR]

A laser system composed of tunable lasers pumped by a copper vapor laser (Oxford Lasers Cu60) is described in this paper. The high resolution obtained with this system has allowed excitation of selective rotational levels of the 15 V and 10 V vibrational bands of the V 1B2 excited electronic state of the CS2 molecule in its vapor phase (∼100 mTorr). The rotational assignment of the excitation spectra was accomplished by observing the dispersed fluorescence. We show that it is not necessary to use a supersonic jet in order to assign the emission spectra of CS2. The goal of this work is to study the highly excited vibrational states of the ground electronic state of CS2 up to the first dissociation limit. For our purpose, there are two important consequences of the particular geometry of the 15 V excitation, which is just below and close to the bending potential barrier of the V 1B2 state. First, a very good Franck–Condon overlap in the excitation and a large Franck–Condon access to high vibrational states, as high as 20 000 cm-1, allow observation of the dispersed fluorescence through a high resolution monochromator with an OMA detector. This avoids the need for more complicated techniques, like SEP spectroscopy. Moreover, we show for the first time, that, in the V 1B2 excited state, a strong Coriolis interaction Q32 Q3 Jc couples the bending (0,3,0) and antisymmetric stretching levels (0,0,1). This breaking of the symmetric–antisymmetric selection rule gives access to the antisymetric stretching levels of the ground electronic state X Σ+g from the 15 V excitation. We also show that, below 12 000 cm-1, the vibrational couplings of CS2 can be described by a model of 2 degrees of freedom, which includes a strong 1:2 Fermi resonance and accidental resonant perturbations between adjacent polyads which is probably a first step in the transition to a chaotic regime in CS2. [ABSTRACT FROM AUTHOR]

Discusses the vibrational spectrum of phosphaethyne revealed by complementary experimental examinations.