Polyatomic molecules, resonant

The question of non-classical manifestations is particularly important in view of the chaos that we have seen is present in the classical dynamics of a multimode system, such as a polyatomic molecule, with more than one resonance coupling. Chaotic classical dynamics is expected to introduce its own peculiarities into quantum spectra [29, 77]. In Fl20, we noted that chaotic regions of phase space are readily seen in the classical dynamics corresponding to the spectroscopic Flamiltonian. Flow important are the effects of chaos in the observed spectrum, and in the wavefiinctions of tire molecule In FI2O, there were some states whose wavefiinctions appeared very disordered, in the region of the... 

Johnson B R, Kittrell C, Kelly P B and Kinsey J L 1996 Resonance Raman spectroscopy of dissociative polyatomic molecules J. Chem. Educ. 100 7743-64... 

I. N. Levine (1975) Molecular Spectroscopy (John Wiley Sons, New York). A survey of the theory of rotational, vibrational, and electronic spectroscopy of diatomic and polyatomic molecules and of nuclear magnetic resonance spectroscopy. 

This chapter deals mainly with (multi)hyphenated techniques comprising wet sample preparation steps (e.g. SFE, SPE) and/or separation techniques (GC, SFC, HPLC, SEC, TLC, CE). Other hyphenated techniques involve thermal-spectroscopic and gas or heat extraction methods (TG, TD, HS, Py, LD, etc.). Also, spectroscopic couplings (e.g. LIBS-LIF) are of interest. Hyphenation of UV spectroscopy and mass spectrometry forms the family of laser mass-spectrometric (LAMS) methods, such as REMPI-ToFMS and MALDI-ToFMS. In REMPI-ToFMS the connecting element between UV spectroscopy and mass spectrometry is laser-induced REMPI ionisation. An intermediate state of the molecule of interest is selectively excited by absorption of a laser photon (the wavelength of a tuneable laser is set in resonance with the transition). The excited molecules are subsequently ionised by absorption of an additional laser photon. Therefore the ionisation selectivity is introduced by the resonance absorption of the first photon, i.e. by UV spectroscopy. However, conventional UV spectra of polyatomic molecules exhibit relatively broad and continuous spectral features, allowing only a medium selectivity. Supersonic jet cooling of the sample molecules (to 5-50 K) reduces the line width of their... 

Polyatomic molecules provide a still richer environment for studying phase control, where coupling between different dissociation channels can occur. Indeed, one of the original motivations for studying coherent control was to develop a means for bond-selective chemistry [25]. The first example of bond-selective two-pathway interference is the dissociation of dimethyl-sulfide to yield either H or CH3 fragments [74]. The peak in Fig. 11 is indicative of a resonance embedded in an elastic continuum (case 4). 

Amat, G., and Nielsen, H. H. (1958b), Vibrational l-Type Doubling and /-Type Resonance in Polyatomic Molecules, J. Mol. Spectrosc. 2, 152. 

Coriolis resonance interactions spect Perturbationof two vibrations of a polyatomic molecule, having nearly equal frequencies, on each other, due to the energy contribution of the Coriolis operator. kor e o las rez-on-ons, in-tor,ak-shonz ) corresponding states phys chem The condition when two or more substances are at the same reduced pressures, the same reduced temperatures, and the same reduced volumes., kar-3 spand ir) stats )... 

Fermi resonance physchem In a polyatomic molecule, the relationship of two vibrational levels that have In zero approximation nearly the same energy they repel each other, and the eigenfunctions of the two states mix. fer-me, rez-3n-3ns fermium chem Asynthetic radioactive element, symbol Fm, with atomic number 100 discovered in debris of the 1952 hydrogen bomb explosion, and now made in nuclear reactors. fer-me-3m )... 

Very large rate constants have been found for near resonant energy transfer between infrared active vibrations in CO2 Such near-resonant transitions and their dependence on temperature have also been studied for collisions between vibrationally excited CO2 and other polyatomic molecules as CH4, C2H4, SF et al. The deactivation cross-sections range from 0.28 for CH3F to 4.3 for SFs at room temperature, and decrease with increasing temperature. 

ORIT in the Photoexcitation Spectrum in Pyrazine The ORIT phenomenon, where a photoabsorption transparency window occurs at certain frequencies due to interference between material waves within a molecule, is briefly considered here. Though ORIT is known for small systems [25,27], it has not been investigated for polyatomic molecules where overlapping resonances... 

There can be a difference between the dissociation of polyatomic molecules and delayed ionization in the nature of the initial excitation. In ZEKE spectroscopy the state that is optically accessed (typically via an intermediate resonantly excited state) is a high Rydberg state, that is a state where most of the available energy is electronic excitation. Such a state is typically directly coupled to the continuum and can promptly ionize, unlike the typical preparation process in a unimolecular dissociation where the state initially accessed does not have much of its energy already along the reaction coordinate. It is quite possible however to observe delayed ionization in molecules that have acquired their energy by other means so that the difference, while certainly important is not one of principle. 

Figure 1. Schematic representation of resonances in the continuum of a polyatomic molecule ABC(X) dissociating into products AB(X, 0) and C. The left-hand side shows an absorption-type cross section <7abs( ) with a rich resonance pattern. The term p(E) is the density of states at the energy E and N r(E) is the number of states at the TS, orthogonal to the dissociation path, that are accessible at energy E. Several experimental schemes for a spectroscopic analysis of resonances are also indicated. (Reprinted, with permission of the Royal Society of Chemistry, from Ref. 34.)...

PHOTODISSOCIATING SMALL POLYATOMIC MOLECULES IN THE VUV REGION RESONANCES IN THE 1E+-1E+ BAND OF OCS... 

The vibrational overlap is both important and difficult to guess. In polyatomic molecules the large number of excited states near resonance will tend to increase the rates of all nonradiative processes, including decay to both A and B. However, intuition tells us that in a very large molecule not all vibrational levels will be important. For example, fluorescence lifetimes of alkyl derivatives of aromatic hydrocarbons are essentially independent of the length of the attached alkyl chain.26... 

F ERMI RESONANCE. In polyatomic molecules. Hvo vibrational levels belonging to different vibrations lor combinations of vibrations) may happen lo have nearly die same energy, and therefore be accidentally degenerate. As was recognized hy Fermi in the case of CO such a "resonance" leads to a perturbation of the energy levels that is very similar to the vibrational perturbations of diatomic molecules. 

An interesting development in molecular rotational relaxation has been the microwave double-resonance method176-178. The technique permits the exploration of the fine detail of the processes which occur in collisions of polyatomic molecules, and results for a number of symmetric tops have been reported. For example, Oka has described experiments on NH3 in which inversion doublets for selected J values were pumped by high microwave power. Pumping disturbs the population of the inversion doublet, and also that of other doublets which are populated from the original pair by collision processes. By absorption measurements of other inversion doublets with steady state irradiation, Oka has shown that in NH3/NH3 collisions, transitions which are allowed by the electric dipole selection rules (A/ = 0, 1, + - —) are preferred. Oka s analysis indicates that relaxation is most favourable in collision with molecules having similar J values, which are termed rotational resonances (R-R transfer). For example the process... 

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