State-to-State Chemistry

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This is a nonlinear process. For absorption of several tens of photons simultaneously, one would expect to take. However, "state-to-state" chemistry does have meaning when one is concerned with the possible product electronic states; for, the dynamics [3], symmetry [4] and spin [5] "selection rules" may inhibit the reactants from forming certain of these states.

In the reactions of.

Determining States of Matter in Chemical Reactions

Reactions of oxygen atoms with aromatic hydrocarbons have received a great deal of attention in recent years because of their potential importance in combustion processes and in atmospheric chemistry. The apparatus used in these experiments consisted of a quadrupole mass spectrometer housed inside a two-chamber, diffusion-pumped, stainless steel vacuum system. The major features of the design were similar to the apparatus employed by Foner and Hudson.

A fundamental collisional process at elevated energies is the dissociation of a molecule into two or more fragments via a collision with a third body. This so called collision-induced dissociation CID has been observed extensively in ion-molecule reactions 1 and, in more recent years, between neutral species CID of the alkali and thallium halides to ion pairs has been studied in this laboratory for the last few years, and the threshold behavior for the process has been determined 4,5. The experiments involve accelerating the M species by the seeded jet method, with H2 as the propellant, and crossing the beam of M with a beam of the cesium or.

To achieve a complete description of the states of reactants or products in a chemical reaction, it is necessary to specify the translational states of the participants. A somewhat coarser level of detail still might include the dependence of the rate on the magnitude of the initial or final relative velocity. Molecular beam reactive scattering experiments are the principal sources of data on translational-energy dependence. For reactants this is achieved through velocity selection and for products by the combination of velocity analysis and angular distributions.

In measurements involving resonant absorption or emission of light, the Doppler effect furnishes a different means of access to velocity information potentially as detailed as that attained by direct velocity analysis. This approach appears especially promising in measurements employing laser-induced fluorescence. The rates of most chemical reactions are accelerated when the temperature is raised, and this has long been interpreted as evidence for an energy requirement the "activation energy" in excess of any thermodynamic requirement.

This activation energy has traditionally been supplied by heating the reagents, but energy transfer among reagent modes is so rapid that no information is obtained to identify the critical mode for reaction. As recently as three years ago, many experts in molecular dynamics thought that chemical activation by means of infrared lasers was not likely to be both practical and useful. There were essentially two reasons for this view. Yet there were reasons to fear that the absorbed infrared energy at all but very low.

There currently is considerable interest in the chemistry of vibrationally excited molecules. By selecting the vibrational states of reagents it is possible to control the rates and pathways of chemical reactions. Such experiments can provide detailed information regarding microscopic reaction mechanisms.

Particular attention has been paid to the gas phase reaction of nitric oxide and ozone. It has been found that the rates of both processes are increased by an order of magnitude when either O3 or NO 6 is vibrationally excited. With the laser off, a steady signal, Idc, was detected. When the. The spin-conserved route associated with the formation of CO2 and C2O, however, was found to be less than 0.

In order to understand the dynamics of this unique process, we employed a cw CO laser to measure the vibrational population of the CO formed in the initial stage of the reaction. A detailed description of the laser-probing apparatus can be found elsewhere 5,6. A Pyrex flash tube was used in the present study to avoid the. The flow rates and pressure in the vessel were 0. These experiments are an improvement over previous work from this laboratory and permit assignment of both the initial vibrational and rotational distributions of the HF and HCl product molecules.

Thus these reactions are chemically activated unimolecular fragmentations with the interesting and important feature that the product UV-Visible chemiluminescence can be used for characterization.

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We are engaged in a detailed study of the dynamics of these reactions. A schematic of the reaction coordinate is shown as Figure 1. Infrared laser induced reactions more often than not lead to products that are consistent with the purely thermal reaction and hence it is difficult to discern whether the laser is acting as more than a heat source. This is particularly true when using C. It is under these latter conditions, however, that we feel we have strong evidence for a non-thermal component in the reaction pathway, although we do not discount a simultaneous thermal component as well.

In an earlier paper [1] on CF2ClCF2Cl we noted a large difference in the reaction rate on irradiation of two separate bands of the compound cm-1 and cm-1 at the band centers. Since the slightly stronger cm-1 band gave the slower rate, it appeared that compartmentalization of energy in the vibrational mode was operating to. During the past few years there has been considerable interest in chemical reactions that produce excited alkali metal atoms.

Chemiluminescence from alkali atoms has been studied recently in both beam experiments where single collision processes can be observed as well as in flames where complex collisional energy transfer plays an important role in the excitation process. These studies were motivated in part by the possibility that these reactions may produce inversion in the excited states of the alkali atoms, suitable for chemical lasers.

We are studying the chemiluminescence produced by the reaction of boron with the alkali fluorides MF , that is. With the reactants in the ground state the exoergicity of these reactions is about 3 ev. This amount of energy can excite only the first p states in sodium and potassium but in rubidium and cesium higher lying excited. Chemiluminescent flames resulting from the reactions between metal atoms and various oxidant gases have been studied extensively in emission under molecular beam and low pressure flow conditions 1. In all cases it is found that the initial reaction products which spontaneously emit light represent a small fraction of the total products.

The technique of laser-induced fluorescence as applied to gas phase reaction products has been well documented by Zare and co-workers 2. Briefly it involves gated optical detection of a pulsed fluorescence signal resulting from pulsed electronic excitation of product molecules by a tunable visible dye laser. However, in the present studies the presence of very bright visible chemiluminescence emanating from the same reaction volume as the laser induced fluorescence, precludes simple. Energy transfer processes in molecules have been of interest to chemists for well over 50 years simply because the majority of chemical reactions proceed through molecular bond breaking.

The rupture of chemical bonds requires energy, but not necessarily just any form of energy. From the journal: Physical Chemistry Chemical Physics. You have access to this article. Please wait while we load your content Something went wrong. Try again? Cited by. Back to tab navigation Download options Please wait Supplementary information PDF K. Article type: Paper. DOI: Download Citation: Phys. He, W. Li, H. Meng, C. Li, G. Guo, X. Qiu and J. Wei, Phys.

State of matter - Wikipedia

Search articles by author Xiaohu He. Wenliang Li. Huiyan Meng. In crystalline solids, particles are packed in a regularly ordered, repeating pattern.

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There are many different crystal structures, and the same substance can have more than one structure. Ice has fifteen known crystal structures, each of which exists at a different temperature and pressure. A solid can transform into a liquid through melting, and a liquid can transform into a solid through freezing.

A solid can also change directly into a gas through a process called sublimation. A liquid is a fluid that conforms to the shape of its container but that retains a nearly constant volume independent of pressure. The volume is definite does not change if the temperature and pressure are constant. When a solid is heated above its melting point, it becomes liquid because the pressure is higher than the triple point of the substance. Intermolecular or interatomic or interionic forces are still important, but the molecules have enough energy to move around, which makes the structure mobile.

This means that a liquid is not definite in shape but rather conforms to the shape of its container. Its volume is usually greater than that of its corresponding solid water is a well-known exception to this rule. The highest temperature at which a particular liquid can exist is called its critical temperature. This process of a liquid changing to a gas is called evaporation. Gas molecules have either very weak bonds or no bonds at all, so they can move freely and quickly. Because of this, not only will a gas conform to the shape of its container, it will also expand to completely fill the container.

Gas molecules have enough kinetic energy that the effect of intermolecular forces is small or zero, for an ideal gas , and they are spaced very far apart from each other; the typical distance between neighboring molecules is much greater than the size of the molecules themselves.