Showing papers in "Annual Review of Physical Chemistry in 1979"

Multiphoton Dissociation of Polyatomic Molecules

Abstract: The dynamics of infrared multiphoton excitation and dissociation of SF/sub 6/ was investigated under collision free conditions by a crossed laser-molecular beam method. In order to understand the excitation mechanism and to elucidate the requirements of laser intensity and energy fluence, a series of experiments were carried out to measure the dissociation yield dependences on energy fluence, vibrational temperature of SF/sub 6/, the pulse duration of the CO/sub 2/ laser and the frequency in both one and two laser experiments. Translational energy distributions of the SF/sub 5/ dissociation product measured by time of flight and angular distributions and the dissociation lifetime of excited SF/sub 6/ as inferred from the observation of secondary dissociation of SF/sub 5/ into SF/sub 4/ and F during the laser pulse suggest that the dynamics of dissociation of excited molecules is dominated by complete energy randomization and rapid intramolecular energy transfer on a nanosecond timescale, and can be adequately described by RRKM theory. An improved phenomenological model including the initial intensity dependent excitation, a rate equation describing the absorption and stimulated emission of single photons, and the unimolecular dissociation of excited molecules is constructed based on available experimental results. The model shows that the energy fluencemore » of the laser determines the excitation of molecules in the quasi-continuum and the excess energy with which molecules dissociate after the laser pulse. The role played by the laser intensity in multiphoton dissociation is more significant than just that of overcoming the intensity dependent absorption in the lowest levels. 63 references.« less

Creation and Disposal of Vibration Energy in Polyatomic Molecules

Abstract: The importance of vibrational excitation in chemical reactions has been known for many years, having been recognized first in the study of the unimolecular reactions of polyatomic molecules. Because a nonlinear polyatomic molecule of N atoms has 3N 6 vibrational degrees of freedom (compared to three rotations and three translations), it is clear that much of the phenomenology of excited molecules must reside in vibration. As in most fields of chemical physics the study of vibrational excitation has grown together from two widely separated areas. Bulk kinetic data (thermal unimolecular reaction rates and sound dispersion) and static spectroscopic energy level experiments are now combined into a large area of interest dominated by spectroscopic measurements of dynamic processes. A knowledge of the chemical, energetic, and spectral properties of poly atomic molecules is important for studies of chemical reactivity, isotope separation, combustion processes, chemical lasers and other technologies, as well as being a splendid stimulus to the ever expanding predictive abilities of chemical theorists. The importance of a knowledge of vibrational processes has increased rapidly with the possibility of using lasers to induce state-specific chemical reactions. Such excitation can result in interesting selectivity only if the resulting molecules retain their state-specific excitation long enough to react. In contrast to the extremely large number of highly selective electronic photochemical reactions, the only comparably selec­ tive effect for vibrational excitation is the isotope selectivity of infrared multiphoton dissociation. Although vibrational excitation is known to enhance strongly the rate of a few bimolecular reactions, there are to date no data showing chemical selectivity based on state-specific excita-

Photosynthesis-The Light Reactions

Abstract: During the past three or four years there has been a substantial advance in our knowledge of the light reactions in photosynthesis. This comes at a time of heightened interest in sunlight as a source of energy for our daily needs. The exploration of potential practical applications in this area will be helped by identifying the common features of the light reactions in higher plants and algae and in the simpler photosynthetic bacteria. These include the processes of light absorption by an antenna of chlorophyll (Chl) and other pigment molecules, the transfer of excitation to a photochemical reaction center, and the initiation of the electron transfer reactions - all within a few picoseconds of the arrival of the photon.

Negative Ion Spectroscopy

Abstract: This article surveys experimental methods for studying the interactions of small, free atomic and molecular negative ions with radiation, and the structural information that can be obtained from such studies. The subject is an old one, having its origins some 40 years ago when Wildt (1) recognized that Hwas a major source of opacity in the solar spectrum. That same year the first book (2) devoted to negative ions appeared. An excellent summary of the early work has been given by Branscomb (3). The past decade, however, has seen major advances in both photon and ion sources for negative ion spectroscopic experiments, which have enabled many new studies of negative ion spectroscopy. In contrast to neutral molecules, which have an infinite number of bound states, negative ions have only one or a few bound states. One is thus forced to study bound-free transitions in negative ions, namely photodetachment. There are basically two types of photodetachment experiments, photodetachment threshold spectroscopy and photoelec­ tron spectroscopy of negative ions. In principle, both types of experi­ mental data may be analyzed to obtain electron affinities and molecular structural iflformation. These methods and the types of information obtainable from such data form the basis for this review. Space limitations preclude coverage of all topics in this field. A critical review of atomic electron affinities (4) appeared a few years ago and will be updated in the near future. There have been a number of earlier reviews of molecular electron affinities (5-9) and a major new

Spectroscopy of Molecular Ions in Noble Gas Matrices

Abstract: Molecular ions are of considerable chemical and physical interest for comparing the spectroscopic and bonding properties of neutral mole­ cules with their positive and negative molecular ions. The study of molecular ions in the gas phase by photoelectron, photoionization mass, and ion cyclotron resonance spectroscopies can be complemented by infrared and optical absorption spectra of the molecular ion trapped in a solid inert gas host. Further ion studies with tunable infrared lasers will be greatly aided by the vibrational data obtained for molecular ions in noble gas solids. The ion-matrix interaction is of fundamental and practical interest as matrix. spectra of ions are related to the gas phase. Charged species in matrices form two general classes described in the literature as "isolated" and "chemically bound" with respect to the counterion. The first ionic species characterized in matrices, Li + 0;, is of the latter type where the lithium cation and the superoxide anion are Coulombically bound together (1, 2), and charge transfer occurs be­ cause this electrostatic attraction more than makes up for the difference between the ionization energy of lithium and the electron affinity of oxygen. The next molecular ions identified in matrices, B2H; and �-, are of the isolated type where the cation is separated by an unde­ termined number of matrix atoms from the anion (3, 4). These ionic systems have been characterized as "Coulomb ion pairs," which exist because of essentially zero overlap between the wavefunctions for the electron on the recipient molecule and the cation that provided the electron (5). Clearly, the formation of ions of the isolated type requires

Developments in Extended X-Ray Absorption Fine Structure Applied to Chemical Systems

Abstract: level, is given by the various spectroscopies, spanning the electromagnetic spectrum. In the present review we describe a structure probe based on a spectroscopy (X-ray absorption spectroscopy) but for which structure information can be deduced due to a diffraction-like phenomenon. Specifically, extended X-ray absorption fine structure (EXAFS ) spectroscopy consists of measuring the absorption spectrum of matter for X rays with photon energies (or wavelengths) in the vicinity of the absorption edges that char acterize the elements present in the material. Such spectra show modulation, called fine structure, of the X-ray absorption coefficient and this is present because photoelectrons released from the X ray absorbing atoms backscatter coherently from neighboring attoms. In this sense, EXAFS spectroscopy, although in­ volving X rays, is a structure probe because of an electron scattering

Singlet-Triplet Energy Separations in Some Hydrocarbon Diradicals

Abstract: A diradical is literally a molecule with two unpaired electrons. We prefer the somewhat broader definition of a diradical as a molecule in which two electrons occupy a degenerate or nearly degenerate pair of orbitals. Group theory predicts a symmetry enforced degeneracy for a pair of orbitals, occupied by a total of two electrons, in the most symmetrical molecular geometry of methylene (CH2), cydobutadiene (CBD), and trimethylenemethane (TMM) (see Figure 1). Thus, we consider all three of these molecules to be diradicals. CH2 is the simplest molecule containing a divalent carbon atom. Although a very large number of other carbenes have been �!udied (1-3), in this review we confine our discussion to the parent hydro­ carbon. CBD yields to cyclopropenyl anion (4, 5) the position of being the smallest fully conjugated cyclic hydrocarbon containing 4n pi elec­ trons; but the former annulene has received more attention. Therefore, our discussion of molecules that are diradicals by virtue of being antiaromatic annulenes focuses on CBD. We have also chosen to discuss another four pi electron system, TMM, as representative of the class of diradicals that are fully conjugated hydrocarbons but for which no classical Kekule structures can be written (6). TMM is the simplest such molecule and, again, the best studied.

Theoretical Studies of Molecular Electronic Transition Lasers

Abstract: The last few years have seen the discovery and rapid development of numerous electronic transition lasers, a new type of laser that offers powerful and efficient sources of coherent visible and ultraviolet radia­ tion for applications in photochemistry,. isotope separation, and con­ trolled thermonuclear fusion. In electronic transition lasers, the lasing occurs between two electronic states of a molecule, where the emission energy corresponds to a photon in the visible or ultraviolet region of the spectrum. Successful laser action requires efficient formation of the upper laser state and rapid depletion of the lower laser state. Rapid depletion of the lower laser level is necessary to maintain the population inversion in the laser, thereby maximizing the energy output. In "bound-free" emission a transition terminates on an unbound lower state (or at least on a repulsive portion of the potential energy curve of the lower state) and the molecule dissociates

Renormalized Kinetic Theory of Dense Fluids

Abstract: The purpose of this review, which belongs to the series on time-depen­ dent statistical mechanics (1-4), is to assess the current status of kinetic theory description of thermal fluctuations in a dense fluid. The quanti­ ties of interest are the space-time correlation functions through which one can study theoretically and experimentally the nonequilibrium properties of many-body systems. Besides the review articles in these volumes, there exist recent reviews (5, 6) and monographs (7-9) dealing with the methods for calculating time correlation functions and the study of dynamics of simple liquids. Reviews (10-12) and a monograph (13) on the kinetic theory approach have also appeared. In the present discussions we focus on fluctuations at finite wavelengths and frequen­ cies and emphasize the direct confrontation between theoretical predic­ tions and experimental data. The problem to be addressed is the derivation of a kinetic equation for fluids over a wide range of density and without restrictions on the range of space-time variations, and the demonstration of the validity of such an equation. For dilute fluids the appropriate kinetic equation is the well-known linearized Boltzmann equation or its generalization to arbitrary frequency and wavelength. These equations involve only un­ correlated binary collisions, and they are computationally tractable. For dense fluids an appropriate equation that treats the dynamics realisti­ cally and is yet tractable for practical calculations is still a subject of active investigation. The development and analysis of equations of this kind is therefore the central theme of our review. There exists a reasonable amount of experimental results to indicate what are the important effects to consider in developing a kinetic theory description of dense fluids. These effects are not unique to kinetic

Unimolecular Ion Decomposition

Abstract: The objective of this review is to discuss the unimolecular decomposi­ tion of polyatomic ions occurring over a wide range of time scales. Selected experimental techniques are described together with the results obtained from those studies. The interaction of high kinetic energy ions with neutral molecules (1) is not discussed here. Decomposition of negative ions is also omitted. Unimolecular ion decompositions have traditionally been studied by the various methods of mass spectrometry. The experiments can be divided into two categories, one where ions are prepared with known internal energy content (Section 2), and the other where decomposition is characterized by the lifetime (Section 3). The rate of formation of a fragment ion as a function of time is an important experimental observable and is directly accessible to field ionization techniques. Consider the decomposition of the molecular ion at time t, then k(t), the normalized rate constant is expressed by dNJt)/ dt dIJt)/ dt k(t)= Nm(t) = 1m ' 1.

Coherent Transient Effects in Optical Spectroscopy

Abstract: Coherent transient effects appear when a sample is illuminated by pulses of radiation and one observes the absorption or coherent emis­ sion by the sample on a time scale short compared to its relaxation time. Depending upon the type of pulses used, a wide variety of different transient effects can be produced, providing researchers with a set of powerful and sensitive tools that can be used to obtain many different kinds of spectroscopic data. These effects were first seen in nuclear magnetic resonance with the discovery of transient nutation in 1949 and spin echoes and free induction decay in 1950 (1, 2). Since that time, pulsed nuclear magnetic resonance has grown into a mature and important branch of NMR spectroscopy.� The history of coherent transient effects in optical spec­ troscopy is considerably shorter because the required coherent optical sources were not available until the advent of the laser in 1960. The first experimental breakthrough occurred in 1964 with the observation of photon echoes (the optical analog of spin echoes) in ruby by Kurnit, Abella & Hartmann (3, 4). Four years later coherent transient effects were also observed in gases (5, 6). These early experiments, which used pulsed laser sources, were quite difficult, however, and it was not until the 1970s with the development of new experimental techniques and better electronics that the field really began to grow and attract a substantial number of researchers. At present, this area is flourishing with many new results being produced in all areas of spectroscopy. For example, in recent years optical coherent transient spectroscopy has

Modeling Chemical Processes in the Stratosphere

Abstract: The authors review the chemical reactors occurring in the stratosphere, models of these chemical reaction kinetics, and the possible aspects of some pollutants upon reaction kinetics. Models for stratospheric chemical reactions and applications of these models are included. (DS)