Atomic, Molecular, and Optical Physics (1986) / Chapter Skim
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5 Molecular Physics
5 Molecular Physics
Pages 88-109
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From page 88... ...
We have attempted to portray here the activities in molecular physics that lie closest to the physics-chemistry interface. THE NEW SPECTROSCOPY Within the last decade laser spectroscopy has been combined with innovative molecular-beam techniques to create and study a multitude of new molecular species.
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From page 89... ...
, the basis of the traditional understanding of molecular structure in terms of potential energy surfaces, no longer applies. Fortunately, a highly successful theoretical framework for understanding Rydberg molecules has been created (multichannel quantum defect theory, mentioned in Chapter 4 in the section on Atomic Structure)
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From page 90... ...
VITO Pump Laser 5800 A C} ~~ ~ Phot 0 d lode Signal 1 1 ~ ~ E=22.4 / E=o(Arb a:/ B -STAT E Energy Level' E ~ / E-33.8 W 1 ~\ E- 25.9 Probe Laser Frequenc y - V >> O Probe Laser 5 500 A Hea t pipe" , Na2 Later o 8000 A Four-M irror Rlog Resonator
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From page 91... ...
The study of open-core molecules provides a comprehensive picture for the electronic charge distribution in these states. The picture reveals the underlying simplicity of the atomic-ion-in-molecule electronic structure that lies concealed beneath an extremely complex energy level structure.
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From page 92... ...
Calculated constants such as spin-orbit couplings or electric dipole moments are valuable for identifying the electron configuration of an electronic state; oscillator strengths can sometimes be calculated more accurately than they can be measured. Hydrogen-Bonded Molecules Midway between the strong chemical forces that create chemical bonds and the weak forces of van der Waals interactions there exists a class of forces that is responsible for molecular aggregates and polymers.
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From page 93... ...
Through high-resolution microwave and infrared spectroscopy of hydrogen-bonded complexes, accurate models of the pair potential for H bonding, and also for van der Waals bonding, can be constructed. This pair potential governs many of the phenomena of condensedphase chemistry: bulk association, conformation and steric effects, salvation, solubility, and physisorption.
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From page 94... ...
The explosive growth of multiple-laser techniques for systematically obtaining high-quality, readily assignable spectral data for highly excited rotation-vibration levels is causing a complete rethinking of the problem of how polyatomic molecules vibrate. MOLECULAR PHOTOIONIZATION AND ELECTRON-MOLECULE SCATTERING Understanding the joint motion of electrons and nuclei in molecular fields is the essence of molecular physics.
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From page 95... ...
Molecular photoionization studies are broad in scope. Here we discuss three topics of particular interest: autoionization, shape resonances, and resonant multiphoton ionization.
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From page 96... ...
The input to a MQDT calculation is a small set of physical parameters quantum defects and dipole amplitudes that characterize the short-range interactions between the excited electron and the core. From these few parameters, MQDT can yield values for many quantities that are directly related to the observables, for instance total photoionization cross sections, vibrational branching ratios, and photoelectron angular distributions.
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From page 97... ...
An excellent example in the strong interplay of theory and experiment is the prediction and confirmation of large changes in vibrational branching ratios and in photoelectron angular distributions induced by shape resonance. These arise because shape resonances are so sensitive to the internuclear separation that they behave differently in the individual vibrational ionization channels.
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From page 98... ...
Resonant multiphoton ionization reveals the photoionization dynamics of excited states that are fully specified quantum mechanically. Photoionization branching ratios, photoelectron angular distributions, alignment, and fragmentation all the important properties in single-photon ionization can now be studied by photoionization of excited molecular states.
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From page 99... ...
Several models have been suggested, invoking virtual states and other mechanisms, but the issue remains unsettled. MOLECULAR DYNAMICS Chemical reactions involve complex many-body interactions.
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From page 100... ...
The study of state-to-state collisions in the past decade constitutes an important advance in basic molecular physics. The results have already provided new insights into molecular dynamics, and they are expected to be valuable in applications involving energy transfer in gases.
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From page 101... ...
Experiments such as these guide the development of the theory of energy transfer and provide an important step toward understanding the precise steps that occur in a chemical reaction. The information is also useful for understanding the drag on airplanes and spacecraft.
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From page 102... ...
Radiative collisions provide a new and flexible probe for studying a chemical reaction in progress. One can describe the collision in terms of the radiative excitation of an atom whose energy levels are tuned into resonance with the laser light by the changing perturbation of a second atomic species.
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From page 103... ...
The relative rates at which these internal product states are formed are important for applications ranging from research into new chemical species to industrial processes and the creation of chemical lasers. In the last decade, much has been learned about the dynamics of chemical reactions in the gas phase.
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From page 104... ...
The energy levels appear to evolve in a simple and orderly fashion. The lower right-hand drawing shows energy levels for a highly excited sodium atom~ssentially a perturbed hydrogen atom in an electric field.
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From page 105... ...
The study of this phenomenon, and of other quantum effects in reactive scattering, will undoubtedly lead to better control of chemical reactions and, one hopes, the eventual development of systems in which the various state-to-state reactions can be selectively activated. Resonances in a Simple Reaction Complex Some time ago chemical physicists developed techniques for calculating cross sections for simple chemical reactions such as H2 + F > HE + H
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From page 106... ...
Now, however, the structure and molecular motion during dissociation can be observed. Because the latter half of many chemical reactions involves a dissociative state whose products fly apart, the photofragmentation process has come to be known as a "half-collision." Laser light is used to break selectively the bonds in simple molecules.
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From page 107... ...
SOME NOVEL MOLECULAR SPECIES A large variety of novel molecular species and molecules in unusual classes of states have emerged from the laboratories of molecular physicists within the past few years. In addition to the species
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From page 108... ...
Molecular Ions Molecular ions play key roles in the chemistry of solutions, in atmospheric chemistry, in the interstellar medium, in plasmas, and in flames. Until recently the experimental study of ions in the gas phase was extremely difficult because ions could not be prepared at high concentration or in isolation from other molecular species.
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From page 109... ...
By providing an opportunity to test approximate theories on a large class of relatively simple systems, van der Waals molecules provide an advance toward understanding molecular structure in all of its manifestations, including the liquid and solid states, and toward understanding chemical reactions.
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