Collisions, optical spectroscopy

Note The acronyms used here are OSPED (optical spectroscopy in a pulsed electrical discharge), FAMS (flowing afterglow mass spectrometry), SIFT (selected ion flow tube), TRAPI (time-resolved atmospheric pressure ionization mass spectrometry), PHPMS (pulsed high-pressure ionization mass spectrometry), ICRMS (ion cyclotron resonance mass spectrometry), and ADO (averaged dipole orientation collision rate theory). 

Spectroscopic techniques have been applied most successfully to the study of individual atoms and molecules in the traditional spectroscopies. The same techniques can also be applied to investigate intermolecular interactions. Obviously, if the individual molecules of the gas are infrared inactive, induced spectra may be studied most readily, without interference from allowed spectra. While conventional spectroscopy generally emphasizes the measurement of frequency and energy levels, collision-induced spectroscopy aims mainly for the measurement of intensity and line shape to provide information on intermolecular interactions (multipole moments, range of exchange forces), intermolecular dynamics (time correlation functions), and optical bulk properties. 

For the measurements of short-range diffusion, the collision rate can also be obtained by optical spectroscopy from fluorescence quenching. In the isotropic case of diffusion-controlled dynamic quenching, the change in fluorescence intensity obeys the equation ... 

The theory of electric polarizability is of fundamental importance to the rational approach and interpretation of large classes of phenomena [1]. In particular, these properties are of fundamental importance to intermolecular interaction studies [2], nonlinear optics [3], collision-induced spectroscopy [4], and the simulation of fluids [5,6]. They are also routinely associated with general molecular characteristics as hardness [7], softness [8], hypersoftness [9], stiffness [10], and compressibility [11]. Understandably, polarizability is also linked to reactivity [12]. Another important field is QSAR, QSPR studies, and the understanding of pharmacological activity [13, 14]. 

In addition to measuring total recombination coefficients, experimentalists seek to determine absolute or relative yields of specific recombination products by emission spectroscopy, laser induced fluorescence, and optical absorption. In most such measurements, the products suffer many collisions between their creation and detection and nothing can be deduced about their initial translational energies. Limited, but important, information on the kinetic energies of the nascent products can be obtained by examination of the widths of emitted spectral lines and by... 

The coherent fs time-resolved CARS method is highly sensitive for the investigation of collision induced (or pressure dependent) changes in optical line shapes especially when line mixing occurs and frequency resolved measurements come to their limits [7]. The fs-CARS spectroscopy is applied to various collision systems (N2-N2, N2-rare gas, C2H2-C2H2, CO-CO)... 

Phase- and Energy-Changing Collisions in Iodine Gas Studies by Optical Multiple-Pulse Spectroscopy, E. T. Sleva and A. H. Zewail, Chem. Phys. Lett. 110, 582 (1984). 

R. Schinke Photodissociation Dynamics 2. L. Frommhold Collision-Induced Absorption in Gases 3. T. F. Gallacher Rydberg Atoms 4. M. Auzinsh and R. Ferber Optical Polarization of Molecules 5.1. E. McCarthy and E. Weigold Electron-Atom Collisions 6. V. Schmidt Electron Spectrometry of Atoms using Synchrotron Radiation 7. Z. Rudzikas Theoretical Atomic Spectroscopy... 

The study of reactive excited states of van der Waals complexes is the link between the laser-assisted collision and the photodissociation approach it brings the collisional problem into a much simpler photodissociation problem. Here, a cold complex which has a defined geometry is formed between the collision partners and optically excited to trigger the reactive process. This creates the photodissociation of a molecule with very weakly bound ground state. The van der Waals spectroscopy has already allowed the accurate determination of the interatomic potential [Na-Ar (Smalley et al. 1977 Tellinghuisen et al. 1979), HgAr (Breckenridge et al. 1985, 1994 Fuke et al. 1984)]. More complex collisional... 

An important aspect of this kind of experiment is the time evolution of the reactive excited complex. It can be expected that for a reaction near the threshold, fine tuning of the optical excitation should yield to drastic changes in the reactive decay time as the spectroscopy already shows for the Ca-FICl system (Keller 1991, Soep et al. 1991, 1992). Real time evolution of binary reactive collisions can be studied through van der Waals complexes, since time t = 0 is defined by the excitation laser, as well as the starting internuclear distance between reactants which is fixed by the ground state geometry. This approach has been used for... 

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