Ultraviolet photon dissociation

For an ion, the cross section for electron capture (EC) roughly increases with the square of the ionic charge [145]. This makes multiply charged ions as produced by electrospray ionization (ESI, Chap. 12) the ideal targets for this process. When a UV laser of 193 nm wavelength (6.4 eV per photon) in the course of ultraviolet photon dissociation (UVPD) experiments erroneously hit a metal surface, elec-... 

The above examples should suffice to show how ion-molecule, dissociative recombination, and neutral-neutral reactions combine to form a variety of small species. Once neutral species are produced, they are destroyed by ion-molecule and neutral-neutral reactions. Stable species such as water and ammonia are depleted only via ion-molecule reactions. The dominant reactive ions in model calculations are the species HCO+, H3, H30+, He+, C+, and H+ many of then-reactions have been studied in the laboratory.41 Radicals such as OH can also be depleted via neutral-neutral reactions with atoms (see reactions 13, 15, 16) and, according to recent measurements, by selected reactions with stable species as well.18 Another loss mechanism in interstellar clouds is adsorption onto dust particles. Still another is photodestruction caused by ultraviolet photons produced when secondary electrons from cosmic ray-induced ionization excite H2, which subsequently fluoresces.42... 

Photodissociation dynamics [89,90] is one of the most active fields of current research into chemical physics. As well as the scalar attributes of product state distributions, vector correlations between the dissociating parent molecule and its photofragments are now being explored [91-93]. The majority of studies have used one or more visible or ultraviolet photons to excite the molecule to a dissociative electronically excited state, and following dissociation the vibrational, rotational, translational, and fine-structure distributions of the fragments have been measured using a variety of pump-probe laser-based detection techniques (for recent examples see references 94-100). Vibrationally mediated photodissociation, in which one photon... 

Rate constants for reaction of the CH radical with a number of atomic and molecular collision partners have been reported, with multiple-photon dissociation of suitable precursor molecules using either infrared or ultraviolet " laser radiation used as the pulsed photolysis source, and laser-induced fluorescence near 431 nm employed as a sensitive time-resolved detection method. A similar technique has been used to measure removal rates of CH2 and CDj with... 

Deuterium and Hydrogen Lamps. A continuum spectrum in the ultraviolet region is produced by electrical excitation of deuterium or hydrogen at low pressure. The mechanism by which a continuum spectrum is produced involves initial formation of an excited molecular species followed by dissociation of the excited molecule to give two atomic species plus an ultraviolet photon. The reactions for deuterium are... 

The extremely wide range of possible dissociation energies necessitates the use of different kinds of light source to break molecular bonds. Van der Waals molecules can be fragmented with single infrared (IR) photons whereas the fission of a chemical bond requires either a single ultraviolet (UV) or many IR photons. The photofragmentation of van der Waals molecules has become a very active field in the last decade and deserves a book in itself (Beswick and Halberstadt 1993). It is a special case of UV photodissociation and can be described by the same theoretical means. In Chapter 12 we will briefly discuss some simple aspects of IR photodissociation in order to elucidate the similarities and the differences to UV photodissociation. 

In all but one [17] of the 1S-2S experiments to date, the atoms have been observed in a gas cell. Molecular hydrogen is dissociated in a gas discharge, and the atoms travel by gas flow and diffusion into the observation cell. The signal is observed by monitoring collision induced vacuum ultraviolet Lyman-a photons. 

Abstract Photochemistry is concerned with the interaction between light and matter. The present chapter outlines the basic concepts of photochemistry in order to provide a foundation for the various aspects of environmental photochemistry explored later in the book. Electronically excited states are produced by the absorption of radiation in the visible and ultraviolet regions of the spectrum. The excited states that can be produced depend on the electronic structure of the absorbing species. Excited molecules can suffer a variety of fates together, these fates make up the various aspects of photochemistry. They include dissociation, ionization and isomerization emission of luminescent radiation as fluorescence or phosphorescence and transfer of energy by intramolecular processes to generate electronic states different from those first excited, or by intermo-lecular processes to produce electronically excited states of molecules chemically different from those in which the absorption first occurred. Each of these processes is described in the chapter, and the ideas of quantum yields and photonic efficiencies are introduced to provide a quantitative expression of their relative contributions. 

Photoionization time-of-flight mass spectrometry is used almost exclusively in all experiments described in this review. The ionizing laser sources have included excimer lasers for photon energies up to 7.87 eV and tunable ultraviolet sources up to 6.5 eV. In some early studies, multiphoton ionization was used, but it has become quite clear that this usually results in dissociative ionization. Such effects have been observed in many systems, ranging from Si49,5o ( 51 jQ jjjg transition metals. Thus single-photon ionization has... 

Due to the relatively high energies needed to dissociate or ionize atmospheric gases, photochemistry is mainly initiated by less than one per cent of the solar photons, more specifically, those whose characteristic wavelength is in the X-ray, ultraviolet, or, for certain molecules, in the visible region. 

Absorption of ultraviolet and visible photons by atmospheric molecules can induce transitions into electronically excited states, which may then photodissociate (see Chapter 2). The dissociation products play a crucial role in the photochemistry of the atmosphere. 

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