Photodissociation Laser Spectroscopy

The earliest molecular beam infrared experiments on Van der Waals complexes used photodissociation spectroscopy a molecular beam is irradiated witli a tunable infrared laser and tire molecular beam intensity is measured as a function of... 

We use laser photofragment spectroscopy to study the vibrational and electronic spectroscopy of ions. Our photofragment spectrometer is shown schematically in Eig. 2. Ions are formed by laser ablation of a metal rod, followed by ion molecule reactions, cool in a supersonic expansion and are accelerated into a dual TOE mass spectrometer. When they reach the reflectron, the mass-selected ions of interest are irradiated using one or more lasers operating in the infrared (IR), visible, or UV. Ions that absorb light can photodissociate, producing fragment ions that are mass analyzed and detected. Each of these steps will be discussed in more detail below, with particular emphasis on the ions of interest. 

Ab initio calculations gave vibrational wavenumbers for 9 isomers of CNNS.524 A CARS study of the effects of 266 nm. pulsed laser photodissociation of NCNCS showed that the vapour contained both NCNCS and CNCN.525 IR and Raman spectra, with factor group analysis, gave vibrational assignments for crystalline ammonium dicyanamide, NH4[N(CN)2].526 Variable-temperature Raman spectroscopy was used to follow the solid state transformation of NI I4[N(CN)2] into NCN=C(NH2)2.527 Ab initio and/or DFT calculations gave vibrational wavenumbers for CH2=CH-N=C=X (X = O, Se) 528 NN-C(CN)2 529 nitroso-azide, NNN-N=0, and nitro-azide, NNN-N02.530... 

Laser surface chemistry has been used as a basis for many new methods in surface processing, for example, photochemical deposition of metals and photochemical etching of solid substrates, which are potentially useful techniques for the microelectronics industry (1-3). However, molecular dynamical studies of UV photodissociation of adsorbates on solid surfaces have been very scarce (4-7). We have studied UV laser photodissociation of small molecules on solid surfaces using photofragment spectroscopy. 

This chapter is concerned with experimental investigations of the dynamics of the dissociation of polyatomic neutral molecules carried out by the technique of laser Doppler spectroscopy, in bulk and under crossed-beam condition. Photodissociation is a basic process in the interaction of light with molecules, of interest in itself as an elementary molecular process and also with respect to a variety of applications in different fields. The interest has increased considerably in recent years, first, because the experimental investigation of photodissociation is rapidly advancing by the use of the laser, and second, because the laser makes possible to achieve photodissociation, state, and isotope selectively, by new excitation mechanisms. These are, aside from the common one-photon absorption, stepwise... 

The structure of molecular complexes in their electronic ground state can be obtained from direct IR laser absorption spectroscopy in pulsed supersonic-slit jet expansions [9.47]. This allows one to follow the formation rate of clusters and complexes during the adiabatic expansion [9.48]. Selective photodissociation of van der Waals clusters by infrared lasers may be used for isotope separation [9.49]. 

A very widely employed method for the measurement of spin-orbit state-specific rate constants is the time-resolved measurement of the concentrations of individual atomic levels after formation of these species from a suitable precursor, either by flash photolysis [13], or, more recently, by laser photodissociation. The concentrations of the various atomic reactant states are monitored by atomic absorption or fluorescence spectroscopy using atomic emission sources [14], or, for spin-orbit-excited states, by observation of the spontaneous infrared emission [15-18]. Recently, Leone and co-workers have utilized gain/absoiption of a colour centre and diode infrared laser to probe the relative populations of ground and spin-orbit excited halogen atoms produced in a chemical reaction [19] and also by photodissociation [20],... 

Provided a tunable laser light source is available, the IRMPD experiment can be performed in a wavelength-dependent manner to reveal the absorption characteristics of gaseous ions. Different from IR spectroscopy of condensed phase samples, in mass spectrometry the occurrence of IR absorption is observed by detection of specific fragment ions and their abundances as a function of laser wavelength. This technique is known as infrared photodissociation spectroscopy (IRPD) [139,156]. 

Fig. 9.35. Setup for infrared photodissociation spectroscopy. In this particular setup (FELIX at Utrecht, Netherlands) the light from a free-electron laser is guided into an open cylindrical ICR cell in a way that the IR light undergoes multiple reflections on the inner surfaces to extend its path for absorption by the ions inside the cell. Reproduced horn Ref. [139] with permission. John Wiley Periodicals, Inc., 2009.

Infrared photodissociation spectroscopy of protonated ammonia cluster ions.NH/(NHj)n ( = 5-8), by using infrared free electron laser. J. 

Flowever, in order to deliver on its promise and maximize its impact on the broader field of chemistry, the methodology of reaction dynamics must be extended toward more complex reactions involving polyatomic molecules and radicals for which even the primary products may not be known. There certainly have been examples of this notably the crossed molecular beams work by Lee [59] on the reactions of O atoms with a series of hydrocarbons. In such cases the spectroscopy of the products is often too complicated to investigate using laser-based techniques, but the recent marriage of intense syncluotron radiation light sources with state-of-the-art scattering instruments holds considerable promise for the elucidation of the bimolecular and photodissociation dynamics of these more complex species. 

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