Photodissociation, photoionization/mass spectrometry

Slow dissociation rates (10 -10 s ) have been measured in Dunbar s laboratory by time-resolved photodissociation, which consists of trapping ions in an ICR cell during a variable delay time after a phot-odissociating photon pulse. The technique called time-resolved photoionization mass spectrometry , developed by Lifshitz, consists of trapping photoions in a cylindrical trap at very low pressure to avoid bimolecular collisions, and then ejecting them into a mass filter after a variable delay covering the microsecond to millisecond range. When the dissociation rate constant becomes lower than ca. 10 s competition with infrared fluorescence takes place and limits the lifetime of the decomposition process. This has to be taken into account to extract the dissociation rate constant from the experimental data. 

The general principle of detection of free radicals is based on the spectroscopy (absorption and emission) and mass spectrometry (ionization) or combination of both. An early review has summarized various techniques to detect small free radicals, particularly diatomic and triatomic species.68 Essentially, the spectroscopy of free radicals provides basic knowledge for the detection of radicals, and the spectroscopy of numerous free radicals has been well characterized (see recent reviews2-4). Two experimental techniques are most popular for spectroscopy studies and thus for detection of radicals laser-induced fluorescence (LIF) and resonance-enhanced multiphoton ionization (REMPI). In the photochemistry studies of free radicals, the intense, tunable and narrow-bandwidth lasers are essential for both the detection (via spectroscopy and photoionization) and the photodissociation of free radicals. 

In the U.S.S.R. photo-mass spectrometry has been used in a study of the photoionization and photodissociation of methylated benzene derivatives and the aromatic amines,16 and recently the hydrazine derivatives17 (Sec. III). Moreover, for many of the aromatic compounds the kinetic energy distribution of the photoelectrons has been measured in both the gaseous and the solid state (Secs. IV and V). 

Femtosecond photodissociation dynamics of nitroethane and l-nitropropane have been studied in the gas phase and in solution by resonance Raman spectroscopy, with excitation in the absorption band around 200 nm. At such short time-scales it is possible to detect changes in the two N-O bond lengths in the Franck-Condon region, prior to C-N bond cleavage. Photolyses of nitroalkanes at 193 nm have been monitored by photoionization of the fragments and time-of-flight mass spectrometry. Both C-N and N-O bond dissociation pathways are observed and, under the conditions of free jet expansion, primary products such as pentyl and hexyl radicals are stabilized and can be detected. 

Williams, E.R. and McLafferty, F.W. (1990) 193-nm laser photoionization and photodissociation for isomer differentiation in Fourier-transform mass-spectrometry. J. Am. Soc. Mass Spectrom., 1, 361-365. 

The goal of this book is to present in a coherent way the problems of the laser control of matter at the atomic-molecular level, namely, control of the velocity distribution of atoms and molecules (saturation Doppler-free spectroscopy) control of the absolute velocity of atoms (laser cooling) control of the orientation, position, and direction of motion of atoms (laser trapping of atoms, and atom optics) control of the coherent behavior of ultracold (quantum) gases laser-induced photoassociation of cold atoms, photoselective ionization of atoms photoselective multiphoton dissociation of simple and polyatomic molecules (vibrationally or electronically excited) multiphoton photoionization and mass spectrometry of molecules and femtosecond coherent control of the photoionization of atoms and photodissociation of molecules. 

Photoionization and Photodissociation Methods in Mass Spectrometry Plasma Desorption Ionization in Mass Spectrometry... 

See also Environmental Applications of Eiectronic Spectroscopy Fragmentation in Mass Spectrometry ion Dissociation Kinetics, Mass Spectrometry ion Energetics in Mass Spectrometry Laser Appiications in Eiectronic Spectroscopy Metastabie ions Muitipho-ton Spectroscopy, Appiications Negative ion Mass Spectrometry, Methods Photoeiectron-Photoion Coincidence Methods in Mass Spectrometry (PEPiCO) Photoionization and Photodissociation Methods in Mass Spectrometry Spectroscopy of ions Time of Flight Mass Spectrometers. 

See also Laser Applications in Electronic Spectroscopy Laser Spectroscopy Theory Light Sources and Optics Multiphoton Excitation in Mass Spectrometry Muitiphoton Spectroscopy, Applications Optical Frequency Conversion Pharmaceuticai Appiications of Atomic Spectroscopy Photoelectron Spectroscopy Photoionization and Photodissociation Methods in Mass Spectrometry Pyroiysis Mass Spectrometry, Methods Time of Flight Mass Spectrometers X-Ray Spectroscopy, Theory Zero Kinetic Energy Photo-eiectron Spectroscopy, Appiications Zero Kinetic Energy Photoelectron Spectroscopy, Theory. 

See also Ion Energetics in Mass Spectrometry Ion Imaging Using Mass Spectrometry Ion Structures in Mass Spectrometry Isotopic Labelling in Mass Spectrometry Metastable Ions Photoionization and Photodissociation Methods in Mass Spectrometry Stereochemistry Studied Using Mass Spectrometry. 

---