Photodissociation, photoionization/mass

The rationalization of mass spectrometric investigations of nitro compounds has benefited significantly from numerous studies applying techniques adopted from photochemistry, such as photodissociation, photoionization and photoelectron photoion coincidence spectroscopy. 

J. Berkowitz and B. Ruscic, Photoionization Mass Spectrometric Studies of Free Radicals (C. Y. Ng, ed.) Vacuum Ultraviolet Photoionization and Photodissociation of Molecules and Clusters, World Scientific Singapore 1991, pp. 1-41. 

Recently in our laboratory we have initiated a program to study the photoabsorption processes of metal vapors throughout the UV and EUV region. Our research interests are (1) to obtain the absolute cross section measurement of atomic and molecular metal vapors, (2) to study the photoionization processes of molecular metal species, and (3) to study the photodissociation processes of molecular metal ions. Several experimental methods such as the heat-pipe absorption spectroscopy, photoionization mass spectroscopy, and electron-ion coincidence technique, will be used in the study. This report summarizes our first experiment using heat-pipe absorption spectroscopy. 

To obtain information concerning energy disposals, product kinetic and internal energy distributions, and identification of primary product structures resulting from the UV photolysis of organosulfur pollutants, we have performed UV laser photodissociation and photoionization mass spec-trometric studies of a series of sulfur-containing species in recent years [40-45,49,50,54-61]. Ab initio calculations [49,50,55-59,65,66,69] have also been made to compare with the energetic measurements and to interpret the experimental observations. This review mostly summarizes and evaluates the results of these experimental and theoretical studies. Related results obtained by other laboratories are also discussed. 

Figure 22. Cross section view of the TOF mass spectrometer (I) photodissociation-photoionization chamber (2) photodissociation and photoionization region (3) beam-source chamber (4) pulsed valve (5) skimmer (6) TOF tube and (7) microchannel plate detector. Taken from ref. 58.

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. 

However, a shortcoming with the VUV photoionization approach is that absolute PI cross-sections are very often not known, and therefore branching ratios cannot be estimated. As matter of fact, studies of photodissociation processes by soft PI using synchrotron light are usually accompanied by measurements carried out using classic (hard) El ionization, where much data have to be taken at all possible fragment masses in order to estimate branching ratios.14-16,20... 

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). 

The photoionization of a molecule to yield an electron and an unfragmented ion may be considered to be the simplest of all photodissociation reactions, and therefore also one of the simplest of the radiationless processes in an isolated molecule. In addition, because the products are charged, a combination of mass spectrometric and photometric data yields information about photoionization reactions not now available for molecular fragmentation reactions. For example, the reaction cross sections for generation of specific charged products and the total photon absorption cross section may be measured and compared, thereby yielding the residual cross section corresponding to radiationless processes other than photoionization. From this information we can deduce some of the consequences of the competition between several radiationless processes in an isolated molecule. 

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. 

As will be seen in Chapter 8, equations identical to Eq. (7.2.13) and Eq. (7.2.17) describe the photoelectron angular distributions observed in photoionization processes. However, owing to the difference in photofragment masses (atom vs. electron), the photoelectron angular distributions sample the photofragmentation dynamics very differently from photodissociation angular distributions. 

The general expressions given by Yang (1948), Eqs. (7.2.13) and (7.2.17), provide a valid description for all photofragmentation products, atoms in the case of photodissociation, electrons in the case of photoionization. In photoionization, the masses of the photofragments are in the ratio m/M. 

It was later shown in two independent studies that the relative importance of NbmC met-cars and nanocrystals in the mass spectrum is critically dependent on the experimental conditions. It has been proved that the concentration of hydrocarbon in the carrier gas, the nature of the laser used as the vaporization source, the laser power selected, and, finally, the direet detection of cluster ions or the photoionization of neutral species drastically influence the relative proportion of met-cars to nanocrystals. This selectivity has been attributed to distinct mechanisms of cluster growth for met-cars and nanocrystals (Section 5.9.2.3). Laser-induced photodissociation of 3 X 3 X 3 nanocrystals M14C13+ (M = Ti, V) and larger clusters assumed to have a fee crystal structure has been reported by Pilgrim and Duncan. The titanium carbide cluster corresponding to the 1044-amu peak in the mass spectrum has been assigned to the 3 x 3 x 4 fee fragment in which one tita-... 

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. 

In terms of the hardware, TRMS methods described in this book use most common types of ion sources and analyzers. Electrospray ionization (ESI), electron ionization (El), atmospheric pressure chemical ionization (APCI), or photoionization systems, and their modified versions, are all widely used in TRMS measurements. The newly developed atmospheric pressure ionization schemes such as desorption electrospray ionization (DESI) and Venturi easy ambient sonic-spray ionization (V-EASI) have already found applications in this area. Mass analyzers constitute the biggest and the most costly part of MS hardware. Few laboratories can afford purchasing different types of mass spectrometers for use in diverse applications. Therefore, the choice of mass spectrometer for TRMS is not always dictated by the optimum specifications of the instrument but its availability. Fortunately, many real-time measurements can be conducted using different mass analyzers equipped with atmospheric pressure inlets - with better or worse results. For example, triple quadrupole mass spectrometers excel at quantitative capabilities however, in many cases, popular ion trap (IT)-MS instruments can be used instead. On the other hand, applications of TRMS in fundamental studies often require a particular type of instrument (e.g., Fourier transform ion cyclotron resonance mass spectrometer for photodissociation studies on trapped ions). 

Time-resolved ionization offers several advantages as a probe of these wavepackets [41, 42, 343, 360]. For example, the ground state of an ion is often better characterized than higher excited states of the neutral molecule, particularly for polyatomics. Ionization is also universal and hence there are no dark states. Furthermore, ionization provides both ions and photoelectrons and, while ion detection provides mass and kinetic-energy resolution in time-resolved studies [508], photoelectron spectra can provide complementary information on the evolution of the wavepacket [22, 63, 78, 132, 201, 270, 271, 362, 363, 377]. Its utihty for real-time probing of molecular dynamics in the femtosecond regime has been nicely demonstrated in studies of wavepackets on excited states of Na2 [22], on the B state of I2 [132], and on the A state of Nal [201]. Femtosecond photoelectron-photoion coincidence imaging studies of photodissociation dynamics have been reported [107]. 

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... 

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