Photoionization molecular processes

By employing a laser for the photoionization (not to be confused with laser desorption/ ionization, where a laser is irradiating a surface, see Section 2.1.21) both sensitivity and selectivity are considerably enhanced. In 1970 the first mass spectrometric analysis of laser photoionized molecular species, namely H2, was performed [54]. Two years later selective two-step photoionization was used to ionize mbidium [55]. Multiphoton ionization mass spectrometry (MPI-MS) was demonstrated in the late 1970s [56—58]. The combination of tunable lasers and MS into a multidimensional analysis tool proved to be a very useful way to investigate excitation and dissociation processes, as well as to obtain mass spectrometric data [59-62]. Because of the pulsed nature of most MPI sources TOF analyzers are preferred, but in combination with continuous wave lasers quadrupole analyzers have been utilized [63]. MPI is performed on species already in the gas phase. The analyte delivery system depends on the application and can be, for example, a GC interface, thermal evaporation from a surface, secondary neutrals from a particle impact event (see Section 2.1.18), or molecular beams that are introduced through a spray interface. There is a multitude of different source geometries. 

Electron dynamics of photoionization states in the course of nonadiabatic transition is one of the important molecular processes. It can be applied to the studies on the following dynamical processes Tracking the history of electronic and vibrational states wandering among the excited states in laser fields with designed optical pulses. These time-dependent transient states should be able to be monitored in terms of time-resolved photoelectron spectroscopy [14, 353]. Inner shell ionization from a deep core level and the concomitant electronic state avalanches (decay) towards the hole, which may be accompanied by autoionization, is also quite interesting. In a... 

Dithiiranethione is an isomer of CS3 and its preparation and structure have been drawing much attention [75JCS(P2)559]. In the chemiionization process of CS + CS2 using the molecular beam photoionrzation method, the photoionization efficiency curve of CS3 was observed (80JCP4242). 

In Eq. (12), l,m are the photoelectron partial wave angular momentum and its projection in the molecular frame and v is the projection of the photon angular momentum on the molecular frame. The presence of an alternative primed set l, m, v signifies interference terms between the primed and unprimed partial waves. The parameter ct is the Coulomb phase shift (see Appendix A). The fi are dipole transition amplitudes to the final-state partial wave I, m and contain dynamical information on the photoionization process. In contrast, the Clebsch-Gordan coefficients (CGC) provide geometric constraints that are consequent upon angular momentum considerations. 

A third source of error is associated with the fragmentation pattern caused by dissociation of the molecular ions formed in the source region of the spectrometer. Under severe conditions these processes may proceed with substantial isotopic fractionation, and this obscures the measurements of isotopic composition at the collector. To some extent careful standardization of the instrumental conditions may ensure that errors from fragmentation are systematic, and thus cancel (at least to some extent). Alternatively, softer ionization methods can be used to prevent most or all of the fragmentation. The bottom spectrum in Fig. 7.7 illustrates this approach it shows the mass spectrum of chlorobenzene obtained by photoionization. Only the parent molecular ions are observed. It should be kept in mind, however, that softer ionization usually yields smaller ion currents and consequently statistical counting errors increase. 

The simple collision theory for bimolecular gas phase reactions is usually introduced to students in the early stages of their courses in chemical kinetics. They learn that the discrepancy between the rate constants calculated by use of this model and the experimentally determined values may be interpreted in terms of a steric factor, which is defined to be the ratio of the experimental to the calculated rate constants Despite its inherent limitations, the collision theory introduces the idea that molecular orientation (molecular shape) may play a role in chemical reactivity. We now have experimental evidence that molecular orientation plays a crucial role in many collision processes ranging from photoionization to thermal energy chemical reactions. Usually, processes involve a statistical distribution of orientations, and information about orientation requirements must be inferred from indirect experiments. Over the last 25 years, two methods have been developed for orienting molecules prior to collision (1) orientation by state selection in inhomogeneous electric fields, which will be discussed in this chapter, and (2) bmte force orientation of polar molecules in extremely strong electric fields. Several chemical reactions have been studied with one of the reagents oriented prior to collision. ... 

Ions related to the parent molecule by loss of two H atoms and likewise of an H atom and a CH3 radical have been observed by mass spectrometry in the photoionization products of the paraffin hydrocarbons.9 It seems that entire molecules, as H2 or CH4, can be dissociated from the labile parent molecular ion, provided sufficient energy is imparted to suitable deformation vibrational modes. Similar disruption processes with the appearance of molecular ions which have lost two H atoms are observed for the alkyl derivatives of hydrazine.17... 

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. 

For the purposes of this review it is convenient to focus attention on that class of molecules in which the valence electrons are easily distinguished from the core electrons (e.g., -n electron systems) and which have a large number of vibrational degrees of freedom. There have been several studies of the photoionization of aromatic molecules.206-209 In the earliest calculations either a free electron model, or a molecule-centered expansion in plane waves, or coulomb functions, has been used. Only the recent calculation by Johnson and Rice210 explicitly considered the interference effects which must accompany any process in a system with interatomic spacings and electron wavelength of comparable magnitude. The importance of atomic interference effects in the representation of molecular continuum states has been emphasized by Cohen and Fano,211 but, as far as we know, only the Johnson-Rice calculation incorporates this phenomenon in a detailed analysis. 

Electronic states which result from the promotion of an electron from a localized molecular orbital to a Rydberg orbital are Rydberg states . Although such states can be observed only in the gas phase, it is likely that they are involved in some photoionization processes. 

In order to study the photochemical action of solar radiation on tropospheric, stratospheric, and mesospheric constituents, the solar spectrum must be divided in various ranges.1 The radiation at wavelengths less than 100 nm, which is absorbed by nitrogen and oxygen in the thermosphere above 100 km, leads essentially to ionization processes and is, therefore, not considered there. Only X-rays of wavelengths less than 1 nm can penetrate into the atmosphere below 100 km, and lead indirectly to the dissociation of molecular constituents. Nevertheless, their principal role is the photoionization in the D region of the ionosphere below 100 km where the solar line Lyman-a at 121.6 nm ionizes the nitric oxide molecule, NO. 

In simple ionic crystals, such as the alkali halides, especially those containing ionic impurities, the process often stops at the photoionization stage, but in most molecular crystals, and even molecular ionic... 

The discovery of confinement resonances in the photoelectron angular distribution parameters from encaged atoms may shed light [36] on the origin of anomalously high values of the nondipole asymmetry parameters observed in diatomic molecules [62]. Following [36], consider photoionization of an inner subshell of the atom A in a diatomic molecule AB in the gas phase, i.e., with random orientation of the molecular axis relative to the polarization vector of the radiation. The atom B remains neutral in this process and is arbitrarily located on the sphere with its center at the nucleus of the atom A with radius equal to the interatomic distance in this molecule. To the lowest order, the effect of the atom B on the photoionization parameters can be approximated by the introduction of a spherically symmetric potential that represents the atom B smeared over... 

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