Dynamics molecular photoionization

In summary, REMPI spectroscopy is now well established as a valuable method for investigating the structure and, in favourable cases, the decay dynamics of many of the long-lived excited electronic states (most notably Rydberg states) of small and medium-sized gas-phase molecules. In addition, for example, structural data for the resulting ions and/or insight into molecular photoionization dynamics can be obtained when the method of REMPI is combined with kinetic energy analysis of the accompanying photoelectrons (see Section 9.3). 

In Chap. 2, we formulate our basic framework of the chemical theory based on the Born-Oppenheimer approximation. We briefly discuss how valid or how accurate the Born-Oppenheimer approximation for bound states is. Also a theory of electron scattering by polyatomic molecules within the Born-Oppenheimer framework (or the so-called fixed nuclei approximation) is presented. This is one of the typical theories of electron dynamics, along with the theory of molecular photoionization. 

The fixed nuclear approximation is extremely useful, not only for the bound states, but also for the treatment of electronic scattering by molecules. For instance, stationary-state scattering theories within the fixed nuclear approximation have been extensively developed for molecular photoionization [18, 250, 336, 516] and electron scattering from polyatomic molecules [210, 418, 419]. These scattering phenomena are quite important in that they are widely foimd in nature as elementary processes and even in industrial applications using plasma processes. These scatterings may be referred to as stationary-state electron dynamics in fixed nuclei approximation. 

Another important electron dynamics induced by photons is molecular photoionization. The theory we use will be rather precisely described later in Chap. 3. 

The photoionization matrix elements, Cnc,kix(R, ), in Eq. (3.59) is explicitly dependent on the nuclear geometry and photoelectron momentmn, and is the essential ingredient in describing the molecular photoionization dynamics. It is built from geometry-dependent partial-wave dipole matrix elements between the neutral and ionized states, Inc,kiXfi(R),... 

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... 

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. 

At a fundamental level, it has been shown that PECD stems from interference between electric dipole operator matrix elements of adjacent continuum f values, and that consequently the chiral parameters depend on the sine rather than the cosine of the relative scattering phases. Generally, this provides a unique probe of the photoionization dynamics in chiral species. More than that, this sine dependence invests the hj parameter with a greatly enhanced response to small changes in scattering phase, and it is believed that this accounts for an extraordinary sensitivity to small conformational changes, or indeed to molecular substitutions, that have only a minimal impact on the other photoionization parameters. 

B. Molecular Beam-Photoionization-Time-of-Flight Mass Spectrometry Studying Dynamics and Bonding... 

Figure 4. The MF PADs for single-photon ionization of ai and <22 symmetry orbitals of a model C3V molecule for light linearly polarized along different axes of the molecule (indicated in parentheses). Note that no photoionization can occur from the <22 orbital for light polarized along the z axis (molecular symmetry axis). The same dynamical parameters as for the calculations of the LF PADs shown in Fig. 3 were used. For further details see Ref. [55],...

The majority of recent PES studies of the halomethanes has concerned photoionization dynamics, and has employed synchrotron radiation sources. Halomethanes are aptly suited for such studies because they exhibit a relatively small number of well-resolved PES bands and contain atoms of very different atomic number, Z. This variation of Z permits fine tuning of the molecular ion potential and opens a window for the study of photoelectron-ion interactions. In addition, the Br3d and I4d shells have large photoionization cross-sections in the SXR region, thus extending the scope of PES studies from the valence to the outer-core electrons. 

The study of ion-molecule reactions using state-selected reagents has become a very exciting area of molecular dynamics. We have developed an experimental apparatus in Orsay which utilizes the properties of our tunable synchrotron radiation source at LURE to prepare ions in selected vibronic levels and then to study their reactions. The ions are state-selected using the TPEPICO (threshold-photoelectron/photoion coincidence) method [1]. 

With femtosecond pump-and-probe experiments fast motion pictures of a vibrating molecule may be obtained, and the time behavior of the wave packets of coherently excited and superimposed molecular vibrations can be mapped. This is illustrated by the following examples dealing with the dynamics of molecular multiphoton ionization and fragmentation of Na2, and its dependence on the phase of the vibrational wave packet in the intermediate state [821]. There are two pathways for photoionization of cold Na2 molecules in a supersonic beam (Fig. 6.100) ... 

Lasers are the precision tools of photochemistry and they have been used to both pump (initiate) and probe (analyse) chemical processes on time-scales that are short enough to allow the direct observation of intramolecular motion and fragmentation (i.e. on the femtosecond time-scale). Thus, laser-based techniques provide us with one of the most direct and effective methods for investigating the mechanisms and dynamics of fundamental processes, such as photodissociation, photoionization and unimolecu-lar reactions. Avery wide variety of molecular systems have now been studied using laser techniques, and only a few selected examples can be described here. 

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