Photoexcitation and ionization

Recently, stepwise laser photoexcitation and ionization has been used to identify Rydberg series in atomic uranium. 

Similar arguments apply to charge exchange and photoexcitation, and the basic result is the same the cross section for the production of Rydberg atoms is the continuation below the limit of the ionization cross section, leading to an n 3 dependence of the excitation cross section. 

However, the direct ionization of the analyte is generally characterized by a weak efficiency. This can be partially explained by the solvent property to absorb photons producing photoexcitation without ionization. This reduces the number of photons available for the direct ionization of the sample, thus reducing the ionization efficiency. Consequently, ionization using doping molecules has also been described. It has indeed been shown that dopant at relatively high concentrations in comparison with the sample allows generally an increase in the efficiency of ionization from 10 to 100 times. This indicates that the process is initiated by the photoionization of the dopant. The dopant must be photoionizable and able to act as intermediates to ionize the sample molecules. The most commonly used dopants are toluene and acetone. Thus, two distinct APPI sources have been described direct APPI and dopant APPI. 

LEI utilizes a pulsed dye laser to promote analyte atoms to a bound excited state. Laser excitation enhances the thermal (collisional) ionization rate of the analyte atom, producing a measurable current in the flame 12). The laser-related current is detected with electrodes and is a measure of the concentration of the absorbing species. LEI may proceed by photoexcitation (via one or more transitions) and thermal ionization or a combination of thermal excitation, photoexcitation, and thermal ionization. 

In its simplest form, LEI is a two-step process (see Fig. 3). It involves three quantum states the atomic ground state, an atomic excited state, and an ionic ground state. For excited levels very near the ionization potential, ionization rates approach collision rates, giving ion yields near unity. The essential steps for LEI, photoexcitation and thermal ionization, are not the only processes occurring in an atmospheric pressure flame. An excited atom can also be collisionally deactivated or fluoresce. A detailed description of signal production requires a complex expression involving several competing rate constants 25). 

Figure 3. Slow dissociation of naphthalene-dg ions resolved by TRPD. Naphthalene ions were generated by multiphoton ionization of naphthalene vapor at 193 nm. After thermalization for about 5 s at 3 x 10 torr, they were photoexcited by two photons at 355 nm, using an 8-ns pulse from the Nd YAG laser. The plots show the extent of fragment ion signal observed at several delay times following the laser pulse and the competitive slow fragmentation to give the C,oD7, CioDj, and CgDj products.

Clearly, the 54 amu species is not formed from the 82 amu entity rather both derive from the very rapid decay of a common precursor, the photoexcited cyclohexene. The 82 amu species may then be associated with the diradical intermediate on the nonconcerted pathway, and the 54 signal with an excited form of butadiene, one vulnerable to ionization when probed with a A, = 615-nm photon (46 kcal/mol). 

TRPES has been recently reviewed and details of the experimental method and its interpretation can be found elsewhere [5], Trans-azobenzene was introduced via a helium supersonic molecular beam into the interaction region of a magnetic bottle photoelectron spectrometer. The molecules were photoexcited by a tunable femtosecond laser pulse (pump pulse) with a wavelength of 280-350nm. After a variable time delay, the excited molecules were ionized by a second femtosecond laser pulse (probe pulse) with a wavelength of 200 or 207nm. The emitted photoelectrons were collected as a function of pump-probe time delay and electron kinetic energy. 

Discussion of Photoelectron and Photofragment Images. The simplest picture for photoexcitation of a molecular Rydberg state would be that of a vertical transition (Av = 0), producing only O2, X(2Ilg)(t = 2) (direct ionization) in the example case. Here electronic motion (ionization) is assumed to be much faster than nuclear motion (dissociation). 02 is much more complicated, of course, and some of the deviations from the simplistic picture could be due not only to the molecule but also to the unconventional three-photon preparation scheme. It is thus important to consider the differences in one-photon and stepwise (2 + 1) excitation. Even with direct one-photon excitation at the energy equivalent of three laser photons, it is known, [78] for example, that the quantum yield for ionization is only 0.5 the other half of the molecules do, in fact, dissociate. 

In Chapter 3 we considered briefly the photoexcitation of Rydberg atoms, paying particular attention to the continuity of cross sections at the ionization limit. In this chapter we consider optical excitation in more detail. While the general behavior is similar in H and the alkali atoms, there are striking differences in the optical absorption cross sections and in the radiative decay rates. These differences can be traced to the variation in the radial matrix elements produced by nonzero quantum defects. The radiative properties of H are well known, and the radiative properties of alkali atoms can be calculated using quantum defect theory. 

A good starting point is photoexcitation from the ground state of H. The problem naturally divides itself into two regimes below the energy of classical ionization limit, where the states are for all practical purposes stable against ionization, and above it where the spectrum is continuous. 

The photoexcitation of nonhydrogenic atoms is in many ways similar to the photoexcitation of H. For example, the strong field mixing resonances observed in Rb, Ba, and Na are well described by a hydrogenic theory.6,7,13 However, all features of the photoexcitation spectra of nonhydrogenic atoms are not equally well described by a hydrogenic theory, and we now describe the deviations. It is convenient to consider three spectral regions, below the classical ionization limit, above the classical ionization limit but below the zero field limit, and above the zero field limit. 

Cso is a strong electron acceptor capable of taking on as many as six electrons and the photoexcited Cgo is an even stronger acceptor than the Cgo in the ground state. On the other hand, strained Si—Si bonds can act as the source of electrons which shows low oxidation potentials and high ionization potentials (Table 2). 

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