Molecular photoionization resonances

Multiphoton ionization MPI Photoionization Atomic and molecular ions Resonance-enhanced MPI is highly selective Trace analysis... 

P.W. Langhoff, Stieltjes-Tchebycheff moment-theory approach to molecular photoionization studies, in T. Rescigno, V. McKoy, B. Schneider (Eds.), Electron-Molecule and Photon-Molecule Collisions, Plenum, New York, 1979 A.U. Hazi, Stieltjes-moment-theory technique for calculating resonance widths, in T. Rescigno, V. McKoy, B. Schneider (Eds.), Electron-Molecule and Photon-Molecule Collisions, Plenum, New York, 1979. 

In sunutary, we have used prototype studies on N2 to convey the progress made in the study of shape resonances in molecular fields, particularly in molecular photoionization. This Included the identification of shape resonant features in photoionization spectra of molecules and the accrual of substantial physical insight into their properties, many of which are peculiar to molecular fields. 

The polarization of the photoelectron spin in atomic and molecular photoionization processes encodes supplementary information about the relative magnitudes of electron transition moments. In atoms, the spherical symmetry permits complete characterization of the transition moments that contribute to the total excitation cross section for each resonance (Heinzmann, 1980). However, in molecules, the Z-mixing caused by the absence of spherical symmetry makes it impossible to completely characterize a resonance excitation mechanism from partial cross sections, angular distribution, and spin-polarization measurements. 

To achieve highly selective and sensitive detection of molecules, it was suggested that one should use laser resonance excitation of molecular vibrations, followed by photoionization of the vibrationaUy excited molecules, that is, vibrationally mediated photoionization (Ambartzumian and Letokhov 1972). As a matter of fact, it proved very difficult to realize this idea because the shift of the VUV absorption bands of the molecules caused by their vibrational excitation was too small. However, there exists another possibility for polyatomic molecules, namely multiphoton (MP) excitation of high-lying vibrational states by high-power IR laser pulses timed to resonance with the pertinent vibrational transitions (Chapter 11). The highly excited vibrational states can be photoionized by another UV or VUV laser pulse. This molecular-photoionization experiment was performed in accordance with the scheme... 

Fig. 10.5 Schemes for resonance molecular photoionization via high-lying vibrational states by way of multiphoton resonance vibrational excitation with IR laser radiation (a) multiphoton IR + VUV excitation, (b) IR multiphoton excitation, and (c) IR multistep + VUV excitation.

For pump-probe photoionization (PPI, Fig.l) the first laser pulse is tuned into resonance with the (vibrationless) electronic transition of the molecule, the second pulse is red-shifted in wavelength, so that the enhanced (1+1 ) photoion signal can be easily identified. When a time-of-flight mass spectrometer is used for detection the mass-selective photoion signal as a function of time delay can be recorded as the RCS spectrum of the electronically excited state, which is particularly useful for the specific investigation of molecular clusters. 

A number of techniques have been used previously for the study of state-selected ion-molecule reactions. In particular, the use of resonance-enhanced multiphoton ionization (REMPI) [21] and threshold photoelectron photoion coincidence (TPEPICO) [22] has allowed the detailed study of effects of vibrational state selection of ions on reaction cross sections. Neither of these methods, however, are intrinsically capable of complete selection of the rotational states of the molecular ions. The TPEPICO technique or related methods do not have sufficient electron energy resolution to achieve this, while REMPI methods are dependent on the selection rules for angular momentum transfer when a well-selected intermediate rotational state is ionized in the most favorable cases only a partial selection of a few ionic rotational states is achieved [23], There can also be problems in REMPI state-selective experiments with vibrational contamination, because the vibrational selectivity is dependent on a combination of energetic restrictions and Franck-Condon factors. 

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... 

Photoionization can result either from the direct interaction of a photon with the ionizing electron or by an indirect process. An example of the latter is autoionization where a photon is absorbed to produce an excited state of the molecule, M, by a resonance transition. The excited state then subsequently decays to the molecular ion and a photoelectron (equation 4) ... 

The recently introduced atmospheric-pressure laser ionization system (APLI) can be considered as a modification of APPI (Ch. 5.7.3). In APLI, the one-step photoionization of APPI is replaced by a two-photon process in resonantly-enhanced multi-photon ionization [148]. Enhanced response for polycyclic aromatic hydrocarbons (relative to APCI) was demonstrated. Molecular ions rather than protonated molecules are generated in APLI (cf. Ch. 6.5). 

A number of less commonly used analytical techniques are available for determining PAHs. These include synchronous luminescence spectroscopy (SLS), resonant (R)/nonresonant (NR)-synchronous scan luminescence (SSL) spectrometry, room temperature phosphorescence (RTP), ultraviolet-resonance Raman spectroscopy (UV-RRS), x-ray excited optical luminescence spectroscopy (XEOL), laser-induced molecular fluorescence (LIMP), supersonic jet/laser induced fluorescence (SSJ/LIF), low- temperature fluorescence spectroscopy (LTFS), high-resolution low-temperature spectrofluorometry, low-temperature molecular luminescence spectrometry (LT-MLS), and supersonic jet spectroscopy/capillary supercritical fluid chromatography (SJS/SFC) Asher 1984 Garrigues and Ewald 1987 Goates et al. 1989 Jones et al. 1988 Lai et al. 1990 Lamotte et al. 1985 Lin et al. 1991 Popl et al. 1975 Richardson and Ando 1977 Saber et al. 1991 Vo-Dinh et al. 1984 Vo- Dinh and Abbott 1984 Vo-Dinh 1981 Woo et al. 1980). More recent methods for the determination of PAHs in environmental samples include GC-MS with stable isotope dilution calibration (Bushby et al. 1993), capillary electrophoresis with UV-laser excited fluorescence detection (Nie et al. 1993), and laser desorption laser photoionization time-of-flight mass spectrometry of direct determination of PAH in solid waste matrices (Dale et al. 1993). 

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