Ionization potential photoionization spectroscopy

Photoionization Spectroscopy of Dichromium and Dimolybdenum Ionization Potentials and Bond Energies. 

We have already shown that excitation energies can be diagrammatically decomposed to yield simpler quantities such as ionization potentials and electron affinities plus some remaining diagrams. MB-RSPT permits the use of this treatment for even more complex processes. In this section, we present the applicability of the theory to double ionizations observed in Auger spectra as well as excitations accompanying photoionization (shake-up processes) observed in ESCA and photoelectron spectroscopy. A detailed description of this approach is given in Refs.135,136. Here we shall present only the formal description. 

This is a severe drawback in the case of equilibrium studies of metal molecules since, as a rule, such molecules are minor vapor components and maximum sensitivity is required for their thermodynamic evaluation. However, very precise ionization potentials can be measured using photoionization spectroscopy (5,28). Berkowltz (28) reviewed early work concerning alkali metal dimers. Herrmann et al. ( ) have measured the ionization potentials of numerous sodium, potassium and mixed sodium-potassium clusters. For most of these clusters the atomization energies of the neutral molecules are not known. Therefore, the dissociation energies of the corresponding positive ions cannot be calculated. 

The gas phase ionization potentials (IP) of the nucleobases have been determined by means of photoelectron spectroscopy and photoionization mass spectrometry [32]. Values for the first vertical and adiabatic ionization potentials are summarized... 

The ionization potentials of a number of cyclopropylalkenes have been measured using photoionization or photoelectron spectroscopy and some of these data are summarized in Table 8. It was proposed by Eaton and Traylor that these values could be used as a quantitative measure of the 7c-donor ability of cyclopropyl to the radical cation resulting from ionization. As can be seen from the data in Table 8 there is a cumulative, but not strictly additive, reduction in the ionization potentials relative to a standard such as 2-butene (9.13 eV) , as the number of cyclopropyl groups increases. 

The reaction AB —) I hv AB e is the basis of photoelectron spectroscopy and photodetachment methods. Many precise and accurate ionization potentials of molecules have been obtained by studying the photoionization of neutral molecules. The same principles apply to the photon methods for determining electron affinities, except that negative ions are studied. The electron affinities of over 1,000 atoms, radicals, clusters, and small molecules have been determined using... 

Snl4 [52] deviate by less than 0.2 eV from the ionization potentials obtained by photoionization and photoelectron spectroscopy as shown in the references given in parenthesis. 

We will discuss the application of multistep laser excitation and ionization to determine the physical properties mentioned above in the lanthanides and actinides with emphasis on the determination of accurate ionization potentials. The discussion will point out how the laser techniques can circumvent many of the experimental obstacles that make these measurements difficult or impossible by conventional spectroscopy. The experimental apparatus and techniques described can be employed to measure all the properties and they are typical of the apparatus and techniques employed generally in multistep laser excitation and ionization. We do not claim completeness for literature cited, especially for laser techniques not involving photoionization detection. 

Ionization potentials of atoms are usually obtained by the determination of a photoionization threshold or more accurately by the observation of long Rydberg progressions. With the exception of a few of these elements with simple spectra, obtaining such measurements for lanthanides and actinides is difficult if not impossible by conventional spectroscopy. Therefore, very accurate ionization limits were not available for the majority of these elements.( 6)... 

The techniques that have been most employed for investigating the electronic properties of small particles are photoemission (UPS, XPS), soft X-ray spectroscopy, EXAFS, photoionization mass spectrometry, and AES (23, 111, 240, 257d,e). While there is some controversy from theoretical work about the minimum particle size required to give bulk properties—from 10 (258) to several hundred atoms (259)—there seems to be a consensus that a cluster of about ISO atoms or more is required to observe a photoemission spectrum similar to that of the corresponding bulk metal (23, 260). When other properties are considered (ionization potential, density of states, valence bandwidth, etc.), the agreement is less satisfactory between the results obtained with different techniques (23). 

Selected values of the adiabatic (ad) ionization potential of the outer-valence orbital Sa from different methods of measurement are given in order of increasing magnitude In the table below (PES = photoelectron spectroscopy, PIMS = photoionization mass spectrometry, also simulated by dipole (e, e+lon) spectroscopy, PA = photoabsorption). Other adiabatic and the vertical (vert) values for 5a and values for the other outer-valence orbital 2e are reported in separate sections on PES (also simulated by binary (e, 2e) spectroscopy) and MS work added to the table ... 

Mar Marijuissen, A., ter Meulen, J.J. Determination of the adiabatic ionization potentials of Si2 and SiCl by photoionization efficiency spectroscopy, Chem. Phys. Lett. 263 (1996) 803-810. 

Ionization of the SiCl radical may produce the ground state of SiCl. Marijnissen and ter Meulen [96Mar] have measured the adiabatic ionization potentials IP of the two isotopomers Si Cl and Si Cl using (1+1) two-color photoionization efficiency spectroscopy (PIE). They started from the 2 =1/2 and Q = 3/2 spin-orbit components of the X II ground state of SiCl, with the following results ... 

The adiabatic first ionization potential Ej=12.71 0.01 eV was measured for HOF by photoionization mass spectrometry [1], and for HOF and DOF by photoelectron (PE) spectroscopy [8]. Ej = 12.69 0.03 eV from an older PE study may be affected by a small H2O impurity [2], and therefore the former value [1] is preferred [2]. The vertical ionization potential is about 13.0 eV [2]. HOF does not fit into a linear relation between vertical ionization potentials and atomic charges (calculated semiempirically) for HOX molecules [9]. 

Ionization potentials of NH2 were measured by photoelectron spectroscopy (PES), by photoionization (PI) and electron impact (El) mass spectrometry, and were calculated by ab initio methods (MCSTEP, MP4, MCSCF). The first two adiabatic ionization potentials of NH2 are listed in the following table. 

The adiabatic ionization potential of 9.589 0.007 eV, determined by photoionization mass spectrometry [7], is in excellent agreement with one obtained by photoelectron spectroscopy (see above). Other values for the first ionization potential obtained from various electron impact experiments range from about 9.6 to 9.9 eV [8 to 14], with the most recent measurements giving 9.8 0.05 [11] and 9.65 0.08eV [8]. A value of 9.62 eV was derived from two Rydberg transitions in the UV absorption spectrum (see p. 54) [15]. 

The PH + ion formed by electron impact on PH3 was mass spectrometrically detected. An ionization efficiency curve between 30 and 50 eV led to appearance potentials of 34.0 eV (square root plot) or 32.7 eV (linear extrapolation) [1] both values are tabulated in [2] for the first value, see also [3]. Partial PH3 ionization cross sections for PH + and PD3 ionization cross section ratios, PDl /PD, were obtained for electron energies up to 180 eV and showed appearance potentials of 35.0 0.5 eV for PH + and 34.9 0.5 eV for PD + (square root plots) [4]. The PH + ion was also mass spectrometrically detected after ionization of PH3 by high-energy electrons (8 keV). Oscillator strengths were obtained by dipole (e,e+ion) spectroscopy simulating photoionization [5]. 

Fig. 1.1. Principles of the real-time multiphoton ionization (MPI) (a) and NeNePo (b) spectroscopic technique, (a) Principle of time-resolved MPI spectroscopy. A wave packet is prepared in an excited state of the neutral system by a pump pulse. Since in general the transition probability to the ion state is a function of the wave packet s location on the potential-energy surface, the propagation of the wave packet can be probed by a second, time-delayed pulse, (b) Principle of the time-resolved NeNePo process. Starting in the anion s potential-energy surface, an ultrashort pump pulse detaches an electron cuid prepares a wave packet in the neutrcd. After a certain delay time At a probe pulse photoionizes the neutral. The time-dependent signal of the cation s intensity is detected. For convenience, this method is called NeNePo , Negative-to-Neutrcd-to-Positive...

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