Trimers ionization potentials

Alkylphosphines have been studied (99, 231), as have cyclopolyphos-phines (120). The reaction products of phosphorus with various olefins were investigated using mass spectral methods (108), and a large number of phospholines have been reported (181, 237). The mass spectrum of the trimer of tetramethylphosphinoborine showed that B—P, C—B, and C—P bonds were broken first, and with the pentamer of the dimethyl compound, B—H, B—P, and C—P bonds were broken (101, 232). The spectra of thiophosphorustri-A -methylimide and its arsenic analog were reported by Holmes and Forstner (130). Ionization potentials have been recorded for many perfluoroalkylarsines and some of the related alkyl compounds (79). 

The ultraviolet photoelectron spectra of diatomic alkali halide molecules are reviewed and interpreted. Data for lithium halide dimers, 112X2> are presented and it is shown that the dimers have significantly larger ionization thresholds than the corresponding monomers. Some historical controversies regarding the presence of dimers and their ionization energies are clarified. Photoionization mass spectrometry is used to determine the adiabatic ionization potential of lithium chloride trimer, in order to probe the trend of I.P. with cluster size. The predictions of Hartree-Fock, Xa and ionic model calculations on this point are presented. 

This manuscript summarizes the most significant current experimental information regarding the photoionization behavior of alkali halide monomers, dimers and trimers. Some data exist on higher ionization potentials, (30) and ionic model calculations have also been applied to these higher levels. (19, 30)... 

The transition from the atom to the cluster to the bulk metal can best be understood in the alkali metals. For example, the ionization potential (IP) (and also the electron affinity (EA)) of sodium clusters Na must approach the metallic sodium work function in the limit N - . We previously displayed this (1) by showing these values from the beautiful experiments by Schumacher et al. (36, 37) (also described in this volume 38)) plotted versus N". The electron affinity values also shown are from (39), (40) and (34) for N = 1,2 and 3, respectively. A better plot still is versus the radius R of the N-mer, equivalent to a plot versus as shown in Figure 1. The slopes of the lines labelled "metal sphere" are slightly uncertain those shown are 4/3 times the slope of Wood ( j ) and assume a simple cubic lattice relation of R and N. It is clear that reasonably accurate interpolation between the bulk work function and the IP and EA values for small clusters is now possible. There are, of course, important quantum and statistical effects for small N, e.g. the trimer has an anomalously low IP and high EA, which can be readily understood in terms of molecular orbital theory (, ). The positive trimer ions may in fact be "ionization sinks" in alkali vapor discharges a possible explanation for the "violet bands" seen in sodium vapor (20) is the radiative recombination of Na. Csj may be the hypothetical negative ion corresponding to EA == 1.2 eV... 

The first studies have been done with molecules and clusters of silver atoms [418], because there is considerable information about them from experiment and from theory. Only the clusters Aga, Ag5, Agy, and Agg have been looked at, because their ionization potentials are low enough to ionize them by two-photon absorption with the laser sources available (see Sect. 2.1.2). However, these are in some respects an ideal series because they provide several different structural relationships among the negative, neutral, and positive species [84, 419]. As is shown in Sect. 5.1, the silver trimer can... 

The first experiments were carried out using the silver trimer anion. Mass-selected Ag3 ions were produced with an intensity of about 2 nA and stored in the ion trap. The detachment was performed at wavelengths of 420 nm, 415 nm, 400 nm, and 390 nm, so that one-photon detachment of the anions was possible. The ionization was done nonresonantly using two photons of the same wavelength. The energy of two photons of 420 nm is only slightly above the ionization potential of the silver trimer, thus allowing a very soft ionization. 

Combining a microscopic electronic theory with molecular dynamics simulations in the Born-Oppenheimer approximation, Bennemann, Garcia, and Jeschke presented the first theoretical results for the ultrafast structural changes in the silver trimer [135]. They determined the timescale for the relaxation from the linear to a triangular structure initiated by a photodetachment process and showed that the time-dependent change of the ionization potential (IP) reflects in detail the internal degrees of freedom. 

Fig. 3.35. Principle of real-time TPl spectroscopy of a triatomic s system such as the Jahn-Teller distorted Nas. A wave packet is prepared in an excited state of the neutral trimer 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 ionization pulse. While for a delay time At the ionization yield might be maximum, there is zero ionization probability for At2...

In order to probe the intriguing catalytic behavior of benzoic acid, studies were conducted with the acid and the sodium salt, sodium benzoate, to determine the whether the protic nature of the catalyst was significant [31]. Complete dissociation of the acid in the molten trimer under reaction conditions was viewed as a matter of speculation, and the presence of protons did not provide any clear insight into the role of the benzoate ion. Sodium benzoate was safely assumed to be dissociated. The data recorded for both catalysts suggested similar kinetic behavior, which in turn implied that the potential degree of ionization of the catalyst does not play a role in the catalysis. 

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