Molecules photoionization spectroscopy

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. 

Fig.6.28a-d. Level schemes of ionization spectroscopy (a) photoionization (b) excitation of autoionizing Rydberg levels (c) two-photon ionization of excited molecules (d) experimental arrangement for photoionization spectroscopy in a molecular beam... 

Time-of-flight mass spectrometers have been used as detectors in a wider variety of experiments tlian any other mass spectrometer. This is especially true of spectroscopic applications, many of which are discussed in this encyclopedia. Unlike the other instruments described in this chapter, the TOP mass spectrometer is usually used for one purpose, to acquire the mass spectrum of a compound. They caimot generally be used for the kinds of ion-molecule chemistry discussed in this chapter, or structural characterization experiments such as collision-induced dissociation. Plowever, they are easily used as detectors for spectroscopic applications such as multi-photoionization (for the spectroscopy of molecular excited states) [38], zero kinetic energy electron spectroscopy [39] (ZEKE, for the precise measurement of ionization energies) and comcidence measurements (such as photoelectron-photoion coincidence spectroscopy [40] for the measurement of ion fragmentation breakdown diagrams). 

Methane-to-methanol conversion by gas-phase transition metal oxide cations has been extensively studied by experiment and theory see reviews by Schroder, Schwarz, and co-workers [18, 23, 134, 135] and by Metz [25, 136]. We have used photofragment spectroscopy to study the electronic spectroscopy of FeO" " [47, 137], NiO [25], and PtO [68], as well as the electronic and vibrational spectroscopy of intermediates of the FeO - - CH4 reaction. [45, 136] We have also used photoionization of FeO to characterize low lying, low spin electronic states of FeO [39]. Our results on the iron-containing molecules are presented in this section. 

Dicyanofuroxan (3,4-dicyano-l,2,5-oxadiazole 2-oxide), the precursor to the NCCNO (see structure 11) species, has been studied in the solid and gas phases to obtain both structural and electronic information. The solid-state structure determined by X-ray diffraction gives an orthorhombic space group PnaZ, with a= 10.2578(14), b = 10.8818(12), and c= 10.2259(15) A. There are two independent molecules with similar geometries in the asymmetric unit. The gas-phase molecule was characterized by Hel photoelectron, Hel and HLtti(3i7 photoionization, and IR spectroscopy. The... 

For many years, investigations on the electronic structure of organic radical cations in general, and of polyenes in particular, were dominated by PE spectroscopy which represented by far the most copious source of data on this subject. Consequently, attention was focussed mainly on those excited states of radical ions which can be formed by direct photoionization. However, promotion of electrons into virtual MOs of radical cations is also possible, but as the corresponding excited states cannot be attained by a one-photon process from the neutral molecule they do not manifest themselves in PE spectra. On the other hand, they can be reached by electronic excitation of the radical cations, provided that the corresponding transitions are allowed by electric-dipole selection rules. As will be shown in Section III.C, the description of such states requires an extension of the simple models used in Section n, but before going into this, we would like to discuss them in a qualitative way and give a brief account of experimental techniques used to study them. 

In ordinary spectroscopy one measures the frequency (or wavelength) and therefore the energy of photons absorbed or emitted (or scattered, as in the Raman effect) by molecules. In photoelectron spectroscopy (PES) one measures the energy of electrons emitted by molecules when they are photoionized by the absorption of high-energy (UV or x-ray) photons. If M stands for a molecule, the photoionization process can be symbolized by... 

Prof. Schlag ( ZEKE Spectroscopy, this volume) has introduced a new sequential technique of ZEKE spectroscopy In the first step, a negative ion M is photoionized, yielding the neutral core M of the excited Rydberg state of the anion M. In the second step, M is further photoionized, yielding the cationic core M+ of the excited Rydberg state of the neutral molecule M. The overall sequence is thus... 

With more conventional photoelectron spectroscopy (PES), the photon energy is fixed and the distribution of kinetic energies of the electrons produced by photoionization determined. The latter distribution reflects the distribution of ion internal states produced. Photoelectron spectroscopy has been applied to determine vibrational-state distributions at the same photon wavelengths at which an ion-molecule reaction was studied using photoionization methods (Fig. 4).86d... 

Photoelectron spectroscopy uses the photoionization of a neutral molecule M to its radical cation M ,... 

The molecules and their first vertical ionization energies listed (Table 3) supplement those already presented as examples. The parent silylene has been generated by the SiFLt + 2F — SiH2 + 2HF reaction and investigated by photoionization mass spectroscopy, which yields the adiabatic onset value quoted73. Silyl derivatives such as C(SiH3)4 have been thermally decomposed to detect new routes to amorphous,... 

More direct evidence for M—CO n bonding can be obtained by photoelectron spectroscopy. For Cr(CO)6 and W(CO)6, the photoionization of one of those electrons that must be responsible for whatever M—CO 77 bonding exists was carried out and the effect of removing such an electron on the frequency of the totally symmetric M—C stretching vibration in the resulting M(CO)6 ion measured. The frequency of this vibration was, in each case, found to be significantly lower than that of the corresponding vibration in the neutral M(CO)6 molecule, that is, by 10%... 

Photoelectron spectroscopy is the study of the electron kinetic energy spectrum produced upon photoionization of molecules with monochromatic radiation. We may write the basic photoelectric equation as... 

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