Photoelectron Spectroscopy for Molecules

FIGURE 6.29 The photoelectron spectrum of H2 shows a series of peaks corresponding to vibrational excitation of Hj. 

We know from Section 4.7 that the vibrational energy is quantized, and we treat it as a simple harmonic oscillator E = nhvyn, where n = 0, 1,2, 3,. .. is the vibrational quantum number. As a result of the vibrational excitation, the peak i in the spectrum is actually a series of narrower peaks the separation between adjacent peaks depends on the vibrational frequency of the diatomic ion  

FIGURE 6.30 The photoelectron spectrum of H2 measures the ionization energy of the a-gt, molecular orbital. The procedures for obtaining orbital energies from measured ionization energies are described in the text. Photoelectron spectroscopy gives no information about the unoccupied orbital. 

FIGURE 6.31 Peaks in the photoelectron spectrum of HCI measured with He(l) photons at 21.2 eV. 

These vibrational fine-structure peaks on the PES data at first appear to be a nuisance, but in fact, they greatly aid in relating experimental data to particular MOs. The connection is made through the concept of bond order introduced in Section 6.2. We illustrate the procedure for three separate cases. 

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In addition to the natural improvements expected in the accuracy of the measurements, and the increased scope in the types of systems examined, new techniques go beyond the issue of thermochemistry to allow for very detailed studies of reaction dynamics. The investigation by Zewail and co-workers of the reactivity of planar COT" on the femtosecond time scale is likely only the beginning. Time-resolved photoelectron spectroscopy, for example, has recently been used to map the potential energy surfaces for the dissociation of simple ions IBr and l2. " Although applications in the field of organic reactive molecules are likely far off, they are now possible. 

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%... 

These cases illustrate just some of the kinds of information provided by photoelectron spectroscopy for the characterization of organo-metallic molecules. Much of this information is extremely basic to the understanding of organometallic molecules, but is generally not available from other experimental techniques. For example, the formal oxidation state of the metal is indicated by the valence ionizations, where the number of ionizations identified with high... 

The first five ionization potentials observed from UV photoelectron spectroscopy for the two molecules (B2Hg Lloyd and Lynaugh, 1970 C2H4 Branton et ai, 1970) are compared in Table 2.6 with the negative of the orbital energies resulting from the theoretical MO-SCF calculations... 

The adiabatic picture developed above, based on the BO approximation, is basic to our understanding of much of chemistry and molecular physics. For example, in spectroscopy the adiabatic picture is one of well-defined spectral bands, one for each electronic state. The smicture of each band is then due to the shape of the molecule and the nuclear motions allowed by the potential surface. This is in general what is seen in absorption and photoelectron spectroscopy. There are, however, occasions when the picture breaks down, and non-adiabatic effects must be included to give a faithful description of a molecular system [160-163]. 

The simplest, and perhaps the most important, information derived from photoelectron spectra is the ionization energies for valence and core electrons. Before the development of photoelectron spectroscopy very few of these were known, especially for polyatomic molecules. For core electrons ionization energies were previously unobtainable and illustrate the extent to which core orbitals differ from the pure atomic orbitals pictured in simple valence theory. 

Surface analysis has made enormous contributions to the field of adhesion science. It enabled investigators to probe fundamental aspects of adhesion such as the composition of anodic oxides on metals, the surface composition of polymers that have been pretreated by etching, the nature of reactions occurring at the interface between a primer and a substrate or between a primer and an adhesive, and the orientation of molecules adsorbed onto substrates. Surface analysis has also enabled adhesion scientists to determine the mechanisms responsible for failure of adhesive bonds, especially after exposure to aggressive environments. The objective of this chapter is to review the principals of surface analysis techniques including attenuated total reflection (ATR) and reflection-absorption (RAIR) infrared spectroscopy. X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), and secondary ion mass spectrometry (SIMS) and to present examples of the application of each technique to important problems in adhesion science. 

Unlike the stable molecule N2O, the sulfur analogue N2S decomposes above 160 K. In the vapour phase N2S has been detected by high-resolution mass spectrometry. The IR spectrum is dominated by a very strong band at 2040 cm [v(NN)]. The first ionization potential has been determined by photoelectron spectroscopy to be 10.6 eV. " These data indicate that N2S resembles diazomethane, CH2N2, rather than N2O. It decomposes to give N2 and diatomic sulfur, S2, and, hence, elemental sulfur, rather than monoatomic sulfur. Ab initio molecular orbital calculations of bond lengths and bond energies for linear N2S indicate that the resonance structure N =N -S is dominant. 

One of the most direct methods is photoelectron spectroscopy (PES), an adaptation of the photoelectric effect (Section 1.2). A photoelectron spectrometer (see illustration below) contains a source of high-frequency, short-wavelength radiation. Ultraviolet radiation is used most often for molecules, but x-rays are used to explore orbitals buried deeply inside solids. Photons in both frequency ranges have so much energy that they can eject electrons from the molecular orbitals they occupy. 

If two different three-dimensional arrangements in space of the atoms in a molecule are interconvertible merely by free rotation about bonds, they are called conformationsIf they are not interconvertible, they are called configurations Configurations represent isomers that can be separated, as previously discussed in this chapter. Conformations represent conformers, which are rapidly interconvertible and are thus nonseparable. The terms conformational isomer and rotamer are sometimes used instead of conformer . A number of methods have been used to determine conformations. These include X-ray and electron diffraction, IR, Raman, UV, NMR, and microwave spectra, photoelectron spectroscopy, supersonic molecular jet spectroscopy, and optical rotatory dispersion (ORD) and CD measurements. Some of these methods are useful only for solids. It must be kept in mind that the conformation of a molecule in the solid state is not necessarily the same as in solution. Conformations can be calculated by a method called molecular mechanics (p. 178). 

Even the photoelectron spectroscopy of closed-shell molecules is valuable for the physical chemistry of radicals because a difference between the nth and the first adiabatic ionization potentials determines the excitation energy in a radical cation for a transition from the ground doublet state to the (n — 1) excited doublet state. 

In the case of selective oxidation catalysis, the use of spectroscopy has provided critical Information about surface and solid state mechanisms. As Is well known( ), some of the most effective catalysts for selective oxidation of olefins are those based on bismuth molybdates. The Industrial significance of these catalysts stems from their unique ability to oxidize propylene and ammonia to acrylonitrile at high selectivity. Several key features of the surface mechanism of this catalytic process have recently been descrlbed(3-A). However, an understanding of the solid state transformations which occur on the catalyst surface or within the catalyst bulk under reaction conditions can only be deduced Indirectly by traditional probe molecule approaches. Direct Insights Into catalyst dynamics require the use of techniques which can probe the solid directly, preferably under reaction conditions. We have, therefore, examined several catalytlcally Important surface and solid state processes of bismuth molybdate based catalysts using multiple spectroscopic techniques Including Raman and Infrared spectroscopies, x-ray and neutron diffraction, and photoelectron spectroscopy. 

Photoelectron spectroscopy has routinely been used to determine the ionization energies of stable molecules. It has also been adapted for the investigation of reactive species, including radicals, biradicals, and carbenes, usually generated chemically or by using pyrolysis. ... 

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