Polyatomic molecules photoelectron spectroscopy

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. 

The major changes in the new edition are as follows There are three new chapters. Chapter 1 is a review and summary of aspects of quantum mechanics and electronic structure relevant to molecular spectroscopy. This chapter replaces the chapter on electronic structure of polyatomic molecules that was repeated from Volume I of Quantum Chemistry. Chapter 2 is a substantially expanded presentation of matrices. Previously, matrices were covered in the last chapter. The placement of matrices early in the book allows their use throughout the book in particular, the very tedious and involved treatment of normal vibrations has been replaced by a simpler and clearer treatment using matrices. Chapter 7 covers molecular electronic spectroscopy, and contains two new sections, one on electronic spectra of polyatomic molecules, and one on photoelectron spectroscopy, together with the section on electronic spectra of diatomic molecules from the previous edition. In addition to the new material on matrices, electronic spectra of polyatomic molecules, and photoelectron... 

TIME-RESOLVED PHOTOELECTRON SPECTROSCOPY OF NONADIABATIC DYNAMICS IN POLYATOMIC MOLECULES... 

As discussed in Section II, the excited-state dynamics of polyatomic molecules is dictated by the coupled flow of both charge and energy within the molecule. As such, a probe technique that is sensitive to both nuclear (vibrational) and electronic configuration is required in order to elucidate the mechanisms of such processes. Photoelectron spectroscopy provides such a technique,... 

As a first step toward answering this question, it seems reasonable to investigate a high-symmetry polyatomic. BH3 is an obvious candidate, given its D3h equilibrium geometry (experimentally verified by photoelectron-spectroscopy studies of BH3 [38] and by gas-phase spectroscopic observations of the neutral molecule itself [39][40]). Besides, its small size makes all-electron OBS-GMCSC calculations on it easily feasible, nowadays, even on a run-of-the-mill Personal Computer. Moreover, the fact that BH3 is an electron-deficient system, and of course that it spontaneously dimerizes to the ever-intriguing diborane, implies that a study of its electronic structure may be of intrinsic interest. 

These examples show that photoelectron spectroscopy is useful in testing theoretical models for bonding because it directly measures ionization energies that can be correlated with theoretical orbital energies through Koopmans s theorem. These methods are readily extended to polyatomic molecules. 

Photoionization and Electron Detachment.—Photoelectron spectroscopy is not within the scope of this volume, but some papers on photoionization which may be of some interest are listed here. Several theoretical treatments of photoionization in atoms and small molecules have appeared.213 Photoionization in the rare gases,214 atomic hydrogen,215 alkali-metal and alkaline-earth atoms,21 magnesium atoms,217 group II atoms,218 mercury atoms,219 molecular hydrogen,220 several polyatomic molecules,221 and solvated electrons 222 has been discussed. Multiphoton ionization in the rare gases,223 benzene,224 and in molecular crystals has also been discussed.225 ... 

It has been shown that the simple approach of PIPICO measurements works satisfactory only for small molecules, where the number of possible iission processes is limited. In the case of polyatomic molecules and clusters, this approach leads to ambiguities. Therefore, the use of multicoincidence experiments, where the individual flight times of the correlated cations, tA and are measured separately, gives more detailed information on the fission mechanisms. One important approach, that is discussed in detail below, is Photoelectron-photoion-photoion-coincidence (PEPIPICO) spectroscopy (cf. Secs. 3.3 and 4.3). The results that are obtained from PEPIPICO spectra are three-dimensional shapes of correlated cation pairs (cf. Sec. 4.3), where the relative intensity of these processes is plotted over the flight times of the correlated cations. Alternatively, the intensity of the coincidence signals can be represented in contour plots, as shown in Fig. 6. Note that only one half of the array is shown because of the mirror symmetry with respect to the main diagonal. 

To illustrate the practical application of the theoretical formalism, we have discussed several representative examples that demonstrate the potential of femtosecond time-resolved spectroscopy. Since typically only a few degrees of freedom (say, 2-4) are involved in the ultrafast photodynamics via a conical intersection, explicit simulations of spectroscopic experiments for polyatomic molecules are nowadays possible on a routine basis. As an example of a state-of-the-art simulation of a femtosecond experiment on a polyatomic system, we have presented first-principles calculations of the time-resolved photoelectron spectra of pyrazine. Although in this case the interpretation was found to be quite complicated, the investigations nevertheless demonstrated the considerable potential of a joint experimental/theoretical study of the dynamics at conical intersections. Employing... 

Stolow, A. (2003a). Femtosecond time-resolved photoelectron spectroscopy of polyatomic molecules. Ann. Rev. Phys. Chem. 54, 89. 

Ionization potentials of transient species have been measured by photoelectron spectroscopy and this technique may be sufficiently sensitive to be used for monitoring energy states of diatomic and perhaps polyatomic product molecules. Although quantitative energy disposal data have not been reported yet, the photoelectron spectroscopic method appears to be useful for diatomics which cannot be observed by conventional spectroscopic techniques. Like laser-induced fluorescence, the Franck-Condon factors of the ground state and the ion must be known to obtain vibrational populations from the intensity data. 

---