Photodissociation electronic spectroscopy

We use laser photofragment spectroscopy to study the vibrational and electronic spectroscopy of ions. Our photofragment spectrometer is shown schematically in Eig. 2. Ions are formed by laser ablation of a metal rod, followed by ion molecule reactions, cool in a supersonic expansion and are accelerated into a dual TOE mass spectrometer. When they reach the reflectron, the mass-selected ions of interest are irradiated using one or more lasers operating in the infrared (IR), visible, or UV. Ions that absorb light can photodissociate, producing fragment ions that are mass analyzed and detected. Each of these steps will be discussed in more detail below, with particular emphasis on the ions of interest. 

The second example of the application of TDDFT to the electronic spectroscopy of metalloporphyrins concerns a CO-ligated iron porphyrin, a system that models the active centers of hemoproteins, recently investigated by Head-Gordon et al [146, 147] in the context of a theoretical study of the initial step of the photodissociation pathway of CO-ligated heme. 

In order to illustrate the complexity of excited states reactivity in transition metal complexes two selected examples are reported in the next section dedicated to the ab initio (CASSCF/MR-CI or MS-CASPT2) study of the photodissociation of M(R)(CO)3(H-DAB) (M=Mn, R=H M=Re, R=H, Ethyl) complexes. Despite the apparent complexity and richness of the electronic spectroscopy, invaluable information regarding the photodissociation dynamics can be obtained on the basis of wave packet propagations on selected 1-Dim or 2-Dim cuts in the PES, restricting the dimensionality to the bonds broken upon visible irradiation (Metal-CO or Metal-R). The importance of the intersystem crossing processes in the photoreactivity of this class of molecules will be illustrated by the theoretical study of the rhenium compound. 

Part 3. Ion Spectroscopy. In Chapter 9, we return to the theme of ion photodissociation, which was included also in Volume IV, Part 6, in an exploration of trapped-ion photodissociation, electron photodetachment, and fluorescence. Trapped-ion fluorescence may offer an alternative approach for the elucidation of ion conformation. Whereas these spectroscopic experiments require high ion densities, much attention is directed to the spectroscopic study of single ions confined in an ion trap. Chapters 10 and 11 are illustrative of such studies, with the former devoted to the study of a single molecular ion in a linear ion trap and the latter to a single atomic ion in Paul-type ion traps. While both types of studies require extensive cooling of the subject ion, once such cooling has been achieved, the ions can remain confined for many hours. 

Abstract In this review we discuss the theory and application of methods of excited state quantum chemistry to excited states of transition metal complexes. We review important works in the field and, in more detail, discuss our own studies of electronic spectroscopy and reactive photochemistry. These include binary metal carbonyl photodissociation and subsequent non-adiabatic relaxation, Jahn-Teller and pseudo-Jahn-Teller effects, photoisomerization of transition metal complexes, and coupled cluster response theory for electronic spectroscopy. We aim to give the general reader an idea of what is possible from modem state-of-the-art computational techniques applied to transition metal systems. 

See also Laser Applications in Electronic Spectroscopy Laser Spectroscopy Theory Light Sources and Optics Multiphoton Excitation in Mass Spectrometry Muitiphoton Spectroscopy, Applications Optical Frequency Conversion Pharmaceuticai Appiications of Atomic Spectroscopy Photoelectron Spectroscopy Photoionization and Photodissociation Methods in Mass Spectrometry Pyroiysis Mass Spectrometry, Methods Time of Flight Mass Spectrometers X-Ray Spectroscopy, Theory Zero Kinetic Energy Photo-eiectron Spectroscopy, Appiications Zero Kinetic Energy Photoelectron Spectroscopy, Theory. 

Molecular electronic absorption and emission spectroscopy, photodissociation, Raman spectroscopy, and vibrational overtone spectroscopy have been examined from a time-dependent perspective for their implications for potential energy surfaces and dynamics. A semiclassical point of view greatly aids intuitive understanding and computation of these various spectroscopies from limited local and regional knowledge of the potential surface. 

Vibrationally mediated photodissociation (VMP) can be used to measure the vibrational spectra of small ions, such as V (OCO). Vibrationally mediated photodissociation is a double resonance technique in which a molecule first absorbs an IR photon. Vibrationally excited molecules are then selectively photodissociated following absorption of a second photon in the UV or visible [114—120]. With neutral molecules, VMP experiments are usually used to measure the spectroscopy of regions of the excited-state potential energy surface that are not Franck-Condon accessible from the ground state and to see how different vibrations affect the photodissociation dynamics. In order for VMP to work, there must be some wavelength at which vibrationally excited molecules have an electronic transition and photodissociate, while vibrationally unexcited molecules do not. In practice, this means that the ion has to have a... 

A second role for mass spectrometry in the investigation of reactive intermediates involves the nse of spectroscopy. Althongh an important nse of ion spectroscopy is the determination of thermochemical properties, including ionization energies (addition or removal of an electron), as in photoelectron or photodetachment spectroscopy, and bond dissociation energies in ions, as in photodissociation methods, additional spectroscopic data can also often be obtained, inclnding structural parameters such as frequencies and geometries. 

Hydroxyl radical (OH) is a key reactive intermediate in combustion and atmospheric chemistry, and it also serves as a prototypic open-shell diatomic system for investigating photodissociation involving multiple potential energy curves and nonadiabatic interactions. Previous theoretical and experimental studies have focused on electronic structures and spectroscopy of OH, especially the A2T,+-X2n band system and the predissociation of rovibrational levels of the M2S+ state,84-93 while there was no experimental work on the photodissociation dynamics to characterize the atomic products. The M2S+ state [asymptotically correlating with the excited-state products 0(1 D) + H(2S)] crosses with three repulsive states [4>J, 2E-, and 4n, correlating with the ground-state fragments 0(3Pj) + H(2S)[ in... 

Dissociation of axial ligands has been followed by picosecond spectroscopy for a number of metalloporphyrins. For the well-known photodissociation of O2 and CO from hemoglobin and myoglobin the photoproducts appear very early < 10 psec. Dissociation of basic axial ligands such as pyridine and piperadine occurs within the lifetime of the excited state for Ni(II), Co(III) as well as for Fe(II) porphyrins. Whether the ejected species is "hot" with energy from the electronic deactivation of the porphyrin is not known, but the dissociation process does not appear to be dependent upon the wavelength of the excitation pulse (30,32). 

The approach taken in our laboratory combines both of these trends. Specifically, we have developed a new experiment that allows us to study, for the first time, the photodissociation spectroscopy and dynamics of an important class of molecules reactive free radicals. This work is motivated in part by the desire to obtain accurate bond dissociation energies for radicals, in order to better determine their possible role in complex chemical mechanisms such as typically occur in combustion or atmospheric chemistry. Moreover, since radicals are open-shell species, one expects many more low-lying electronic states than in closed-shell molecules of similar size and composition. Thus, the spectroscopy and dissociation dynamics of these excited states should, in many cases, be qualitatively different from that of closed-shell species. 

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