Continuum dissociation

Massmann and co-workers [102] undertook detailed studies of these types of spectra in graphite furnaces, and scanned the dissociation continua of numerous molecules, including the alkali halides. As this kind of background absorption is continuous , it does not represent any problem for HR-CS AAS, and it can be corrected easily by all BC systems used in LS AAS, unless the background absorption is changing too rapidly or it is exceeding the correction limits discussed earlier. More details may be found in Reference [150], 

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Many common dimers possess just a few rotovibrational levels, but some, like Xe2, possess thousands of rotovibrational states. Molecular transition frequencies vary from just a few wavenumbers to tens of wavenumbers. Besides these rotovibrational bands, dissociation continua exist, see Chapters 5 and 6 for details. Experimentally, very little is known about these dimer signatures in the translational band, presumably because high-resolution work in the far infrared is difficult. Furthermore, because of the feebleness of induced dipoles, high pressures are commonly necessary for a recording of the absorption spectra. This fact tends to pressure-broaden dimer lines to a point where their observation may be impossible. 

To make the model as simple as possible we consider N degenerate states which can dissociate. In general, there can be K, K 1, dissociation continua. These will in general be distinct continua because there can be different possible internal states of the products. A useful way to think about K is as the number N (E — E0) of states at the transition state. If we uniquely correlate the states at the transition state with the states of the products (118) then this identification is obvious. Otherwise, we recall that there is no obvious way to specify the location of the transition state except for its property of being the bottleneck for the reactive events. It follows from this (known as the variational transition state (119)) point of view that the transition state is to be located where N (E — E0) is minimal (120). 

Recent developments of the supersonic-nozzle technique opened a new field the study of dynamics of electronically excited van der Waals complexes. This problem is closely related to that of collisions involving electronically excited molecules. The analogy clearly appears when the system composed of an excited molecule A and of a perturber M is described in the reference of its center of mass. The electronic excitation of the free molecule (followed by a collision) corresponds to the electronic transition between ground- and excited-state dissociative continua (Fig. 5). 

Collision-induced j> /> transitions in the excited molecule M will result from the coupling between dissociative continua of the s> and /> states of the M. ..A complex. This coupling strength depends on the relative velocities of colliders, impact parameters, orientation of molecular axes, etc. The theoretical treatment of the problem necessitates thus an averaging over the statistical distribution of all relevant parameters. 

Specifically, in Ref. [145] state-specific HF calculations allowed the understanding of formation and the consequences of excited states in the tri-atomic molecules HeH2, NeH2 and ArH2, which are formed transiently in the dissociative continua of the ground state. 

In this chapter, we turn to problems of quantum chemistry and of many-electron atomic and molecular physics for which fhe desideratum is the quantitative knowledge and easy conceptual understanding of dynamical processes and phenomena thaf depend explicifly on time. We focus on a theoretical and computational approach which computes q>(q,t) by solving nonperturbatively the many-electron TDSE for unstable states of atoms and small molecules. The time evolution of fhese states is caused either by the time-independent Hamiltonian, Ham ( -g-/ case of time-resolved autoionization—see below) or by the time-dependent Hamiltonian, H t) = Ham + Vext(f), where Vext(f) is the sum of the identical one-electron operators that couple the field of a strong pulse of radiation to the electronic and nuclear moments of N-electron atomic or molecular states of inferest, thereby producing, during and at the end of the interaction, final stafes in the ionization or the dissociation continua. 

In unimolecular fragmentation reactions we have coupling of quasi-bound or resonance scattering states to dissociation continua, considering the fragments as part of a single supermolecule [equation (34)]. A are highly excited, metastable... 

The optical model " of unimolecular resonances uses the partitioning of Hilbert space into Q and P subspaces, where Q and P represent the projection of the Hilbert space onto bound states and dissociative continua, respectively ... 

Another experimental pointer to the role played by the quantal description of the irradiated molecule relates to the observation of fragments like and CS+ when 40-fs pulses are used. Such ions cannot be produced by direct field ionization of neutral CS2 because Franck-Condon factors indicate that the dissociation continua of the X, A, and B electronic states of CSj are vertically inaccessible. The next ionic state C lies at 16.2 eV from the ground state of the neutral, well above the dissociation limits S+ + CS and S + CS and is hence predissociative. It is the lengthening of the C-S bond that occurs in the El process that properly accounts for the and CS yields in longer pulses population of excited electronic states of CSj then becomes likely, and it is these excited cation states that act as precursors for S and CS. The nonappearance of these fragment ions in the case of the 10-fs spectrum is an unambiguous signature that the El process is switched off in the ultrashort domain. 

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