Local-mode excitation

Local-Mode Excitation In local-mode sampling an individual local mode such as a CH bond in benzene is excited.This type of trajectory calculation has been performed to determine the population of a local-mode state v) versus time, from which the absorption linewidth of the overtone state may be determined.Good agreement has been found with both experimental and quantum-mechanically calculated overtone line-widths for benzene and linear alkanes. 

Excited Vibrational Spectrum of Sulfur Dioxide. II. Normal to Local Mode Transition and Quantum Stochasticity. 

Henry, B. R. (1977), Use of Local Modes in the Description of Highly Vibrationally Excited Molecules, Acc. Chem. Res. 10, 207. 

Specifically, the various papers working within both the adiabatic and the Condon approximations, and using the (frequent) assumption of harmonic vibrations, can still differ in how many and what type (optical, acoustic, or local) modes they consider and in how they approximate the four separate integrals on the right-hand side of Eq. (40). And the choice of modes applies to both the ground and the excited states (so does the choice of electronic wave functions, but this choice is implicit in the evaluation of the electronic integrals.) It is this choice regarding the two states that was emphasized in connection with Fig. 15 (Section 10b). It can be seen that even within the stated approximations (adiabatic, Condon, harmonic) there is an appreciable number of permutations and combinations. 

To explain the idea of the method [13,14], let us consider a two-phonon decay of a highly excited local mode caused by the interaction /7irl( = QY.rf V3 A,, where V3 lI/ are the cubic anharmonicity interaction parameters, Q is the coordinate... 

Let us consider first the decay of a strongly excited local mode due to simultaneous emission of k > 2 phonons. The Hamiltonian of the system under consideration is... 

Fig. 1.9. Absorption cross section (arbitrary units) of H20 initially prepared in the 04-0) state as a function of the photolysis wavelength A2. The first two quantum numbers specify the excitation of the two stretching modes (using a local mode assignment, see Chapter 13), the minus sign indicates the symmetry with respect to the interchange of the two H atoms, and the third quantum number denotes the bending state. Adapted from Vander Wal, Scott, and Crim (1991) calculations by Weide, Hennig, and Schinke (1989).

Fig. 10.5. Measured rotational state distributions of OH following the dissociation of the three lowest bending states of H2O (open circles). In addition to the bending quanta H20(X) also contains 4 respectively 3 quanta of OH stretching excitation. The local mode nomenclature nm k) is explained in Section 13.2. The total angular momentum is zero in all cases. The filled circles represent the harmonic oscillator approximation defined in the text. Reproduced from Schinke, Vander Wal, Scott, and Crim (1991).

Fig. 14.7. High resolution Raman spectrum for D20 excited with a 171 nm photon. Local mode assignment is used the first two quantum numbers indicate the OH stretching modes, the plus sign indicates the symmetry, and the third number represents the bending quantum number. The latter is zero in all cases and therefore not mentioned in the text. The energies are measured with respect to the 00+0) vibrational ground state. Reproduced from Sension et al. (1990).

Fig. 14.8. Raman spectra for H2S measured for several excitation wavelengths. The maximum of the absorption spectrum is around 195 nm. Local mode assignment is used as described in Figure 14.7 for H20. Note the striking dependence on the excitation wavelength which is given at the left-hand side of each spectrum Reproduced from Brudzynski, Sension, and Hudson (1990).

An inherent limitation of mode-selective methods is that Nature does not always provide a local mode that coincides with the channel of interest. One way to circumvent the natural reactive propensities of a molecule is to exploit the coherence properties of the quantum mechanical wave function that describes the motion of the particle. These properties may be imparted to a reacting molecule by building them first into a light source and then transferring them to the molecular wave function by means of a suitable excitation process. 

The initial step in the double-resonance scheme is the excitation of a local mode hydrogen stretch vibration localized in a hydrogen halide moiety. In principle, this can be done either at the fundamental or one of the overtones. With presently available Ti sapphire lasers and parametric oscillators (OPOs), it is possible to saturate fundamentals and first overtones, thus ensuring maximum population transfer. Second overtones cannot be pumped as efficiently, but offer enormous discrimination against background and can be used to shift frequencies out of the vacuum ultraviolet and into a more user-friendly part of the ultraviolet. Thus, first and second overtones are very attractive. 

These results show that one could represent the motion of the complex by one-dimensional modes on surfaces correlated to excited atomic calcium (see Figure 4-7), and the spectra are characteristic of the entrance valley of this reaction. On the other hand, the chemiluminescent reaction of excited calcium with HCl is known to occur with very high cross sections 25 A2 ( D2) (Brinckman et al. 1980) to 68 A2 (1P) (Rettner and Zare 1982). These cross sections agree with a passage of the ionic covalent crossing without a barrier at 3.5 A. Hence, there should be a smooth passage from the reagents to the products and the observation of action spectra with distinct features, not a continuum, can be interpreted as the excitation of local modes perpendicular to the reaction coordinate. 

Alternatively, such prepared excited states may prove useful photochemically under particular circumstances. This is especially true for local-mode-type molecules [461, 462], that is, molecules for which vibrational eigenstates resemble localized excitation in individual bonds. As an example, in the case of HOD, the large frequency difference between the OH and OD oscillators is such that intramolecular vibrational relaxation does not destroy the localized excitation. (Similar effects arise if one excites a resonance state that displays local behavior see, for example, Ref. [463] for an ABA-type molecule.) As shown theoretically [464, 465], and confirmed experimentally [53-60], preparation of the OH stretch followed by an excitation laser leading to dissociation gives a marked enhancement of the H atom photodissociation in many molecules. 

In the present paper, we show that it is possible to calculate both vibrational and electronic transitions of H2SO4 with an accuracy that is useful in atmospheric simulations. We calculate the absorption cross sections from the infrared to the vacuum UV region. In Section 2 we describe the vibrational local mode model used to calculate OH-stretching and SOH-bending vibrational transitions as well as their combinations and overtones [42-44]. This model provides frequencies and intensities of the dominant vibrational transitions from the infrared to the visible region. In Section 3 we present vertical excitation energies and oscillator strengths of the electronic transitions calculated with coupled cluster response theory. These coupled cluster calculations provide us with an accurate estimate of the lowest... 

A brief review and reassessment of data on the photophysics of benzene has been presented by Pereira. Evidence for the l E2g valence state has been obtained by u.v. two-photon spectroscopy.Slow electron impact excites fluorescence in thin films of benzene at 77 K as well as emission from isomers." The fluorescence yields and quenching by chloroform of alkyl-benzenes and 1-methylnaphthalene after excitation into Si, Sz, and S3 states and after photoionization have been measured. The channel-three process has been reconsidered in terms of the effects of local modes and Morse oscillator potentials. Excited-state dipole moments of some monosubstituted benzenes have been estimated from solvent effects on electronic absorption spectra, Structural imperfections influence the photochemistry of durene in crystals at low temperatures. Relaxation time studies on excited oxido-substituted p-oligophenylenes have been made by fluorescence depolarization... 

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