Doorway modes

In this contribution we demonstrate vibrationally induced molecular dissociation in two polyatomic molecules, namely chromium hexacarbonyl and diazomethane (Fig. 1), employing fs mid-infrared laser sources [1,2]. As will be shown below, in both cases the initially excited mode does not lead directly to reaction but provides the doorway to access the right combination of modes. Thus both reactions have three steps (1) Stepwise multiphoton excitation of the doorway mode, (2) sharing of energy with other modes, and (3) formation of... 

Fig. 10. (a) A ID lattice of rigid diatomic molecules. The dispersion curve for acoustic phonons that result from translations runs from zero frequency to a cut-off termed the Debye frequency. The vibron has no dispersion. Adding flexibility to the molecules introduces dispersion in the vibron state and narrows the gap between vibrons and phonon as shown at right, (b) A 3D lattice of flexible naphthalene molecules. The 12 phonons overlap significantly with the two or three lowest energy vibrations, termed doorway modes. Doorway modes are coupled to both phonons and higher frequency vibrations associated with bond breaking. Adapted from ref. [91]. 

Fig. 13. Multiphonon and doorway mode models for phonon pumping of vibrations. The continuous states at right represent phonons. States from zero frequency to ttfe (lighter color) represent phonon fundamentals higher energy states are phonon combinations and overtones,...

Dlott and Payer s doorway model of phonon pumping [50] suggested two essentially equivalent methods for determining the rate of phonon pumping of doorway vibrations for large polyatomic molecules where only two phonons are needed to pump a doorway vibration. The anharmonic term in the Hamiltonian responsible for two-phonon doorway mode pumping is a cubic anharmonic term of the form [21,50],... 

Aromatic alcohol clusters have been well-studied, also for methodical reasons. The UV chromophore can be exploited for sensitive detection of the IR spectrum [35, 36, 120, 179]. Time-domain experiments become possible [21], which show that the initial energy flow out of the O—H stretching mode occurs primarily via C—H stretching and bending doorway states. Like in the case of carboxylic acid dimers [245], the role of the hydrogen bond is to shift the O—H stretching mode closer to these doorway states and thus to accelerate the initial energy flow. 

As is ISC, IC is very slow for electronic states with similarly shaped potential energy surfaces. When the potential surfaces have very different shapes, there will be a small number of vibrational doorway states that are especially effective in coupling to the bright state. Conical intersections are a special class of potential surfaces of very different shapes. But even when potential surfaces have very different shapes, many normal coordinate displacements and the associated vibrational normal modes will have nearly identical forms on both surfaces. These normal modes are Franck-Condon inactive and do not contribute to IC. The normal coordinate displacements that express the differences in shapes of the potential surfaces are embodied in vibrational normal modes that are Franck-Condon active. These modes are called promoting modes because, when such a mode on one potential surface is plucked from an eigenstate on the other surface, intramolecular dynamics is promoted or initiated. 

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