Gas-condensed matter shift

In the crystal containing molecules with a center of inversion and only in the dipole approximation for the intramolecular interaction, the parts of the total Hamiltonian H4 = H = He = 0, and at the same time the value T>f, is equal to zero. However, it is known that the value Df determining the so-called gas-condensed matter shift even exists in crystals with center of inversion (and thus due to the contribution of the nondipole intermolecular interaction) and can be of the order of 0.1 eV (in anthracene crystals the value of the gas-condensed matter shift is about 2000 cm-1). This indicates that even in centrosymmetric crystals the terms in the total Hamiltonian H4, Hs, He may be neglected only with some care. 

Here Eq is the energy of the molecular excitation where also the gas-condensed matter shift is taken into account, and Pn, Pn are the exciton creation and annihilation Pauli operators on the molecule n, where n labels the unit cells of the... 

Gas condensed matter shift and the possibility of governing spectra of Frenkel excitons in thin layers... 

To go further we recollect how the gas-condensed matter shift can be calculated. It is known from the theory of molecular (Frenkel) excitons (5) that this shift appears due to the difference between the energies of interaction of the excited molecule (molecular state /) and the unexcited molecule (molecular ground state 0) with all other molecules of the same crystal in the ground state ... 

For the crystals formed by molecules without an inversion center the long-range dipole-dipole interaction of static dipoles becomes important. For such crystals the thickness of the surface layer, where the change of the gas-condensed matter shift can be important, increases and the special role of the outermost monolayer becomes weaker. 

The excited electronic state of the outermost molecular monolayer in anthracene is clearly seen in emission at low temperature. The monolayer next to the surface is blue-shifted by 10 cm-1 and the following one by 2 cm-1. The nature of these blue-shifts is now well understood and is related with the absence of neighbors for molecules located near the surface from the vacuum side. Therefore, for these molecules the gas-condensed matter (G-CM) shift of the electronic transition frequency is smaller than the G-CM shift in the bulk (see Fig. 9.1 we assume that the surface corresponds to the (a, b) plane of the anthracene crystal). For temperatures low compared with the blue-shift, the... 

The vibrational levels of a molecule in a condensed matter system are influenced by the surrounding medium through intermolecular interactions. The time-averaged forces exerted by the solvent on a molecular oscillator cause a static shift in the vibrational absorption frequency. The frequency shifts of the vibrational transitions of a molecule between the gas phase and a condensed matter environment is an indicator of the effect of the solvent on the internal mechanical degrees of freedom of a solute. 

One very important area is the identification of molecular species in astronomical sources. A surprising number of polyatomic molecules and ions, a selection of which are listed in Table 7.1, have been identified in the so-called molecular clouds, which appear to contain regions with moderate effective temperatures (tens or even hundreds of Kelvins) and which may represent an early stage in the condensation of interstellar matter into stars. Parts of an emission spectrum from such a source arc shown in Figure 7.8. Doppler shifts of the frequencies of lines in these rotation spectra give accurate estimates of the velocities of the gas clouds relative to Earth. 

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