Molecules, large vibrational excitation

In section 3.1 we discussed the present picture of the electronic arrangement of CO chemisorbed on a metal surface, which was schematically shown in Fig. 3. When the molecule is vibrationally excited charge is oscillating between the 2n resonance and the metal. This gives rise to the large increase in the dynamical dipole moment, as was discussed in that section. These local charge... 

The dipolar or induced dipolar natime of molecules means that the impacting electron can cause rotational excitation but, because of conservation of momentum, very little of the kinetic energy of the electron can be imparted and little direct vibrational excitation can occur (Cottrell, 1965). Further, although ion-sources frequently operate at fairly high temperatures, the population of vibrationally excited states of molecules even at 500°K is very low and the source of the large vibrational excitation of ions must be sought elsewhere. For illustrative... 

When a large molecule is vibrationally excited or when a simpler one is excited to a high vibrational level, the excited state is immersed in a dense vibrational manifold that furnishes many vibration (V-V) deexcitation channels. The simplest example of V-V transfer between higher vibrational levels is provided by the exchange of a vibrational quantum between two diatomic molecules,... 

In this section, results that have been obtained from direct kinetic studies of processes involving vibrationally excited molecules are considered in relation to the factors that can determine the dynamics of molecular collisions. The number of investigations that could be mentioned in a review of this kind is already rather large. However, in order to be able to discuss some systems in reasonable depth, sdective, rather than comprehensive, coverage of the litoature is provided. The selection concentrates on systems where reasonably accurate kinetic data have been measured and on examples that illustrate the range of detailed processes that can occur when molecules in vibrationally excited states collide with potentially reactive species. 

Raman line intensities are proportional to the number density N of molecules in the initial state /c>, which is in turn proportional to the pertinent Boltzmann factor for that state at thermal equilibrium. Consequently, the relative intensities of a Stokes transition /c> - m> and the corresponding anti-Stokes transition m> -> /c> are 1 and exp — hoj kjkT), respectively. (The factor coicol varies little between the Stokes and anti-Stokes lines, because the Raman frequency shifts are ordinarily small compared to cui.) Hence the anti-Stokes Raman transitions (which require molecules in vibrationally excited initial states) are considerably less intense than their Stokes counterparts, particularly when the Raman shift (o k is large. In much of the current vibrational Raman literature, only the Stokes spectrum is reported (cf Fig. 10.1). 

An important area that has yet to be fully explored is the effect of the flexibility of water molecules. The intennolecular forces in water are large enough to cause significant distortions from the gas-phase monomer geometry. In addition, the flexibility is cmcial in any description of vibrational excitation in water. 

In a large molecule fhe vibrational and rofafional levels associated wifh any elecfronic sfafe become so exfremely congested af high vibrational energies fhaf fhey form a pseudo-continuum. This is illusfrafed for Sq, Si and fhe lowesf excited friplef sfafe T, lying below Sj,... 

In addition to the previously mentioned disadvantages, all of these methods have another drawback in the large molecule photofragment velocity measurements. For example, in the studies of UV photon photodissociation of polyatomic molecules, like alkene and aromatic molecules, molecules excited by the UV photons quickly become highly vibrationally excited in the ground electronic state through fast internal conversion, and dissociation occurs in the ground electronic state. 

The delay time between the pump and the probe laser pulses is usually very short in these experiments. The delay time is less than 5 ns when the pump and the probe laser pulses are the same, and the delay time is as long as several hundred nanoseconds when the pump and the probe laser pulses are from two different sources. The short delay time ensures that the fragments flying with different velocities are equally sampled before they leave the detection region. Since the delay time is much shorter than the lifetime of the excited molecules (.A ), most of these molecules do not dissociate into fragments when the probe laser pulse arrives. As a result, the probe laser can easily cause dissociative ionization of the vibrationally excited molecules due to their large internal energy. 

In very small molecules such as CH4 or C2H2 the molecule vibrates as a whole and all atoms are involved equally in vibrational excitation and not all vibrations can be seen. Generally different groups of large molecules are not excited to the same extent. Polar groups takes preference and as a result the IR spectra of large molecules show IR bands of group vibration rather than of molecular vibration. 

An ordered monolayer of molecules having a large dynamical dipole moment must not be regarded as an ensemble of individual oscillators but a strongly coupled system, the vibrational excitations being collective modes (phonons) for which the wavevector q is a good quantum number. The dispersion of the mode for CO/Cu(100) in the c(2 x 2) structure has been measured by off-specular EELS, while the infrared radiation of course only excites the q = 0 mode. 

Very large rate constants have been found for near resonant energy transfer between infrared active vibrations in CO2 Such near-resonant transitions and their dependence on temperature have also been studied for collisions between vibrationally excited CO2 and other polyatomic molecules as CH4, C2H4, SF et al. The deactivation cross-sections range from 0.28 for CH3F to 4.3 for SFs at room temperature, and decrease with increasing temperature. 

Since the CO2 laser line corresponds to a transition between two excited vibrational levels, only those CO2 molecules can be excited by absorption of the laser line which are in the (OOl)-level, populated at 300 ° K with about 1 % of the total number of molecules. In spite of this low population density, the laser-excited fluorescence method is easily achieved because of the large exciting laser intensity. 

In particular, Shapiro and others calculated state-to-state photodissociation cross sections from vibrationally excited states of HCN and DCN [58], N2O [59], and O3 [60]. Eor instance, the detailed product-vibrational state distributions and absorption spectra of HCN(DCN) were compared [58]. These results were obtained employing a half-collision approximation, where the photodissociation could be depicted as consisting of two steps, that is, absorption of the photon and the dissociation, as well as an exact numerical integration of the coupled equations. In particular, it was predicted that large isotope effects can be obtained in certain regions of the spectrum by photodissociation of vibrationally excited molecules. 

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