Rovibrationally excited molecules

Bar, I. and S. Rosenwaks (2001). Controlling bond cleavage and probing intramolecular dynamics via photodissociation of rovibrationally excited molecules. Int. Rev. Phys. Chem. 20,711. 

Figure 7. Potential energy diagram of CH2O. After excitation to specific rovibrational levels of Si, internal conversion leads to highly excited molecules in the ground electronic state So, whereas intersystem crossing populates the lowest triplet state Ti.

PHOTOINDUCED BOND CLEAVAGE AS A PROBE OF MODE SPECIFICITY AND INTRAMOLECULAR DYNAMICS IN ROVIBRATIONALLY EXCITED TRIATOMIC TO 10 ATOM MOLECULES... 

The terms mode-selective and bond-selective dissociation refer to the control of the dissociation products in VMP. The terms are usually used as synonyms although, strictly speaking, the former should refer to selective preexcitation of a vibrational mode and the latter to the resulting selective bond cleavage. Control of the dissociation products in VMP has been extensively reviewed [28-31] and our discussion will focus on molecules studied (or continued to be smdied) after the latest comprehensive review was published [31], An exception will be a short overview on the VMP of water isotopologues since it was the extensive theoretical and experimental investigations of these molecules, in particular H2O and HOD, that opened a new era of detailed smdies of state-to-state photodissociation out of specific rovibrationally excited states of polyatomic molecules. 

We have also learned that VMP is an effective tool in molecular spectroscopy and molecular dynamics studies. It is effective, in particular, for determination of IVR lifetimes and for studying the vibrational spectroscopy of states that are difficult to study applying other methods. The above-mentioned limit of the size of the molecule is irrelevant here. For observing the mode selectivity in VMP, the vibrational excitation has to survive IVR in order to retain the selectivity since the subsequent electronic excitation has to be from the excited vibrational state. In contrast, monitoring vibrational molecular dynamics relies only on the efficacy of the excitation of the specific rovibrational state. When IVR is fast and rovibrational distribution reaches equilibrium, the subsequent electronic excitation will still reflect the efficacy of the initial rovibrational excitation. In other words, whereas fast IVR precludes mode selectivity, it facilitates the unraveling of the vibrational molecular dynamics. 

The subexcitation electrons lose their energy in small portions, which are spent on excitation of rovibrational states and in elastic collisions. In polar media there is an additional channel of energy losses, namely, the dipole relaxation of the medium. The rate with which the energy is lost in all these processes is several orders of magnitude smaller than the rate of ionizaton losses (see the estimates presented in Section II), so the thermalization of subexcitation electrons is a relatively slow process and lasts up to 10 13 s or more. By that time the fast chemical reactions, which may involve the slow electrons themselves (for example, the reactions with acceptors), are already in progress in the medium. For this reason, together with ions and excited molecules, the subexcitation electrons are active particles of the primary stage of radiolysis. 

Morrison, M.A. (1986). A first-order nondegenerate adiabatic theory for calculating near-threshold cross sections for rovibrational excitation of molecules by electron impact, J. Phys. B 19, L707-L715. 

Thiimmel, H.T., Grimm-Bosbach, T., Nesbet, R.K. and Peyerimhoff, S.D. (1995). Rovibrational excitation by electron impact, in Computational Methods for Electron-Molecule Collisions, eds. W.M. Huo and F. Gianturco (Plenum,... 

The vibrational heating efficiency of LiH molecules in collisions with He atoms was the subject of further study [34], The excitation and relaxation rates over a broad range of temperatures were reported, together with the average energy transfer indices. It was found that in spite of the weak nature of the van der Waals interaction, the strong anisotropy of the surface leads to rovibrational excitation rates which are larger, for example, than those exhibited by the He-CO [35] or He-N2 [36] systems. 

The alumina flow tube is used to run at temperatures up to 1800 K since it has a higher thermal stability than the quartz one. While the furnace can reach a temperature of 1875 K, the maximum operating temperature has been limited to 1800 K because lifetime of the heating elements decreases substantially as the maximum temperature is approached. Hot alumina reacts with several of the neutrals, including NO. The industrial grade quartz tube, on the other hand, has been found to be less reactive but has an upper temperature limit of 1400 K. At present, we routinely use the quartz flow tube since most molecules of interest have appreciable levels of rovibrational excitation at 1400 K. If higher temperatures are required to elucidate the chemistry for a particular reaction system, it takes about 1-2 days to switch over to the alumina flow tube. 

For molecules and ions having more than one atom, the extra energy can make the component bonds rotate and vibrate faster (rovibrational energy). Isolated atoms, having no bonds, cannot be excited in this way. 

As excited atoms, molecules, or ions come to equilibrium with their surroundings at normal temperatures and pressures, the extra energy is dissipated to the surroundings. This dissipation causes the particles to slow as translational energy is lost, to rotate and vibrate more slowly as rovibrational energy is lost, and to emit light or x-rays as electronic energy is lost. 

Molecules vibrate at fundamental frequencies that are usually in the mid-infrared. Some overtone and combination transitions occur at shorter wavelengths. Because infrared photons have enough energy to excite rotational motions also, the ir spectmm of a gas consists of rovibrational bands in which each vibrational transition is accompanied by numerous simultaneous rotational transitions. In condensed phases the rotational stmcture is suppressed, but the vibrational frequencies remain highly specific, and information on the molecular environment can often be deduced from hnewidths, frequency shifts, and additional spectral stmcture owing to phonon (thermal acoustic mode) and lattice effects. 

Radiometry. Radiometry is the measurement of radiant electromagnetic energy (17,18,134), considered herein to be the direct detection and spectroscopic analysis of ambient thermal emission, as distinguished from techniques in which the sample is actively probed. At any temperature above absolute zero, some molecules are in thermally populated excited levels, and transitions from these to the ground state radiate energy at characteristic frequencies. Erom Wien s displacement law, T = 2898 //m-K, the emission maximum at 300 K is near 10 fim in the mid-ir. This radiation occurs at just the energies of molecular rovibrational transitions, so thermal emission carries much the same information as an ir absorption spectmm. Detection of the emissions of remote thermal sources is the ultimate passive and noninvasive technique, requiring not even an optical probe of the sampled volume. 

It is not possible to discuss highly excited states of molecules without reference to the recent progress in nonlinear dynamics.2 Indeed, the stimulation is mutual. Rovibrational spectra of polyatomic molecules provides both an ideal testing ground for the recent ideas on the manifestation of chaos in Hamiltonian systems and in turn provides many challenges for the theory. 

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