Vibrational de-excitation

An important further consequence of curvature of the interaction region and a late barrier is tliat molecules that fail to dissociate can return to the gas-phase in vibrational states different from the initial, as has been observed experunentally in the H2/CU system [53, ]. To undergo vibrational (de-)excitation, the molecules must round the elbow part way, but fail to go over the barrier, eitlier because it is too high, or because the combination of vibrational and translational motions is such that the molecule moves across rather than over the barrier. Such vibrational excitation and de-excitation constrains the PES in that we require the elbow to have high curvature. Dissociation is not necessary, however, for as we have pointed out, vibrational excitation is observed in the scattering of NO from Ag(l 11) [55]. 

Experiments to measure the vibrational de-excitation of NO at a metal surface are much more challenging than those already described. One needs both a source of vibrationally-excited molecules as well as means of detecting the results of the scattering interaction, necessitating the use... 

Figure 3.44. Dissociation of 02 adsorbed on Pt(lll) by inelastic tunneling of electrons from a STM tip. (a) Schematic ID PES for chemisorbed Of dissociation and illustrating different types of excitations that can lead to dissociation, (b) Schematic picture of inelastic electron tunneling to an adsorbate-induced resonance with density of states pa inducing vibrational excitation (1) competing with non-adiabatic vibrational de-excitation that creates e-h pairs in the substrate (2). (c) Dissociation rate Rd for 0 as a function of tunneling current I at the three tip bias voltages labeled in the figure. Solid lines are fits of Rd a IN to the experiments with N = 0.8, 1.8, and 3.2 for tip biases of 0.4, 0.3, and 0.2 V, respectively and correspond to the three excitation conditions in (a). Dashed lines are results of a theoretical model incorporating the physics in (a) and (b) and a single fit parameter. From Ref. [153].

The theoretical model developed to explain these experiments is based on inelastic tunneling of electrons from the tip into the 2ir adsorbate resonance that induces vibrational excitation in a manner similar to that of the DIMET model (Figure 3.44(b)). Of course, in this case, the chemistry is induced by specific and variable energy hot electrons rather than a thermal distribution at Te. Another significant difference is that STM induced currents are low so that vibrational excitation rates are smaller than vibrational de-excitation rates via e-h pair damping. Therefore, coherent vibrational ladder climbing dominates over incoherent ladder climbing,... 

Theoretical work produces an almost embarrassing wealth of information. In addition to diffraction intensities, and the probabilities of vibrational and rotational transitions, we can obtain combinations of these, e.g. vibrational de-excitation accompanying rotational excitation. These coupled changes probe the PES very precisely in particular regions. If we consider combined rotational-vibrational changes,... 

Figure 10 A typical trajectory showing rotational excitation accompanying vibrational de-excitation (i.e. a vibration to rotational energy transfer) [71]. The top panel shows the evolution in the Z (molecule-surface distance) and r (molecular bond length) coordinates. In the lower panel, the motion is projected onto the r — 0 (molecular bond orientation) plane. Coupling of vibrations and rotations occurs because the molecule attempts to dissociate at an unfavourable bond angle.

Figure 12 The dependence of rotational excitation, vibrational de-excitation and dissociation on the initial orientation of molecules incident in the v = 1, / = 0 state on a model Cu(l 11) surface. There is a clear orientation dependence of specific processes [60],...

Perhaps the most likely immediate commercial application of carbonyl fluoride, however, arises from its spectroscopic properties. Irradiation of mixtures of COF, and H, (or D,), over a wide pressure range and at ambient temperature, with the multiline output of a continuous wave CO, laser, results in the generation of excited state HF (DF) which lases [1387]. Energy transfer from the R, line (970 cm" ) of CO, (which is close in energy to the c, band of COF,) causes the dissociation of the COF, to CO and two excited state fluorine atoms which subsequentiy react with the dihydrogen (or dideuterium). However, COF, itself has been found to effect rapid vibrational de-excitation of HF [239], an observation that suggests that the COF,/H, route to the HF laser may be of limited practicality. 

A possible smoking gun would be infrared (IR)-induced PT, e.g. the driving of the acid-base PT reaction AH + B —> A + HB+, Eq. (10.1), by the IR excitation of the AH proton vibration. In a traditional perspective, the only role of the solvent would be to thwart the IR-induced reaction by vibrational de-excitation of the AH vibration. In the nontraditional perspective however, the nonequilibrium solvent can assist the IR-induced reaction [65]. This is currently under study via detailed simulations. 

The absence of ion translational energy effects on the C8 distribution is also indicated by the lack of an applied field effect. We conclude that vibrational de-excitation is occurring and that argon is much more effective for vibrational de-excitaton than is neon. Such a large difference in vibrational deactivation efficiency between neon and argon is somewhat surprising. However, deactivation of ions may be more sensitive to polarizability effects than is deactivation of neutral molecules. 

Table III shows that the reaction selectivity of tert-C4H9+ can also be increased by lowering the liquid temperature. Lowering the temperature lowers the isobutylene vapor pressure. At 0.04 torr and above C4H8+ makes one or more gas-phase collisions with isobutylene to form vibra-tionally excited tert-C4H9 However, below 0.04 torr more and more of the C4H8+ ions are directly injected into the liquid. Reaction 1 then occurs in the liquid phase and the product tert-C4H9+ ion is vibrationally de-excited. At vapor pressures of only 0.004 torr the reaction selectivity reaches 90%. This suggests that thermal energy tert-C4H9+ ions are indeed quite selective in their reaction with isobutylene. In fact, 2,4,4-trimethylpentene-2 accounts for 84% of all the C8 products.

In most cases of vibrational de-excitation in large rigid molecules we are dealing with (i <- 0) transitions where i > > 2. As shown in section... 

Raman is another vibrational spectroscopy, complementary to IR in that it is sensitive to the excitation of bonds that are non-polar but polarizable, while IR requires a dipole moment. The specimen is illuminated by a laser and most light is scattered elastically. Some small part is shifted down in frequency (Stokes) by a bond becoming vibrationally excited, and an even smaller part is shifted up (anti-Stokes) by vibrational de-excitation. The whole spectrum allows compounds and polymers to be identified and... 

---