Rotational excitation probability

Figure 16 The natural logarithm of the rotational excitation probability as a function of the reciprocal surface temperature, i.e. an Arrhenius plot. Experimental results [79] are shown by large symbols with thick lines, theoretical results [80] obtained from classical simulations are shown by small symbols connected with faint lines.

Cowin JP, Yu CF, Sibener SJ, Wharton L (1983) HD scattering from Pt(lll) Rotational excitation probabilities. J Chem Phys 79 3537... 

Observed angular distributions were quasi-specular and scattered rotational distributions were strongly dependent upon the incidence energy, both observations indicating the direct nature of the interaction. The most important observation of the work was the approximately Arrhenius surface temperature dependence of the vibrational excitation probability, exhibiting an effective activation energy close to the vibrational excitation energy of the scattered molecule (see Fig. 2). The authors also showed that the... 

Contrary to a conclusion (315) that CN carries most of the excess energy as translational energy, Ling and Wilson (638) have found that CN radicals are produced in two different internally excited states, one probably in the <42n state (60%) and the other in the vibrationally and rotationally excited... 

The results of all of these studies are summarized in Table 2. They show that, in the work with excimer lasers, a higher fraction of the radicals are detected in the higher vibrational levels. This higher observed fraction in the upper vibrational levels probably does not occur because of any discrepancy between the different studies but is the result of the difficulty of detecting small amounts of vibrationally excited radicals in the presences of large amounts of v" = 0 radicals that have a substantial amount of rotational excitation. Fisher et al. had to make a special effort to determine the vibrationally excited radicals in the presence of rotationally excited CN(v" = 0). 

The second main difficulty concerns simultaneous rotational excitation, which does play a role, though probably a minor one, in the vibrational relaxation of hydrides. Presently, there is no satisfactory theory, and little direct experimental evidence, to show that rotational excitation is strongly coupled to vibrational relaxation. 

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 11 A comparison of theoretical scattering probabilities (solid bnes) with those of experiment (symbols) [72], The top panel shows scattering into the same quantum state as the initial state for which there is good agreement. The bottom panel shows that theory greatly overestimates the probability of scattering into a rotationally excited state.

Figure 14 Experimental demonstration of the thermal effects in H2 dissociation and scattering on Cu surfaces. A shows the Arrhenius dependence of the dissociation probability at die translational energies indicated [44]. From these curves, a translational dependent activation can be extracted, as in B [44]. The dependence is clearly linear with a slope of — 1. Scattering probabilities for rotational excitation also show and Arrhenius dependence, as shown in C. The activation energy extracted from this has a very strong state dependence.

Ethylene molecules are known to physisorb at low crystal temperature. The binding energy in this state was estimated to be 0.25 eV from isothermal desorption experiments on Ag(l 0 0) [84]. Near edge X-rays absorption fine structure showed that the admolecules occupy the fourfold hollows on Ag(l 0 0) with the axis parallel to the surface [85, 86]. The sticking probability into the physisorption well is inhibited for rotationally excited gas-phase molecules [84]. 

Vibrational and Rotational Excitation in Gaseous Collisions probability of excitation or deexcitation is determined from... 

Another mechanism responsible for the optically induced anisotropy is angular redistribution (AR) of molecules. This mechanism has been widely developed to explain photoinduced birefringence and dichroism. In most experimental cases, there is evidence of some rotation of molecules during the photoisomerization cycle (see Reference 2, for example). This rotation results in AR, because the molecules remain longer in states with lower excitation probability, and so more molecules are accumulated perpendicular to the pump polarization. The AR process is initiated by the AHB, and these two processes should be studied simultaneously in the framework of general... 

---