Dynamics of recoil calculated for dissociating HI with the H-atom recoiling (with an energy of 3.4 eV in (a)-(c) and 6 eV in (d)) toward the LiF (001) surface below. Panels (a) and (b) show the marked effect on the recoil dynamics of a small increase in the downward tilt of the I-H axis. Panels (c) and (d) stress a different factor; the effect of the recoil energy of the H-atom at a constant tilt angle to the surface; the surface has a different corrugation in contours of low potential-energy (experienced by the 3.4 eV H-atom in panel (c)) as compared with high potential-energy (experienced by the 6 eV H-atom in panel (d)). Reprinted with permission from V. J. Barclay et al., Faraday Discuss. 96, 129 (1993). Copyright © 1993 The Royal Society of Chemistry.
(a) Linear and bent configurations for a pair of HBr molecules physisorbed on LiF (001). (b) Reaction probability ( + BrH′ → HBr + H′) vs collision energy for the linear and bent configurations with low impact-parameter, compared with gas-reaction averaged over all impact parameters. Reprinted with permission from E. B. D. Bourdon et al., Faraday Discuss. Chem. Soc. 82, 343 (1986). Copyright © 1986 The Royal Society of Chemistry.
(a) Trajectory for the abstraction reaction T + HR = TH + R (T = 3 amu, H = 1 amu, R = 15 amu) at a collision energy of 2 eV (Ref. 32). (b) Trajectory of reaction A + BC = AB + C in which A = K, B = C = Br; the reaction is therefore K + Br2 = KBr + Br.31 (c) Trajectory of the same reaction as above but with potential so constructed as to allow the atom A to “migrate” in mid-trajectory from B to C. Reprinted with permission from P. J. Kuntz et al., J. Chem. Phys. 50, 4623 (1969). Copyright © 1969 American Institute of Physics.
(a) An example of a migratory trajectory36 in the reaction H + ICl → HCl + I, for which much experimental information exists on the internal excitation of the HCl, obtained from its infrared chemiluminescence.3,35 At step 36 in the trajectory the H-atom rather than being repelled by Cl of ICl is able to insert between Cl and I, and thereby start its approach to form highly rotationally and vibrationally excited HCl via “migration” from I to Cl. The experiments also show differently excited HCl (low rotation and vibration) due to “direct” reaction with the Cl-end of ICl.36 (b) “Force versus time” plot indicating that there can be a long-lived complex for this simple Cl-atom exchange reaction, when it occurs by way of migration from the I-end of the molecule. Reprinted with permission from J. C. Polanyi et al., Faraday Discuss. Chem. Soc. 67, 66 (1979). Copyright © 1979 The Royal Society of Chemistry.
The translational energy of the H-atoms ejected from the aligned surface reaction of Na + BrH = NaBr + H taking place in a Na..BrH complex at a LiF(001) is plotted along the vertical axis in eV of translational energy. The “structure” in the translational recoil corresponds to the size of the vibrational quanta left-behind in the NaBr complex, remaining on the surface. The vibrational quantum numbers for the vibrationally hot NaBr (inferred from the peaks in the H-atom translation) are given in the abscissa of the figure. Reprinted with permission from S. Dobrin et al., J. Chem. Phys. 119, 9795 (2003). Copyright © 2003 American Institute of Physics.
Hot-atom induced recoil of (ad) + CO (ad) = CO2(g) in SAR, as envisaged on the basis of an experiments at a Pt(111) surface. Reprinted with permission from C. Akerlund et al., J. Chem. Phys. 104, 7359 (1996). Copyright © 1996 American Institute of Physics.
Directed recoil of an -atom toward CO(ad), the atomic oxygen being formed photolytically from aligned O2(ad) at a Pt step-edge as pictured in Ref. 50. Reprinted with permission from C. E. Tripa and J. T. Yates, Jr., Nature (London) 398, 591 (1999). Copyright © 1999 Macmillan Publishers Ltd.
The figure exemplifies SAR of (ad) + HI(ad) = HCl(g) + I(ad) in a 3.3 Å impact parameter collision. The two-state (ground- and anionic-state) model described in the text is applied to the case of recoiling in the plane of the copper surface away from planar physisorbed p-dichlorobenzene rotated at +5º clockwise, away from the  axis of Cu(110), toward physisorbed HI(ad), The HI(ad) was computed by VASP (with van der Waals attraction to an five-layer copper slab) and was found to direct its HI axis initially at 54º away from the plane of the copper surface. The axis linking I or Cl to H is indicated by a short black line, to make the alignment evident. The first 80 fs were spent on the anionic excited state, subsequent times on the ground state shown at the indicated time-lapses, starting from t0.
The central picture (panel (b)) gives a map of the successive locations of I. H and Cl for the reactive event shown in Fig. 8, mapped onto the surface plane. The time-intervals in the figure between the indicated points are first from t0 to t1 (time spent in the anionic excited state) 80 fs, then at 50 fs intervals following t1. After 80 fs in the anionic state the translational energy is 1.32 eV. The three SAR encounters shown in panels (a)-(c) of the figure were obtained by setting the Cl..Cl axis of the planar p-dichlorobenzene (pDCB) molecule, which can readily be rotated in the surface plane (with <20 meV energy-change), at −15º, +5º, and +10º to the  axis of the Cu(110) (positive rotation is clockwise). The corresponding impact parameters for the SAR trajectories are shown in each panel of Fig. 9. Panel (a): At the lowest impact parameter 0.9 Å collides with HI to internally excite it within the period indicated by the dotted oval, thereby dissociating the HI (the H can be seen to leave behind the I). Panel (b): At the intermediate impact parameter vibrating-rotating HCl persists without termination, ultimately leaving the surface as HCl(g) for the gas-phase. The inset in panel (b) shows the H-end of vibrationally excited HCl colliding with the surface without reaction, at about position 4 (t4 = 230 fs in the previous figure). Panel (c): At the higher impact parameter of 3.9 Å the passes by the HI, picking up the H-atom to form a short-lived vibrationally excited HCl (within the dotted circle) that, when it encounters the surface at its H-end, with H–Cl undergoing a vibrational stretch, reacts to liberate H which, traverses the surface along the prior H–Cl axis.
(a) Top view at the left, and side view at right, for (at 0 fs) computed geometry of chemisorbed chlorophenyl, ClPh(ad), which has Cl tilted upward away from the surface. Adjacent is physisorbed HI(ad), for which computation shows H tilted up. By 150 fs H-atom transfer is in progress. By 500 fs HCl has been released, without further interaction with the surface. (b) The corresponding 2D trajectories of Cl, H, I atoms (point #1 is at 0 fs, point #2 is at 70 fs, further points are at 50 fs intervals).
Schematic proposal for transition state spectroscopy by femtosecond laser adsorption during SAR of + H2S = H2 + HS.61 Lingering of the trajectory at a turning point on the lower potential-energy surface is shown as leading to enhanced absorption at time t2. Reprinted with permission from J. C. Polyani and A. H. Zewail, Acc. Chem. Res. 28, 119 (1995). Copyright © 1995 American Chemical Society.
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