The rotational and vibrational state distributions of the H2 product from the reactions of translationally excited H atoms with HCl, HBr, and HI at 1.6 eV are probed by coherent anti-Stokes Raman scattering spectroscopy after only one collision of the fast H atom. Despite the high collision energy, only the very exoergic (ΔH=−1.4 eV) hydrogen atom abstraction involving HI leads to appreciable H2 product vibrational excitation. For this reaction the H2 vibrational distribution is strongly inverted and peaks in v′=1, with 25% of the total available energy partitioned to vibration. For the mildy exoergic (ΔH=−0.72 eV) reaction with HBr and the nearly thermoneutral (ΔH=−0.05 eV) reaction with HCl, very little energy appears in H2 vibration, 9% and 2%, respectively, and the vibrational state distributions peak at v′=0. However, in all three reactions a significant fraction, 18% to 21%, of the total energy available appears as H2 rotation. All three reactions show a strong propensity to conserve the translational energy, that is the translational energy of the H2+X products is very nearly the same as that of the H+HX reactants. For the reactions with HCl, HBr, and HI the average translational energy of the products is 1.3, 1.7, and 1.7 eV, respectively, and the width of the translational energy distribution is only about 0.5 eV full width at half maximum. The energy disposal in all three reactions is quite specific, despite the fact that this high collision energy is well above the barrier to reaction in all three systems and a large number of product quantum states are energetically accessible. Only a few of these energetically allowed final states are appreciably populated. Although detailed theoretical calculations will be required to account completely for the state specifity, quite simple models of the reaction dynamics can explain much of the dynamical bias that we observe.

We report measurement of the HD product quantum state distributions and absolute cross section for the H+CD4→HD(v′,J′)+CD3 reaction at a collision energy of 1.5 eV. The total reaction cross section is small, 0.14±0.03 Å2, making the experimental measurements difficult. The HD quantum state distribution peaks at low J′ in both v′=0 and v′=1, the only vibrational states in which product is observed. Very little of the 1.5 eV available energy appears as internal excitation of the HD product molecule, 7% in HD product vibration and 9% in rotation. However, linear surprisal analysis shows that this limited internal energy disposal in the HD product in some ways exceeds that expected statistically, since two of the best-fit surprisal parameters (Θr=2.9±0.6 for v′=0, Θr=−1.9±0.5 for v′=1, λv=−2.2±0.6 ) are negative. The HD rovibrational state distribution shows an anomalous positive correlation of product vibrational and rotational excitation. Those molecules formed in the vibrationally excited state, v′=1, have significantly more rotational energy (〈Erot〉=0.17 eV) than those molecules formed in the vibrational ground state, v′=0 (〈Erot〉=0.13 eV). This behavior runs counter to the otherwise universal behavior for direct bimolecular reactions—a negative correlation of product vibrational and rotational excitation. We speculate as to the source of this anomalous energy disposal.

The friction of a clean diamond tip on diamond (111) and (100) surfaces is studied using an ultrahigh vacuum force microscope that simultaneously measures forces parallel and perpendicular to the surface. The 30 nm radius diamond tip is fabricated by chemical vapor deposition. The attractive normal force curve between the tip and surface agrees well with calculated dispersion interactions. The frictional force exhibits periodic features, which on the (100) surface are tentatively associated with a 2×1 reconstructed surface convoluted over an asymmetric tip shape. The (111) surface shows features that cannot be simply related to the surface structure. As the tip is scanned back and forth along a line, the same features are observed in each direction, but offset, suggesting the presence of a conservative force independent of the direction of motion as well as a nonconservative force. The friction is approximately ≂3×10−9 N independent of loads up to 1×10−7 N.

Articles you may be interested inPhotogeneration of ions via delocalized charge transfer states. II. HX2 − (X=Cl,Br,I) in rare gas matrices J. Chem. Phys. 98, 3914 (1993); 10.1063/1.464018 Statetostate dynamics of H+HX collisions. I. The H+HX→H2+X (X=Cl,Br,I) abstraction reactions at 1.6 eV collision energy J. Chem. Phys. 90, 4795 (1989); 10.1063/1.456574Experimental study of the dynamics of D+H2 reactive and inelastic collisions below 1.0 eV relative energyWe report measurement of product state distributions for the rotationally and/or vibrationally excited HX formed in collisions oftranslationally hot H atoms with HX (X = CI, Br, and I) at 1.6 eV collision energy. The product state distributions are probed after only one collision of the fast H atom, using coherent anti-Stokes Raman scattering spectroscopy. Whether proceeding by inelastic collisions or reactive exchange, the transfer of translational energy to vibrational and rotational energy is quite inefficient in H + HX collisions at 1.6 eV. For all three hydrogen halides only 2-3% ofthe initial translational energy appears as HX vibration.For H + HC} only 6% ofthe initial energy is converted to HCI rotational energy, while for H + HBr and H + HI, this percentage is twice as large, 11-12%, but still small. The indistinguishability of the two H atoms involved makes it impossible to distinguish reactive exchange from inelastic energy transfer in these H + HX collisions. However, the difference in rotational energy partitioning for H + HBr and H + HI as compared with H + HCI, suggests that reactive exchange is dominant in the former and inelastic energy transfer dominates in the latter. The total cross sections for the combined energy transfer/reactive exchange do not change much with the identity of X, being 13 ± 3, 11 ± 2, and 11 ± 2 ft.?, for H + HCI, H + HBr, and H + HI, respectively.

The rotational and vibrational quantum state distributions for the H2 products of the H+HR→H2+R reactions (HR=C2H6 and C3H8 ) at 1.6 eV collision energy have been measured using coherent anti-Stokes Raman scattering. Total reaction cross sections have also been determined. For the total cross sections we find 1.5±0.5 Å2 for the ethane reaction and 2.9±0.8 Å2 for the propane reaction. Although several vibrational states are energetically accessible, we observe H2 products only in v′=0 and v′=1, with the majority in the ground vibrational state. The H2 products are on average rotationally cold as well, and 20% or less of the total energy is partitioned to H2 internal energy. However, the quantum state distributions show a positive correlation of H2 product rotational and vibrational energy. That is, the average rotational energy of the H2 in v′=1 is substantially greater than the average rotational energy of the H2 in v′=0. Comparison with state-to-state dynamics results previously obtained for the kinematically and energetically similar H+CD4→HD+CD3 and H+HCl→H2+Cl reactions seems to indicate that this anomalous energy disposal is an intrinsic characteristic of H + alkane hydrogen atom abstraction reactions at high collision energy. We speculate that this anomalous behavior is the result of inelastic encounters between the nascent H2 and alkyl radical products in the reaction exit channel.