Cartwheel mode

In contrast to the vibrational effect, the rotational effect on hydrogen dissociation on Cu is much less understood, until very recently. Most 3D quantum calculations have used the plane rotor model, which is not appropriate for studying rotational effects. The studies of Refs. 113, 114, and 117 using the spherical rotor treatment have obtained important results on the effect of rotational orientation and the nuclear symmetry. The rotational orientational effect is clearly shown in Fig. 15, where reaction probabilities for different initial rotational orientation states are plotted as a function of kinetic energy. Significant enhancement of reaction probability is seen for the state with j = m ( helicopter mode) while the m = o ( cartwheel mode) is least effective for dissociation. 

As mentioned above, the suppression of sticking in H2/Pd(100) by rotation is due to the cartwheel mode (mj = 0), whereas the helicopter ( w/ = J) mode enhances reactivity, i.e., quantum results suggest a strong steric effect. In order to verify this steric effect, the rotational quadrupole alignment parameter defined as ... 

Increased rotational energy helps to dissociate the molecule, as Figure 6.13 shows. Calculations indicate that rotational motion in a plane perpendicular to the surface (the cartwheel mode) is restricted, because it affects the interaction between the molecule and the surface. Rotation parallel to the surface (the helicopter mode ) is virtually free, however, and allows for stretching of the H-H bond as required for dissociation. 

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