4.4 Basic Features of Atom–Diatom Reactions
147
These considerations are useful when dealing with multidimensional systems and
long lasting events to the end of estimating the error made by truncating the time
integration on the accuracy of the computed reactive probability.
4.4.5 The Last Mile to the Experiment
The reactions already mentioned in this chapter
(a) N + N 2 → N 2 + N
(b) H + H 2 → H 2 + H
(c) H + Cl 2 → HCl + H
(d) Li + FH → LiF + H
are typical prototypes of atom–diatom reactive systems exhibiting in a different
fashion-related distinctive features of related experimental data.
In particular, Reaction (a) is a typical heavy heavy-heavy (HHH) atom symmetric
system with a dominant collinear minimum energy path. We have already seen, in
the previous subsections, that the midpoint location of the barrier to reaction of this
system leads to an even efficacy of translation and vibrational energy in promoting
reactivity. At the same time, the isoenergicity of the system once past the threshold
allows a non-negligible back reflection at certain energies that, however, due to the
heavy masses involved do not show up as sharp spikes. They rather show up as shallow
undulations which are completely smoothed in the highly averaged structure of the
rate coefficient when plotted as a function of temperature (as shown in Fig. 4.18).
As to Reaction (b) (that is a typical light light-light (LLL) atom symmetric system
also showing a dominant collinear minimum energy path), the light masses involved
lead to a fine structure of the probability when plotted as a function of energy (as
shown in Figs. 4.12 and 4.13). This is made possible by the frequent bouncing back
and forth of the intermediate particle within the small skewed angle of the reaction
channel for these systems.
Completely different is the behavior of Reaction (c) that is a typical light heavyheavy (LHH) asymmetric exoergic system with a dominant collinear minimum
energy path and an early barrier. The early location of the barrier enables collision energy to play a dominant role in promoting reaction. This agrees with the fact
that a large excess of translational energy may cause the incoming particle to be
reflected back so fast (short interaction time) as from a hard repulsive wall.
147
These considerations are useful when dealing with multidimensional systems and
long lasting events to the end of estimating the error made by truncating the time
integration on the accuracy of the computed reactive probability.
4.4.5 The Last Mile to the Experiment
The reactions already mentioned in this chapter
(a) N + N 2 → N 2 + N
(b) H + H 2 → H 2 + H
(c) H + Cl 2 → HCl + H
(d) Li + FH → LiF + H
are typical prototypes of atom–diatom reactive systems exhibiting in a different
fashion-related distinctive features of related experimental data.
In particular, Reaction (a) is a typical heavy heavy-heavy (HHH) atom symmetric
system with a dominant collinear minimum energy path. We have already seen, in
the previous subsections, that the midpoint location of the barrier to reaction of this
system leads to an even efficacy of translation and vibrational energy in promoting
reactivity. At the same time, the isoenergicity of the system once past the threshold
allows a non-negligible back reflection at certain energies that, however, due to the
heavy masses involved do not show up as sharp spikes. They rather show up as shallow
undulations which are completely smoothed in the highly averaged structure of the
rate coefficient when plotted as a function of temperature (as shown in Fig. 4.18).
As to Reaction (b) (that is a typical light light-light (LLL) atom symmetric system
also showing a dominant collinear minimum energy path), the light masses involved
lead to a fine structure of the probability when plotted as a function of energy (as
shown in Figs. 4.12 and 4.13). This is made possible by the frequent bouncing back
and forth of the intermediate particle within the small skewed angle of the reaction
channel for these systems.
Completely different is the behavior of Reaction (c) that is a typical light heavyheavy (LHH) asymmetric exoergic system with a dominant collinear minimum
energy path and an early barrier. The early location of the barrier enables collision energy to play a dominant role in promoting reaction. This agrees with the fact
that a large excess of translational energy may cause the incoming particle to be
reflected back so fast (short interaction time) as from a hard repulsive wall.