Mechanism and Experimental Observability of Global Switching Between Reactive and Nonreactive Coordinates at High Total Energies

We present a mechanism of global reaction coordinate switching, namely, a phenomenon in which the reaction coordinate dynamically switches to another coordinate as the total energy of the system increases. The mechanism is based on global changes in the underlying phase space geometry caused by a switching of dominant unstable modes from the original reactive mode to another nonreactive mode in systems with more than 2 degrees of freedom. We demonstrate an experimental observability to detect a reaction coordinate switching in an ionization reaction of a hydrogen atom in crossed electric and magnetic fields. For this reaction, the reaction coordinate is a coordinate along which electrons escape and its switching changes the escaping direction from the direction of the electric field to that of the magnetic field and, thus, the switching can be detected experimentally by measuring the angle-resolved momentum distribution of escaping electrons.

Figure 1

(a) The cylindrical stable and unstable manifolds (indicated by red and green, respectively) emanating from the NHIM (indicated by the blue dot), projected on the coordinate space, at the total energy $E=0.001$ above the Stark saddle. The proton is indicated by the yellow circle. (b) The stable and unstable manifolds (indicated by red and green, respectively) of the invariant plane (indicated by blue) at the total energy $E=1.45$, projected on the coordinate space.

Figure 2

(a),(b) Ratio between the normal and tangential stretching rates of the invariant plane on the Poincaré surface $x_2=0,(d{x}_2/dt)>0$, at the total energy (a) $E=0.5$ and (b) $E=1.45$. The proton is indicated by the yellow circle and the periodic orbit by the green cross in each figure. (c) Volume fraction in the invariant plane where the normal stretching rate dominates the tangential stretching rate. Those at energies $E=0.5$ and $E=1.45$ are indicated by the arrows. The standard error of the volume fraction is less than $3.16\times {10}^{-3}$ for all of the energies.

Figure 3

(a) Distributions of the $P_3$ values of the escaping electrons at the total energies $E=0.3,0.5,0.7,1.0,1.2,1.5$. (b) Distributions of the energy ratio between the energy of the $x_3$ component, $E_3$, and that of the others, $E_{12}$, of the escaping electrons at the total energies $E=0.3,0.5,0.7,1.0,1.2,1.5$. In these figures, the standard error is less than $1.73\times {10}^{-4}$ for all of the $P_3$ values and the energies.