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. DOI: 10.1103/PhysRevLett.115.093003 PACS numbers: 32.80.Rm, 05.45.Ac, 45.20.Jj, 82.20.-w In the conventional picture of chemical reactions, a system starts from a reactant and ends up with a product by overcoming a first-rank saddle in between the two. If the total energy of a system is close enough to the potential energy of the saddle, a harmonic approximation of the potential energy surface at the saddle is valid during the time interval in which the system traverses through the saddle. Then, the direction toward which the reaction occurs is determined by the unstable mode at the saddle. As the total energy increases, the nonlinearity of the potential energy starts to play an important role even in the vicinity of the saddle, and the nonlinear couplings between the unstable mode and the other modes become no longer negligible.At the total energy in which the harmonic terms dominate the nonlinear terms, one can still take them into account in a perturbative manner [1][2][3][4][5] so that the resulting perturbed unstable mode is decoupled from the other perturbed modes and the perturbed unstable mode is hyperbolic. A key geometrical object behind the perturbed modes is a normally hyperbolic invariant manifold (NHIM) [6,7], defined as a zero level set of the perturbed unstable mode and its conjugate momentum on an equienergy surface [5]. The NHIM has a pair of stable and unstable normal directions that are linear combinations of this perturbed unstable mode and its conjugate momentum. There, the stable and unstable manifolds emanating in the normal directions determine the directions toward which the reaction occurs. Specifically, if the NHIM has a topology of a hypersphere, these manifolds are cylindrical and, thus, are called a cylindrical stable manifold and a cylindrical unstable manifold, respectively. In that case, every reactive trajectory should run through the cylindrical stable manifold and then run through the cylindrical unstable manifold [5,[8][9][10][11][12].If the total energy increases even further, the perturbative construction of the NHIM starts to fai...