Potential energy surface time-dependent probabilities

The vibrationally excited states of H2-OH have enough energy to decay either to H2 and OH or to cross the barrier to reaction. Time-dependent experiments have been carried out to monitor the non-reactive decay (to H2 + OH), which occurs on a timescale of microseconds for H2-OH but nanoseconds for D2-OH [52, 58]. Analogous experiments have also been carried out for complexes in which the H2 vibration is excited [59]. The reactive decay products have not yet been detected, but it is probably only a matter of time. Even if it proves impossible for H2-OH, there are plenty of other pre-reactive complexes that can be produced. There is little doubt that the spectroscopy of such species will be a rich source of infonnation on reactive potential energy surfaces in the fairly near future. 

Since DFT calculations are in principle only applicable for the electronic ground state, they cannot be used in order to describe electronic excitations. Still it is possible to treat electronic exciations from first principles by either using quantum chemistry methods [114] or time-dependent density-functional theory (TDDFT) [115,116], First attempts have been done in order to calculate the chemicurrent created by an atom incident on a metal surface based on time-dependent density functional theory [117, 118]. In this approach, three independent steps are preformed. First, a conventional Kohn-Sham DFT calculation is performed in order to evaluate the ground state potential energy surface. Then, the resulting Kohn-Sham states are used in the framework of time-dependent DFT in order to obtain a position dependent friction coefficient. Finally, this friction coefficient is used in a forced oscillator model in which the probability density of electron-hole pair excitations caused by the classical motion of the incident atom is estimated. 

Because of the very large well in the potential energy surface, very few accurate cpianturn dynamical results were available five years ago, either by time-dependent or by time-independent methods. Indeed, a. very large number of channels or of grid points are necessary to converge reaction probabilities and cross sections. 

Figure A3.13.il. Illustration of the time evolution of reduced two-dimensional probability densities I I and I I for the excitation of CHD between 50 and 70 fs (see [154] for further details). The full curve is a cut of the potential energy surface at the momentary absorbed energy corresponding to 3000 cm during the entire time interval shown here ( 6000 cm , if zero point energy is included). The dashed curves show the energy uncertainty of the time-dependent wave packet, approximately 500 cm . Left-hand side excitation along the x-axis (see figure A3.13.5). The vertical axis in the two-dimensional contour line representations is...

Fig. 1.1. Principles of the real-time multiphoton ionization (MPI) (a) and NeNePo (b) spectroscopic technique, (a) Principle of time-resolved MPI spectroscopy. A wave packet is prepared in an excited state of the neutral system by a pump pulse. Since in general the transition probability to the ion state is a function of the wave packet s location on the potential-energy surface, the propagation of the wave packet can be probed by a second, time-delayed pulse, (b) Principle of the time-resolved NeNePo process. Starting in the anion s potential-energy surface, an ultrashort pump pulse detaches an electron cuid prepares a wave packet in the neutrcd. After a certain delay time At a probe pulse photoionizes the neutral. The time-dependent signal of the cation s intensity is detected. For convenience, this method is called NeNePo , Negative-to-Neutrcd-to-Positive...

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