Above Threshold Dissociation

Chu S I 1991 Complex quasivibrational energy formalism for intense-field multiphoton and above-threshold dissociation—complex-scaling Fourier-grid Hamiltonian method J. Chem. Phys. 94 7901... 

Figure 1. Morse potential V(q), vibrational levels Ev, and wave functions < (q) for the model OH (adapted from Ref. 14). The arrows indicate various selective vibrational transitions as well as above-threshold dissociations (ATDs) induced by IR femtosecond/picosecond laser pulses, as discussed in Sections III.A-III.D see Figs. 3-5. Horizontal bars on the arrows mark multiple photon energies ha of the laser pulses cf. Table 1. The resulting ATD spectrum is illustrated by the insert above threshold.

The resulting phenomena of above-threshold dissociation (ATD) resemble those of above-threshold ionization (ATI) see Ref. 34. 

M. Thachuk, M. Yu. Ivanov, and D. M. Wardlaw, /. Chem. Phys., 105, 4094 (1996). A Semiclassical Approach to Intense-Field Above-Threshold Dissociation in the Long Wavelength Limit. 

A. Giusti-Suzor, X. He, O. Atabek, F.H. Mies, Above-threshold dissociation ofH in intense laser fields, Phys. Rev. Lett. 64 (1990) 515. 

G. Jolicard, O. Atabek, Above-threshold-dissociation dynamics of with short intense laser pulses, Phys. Rev. A 46 (1992) 5845. 

Wave function methods are prohibitive for more than a few electrons in the regime of (not too high) laser intensities where the electron-electron interaction is still important. TDDFT is essentially the only practical scheme avail-able " for dealing with many electrons in these kinds of time-dependent fields. There exists a host of phenomena that TDDFT might be able to predict including high harmonic generation, multiphoton ionization, above-threshold ionization, above-threshold dissociation, etc., but these predictions will be possible only if accurate approximations are available. 

ATD above-threshold dissociation DEP differenti2d electrolytic potentiometry... 

Miyawaki J, Yamanouchi K and Tsuchiya S 1993 State-specific unimolecular reaction of NO2 just above the dissociation threshold J. Chem. Phys. 99 254-64... 

At low energies the abstraction process dominates and at higher energies the exchange mechanism becomes more important. The cross-sections for the two processes crossing at 10 eV. The END calculations yield absolute cross-sections that show the same trend as the experimentally determined relative cross-sections for the two processes. The theory predicts that a substantial fraction of the abstraction product NHjD, which are excited above the dissociation threshold for an N—H bond actually dissociates to NH2D" + H or NH3 during the almost 50-ps travel from the collision chamber to the detector, and thus affects the measured relative cross-sections of the two processes. 

Next, we discuss the J = 0 calculations of bound and pseudobound vibrational states reported elsewhere [12] for Li3 in its first-excited electronic doublet state. A total of 1944 (1675), 1787 (1732), and 2349 (2387) vibrational states of A, Ai, and E symmetries have been computed without (with) consideration of the GP effect up to the Li2(63 X)u) +Li dissociation threshold of 0.0422 eV. Figure 9 shows the energy levels that have been calculated without consideration of the GP effect up to the dissociation threshold of the lower surface, 1.0560eV, in a total of 41, 16, and 51 levels of A], A2, and E symmetries. Note that they are genuine bound states. On the other hand, the cone states above the dissociation energy of the lower surface are embedded in a continuum, and hence appear as resonances in scattering experiments or long-lived complexes in unimolecular decay experiments. They are therefore pseudobound states or resonance states if the full two-state nonadiabatic problem is considered. The lowest levels of A, A2, and E symmetries lie at —1.4282,... 

Unfortunately, as with all oversimplified theories, there are limitations for the application of the latter equation to ions close to the dissociation threshold, hi these cases, the number of degrees of freedom has to be replaced by an effective number of oscillators which is obtained by use of an arbitrary correction factor. [7] However, as long as we are dealing with ions having internal energies considerably above the dissociation threshold, i.e., where E - Eo)/E = 1, the relationship is valid and can even be simplified to give the quasi-exponential expression... 

Figure 5. Series of IR femtosecond/picosecond laser pulses for the sequence of transitions OH(u = 10)- OH(u = 15) - 0 + H for the isolated model OH cf. Fig. 1 and Table I. The notations are as in Fig. 3 populations Pwen(t) = Xoio Po( ) and PCOnt(t) = lPweiKO indicate the total populations of bound and continuum states embedded in the potential well and above the dissociation threshold, respectively. The resulting spectrum of ATD is shown in Fig. 1.

The coupling between the angle y and the dissociation coordinate R is always large if Jacobi coordinates are used. At low energies deep inside the well, this coupling is linear and normal coordinates are usually better suited for interpretation and assignment than are Jacobi coordinates. However, if the molecular dynamics above the dissociation threshold is studied, the normal-mode picture breaks down and scattering coordinates have to be employed. 

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