Photodissociation wavepacket dynamics

Figure 3, Wavepacket dynamics of the photodissociation of NOCl, shown as snapshots of the density (wavepacket amplitude squared) at various times, The coordinates, in au, are described in Figure b, and the wavepacket is initially the ground-state vibronic wave function vertically excited onto the 5i state. Increasing corresponds to chlorine dissociation. The density has been integrated over the angular coordinate. The 5i PES is ploted for the geometry, 9 = 127, the ground-state equilibrium value,...

Several approaches have been used to calculate the effect of isotopic substitution on the absorption cross section of N2O. The zero point energy (ZPE) model [83] as described in Section 4, 2D and 3D wavepacket dynamics [110,118], a semi-empirical model [85,86] and an extended reflection principle model [84] have been used to explain the differences in the absorption that stem from isotopic substitution. In the following the emphasis lies on the enhanced understanding of the isotopic differences in the photodissociation reactions, which arises from employing wavepacket propagation calculations for the various isotopologues. 

Heller [1. 2. 3] introduced and popularised wavepacket dynamics in the context of the theory of nioleculm iihotodissociation. In a photodissociation process, the molecule starts in a. well defined initial state and ends up in a firiiil scattering state. The initial bound state vibrational-rotatioiuil wavefunction provides a natural initial wavepacket in this case. 

Gray, S.K. (1992) Wavepacket dynamics of resonance decay An iterative equation approach with application to HCO —> H + CO, J. Chem. Phys. 96, 6543-6554. Guo, H. (1993) Timc-dcpcndcnt quantum dynamical study of the photodissociation of hypoclilorous acid, J. Phys. Chem. 97, 2602-2608. 

We next study how the results of wavepacket dynamics can be tracked experimentally as a real-time history of chemical or physical events. Femtosecond time-resolved spectroscopy enables us to observe nuclear dynamics and to chart the path of chemical reactions in real time, and has been exploited in numerous applications ranging from fundamental studies of real-time motion in the photodissociation of diatomic molecules to stud-... 

Guo, H. (1991). A three-dimensional wavepacket study on photodissociation dynamics of methyl iodide, Chem. Phys. Lett. 187, 360-366. 

First, we present the dynamics of the initial wavepacket a. Initially the system stands at the equilibrium position of the electronic ground X. The temporal evolution of the wavepacket Pe generated in the electronic excited state is shown in the left-hand column of Fig. 5.9. Apparently, tp originates in the Frank-Condon (FC) region, which is located at the steep inner wall of the electronically excited A state. The repulsive force of the potential l 0 the drives e(t) downhill toward the saddle point and then up the potential ridge, where Pe(t) bifurcates into two asymptotic valleys, with Ye = 0.495 in channel f. The excitation achieved using this simple quadratically chirped pulse is not naturally bond-selective because of the symmetry of the system. The role played by our quadratically chirped pulse is similar to that of the ordinary photodissociation process, except that it can cause near-complete excitation (see Table 5.1 for the efficiency). This is not very exciting, however, because we would like to break the bond selectively. 

The HCl and DCl spectra calculated with the reflection principle model have too low intensities in comparison to the wavepacket results, which are for the same conditions. The differences in the results demonstrate that the dynamics of the system play an important role even for direct photodissociation reactions like HCl and DCl. 

In both cases, la and lb, the total photodissociation cross section is completely determined by the short-time dynamics in the Franck-Condon region. In contrast, the partial cross sections, which determine the vibrational, rotational, and electronic-state distributions of the products, involves longer time dynamics. To obtain all of the relevant information about the reaction, the wavepacket evolution must be followed out into the product region of the potential energy surface and projected onto the various different vibrational and rotational states of the fragments. The partial cross section for scattering... 

Basic questions are analyzed, as is the case for the photochemistry of formaldehyde. Contrary to previous results, direct quantum dynamics simulations showed that the H2 + CO H + HCO branching ratio in the Si/Sq nonadiabatic photodissociation of formaldehyde is controlled by the direction and size of the mean momentum of the wavepacket when it crosses the seam of conical intersection. In practice, if the wavepacket falls down from the barrier to the conical intersection with no initial momentum the system leads to H2 + CO, while an extra momentum toward products favors... 

Time-resolved ionization offers several advantages as a probe of these wavepackets [41, 42, 343, 360]. For example, the ground state of an ion is often better characterized than higher excited states of the neutral molecule, particularly for polyatomics. Ionization is also universal and hence there are no dark states. Furthermore, ionization provides both ions and photoelectrons and, while ion detection provides mass and kinetic-energy resolution in time-resolved studies [508], photoelectron spectra can provide complementary information on the evolution of the wavepacket [22, 63, 78, 132, 201, 270, 271, 362, 363, 377]. Its utihty for real-time probing of molecular dynamics in the femtosecond regime has been nicely demonstrated in studies of wavepackets on excited states of Na2 [22], on the B state of I2 [132], and on the A state of Nal [201]. Femtosecond photoelectron-photoion coincidence imaging studies of photodissociation dynamics have been reported [107]. 

Since the first experiments on the I-CN [21] bond cleavage and the wavepacket oscillations between the ionic and covalent potentials in the photodissociation of Nal [22, 23], pump-probe techniques have been applied to a wide range of important photochemical processes. However, the data obtained Ifom such experiments are often difficult to interpret and theoretical modeling is needed to get further insight into the excited state dynamics of the systems of interest at the atomistic level. In this context, the development of efficient and accurate computational methods for the description of ground and excited electronic states of mid-size molecular systems in a balanced way [24, 25], has greatly facilitated the theoretical study of photochemical processes. 

Takayanagi T, Shiga M. (2003) Photodissociation of CI2 in helium clusters an application of hybrid method of quantum wavepacket d5uiamics and path integral centroid molecular dynamics. Chem. Phys. Lett. 372 90-96. 

The theoretical model for photodetachment is similar to that used to describe photodissociation outlined in the last section. As illustrated in Fig. 3.7, the initial wave packet on the neutral PES was chosen as the ground vibrational state of cis-HOCO, which has a lower energy than its tram counterpart. The anion vibrational eigenfunction was determined on a newly developed anion PES at the same CCSD(T)-F12/AVTZ level [130], as used to construct the neutral PES [100, 101]. The neutral wave packet was propagated to yield probabilities to both the HO-I-CO and H-I-CO2 asymptotes with a flux method [108] and the cosine Fourier transform of the Chebyshev autocorrelation function yielded the energy spectrum [44]. The discretization of the Hamiltonian and wavepacket, and the propagation were essentially the same as in our recent reaction dynamics study [107]. 

Balint-Kurti, G. G. (2008). Time-dependent and time-independent wavepacket approaches to reactive scattering and photodissociation dynamics. International Reviews in Physical... 

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