Pump-dump control

Figure Al.6.31. Multiple pathway interference interpretation of pump-dump control. Since each of the pair of pulses contains many frequency components, there are an infinite number of combination frequencies which lead to the same fmal energy state, which generally interfere. The time delay between the pump and...

Figure 39. Pump-dump control of NaK molecule by using two quadratically chirped pulses. The initial state taken as the ground vibrational eigenstate of the ground state X is excited by a quadratically chirped pulse to the excited state A. This excited wavepacket is dumped at the outer turning point at t 230 fs by the second quadratically chirped pulse. The laser parameters used are = 2.75(1.972) X 10-2 eVfs- 1.441(1.031) eV, and / = 0.15(0.10)TWcm-2 for the first (second) pulse. The two pulses are centered at t = 14.5 fs and t2 = 235.8 fs, respectively. Both of them have a temporal width i = 20 fs. (See color insert.) Taken from Ref. [37].

Figure 40. Pump-dump control of NaK by using two quadraticaUy chirped pulses. The initial state and the first step of pump are the same as in Fig. 39. The excited wave packet is now dumped at R 6.5cio on the way to the outer turning point. The parameters of the second pulse are a ) = 1.929 X 10 eVfs , = 1.224eV, and I = 0.lOTWcm . The second pulse is centered at...

FIGURE 13 A pump-dump control scheme, used to control the branching ratio of the dissociation of a triatomic molecule, ABC. 

An intuitive method for controlling the motion of a wave packet is to use a pair of pump-probe laser pulses, as shown in Fig. 13. This method is called the pump-dump control scenario, in which the probe is a controlling pulse that is used to create a desired product of a chemical reaction. The controlling pulse is applied to the system just at the time when the wave packet on the excited state potential energy surface has propagated to the position of the desired reaction product on the ground state surface. In this scenario the control parameter is the delay time r. This type of control scheme is sometimes referred to as the Tannor-Rice model. 

In order to demonstrate the efficiency and robustness of the formulations presented in Sect. 5.2 and 5.3, three practical applications are considered here. In the first part of this section, we consider the complete excitation of a wavepacket from a nonequilibrium displaced position, which is directly related to the idea of bond-selective breaking, as explained in the Introduction . This is demonstrated numerically by taking diatomic molecules LiH and NaK as examples. In the second part, we consider the complete pump-dump control and creation of a localized wavepacket using quadratic chirping within the pump-dump mechanism. The bond-selective photodissociation of the H20 molecule is discussed in the third part of this subsection as an example of a multidimensional system. 

Fig. 5.7. Pump-dump control of NaK molecule using two quadratically chirped pulses. The initial state is taken as the ground vibrational eigenfunction of the ground state X1S+ and this is excited by a quadratically chirped pulse to the excited state A1E+. The excited wavepacket is dumped at the outer turning point t cs 230 fs by the second quadratically chirped pulse. The laser parameters used are...

Their approach, which founded the fundamentals of pump-dump control and of optimal control, is the subject of Chapter 4. 

COUNTERING PARTIALLY COHERENT LASER EFFECTS IN PUMP-DUMP CONTROL... 

To examine a particular case, consider the pump—dump control discussed in Section 3.5 and Chapter 4. Here a sequence of two pulses first excite a molecule to a set of intermediate bound states with wave func and energy and then to dissociate these states. We denote the two (k = 1,2) by... 

Further, as discussed in Section 3.1, the inability to control the product ratio by shaping the pulse can be overcome by photodissociating not just one EX) bound . state but a superposition of several bound states )) (as was done, e.g., with bichro-, matic control). Such a superposition state can be created separately by an initial preparation pulse, as in the case of pump-dump control scenario Sections 3.5 and. 4.1). Alternatively, the superposition state can be created by the photolysis pulse itself (by, e.g., a stimulated Raman process), provided that the bandwidth of the -pulse is comparable to the energy spacings between the Ef) levels. r, ... 

Encouraged by the confirmation of the control concept, two-parameter control was considered in order to manipulate different processes in dimers and diatomic molecules. In addition to the pump-probe time delay, the second control parameter involved the pump [72, 73] or probe [66, 67] wavelength, the pump-dump delay [69, 74, 75], the laser power [121], the chirp [68, 76], or the temporal width [70] of the laser pulse. Optimal pump-dump control of K2 has been carried out theoretically in order to maximize the population of certain vibrational levels of the ground electronic state using one excited state as an intermediate pathway [71, 292-294]. The maximization of the ionization yield in mixed alkali dimers has been performed first experimentally using closed-loop learning control [77,78, 83] (CLL) and then theoretically in the framework of optimal control theory (OCT) [84]. 

The analytic form of the pump and dump pulses in the optimal phase-unlocked pump-dump control is />( >)( )= p( >)(f) exp (—/cOe f)- -exp megt), where Tipp) is a slowly varying envelope of the helds, and (s>eg is the energy difference between the minima of the excited and the ground states. The objective in the ground state is represented in the Wigner formulation by an operator A = A(r) g)(g. A(r) is the Wigner transform of the objective in the phase space T = of coordinates and momenta, and... 

Using the strategy for optimal pump-dump control based on the intermediate target, we have shown that the isomerization pathway through the conical intersection can be suppressed and that optimized pulses can drive the isomerization process to the desired objective (isomer 11). This means that the complex systems are amenable to control, provided that the intermediate target exists. Furthermore, the analysis of the MD and of the tailored pulses allows for the identification of the mechanism responsible for the selection of appropriate vibronic modes necessary for the optimal control. 

In Sect. 6.3, we first provide the pulse-design scheme to induce and control 7T-electron rotation in a chiral aromatic molecule. Next, on the basis of dynamical simulations in a semiempirical model, we demonstrate that the initial direction of 7T-electron rotation depends on the spatial configuration of each enantiomer with respect to the polarization direction of a linearly polarized laser pulse and then 7T electrons continue to rotate clockwise and counterclockwise (or counterclockwise and clockwise) in turn. Moreover, a pump-dump control scheme to prevent the switching of the rotation direction and realize a consecutive unidirectional JT-electron rotation is presented. 

Unidirectional Rotation by the Pump-Dump Control Method... 

The numerical results shown in Fig. 6.2 confirm that the rotation direction of n electrons temporally changes between clockwise and counterclockwise in the case of a single-pulse control. Switching of the rotation direction can be prevented efficiently, and unidirectional rotation of Jt electrons can be realized consecutively in a simple manner. In the three-level model analysis in a short-pulse limit, as already stated, the pulse with e+ (e ) creates a coherent superposition L) - - H) ( L) — H)), and L) - - H) created by a pump pulse with e+ evolves as L) - - H) L) — i H). Then the population in —) can be dumped to G) by applying a dump pulse with e just after the created state has completely shifted as L) — i H) L) — H). Thus, only clockwise rotation can be generated. Figure 6.3 shows the results of a pump-dump control simulation of an R enantiomer of DCPH. The values of the parameters of the pump pulse were/ = 2.24 GVm i, = 19.4 fs, co = 7.72 eVM, and e = e+, and those of the dump pulse were / = 2.37 GVm , tj = 19.4 fs, co = 7.72 eV/h, and e = e. The delay time between the pulses was 19.4 fs. 

The pump-dump control concept [17,18] has been realized in different experiments in the gas and the condensed phase (see review [71] and references therein, [30]). The concept includes three successive steps. First step excitation of the system from the ground state (reactant) to an excited state with a femtosecond pump pulse short enough to create a wavepacket in the excited state. Second step field free evolution of the system. Third step interaction with a second pulse to dump the vibrational wavepacket to the target state/region on the electronic ground state. 

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