David Cardoza

Gaining mechanistic insight from closed loop learning control: The importance of basis in searching the phase space

This paper discusses different routes to gaining insight from closed loop learning control experiments. We focus on the role of the basis in which pulse shapes are encoded and the algorithmic search is performed. We demonstrate that a physically motivated, nonlinear basis change can reduce the dimensionality of the phase space to one or two degrees of freedom. The dependence of the control goal on the most important degrees of freedom can then be mapped out in detail, leading toward a better understanding of the control mechanism. We discuss simulations and experiments in selective molecular fragmentation using shaped ultrafast laser pulses.

Interpreting closed-loop learning control of molecular fragmentation in terms of wave-packet dynamics and enhanced molecular ionization

We interpret a molecular fragmentation experiment using shaped, ultrafast laser pulses in terms of enhanced molecular ionization during dissociation. A closed-loop learning control experiment was performed to maximize the CF3+CH3+ production ratio in the dissociative ionization of CH3COCF3. Using ab inito molecular structure calculations and quasistatic molecular ionization calculations along with data from pump-probe experiments, we identify the primary control mechanism which is quite general and should be applicable to a broad class of molecules.

Transformations to diagonal bases in closed-loop quantum learning control experiments

This paper discusses transformations between bases used in closed-loop learning control experiments. The goal is to transform to a basis in which the number of control parameters is minimized and in which the parameters act independently. We demonstrate a simple procedure for testing whether a unitary linear transformation (i.e., a rotation amongst the control variables) is sufficient to reduce the search problem to a set of globally independent variables. This concept is demonstrated with closed-loop molecular fragmentation experiments utilizing shaped, ultrafast laser pulses.

Dissociative wave packets and dynamic resonances.

The authors examine the role of dynamic resonances in laser driven molecular fragmentation. The yields of molecular fragments can undergo dramatic changes as an impulsively excited dissociative wave packet passes through a dynamic resonance. The authors compare three different kinds of dynamic resonances in a series of molecular families and highlight the possibility of characterizing the dissociative wave function as it crosses the resonance location.

Controlling molecular fragmentation: Adiabatic rapid passage as a mechanism for charge transfer

Control over CF₃ ⁺/CHBr₂ ⁺ in laser driven fragmentation of CHBr₂COCF₃, is observed. Pump-probe spectroscopy reveals a charge transfer mechanism. This mechanism may allow for a measurement of the wave function for a dissociating polyatomic molecule.

Understanding Learning Control in a Molecular Family

Experiments demonstrate coherent control over a wide variety of atomic and molecular systems using shaped ultrafast laser pulses. In many cases, where the system Hamiltonian is not known well enough to predict optimal control fields a priori, learning algorithms are used. Although learning algorithms are successful at discovering optimal laser pulses for control, it is generally difficult to understand the physical mechanisms underlying control. Although a few pioneering experiments have been able to uncover the control mechanism exploited by an optimal pulse, the lack of systematic techniques to gain insights into control mechanisms and to develop predictive models for control remains a significant barrier to achieving the ultimate goal of laser selective chemistry. This chapter uses a combination of learning algorithm modifications, pump-probe spectroscopy, ab initio molecular structure calculations, and quasi-static molecular ionization calculations to decipher the mechanism underlying control in l, l, l-trifluoroacetone. This predicts control in similar molecules such as tri-deuterated acetone and 1,1,1-trichloroacetone.

Systematic Behavior in Chemical Reactions Driven by Shaped Ultrafast Laser Pulses

We investigate learning control over molecular fragmentation in a series of similar molecules. We interpret the physical mechanism underlying control in terms of an intuitive picture that is based upon ab initio molecular structure calculations.