Closing the Loop on Bond Selective Chemistry Using Tailored Strong Field Laser Pulses

Abstract

Strong field, closed-loop control of gas-phase photochemical reactivity is the focus of this article. The control of chemical reactivity is now possible using tailored laser pulses to circumvent previous laser bandwidth limitations. As an illustration of this capability, ketone rearrangements and dissociation reactions are considered. To introduce the experiments we discuss both optimal control theory (OCT) and optimal control experiments (OCE) with an emphasis on closed-loop control methods using near-infrared fs pulses. Because the experiments are in the strong field regime, we present the current state of the understanding of the electronic and nuclear photophysical processes that occur when polyatomic molecules are subjected to laser intensities ranging between 10 13 and 10 15 W cm -2 . Photoelectron spectroscopy measurements are presented that begin to elucidate the control mechanisms. These delineate the order of the multiphoton process, the presence of transient shifting of excited electronic state energies (on the order of 5 eV), and the phenomena of lifetime broadening of electronic states. Recent experiments probing the energy partitioning to nuclear modes are presented with an emphasis on detecting the final kinetic energy of fragment ions. The advances in laser pulse shaping technology slaved to pattern recognition learning algorithms have opened up the prospect of studying the dynamics and chemical manipulation of virtually any system that can be introduced into the closed-loop apparatus. Rather than operating under the limitation of finding the molecule to suit the laser capabilities, the closed-loop learning control procedure operating in the strong field regime now makes it possible to merely tailor the control laser to meet the molecule's dynamical capabilities in keeping with the chemical objectives. The prospects are very bright for exploring chemical reactivity with these tools.

Searching for quantum optimal controls under severe constraints

“…Applications of quantum control in the laboratory have grown dramatically over the past fifteen years [1][2][3][4][5][6][7][8][9]. Successful optimal control experiments (OCEs) have included selective control of molecular vibrational [10][11][12][13][14][15][16][17] and electronic states [18][19][20][21][22][23][24][25][26][27], preservation of quantum coherence [28,29], control of photoisomerization reactions [30][31][32][33][34][35], selective manipulation of chemical bonds [36][37][38][39][40][41][42][43][44], high-harmonic generation and coherent manipulation of the resulting soft X-rays [45][46][47][48]…”

Strong field quantum control by selective population of dressed states

“…An alternative approach based on the use of the combination of pulse shaping techniques [12] with adaptive feedback learning loops (closed loop) was suggested [13] for the case when the underlying potential surfaces are unknown. Implementations of this technique demonstrate the optimization of almost any conceivable physical quantity [14][15][16][17][18][19][20][21][22][23][24][25][26]. However, it is not clear whether this methodology is suitable to extract the underlying physical mechanism from the electrical fields obtained during the optimization process.…”

Control Goal Selection through Anticorrelation Analysis in the Detection Space

“…2 The laser-molecule interaction results in energy deposition into all internal degrees of freedom (rotational, vibrational, and electronic excitation), resulting in ionization and photodissociation. Recent reports demonstrate that different laser pulse shapes lead to different molecular fragmentation distributions.…”

“…Recent reports demonstrate that different laser pulse shapes lead to different molecular fragmentation distributions. 1,2 The ability to manipulate mass spectral fragmentation patterns using shaped laser pulses establishes the potential for the selective excitation, photodissociation, and ionization of molecules.…”

“…From a practical standpoint, the strong-field laser pulse shapes that control various fragment ratios can substantially facilitate the analytical identification of a molecule in a complex mixture, as may be observed, for example, in toxic chemical detection systems. Selecting controllable product states is relatively easy when the dissociation channels of the sample species are few, as is the case for a reasonably small molecule such as CH 2 BrCl, which has only a few prominent peaks in its mass spectrum. 3 The task of selecting controllable product states becomes more challenging as the number of fragmentation channels increases.…”