Two‐pulse laser control of bond‐selective fragmentation

Enhancing the branching ratios in the dissociation channels for O(16)O(16)O(18) molecule by designing optimum laser pulses: A study using stochastic optimization

We propose a strategy of using a stochastic optimization technique, namely, simulated annealing to design optimum laser pulses (both IR and UV) to achieve greater fluxes along the two dissociating channels (O(18) + O(16)O(16) and O(16) + O(16)O(18)) in O(16)O(16)O(18) molecule. We show that the integrated fluxes obtained along the targeted dissociating channel is larger with the optimized pulse than with the unoptimized one. The flux ratios are also more impressive with the optimized pulse than with the unoptimized one. We also look at the evolution contours of the wavefunctions along the two channels with time after the actions of both the IR and UV pulses and compare the profiles for unoptimized (initial) and optimized fields for better understanding the results that we achieve. We also report the pulse parameters obtained as well as the final shapes they take.

Coherent Control of Photofragment Distributions Using Laser Phase Modulation in the Weak-Field Limit

The possibility of quantum interference control of the final state distributions of photodissociation fragments by means of pure phase modulation of the pump laser pulse in the weak-field regime is demonstrated theoretically for the first time. The specific application involves realistic wave packet calculations of the transient vibrational populations of the Br2(B, v(f)) fragment produced upon predissociation of the Ne-Br2(B) complex, which is excited to a superposition of resonance states using pulses with different linear chirps. Transient phase effects on the fragment populations are found to persist for long times (about 200 ps) after the pulse is over due to interference between overlapping resonances in Ne-Br2(B).

Pulse-train control of photofragmentation at constant field energy

We consider a phaselocked two-pulse sequence applied to photofragmentation in the weak-field limit. The two pulses are not overlapping in time, i.e., the energy of the pulse-train is constant for all time delays. It is shown that the relative yield of excited Br (*) in the nonadiabatic process: I + Br* ← IBr → I + Br, changes as a function of time delay when the two excited wave packets interfere. The underlying mechanisms are analyzed and the change in the branching ratio as a function of time delay is only a reflection of a changing frequency distribution of the pulse train; the branching ratio does not depend on the detailed pulse shape.

Biomolecular structure manipulation using tailored electromagnetic radiation: a proof of concept on a simplified model of the active site of bacterial DNA topoisomerase

We report on the viability of breaking selected bonds in biological systems using tailored electromagnetic radiation.

Quantum Simulations for Isotope Effects of IR + UV Laser Pulses on Symmetry and Selective Hydrogen Bond Breaking

Intense few-cycle femtosecond (fs) infrared (IR) laser pulses yield dynamical symmetry breaking of oriented strong symmetric hydrogen bonds A···H···A due to nearly coherent vibrational excitation. Consequently, the system evolves with alternate stretches or compressions of the competing bonds A···H and H···A. For a specific stretch of, say, the H···A bond, an ultrashort ultraviolet (UV) laser pulse may induce a nearly instantaneous electronic excitation and/or photo-detachment by means of a Franck–Condon(FC)-type transition to a dissociative potential energy surface of the excited state. The stretched bond, H····A, will then dissociate selectively, yielding preferably the products AH+A. A minor fraction of the other products A+HA may also be formed due to competing wavepacket dispersion. The corresponding isotope effects are investigated by means of the representative laser driven molecular wavepackets which are propagated on ab initio potential energy surfaces and with ab initio dipole coupling for the model systems, FHF− versus FDF−. The resulting quantum dynamics for both systems are nearly equivalent if the driving IR laser fields are scaled with decreasing carrier frequency and amplitudes and increasing durations for the corresponding increasing masses of the isotopomers.

Laser control of reactions of photoswitching functional molecules

Laser control schemes of reactions of photoswitching functional molecules are proposed based on the quantum mechanical wave-packet dynamics and the design of laser parameters. The appropriately designed quadratically chirped laser pulses can achieve nearly complete transitions of wave packet among electronic states. The laser parameters can be optimized by using the Zhu-Nakamura theory of nonadiabatic transition. This method is effective not only for the initial photoexcitation process but also for the pump and dump scheme in the middle of the overall photoswitching process. The effects of momentum of the wave packet crossing a conical intersection on the branching ratio of products have also been clarified. These control schemes mentioned above are successfully applied to the cyclohexadiene/hexatriene photoisomerization (ring-opening) process which is the reaction center of practical photoswitching molecules such as diarylethenes. The overall efficiency of the ring opening can be appreciably increased by using the appropriately designed laser pulses compared to that of the natural photoisomerization without any control schemes.

Laser control of electronic transitions of wave packet by using quadratically chirped pulses

An effective scheme is proposed for the laser control of wave packet dynamics. It is demonstrated that by using specially designed quadratically chirped pulses, fast and nearly complete excitation of wave packet can be achieved without significant distortion of its shape. The parameters of the laser pulse can be estimated analytically from the Zhu-Nakamura theory of nonadiabatic transition. If the wave packet is not too narrow or not too broad, then the scheme is expected to be utilizable for multidimensional systems. The scheme is applicable to various processes such as simple electronic excitation, pump-dump, and selective bond breaking, and it is actually numerically demonstrated to work well by taking diatomic and triatomic molecules (LiH, NaK, H(2)O) as examples.

Quantum model simulations of symmetry breaking and control of bond selective dissociation of FHF− using IR+UV laser pulses

Symmetry breaking and control of bond selective dissociation can be achieved by means of ultrashort few-cycle-infrared (IR) and ultraviolet (UV) laser pulses. The mechanism is demonstrated for the oriented model system, FHF-, by nuclear wave packets which are propagated on two-dimensional potential energy surfaces calculated at the QCISD/d-aug-cc-pVTZ level of theory. The IR laser pulse is optimized to drive the wave packet coherently along alternate bonds. Next, a well-timed ultrashort UV laser pulse excites the wave packet, via photodetachment of the negative bihalide anion, to the bond selective domain of the neutral surface close to the transition state. The excited wave packet is then biased to evolve along the pre-excited bond toward the target product channel, rather than bifurcating in equal amounts. Comparison of the vibrational frequencies obtained within our model with harmonic and experimental frequencies indicates substantial anharmonicities and mode couplings which impose restrictions on the mechanism in the domain of ultrashort laser fields. Extended applications of the method to randomly oriented or to asymmetric systems XHY- are also discussed, implying the control of product directionality and competing bond-breaking.