Moshe Shapiro

Laser control of unimolecular decay yields in the presence of collisions

We show that coherent light can be used to control the relative product yield in unimolecular photodissociation, even in the presence of collisions. That is, coherence contributions to the photodissociation, which are responsible for control over the reaction, are sufficiently robust to survive inhomogeneous broadening effects over a substantial temperature range. A number of experimental implementations for control in the presence of collisions are proposed.

Theory of enantiomeric control in dimethylallene using achiral light

Extensive control over enantiomer populations using achiral light is computationally demonstrated for J, MJ-selected 1,3 dimethylallene. In particular, by altering the detuning of one of three lasers incident on an J, MJ-polarized racemic mixture, one can alter the enantiomeric excess from ≈93% of the L enantiomer to ≈93% of the D enantiomer.

Interference control without laser coherence: Molecular photodissociation

Control over channel‐specific line shapes and branching ratios in photodissociation is shown to be achievable by irradiating a molecule with two intense cw lasers whose relative phase need not be well defined. Control results from quantum interference between nonlinear pathways induced by the intense fields, within which the relative laser phase cancels. The interference, and hence the product yields, can be manipulated by changing the relative frequencies and intensities of the two lasers. In this paper this theory of high field control is developed, and computations on the photodissociation of Na2 are presented. Control over product yields is shown to be extensive, even with inclusion of rotational states. For example, the branching ratio between the Na(3s)+Na(3p) and Na(3s)+Na(4s) products can change by as much as a factor of 10 as the frequencies are tuned.

Coherent control of the CH2Br+I←CH2BrI→CH2I+Br branching photodissociation reaction

Coherent control over branching in the photodissociation of collinear CH2BrI to yield either CH2Br+I or CH2I+Br is examined computationally. Quantum photodissociation calculations, using two excited potentials surfaces, are carried out using a new method incorporating negative imaginary absorbing potentials within the artificial channel method. Extensive control over the I/Br branching ratio is shown to result as experimentally controllable laser amplitudes and phases are varied. Such control is observed for excitation from either initial superpositions of chaotic or regular CH2BrI bound states.

Coherent control of resonance-mediated reactions: F+HD.

Cross sections resulting from scattering that proceeds via an intermediate resonance are shown to be exceptionally controllable using a coherent superposition of only two initial states. Full quantum computations on F+HD(v=0;j=0,1)-->H+DF, D+HF, which exhibits a resonance in one of the reactive channels, support the formal arguments, showing that control is indeed vast. In this case the ratio of reactive integral cross sections can be altered by a factor of 62 (compared to a noncoherent factor of only 3.3), while the ratio of reactive differential cross sections can be altered by a factor of over 6000 (compared to a noncoherent factor of less than 7). These results constitute the first prediction of extensive quantum control in a collisional process.

Interference control of photodissociation branching ratios. Two-color frequency tuning of intense laser fields

Abstract Control over product probabilities and channel specific line shapes in molecular photodissociation is shown to result from quantum interference effects which can be manipulated by varying the frequencies of two intense laser fields. The laser fields, whose relative phase need not be well defined, have frequencies centered around two transitions: one between the continuum and an initially populated state and the second between the continuum and an initially unpopulated molecular bound state. Computations on Na2 photodissociation show that control over product yields is extensive, with the branching ratio changing by a factor of ten as the frequencies are tuned over a convenient range.

Two-pulse coherent control of electronic branching in Li2 photodissociation

Control over the product branching ratio in the photodissociation of Li2 into Li(2s)+Li(2p) and Li(2s)+Li(3p) channels is explored computationally using the pump–pump coherent control scenario. Extensive control over the Li(3p)/Li(2p) branching ratio is demonstrated as the delay time between the two pulses is varied. The pulse width dependence is examined and better control is found to result from a narrow pump pulse which excites a superposition of only two levels, followed by a broad dissociation pulse.

Theory of “laser distillation” of enantiomers: Purification of a racemic mixture of randomly oriented dimethylallene in a collisional environment

Enantiomeric control of 1,3 dimethylallene in a collisional environment is examined. Specifically, our previous laser distillation scenario wherein three perpendicular linearly polarized light fields are applied to excite a set of vib-rotational eigenstates of a randomly oriented sample is considered. The addition of internal conversion, dissociation, decoherence, and collisional relaxation mimics experimental conditions and molecular decay processes. Of greatest relevance is internal conversion which, in the case of dimethylallene, is followed by molecular dissociation. For various rates of internal conversion, enantiomeric control is maintained in this scenario by a delicate balance between collisional relaxation of excited dimethylallene that enhances control and collisional dephasing, which diminishes control.

PUMP-DUMP COHERENT CONTROL WITH PARTIALLY COHERENT LASER PULSES

The theory of coherent control of photodissociation with partially coherent laser pulses is developed and applied to the pump–dump control scenario of a collinear model of DH2. The coherence characteristics of the pump pulse are shown to be crucial for maintaining control over the product yield, whereas the coherence properties of the dump pulse are only of secondary importance. Control is shown to survive for partially coherent laser pulses, but only for a range of incoherence which precludes control with typical nanosecond pulsed dye lasers.

Theory of resonant two‐photon dissociation of Na2

The quantum scattering theory of resonant two‐photon (ω1+ω2) dissociation is developed and applied to Na2 photodissociation. In the energy range considered, photodissociation primarily occurs via excitation to the Au20091Σu state, intersystem crossing to the bu20093Πu state, and subsequent excitation to the triplet continuum. Photodissociation rates to produce Na(3s)+Na(3d), Na(3s)+Na(4s), and Na(3s)+Na(3p) are reported as a function of both ω1 and ω2. Characteristic features due to spin–orbit coupling and to multiple product production are observed and discussed.