Selective photodissociation in diatomic molecules by dynamical Stark-shift control

Selective population transfer in electronic states of dissociative molecular systems is illustrated by adopting a control scheme based on Stark-chirped rapid adiabatic passage (SCRAP). In contrast to the discrete N-level system, dynamical Stark shift is induced in a more complex manner in the molecular electronic states. Wavepacket dynamics on the light-induced potentials, which are determined by the detuning of the pump pulse, can be controlled by additional Stark pulse in the SCRAP scheme. Complete population transfer can be achieved by either lowering the energy barrier along the adiabatic passage or placing the initial wavepacket on a well-defined dressed state suitable for the control. The determination of the pulse sequence is sufficient for controlling population transfer to the target state.     

Complete quantum control of the population transfer branching ratio between two degenerate target states

A five-level four-pulse phase-sensitive extended stimulated Raman adiabatic passage scheme is proposed to realize complete control of the population transfer branching ratio between two degenerate target states. The control is achieved via a three-node null eigenstate that can be correlated with an arbitrary superposition of the target states. Our results suggest that complete suppression of the yield of one of two degenerate product states, and therefore absolute selectivity in photochemistry, is achievable and predictable, even without studying the properties of the unwanted product state beforehand.     

Measurement-assisted coherent control

Two advantageous roles of the influence of measurement on a system subject to coherent control are exposed using a five-level model system. In particular, a continuous measurement of the population in a branch state in the Kobrak-Rice extended stimulated Raman adiabatic passage scheme is shown to provide a powerful means for controlling the population transfer branching ratio between two degenerate target states. It is demonstrated that a measurement with a large strength may be used to completely shut off the yield of one target state and that the same measurement with a weak strength can dramatically enhance the robustness of the controlled branching ratio against dephasing.     

Ultrafast control of the internuclear distance with parabolic chirped pulses

Recently, control over the bond length of a diatomic molecule with the use of parabolic chirped pulses was predicted on the basis of numerical calculations [Chang; et al. Phys. Rev. A 2010, 82, 063414]. To achieve the required bond elongation, a laser scheme was proposed that implies population inversion and vibrational trapping in a dissociative state. In this work we identify two regimes where the scheme works, called the strong and the weak adiabatic regimes. We define appropriate parameters to identify the thresholds where the different regimes operate. The strong adiabatic regime is characterized by a quasi-static process that requires longer pulses. The molecule is stabilized at a bond distance and at a time directly controlled by the pulse in a time-symmetrical way. In this work we analyze the degree of control over the period and elongation of the bond as a function of the pulse bandwidth. The weak adiabatic regime implies dynamic deformation of the bond, which allows for larger bond stretch and the use of shorter pulses. The dynamics is anharmonic and not time-symmetrical and the final state is a wave packet in the ground potential. We show how the vibrational energy of the wave packet can be controlled by changing the pulse duration.     

Selective excitation of LI2 by chirped laser pulses with all possible interstate radiative couplings

We have numerically explored the feasibility and the mechanism of population transfer to the excited E (1)Σ(g) electronic state of Li(2) from the v=0 level of the ground electronic state X (1)Σ(g) using the A (1)Σ(u) state as an intermediate. In this system, the use of transform limited pulses with a frequency difference greater than the maximum Rabi frequency does not produce population transfer when all possible radiative couplings are taken into account. We have employed two synchronous pulses far detuned from the allowed transition frequencies, mainly with the lower frequency pulse positively chirped, and both pulses coupling the successive pair of states, X-A and A-E. The adiabaticity of the process has been investigated by a generalized Floquet calculation in the basis of 12 field dressed molecular states, and the results have been compared with those obtained from the full solution of time dependent Schro?dinger equation. The conventional representation of the process in terms of three (or four) adiabatic potentials is not valid. It has been found that for cases of almost complete population transfer in full calculations with the conservation of the vibrational quantum number, adiabatic passage is attained with the 12 state Floquet model but not with the six state model. The agreement between the full calculations and the 12 state Floquet calculations is generally good when the transfer is adiabatic. Another characteristic feature of this work is the gaining of control over the vibrational state preparation in the final electronic state by careful tuning of the laser parameters as well as the chirp rate sign. This causes time dependent changes in the adiabatic potentials and nonadiabatic transfers can be made to occur between them.     

Assisted adiabatic passage revisited

We report the results of studies of various aspects of the counter diabatic field paradigm, a recipe that generates adiabatic population transfer between levels by assisting a given field so that the total field generates the population transfer that the adiabatic approximation suggests for the given field. The sensitivity of this recipe to the pulse parameters and to Gaussian stochastic phase fluctuations between the fields has been investigated. Ladder-climbing population transfer between the vibrational levels of a nonrotating Morse oscillator is examined, and a numerical demonstration of the efficiency of the scheme is given.     

Steering population flow in coherently driven lossy quantum ladders

We present a detailed theory of a technique for the adiabatic control of the population flow through a preselected decaying excited level in a three-level ladder quantum system, as was experimentally demonstrated recently by Garcia-Fernandez et al. [Phys. Rev. Lett. 95, 043001 (2005)]. Specifically, we consider a three-state excitation chain of bound states, 1-2-3, of successively increasing excitation energy, in which probability loss via fluorescence occurs from states 2 and 3. We describe a laser excitation scheme that can, by adjustment of laser parameters, alter at will the relative fraction of population that, starting from state 1, is ultimately lost through states 2 and 3. We present analytical results for the conditions under which quasiadiabatic passage can take place.     

Coherent population transfer in molecules coupled with a dissipative environment by intense ultrashort chirped pulse. II. A simple model

We have developed a simple and physically clear picture of adiabatic rapid passage (ARP) in molecules in solution by careful examination of all the conditions needed for ARP. The relaxation effects were considered in the framework of the Landau-Zener model for random crossing of levels. The model enables us to include into consideration non-Markovian Gaussian-correlated noise. It explains all the numerical results obtained in the first paper of the series [B. D. Fainberg and V. A. Gorbunov, J. Chem. Phys. 117, 7222 (2002)], in particular, that for positive chirp pulse excitation relaxation favors more efficient population transfer with respect to the relaxation-free system with frozen nuclear motion. We also relate parameters of non-Markovian Gaussian-correlated noise with irreversible dephasing time of an optical transition by calculating the photon echo signal attenuation.     

Adiabatic passage in a three-state system with non-Markovian relaxation: the role of excited-state absorption and two-exciton processes

The influence of excited-state absorption (ESA) and two-exciton processes on a coherent population transfer with intense ultrashort chirped pulses in molecular systems in solution has been studied. A unified treatment of adiabatic rapid passage (ARP) in such systems has been developed using a three-state electronic system with relaxation treated as a diffusion on electronic potential energy surfaces. We have shown that ESA has a profound effect on coherent population transfer in large molecules that necessitates a more accurate interpretation of experimental data. A simple and physically clear model for ARP in molecules with three electronic states in solution has been developed by extending the Landau-Zener calculations putting in a third level to random crossing of levels. A method for quantum control of two-exciton states in molecular complexes has been proposed.     

A theoretical study on quantum control of photodissociation dynamics by ultrashort chirped laser pulses

By a wavepacket propagation, we demonstrate the possibility of controlling the photodissociation branching ratio between two fragment channels by a single ultrashort linearly chirped laser pulse. It is found that a negatively chirped pulse of a moderate chirp rate completely prohibits the production of one of the photofragment channels. Two characteristics of chirped laser pulses contribute to this remarkable effect: the mechanism of adiabatic rapid passage (ARP) for the population transfer between the ground and excited states and the intrapulse pump‐dump process for determining the branching ratio. ©1999 John Wiley & Sons, Inc. Int J Quant Chem 72: 525–532, 1999     

Adiabatic passage by light-induced potentials in polyatomic molecules

In this paper we study the first application of adiabatic passage by light-induced potentials in polyatomic molecules. We analyze the effects of increasing the dimensionality of the system on the adiabatic requirements of the method and the role of intramolecular coupling among the vibrational modes. By using a model of two-dimensional displaced harmonic oscillators with or without rotation of the normal mode axis of the excited states (Duschinsky effect) we find that (1) it is possible to selectively transfer the vibrational population by adiabatic elongation of the bonds, (2) the adiabatic demands depend mainly on the energy barrier between the ground and excited electronic configurations, and not on the dimension of the system, (3) in the presence of intramolecular couplings the selective transfer can be achieved but at the cost of increasing the duration and/or the intensity of the pulses, which are needed to overcome small avoided crossings, and (4) the problem of selectivity becomes more important as the vibrational energy of the initial wave function increases.     

Driving the Bloch vector of a single molecule: towards a triggered single photon source

We propose the method of rapid adiabatic passage to prepare a single molecule in its fluorescing excited state. Spontaneous emission from this state gives rise to a single photon. Since the adiabatic passage can be performed on command, the molecule can be used as a triggered single photon source. Preliminary experiments and quantum Monte-Carlo simulations demonstrate the feasibility of this scheme.     

Theory of laser enhancement of ultracold reactions: the fermion-boson population transfer by adiabatic passage of 6Li+6Li7Li(Tr=1 mK)-->6Li6Li+7Li(Tp=1 mK)

We present a new theory of population transfer by adiabatic passage. This theory relates laser catalysis to adiabatic passage, enhancing chemical reactions with the freedom to choose the translational energies of the reactants and products separately. The process, A+BC<-->(Planck's over omega(p) )ABC*(v)<-->(Planck's over omega(s))AB+C, involves two laser fields that are slowly varying so the process is adiabatic, and sufficiently intense so the population of the intermediate bound complex (ABC) is minimized. We apply this theory to the collinear exchange reaction (6)Li+(7)Li(2)(T(r))<-->(Planck's over omega(p))((6)Li(7)Li(7)Li)*<-->(variant Planck's over 2piomega(s) ) (6)Li(7)Li(T(p))+(7)Li. We show that at translational energies T(p)=T(r)=1 mK with a narrow energy bandwidth of delta(E)=0.01 mK, we can obtain nearly total (> or =98%) population transfer from the reactant to the product states. This can be done with a pump laser and a Stokes laser in an "intuitive" sequence (t(p)相似文献   

Efficient Preparation of D2 Molecules in v=2 by Stimulated Raman Pumping

We report the preparation of D₂ molecules in v=2 level in molecular beam condition. A single longitudinal mode laser system was used for excitation of D₂ from (v=0, j=0) to (v=2, j=0) with the scheme of stimulated Raman pumping. An excitation efficiency of 25.2% has been achieved, which was determined by the scheme of resonance-enhanced multiphoton ionization. Dependence of relative excitation efficiency on laser energy has been measured. We found that the increasing rate of excitation efficiency became slower as pulse energy of Stokes laser increase, while the excitation efficiency still increases approximately linearly with pump pulse energies up to 60 mJ. The spectral line shapes of Raman transition was also measured at different laser energies and considerable dynamical Stark effect was observed. A single peak was found on the three dimension surface of relative excitation efficiency, indicating the process occurred in the present study is a process of stimulated Raman pumping instead of stimulated adiabatic Raman passage.     

Stationary molecular wave packets at nonequilibrium nuclear configurations

We study different schemes that allow laser controlled adiabatic manipulation of the bond in diatomic molecules by using sequences of nonresonant time-delayed chirped pulses. The schemes rely on adiabatic passage of the vibrational wave packet by laser-induced potential shaping from the ground electronic state to a laser-stabilized dissociative electronic state by two-photon absorption. The degree of control that is possible over the position (bond length) and width (bond spread) of the vibrational wave packet is compared for the different schemes. The dynamics is analyzed detailing the role of the different control knobs and the conditions that allow or break the adiabatic passage.     

Propagation of complex shaped ultrafast pulses in highly optically dense samples

We examine the propagation of shaped (amplitude- and frequency-modulated) ultrafast laser pulses through optically dense rubidium vapor. Pulse reshaping, stimulated emission dynamics, and residual electronic excitation all strongly depend on the laser pulse shape. For example, frequency swept pulses, which produce adiabatic passage in the optically thin limit (independent of the sign of the frequency sweep), behave unexpectedly in optically dense samples. Paraxial Maxwell optical Bloch equations can model our ultrafast pulse propagation results well and provide insight.     

耗散对受激拉曼绝热转移过程的影响

应用最近发展的量子耗散理论,研究了耗散对简单三能级体系的受激拉曼绝热转移过程的影响,并与pump-dump过程比较.计算结果表明,受激拉曼绝热转移的机制能很好地抑制中间态的弛豫与涨落的影响.数值结果也表明了新的量子耗散理论可以正确地描述场与耗散相互耦合的动力学问题.     

Analysis of adiabatic passage by light-induced potentials with chirped laser pulses in three- and four-level diatomic systems

This paper describes an investigation into the process of adiabatic passage by light-induced potentials (APLIP), which was previously suggested as a method for employing two strong picosecond laser pulses to transfer the population between two electronic states. We have extended earlier numerical studies in order to assess the feasibility of an experimental implementation of the APLIP concept. APLIP has been modeled in a three-level model system based on Na2 with chirped pulses, using laser parameters available from a typical chirped pulse amplified Ti:sapphire laser. The model showed that the APLIP process remains essentially unchanged for chirped pulses of equal magnitude and the opposite, or equal and positive sign of chirp as compared to the transform-limited case. We also examined the case of additional electronic states by introduction of a fourth state that lies close to the "target," i.e., final, state. The investigation showed that there are circumstances in which a significant fraction of the population gets transferred to this state which will disrupt the APLIP process. However, by switching to this fourth state as the target state in an experiment, good transfer efficiency is recovered. The results of the extension of the original APLIP modeling to chirped pulses and additional electronic states indicate that an APLIP experimental realization should be feasible in Na2.     

Efficient Coherent Population Transfer of D2 Molecules by Stark-induced Adiabatic Raman Passage

Preparation of a high flux of hydrogen molecules in a specific vibrationally excited state is the major prerequisite and challenge in scattering experiments that use vibrationally excited hydrogen molecules as the target. The widely used scheme of stimulated Raman pumping suffers from coherent population return which severely limits the excitation efficiency. Re-cently we successfully transferred D₂ molecules in the molecular beam from (v=0, J=0) to (v=1, J=0) level, with the scheme of Stark-induced adiabatic Raman passage. As high as 75% of the excitation efficiency was achieved. This excitation technique promise to be a unique tool for crossed beam and beam-surface scattering experiments which aim to reveal the role of vibrational excitation of hydrogen molecules in the chemical reaction.     

Theory of the two step enantiomeric purification of 1,3 dimethylallene

An application of a recently proposed [P. Kral et al., Phys. Rev. Lett. 90, 033001 (2003)] two step optical control scenario to the purification of a racemic mixture of 1,3 dimethylallene is presented. Both steps combine adiabatic and diabatic passage phenomena. In the first step, three laser pulses of mutually perpendicular linear polarizations, applied in a "cyclic adiabatic passage" scheme, are shown to be able to distinguish between the L and D enantiomers due to their difference in matter-radiation phase. In the second step, which immediately follows the first, a sequence of pulses is used to convert one enantiomer to its mirror-imaged form. This scenario, which only negligibly populates the first excited electronic state, proves extremely useful for systems such as dimethylallene, which can suffer losses from dissociation and internal conversion upon electronic excitation. We computationally observe conversion of a racemic mixture of dimethylallene to a sample containing approximately 95% of the enantiomer of choice.