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.     

Polarization-transfer methods in solid-state magic-angle-spinning NMR: adiabatic CN pulse sequences.

An adiabatic double-quantum polarization-transfer experiment is described. It can be characterized as an adiabatic variant of the POST-C7 experiment. A continuous variation of the phase increment between pulses leads to the introduction of a fictitious Zeeman field that allows for an adiabatic passage through the recoupling condition. This results in a chemical-shift-offset-compensated adiabatic experiment, which leads to an efficient and broadbanded polarization transfer or to a double-quantum excitation. Similar variations of other C- or R-type experiments can be envisioned.     

Adiabatic transfer of population in a dense fluid: the role of dephasing statistics

We report the results of simulation studies of the statistics of vibrational dephasing of a YCl (Y=H, D, T, and I) diatom in dense fluid Ar at two temperatures, including the effect of strong field driving on the energy level modulation statistics. The distribution of energy level modulations is found to be non-Gaussian with a high energy tail. Aspects of stimulated Raman adiabatic passage (STIRAP) between the vibrational levels of HCl in dense fluid Ar have been investigated. For HCl with nearly degenerate v=0-->v=1 and v=1-->v=2 transitions, the combined effect of modulation and power broadening reduces the STIRAP efficiency for population transfer from v=0 to v=2 of the order of 30%. However, if the transitions used have very different frequencies, as in the original model studied by Demirplak and Rice [J. Chem. Phys. 116, 8028 (2002)], the STIRAP efficiency for population transfer remains high, of the order of 80%, even with non-Gaussian modulation of energy levels.     

Next‐Generation Heteronuclear Decoupling for High‐Field Biomolecular NMR Spectroscopy

Ultra‐high‐field NMR spectroscopy requires an increased bandwidth for heteronuclear decoupling, especially in biomolecular NMR applications. Composite pulse decoupling cannot provide sufficient bandwidth at practical power levels, and adiabatic pulse decoupling with sufficient bandwidth is compromised by sideband artifacts. A novel low‐power, broadband heteronuclear decoupling pulse is presented that generates minimal, ultra‐low sidebands. The pulse was derived using optimal control theory and represents a new generation of decoupling pulses free from the constraints of periodic and cyclic sequences. In comparison to currently available state‐of‐the‐art methods this novel pulse provides greatly improved decoupling performance that satisfies the demands of high‐field biomolecular NMR spectroscopy.     

Experimental control of excitation flow produced by delayed pulses in a ladder of molecular levels

We study a method for controlling the flow of excitation through decaying levels in a three-level ladder excitation scheme in Na(2) molecules. Like the stimulated Raman adiabatic passage (STIRAP), this method is based on the control of the evolution of adiabatic states by a suitable delayed interaction of the molecules with two radiation fields. However, unlike STIRAP, which transfers a population between two stable levels g and f via a decaying intermediate level e through the interaction of partially overlapping pulses (usually in a Lambda linkage), here the final level f is not long lived. Therefore, the population reaching level f decays to other levels during the transfer process. Thus, rather than controlling the transfer into level f, we control the flow of the population through this level. In the present implementation a laser P couples a degenerate rovibrational level in the ground electronic state X 1Sigma(g)+, v" = 0, j" = 7 to the intermediate level A 1Sigma(u)+, v' = 10, J' = 8, which in turn is linked to the final level 5 1Sigma(g)+, v = 10, J = 9 by a laser S, from which decay occurs to vibrational levels in the electronic A and X states. As in STIRAP, the maximum excitation flow through level f is observed when the P laser precedes the S laser. We study the influence of the laser parameters and discuss the consequences of the detection geometry on the measured signals. In addition to verifying the control of the flow of population through level f we present a procedure for the quantitative determination of the fraction kappa(f) of molecules initially in the ground level which is driven through the final level f. This calibration method is applicable for any stepwise excitation.     

Cross polarization from spins I=12 to spins S=1 in nuclear magnetic resonance with magic angle sample spinning

Spin locking of the nuclear magnetization of a spin with S=1 such as deuterium in the presence of a radio-frequency field under magic angle spinning (MAS) is described in terms of adiabatic modulations of the energy levels. In a brief initial period, part of the initial density operator nutates about the Hamiltonian and is dephased. The remaining spin-locked state undergoes persistent oscillatory transfer processes between various coherences with a periodicity given by the rotation of the sample. While all crystallites in the powder undergo such periodic transfer processes, the phases of the oscillations depend on the angle gamma of the crystallites. The angle gamma is the azimuthal angle defining the orientation of the unique axis of the quadrupolar interaction tensor in a rotor-fixed frame. The theory is extended to describe cross-polarization between spins S=1 and I=12 under MAS. There are four distinct Hartmann-Hahn matching conditions that correspond to four zero-quantum matching conditions, all of which are shifted and broadened compared to their spin S=12 counterparts. These matching conditions are further split into a family of sideband conditions separated by the spinning frequency. The theory allows the calculation of both shifts and broadening factors of the matching conditions, as verified by simulations and experiments.     

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.     

Selective photochemistry via adiabatic passage: degenerate product states with different lifetimes

Two-pulse selective photochemistry that exploits population transfer via adiabatic passage is considered for the case that there are degenerate product states with different lifetimes. As an example, a four-level model system with a complex symmetric Hamiltonian is constructed. Analytical and numerical studies of this model system demonstrate that extensive control over the product branching ratio can be achieved by detuning either the pump pulse or the Stokes pulse while maintaining negligible population in the intermediate state. This control approach represents a significant simplification of both the Kobrak-Rice extended stimulated Raman adiabatic passage scheme and the Chen-Shapiro-Brumer strong-field control scheme.     

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.     

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.     

Enhanced selectivity and yield in multichannel photodissociation reactions: application to CH3I

We develop a method to improve the population transfer and final-channel control of multichannel photodissociation reactions. The method is applied to the photodissociation of methyl iodide, CH3(v)+I*(2P1/2)<--CH3I-->CH3(v)+I(2P3/2). Our method is based on simultaneously exciting many two-photon pathways that lead to the same final outcome, each proceeding via a different intermediate bound state. The selectivity of the final product state(s) is a result of coherently controlled interference between the quantum pathways. The improvement in the population transfer yield from the ground state to the selected dissociative channel(s) is made possible by executing the process in an adiabatic fashion.     

Electron transfer mechanisms in surface dynamical processes

Some topics concerning on the dynamical electron transfer processes between adparticle and surface are discussed based on recent theoretical studies. They include the band effect on the electron transfer probability, the change from the diabatic to adiabatic behavior seen in the field induced desorption (FID), and the effect of the couplings with the medium degrees of freedom on the electron transfer process. It is elucidated how the competition of the energy parameters, i.e., the band width, the inverse of the scattering time and the interaction energy leads to different features of the electron transfer. Natural crossover of the FID behavior from the diabatic to the adiabatic limit is clarified by the generalized kinematic equation based on the quantum model of the electron transfer. Enhancement of the diabatic behavior by the coupling with the heat bath or sorrounding medium is concluded with the stochastic trajectory method and the time-developing operator method.     

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.     

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.     

On the limitations of adiabatic population transfer between molecular electronic states induced by intense femtosecond laser pulses

The possibility to perform a stimulated Raman adiabatic passage process in molecules on the ultrafast time scale is investigated theoretically. Motivated by recent experiments, the mid R:B<--mid R:X electronic transitions in molecular iodine are studied as a prototype example with the goal to selectively induce a population transfer employing two intense and time-delayed ultrashort laser pulses and different coupling schemes. For the purpose of interpretation, the coupled multilevel vibronic problem is reduced to a quasi-three-level system by averaging over the vibrational degree of freedom. It is shown that the vibrational dynamics becomes essential at high field intensities. Considering a 2-dimensional parameter space (intensity and delay time of the femtosecond laser pulses), a strong-field control landscape is constructed.     

Coherent population transfer: Stimulated raman adiabatic passage and the Landau-Zener picture

We show that three-state coherent excitation by pulsed lasers, sometimes termed Stimulated Raman Adiabatic Passage (STIRAP), can be regarded as a limiting case of four-state excitation involving pairs of two-state transitions (pump and Stokes) that couple coherently. A complementary limiting case of our four-state model is recognizable as a pair of Landau-Zener curve crossings. In each of these regimes it is possible to have complete population transfer by adiabatic passage. With the aid of analytical expressions and numerical computations we show the differences between these two mechanisms.     

On the possibility of an STM-induced dissociative electron transfer

The process of dissociative adsorption of a molecule on an electrode in a system of the type in situ scanning tunneling microscopy (STM) is investigated theoretically. It is shown that, in the case of fully nonadiabatic or partly adiabatic electron transfer, the presence of the tip of STM may either accelerate (or even induce) or decelerate the process of dissociation of the molecule, depending on the sign of the bias voltage. The maximum effect takes place in the case of strong interaction of the molecule with both electrodes (fully adiabatic electron transfer). In this limit, diagrams of kinetic modes, which mark off the boundaries between processes of different types possible in a given system, are constructed.Translated from Elektrokhimiya, Vol. 41, No. 3, 2005, pp. 273–284.Original Russian Text Copyright © 2005 by Kuznetsov, Medvedev.     

A generalization of the method of stimulated emission pumping

Drawing on the results of an analysis of the nature of the pulse that generates complete transfer of population from one to another level in a system with a discrete spectrum, a generalization of the method of stimulated emission pumping is proposed. It is shown that a small subset of the Fourier components of the optimal pulse will, if their relative amplitudes are the same as in the optimal pulse, generate almost as efficient a population transfer, thereby generating the opportunity to prepare a system in a selected state with a selected population.