Control of wave packets in Li(2) by shaping the pump and probe pulses for a state-selected pump-probe analysis of the ionization continuum

Wave packet signals in Li(2) prepared by shaped pump pulses are also detected with state-selected shaped probe pulses in the ionization continuum. The results show that the final states are discrete Rydberg states instead of continuum states. Final autoionizing states in the continuum are observed and characterized. By selecting specific resonant rovibrational electronic transitions from the superposition states prepared in the wave packets to the final autoionizing states with the pulse shaping system, the modulation depths of the wave packet signals are increased by as much as 5.20+/-0.03 times. Control of the wave packets is also realized by shaping the probe pulses to select specific resonant transitions between the states in the wave packets and the highly excited Rydberg states. The detected amplitude ratio of one specific vibrational quantum beat to one specific rotational quantum beat can be decreased by ten times.     

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.     

Implementation of quantum gate operations in molecules with weak laser fields

We numerically propose a way to perform quantum computations by combining an ensemble of molecular states and weak laser pulses. A logical input state is expressed as a superposition state (a wave packet) of molecular states, which is initially prepared by a designed femtosecond laser pulse. The free propagation of the wave packet for a specified time interval leads to the specified change in the relative phases among the molecular basis states, which corresponds to a computational result. The computational results are retrieved by means of quantum interferometry. Numerical tests are implemented in the vibrational states of the B state of I2 employing controlled-NOT gate, and 2 and 3 qubits Fourier transforms. All the steps involved in the computational scheme, i.e., the initial preparation, gate operation, and detection steps, are achieved with extremely high precision.     

Coherent ultrafast photoelectron spectroscopy in Br2: an alternative method for measuring Delta v not equal 0 transitions in Rydberg-to-Rydberg excitations

A two-state vibrational wave packet is prepared in a low-lying 4d[12](1 or 2) Rydberg state of jet cooled Br(2) (4d, v(')=3 and v(')=4) by two-photon excitation with 266.5 nm pulses from an ultrafast laser. The wave packet is detected by autoionization following excitation with time-delayed 800 nm pulses to the n=8 (v(+)=4) and n=9 (v(+)=3) Rydberg states in the (2)Pi(12) angular momentum core state. Autoionization of each state occurs to the (2)Pi(32) state of the ion through spin-orbit ionization. Photoelectron spectroscopy is used to differentiate between the n=8 and n=9 ejected photoelectrons. Detection of the wave packet recurrences via the n=8 and n=9 Rydberg states reveals a pi phase-shift difference of the recurrences between the two final states. In each case, Delta v not equal 0 transitions are observed since wave packet recurrences are detected. By fitting the observed phase change of the recurrences with a simple model for the overlap amplitudes and assumptions about the potentials, we estimate, within the context of the model, that approximately 0.6% of the transitions may be attributed to Delta v= +/- 1 transitions between the initial Rydberg superposition state and the final Rydberg detection state.     

Pump-dump iterative squeezing of vibrational wave packets

The free motion of a nonstationary vibrational wave packet in an electronic potential is a source of interesting quantum properties. In this work we propose an iterative scheme that allows continuous stretching and squeezing of a wave packet in the ground or in an excited electronic state, by switching the wave function between both potentials with pi pulses at certain times. Using a simple model of displaced harmonic oscillators and delta pulses, we derive the analytical solution and the conditions for its possible implementation and optimization in different molecules and electronic states. We show that the main constraining parameter is the pulse bandwidth. Although in principle the degree of squeezing (or stretching) is not bounded, the physical resources increase quadratically with the number of iterations, while the achieved squeezing only increases linearly.     

Ultrafast strong field quantum control on K2 dimers

In this theoretical wave packet study, we demonstrate efficient strong field quantum control by Selective Population of Dressed States (SPODS) on K₂ dimers. Shaped femtosecond pulses are used to switch population transfer among different final states with high selectivity and almost unit efficiency. The physical mechanism of this ultrafast molecular strong field control scenario is analyzed in analogy to the interpretation of recent experimental studies of SPODS on atoms. Criteria for the design of SPODS-pulses are presented along with experimental signatures for the observation of SPODS on molecules.     

Coherent spin control of matrix isolated molecules by IR+UV laser pulses: quantum simulations for ClF in Ar

Two coherent sequential IR+UV laser pulses may be used to generate two time-dependent nuclear wave functions in electronic excited triplet and singlet states via single (UV) and two photon (IR+UV) excitation pathways, exploiting spin-orbit coupling and vibrational pre-excitation, respectively. These wave functions evolve from different Franck-Condon domains until they overlap in a domain of bond stretching with efficient intersystem crossing. Here, the coherence of the laser pulses is turned into optimal interferences of the wave packets, yielding the total wave packet at the target place, time, and with dominant target spin. The time resolution of spin control is few femtoseconds. The mechanism is demonstrated by means of quantum model simulations for ClF in an Ar matrix.     

Molecular wave packet interferometry and quantum entanglement

We study wave packet interferometry (WPI) considering the laser pulse fields both classical and quantum mechanically. WPI occurs in a molecule after subjecting it to the interaction with a sequence of phase-locked ultrashort laser pulses. Typically, the measured quantity is the fluorescence of the molecule from an excited electronic state. This signal has imprinted the interference of the vibrational wave packets prepared by the different laser pulses of the sequence. The consideration of the pulses as quantum entities in the analysis allows us to study the entanglement of the laser pulse states with the molecular states. With a simple model for the molecular system, plus several justified approximations, we solve for the fully quantum mechanical molecule-electromagnetic field state. We then study the reduced density matrices of the molecule and the laser pulses separately. We calculate measurable corrections to the case where the fields are treated classically.     

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.     

Vibrationally state-selective spin-orbit transfer with strong nonresonant pulses

By dynamic Stark shift using strong nonresonant pulses, we show that it is in principle possible to prepare arbitrary superposition states of mixed multiplicity. By a proper choice of parameters, the transfer of population is shown to follow the Rabi formula, where the initial and target states are now vibrational states of two light-induced molecular potentials of different multiplicity. Starting from nonstationary wave packets, the spin transfer can proceed via parallel transfer using a single pulse or by sequential transfer using a pulse sequence. A simple model is proposed to analyze the properties of both schemes and the feasibility of their experimental implementation for spin-orbit transitions in Rb2.     

The influence of ultrashort excitations on the vibrational dynamics in a diatomic molecule

The problem of vibrational wave packet dynamics in the system of two electronic states of a diatomic molecule, where the states are coupled by infinitely short light pulses, is solved. The electronic states were modeled by shifted harmonic oscillators with different frequencies. Exact expressions for the probability densities of the wave packets in the ground and excited states were derived. The spatial, spectral, and temporal characteristics of the wave packets, namely, the range of motion, spatial width, mean energy, spectral width (the mean number of vibrational states in a wave packet), and the autocorrelation function, were calculated as functions of the molecular parameters (the frequency ratio and the distance between the potential minima) and of the delay time between the light pulses. The possibility of controlling the mean energy and spectral width of the wave packets in the ground electronic state by varying the delay time is considered. It was shown that "squeezed" wave packets can be prepared in the ground electronic state if the upper electronic state is shallow.     

Rotational coherence imaging and control for CN molecules through time-frequency resolved coherent anti-Stokes Raman scattering

Numerical wave packet simulations are performed for studying coherent anti-Stokes Raman scattering (CARS) for CN radicals. Electronic coherence is created by femtosecond laser pulses between the X(2)Σ and B(2)Σ states. Due to the large energy separation of vibrational states, the wave packets are superpositions of rotational states only. This allows for a specially detailed inspection of the second- and third-order coherences by a two-dimensional imaging approach. We present the time-frequency domain images to illustrate the intra- and intermolecular interferences, and discuss the procedure to rationally control and experimentally detect the interferograms in solid Xe environment.     

Steering dissociation of Br2 molecules with two femtosecond pulses via wave packet interference

The dissociation dynamics of Br2 molecules induced by two femtosecond pump pulses are studied based on the calculation of time-dependent quantum wave packet. Perpendicular transition from X 1Sigma g+ to A 3Pi 1u+ and 1Pi 1u+ and parallel transition from X 1Sigma g+ to B 3Pi 0u+, involving two product channels Br (2P3/2)+Br (2P3/2) and Br (2P3/2)+Br* (2P1/2), respectively, are taken into account. Two pump pulses create dissociating wave packets interfering with each other. By varying laser parameters, the interference of dissociating wave packets can be controlled, and the dissociation probabilities of Br2 molecules on the three excited states can be changed to different degrees. The branching ratio of Br*/(Br+Br*) is calculated as a function of pulse delay time and phase difference.     

Molecular isomerization induced by ultrashort infrared pulses. II. Pump-dump isomerization using pairs of time-delayed half-cycle pulses

We investigate population transfer across the barrier in a double-well potential, induced by a pair of time-delayed single-lobe half-cycle pulses. We apply this setup both to a one-dimensional (1D) quartic model potential and to a three-dimensional potential representing HCN-->HNC isomerization. Overall the results for the two systems are similar, although in the 3D system some additional features appear not seen in the 1D case. The generic mechanism of population transfer is the preparation by the pump pulse of a wave packet involving delocalized states above the barrier, followed by the essentially 1D motion of the delocalized part of wave packet across the barrier, and the eventual de-excitation by the dump pulse to localized states in the other well. The correct timing is given by the well-to-well passage time of the wave packet and its recurrence properties, and by the signs of the field lobes which determine the direction and acceleration or deceleration of the wave packet motion. In the 3D system an additional pump-pump-dump mechanism linked to wave packet motion in the reagent well can mediate isomerization. Since the transfer time and the pulse durations are of the same order of magnitude, there is also a marked dependence of the dynamics and the transfer yield on the pulse duration. Our analysis also sheds light on the pronounced carrier envelope phase dependence previously observed for isomerization and molecular dissociation with one-cycle and sub-one-cycle pulses.     

Preparation and resolution of molecular states by coherent sequences of phase-locked ultrashort laser pulses

We study the application of nonlinear wave packet interferometry to the preparation and resolution of the overlaps of nonstationary nuclear wave functions evolving in an excited electronic state of a diatomic molecule. It is shown that possible experiments with two phase-locked ultrashort pulsepairs can be used to determine a specific vibrational wave packet state in terms of coherent states of the ground electronic state. We apply this scheme to an idealized molecule with harmonic potential energy surfaces and to the X <-- B transition states of the iodine molecule. Our results indicate that this scheme is very promising as a potential tool to quantum control.     

Adiabatic squeezing of molecular wave packets by laser pulses

Strong pulse sequences can be used to control the position and width of the molecular wave packet. In this paper we propose a new scheme to maximally compress the wave packet in a quasistatic way by freezing it at a peculiar adiabatic potential shaped by two laser pulses. The dynamic principles of the scheme and the characteristic effect of the different control parameters are presented and analyzed. We use two different molecular models, electronic potentials modeled by harmonic oscillators, with the same force constants, and the Na(2) dimer, to show the typical yield that can be obtained in compressing the initial (minimum width) molecular wave function.     

Wave packet interferometry and quantum state reconstruction by acousto-optic phase modulation

Studies of wave packet dynamics often involve phase-selective measurements of coherent optical signals generated from sequences of ultrashort laser pulses. In wave packet interferometry (WPI), the separation between the temporal envelopes of the pulses must be precisely monitored or maintained. Here we introduce a new (and easy to implement) experimental scheme for phase-selective measurements that combines acousto-optic phase modulation with ultrashort laser excitation to produce an intensity-modulated fluorescence signal. Synchronous detection, with respect to an appropriately constructed reference, allows the signal to be simultaneously measured at two phases differing by 90 degrees. Our method effectively decouples the relative temporal phase from the pulse envelopes of a collinear train of optical pulse pairs. We thus achieve a robust and high signal-to-noise scheme for WPI applications, such as quantum state reconstruction and electronic spectroscopy. The validity of the method is demonstrated, and state reconstruction is performed, on a model quantum system--atomic Rb vapor. Moreover, we show that our measurements recover the correct separation between the absorptive and dispersive contributions to the system susceptibility.     

用两束超短乜秒激光脉冲选择控制D2分子的转动波包

通过求解D2分子在飞秒激光场中的含时薛定谔方程,研究了室温下D2分子在超快1s秒激光驱动下的的转动波包动力学．选择用第一束超短飞秒脉冲与温度为300K的D2分子系综相互作用产生一个相干转动波包,用第二束超短匕秒脉冲在波包的1／4和3／4恢复周期选择操纵D2分子取向．研究结果表明,通过选择两束超短飞秒脉冲的延迟时间,可以有效控制D2分子转动波包中奇偶态的相对布居,从而选择性的控制D2分子取向．     

Optical control of excited-state vibrational coherences of a molecule in solution: The influence of the excitation pulse spectrum and phase in LD690

Spectral and phase shaping of femtosecond laser pulses is used to selectively excite vibrational wave packets on the ground (S0) and excited (S1) electronic states in the laser dye LD690. The transient absorption signals observed following excitation near the peak of the ground-state absorption spectrum are characterized by a dominant 586 cm(-1) vibrational mode. This vibration is assigned to a wave packet on the S0 potential energy surface. When the excitation pulse is tuned to the blue wing of the absorption spectrum, a lower frequency 568 cm(-1) vibration dominates the response. This lower frequency mode is assigned to a vibrational wave packet on the S1 electronic state. The spectrum and phase of the excitation pulse also influence both the dephasing of the vibrational wave packet and the amplitude profiles of the oscillations as a function of probe wavelength. Excitation by blue-tuned, positively chirped pulses slows the apparent dephasing of the vibrational coherences compared with a transform-limited pulse having the same spectrum. Blue-tuned negatively chirped excitation pulses suppress the observation of coherent oscillations in the ground state.