Excited Wave Packet Research Articles

Two-electron dynamics in nonlinear double excitation of helium by intense ultrashort extreme-ultraviolet pulses

We present a theoretical study for multiphoton double excitation of He atoms exposed to an intense ultrashort extreme-ultraviolet (EUV) pulse, where the photon energy coincides with the transition energy from the ground state to a Rydberg state, by solving the time-dependent Schr\odinger equation in the hyperspherical coordinates. Photoelectron spectra under the conditions comparable with a recent experiment [Hishikawa et al., Phys. Rev. Lett. 107, 243003 (2011)] are calculated and analyzed. We identify the mechanism of the enhanced three-photon absorption probability which is more than one order of magnitude larger than that for the two-photon process in accordance with the experiment. The enhancement is attributed to a propensity rule for double excitation in a two-step mechanism, in which a one-photon absorption by one electron to a Rydberg state is followed by a two-photon absorption by the other electron to an excited orbital, while the first electron remains at nearly the same principle quantum number. Based on the time-dependent perturbation theory, the three-photon absorption probability exhibits peculiar cubic dependence on the pulse duration due to the propensity rule, in contrast to the linear dependence of the two-photon absorption probability. Thus a crossover between the two- and three-photon absorption probabilities takes place for sufficiently intense and long pulses. We also study the time evolution of a doubly excited two-electron wave packet created by an intense ultrashort EUV pulse efficiently using the same enhancement scheme, opening up the possibility of visualizing the correlated motion of two electrons in the time domain.

Theoretical study of multi-photon double electron excitation of He by intense ultrashort EUV pulses

We use the time-dependent hypersperhical close-coupling method to study the double excitation of helium atoms by intense ultrashort XUV pulses. In addition to showing the photoelectron yield spectra, we demonstrate that free electron lasers can be used to efficiently create doubly excited electron wave packets, opening up the possibility of visualizing the correlated motion of electrons in atoms.

Optimized production of ultracold ground-state molecules: Stabilization employing potentials with ion-pair character and strong spin-orbit coupling

We discuss the production of ultracold molecules in their electronic ground state by photoassociation employing electronically excited states with ion-pair character and strong spin-orbit interaction. A short photoassociation laser pulse drives a non-resonant three-photon transition for alkali atoms colliding in their lowest triplet state. The excited state wave packet is transferred to the ground electronic state by a second laser pulse, driving a resonant two-photon transition. After analyzing the transition matrix elements governing the stabilization step, we discuss the efficiency of population transfer using transform-limited and linearly chirped laser pulses. Finally, we employ optimal control theory to find the most efficient stabilization pathways. We find that the stabilization efficiency can be increased by one and two orders of magnitude for linearly chirped and optimally shaped laser pulses, respectively.

Validity of the Bersohn–Zewail model beyond justification

The absorption of probe pulses in ultrafast pump–probe experiments can be determined from the Bersohn–Zewail (BZ) model. The model relies on classical mechanics to describe the dynamics of the nuclei in the excited electronic state prepared by the ultrashort pump pulse. The BZ model provides excellent agreement between the classical trajectory and the average position of the excited state wave packet. By investigating the approximations connecting the nuclear dynamics described by quantum mechanics and the BZ model, we conclude that this agreement goes far beyond the validity of the individual approximations.

Laser adiabatic manipulation of the bond length of diatomic molecules with a single chirped pulse

We propose and test numerically a scheme for controlling the bond distance in a diatomic molecule that requires the use of a single chirped pulse. The laser prepares a superposition state of both nuclear and electronic degrees of freedom, where the main character of the electronic wave function is that of an excited dissociative state. The main limitation of the scheme is the need of ultra broadband pulses, where the bandwidth must be of the order of the dissociation energy to achieve large bond elongations. The scheme can be used to deform the bond during the laser excitation to an arbitrary large and constant value, or to allow slow time-dependent bond elongations. Additionally, the scheme can be used to prepare highly excited vibrational wave packets in the ground potential after the pulse is switched off, at the expense of losing some population that dissociates. These wave packets are initially localized at the outer well of the potential, at energies controllable by the excitation process.

Efficient and robust strong-field control of population transfer in sensitizer dyes with designed femtosecond laser pulses

We demonstrate control of electronic population transfer in molecules with the help of appropriately shaped femtosecond laser pulses. To this end we investigate two photosensitizer dyes in solution being prepared in the triplet ground state. Excitation within the triplet system is followed by intersystem crossing and the corresponding singlet fluorescence is monitored as a measure of population transfer in the triplet system. We record control landscapes with respect to the fluorescence intensity on both dyes by a systematic variation of laser pulse shapes combining second order and third order dispersion. In the strong-field regime we find highly structured topologies with large areas of maximum or minimum population transfer being insensitive over a certain range of applied laser intensities thus demonstrating robustness. We then compare our experimental results with simulations on generic molecular potentials by solving the time-dependent Schrödinger equation for excitation with shaped pulses. Control landscapes with respect to population transfer confirm the general trends from experiments. An analysis of regions with maximum or minimum population transfer indicates that coherent processes are responsible for the outcome of our excitation process. The physical mechanisms of joint motion of ground and excited state wave packets or population of a vibrational eigenstate in the excited state permit us to discuss the molecular dynamics in an atom-like picture.

Superior Z → E and E → Z photoswitching dynamics of dihydrodibenzodiazocine, a bridged azobenzene, by S1(nπ*) excitation at λ = 387 and 490 nm

The ultrafast Z→E and E→Z photoisomerisation dynamics of 5,6-dihydrodibenzo[c,g][1,2]diazocine (1), the parent compound of a class of bridged azobenzene-based photochromic molecular switches with a severely constrained eight-membered heterocyclic ring as central unit, have been studied by femtosecond time-resolved spectroscopy in n-hexane as solvent and by quantum chemical calculations. The diazocine contrasts with azobenzene (AB) in that its Z rather than E isomer is the energetically more stable form. Moreover, it stands out compared to AB for the spectrally well separated S(1)(nπ*) absorption bands of its two isomers. The Z isomer absorbs at around λ = 404 nm, the E form has its absorption maximum around λ = 490 nm. The observed transient spectra following S(1)(nπ*) photoexcitation show ultrafast excited-state decays with time constants τ(1) = 70 fs for the Z and <50 fs for the E isomer reflecting very fast departures of the excited wave packets from the S(1) Franck-Condon regions and τ(2) = 270 fs (320 fs) related to the Z→E (resp. E→Z) isomerisations. Slower transient absorption changes on the time scale of τ(3) = 5 ps are due to vibrational cooling of the reaction products. The results show that the unique steric constraints in the diazocine do not hinder, but accelerate the molecular isomerisation dynamics and increase the photoswitching efficiencies, contrary to chemical intuition. The observed isomerisation times and quantum yields are rationalised on the basis of CASPT2//CASSCF calculations by a S(1)/S(0) conical intersection seam at a CNNC dihedral angle of ≈96° involving twisting and torsion of the central CNNC moiety. With improved photochromism, high quantum yields, short reaction times and good photostability, diazocine 1 and its derivatives constitute outstanding candidates for photoswitchable molecular tweezers and other applications.

Electronic and steric effects on the photo-induced C→E ring-opening of structurally modified furylfulgides

The ultrafast C→E ring-opening reactions of four selectively modified furylfulgides have been studied by means of ultrafast broadband transient absorption spectroscopy after femtosecond laser excitation at λ = 500 nm. A large difference in the dynamics was found in the case of benzannulation at the furyl moiety as an example for an electronic effect by extension of the conjugated π-electron system compared to furylfulgides carrying sterically different alkyl substituents at the central cyclohexadiene (CHD) ring. The measured very similar spectro-temporal absorption maps for the furylfulgides with a methyl or isopropyl group at the CHD ring or an intramolecular alkyl bridge from the CHD to the furyl moiety showed two distinctive excited-state absorptions with slightly different decay times. The first time constant (τ(1) = 0.39-0.57 ps) was assigned to the rapid departure of the excited wavepacket from the Franck-Condon region. The slightly longer second decay time of τ(2) = 0.66-0.92 ps, depending on the compound, was attributed to the electronic deactivation and ring-opening through a conical intersection to the S(0) state. In contrast, the benzannulation at the furyl moiety was found to lead to a bi-phasic excited-state decay with τ(2) = 4.7 ps and a much slower additional contribution of τ(3) = 17.4 ps, ≈25 times longer compared to the normal furylfulgides. The drastic change is attributed to a trapping of excited molecules in a local potential energy minimum en route to the conical intersection.

Inducing changes in the bond length of diatomic molecules by time-symmetric chirped pulses

We show numerically that it is possible to change the structure of a simple molecule, that is, a diatomic molecule, where the bond length is modified at a precise timing with symmetrically chirped laser pulses. In the adiabatic regime, the process is fully time reversible, making it possible to design slow vibrations with large bond elongations. The scheme relies on the preparation of a separable state of both nuclear and electronic degrees of freedom with predominant amplitude on the dissociative (antibonding) electronic wave function. Shorter laser pulses can be used to dynamically induce larger bond elongations, preparing a highly excited vibrational wave packet in the ground potential as the laser is switched off.

Long-lived coherence in carotenoids

We use two-colour vibronic coherence spectroscopy to observe long-lived vibrational coherences in the ground electronic state of carotenoid molecules, with decoherence times in excess of 1 ps. Lycopene and spheroidene were studied isolated in solution, and within the LH2 light-harvesting complex extracted from purple bacteria. The vibrational coherence time is shown to increase significantly for the carotenoid in the complex, providing further support to previous assertions that long-lived electronic coherences in light-harvesting complexes are facilitated by in-phase motion of the chromophores and surrounding proteins. Using this technique, we are also able to follow the evolution of excited state coherences and find that for carotenoids in the light-harvesting complex the ⟨S2|S0⟩ superposition remains coherent for more than 70 fs. In addition to the implications of this long electronic decoherence time, the extended coherence allows us to observe the evolution of the excited state wavepacket. These experiments reveal an enhancement of the vibronic coupling to the first vibrational level of the C–C stretching mode and/or methyl-rocking mode in the ground electronic state 70 fs after the initial excitation. These observations open the door to future experiments and modelling that may be able to resolve the relaxation dynamics of carotenoids in solution and in natural light-harvesting systems.

Theoretical Study of the Influence of Femtosecond Laser Wavelength on the Evolution of a Double-Minimum Electronic Excited State Wave Packet for NaRb

Employing the two-state model and the time-dependent wave packet method, the influence of femtosecond laser wavelength on the evolution of the double-minimum electronic excited state wave packet is numerically investigated. For different laser wavelengths, evolutions of the double-minimum electronic excited state wave packet with time and internuclear distance are different. One can control the evolution of the wave packet by varying the laser wavelength appropriately, which will benefit the light manipulation of atomic and molecular processes. Furthermore, study of the dynamics of the NaRb molecule may yield clues to creating an ultracold molecule.

A Major Stratospheric Sudden Warming Event in January 2009

Abstract The major stratospheric sudden warming (SSW) event of January 2009 is analyzed using the Japan Meteorological Agency (JMA) Climate Data Assimilation System (JCDAS). This SSW event is characterized by the extraordinary predominance of the planetary-scale wave of zonal wavenumber 2 (wave 2). The total amount of the upward Eliassen–Palm (EP) flux for wave 2 was the strongest since the winter of 1978/79. It is found that the remarkable development of the upper troposphere ridge over Alaska played important roles in the SSW in January 2009. During the first development stage, the ridge excited wave packets upward as well as eastward over around Alaska. The eastward-propagating packets intensified a trough over eastern Siberia, which led to the development of the planetary wave over eastern Siberia during the second development stage. The results of this study indicate that the pronounced wave-2 pattern observed in the stratosphere was brought about by accumulative effects of rather localized propagation of wave packets from the troposphere during the course of this SSW event rather than by the ubiquitous propagation of planetary-scale disturbances in the troposphere. The features of the SSW in January 2009 are quite similar to those during the major stratospheric warming event in February 1989: both SSWs are characterized by the predominance of wave 2, the remarkable development of the upper troposphere ridge over around Alaska, and positive SSTs in the eastern part of the North Pacific corresponding to a La Niña condition.

Solving the spectroscopic phase: imaging excited wave packets and extracting excited state potentials from fluorescence data

We develop an inversion scheme for obtaining the signs of transition-dipole amplitudes from fluorescence line intensities. Using the amplitudes thus obtained we show how to extract highly accurate excited state potential(s) and the transition-dipole(s) as a function of inter-nuclear displacements. The same dipole amplitudes can also be used to extract the phase and amplitude of unknown time-evolving wave packets, in essentially a quantum non-demolition manner. The procedure, which is demonstrated for the A((1)∑) and B((1)Π(u)) states of the Na(2) molecule, is shown to yield reliable results even when we are given incomplete or uncertain data. We also demonstrate the success of our approach in extracting double minimum potentials. The inversion scheme is in principle applicable to any polyatomic molecule.

State selection in nonresonantly excited wave packets by tuning from nonadiabatic to adiabatic interaction

We show for rotational alignment of diatomic molecules that the crossover from nonadiabatic to adiabatic limits is well described by a convolution of excitation pulse envelope and sinusoidal molecular response and that it takes place in a uniform way in the region between 0.1 and 1 for the ratio of pulse duration to rotational period. In a nonresonant Raman-type excitation, this crossover is used to manipulate the $J$ composition of a rotational wave packet with respect to the initial thermal distribution. By optimizing the duration of a single pulse, arbitrarily narrow distributions at low $J$ levels can be formed. A double-pulse excitation, where a longer second pulse acts as a selective dump pulse, allows to prepare nonthermal distributions centered at high $J$ values. With the alignment signal on top of an isotropic background, experimental techniques sensitive to the induced anisotropy are optimally suited for implementation. To demonstrate the efficiency of the method, numerical simulations are carried out for rotational alignment in ${^{14}\text{N}}_{2}$ at various experimentally relevant laser intensities. The scheme is transferable to quantum systems with a significant variation in transition frequencies between subsequent levels.

Dynamics of vibrational and electronic coherences in the electronic excited state studied in a negative-time range

Femtosecond pump–probe spectroscopy signals in a negative-time range (i.e., when the probe pulse precedes pump pulse) were analyzed to study the dynamics of electronic and vibrational coherences in the photodissociation of oxyhemoglobin. The decay time of the electronic coherence between the ground and Q-band electronic states is found to be 45 ± 5 fs. The dephasing times of the vibrational modes of several frequencies in the ground state have been determined. Those in the excited state are found to be ∼26 fs. The absence or presence of the modes in the negative-time range due to the excited state wavepacket is explained in terms of the mode coupling strength.

Modeling the generation and gravity-capillary waves using an underwater sound source

This is an experimental study of the acoustic method of surface-wave excitation using an underwater source of high-frequency (950 kHz) sound. The surface waves are excited at the sound-beam modulation frequency (3–55 Hz). For a normal fall onto the free surface, the modulated sound beam efficiently generates waves in the gravity-capillary range. This provides flexible electronic control of the main wave parameters (frequency and amplitude) in the packet and continuous modes. The amplitude-frequency characteristics of the process of surface-wave generation were obtained by numerical calculations (based on equations for the rate of acoustic flux and propagation of gravity-capillary surface waves) and by experiments (based on surface wave measurements by optical and contact methods). Both values are very consistent: on the background of a similar monotonic attenuation with frequency, they have a local dip near the minimum of the phase velocity and oscillation in the frequency range above 20 Hz. The experiments on the excitation of wave packets by single acoustic messages with varying lengths and powers, as well as by falling water drops, indicated that, in all cases, the phase characteristics are satisfactorily consistent with one another and the time needed for the signal to arrive at the measurement point is determined by the group velocity.

Theoretical studies of angle-resolved ion yield spectra of core-to-valence transitions of acetylene

Recent experimental results on angle-resolved photoion-yield spectroscopy (ARPIS) spectra near the core-to-valence excitation in acetylene show significant anisotropies in the spectral profile measured at 0 degrees and 90 degrees regarding to the polarization direction of x-ray photons. In the present work, a theoretical model is proposed to simulate the fine structure and anisotropy in ARPIS. This employs two-dimensional potential energy surfaces of the ground and core-excited states, as well as transition dipole moments, including symmetric and antisymmetric bending modes to account for Duschinsky effect. The ARPIS is simulated by evaluation of the ion flux, which is found as a projection of the excited state wave packet on a particular direction in the molecular frame. Numerical simulations explain qualitatively the angular dependence of the experimental spectra of the 1s-->1pi(g) ( *) and 1s-->3sigma(u) ( *) transitions. The effects of the lifetime of the core-excited state, the direction of the ion flux, and the transition dipole moment are discussed.

Parallel ultrafast E–C ring closure and E–Z isomerisation in a photochromic furylfulgide studied by femtosecond time-resolved spectroscopy

The photo-induced parallel E-->C ring closure and E-->Z isomerisation reactions of the (E) isomer of the photochromic furylfulgide 2-[1-(2,5-dimethyl-3-furyl)-ethylidene]-3-isopropylidene succinic anhydride () in n-hexane have been studied using femtosecond time-resolved spectroscopy. Broadband transient absorption data after femtosecond laser excitation at lambda(pump) = 335 nm provide time constants of 100 fs and 250 fs that belong to the formation of the (C) and the (Z) isomers, respectively, to yield a (C):(Z) product ratio of about 2:1. The results are consistent with a conformer-specific photoreaction of the (E)-isomer of , where one conformer (alpha) undergoes predominantly E-->C ring closure and the other (beta) E-->Z isomerisation, or alternatively with an ultrafast branching of the excited wavepacket of the alpha-conformer within Deltat < 250 fs after the pump pulse. The observed isomerisation times suggest that the ensuing transformations proceed via distinctive conical intersections between the respective potential energy hypersurfaces. Oscillations of the transient absorption with frequencies of approximately 64 and 114 cm(-1) are found and interpreted as excited-state vibrations induced during the E-->Z isomerisation reaction. Slower spectral dynamics at delay times up to approximately 10 ps reflect the cooling of the vibrationally hot reactant and product molecules after their return to their electronic ground states. Time-dependent DFT calculations were performed to shed light on the reaction coordinates involved. The emerging picture for the dynamics of obtained in this work is of interest in the broader context for our understanding of conformer-specific photochemistry and competing ultrafast reactions in polyatomic molecules.

In situ accurate determination of the zero time delay between two independent ultrashort laser pulses by observing the oscillation of an atomic excited wave packet

We propose a novel method that uses the oscillation of an atomic excited wave packet observed through a pump-probe technique to accurately determine the zero time delay between a pair of ultrashort laser pulses. This physically based approach provides an easy fix for the intractable problem of synchronizing two different femtosecond laser pulses in a practical experimental environment, especially where an in situ time zero measurement with high accuracy is required.

Cavity-assisted spontaneous emission as a single-photon source: Pulse shape and efficiency of one-photon Fock-state preparation

Within the framework of exact quantum electrodynamics in dispersing and absorbing media, we have studied the quantum state of the radiation emitted from an initially in the upper state, prepared two-level atom in a high-$\mathit{Q}$ cavity, including the regime where the emitted photon belongs to a wave packet that simultaneously covers the areas inside and outside the cavity. For both continuing atom-field interaction and short-term atom-field interaction, we have determined the spatiotemporal shape of the excited outgoing wave packet and calculated the efficiency of the wave packet to carry a one-photon Fock state. Furthermore, we have made contact with quantum noise theories where the intracavity field and the field outside the cavity are regarded as approximately representing independent degrees of freedom such that two separate Hilbert spaces can be introduced.