CHEMICAL PHYSICS

Attosecond-resolved photoionizationof chiral moleculesS. Beaulieu,1,2* A. Comby,1 A. Clergerie,1 J. Caillat,3 D. Descamps,1 N. Dudovich,4

B. Fabre,1 R. Géneaux,5 F. Légaré,2 S. Petit,1 B. Pons,1 G. Porat,4 T. Ruchon,5

R. Taïeb,3 V. Blanchet,1 Y. Mairesse1

Chiral light-matter interactions have been investigated for two centuries, leading to thediscovery of many chiroptical processes used for discrimination of enantiomers. Whereasmost chiroptical effects result from a response of bound electrons, photoionization canproduce much stronger chiral signals that manifest as asymmetries in the angulardistribution of the photoelectrons along the light-propagation axis. We implemented self-referenced attosecond photoelectron interferometry to measure the temporal profile ofthe forward and backward electron wave packets emitted upon photoionization of camphorby circularly polarized laser pulses. We measured a delay between electrons ejected forwardand backward, which depends on the ejection angle and reaches 24 attoseconds. Theasymmetric temporal shape of electron wave packets emitted through an autoionizing statefurther reveals the chiral character of strongly correlated electronic dynamics.

Bolts and nuts are among the most com-monchiral objects in ourmacroscopicworld.Their chiral nature is used to convert ro-tation to directional translation: Rotatingthe nut on a bolt induces its translation

forward or backward, depending on the rotationdirection. A very similar effect occurs in the mi-croscopic world when enantiopure chiral mole-cules are photoionized by circularly polarizedradiation (1). The ejected photoelectrons tendto go forward or backward relative to the light-propagation axis, depending on the helicity ofthe ionizing light and the handedness of themolecules (2, 3). As a result, the photoelectronangular distribution shows an asymmetry, calledphotoelectron circular dichroism (PECD). PECDis one of the most sensitive probes of static (4)anddynamical (5,6)molecular chirality, producingsignals that are up to two orders of magnitudelarger thanmost circular dichroisms. From a clas-sical point of view, PECD can be seen as the resultof the combined action of the chiral molecularpotential and the circular ionizing electric fieldon the outgoing electron trajectories (7). Quan-

tummechanically, it arises from the interferencebetween partial waves of different parity consti-tuting the outgoing photoelectron wave packet(1). Both interpretations show that subtle dif-ferences in the ionization dynamics can havesubstantial consequences. Thus, PECD has beenproposed as a possible hypothesis to explain thehomochirality of terrestrial life (8): The asym-metric electron ejection induces an asymmetricrecoil of the ions, which can lead to enantiomericseparation when accumulated over hundreds ofmillions of years (9).Photoionization was considered to be instan-

taneous from an experimental point of view untilattosecond technology made it possible to mea-sure the underlying ultrafast electron dynamics.Delays of a few attoseconds were measured be-tween electrons originating fromdifferent atomicorbitals (10), from distinct bands of a solid (11),associated with different vibrational states of amolecular ion (12), or from differing spin-orbitstates (13). The direction of the electron emis-sion also influences the photoionization dy-namics: Delays have been observed between

electrons ejected at different angles (14–16) orfrom different sides of an asymmetric molecule(17, 18).Thephotoionizationprocess involvesmore com-

plicated dynamics when autoionization occurs.In that case, the photoabsorption promotes thesystem into a metastable bound state coupled toequienergetic continuum states through configu-ration interaction. This coupling leads to auto-ionization of themetastable state. The interferenceof direct and indirect photoionization channelsproduces characteristic (Fano) spectral profiles(19) associatedwith complex temporal dynamics.Recent pioneering attosecond experiments in raregases have shown the possibility ofmeasuring thebuildup of Fano lineshapes in the temporal do-main (20) as well as the spectral phase acrossthe resonance (21, 22), allowing the reconstruc-tion of the temporal profile of the electron wavepacket (22).In this study, we aimed to determine whether

or not the electrons ejected forward andbackwardfrom a sample of randomly oriented enantio-pure chiral molecules were temporally synchro-nized. Answering this question, for both direct(Fig. 1, A and C) and indirect (Fig. 1, B and D)photoionization pathways, is a challenging task.Up to now, attosecond-delay measurements inthe gas phase have been conducted on rare gasatoms or di- and triatomic molecules, which wereused as benchmark systems. These experimentshave revealed a strong influence of theweakprobefield on the outcome of the measurement. It isthus necessary to perform accurate theoreticalcalculations to calibrate these measurement-induced delays (10, 23). Such theoretical calcula-tions for large and low-symmetry chiralmolecules

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1Université de Bordeaux, CNRS, Commissariat à l'EnergieAtomique et aux Energies Alternatives (CEA), Centre LasersIntenses et Applications (CELIA), UMR5107, F33405 Talence,France. 2Institut National de la Recherche Scientifique,Varennes, Quebec, Canada. 3Sorbonne Universités, UPMCUniversité Paris 6, CNRS-UMR 7614, Laboratoire de ChimiePhysique Matière et Rayonnement, 75252 Paris, France.4Department of Physics of Complex Systems, WeizmannInstitute of Science, 76100 Rehovot, Israel. 5LaboratoireInteractions, Dynamiques et Lasers (LIDYL), CEA, CNRS,Université Paris-Saclay, CEA Saclay, 91191 Gif-sur-Yvette,France.*Corresponding author. Email: beaulieus@emt.inrs.ca

Fig. 1. Schematic view of the two time scalesof photoionization of two camphor enantiomers.(1S)-(−)-camphor is shown in (A) and (B);(1R)-(+)-camphor in (C) and (D). In directphotoionization [(A) and (C)], the forward (f) andbackward (b) electron wave packets may bedelayed by a few attoseconds because of theasymmetric scattering of the outgoing electronin the chiral molecular potential. In the caseof autoionization [(B) and (D)], the dynamics ofthe autoionizing state can lead to differenttemporal structures of photoelectron wave packetsin the forward and backward directions, on thefemtosecond time scale.

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[e.g., camphor (C10H16O)] are currently far fromreach.To directly access the delays between forward

and backward electron emission, without anymeasurement-induced effects, we implementeda self-referenced photoelectron interferometrytechnique using photoionization by two phase-locked laser fields to detect differential attosecondphotoionization delays with a resolution of 2 as.By independently controlling the chirality of theionizing andprobe light pulses,we fully decoupledthe intrinsic photoionization delays from themeasurement-induced delays.

Photoelectron interferometry

When an intense femtosecond laser pulse ionizesan atom or a molecule, multiple photons can beabsorbed above the ionization threshold [above-threshold ionization (ATI)]. In the spectral domain,ATI produces a comb of photoelectron peaks sep-arated by the laser photon energy (24). Each peakis characterized by a spectral width dw, and theoverall ATI spectrum extends over awidth Dw. Inthe time domain, the ATI process leads to theemission of attosecond electron bursts (Fig. 1,A to C) of duration dt, which form a train. Theoverall duration of the train Dt is set by the laserpulse duration, typically a few tens of femto-seconds. The ATI emission can last longer if anautoionizing state is populated: The lifetime ofthe autoionizing state increases the electronwavepacket duration (Fig. 1, B to D). Characterizingthe temporal dynamics of the ionization processrequires measuring the process on two timescales (dt and Dt). The femtosecond structureof the wave packet is encoded in the spectralintensity and phase within the bandwidth ofeach ATI peak: Dt is related to dw. On the otherhand, the attosecond substructures are encodedin the relative amplitude and phase between thedifferent ATI peaks: dt is associated with Dw.Thus, to obtain a complete picture of the temporaldynamics of the ionization process, it is necessaryto measure the spectral phase of the ATI peaks,both within their bandwidth (dw) and from onepeak to the next (Dw). This is possible with theuse of photoelectron interferometry (22, 25, 26).We first present the basic concepts of photo-

electron interferometry and highlight the richspectroscopic information that it provides aboutthe ionized target. For now, we are leaving asidethe chiral character of the experiment (by inte-grating over the photoelectron ejection angles).The principle of themeasurement is described inFig. 2. The targetmoleculewe chosewas camphor,a bicyclic ketonewhichhasbeenextensively studiedin PECD experiments performed in single-photon(1, 27),multiphoton (28, 29), andATI regimes (30).Camphor has a first ionization potential of 8.76 eVand Rydberg states starting around 6.2 eV: It isionizedby 2+n resonance-enhancedmultiphotontransitions when using a 400-nm ultraviolet (UV)field, with n being the order of the ATI peak. Inthe presence of aweak infrared (IR) 800-nm field(frequencyw), newpeaks, called sidebands, appearbetween the ATI comb. Two quantum paths leadto the same sideband: addition of an IR photon

toATIpeaknor subtraction of one IRphoton fromATI peak n + 1. These two paths interfere, and thesideband amplitude oscillates as a function of therelative delay between theUV and IR fields, at 2wfrequency (26, 31, 32). The phase of the side-band oscillations encodes the relative phase be-tween the two neighboring ATI peaks and, thus,the temporal properties of the photoemissionprocess.The photoelectronswere collected by a velocity-

map imaging (VMI) spectrometer, which mea-sures the angle-resolved photoelectron spectrum(Fig. 2, A and B). The superposition of 800- and400-nm pulses produces an electric field that isstronger in the upper or lower direction, depend-ing on the relative delay between the two fields.As a consequence, the electrons ejected up anddown are modulated in opposite phases (26, 33)

(Fig. 2B). We measured the sideband phase in-dependently on the angularly integrated upperand lower halves of the photoelectron image andaveraged the phase obtained from the upper halfwith the p-shifted phase obtained from the lowerhalf. More details about the experimental setup,raw VMI images and their inversion (34), as wellas data analysis are provided in the supplemen-tary materials (SM).Figure 2D shows the sideband oscillation phase

ϕ2w as a function of the photoelectron kineticenergyE. The first sideband (SB1), which encodesthe phase difference between ATI peaks one andtwo, presents an abrupt p-phase jump around1.9 eV. This is the signature of a resonance asso-ciated with one or the other of the two contrib-uting ATI peaks. The second and third sidebands,which are built, respectively, upon ATI peaks two

Beaulieu et al., Science 358, 1288–1294 (2017) 8 December 2017 2 of 6

Fig. 2. Principle and implementation of photoelectron interferometry. (A) Schematic experi-mental setup. Two phase-locked femtosecond laser pulses with linear or circular polarization arefocused into a jet of enantiopure camphor molecules in the interaction zone of a velocity-mapimaging (VMI) spectrometer. The photoelectrons are accelerated by a set of electrodes ontomicrochannel plates imaged by a phosphor screen and a charge-coupled device camera, whichrecords the two-dimensional (2D) projection of the 3D angular distribution of the photoelectronspectrum with an energy resolution around 0.2 eV at 2 eV. (B) Typical measured photoelectron angulardistribution and evolution of the second sideband as a function of delay between the two fields. Theoscillations in the upper and lower half of the distribution are out of phase, reflecting the up-downasymmetry of the total ionizing electric field. PE, photoelectron energy; a, electron ejection angle.(C) Ionization scheme.The 400-nm pulse (duration: 40 fs; intensity: ∼5 × 1012 W/cm2) induces multiphotonabove-threshold ionization (ATI) of the molecules. The first ionizing transition lies in the vicinity of anautoionizing resonance. The 800-nm pulse (duration: 30 fs; intensity: ∼5 × 1011 W/cm2) producesadditional transitions, leading to the creation of sidebands (SB1 and SB2) between the ATI peaks. V,configuration interaction; Ryd, Rydberg states. (D) Angle-integrated photoelectron spectrum composedof ATI peaks and nth-order sidebands (SBn). The blue lines represent the angularly integratedspectrally resolved 2w-oscillation phases for each sideband, and the red dots denote the spectrally averaged2w-oscillation phases for nonresonant SB2 and SB3. ϕ2w(E), energy-resolved sideband oscillation phase;

ϕ2w(�E), energy-integrated sideband oscillation phase; PES, photoelectron spectrum.

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and three and peaks three and four, have a smoothphase variation across their bandwidth (i.e., withoutany trace of resonance). We can thus conclude thatthe resonance occurs in the formation of the firstATI peak (ATI1) and does not propagate to thehigher ATI peaks, as confirmed by theoreticalcalculations of resonant photoelectron interfer-ometry presented in the SM.We proceeded to investigate the chiral (enan-

tiospecific) photoionization dynamics in the twodifferent regimes identified above: direct atto-second ionization (SB2andSB3) and indirect femto-second ionization in the vicinity of an autoionizingresonance (SB1).

Attosecond delays in nonresonantphotoionization

Westarted by analyzing thedirect photoionizationdynamics, which occur on the attosecond timescale and are encoded in the relative phase be-tween thedifferentATIpeaks. These relativephasescan be obtained by extracting the oscillationphases of the signals averaged over the band-width of each sideband, ϕ2wð�EÞ. Neglecting thevariations over the spectral width is equivalent toassuming that the photoionization process isstrictly periodic from one laser cycle to the next.The spectral homogeneity of the sideband phasesshown in Fig. 2D indicates that this assumptionis reasonable for SB2 and SB3, but not for theresonant SB1, which is discussed later. This schemeis similar to the conventional RABBIT analysis(reconstruction of attosecond beatings by inter-ference of two-photon transitions) (25), here ex-tended to the case of above-threshold ionization(26, 32). Simulations presented in the SM vali-date the analogy between the two techniques.Measuring the phaseϕ2wð�EÞ is equivalent to

measuring the time delay t that maximizes thesignal of each sideband. This delay can be decom-posed as the sum of three contributions that re-flect the three steps of the sideband creation (35)

t = tlight + tW + tcc (1)

The ionization is triggered by absorption of lightat a well-defined time (tlight). Next, the electronscatters in the molecular potential and acquiresa delay tW—the Wigner delay (36). The electronalso interacts with the weak IR field, which in-duces continuum-continuum transitions from themain ATI peaks to the sidebands, introducingan additional delay tcc. Whereas tlight and tcc areinduced by the measurement, tW is intrinsic tothe probed system and is the physical quantityof interest. It represents the delay between anelectron scattering in a given potential and in areference potential, as introduced by Wigner in1955 (36).To resolve the enantiosensitivity of Wigner

delays, we turned to chiroptical measurementscomparing the sideband oscillation phases forelectrons emitted in the forward (f) versus back-ward (b) directions and extracting the differenceDtf/b = tf− tb. This procedure naturally eliminatestlight, which is strictly common to the forward andbackwardelectrons. Furtherdecoupling is achieved

by using different combinations of linearly andcircularly polarized light. The forward/backward(f/b) symmetry can be broken only by the chiralnature of the interaction—that is, if a circularlypolarized light pulse is used. We can thus selec-tively break the f/b symmetry only in the ioniza-tion step by using a circularly polarized UV fieldand a linear IR field. In that case, Dtcc

f/b = 0 andDtf/b = DtW

f/b. Alternatively, we can render theWigner delay f/b symmetric by using a linearlypolarized UV field for ionization and a circularlypolarized IR field probing photons to obtainDtW

f/b = 0 and Dtf/b = Dtccf/b.

The photoelectron images were separated infour quadrants, and the signal was angularlyaveraged in each quadrant. A Fourier analysis ofthe 2w oscillations was conducted to determinethe delay that maximized each sideband, in eachquadrant. We calculated the difference betweenthe delaysmeasured in the forward andbackwarddirections (Dtf/b). This procedure was repeatedfor left and right helicities, as well as for fiveconsecutive measurements of each enantiomer.The delays measured from opposite helicities oropposite enantiomers have opposite signs, re-vealing that the differential f/b ionization delayis a genuine chiral observable. To extract themost accurate value of the differential delay,we averaged the results obtained from the (+)and (−) enantiomers: Dtf/b = [Dtf/b(+) − Dtf/b(−)]/2 (see SM).First, we used a linearly polarized UV ionizing

field and a circularly polarized IR measurementfield. The Wigner component tW of the sideband

delay was thus f/b symmetric (Dtf/b = Dtccf/b). The

results shown in Fig. 3A reveal that the differen-tial f/b continuum-continuum induced delay iszero (within the 2-as accuracy of the presentmea-surement). This means that the laser-inducedtransitions produce essentially the same delay onelectrons emitted in the forward and backwarddirections, without any substantial sensitivity tothe chiral character of the ionic potential. How-ever, a weak influence of the chiral potential is stillnoticeable in this polarization configuration: Theintensity of SB2 averaged over all UV-IR delaysshows a f/b asymmetry (PECD) on the order of0.5% (two times smaller than when the UV iscircularly polarized).To find signatures of chirality in the photo-

ionization delays, we resolved the angular depen-dence of the photoionization dynamics (14, 37, 38).We integrated the photoelectron signal in slices of10° around different ejection angles a from thepolarization plane of the light and thenmeasuredthe associated delays (Dtcc

f/b). The results for SB2are shown inFig. 3B. For electrons emitted beyond70°, the signal was too low to extract reliable val-ues. A weak but nonzero Dtcc

f/b is measured whenelectrons are ejected close to the polarizationplane of the IR laser, reaching 5 ± 2 as at a = 25°.This delay tends to vanish for higher ejectionangles, but the error bars become larger becauseof the lower level of signal. Measurements on SB3show a zero delay, regardless of the ejection angle(see SM). In the commonly accepted intuitivepicture of multicolor photoionization, the linearUV field induces bound-free transitions starting

Beaulieu et al., Science 358, 1288–1294 (2017) 8 December 2017 3 of 6

Fig. 3. Forward/backwarddifferential delays innonresonant photo-ionization of camphor.(A) When the UV fieldis linearly polarizedand the IR field isleft–circularly polarized,the differential delayDtcc

f/b is zero for SB2and SB3. (B) Angularlyresolving the differentialdelay shows that itreaches nonzero valuesfor electrons ejectedclose to the laserpolarization plane (forSB2). (Inset) Angulardistribution of thenormalized SB2 inten-sity. (C) When theUV field is left–circularlypolarized and the IRfield is linearly polarized,the differential delayDtW

f/b is nonzero forSB2. (D) Angular resolu-tion of the differentialdelays from SB2. Errorbars are defined as 95%confidence intervals.

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from themolecular core region, and theIR field subsequently drives continuum-continuum transitions while the elec-tron is escaping from the core region.The continuum-continuum transitionsare thus rather insensitive to thedetailsof the molecular potential. This inter-pretation was recently confirmed bythe observation of a zero continuum-continuum delay between electronsescaping the two sides of an orientedasymmetric molecule in theoreticalcalculations (17). The zero overall de-lay wemeasured between forward andbackward electrons on SB2 and SB3 isconsistentwith thispicture.Nevertheless,the angle-resolvedmeasurementsdem-onstrate that the continuum-continuumtransitions can be slightly influenced bythe core (chiral) region of the potential.Second, we broke the f/b symmetry

in the ionization step by switching thepolarization state of the ionizing UVfield to circular while using a linearlypolarized IR field. In this case, the pho-toionization process is intrinsically f/basymmetric, whereas the continuum-continuum coupling is f/b symmetric:Dtf/b = DtW

f/b. The magnitude of thetemporally averaged PECD on SB2 waslarger than in the previous configu-ration, reaching 1%.Themeasurements(Fig. 3C) show a differential delay ofDtW

f/b = 7 ± 2 as for the second side-band (SB2). Our experiment is thusable to reveal a small f/b asymmetryin the Wigner delay in the photoion-ization of chiral molecules by circu-larly polarized light. TheDtf/b vanishesfor SB3 because of the decrease of boththe f/b asymmetric character of thephotoionization and the absolute Wigner timedelaywith increasingphotoelectron kinetic energy.The evolution of the differentialWigner delays

with photoelectron ejection angle are shown inFig. 3D. Close to a = 0°,DtW

f/b is null, which is notsurprising because the PECD also vanishes in thelaser polarization plane. For electrons emittedin the 60°-to-70° slice, DtW

f/b reaches 24 as. Thisangle-resolved analysis shows that although theaverage difference between forward and back-ward electron ionization times is only 7 as, itstrongly varies with ejection angle and can reachhigher values for electrons emitted away from thelaser polarization plane. Repeating this analysisfor SB3 shows that the differential Wigner timedelay remains zero within the error bars for allphotoelectron ejection angles (see SM). By access-ing the angular dependence of the emission time,our measurements give access to the phase prop-erties of the photoionizationmatrix elements. Thedetermination of the underlying scattering phaseshifts has been a long-standing quest of photo-ionization experiments, and our results show thattheir energy derivatives (i.e., the Wigner delays)are accessible with high accuracy through the useof a relatively simple setup.

The differential Wigner delay is a signature ofthe asymmetric scattering process at the heart ofthe photoelectron circular dichroism phenome-non.Wigner delays are determined by the energyderivative of the scattering phase. We performeda theoretical analysis of the photoionization ofcamphor molecules (see SM for details about thetheoretical model). Our calculations confirm theexistence of asymmetric Wigner delays, even in arandomly oriented ensemble of molecules. Thetheoretical f/b differential Wigner delay DtW

f/b ison the order of 5 as for 2-eV electrons, in agree-ment with the present experimental observa-tion. The evolution of DtW

f/b with respect to thephotoelectron energy, shown in the SM, revealsrich spectroscopic features that are not visiblein the photoelectron spectrum and cannot beeasily distinguished in the PECD signal. Thus,DtW

f/b is a sensitive chiral observable, whichenables tracking of subtle features of the mo-lecular potential (such as the differential Cooperminima) that survive molecular orientationaveraging (see SM). This observation opensprospects for highly sensitive experiments—for instance, through molecular-frame mea-surements (39)—as well as accurate testing

of advanced quantum theories ofmolecular photoionization.The differential f/b analysis and the

control of the chiral symmetry-breakingof the interaction enabled us to fullydecouple the different components ofthe photoionization delays, withoutthe need for any theoretical input,and to reveal a tiny but measurabledelay in the direct photoionization.We proceeded to use photoelectroninterferometry to extract more com-plex dynamics occurringwhen the chi-ral molecules are photoionized in thevicinity of an autoionizing resonance.

Resonant photoionization

Continuum resonances play an essen-tial role in the photoionization ofmostpolyatomicmolecules. They can arisefrom the shape of the molecular po-tential, in a single-electron picture(shape resonances) (40), or frommulti-electron dynamics involving electroncorrelations and couplings betweendifferent channels (19). In both cases,spectrally localized scattering phasejump(s) are expected, reflecting themodification of the ionization dynam-ics. For instance, in chiral molecules,the PECD was recently shown to beenhanced in the vicinity of a Fano res-onance (41).Weused the photoelectroninterferometry technique to directlytrack the asymmetric ionization dy-namics of chiral molecules in the vici-nity of an autoionization resonance.As shown in Fig. 2D, SB1 presents

a sharp p-phase jump around 1.9 eV,which reflects the presence of a res-onance on the first ATI peak. We re-

solved this phase jump in the forward (blue, ϕf )and backward (red, ϕb ) directions, using circu-larly polarized UV (Fig. 4A) or IR (Fig. 4C) light.When the UV light is circularly polarized, thespectral phase exhibits a weak ∼0.75-rad bumpcentered around 2.1 eV. A notable difference wasobserved between the forward and backwardspectral phases. This differenceðDϕf=b ¼ ϕf �ϕbÞ(Fig. 4B) shows a good mirroring between the twoenantiomers.The case where the f/b symmetry is broken by

the weak IR pulse (linear UV and circular IR) ismore intriguing. The spectral phases show a steep∼p jump around 1.9 eV, in opposite directions forforward and backward electrons. After the jump,the phases become nearly identical, as they areseparated by ∼2p (Fig. 4C). The f/b differentialphases ðDϕf=bÞ obtained in the two enantiomersare almost exactly opposite (Fig. 4D). To a first ap-proximation, the presence of this huge asymmetryis unexpected. The circularly polarized field actsduring the continuum-continuum transitions,which should be affected mostly by the long-range (nonchiral) part of the molecular potentialand should, therefore, be f/b symmetric (17). Ourmeasurement demonstrates that in the vicinity

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Fig. 4. Phase-resolved resonant photoionization in camphor.(A and C) Spectral amplitudes (black) and forward and backward

spectral phases [ϕfðEÞ, blue; ϕbðEÞ, red] of SB1 in (1R)-(+)-camphor,using left–circularly polarized UV field and linearly polarized IR field in(A) and linearly polarized UV field and left–circularly polarized IRfield in (C). (B and D) Forward/backward (f/b) asymmetry of thespectral phase (Dϕf/b) in camphor, using left–circularly polarized UVfield and linearly polarized IR field in (B) and linearly polarized UV fieldand left–circularly polarized IR field in (D). In (B) and (D), the dotsrepresent the mean values of the f/b asymmetry of the spectral phase,whereas the solid lines show the error bars, which are defined as95% confidence intervals.

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of a resonance, the f/b symmetry canalsobebrokenduring the continuum-continuumtransitions. Thisfinding is in agreement with a recent theoreticalinvestigation of photoelectron interferometry,which demonstrated that the simple separationof the measured delay (t), in a sum of the con-tributions fromWigner (tW) and continuum-continuum (tcc) delays, no longer held in thepresence of a resonance (42). Argenti et al. dem-onstrated that tW and tcc are entangled in reso-nant photoionization. The measured delay (t) isrepresentative of the two-color photoionizationprocess, and our results show that the circularpolarization of the weak IR field is sufficient toinduce a major symmetry-breaking, in the pres-ence of a resonance.

Asymmetric electron wave packets

We could retrieve the temporal profiles of thetwo-color forward and backward wave packetsby Fourier-transforming their measured spectralamplitudes and phases. To extract the angulardependence of these temporal profiles, we ana-lyzed the oscillations of SB1 as a function of the

electron ejection angle (a), as done previouslyin the nonresonant case (see raw data in SM).Figure 5 shows the resulting angle-resolved wavepackets in the temporal domain. In both polar-ization configurations, the wave packet showsa single temporal peak when the electrons areejected close to the propagation axis of the light(90°) and a double-peak structure when theelectrons are ejected near the laser polariza-tion plane (0°). The latter are signatures of thetemporal interference between the direct non-resonant and the resonant components of theautoionizing wave packets (22). The effect ofthe resonance appears more confined aroundthe laser polarization direction when the UVfield is linearly polarized (Fig. 5B), probably be-cause of a stronger anisotropy of the resonantexcitation compared with the circularly polar-ized case.The chiral nature of the photoionization pro-

cess can be investigated by comparing cuts of thetemporal profile of electrons ejected at positive(forward, blue) and negative (backward, red) an-gles (a) (Fig. 5). When the UV field is circularly

polarized, the two bumps of the forward electronwave packet emitted around 30° maximize ∼400as after the backward wave packet (Fig. 5A). Asimilar delay is measured around 60°, where thewave packet shows a single-peak structure. As theejection angle further increases, the orderingbetween forward andbackward emission reverses,with a ∼−250-as delay around 80°. These subtlefeatures obey chiral inversion when switchingfrom one enantiomer to the other, as shown in theSM. In the other polarization configuration (Fig.5B), the two bumps from the forward andbackwardwave packets are synchronized in timefor electrons ejected close to the laser polariza-tion plane (a = 0°). However, their relative yieldis strongly f/b asymmetric. This means that inthe vicinity of resonances, where tW and tcc areentangled (42), the perturbative IR pulse can beused to break the f/b symmetry and to subse-quently tailor asymmetric electronic wave pack-ets, both in time and space. At larger emissionangles, where the dynamics are governed by asingle nonresonant pathway, the single-peakwavepackets become f/b symmetric. This analysis pro-vides a deep insight into the angular depen-dence of the multielectron dynamics governingautoionization.Comparison of the two polarization configu-

rations used in the measurements (Fig. 5) revealsthat thewave packet asymmetry ismuch strongerwhen the weak IR field is circularly polarized.Weattribute this result to the sequential nature ofthe resonant photoionization process. The lin-early polarized UV photons populate a quasi-bound state embedded in the continuum, whichcan relax by ionization, releasing electrons at theenergy of the first ATI peak (autoionization)(Fig. 2C). However, another process could lead toionization of the quasi-bound state: the absorp-tion of one IR photon releasing an electron withthe energy of the first sideband. This can be seenas a classic PECD experiment, starting from ahighly excited quasi-bound state. Recent experi-ments showed that PECD could be observedwhenbound states excited by linear photons wereionized by circularly polarized photons (5). Thepresent scheme extends this scenario to quasi-bound states. On the other hand, when the UVphotons are circularly polarized, they can inducean asymmetric wave packet in the excited states,a phenomenon called photoexcitation circular di-chroism (43). The ionization of such a wave packetby linearly polarized light produces f/b asymme-tries, but theywere observed to beweaker than thePECD from excited states. This could explain whyweobserve aweakerwavepacket asymmetrywhenthe IR photons are linearly polarized.

Time-frequency analysis

The temporal profile of the wave packets onlyprovides spectrally integrated information aboutthe rich ongoing dynamics. The spectral originof temporal asymmetries can be revealed via atime-frequency analysis (44). The Wigner-Villedistribution (WVD) is particularly noteworthybecause it encodes the quantum interferencebetween different components of a wave packet.

Beaulieu et al., Science 358, 1288–1294 (2017) 8 December 2017 5 of 6

Fig. 5. Angle-resolved temporal profile of the autoionizing photoelectron wave packet. Thecontour plot depicts the temporal profile of the wave packet as a function of the electron ejectionangle. The upright external plots show forward (blue) and backward (red) cuts of the wave packetsalong specific angles. (A) Left–circularly polarized UV field and linearly polarized IR field. (B) Linearlypolarized UV field and left–circularly polarized IR field.

Fig. 6. Wigner-Ville distributions (WVDs) of the autoionizing photoelectron wave packets.Distributions were calculated on an average wave packet obtained by summing the wave packetfrom (1R)-(+)-camphor and the mirrored wave packet from (1S)-(−)-camphor. (A) WVD of aforward wave packet emitted around a = +10° from the linearly polarized UV field. (B) WVD of abackward wave packet emitted around a = −10°. (C) WVD of the forward/backward differentialwave packet.

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For a wave function Y(t), the WVD [W(W, t)] isdefined as

W ðW; tÞ ¼ ∫Yðt � t=2ÞYðt þ t=2ÞeiWtdt ð2Þ

where W is the angular frequency, t is the time,and i is the imaginary unit. Figure 6 shows theWVD of the electron wave functions emittedaround a = +10° [forward, Yf(t)] and a = −10°[backward, Yb(t)] from the laser polarizationplane. The distributions were calculated by aver-aging thewave packet from (1R)-(+)-camphor andthe mirrored wave packet from (1S)-(−)-camphor.A strong negative lobe is present around timet = 0 at the energy of the resonance (1.9 eV),revealing the quantum interference betweenthe direct and indirect ionization components.Hyperbolic fringes converging to the energy ofthe resonance are observed in the leading orfalling edge of the distribution, depending onthe f/b emission direction. The asymmetry ofthese fringes reflects an asymmetric destruc-tive interference between wave packet compo-nents. To isolate the asymmetric part of the wavefunction, we calculated the WVD of the differ-ential wave function DYf/b(t) = Yf(t) − Yb(t)(Fig. 6C). It is strikingly simple, with a tem-porally long and spectrally narrow signal at theenergy of the resonance and almost no negativecomponents. We conclude that although theforward and backward wave packets are eachformed by the coherent superposition of a reso-nant and a nonresonant contribution, the chi-ral character of the wave packet appears to bestrongly dominated by a single resonant path-way, which leads to the disappearance of thesignature of quantummechanical interference inthe WVD. The WVD thus provides insight intothe origin of the asymmetric shaping of photo-electron wave packets during resonant photo-ionization of chiral molecules.

Conclusions and perspectives

Our results show that using circularly polarizedphotons to drive photoionization of chiral mole-cules induces asymmetric delays in the photo-emission, on both femtosecond and attosecondtime scales. In direct nonresonant photoioniza-tion, the f/b asymmetry in the Wigner time de-lays are on the order of few attoseconds. In the

vicinity of an autoionizing resonance driven byelectronic correlation, we have found that theemitted two-color electronwave packet is strong-ly asymmetric, demonstrating the chiral char-acter of this multielectronic effect. By usingthe synergies of molecular and light chiralityin the vicinity of resonances, we demonstratedtailoring of the shape of the released electronwave packets, both in time and space, which is anew scheme for multidimensional attosecondquantum control. The high accuracy of the mea-surements can also be used as a powerful bench-marking tool for quantum theories of molecularphotoionization. Finally, our approach could begeneralized to a broad variety of systems toshed light on the ultrafast symmetry-breakings,which are at the heart of very recent technologicaland scientific breakthroughs, such as in super-conducting chiral nanotubes (45) and chiralspintronics devices (46).

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ACKNOWLEDGMENTS

We thank R. Bouillaud and L. Merzeau for technical assistance andA. L’Huillier for fruitful discussions. This project has received fundingfrom the European Research Council under the European UnionHorizon 2020 research and innovation program 682978 - EXCITERSand from LASERLAB-EUROPE, grant 284464, the EuropeanCommission’s Seventh Framework Programme. We acknowledgethe financial support from the French National Research Agencythrough grants ANR-14-CE32-0014 MISFITS, ANR-14-CE32-0010XSTASE, and IdEx Bordeaux LAPHIA (ANR-10-IDEX-03-02) and theConseil Regional de Nouvelle-Aquitaine (2.1.3-09010502 COLA2project). J.C. and R.T. acknowledge financial support from theLABEX PlasaPar project, the program “Investissements d’Avenir”under the reference ANR-11-IDEX-0004-02, and the programANR-15-CE30-0001-01-CIMBAAD. S.B. acknowledges the NaturalSciences and Engineering Research Council of Canada VanierScholarship. The data presented in the study are available inthe SM.

SUPPLEMENTARY MATERIALS

www.sciencemag.org/content/358/6368/1288/suppl/DC1Supplementary TextFigs. S1 to S15References (47–62)Data S1

3 August 2017; accepted 10 November 201710.1126/science.aao5624

Beaulieu et al., Science 358, 1288–1294 (2017) 8 December 2017 6 of 6

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Attosecond-resolved photoionization of chiral molecules

G. Porat, T. Ruchon, R. Taïeb, V. Blanchet and Y. MairesseS. Beaulieu, A. Comby, A. Clergerie, J. Caillat, D. Descamps, N. Dudovich, B. Fabre, R. Géneaux, F. Légaré, S. Petit, B. Pons,

DOI: 10.1126/science.aao5624 (6368), 1288-1294.358Science 

, this issue p. 1288Sciencetrajectories that spanned up to 24 attoseconds.mirror-image isomers of camphor with circularly polarized light, which revealed the angle-dependent delays betweeninterferometric approach to uncover the temporal dynamics associated with this asymmetry. They probed the

used anet al.asymmetric distribution of trajectories as electrons are ejected during photoionization. Beaulieu circulate like a left- or right-handed corkscrew. Matches or mismatches in this mutual handedness give rise to an

Just as the atoms in a molecule can be arranged in a left- or right-handed manner, the field in a beam of light canClocking departures from chiral origins

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MATERIALSSUPPLEMENTARY http://science.sciencemag.org/content/suppl/2017/12/06/358.6368.1288.DC1

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