We have studied ultrafast nonadiabatic dynamics of excess electrons trapped in the band gap of liquid water using time- and angle-resolved photoemission spectroscopy. Anisotropic photoemission from the first excited state was discovered, which enabled unambiguous identification of nonadiabatic transition to the ground state in 60 fs in H2O and 100 fs in D2O. The photoelectron kinetic energy distribution exhibited a rapid spectral shift in ca. 20 fs, which is ascribed to the librational response of a hydration shell to electronic excitation. Photoemission anisotropy indicates that the electron orbital in the excited state is depolarized in less than 40 fs

Karashima, Shutaro

Suzuki, Toshinori

Yamamoto, Yo Ichi

Physical Review Letters

English

Kyoto University Research Information Repository

TitleResolving Nonadiabatic Dynamics of Hydrated ElectronsUsing Ultrafast Photoemission AnisotropyAuthor(s)Karashima, Shutaro; Yamamoto, Yo Ichi; Suzuki, ToshinoriCitationPhysical Review Letters (2016), 116(13)Issue Date2016-04-01URL http://hdl.handle.net/2433/218322Right© 2016 American Physical Society.Type Journal ArticleTextversionpublisherKyoto UniversityResolving Nonadiabatic Dynamics of Hydrated ElectronsUsing Ultrafast Photoemission AnisotropyShutaro Karashima, Yo-ichi Yamamoto, and Toshinori Suzuki*Department of Chemistry, Graduate School of Science, Kyoto University, Kitashirakawa-Oiwakecho,Sakyo-Ku, 606-8502 Kyoto, Japan(Received 21 June 2015; revised manuscript received 30 January 2016; published 1 April 2016)We have studied ultrafast nonadiabatic dynamics of excess electrons trapped in the band gap of liquidwater using time- and angle-resolved photoemission spectroscopy. Anisotropic photoemission from thefirst excited state was discovered, which enabled unambiguous identification of nonadiabatic transition tothe ground state in 60 fs in H2O and 100 fs in D2O. The photoelectron kinetic energy distribution exhibiteda rapid spectral shift in ca. 20 fs, which is ascribed to the librational response of a hydration shell toelectronic excitation. Photoemission anisotropy indicates that the electron orbital in the excited state isdepolarized in less than 40 fs.DOI: 10.1103/PhysRevLett.116.137601A living cell exposed to high-energy particles or radiationundergoes ionization of cell water to generate H2Oþ and aquasifree electron. The former transfers a proton to a neutralwater molecule, H2Oþ þ H2O → H3Oþ þ OH, to producean OH radical, which reacts with DNA to induce strandbreaks. The latter undergoes dissociative attachment tomolecules within its picosecond lifetime before beingtrapped in the band gap of liquid water. Thus, the dynamicsof electrons in liquid water are of great importance inradiation chemistry and biology [1]. It is also of great interestin the study of quantum physics in soft matter [2–5].The simplest picture of an electron trapped in liquidwater [a hydrated electron; e−ðaqÞ] is a charged particle in aspherical dielectric cavity [6]. This widely accepted picturehas been challenged [7]; however, recent calculations usingdensity functional theory (DFT) suggest that e−ðaqÞ isindeed a cavity state, although the probability density of anexcess electron strongly penetrates into surrounding watermolecules [8–10]. The one-electron wave function of anexcess electron in the ground state (GS) is nodeless like ans orbital, while those of the triply degenerate excited state(ES) have a single node similar to that of p orbitals. The GSis 3.3 eV lower in energy than the vacuum level [11–14],and the ES, which mediates electron trapping from theconduction band, is ca. 1.7 eV higher in energy than theGS. Previous experimental studies [15–18] have shown thatthe ES undergoes a nonadiabatic transition to the GS within1 ps; however, neither an accurate time scale nor themechanistic details have been established [6]. One of thedifficulties faced in previous experiments [15–18] was thatthe ES and GS could not be resolved clearly using transientabsorption and photoemission spectra. Here, we present analternative experimental approach that allows us to resolvethe nonadiabatic dynamics of e−ðaqÞ using time- and angle-resolved photoemission spectroscopy [19].If we use an analogy with photoemission from an atom inthe gas phase, we may anticipate different photoemissionanisotropy for the GS and ES of e−ðaqÞ, because the formercreates a p partial wave while the latter creates s and dwaves. This, however, is too simplistic a view, because aphotoelectron undergoes elastic scattering with solventmolecules prior to emission from the surface. We haveshown in our previous study [19] that, despite its strong scharacter, photoemission from the GS is isotropic, and wehave attributed this to the elastic scattering of photoelec-trons in bulk water. Based on this result, photoemissionfrom the ES may also be expected to be isotropic.Nonetheless, this expectation is wrong, as we present here.In our experiment, an aqueous NaBr solution wasdischarged into a photoelectron spectrometer from a fusedsilica capillary (inner diameter of 15 μm) at a flow rate of0.18 mL=min to create a laminar flow. Three laser pulsesilluminated the laminar flow 1 mm downstream from thenozzle at a repetition rate of 50 kHz. The liquid temperatureis estimated around 278 K. The first pulse (ω1, 200 nm,3–10 nJ) generated e−ðaqÞ in the GS via a charge-transfer-to-solvent reaction from Br− to bulk water. The secondpulse (ω2, 700 nm, 240–320 nJ) optically excited e−ðaqÞ tothe ES. The third pulse (ω3, 350 nm, 60–100 nJ) inducedphotoemission of e−ðaqÞ from the ES and highly vibra-tionally excited states in the GS; the photon energy of ω3(3.54 eV) was only slightly larger than the vertical bindingenergy (VBE) of e−ðaqÞ, so that almost no photoemissionwas induced from the GS at equilibrium. The cross-correlation between ω2 and ω3 pulses was estimated tobe 55–80 fs, depending on the condition of the laser. Theω2 pulses were delayed with respect to ω1 by 200 ps tothermalize e−ðaqÞ prior to the pump (ω2) and probe (ω3)measurements. The polarization direction of ω1 wasperpendicular to the electron detection axis of a time-of-flight (TOF) electron energy analyzer in all measurements.The polarization directions of the ω2 and ω3 pulses,indicated by θ2 and θ3 in Fig. 1(b), were variedPRL 116, 137601 (2016) P HY S I CA L R EV I EW LE T T ER Sweek ending1 APRIL 20160031-9007=16=116(13)=137601(5) 137601-1 © 2016 American Physical Societyindependently using half wave plates. In measurements ofphotoemission anisotropy, the photoionization region of theapparatus was maintained field-free, and photoelectronswere sampled using a 1-mm diameter skimmer placed at2 mm from the liquid surface. The TOF energy analyzerhad an electron flight path of 1.2 m, and electrostatic lensesplaced in the flight tube collimated the electron trajectoriesand directed them to the detector. Thus, the detection solidangle (0.18 sr) was primarily determined by the entranceskimmer. The energy resolution of the analyzer was40 meV. The TOF spectrum was measured using amultichannel scaler. In the following discussion, thepump-probe delay time (Δt23) refers to the time intervalbetween pulses ω2 and ω3. We also performed angle-integrated measurements by adding a SmCo permanentmagnet in the ionization region and a solenoid in the flighttube to construct a magnetic bottle [20].In order to understand the time evolution of the photo-electron kinetic energy (PKE) distribution, let us firstexamine the angle-integrated results. Figure 2 presentstwo-dimensional maps of the PKE distributions measuredas a function ofΔt23 for e−ðaqÞ in (a) H2O and (b) D2O; thetime axis is logarithmic. In both cases, a high PKEcomponent (1.5–2.5 eV) appears immediately after theω2 pulse and decays in about 100 fs, and a low PKEcomponent with a long lifetime of about a picosecond isalso present. The entire dynamics are slower in D2O thanin H2O, indicating that electron dynamics are strongly cou-pled with the vibrations of water molecules. Figures 2(c)and 2(d), respectively, show the global fit for the data inFigs. 2(a) and 2(b) using the following function,fðtÞ ¼XNj¼1CjFðtÞ þ Bexp½t=τ−1⊗ gðtÞ;FðtÞ ¼0 ðt < tdÞexp½−ðt − tdÞ=τj ðtd ≤ tÞ ; ð1Þwhere t is Δt23, C and B are expansion coefficients, and τjand τ−1 are the time constants in the positive and negativetime range, respectively. gðtÞ is a Gaussian-shaped cross-correlation function between ω2 and ω3. The signal in thenegative time region is due to a small background causedby the ω3 − ω2 pulse sequence. Careful examination ofFig. 2(a) reveals that the peak of the PKE distributionrapidly downshifts within 20 fs due to the vibrational wavepacket dynamics of the hydration shell. To express this fastevolution, we introduced the term td into Eq. (1), which willbe discussed in more detail later. The observed time profileswere well reproduced using Eq. (1) with N ¼ 2 and thefollowing best-fit parameters: τ1 ¼ 60 10, τ2 ¼ 52030, and τ−1 ¼ 180 60 fs for H2O and τ1 ¼ 100 20,τ2 ¼ 740 130, and τ−1 ¼ 100 20 fs for D2O. The τ1and τ2 time constants obtained from our analysis are inFIG. 1. (a) Schematic energy diagram and (b) experimentalgeometry.FIG. 2. Two-dimensional map of the photoelectron kineticenergy distribution measured as a function of Δt23 for e−ðaqÞin (a) H2O and (b) D2O. The global fits to these experimental dataare presented in (c) and (d), and the differences between the dataand the fits are shown in (e) and (f), respectively. A constant hasbeen added to the actual delay (tplot ¼ ttrue þ 150 fs) to shift theentire distribution and display the data around t ¼ 0. The timelabel and grids are presented for ttrue. θ2 and θ3 were 0° and 90°,respectively. The cross-correlation time between ω2 and ω3was 75 fs.PRL 116, 137601 (2016) P HY S I CA L R EV I EW LE T T ER Sweek ending1 APRIL 2016137601-2reasonable agreement with previous reports [15–18]. Thedifference between the experimental data and the globalfits are shown in Figs. 2(e) and 2(f) for H2O and D2O,respectively.Figure 3 shows decay-associated spectra at Δt23 ¼ 40and 400 fs obtained by the global fit shown in Fig. 2. The τ1component observed in the high PKE region is readilyassigned to ES, as it appears immediately after the pumppulse, while the τ2 component at low PKE can be assignedto either the vibrationally relaxed ES or the highly vibra-tionally excited GS. The assignment of the τ2 component isthe key question to address in this study.Figure 4 presents time- and angle-resolved photoemis-sion spectra of e−ðaqÞ in H2O at (a) Δt23 ¼ 0, (b) 40,(c) 80, and (d) 400 fs. Upper and lower panels show theresults obtained using θ2 being 0° and 90°, and each of eightfigures shows three PKE distributions measured using θ3being 0° (black), 54° (green), and 90° (blue). The photo-emission intensity at short time delay is stronger forθ3 ¼ 0° than 90°, indicating that photoemission preferen-tially occurs along the electric field direction of the probepulse. This accords with classical electrodynamics, whichis seen most typically in photoemission. The θ3 dependencevanishes at 400 fs, when the τ2 component dominates.Figure 4 clearly shows that the photoemission anisotropy isassociated with the τ1 component (ES).The observed distributions can be simulated using thedecay-associated spectra in Fig. 3 and anisotropy param-eters assumed for the τ1 and τ2 components. The angulardependence, Iðθ3Þ, upon two-photon photoemission isgenerally expressed asIðθ3Þ ∝ 1þ β2P2ðcosθ3Þ þ β4P4ðcosθ3Þ; ð2ÞwhereE is the PKE, βn is an expansion coefficient, andPn isthe nth order Legendre polynomial. In our analysis, wehave neglected the β4 term, which is usually small.The solid lines in both the upper and lower panels ofFigs. 4(b)–4(d) are distributions simulated by assumingβ2 ¼ 0.16 and 0, respectively, for the τ1 and τ2 components,which are in excellent agreement with the experimentaldata. The difference in photoemission anisotropy betweenthe τ1 and τ2 components suggests that they correspond todifferent electronic states. Since the τ1 component corre-sponds to the ES and photoemission from the GS is knownto be isotropic, the τ2 component is assigned to a highlyvibrationally excited state in the GS. Thus, we concludethat e−ðaqÞ inH2Oundergoes a nonadiabatic transition fromthe ES to the GS in 60 10 fs.Why does the ES exhibit photoemission anisotropy, evenif the GS does not? We conjecture that it originates from alarger electron density for the ES at the top molecular layerof the liquid. Uhlig et al.have calculated the spin density inthe GS and ES of e−ðaqÞ near the water surface [21], andthey showed that the spin density in the inner cavity is 40%in the GS while it is less than 7% in the ES. In contrast, inthe gas phase, the fraction is 11% in the GS, while it is20%–27% in the ES [21]. Although these fractions willstrongly fluctuate with thermal motion of the hydrogen-bonding network in liquid water, a more spread-outelectron distribution in the ES than in the GS is alwaysFIG. 3. Decay-associated spectra of τ1 (blue, dashed line) andτ2 (green, dashed-dot line) components and their sum (black,solid line) at Δt23 values of (a) 40 and (b) 400 fs determined byglobal fitting using Eq. (1) with N ¼ 2 for e−ðaqÞ in H2O.FIG. 4. Photoelectron kinetic energy distributions observed fore−ðaqÞ in H2O forΔt23 ¼ 0 values of (a) 0, (b)þ40, (c)þ80, and(d) þ400 fs. θ2 is 0° and 90° for the upper and lower panels,respectively. The black, green, and blue colors correspond,respectively, to θ3 of 0°, 54°, and 90°. The dots and error barsrepresent the experimental data and their standard deviation,while the solid lines are simulations using the decay associatedspectra and the anisotropy parameter (β2) assumed for the τ1component. The anisotropy parameter of the τ2 component wasassumed to be zero. The cross-correlation time between ω2 andω3 was 55 fs.PRL 116, 137601 (2016) P HY S I CA L R EV I EW LE T T ER Sweek ending1 APRIL 2016137601-3predicted for a particle in an attractive potential with a finitebarrier. The β2 value of 0.16 is relatively low, becausephotoemission from e−ðaqÞ at larger depths becomesisotropic. The probing depth of photoemission spectros-copy is still under investigation [22–24]; however, it is onthe order of nanometers in the PKE region of 5 eV, which isgreater than the radius of gyration of e−ðaqÞ (0.25 nm).The linearly polarized ω2 pulses create orbital alignmentof the ES. It is anticipated that the p orbital alignmentperpendicular to the liquid surface is more effective than theparallel alignment to increase the excess electron densityat the top molecular layer and to enhance photoemissionanisotropy. Therefore, we initially employed θ2 ¼ 0 (theupper panels in Fig. 4). Then, we performed similarmeasurements for θ2 ¼ 90 (the lower panels in Fig. 4),which prepares the p orbital parallel to the liquid surface allaround the cylindrical liquid microjet. As seen in Fig. 4(a),the distribution at t ¼ 0 fs exhibits higher anisotropy forθ2 ¼ 0 than for 90. More specifically, the β2 values of the τ1component were 0.32 and 0.24 for θ2 ¼ 0 and 90,respectively. However, the distributions observed afterthe time delay of 40 fs [Figs. 4(b)–4(d)] were independentof θ2 and expressed using the anisotropy parametersβ2 ¼ 0.16 and 0 for the τ1 and τ2 components, respectively.The results indicate that the orbital alignment of the ESinfluences the photoemission angular anisotropy, and thatthe orbital is depolarized in less than 40 fs.Close examination of Fig. 2 reveals that the peak energyof the PKE distribution downshifts within 20 fs. In ouranalysis, we expressed the spectral shift by introducing tdin Eq. (1). The td values determined using the global fit(N ¼ 2) are shown in Fig. 5. The value of td is nearly zeroat the highest PKE and increases for lower PKE, indicatingthat the highest PKE signal is from the Franck-Condonregion in the ES, and the delayed appearance of the lowPKE signal is due to wave packet motion in the ES. Thedownshift in the PKE indicates that the potential gradientalong the reaction coordinate is greater in the ES than thefinal state of neutral water, so that the VBE of the ESincreases along this coordinate. The td value is systemati-cally greater for D2O; at a PKE of 1 eV, the maximum valueof td is ca. 17 fs in H2O, while it is 25 fs in D2O. Theisotope effect suggests that the ultrafast wave packetmotion responsible for the PKE shift is primarily librationof water molecules [5]. The photoelectron total intensitydoes not exhibit 10–20 fs time constants, so that there isnegligible population decay due to the ES-GS nonadiabatictransitions within this ultrafast solvent response time.As we have seen in Fig. 4, photoemission anisotropydiminishes within the same time scale.In conclusion, photoexcited e−ðaqÞ in water undergoesultrafast solvent response of a hydration shell and depo-larization of the p orbital followed by a nonadiabatictransition to the GS in sub-100 fs. The subsequent vibra-tional relaxation from highly vibrationally excited states inthe GS occurs in a subpicosecond time scale. This studyconfirms that the ES-GS nonadiabatic reaction time isshorter than the theoretical estimates [6] reported so far.This work was supported by JSPS KAKENHI GrantNo. 15H05753. Y. I. Y. is supported by the ResearchFellowship of Japan Society for the Promotion ofScience for Young Scientists. We thank M. Hayashi forcontribution in the early stage of this work and S. Adachifor technical assistance.*suzuki@kuchem.kyoto‑u.ac.jp[1] E. Alizadeh and L. Sanche, Chem. Rev. 112, 5578 (2012).[2] A. Migus, Y. Gauduel, J. L. Martin, and A. Antonetti, Phys.Rev. Lett. 58, 1559 (1987).[3] F. J. Webster, J. Schnitker, M. S. Friedrichs, R. A. Friesner,and P. J. Rossky, Phys. Rev. Lett. 66, 3172 (1991).[4] C. Silva, P. K. Walhout, K. Yokoyama, and P. F. Barbara,Phys. Rev. Lett. 80, 1086 (1998).[5] M. F. Emde,A.Bultuska, A.Kummrow,M. S. Pshenichnikov,and D. A. Wiersma, Phys. 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Resolving Nonadiabatic Dynamics of Hydrated Electrons Using Ultrafast Photoemission Anisotropy

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Resolving Nonadiabatic Dynamics of Hydrated Electrons Using Ultrafast Photoemission Anisotropy

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