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Femtosecond Time and Angle-Resolved Photoelectron Spectroscopy of Aqueous Solutions

Toshinori SuzukiKyoto University

photoelectron

Electron dynamics in water 　　　　　　　　　　　　　　

Pure waterH-Bond Network

・ Elucidation of Hydration around solutes・ Chemical Reaction・ Real-time dynamics

Quantum Chemistry, MD Simulation

Change of H-bonding

Biomolecule & Water

Water and Hydration

Energy Conversion

Life Innovation

Green Innovation

Photoelectric effect

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Kinetic Energy = hn – eBE eBE: electron binding energy

Sample

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Example: Charge Transfer from DABCO to Water

0.1M DABCO

hvprobe

260nm(4.77eV)

hvpump

226nm

0

eBE S2

electron kinetic energy

Dt

hydratedelectron

vacuum level

S0

Time-Resolved Photoelectron Spectroscopy of Aqueous Solutions

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e-

liquid beam

detectorflight tube ~ 1m

femtosecondlaser pulses

Magnet

e-

TOF e- energy analyzer

liq. N2 trap

nozzle

detector

Time-Resolved Photoelectron Spectroscopy of Aqueous Solutions

10-4 Torr

10-6 Torr

50% collection efficiency

f0.5 mm aperture

TMP

TMP

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Example: Charge Transfer from DABCO to Water

0.1M DABCO

hvprobe

260nm(4.77eV)

hvpump

226nm

0

eBE S2

electron kinetic energy 3.4 eV: Signature of hydrated electron

Dt

hydratedelectron

vacuum level

S0

Electronic transitions of hydrated DABCO has characters of charge transfer to solvent

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S1

S2, S3, S4

Yamamoto et al., Phys. Rev. Lett. 112, 187063 (2014)

p Rydberg

s Rydberg

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Angle-Resolved Measurement

Rotating detection axis or Rotating polarization axis

This Work

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Time and Angle-Resolved Measurement

Low collection efficiency

probe

analyzer

100 kHz laser

Rotating polarization

No magnet

10-3 str detection solid angle

Phys. Rev. Lett. 112, 187063 (2014)

1°

f 40 mm

1.2m

100 fs 200 fs 300 fs 3 ps (hydrated electron)

Angle-resolved experiment: 4 selected time delays

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Polarization dependence of photoelectron spectrum @100fsAqueous DABCO solution

eBE (eV)4321

1.0

0.5

0.0

4321

1.0

0.5

0.0

43210-1

1.0

0.5

0.0

0 10 20 30 40 50 60 70 80 90

S(E,q

) (a

rb. u

nits

)

4321

1.0

0.5

0.0

4321

1.0

0.5

0.0

4321

1.0

0.5

0.0

4321

1.0

0.5

0.0

4321

1.0

0.5

0.0

4321

1.0

0.5

0.0

4321

1.0

0.5

0.0

4321

1.0

0.5

0.0

e-

probe

pump 　　

detection axis

@ 100 fs

anisotropic

Strong photoemission parallel to the probe laser polarization at low eBE

parallel

vertical

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eBE (eV)

S(E,q

) (a

rb.

units

)Time evolution

Dt = 100 fs Dt = 200 fs

Dt = 300 fs Dt = 3 ps

43210-1

1.0

0.5

0.0

0 10 20 30 40 50 60 70 80 90

anisotropic

parallel

vertical

hydrated electron

Phys. Rev. Lett. 112, 187063 (2014)

Elastic and inelastic scattering reduces photoemission anisotropy

e-

species at the surfacespecies in the bulk

2~5 nm

probing depth

e-

anisotropy is reduced by elastic scattering by water.

e-

anisotropy is observed

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DABCO is segregated on liquid surface

Probing depth is limited.

hydrophobic

160

140

120

100

80

60

40

20

0

409408407406405404403402

DABCO

NH4+

eBE (eV)Pho

toel

ectr

on s

igna

ls (

arb.

uni

ts)

N1s

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NH4+ is not surface active.

Gopalakirishnan et al., J. Phys. Chem. B, 109, 8861 (2005)

DABCO has an enhanced molecular density on the liquid surface.

More signal Less signal

Soft X-ray photoelectron spectrum

Our electron scattering calculations on DABCO with 64 water molecules predicts isotropic photoemission

DABCO should be on the liquid surface

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Phys. Rev. Lett. 112, 187063 (2014)

isotropic

anisotropic

anisotropic

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CTTS reaction occurs to the bulk side

probe laser polarization

e-

e-aq

e-

CTTS reaction

Summary

The first time- and angle-resolved photoelectron spectroscopy of aqueous solutions has been demonstrated.

Photoemission was anisotropic from chemical species near the surface.

No evidence was found for an electron trapped on liquid water surface.

Coworkers & Funding

Yo-Ichi Yamamoto Yoshi-Ichi Suzuki

Shutaro Karashima Takuya Horio

Gaia Tomasello (Wurzburg) Roland Mitric (Wurzburg)

JST-CREST