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Observation of Ultrafast Charge Migration in an Amino Acid. Louise Belshaw. Queen’s University, Belfast. Observation of Ultrafast Charge Migration in an Amino Acid. Outline. Why biomolecules with attosecond lasers? Phenylalanine How: experimental pump – probe setup

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slide1

Observation of Ultrafast Charge Migration in an Amino Acid

Louise Belshaw

Queen’s University, Belfast

slide2

Observation of Ultrafast Charge Migration in an Amino Acid

Outline

Why biomolecules with attosecond lasers?

Phenylalanine

How: experimental pump – probe setup

Results with phenylalanine

Conclusions

slide3

Ultrafast Dynamics in Biomolecules

Why biomolecules with attosecond lasers?

Ultrafast Dynamics: responsible for many important, fundamental processes in biomolecules, for example:

  • excited energy redistribution in DNA:
    • - strong UV absorption – excited state energy
    • - ultrafast decay → prevents creation of harmful products
  • charge transfer/migration – facilitates transmission of information
    • movement of electron hole across peptide backbone
    • ‘wires’ together distant atoms

Why use ultrafast lasers?

  • Short Pulses: femtoseconds, attoseconds (recently 67 as!)
    • Time resolution: Observing the fastest processes in molecules
    • Control

F. Remacle and R.D. Levine PNAS103, 6793 (2006).

slide4

Ultrafast Dynamics in Biomolecules

2. Phenylalanine

Chosen molecule: phenylalanine

Why?

  • ‘Model’ for charge migration in biomolecular systems
  • Two charge acceptor sites:
    • Similar binding energy
    • Separated by two singly bonded carbons

Similar binding energy…

Consider states 1, 2:

ψHOLE(t) = c1 exp (-iE1t / ℏ) + c2exp (-iE2/ ℏ)

Then, hole charge density:

│ψHOLE(t)│2 = │c1│2 + │c2│2 + 2│c1c2*│cos(E2 – E1)t / ℏ)

ΔE = E2 – E1

f = ΔE / h

T = h / ΔE

If ΔE = 1 eV, T = 4 fs;

If ΔE = 0.1 eV, T = 40 fs.

slide5

Pump - Probe Set-Up in Politecnicodi Milano

VIS/NIR from previous stages

τ = 6fs, λ = 500-950 nm

High Harmonic Generation

Beamsplitter

slide6

Pump - Probe Set-Up in Politecnicodi Milano

XUV

τ = 1.5 fs

VIS/NIR from previous stages

τ = 6fs, λ = 500-950 nm

High Harmonic Generation

Beamsplitter

VIS/NIR Pulse

slide7

Pump - Probe Set-Up in Politecnicodi Milano

XUV

τ = 1.5 fs

VIS/NIR from previous stages

τ = 6fs, λ = 500-950 nm

High Harmonic Generation

Beamsplitter

Produce Gas Phase Sample

VIS/NIR Pulse

Delay Stage

τD

VIS/NIR

τ = 6 fs

slide8

Production of a Gas Phase Sample

Laser Induced Acoustic Desorption (LIAD)

  • sample deposited on thin foil
  • foil back irradiated
  • neutral plume created
  • studied in pump-probe scheme
  • products extracted and analysed
  • produces neutral intact molecules
  • fs interaction with sample only (no matrix)
  • photo-sensitive molecules can be studied

LIAD:

C.R. Calvert et al,

Phys. Chem. Chem. Phys.

14, 6289 (2012).

slide9

Laser Pulse Interaction with Phenylalanine

Two laser pulses:

1. XUV: 16 - 40 eV, 1.5 fs

2. VIS/NIR: 1.3 – 2.5 eV, 6 fs

How do these interact with phenylalanine?

1. XUV: 16 - 40 eV, 1.5 fs

Single Photon Ionisation

All outer shell electrons

Plus

Some inner shell electrons

slide10

Laser Pulse Interaction with Phenylalanine

Two laser pulses:

1. XUV: 16 - 40 eV, 1.5 fs

2. VIS/NIR: 1.3 – 2.5 eV, 6 fs

How do these interact with phenylalanine?

1. XUV: 16 - 40 eV, 1.5 fs

Single Photon Ionisation

All outer shell electrons

Plus

Some inner shell electrons

Fragmentation dependent upon location of charge in the molecule:

charge in π1: includem/q = 65, 77, 91, 103

charge in nN: include m/q = 120, 74

slide11

Laser Pulse Interaction with Phenylalanine

Two laser pulses:

1. XUV: 16 - 40 eV, 1.5 fs

2. VIS/NIR: 1.3 – 2.5 eV, 6 fs

How do these interact with phenylalanine?

2. VIS/NIR: 1.3 – 2.5 eV, 6 fs

Multiphoton, Tunnelling

Ionises from only the highest occupied molecular orbitals

slide12

Laser Pulse Interaction with Phenylalanine

Two laser pulses:

1. XUV: 16 - 40 eV, 1.5 fs

2. VIS/NIR: 1.3 – 2.5 eV, 6 fs

How do these interact with phenylalanine?

2. VIS/NIR: 1.3 – 2.5 eV, 6 fs

Multiphoton, Tunnelling

Only highest occupied molecular orbitals

Mostly nN fragments

Ionisation favoured from amine group

slide13

Pump – Probe Experiment in Phenylalanine

Experimental Scheme

Ionise first (pump) with XUV pulse

Probe with VIS/NIR

Follow the fragments’ yields as a function of the delay, τD, between pump and probe.

Probe with VIS/NIR

Probing excitation in phenyl group

(once charged, absorbs strongly in VIS)

Probing charge on the amine group through ionisation

Figure: R. Weinkaufet al,

J. Phys. Chem. 100, 18567 (1996).

slide14

Pump – Probe Results in Phenylalanine

Dynamics on the timescale τ = 80 fs

Temporal dependence with changing delay between XUV and VIS/NIR pulses of a number of fragments in the spectra

Ion

Yield

Ion

Yield

Time delay, τD

No time dependence in yield for nN fragments.

L. Belshaw et al,

J. Phys. Chem. Lett.

3, 3751 (2012).

Time delay, τD

Increase in yield for π1 fragments

slide15

Pump – Probe Results in Phenylalanine

Dynamics on the timescale τ = 80 fs

Temporal dependence with changing delay between XUV and VIS/NIR pulses of a number of fragments in the spectra

τ = 80 ± 20 fs

Internal Conversion to the π1 state following initial ionisation by XUV

τD < 0

No absorption in neutral phenyl;

Once charged, absorbs strongly in VIS.

Ion

Yield

Increasing population in π1 : opens up absorption by VIS/NIR.

L. Belshaw et al,

J. Phys. Chem. Lett.

3, 3751 (2012).

Time delay, τD

τD > 0

Increase in yield for π1 fragments

slide16

Pump – Probe Results in Phenylalanine

Dynamics on the timescale τ = 30 fs

Observed in the yield of the doubly charged immonium ion, m /q = 60

τ = 30 ± 5 fs

L. Belshaw et al,

J. Phys. Chem. Lett.

3, 3751 (2012).

slide17

Pump – Probe Results in Phenylalanine

Dynamics on the timescale τ = 30 fs

charge migration

m/q = 60

4

3

2

1

0

Delay, τD

Probe with VIS/NIR

m/q = 60

L. Belshaw et al,

J. Phys. Chem. Lett.

3, 3751 (2012).

Probing charge on the amine group through ionisation

-200

-100

0

100

200

300

slide18

Pump – Probe Results in Phenylalanine

Dynamics on the timescale τ = 30 fs

charge migration

m/q = 60

4

3

2

1

0

Delay, τD

m/q = 60

L. Belshaw et al,

J. Phys. Chem. Lett.

3, 3751 (2012).

consequence of the sensitivity of charge migration to nuclear rearrangement

τ = 30 fs

-200

-100

0

100

200

300

slide19

Conclusions

We have identified two separate ultrafast processes in phenylalanine molecules:

80 ± 20 fs internal conversion

30 ± 5 fs charge migration

Attosecond pump pulses

Few-cycle femtosecond probe pulses

Double Ionisation technique

powerful scheme for studying

charge migration

slide20

Ultrafast Dynamics Research

www.ultrafastbelfast.co.uk

Dr. Jason Greenwood

Prof. Ian Williams

Martin Duffy

Louise Belshaw

Prof. Mauro Nisoli

Dr. Francesca Calegari

Andrea Trabattoni

Thanks for Listening

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