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Observation of Ultrafast Charge Migration in an Amino Acid

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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Observation of Ultrafast Charge Migration in an Amino Acid

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  1. Observation of Ultrafast Charge Migration in an Amino Acid Louise Belshaw Queen’s University, Belfast

  2. 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

  3. 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).

  4. 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.

  5. Pump - Probe Set-Up in Politecnicodi Milano VIS/NIR from previous stages τ = 6fs, λ = 500-950 nm High Harmonic Generation Beamsplitter

  6. 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

  7. 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

  8. 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).

  9. 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

  10. 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

  11. 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

  12. 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

  13. 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).

  14. 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

  15. 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

  16. 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).

  17. 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

  18. 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

  19. 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

  20. 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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