SDG, Durham, January 2013
Observation of Ultrafast Charge Migration in an
Amino Acid
Louise Belshaw
Queen’s University, Belfast
www.ultrafastbelfast.co.uk
LOUISE BELSHAW
OBSERVATION OF ULTRAFAST CHARGE MIGRATION IN AN AMINO ACID
SDG, Durham, January 2013
Observation of Ultrafast Charge Migration in an
Amino Acid
Outline
1. Why biomolecules with attosecond lasers?
2. Phenylalanine
3. How: experimental pump – probe setup
4. Results with phenylalanine
5. Conclusions
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LOUISE BELSHAW
OBSERVATION OF ULTRAFAST CHARGE MIGRATION IN AN AMINO ACID
SDG, Durham, January 2013
Ultrafast Dynamics in Biomolecules
1. 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?
F. Remacle and R.D. Levine
PNAS 103, 6793 (2006).
• Short Pulses: femtoseconds, attoseconds (recently 67 as!)
• Time resolution: Observing the fastest processes in molecules
• Control
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LOUISE BELSHAW
OBSERVATION OF ULTRAFAST CHARGE MIGRATION IN AN AMINO ACID
SDG, Durham, January 2013
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)
ΔE = E2 – E1
f = ΔE / h
T = h / ΔE
Then, hole charge density:
│ψHOLE(t)│2 = │c1│2 + │c2│2 + 2│c1c2*│cos(E2 – E1)t / ℏ)
If ΔE = 1 eV, T = 4 fs;
If ΔE = 0.1 eV, T = 40 fs.
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LOUISE BELSHAW
OBSERVATION OF ULTRAFAST CHARGE MIGRATION IN AN AMINO ACID
SDG, Durham, January 2013
Pump - Probe Set-Up in Politecnico di Milano
VIS/NIR from previous
stages
τ = 6fs, λ = 500-950 nm
High Harmonic
Generation
Beamsplitter
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LOUISE BELSHAW
OBSERVATION OF ULTRAFAST CHARGE MIGRATION IN AN AMINO ACID
SDG, Durham, January 2013
Pump - Probe Set-Up in Politecnico di Milano
VIS/NIR from previous
stages
τ = 6fs, λ = 500-950 nm
High Harmonic
Generation
XUV
τ = 1.5 fs
Beamsplitter
VIS/NIR Pulse
www.ultrafastbelfast.co.uk
LOUISE BELSHAW
OBSERVATION OF ULTRAFAST CHARGE MIGRATION IN AN AMINO ACID
SDG, Durham, January 2013
Pump - Probe Set-Up in Politecnico di Milano
VIS/NIR from previous
stages
τ = 6fs, λ = 500-950 nm
High Harmonic
Generation
Beamsplitter
XUV
τ = 1.5 fs
Produce Gas
Phase Sample
VIS/NIR Pulse
Delay Stage
τD
VIS/NIR
τ = 6 fs
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LOUISE BELSHAW
OBSERVATION OF ULTRAFAST CHARGE MIGRATION IN AN AMINO ACID
SDG, Durham, January 2013
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
LIAD:
 produces neutral intact molecules
 fs interaction with sample only (no matrix)
 photo-sensitive molecules can be studied
C.R. Calvert et al,
Phys. Chem. Chem. Phys.
14, 6289 (2012).
www.ultrafastbelfast.co.uk
LOUISE BELSHAW
OBSERVATION OF ULTRAFAST CHARGE MIGRATION IN AN AMINO ACID
SDG, Durham, January 2013
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
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LOUISE BELSHAW
OBSERVATION OF ULTRAFAST CHARGE MIGRATION IN AN AMINO ACID
SDG, Durham, January 2013
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: include m/q = 65, 77, 91, 103
charge in nN: include m/q = 120, 74
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LOUISE BELSHAW
OBSERVATION OF ULTRAFAST CHARGE MIGRATION IN AN AMINO ACID
SDG, Durham, January 2013
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
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LOUISE BELSHAW
OBSERVATION OF ULTRAFAST CHARGE MIGRATION IN AN AMINO ACID
SDG, Durham, January 2013
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
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LOUISE BELSHAW
OBSERVATION OF ULTRAFAST CHARGE MIGRATION IN AN AMINO ACID
SDG, Durham, January 2013
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. Weinkauf et al,
J. Phys. Chem. 100, 18567 (1996).
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LOUISE BELSHAW
OBSERVATION OF ULTRAFAST CHARGE MIGRATION IN AN AMINO ACID
SDG, Durham, January 2013
Pump – Probe Results in Phenylalanine
Dynamics on the timescale τ = 80 fs
L. Belshaw et al,
J. Phys. Chem. Lett.
3, 3751 (2012).
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.
Time delay, τD
Increase in yield for π1 fragments
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LOUISE BELSHAW
OBSERVATION OF ULTRAFAST CHARGE MIGRATION IN AN AMINO ACID
SDG, Durham, January 2013
Pump – Probe Results in Phenylalanine
L. Belshaw et al,
J. Phys. Chem. Lett.
3, 3751 (2012).
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
Ion
Yield
Time delay, τD
Increasing
population in π1 :
opens up absorption
by VIS/NIR.
No absorption in
neutral phenyl;
Once charged, absorbs
strongly in VIS.
τD > 0
Increase in yield for π1 fragments
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LOUISE BELSHAW
OBSERVATION OF ULTRAFAST CHARGE MIGRATION IN AN AMINO ACID
SDG, Durham, January 2013
Pump – Probe Results in Phenylalanine
L. Belshaw et al,
J. Phys. Chem. Lett.
3, 3751 (2012).
Dynamics on the timescale τ = 30 fs
Observed in the yield of the doubly charged immonium ion, m /q = 60
τ = 30 ± 5 fs
www.ultrafastbelfast.co.uk
LOUISE BELSHAW
OBSERVATION OF ULTRAFAST CHARGE MIGRATION IN AN AMINO ACID
SDG, Durham, January 2013
Pump – Probe Results in Phenylalanine
L. Belshaw et al,
J. Phys. Chem. Lett.
3, 3751 (2012).
Dynamics on the timescale τ = 30 fs
charge
migration
m/q = 60
4
3
2
1
0
-200
-100
0
100
200
300
Delay, τD
m/q = 60
Probe with VIS/NIR
Probing charge on the amine group
through ionisation
www.ultrafastbelfast.co.uk
LOUISE BELSHAW
OBSERVATION OF ULTRAFAST CHARGE MIGRATION IN AN AMINO ACID
SDG, Durham, January 2013
Pump – Probe Results in Phenylalanine
L. Belshaw et al,
J. Phys. Chem. Lett.
3, 3751 (2012).
Dynamics on the timescale τ = 30 fs
charge
migration
m/q = 60
4
3
2
1
0
-200
-100
0
100
200
300
Delay, τD
m/q = 60
τ = 30 fs
consequence of the sensitivity of charge
migration to nuclear rearrangement
www.ultrafastbelfast.co.uk
LOUISE BELSHAW
OBSERVATION OF ULTRAFAST CHARGE MIGRATION IN AN AMINO ACID
SDG, Durham, January 2013
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
www.ultrafastbelfast.co.uk
LOUISE BELSHAW
OBSERVATION OF ULTRAFAST CHARGE MIGRATION IN AN AMINO ACID
SDG, Durham, January 2013
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
www.ultrafastbelfast.co.uk
LOUISE BELSHAW
OBSERVATION OF ULTRAFAST CHARGE MIGRATION IN AN AMINO ACID