making molecules sing: quantum beats and interference as probes of molecular structure katharine...
TRANSCRIPT
![Page 1: Making molecules sing: Quantum beats and interference as probes of molecular structure Katharine Reid Inaugural lecture, May 27 th 2010](https://reader033.vdocuments.site/reader033/viewer/2022051417/56649d045503460f949d7ae6/html5/thumbnails/1.jpg)
Making molecules sing: Quantum beats and interference as
probes of molecular structure
Katharine Reid
Inaugural lecture, May 27th 2010
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Wolverley High School, Kidderminster
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Science at Wolverley
Melvyn Kershaw
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University of Sussex
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Chemical Physics at Sussex
Harry Kroto Tony StaceTony McCafferyJohn Murrell
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Light Sources
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Flashlamp Laser
Broad bandIncoherentPulsed
Narrow bandCoherentContinuous
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Spectral profile and bandwidth
Flashlamp Continuous laser
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Molecules in excited states
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Molecules in excited states
Laser induced fluorescence
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State-selection and detection
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excitation collision emission
Probing the dynamics of chemical reactions
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Dick Zare
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Stanford University
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Interference
Thomas Young A double slit
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Interference
Constructive: bright spot
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Interference
Destructive: dark spot
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d
L
y
Interference
The first bright spot occurs at y = L/d
If = 530 nm (green), L = 14 m and d = 50 microns
Maximum of first bright spot is at y = 14.8 cm
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Electron waves
Atomic orbitals – bound electrons
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Ejecting electrons with light
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Simplifying spectra
Thermal congestion
State selection
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Photoelectron interference patterns
Laser polarization direction
Constructive interference
Destructive interference
These patterns provide unique information on molecular structure
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University of Nottingham
Lunch time near the School of Chemistry, University Park!!
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The “younger chemists” c. 1996
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Who is the odd one out?
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Time-resolved measurements
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Photoinitiation of H2 + Cl2
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Flash photolysis
The Nobel Prize in Chemistry 1967
Eigen Norrish Porter
"for their studies of extremely fast chemical reactions, effected by disturbing the equilibrium by means of very short pulses of energy"
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Understanding photochemistry
vision photosynthesis solar cells
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Laser Bandwidth
narrow bandcontinuous/long pulse
broad bandshort pulse
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State-selection ...
Narrow band
Long pulse
Broad band
Short pulse
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Vibrational states in polyatomic molecules
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On excitation only certain vibrational states can be prepared
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Vibrations can be coupled
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Time-resolved measurements
t = 0 t1 t2
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Nano what?
1 ps = 1000 fs
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Femtochemistry
The Nobel Prize in Chemistry 1999
"for his studies of the transition states of chemical reactions using femtosecond spectroscopy”
Ahmed Zewail
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Intramolecular vibrational energy redistribution
Timescale: tens of picoseconds
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Questions
1. What is the timescale?2. What is the mechanism (which dark states are
involved)?3. Can we influence the process? (Bond-selective
chemistry, coherent control, mode-specificity)4. What can we learn about chemical reactivity?
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The experiments!
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Toluene absorption spectrum
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Photoelectron imaging
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t = 0 t1 t2
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0 ps
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1 ps
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2 ps
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3 ps
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4 ps
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5 ps
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6 ps
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Time-resolved photoelectron spectra
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Peak intensities versus time
Time delay / picoseconds
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Quantum beats
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Beating patterns
E4 329.63 HzF4 349.23 Hz
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Making molecules sing?
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Coupled vibrational states
Three states = two observable frequencies
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Analysis of quantum beats in toluene
From this we can learn:
1. The timescale of the dynamics2. The “coupling matrix elements”3. The exact vibrational energies
More importantly, we have developed and tested a method that can be used to interrogate more complicated dynamical processes.
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Toluene at higher excitation energy ...
Intensity decreases with time
Intensity increases with time
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Where to from here?
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Support
Neil Barnes
Mike Towrie
Pavel Matousek
Kate Ronayne
The workshop
Students
Dave Townsend
Paul Whiteside
Chris Hammond
Paul Hockett
Mick Staniforth
Alistair Green
Jonathan Midgley
Postdocs
Simon Duxon
Tom Field
Jon Underwood
Julia Davies
Susan Bellm
Adrian King
Help with this lecture
Neil Barnes
Paul Gaetto
Collaborators
Ivan Powis
Tim Wright
Thanks to:
Mentors
Melvyn Kershaw
Tony McCaffery
Dick Zare
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And thanks to everyone for their support.