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Making molecules sing: Quantum beats and interference as
probes of molecular structure
Katharine Reid
Inaugural lecture, May 27th 2010
Wolverley High School, Kidderminster
Science at Wolverley
Melvyn Kershaw
University of Sussex
Chemical Physics at Sussex
Harry Kroto Tony StaceTony McCafferyJohn Murrell
Light Sources
Flashlamp Laser
Broad bandIncoherentPulsed
Narrow bandCoherentContinuous
Spectral profile and bandwidth
Flashlamp Continuous laser
Molecules in excited states
Molecules in excited states
Laser induced fluorescence
State-selection and detection
excitation collision emission
Probing the dynamics of chemical reactions
Dick Zare
Stanford University
Interference
Thomas Young A double slit
Interference
Constructive: bright spot
Interference
Destructive: dark spot
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
Electron waves
Atomic orbitals – bound electrons
Ejecting electrons with light
Simplifying spectra
Thermal congestion
State selection
Photoelectron interference patterns
Laser polarization direction
Constructive interference
Destructive interference
These patterns provide unique information on molecular structure
University of Nottingham
Lunch time near the School of Chemistry, University Park!!
The “younger chemists” c. 1996
Who is the odd one out?
Time-resolved measurements
Photoinitiation of H2 + Cl2
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"
Understanding photochemistry
vision photosynthesis solar cells
Laser Bandwidth
narrow bandcontinuous/long pulse
broad bandshort pulse
State-selection ...
Narrow band
Long pulse
Broad band
Short pulse
Vibrational states in polyatomic molecules
On excitation only certain vibrational states can be prepared
Vibrations can be coupled
Time-resolved measurements
t = 0 t1 t2
Nano what?
1 ps = 1000 fs
Femtochemistry
The Nobel Prize in Chemistry 1999
"for his studies of the transition states of chemical reactions using femtosecond spectroscopy”
Ahmed Zewail
Intramolecular vibrational energy redistribution
Timescale: tens of picoseconds
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?
The experiments!
Toluene absorption spectrum
Photoelectron imaging
t = 0 t1 t2
0 ps
1 ps
2 ps
3 ps
4 ps
5 ps
6 ps
Time-resolved photoelectron spectra
Peak intensities versus time
Time delay / picoseconds
Quantum beats
Beating patterns
E4 329.63 HzF4 349.23 Hz
Making molecules sing?
Coupled vibrational states
Three states = two observable frequencies
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.
Toluene at higher excitation energy ...
Intensity decreases with time
Intensity increases with time
Where to from here?
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
And thanks to everyone for their support.