Making molecules sing: Quantum beats and interference as probes of molecular structure Katharine Reid Inaugural lecture, May 27 th 2010
Wolverley High School, Kidderminster
Science at Wolverley Melvyn Kershaw
University of Sussex
Chemical Physics at Sussex Harry KrotoTony StaceTony McCafferyJohn Murrell
Light Sources
FlashlampLaser Broad band Incoherent Pulsed Narrow band Coherent Continuous
Spectral profile and bandwidth FlashlampContinuous laser
Molecules in excited states
Laser induced fluorescence
State-selection and detection
excitationcollisionemission 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 H 2 + Cl 2
Flash photolysis The Nobel Prize in Chemistry 1967 EigenNorrishPorter "for their studies of extremely fast chemical reactions, effected by disturbing the equilibrium by means of very short pulses of energy"
Understanding photochemistry visionphotosynthesissolar cells
Laser Bandwidth narrow band continuous/long pulse broad band short 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 t1t1 t2t2
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 t1t1 t2t2
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 E Hz F 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 dynamics 2.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.