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.