 Professor T.P. Softley
Chemistry Research Laboratory
Telephone: 44 (0) 1865 275 428
Research Group Web Site
There are three principal areas of interest in our group:
- Spectroscopic properties and applications of highly excited “Rydberg” states
of molecules, including scattering with solid surfaces.
- Molecular deceleration and ultracold chemistry – reactive
collisions of molecules in the gas phase at very low velocity,
corresponding to sub-Kelvin temperatures.
- Dynamics of photodissociation and photoionization processes.
These are all fundamental studies at the frontier between physical
chemistry and atomic/molecular physics, and a combination of
experimental and theoretical/computational research is carried
out in the group.
(a) In a molecular Rydberg state, one electron
has been excited into a distant, high-energy orbital, and its
behaviour is similar to, but subtly different from, the behaviour
of a high-energy state of the H atom. Atomic-like quantum numbers
(n, l, ml) can be assigned to the
Rydberg electron, while the remainder of the molecule (the cationic
core) vibrates and rotates independently. We use ultraviolet
or vacuum ultraviolet laser systems to populate these highly
excited states of molecules and to obtain spectroscopic information
on the energy levels and dynamics (i.e., the decay of the states).
We also use quantum mechanical methods, for example multichannel
quantum defect theory, to try to understand the spectroscopic
and dynamical behaviour.
Rydberg
states have very unusual properties: In particular, the Rydberg
electron orbital is easily perturbed by external electromagnetic
fields giving the opportunity to control the charge distribution
in the molecule. A very large dipole moment can be created with
a selected orientation, providing a handle by which to manipulate
the translational motion with inhomogeneous electric fields:
for example beams of neutral Rydberg molecules can be deflected
or focused in a similar manner to charged particles. The Rydberg
electron can also be easily removed by an applied field to produce
a molecular ion that can be detected or used in subsequent experiments.
We are interested in pursuing a range of applications of the
exotic properties of Rydberg states. Current experiments in our
group include (i) the study of scattering of H2 Rydberg molecules
at metal surfaces, and (ii) the use of electric fields to decelerate
beams of Rydberg molecules (see (b)).
(b) In the rapidly developing field of ultracold molecules
and ultracold chemistry, the aim is to produce molecules
in the gas phase with a translational energy distribution that
would be characteristic of temperatures in the milliKelvin
or lower range. Novel physics is expected to arise from the
interactions between molecules when their deBroglie wavelength
becomes large compared to characteristic molecular dimensions.
But in order to study reactive processes at very low temperatures,
we must focus on reactions in which the activation barrier
is small or negligible. Many reactions of free-radical species
fall into this category of barrierless reactions. An example
of a reaction we plan to study is the insertion reaction CH
+ NH3 ® CH2NH + H which has no
activation barrier and hence reactive scattering can occur
even at extremely low temperatures. We are also developing
approaches to studying low-energy collisions between ions and
neutral molecules - another category of activationless process
- and between low-energy molecular beams and metallic surfaces.
Laser
cooling methods have already been developed for atoms, leading
to the observation of Bose-Einstein condensation (and the award
of two Nobel prizes in physics). Unfortunately these techniques
are not applicable to neutral molecules. The main approach we
are adopting involves using electrostatic fields to decelerate
molecules. This relies on the Stark effect – the perturbation
of energy levels by an electric field – and on the existence
of a sizeable dipole moment in the molecule. A molecule in a
quantum state for which the energy increases with increasing
field will slow down as it moves into a high-electric-field region
through the conversion of kinetic energy into potential energy.
In our lab we have two variants of this approach; one uses a
130-stage “Stark decelerator” in which a beam of
dipolar ground state molecules such as NH3 passes through a sequence
of very strong fields and is eventually brought to a standstill
in an electrostatic trap. The other approach involves Rydberg
state excitation of the molecules and the creation of a very
large dipole moment; the molecules can then be decelerated in
a single stage device with modest inhomogeneous electric fields.
In the future this work will lead to highly controlled ultracold
scattering experiments both in the gas phase and at solid surfaces.
(c) In a photodissociation event in the gas
phase (e.g., CH3CHO + hn ® CH3 +
HCO) the fragments fly out from the parent centre of mass with
a distribution of energies, orientations and angular momenta.
If the photodissociation is initiated using a laser at a well-defined
spatial position, then the subsequent trajectories of the products
can be followed using ion imaging techniques. The product molecules
are ionized using laser multiphoton excitation and the three
dimensional distribution of ions is ‘pancaked’ onto
a spatially-sensitive two-dimensional detector. The information
contained in these ion images reveals how the excess energy is
distributed amongst the internal degrees of freedom (rotation
and vibration) of the products; this allows us to understand
details of the forces exerted on the fragments at the moment
the molecule is falling apart and hence learn about the potential
energy surfaces that control the dynamics of the dissociation
process. A question of particular interest is the correlation in
the vibrational-rotational motion between the pair of product
molecules. We are also interested in the study of near-threshold
photodissociation – in which
the photon energy almost matches the dissociation energy – because
in this case the fragments separate with very low translational
energy. As such, near threshold photodissociation processes can
be considered as a form of ultracold reaction (see (b) above). Experimentally
we have developed a variation on the ion-imaging technique in
which the fragments are excited to Rydberg states rather than
being ionized. This “Rydberg tagging” promises improvements
in energy resolution, especially for near-threshold processes,
as well as prospects for detecting photofragments which cannot
be easily ionized using multiphoton techniques.
An
additional area of interest is the experimental and theoretical
study of near-thresholdphotoionization processes
and the role of Rydberg states in those processes. Rydberg states
of molecules exist not only just below the ionization limit energetically,
but also just above it if the ionic core of the Rydberg state
has non-zero vibration rotation energy. Thus photoionization
often occurs via an indirect process involving excitation of
a Rydberg state, which subsequently decays by autoionization.
A further factor to consider is the competition between autoionization
and predissociation (into neutral fragments) of Rydberg states;
this competition is of relevance not only to the photoionization
probability, but also to the important chemical process of dissociative
recombination (e.g., H2+ + e- ® H
+ H). A range of experiments give information on autoionization
and predissociation processes near the ionization thresholds,
including high resolution spectroscopy, imaging of dissociation
fragments and photoelectron angular distributions. We make use
of Multichannel Quantum Defect Theory (MQDT) as a means to simulate
and interpret this data and we are particularly interested in
extending the application of the theory, which has mainly been
applied to diatomic systems, to polyatomic species such as NH3. |
