Research Guides

Department of Chemistry University of Oxford

Professor T.P. Softley

There are three principal areas of interest in our group:

* Ultracold molecules and ultracold chemistry – reactive collisions of molecules in the gas phase at very low velocity, corresponding to sub-Kelvin temperatures.
* Spectroscopic properties and applications of highly excited “Rydberg” states of molecules, especially their interaction with solid surfaces.
* Dynamics of photodissociation and photoionization processes and their use in producing cold atoms and molecules.
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 the rapidly developing field of ultracold molecules and ultracold chemistry, the aim is to produce and study molecules in the gas phase with a translational energy distribution that would be characteristic of temperatures in the milliKelvin range or lower. Novel physics is expected to arise from the interactions between molecules when their deBroglie wavelength becomes large compared to characteristic molecular dimensions.  For reactive collisions, a wave description of the interacting system must replace the classical picture of colliding particles, and reaction rates will reflect the occurrence of tunneling and quantum reflection, the highly restricted angular momentum of the collisions and the existence of scattering resonances at low collision energies. The population of only the lowest internal quantum states (vibration and rotation) will also have a significant effect on the reaction dynamics.   The study of cold collisions provides a fundamental test for quantum theories of chemical reactions. It also offers the ultimate in control over the degrees of freedom that determine chemical processes.

In order to study reactions 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 (CH + NH3 → CH2NH + H is an example of a reaction we aim to study).  Ion-molecule collisions are another general category of barrierless reaction and we have developed a novel experimental apparatus which combines laser cooled atomic ions with low velocity neutral molecules to study reactions such as Ca+  + CH3F → CaF+ +  CH3. Individual ions can be imaged through fluorescence microscopy in the ion trap and reaction rates measured by direct monitoring of the disappearance of the reactant ions. The sensitivity is so high that reaction rates as low as one reactive collision per minute can be observed.

Laser cooling methods which are applicable to atoms and atomic ions (leading to the observation of Bose-Einstein condensation  and the award of two Nobel prizes in physics) are unfortunately not applicable to neutral molecules. The main approach we are adopting  to make cold neutral molecules involves using electrostatic fields to impart forces on molecules which change their velocity in direction or magnitude. 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 experience a repelling force as it moves into a high-electric-field region through the conversion of kinetic energy into potential energy, leading to its deceleration or deflection.  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 incrementally loses kinetic energy at each stage. In this way a sample of molecules with precisely controlled and very low velocity can be produced after passage through 130 high-field stages. The other approach involves taking a sample of molecules at room temperature with a Maxwell-Boltzmann velocity distribution and passing them through a bent electrostatic quadrupole guide; the guiding of the molecules around the bend is successful only for the slowest molecules in the sample, and thus the low-velocity tail of the Maxwell-Boltzmann distribution is selected. Both these sources are capable of producing molecules with a mean kinetic energy equivalent to a translational temperature in the milliKelvin range. Our objective is to take advantage of the development of these cold molecule production techniques to study chemistry in an exotic physical regime.
 

(b) 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, wavepacket propagation or classical trajectory simulation 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 with much smaller fields than ground state molecules. 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 pursuing a range of applications of the exotic properties of Rydberg states. The interaction of a Rydberg state with a solid surface is of particular interest currently.  With metallic surfaces, charge transfer of the Rydberg electron to the conduction band or surface states of the metal is the predominant process occurring and this process has relevance to plasma-surface interactions, ion sputtering processes, the understanding of electronically excited states of surfaces, and the control of the novel chemical processes. The ease of manipulation of the Rydberg wavefunction leads to new experimental approaches to study charge transfer dynamics at interfaces.  Our current experiments involve the interaction  of H atom and H2 molecule Rydberg beams with a range of surface types – metals, semiconductors, thin layers and nanostructures. We aim to characterize and understand the fundamental physical and chemical processes occurring and use this knowledge to develop new applications in controlled chemistry, surface analysis, vapour deposition etc.


(c) In a photodissociation event in the gas phase (e.g., CH3CHO + hν → 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 (their energies and angular distributions) 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.  By matching the velocity of the resulting fragments with the original parent molecular beam velocity, the backwards flying fragments experience a velocity cancellation to bring them to rest (or near to rest) in the laboratory frame. We are currently investigating the use of photodissociation of halogen molecules to produce cold halogen atoms for studies of cold chemical processes (see (a) above).        

An additional area of interest is the experimental and theoretical study of near-threshold photoionization 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 and its isotopomers NH2D, ND2H and ND3. In the context of cold collisions we are also interested in using photoionization as a mechanism for producing quantum state selected molecular ions.

 

 

2005 – 2010

 “ Level crossings in the ionization of H2 Rydberg molecules at a metal surface”
E. A. McCormack, M. S. Ford and T. P. Softley, J. Phys. Chem. In Press (2010).

“The PFI ZEKE and photoionization spectra of ND3, with an ab initio MQDT simulation”
L. Duggan, M. Raunhardt, M. Schafer, U. Hollenstein, T. P. Softley and F. Merkt, Molec Phys. 108  1069 (2010)
 
“Ultracold molecules and ultracold chemistry”
M. T. Bell and T. P. Softley, Molecular Physics, 107, 99 (2009).

“Combination of Coulomb Crystal and Quadrupole velocity selector for the study of reactions at sub-Kelvin temperatures”. M. Bell, A. Gingell, S. Willitsch, J. Oldham and T. P. Softley, Faraday Discissions 142, 73 (2009).
 
“Wavepacket Propagation study of the charge transfer dynamics of Rydberg atoms with a metal surface”, E. So, M. Bell and T. P. Softley, Phys Rev A,  79 012901 (2009).

  “Chemical applications of laser- and sympathetically cooled ions in ion traps”, S. Willitsch, M. T. Bell, A. D. Gingell and T. P. Softley, Phys. Chem. Chem. Phys. 10 7228 (2008)

"Cold Reactive collisions between Laser Cooled Ions and Velocity-Selected Neutral Molecules", S. Willitsch, M. Bell, A. Gingell, S. R. Procter and T. P. Softley, Physical Review Letters, 100, 043203 (2008).
 
“Spectroscopic study and multichannel quantum defect theory analysis o f the Stark effect in Rydberg states of neon”
M. Gruetter, O, Zehnder, T. P. Softley and F. Merkt, J. Phys. B, 41, 115001, (2008).

 “The Stark effect in the predissociating Rydberg states of NO”
B. B. Clarson, S. R. Procter, A. L. Goodgame and T. P. Softley, Molec. Phys. 106, 1317 (2008).
 
“Rydberg Matter" T. P. Softley and F. Merkt, Molec. Phys., 105, 923 (2007).

"Cooling effects in the Stark deceleration of Rydberg atoms/moelcules with time-dependent electric fields" Y. Yamakita, R. Takahashi, K. Ohno, S. R. Procter, G. Maguire and T. P. Softley, J. Phys. Conf. Ser., 80, 012045 (2007).

"Fully state-selected Velocity Map Imaging study of the near threshold photodissociation of NO2: variation of the angular anisotropy parameter" S. J. Matthews, S. Willitsch and T. P. Softley, Phys. Chem. Chem. Phys. 9, 5656 (2007).

"Rovibronic photoionization dynamics of ammonia isotopomers" U. Hollenstein, F. Merkt, L. Meyer, T. P. Softley and S. Willitsch, Molec. Phys., 105, 1711 (2007).

"Interaction of Rydberg molecules with a metal surface" G. R. Lloyd, S. R. Procter, E. McCormack and T. P. Softley, J. Chem. Phys., 126, 184702 (2007).

"The Stark effect and translational control of hydrogen molecules" Y. Yamakita, S. R. Procter and T. P. Softley, J. Plasma Fusion Res. Series, 7, 64 (2006).

"Ionization of Hydrogen Rydberg molecules at a metal surface" G. R. Lloyd, S. R. Procter and T. P. Softley, Phys. Rev. Lett., 95, 133202 (2005).

"Multichannel quantum defect theory simulations of the Rydberg spectra of HCO" S. A. Brownbill and T. P. Softley, Mol. Phys., 103, 2347 (2005)

"Controlling the motion of hydrogen molecules: design of a two-dipole Rydberg decelerator" T. P. Softley, S. R. Procter, Y. Yamakita, G. Maguire and F. Merkt, J.Elec. Spec. Rel. Phen., 144, 113 (2005).

"Velocity-map imaging study of the photodissociation of acetaldehyde" H. A. Cruse and T. P. Softley, J. Chem. Phys., 122, 124303 (2005).

2000 - 2004

"Demonstration of the combination of slice-imaging and Rydberg tagging for studies of photodissociation dynamics" H. A. Cruse and T. P. Softley, J. Chem. Phys., 121, 4089 (2004).

"Deflection and deceleration of hydrogen Rydberg molecules in inhomogeneous electric fields" Y. Yamakita, S. R. Procter, A. L. Goodgame, T. P. Softley and F. Merkt, J. Chem. Phys. 121, 1419 (2004).

"Non-hydrogenic effects in the deceleration of Rydberg atoms in inhomogeneous electric fields" E. Vliegen, H. J. Worner, T. P. Softley and F. Merkt, Phys. Rev. Lett., 92, 033005 (2004).

"Applications of molecular Rydberg states in chemical dynamics and spectroscopy" T. P. Softley, Int. Rev. Phys. Chem., 23, 1 (2004).

"The Role of Phase in molecular Rydberg wave packet dynamics" R. A. L. Smith, V. G., Stavros, J. R. R., Verlet, H. H., Fielding, D. Townsend, and T. P. Softley, J. Chem. Phys. 119, 3085 (2003).

"Controlling the motion of hydrogen molecules" S. R. Procter, Y. Yamakita, F. Merkt, and T. P. Softley, Chem. Phys. Lett., 374, 667, (2003).

"High-resolution threshold-ionization spectroscopy of NH3" R. Seiler, U. Hollenstein, T. P. Softley, and F. Merkt, J. Chem. Phys. 118, 10024 (2003).

"Study of the Stark effect in the v+ = 1 autoionizing Rydberg states of NO" A.L. Goodgame, H. Dickinson, S.R. Mackenzie and T.P. Softley, J. Chem. Phys., 116 4922 (2002).

"Spatial discrimination of Rydberg tagged molecular photofragments in an inhomogeneous electric field" O. L. A. Monti, H. A. Cruse, S. R. Mackenzie and T. P. Softley, J. Chem. Phys. 115, 7924 (2001).

"Multichannel quantum defect theory (MQDT) analysis of the (2+1') Mass Analyzed Threshold Ionization (MATI) Spectroscopy of NH3." H. Dickinson, D. Rolland and T. P. Softley, J. Phys. Chem. A 105, 5590 (2001).

"(2+1') Mass Analyzed Threshold Ionization (MATI) Spectroscopy of the CD3 radical" H. Dickinson, T. Chelmick and T.P. Softley, Chem. Phys. Lett., 338, 37 (2001).

"High Resolution photoionization spectroscopy of vibrationally excited ArNO" O. L. A. Monti, H. A. Cruse, S. R. Mackenzie and T. P. Softley, Chem. Phys. Lett., 333, 146 (2001).

"Deflection of krypton Rydberg atoms in the field of an electric dipole" D. Townsend, A. L. Goodgame, S. R. Procter, S. R. Mackenzie and T. P. Softley , J. Phys B: At. Mol. Opt. Phys. 34, 439 (2001).

"(2+1') REMPI and (2+1') MATI Spectroscopy of H2O" H. Dickinson, S. R. Mackenzie and T. P. Softley, Phys. Chem. Chem. Phys. 2, 4669, (2000).

"The dynamics of high Rydberg states in the presence of time-dependent inhomogeneous fields" S. R. Procter, M. J. Webb and T. P. Softley, Faraday Discussion 115, 277, (2000).

"Rapidly fluctuating anisotropy parameter in the near-threshold photodissociation of NO2" O. L. A. Monti, H. Dickinson, S. R. Mackenzie and T. P. Softley J. Chem. Phys. 112, 3699 (2000).

"Rydberg state decay in inhomogeneous electric fields" T. P. Softley and R. J. Rednall, J. Chem. Phys. 112, 7992 (2000).