Velocity Map Imaging of Ultrafast Molecular Dynamics Using Coulomb Explosion Imaging

The development of lasers capable of producing intense, coherent, femtosecond pulses of light has allowed for a wide range of detailed studies into chemical dynamics on their native timescales. This project proposes to investigate the structures and dynamics of gas phase molecules using a technique known as Coulomb explosion imaging. Here, an intense, ultrashort laser pulse is used to rapidly remove several electrons from the target molecule. The resulting highly charged cation is very unstable with respect to fragmentation - known as a Coulomb explosion. If the explosion occurs before any significant distortion or rearrangement of the molecule, the original structure of the molecule can be determined through measuring the momenta of resulting fragments, and any correlations between them. This has been used recently in the Brouard group to directly determine the chirality of gas phase molecules ( J. Chem. Phys., 2016, 144, 161105). The technique can be extended to a pump-probe regime, allowing structural dynamics to be imaged in a time resolved manner. The group recently used this to study photoinduced torsional motion in a biphenyl derivative (Phys. Rev. Lett., 2014, 133, 073005). The proposed research aims to extend the above work to study a range of photoinduced processes, such as photoisomerisations and charge-transfers. This will be facilitated by a new femtosecond laser (Ti:Sa) which has very recently been installed into the Brouard group lab in Oxford. This laser system will be used to induce Coulomb explosions in the molecules targeted. These experiments will make use of a technique known as velocity map imaging (VMI), in which ions generated with the same velocity are focussed onto the same point on a position sensitive detector. Detected ion hits will be recorded using the novel Pixel Imaging Mass Spectrometry (PImMS) or Timepix sensors. PImMS is an event-triggered detector developed in Oxford, which comprises an array of pixels allowing the logging of an event's position (x,y) and time (t, to a precision of 12.5 ns) (J. Inst., 2012, 7, C08001). Timepix is a similar detector, with the latest sensor improving the timing precision to approximately 1 ns. These sensors provide significant improvements over the CCD cameras conventionally used in VMI experiments, as they allow for all ion hits to be recorded in a single experimental cycle. Recording multiple ion hits (separated by their characteristic time of flight) in a single experiment allows for correlations between momenta of different ions to be elucidated, using coincident or covariance imaging techniques. These correlations can provide a great wealth of structural information, allowing for structure and dynamics to be studied with far greater detail. Experiments using the new Oxford femtosecond laser will initially be focussed on one-colour Coulomb explosions. A range of small molecules will be targeted to investigate their Coulomb explosion dynamics, with a view to building towards more structurally complex targets. These one colour experiments will also enable promising molecules for time-resolved studies into their photodynamics to be identified. A significant portion of the time-resolved pump-probe experiments will be carried out in collaboration at the FLASH free electron laser in DESY, Hamburg. This collaboration incorporates a wide range of groups, such as Rolles and Rudenko (Kansas State), Stapfeldt (Aarhus) and Bari and Boll (DESY). The aims of this collaboration include studies of ring-opening reactions, and studies into the charge-transfer dynamics of peptides. Planned experiments in collaboration with the Neumark and Leone groups (UC Berkeley) aims to combine novel attosecond lasers with the PImMS detector to study ultrafast (sub femtosecond) charge migration. This project falls within the EPSRC Chemical Reaction Dynamics and Mechanisms research area.