Time-resolved Velocity Map Imaging using Pixel Imaging Mass Spectrometry

This project will focus on time-resolved ion imaging at the atto- to nanosecond systems and falls within the EPSRC Physical Sciences research area. Velocity map imaging (VMI) has revolutionised the field of photodissociation dynamics by providing direct images of scattered photofragments. These reveal the fundamental partitioning of energy during photochemical reactions, and correlation of such information for different ions during a single experiment allows information to be gleaned about reaction mechanisms and structural changes over very short timescales. Coupling VMI with the Pixel Imaging Mass Spectrometry (PImMS) time-stamping sensor allows the full three-dimensional velocity distribution of multiple mass-to-charge fragments to be directly recorded. As a result, several applications of the PImMS camera with VMI imaging will be carried out during my project.
Nanosecond time resolved imaging will be conducted in Oxford using a pump-probe scheme to photodissociate molecular chlorine. PEDA (post extraction differential acceleration) slice imaging of the subsequent atomic chlorine is a technique that enhances the time of flight resolution, which in conjunction with the application of the PImMS sensor with multi-mass imaging and time stamping capabilities, can obtain a series of images. A 3D image of the atomic chlorine is thus obtained and angular momentum polarisabilities determined.
The project will look to study systems on a femtosecond timescale using the free-electron laser FLASH at a synchrotron facility in Hamburg. These will use the PImMS sensor to undergo a series of experiments employing the use of time resolved imaging to create a "molecular movie": where a molecule's structure is altered via photoexcitation and the resulting images recorded on a femtosecond timescale to create an effective video of the structural change and subsequent dynamics.
Femtosecond time-resolved imaging will also be undertaken in collaboration with Professor Henrik Stapelfeldt in Aarhus; expanding on previous work using the PImMS sensor. This work is built upon studying isomers and identifying benzene derivatives following a femtosecond laser-induced Coulomb explosion, with an ultimate goal of distinguishing chiral molecules using an ultra-short timescale. Ions of different mass can be correlated to one another using the PImMS sensor through covariance maps, meaning that one can see where a particular ion hits the detector relative to a reference ion. The inference of this is that the particular structure of the sample can be deduced through a number of these covariance maps.
The utilisation of attosecond time- resolved imaging in collaboration with Berkeley is another possibility to be explored. This corresponds to electronic dynamics and would allow imaging of electrons during charge redistribution, gaining unprecedented insight into the dynamics of this ultra-fast, little understood process. Understanding of how electrons reorganise is of fundamental importance to many emerging fields such as molecular electronics.