New developments in velocity-map imaging mass spectrometry based on the Pixel Imaging Mass Spectrometry camera: Correlations between multiple ions and electrons and imaging molecular structure and following photoinduced processes on a femtosecond time scale
Final Report Abstract
The Pixel Imaging Mass Spectrometry (PImMS) camera was employed to develop three-dimensional imaging techniques. The PImMS camera records the ion arrival time together with the spatial position on the detector, and possible applications use the time information in different ways. (1) Three-dimensional imaging. The PImMS camera with its ability to capture both the two-dimensional impact position of charged particles and their time-of-flight was used in a proof-of-principle three-dimensional imaging experiment on the photo-dissociation of OCS at 230 nm. Coupling the PImMS camera with velocitymap imaging allows to directly record the three-dimensional Newton sphere of fragment ions; in this case the Newton sphere is captured in 12 slices, each with a width of 12.5 nanoseconds. To enhance the resolution in the third dimension, post-extraction Newton sphere inversion slicing is planned. (2) Coulomb explosion covariance imaging. Coulomb explosion imaging is a powerful tool to investigate the structure of a molecule; implemented in a pump-probe scheme, it allows to study structural changes on a femtosecond time scale. The ability of the PImMS camera to capture the full time-of-flight and velocity-map information in each acquisition cycle allows to separate different ionic fragments by time-of-flight and to correlate the twodimensional momentum images of different fragments. The obtained information grants insights into the structure of the parent molecule immediately prior to fragmentation, as well as into the Coulomb explosion process itself. Ion trajectory calculations based on pairwise Coulomb repulsion and their comparison with the experimental data result in a more detailed understanding of the underlying processes. A biphenyl derivative has been chosen for the proof-of-principle work. The molecules were aligned by a nanosecond laser pulse and subsequently Coulomb exploded by a femtosecond laser pulse. The fragment ions were velocity-mapped onto a MCP/P47 detector and imaged using the PImMS camera. The recorded data include the complete time-of-flight and velocity-map information for all fragment ions. Any mass of interest can be selected during data analysis, and covariance images were generated that show coincidental velocity correlations between different fragments. The resulting information yields insights into the molecular structure of the biphenyl derivative, revealing the positions of the different substituents on the two phenyl rings, as well as the dihedral angle. Furthermore, femtosecond time-resolved experiments were performed to follow the molecular dynamics after a kick pulse had initiated torsional motion in the molecules. Next, the photo-induced trans-cis isomerization of a stilbene-type molecule was chosen as a test system to follow structural changes in photo-switches in real-time. A first comparison of experimental and simulated data looks promising, but the data analysis is still in progress. The Coulomb-explosion covariance imaging technique developed within the project holds much promise for the measurement of characteristic structural parameters, particularly those relating to bond angles, of relatively large polyatomic systems on a femtosecond time scale. The covariance analysis has been extended to include correlations between three ions, which may, depending on the molecular structure, oblivate the need for prealignment in the laboratory.
Publications
- “Covariance imaging experiments using a pixel-imaging mass-spectrometry camera”, Phys. Rev. A 89, 011401(R) (2014)
C.S. Slater, S. Blake, M. Brouard, A. Lauer, H. Stapelfeldt et al.
(See online at https://doi.org/10.1103/PhysRevA.89.011401) - “Dynamic Stark Control of Torsional Motion by a Pair of Laser Pulses”, PRL 113, 073005 (2014)
L. Christensen, J. Nielsen, C.S. Slater, A. Lauer, M. Brouard, H. Stapelfeld et al.
(See online at https://doi.org/10.1103/PhysRevLett.113.073005) - „Fast sensors for time-of-flight imaging applications“, Phys. Chem. Chem. Phys. 16, 383 (2014)
C. Vallance, M. Brouard, A. Lauer, C.S. Slater et al.
(See online at https://doi.org/10.1039/c3cp53183j) - “Coulomb-explosion imaging using a pixel-imaging mass-spectrometry camera”, Phys. Rev. A 91, 053424 (2015)
C.S. Slater, S. Blake, M. Brouard, A. Lauer, H. Stapelfeldt et al.
(See online at https://doi.org/10.1103/PhysRevA.91.053424)
