Session B02: Velocity Mapping Imaging II

Covariance-map imaging: a new dimension for chemical dynamics studies

Since velocity-map imaging was first developed over 20 years ago it has revolutionised experimental work in the field of chemical reaction dynamics.  The approach allows the complete scattering distribution for a chosen reaction product to be imaged directly in the gas phase in a single measurement, and has provided fascinating and often quantum-state resolved insights into the mechanisms of a wide variety of photochemical, bimolecular, and surface reactions.

The development of universal ionization methods and ultrafast time-of-flight imaging sensors has ushered in the era of multi-mass velocity-map imaging, in which multiple reaction products can be imaged in a single experiment.  As well as offering a vast reduction in data acquisition time, multi-mass imaging data sets can be analysed using a statistical data processing technique known as covariance mapping to reveal correlations amongst the various detected reaction products.

This talk will provide an introduction to covariance mapping, and will show how it can be used in a variety of applications either to unravel complex multi-step reaction mechanisms or to provide information on gas-phase molecular structures.   

Coulomb explosion dynamics of thioesters

Laser-induced Coulomb explosion imaging (CEI), a powerful probing technique commonly used in ultrafast experiments, has been coupled to covariance map analysis to unravel the multi-body dissociation dynamics of complex molecular systems. In order to extract dissociation dynamics from such images, it is crucial to determine the correlation between fragments’ momenta produced at each dissociative ionization event. In the high count rate regime, to calculate the correlation between momenta of different ionic fragments, we need to use statistical methods such as covariance map analysis. Here, we used a coincidence detection technique which was initially developed by Lee et al.¹ to acquire 3D multi-mass momentum images necessary to determine the covariance between ion pairs’ momenta. The covariance images are acquired using the reconstructed multi-mass images of coincident events. Using the combination of aforementioned techniques, we have been able to study the dissociation dynamics of two thioester molecules which show interesting double-ionization decay pathways². In these studies, we induced Coulomb explosion in chlorocarbonylsulfenyl chloride and methoxycarbonylsulfenyl chloride molecules using high intensity laser beams. We have been able to reveal several fragmentation channels using two and three-fold covariance images and identify the origin of surprising correlations seen previously in the latter system. These experimental results are supported with ab initio calculations of the decomposition pathways of multiply charged parent ions.

1.           S. K. Lee, F. Cudry, Y. F. Lin, S. Lingenfelter, A. H. Winney, L. Fan and W. Li, Rev. Sci. Inst, 2014.

2.           M. F. Erben, M. Geronés, R. M. Romano and C. O. Della Védova, J. Phys. Chem. A, 2007.



    

Three-dimensional electron and ion momentum imaging with a double-sided coincidence VMI

Time-resolved experiment, be it at free-electron laser facilities or with table-top laser sources, are often hampered by limited beamtime and/or unstable laser conditions that make it difficult to repeat measurements of different observable under identical conditions. It is therefore beneficial to measure multiple observables during the same scan and possible even in coincidence. I will present several recent examples of electron and ion momentum imaging experiments performed using double-sided coincidence VMI setups [1,2] with free-electron lasers, laboratory HHG sources, and synchrotron radiation with the goal to study ultrafast photochemical reactions in gas-phase molecules [3-5].

[1] B. Erk et al., J. Synchr. Rad. 25, 1529-1540 (2018).

[2] U. Ablikim et al., Rev. Sci. Instr. 90, 055103 (2019).

[3] J.W.L. Lee et al., Nature Communications 12, 6107 (2021)

[4] S. Pathak et al., J. Phys. Chem. Lett. 11, 10205-10211 (2020)

[5] R. Forbes, et al., J. Phys. B. 53, 224001 (2020)   

Picosecond vs. Femtosecond: are all Laser Desorption Ionization created equal?

Three-dimensional (3D) momentum imaging was used to investigate the ionization dynamics of matrix-assisted laser desorption/ionization (MALDI) initiated by both picosecond and femtosecond laser pulses. Over the past three decades a significant effort has been focused on increasing the sensitivity of this method, which remains low for large biomolecules. Traditional MALDI experiments utilize UV pulses to initiate laser desorption of molecules/ions from the surface into the gaseous plume. In the plume, the facilitation of charge transfer is mainly due to collisions between matrix and analyte species. While it is customary to use nanosecond lasers, femtosecond lasers have been implemented in several MALDI studies. The orders of magnitude difference between these pulse durations would suggest that different ionization mechanisms are at play. Here, we have acquired the first 3D momentum images of desorbed ions from 2,5-dihydroxybenzoic (DHB) acid, a common MALDI matrices. The momentum distributions and relative TOF spectra revealed striking difference between the desorption processes initiated by picosecond and femtosecond pulses. The lack of initial ion momentum in all three dimensions from femtosecond pulses activation indicate a suppression of plume formation. This feature can be exploited to enhance the analyte detection sensitivity of MALDI.   

The primary steps of ion solvation

I will present recent experimental results that have enabled us to observe the solvation dynamics of a single Na + ion in liquid helium. A single Na + ion is created instantly at the surface of a liquid He nanodroplet and we measured in real time the gradual attachment of individual He atoms to the ion. Our results show that the first 12 He atoms bind to Na + with a constant rate of one atom per 0.75 ps. The experimental findings are in agreement with numerical simulations based on time-dependent density-functional theory. These simulations show that the first solvation shell of the Na + ion contains 12 He atoms and is formed in about 10 ps.   

All-Optical Three-Dimensional Electron Momentum Imaging

To achieve an efficient 3-D imaging detection of electrons/ions in coincidence, a conventional 2D imaging detector (MCP/phosphor screen) and a fast frame camera are used in the 3D velocity map imaging (VMI) technique[1, 2] . However, it is still difficult to obtain two separate TOF events for two electrons using a conventional MCP detector coupled with a photomultiplier tube (PMT). This is because the phosphor screen is usually made of P47 phosphor which has longer decay time and thus not good to achieve high temporal resolution. Furthermore, due to the very short time separation interval between two electrons, it is imperative to use different phosphor/scintillator for improved 3D electron momentum imaging.

Herein, we demonstrate that a scintillator screen coated with poly-para-phenylene laser dye (Exalite 404) can be used to achieve a greatly improved TOF resolution, which is sufficienct for 3D electron imaging.. A silicon photomultiplier tube (si-PMT) is also adopted to suppress the ringing in electric signals, typically associated with MCP pick-off.. The shorter emission lifetime of the poly-paraphenylene dye compared to the conventional P47 phosphor helps achieve an unprecedented dead time (~0.48 ns). This has greatly enhanced the multi-hit capability of the 3D VMI technique in detecting two or more electrons in coincidence.