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Tidspunkt

Torsdag 24. oktober 2024, kl. 10:15 - 12:00

Sted

1520-732

Af Anna Katrine Mathiassen

Claire Vallance will give a talk titled: Imaging the mechanisms of dissociative electron ionisation

Electron ionisation (EI) is important in a variety of naturally-occurring and man-made plasmas, including the Earth’s upper atmosphere, interstellar gas-clouds, gas discharges, and industrial and fusion plasmas. It is also widely used as an ionisation technique in mass spectrometry, and plays a key role in radiation damage to biological tissue. In contrast to photoionisation, EI is a collisional process. As such, it does not obey optical selection rules, and the amount of energy transferred from the electron to the molecule is often unknown, posing additional challenges when attempting to interpret experimental data. Recent experimental advances have made it possible to study the dynamics of EI by multi-mass velocity-map imaging. This approach yields complete scattering distributions for the ionic products of an electron-molecule collision as a function of the incident electron kinetic energy, providing considerable insight into the detailed dynamics of the EI process and subsequent dissociation of the molecular ion.

While the majority of parent ions formed by EI are singly-charged, at higher energies it also becomes possible to form multiply charged ions, which dissociate to give two or more charged products. In such cases covariance analysis of the data set (see Figure 1 for an example) provides further insight into the dynamics by revealing correlations between pairs of ions formed in the same event.

This talk will provide an overview of velocity-map imaging and covariance-map imaging as applied to the study of EI processes. Using the examples of CF₃I and C₂F₆, we will summarise our current working knowledge of the initial ionisation process, as well as illustrating how these new techniques can be used to unravel the complex multi-step dissociation mechanisms that ensue.

Till Jahnke will give a talk titled: Accessing the ultrafast time domain using coincidence measurements

Recording real-time movies of dynamical processes in molecules, as, for example, progressing chemical reactions, has been a driving force for many disciplines in fundamental sciences during the last decades. Comparably new are experimental techniques, that address single molecules in the gas phase and that involve coincident single-particle detection for imaging these dynamics are Coulomb explosion imaging and Photoelectron diffraction imaging. The latter employs the interference pattern of (photo)electrons emitted from molecules to infer the initial molecular geometry, the former uses (for example) ultrashort light pulses to (heavily) fragment the inspected molecules in order to gather such information from the breakup pattern. X-ray free-electron lasers are able to produce ultrashort light pulses with highest intensity, which are perfectly suitable to perform measurements along these lines. In particular, these light sources allow for time-resolved studies in a pump-probe scheme by adding ultrashort UV pulses that are synchronized with the X-ray flashes.

Since almost five years a dedicated COLTRIMS reaction microscope [1,2] is available at the SQS-instrument of the European X-ray free-electron laser, which was used recently to perform Coulomb explosion imaging (see Fig. 1) and Photoelectron diffraction imaging measurements. Some examples will be presented in the talk. The COLTRIMS technique is a powerful coincidence measurement approach, which allows in addition to examine in some cases molecular dynamics on a femtosecond time scale without the need for ultrashort light pulses. The talk will present some examples of synchrotron-based measurements along this theme, as well.

Two seminars on AMO physics / physical chemistry

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