On the Diffractive Imaging and Rotational-State-Selective Fragmentation of CF2I2

Abstract

Understanding how molecular structure and internal quantum-state preparation govern chemical reactivity is a central objective of molecular science. Experimental access to both atomic geometry and state-resolved reaction dynamics is therefore essential for revealing the mechanisms of molecular processes. X-ray diffraction imaging provides direct structural information with sub-angstrom sensitivity, whereas state-selected fragmentation measurements probe how rotational and vibrational excitation influence competing dissociation pathways. This dissertation combines these complementary approaches to investigate isolated Diiododifluoromethane (CF2I2) molecules under controlled gas-phase conditions. Owing to its two heavy iodine centers and its relevance as a model for reactive iodine species in biogenic polyhalomethanes, CF2I2 provides an attractive system for both X-ray diffraction imaging and fragmentation-dynamics studies.

The first part of the dissertation focuses on diffraction-image analysis of aligned molecular ensembles and addresses two key aspects. First, three-dimensional rotational control of CF2I2 is investigated. Molecular alignment mitigates the loss of structural information caused by averaging over random orientations and enables direct access to molecular-frame information. An analytical expression is presented for evaluating the degree of three-dimensional alignment in the pendular limit under linearly and elliptically polarized laser fields over a range of rotational temperatures, thereby avoiding extensive numerical simulations. Second, the calculated degree of alignment is used to describe the angular distribution of the molecular ensemble, which is then employed to simulate diffraction images including realistic noise. Molecular structural parameters are retrieved from these diffraction patterns using a forward-analysis coordinate-descent algorithm. In addition, sensitivity analysis and shot-noise-limited image reconstruction are developed for the quantitative interpretation of diffraction data. These methods establish practical criteria for the number of scattering events required to obtain interpretable images and demonstrate that molecular alignment, combined with statistical analysis, significantly improves structural determination for gas-phase diffraction experiments at advanced light sources such as X-ray Free-Electron Lasers.

The second part investigates rotational-state-dependent fragmentation dynamics of CF2I2 following strong-field ionization. Rotationally cold and state-selected molecular ensembles are prepared using electrostatic deflection, enabling systematic control of the initial rotational-state distribution. Spatially resolved ion-yield measurements reveal pronounced changes in fragment branching ratios with rotational-state population, demonstrating that rotational excitation can modify access to competing ionization and dissociation pathways. These observations provide evidence that molecular rotation plays an active role instrong-field fragmentation dynamics and highlight the importance of initial-state preparation in determining chemical reaction outcomes.