Dissociation dynamics of highly excited molecules: Theory and Experiment

This thesis presents studies on dissociation of two model molecules: Butadiene and Cyclopropane. Tunable synchrotron radiation was used to ionize or excite the molecules in the gas phase, and the momentum correlation of the resulting fragment ions were measured using a 3D momenta coincident ion spectroscopy. The experimental results were interpreted with the aid of ab-initio quantum calculation. This allows us to gain insight into the fundamental processes behind the molecular dissociation, that how correlated electronic and nuclear dynamics drive molecular dissociation.Tunable XUV-radiation was used to doubly ionize molecules to different states.By Comparing experimental and theoretical values for appearance energy and kinetic energy released of the dissociation channels, electronic gateway state of each dissociation channels were identified. By analysing the momentum vector of ion pairs as a function of photon energy and internal energy sharing in the dissociative double ionization channels, mechanisms of double ionization (direct or indirect) processes were investigated. The studies shed light on the electron-electron and electron-nuclear correlation effects in the molecules.Tunable X-rays were used to selectively excite a localized core electron to different valence orbitals, and the subsequent autoionization and dissociation processes were studied by analysing the correlated momentum of ionic fragments. In butadiene, the dependence of molecular dissociation on the initial site of core-hole was studied for the chemically shifted terminal and central carbon core-electrons excitation.In cyclopropane the dependence of molecular dissociation on the changing of the molecular bonding character was studied for different core-to-valence excitation.The studies indicated the importance of the ultra-fast nuclear dynamics initiatedwithin a few femtosecond core-hole lifetime changing the picture of electron-electron correlation in autoionization processes and leading to specific dissociation channels

Dissociative double-photoionization of butadiene in the 25-45 eV energy range using 3-D multi-coincidence ion momentum imaging spectrometry

Dissociative double-photoionization of butadiene in the 25-45 eV energy range has been studied with tunable synchrotron radiation using full three-dimensional ion momentum imaging. Using ab initio calculations, the electronic states of the molecular dication below 33 eV are identified. The results of the measurement and calculation show that double ionization from π orbitals selectively triggers twisting about the terminal or central C–C bonds. We show that this conformational rearrangement depends upon the dication electronic state, which effectively acts as a gateway for the dissociation reaction pathway. For photon energies above 33 eV, three-body dissociation channels where neutral H-atom evaporation precedes C–C charge-separation in the dication species appear in the correlation map. The fragment angular distributions support a model where the dication species is initially aligned with the molecular backbone parallel to the polarization vector of the light, indicating a high probability for double-ionization to the “gateway states” for molecules with this orientation

Molecular dynamics of NH3 induced by core-electron excitation

Nuclear motion in the N1s(-1)4a(1)(1) core-excited state of ammonia is investigated by studying the angular anisotropy of fragments produced in the decay of the highly excited molecule and compared with predictions from ab initio calculations. Two different fragmentation channels (H+/NH2+ and H+/NH+/H) reveal complex nuclear dynamics as the excitation photon energy is tuned through the 4a(1) resonance. The well-defined angular anisotropy of the fragments produced in the dissociation of the molecular dication species suggests a very rapid nuclear motion and the time scale of the nuclear dynamics is limited to the low fs timescale