Fragmentation Dynamics of Triatomic Molecules in Femtosecond Laser Pulses Probed by Coulomb Explosion Imaging

Abstract

In this thesis we have utilized few-cycle pulses in the range 10-15s, to initiate CE to allow us to image the structure, dynamics, and kinetics of ionization and dissociation of triatomic molecules. We have made a series of measurements of this process for CO2 and N2O, by varying the laser pulse duration from 7 to 500 fs with intensity ranging from 2.5×1014 to 4×1015 (W/cm2), in order to identify the charge states and time scales involved. This is a new approach in CEI introducing a multi-dimensional aspect to the science of non-perturbative laser-molecule interaction. We refer to this approach as FEmtosecond Multi-PUlse Length Spectroscopy (FEMPULS). The use of a time and position sensitive detector allow us to observe all fragment ions in coincidence. By representing the final fragmentation with Dalitz and Newton plots, we have identified the underlying break up dynamics. Momentum conservation has been used to extract the correlated fragment ions which come from a single parent ion. This is achieved by considering that the total momentum of all correlated fragments must add up to zero. One of the main outcomes of our study is observation of charge resonance enhanced ionization (CREI) for triatomic molecules. In the case of CO2, we found that for the 4+ and higher charge states, 100 fs is the time scale required to reach the critical geometry RCO= 2.1Å and ӨOCO =163º (equilibrium CO2 geometry is RCO= 1:16Å and ӨOCO =172º. The CO23+ molecule, however, appears always to begin dissociation from closer than 1.7 Å indicating that dynamics on charge states lower than 3+ is not sufficient to initiate CREI. Finally, we make quantum ab initio calculations of ionization rates for CO2 and identify the electronic states responsible for CREI. Total kinetic energy (KER) has been measured for channels (1, 1, 1) to (2, 2, 2) and it was found that the (1, 1, 1) channel is not Coulombic, while (2, 2, 2) channel is very close to Coulombic (KER close to 90% of the coulombic potential). As another outcome of our study, for the case of N2O, we observed for the first time that there are two stepwise dissociation pathways for N2O3+: (1) N2O3+ → N++ NO2+ → N+ + N++ O+ and (2) N2O3+ → N22++O+ → N+ + N++ O+ as well as one for N2O4+ → N2++ NO2+ → N2+ + N++ O+. The N22+ stepwise channel is suppressed for longer pulse length, a phenomenon which we attribute to the influence which the structure of the 3+ potential has on the dissociating wave packet propagation. Finally, by observing the KER for each channel as a function of pulse duration, we show the increasing importance of CREI for channels higher than 3+.