Ultra-fast Dynamics of Small Molecules in Strong Fields

Title of dissertation: ULTRA-FAST DYNAMICS OF SMALL MOLECULES IN STRONG FIELDS Kun Zhao, Doctor of Philosophy, 2006 Dissertation directed by: Professor Wendell T. Hill, III Institute for Physical Science and Technology & Department of Physics Correlation detection techniques (image labeling, coincidence imaging, and joint variance) are developed with an image spectrometer that is capable of collecting charges ejected over 4π sr and a digital camera that can be synchronized with the laser repetition rate at up to 735 Hz. With these techniques, molecular decay channels ejecting atomic fragment ions with different momenta (energies) can be isolated; thus the initial molecular configurations (bond lengths and/or bond angles) and orientations as well as their distributions can be extracted. These techniques are applied to study strong-field induced dynamics of diatomic and triatomic molecules. Specific studies included the measurement of (1) the Coulomb explosion energy as a function of bond angle in linear (CO2) and bent (NO2) triatomics and (2) the ejection anisotropy relative to the laser polarization axis during Coulomb explosions in both triatomic (CO2 and NO2) and diatomic (H2, N2 and O2) systems. All the experiments were performed with 100 fs, 800 nm laser pulses focused to intensities of 0.1 ∼ 5.0× 10 W/cm. The explosion energy of NO2 decreases monotonically by more than 25% from the smallest to the largest bond angles. By contrast, the CO2 explosion energies are nearly independent of bond angle. The enhanced ionization and static screening models in two-dimension with three charge centers were developed to simulate the explosion energies as a function of bond angle. The predictions of both models are consistent with the measurements of CO2 and NO2. At the same time, the observed explosion signals as a function of bond angle for both CO2 and NO2 show largeamplitude vibrations. The peaks of the explosion signal distributions for both CO2 and NO2 appear near the equilibrium bond angles of the neutral systems. The ejection angular distributions in triatomic (CO2 and NO2) and diatomic (H2, N2, and O2) Coulomb explosions were measured; the contribution made to the ejection anisotropy by geometric and dynamic alignment was studied by comparing the images obtained with linearly and circularly polarized fields. Different angular distributions of the molecules are consistent with different ionization stages, induced dipole moments and rotational constants. A narrower distribution of H2 than other diatomics is confirmed by its nearly complete dynamic alignment in the field. The results also show that the large dynamic alignment in the linear triatomic CO2 is consistent with the fact that more electrons have been removed and the precursor molecular ion spends more time in the field prior to the explosion than diatomic systems such as N2 and O2. ULTRA-FAST DYNAMICS OF SMALL MOLECULES IN STRONG FIELDS