The strong-field ionization mechanisms of molecules and clusters
Open Access
- Author:
- Sayres, Scott Grayson
- Graduate Program:
- Chemistry
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- August 31, 2010
- Committee Members:
- Albert Welford Castleman Jr., Dissertation Advisor/Co-Advisor
Albert Welford Castleman Jr., Committee Chair/Co-Chair
James Bernhard Anderson, Committee Member
Nicholas Winograd, Committee Member
Jorge Osvaldo Sofo, Committee Member - Keywords:
- high charge states
mass spectrometry
Coulomb explosion
strong-field ionization
multiphoton ionization - Abstract:
- The interaction of light and matter is of fundamental interest to the fields of both physics and chemistry. Advances in laser technology continually push the interaction of light and matter to higher energies, reaching unexplored intensities in which new science is emerging. The work detailed in this thesis pertains to the ionization mechanisms that occur in atoms, molecules, and clusters for a wide range of laser intensities. Presented in Chapter 1 is a brief introduction to ionization, discussing the mechanisms in which it proceeds in different mediums including atoms, molecules, and clusters. At low laser intensities, the responsible ionization mechanism for atoms and molecules is the sequential absorption of photons by the molecule until the energy is high enough to kick off an electron, known as multiphoton ionization. However, at higher intensities the electric field (ponderomotive energy) of the laser becomes comparable to the attractive forces of the nuclei and ionization proceeds through the phenomena known as quantum tunneling. Tunneling ionization relies on the small probability that the electron exists outside a barrier. A large enhancement in the ionization rate, in which electrons are ejected at an energy that is orders of magnitude lower than expected, is observed in clustered species at very high intensities and is not yet completely understood. The focus of this dissertation research is to investigate the influence of laser intensity on the production of highly charged ions from molecules and clusters in order to gain insight into the mechanisms involved. Chapter 2 provides an in depth explanation to the experimental apparatus and technique used to measure the ionization dynamics. The experiments conducted herein required use of a femtosecond laser system coupled to a time-of-flight mass spectrometer uniquely coupled to allow for the intensity to be selected. In the experiments conducted here, an ultra-intense femtosecond laser is focused onto a molecular/cluster beam, instantly heating the electrons to roughly a million kelvin causing ionization. The intensity dependence on the extent of ionization is measured in a vacuum chamber via a time-of-flight mass spectrometry. A variety of theoretical simulations used for comparison to the experimental results are explained in Chapter 3. This involves the precise determination of the electronic wavefunction through electronic structural software. Additionally, a series of tunneling models are detailed and used for the experiments. The ionization and fragmentation dynamics of a simple molecule, silane, are discussed in Chapter 4. The ionization potential of silane is found to be in agreement with other techniques. Moreover, the appearance potentials of the fragments have been measured and are also in relatively good agreement with literature results, as well as calculated adiabatic values. The branching ratio is determined to proceed to SiH3+ to a larger extent than with chemical vapor deposition, which could help in the production of defect free thin film transistors and solar cells. In Chapter 5, an in depth investigation to the ionization mechanism of ammonia is detailed and compared to the mechanism observed in ammonia clusters. The ionization of ammonia was found to proceed through tunneling of electrons from the highest occupied molecular orbital (HOMO). Fragments of NH2+ and NH+ are observed to appear from electrons tunneling from an inner orbital, HOMO-1. Thus, ionization is observed to originate from multiple orbitals, with the orbital contributions separated by m/z in the mass spectrometer. Upon clustering, complete removal of all valence electrons was observed orders of magnitude lower in laser intensity than expected. This extreme ionization enhancement demonstrates tunneling from all valence orbitals. Ionization occurs non-sequentially indicating that once an orbital is accessed by the laser, it is emptied of electrons. Chapter 6 discusses the measurement of the ionization potentials of small silicon clusters. The intensity scan method can be used to determine the ionization potential of stable clusters and even be used as a new technique for discovering magic clusters. The high charge states are found to behave according to as if their ionization potentials were lowered by ~40%. They also demonstrate a nonsequential ionization mechanism. Chapter 7 discusses the ionization mechanisms for the Group Vb transition metal (V, Nb, and Ta) oxide clusters. The exchange of the metal ion does not lead to extensive changes in the energy requirements of the high charge states. However, the more stable the cluster distribution, the further the ionization proceeds. The appearance of the high charge states of the oxygen are not only enhanced, but demonstrate disagreement with the expected appearance ordering from the ADK model. The ionization mechanism is attributed to arising from frozen orbitals. As many orbitals are similar in energy, this leads to a steady flowing of electrons from the clusters at low laser intensities, before significant movement occurs. Finally, Chapter 8 brings this thesis to a conclusion. By systematically varying the laser intensity, it has been possible to not only measure the ionization potential for a wide variety of systems, but also to obtain mechanistic information. Tunneling has been witnessed to occur from multiple molecular orbitals and is greatly enhanced by the clustering phenomena, occurring orders of magnitude lower in laser intensity than expected.