George N. Gibson - US grants

The proposal calls for the development of a versatile high- repetition rate short-pulse laser system and the associated ability to carry out correlation and coincidence measurements on electron and ion time-of-flight signals. These tools will be applied to a wide range of problems including: Correlation techniques to unambiguously determine dissociation pathways of highly ionized diatomic molecules in strong laser fields, Curve crossing in transient states of molecular ions, and Fundamental measurements in the above-threshold-ionization of atoms by strong laser fields. ***

The Physics Department is furnishing two dedicated computer laboratory rooms, each with 13 multimedia computer workstations and a portable capture workstation. The 26 workstations are being networked to a file server that contains a library of video clips. The department is also purchasing Pasco Science Workshop Interfaces, probes, dynamica tracks, and carts for MBL experiments. Video cameras, VCRs, laserdisc players, and video clips are available for students to do video experiments. Two student workstations are on an NT-shaped worktable with a primary group of four students collaborating on an experiment. Six worktables are placed in a U formation around a lab room so that the instructor can monitor students' progress at a glance. The Tools for Scientific Thinking curriculum for MBL experiments and Workshop Physics curriculum for video experiments are being adapted for the use of multisection elementary physics courses with large lecture sizes. These curricula encourage students to take an active role in their learning and construct physical knowledge from actual observation.

This proposal seeks funding to build a high energy, cavity dumped laser system using EO modulators. The work is an extension of previous accomplishment by the PI under a NSF CAREER award (PHY-9502935). The PI has demonstrated an EO cavity-dumped laser system that has produced 200nJ of energy in 20 fsec at 1 KHz repetition rate, which is twice the energy of the state-of-the-art acuosto-optic (AO) cavity-dumped laser system. The advantage of the EO cavity-dumping over that of AO is that the latter suffers instability at high powers due to the nonlinear process of the AO crystal. The PI predicted that the theoretical maximum energy of an EO cavity dumped laser system is still 50 times what has been demonstrated, and that a repetition rate as high as 10 MHz is possible. This proposal is to improve the original design and to achieve even higher energy of the laser system than he has demonstrated.

This project in laser-molecule interactions utilizes a femtosecond laser system to probe the behavior of molecules in strong laser fields. The work will attempt to disentangle various effects that influence this behavior, such as bond softening, light-induced bound states, enhanced ionization by electron localization, charge-asymmetric dissociation, and rescattering. Measurements will employ a new technique called correlated ion spectroscopy.

With this award from the Major Research Instrumentation (MRI) Program, the Department of Physics at the University of Connecticut will acquire a tunable ultrafast laser and a femtosecond time-resolved optical spectrometer. This equipment will enhance research in the following areas: a) photosynthetic energy transfer; b) biological photoconversion; and c) high-intensity laser physics. Faculty and students from Bowdoin College, a primarily undergraduate institution, will also carry out research studies using this instrumentation.

This instrumentation will allow participating students and postdoctoral associates to obtain experience in the techniques for the study of ultrafast processes applied to biological materials and strong field physics.

This research project focuses on the study of the nature and extent of the strong field excitation of molecules. Recent theoretical work shows that diatomic molecules possess an electronic structure in which the AC Stark shift is greatly reduced and the multi-photon coupling is greatly enhanced to such an extent that high-order (n>10) multi-photon pi pulses can be produced through a resonant process. The behavior of molecules in strong laser fields as a function of laser wavelength will be studied using an optical parametric amplifier, and ultraviolet fluorescence spectroscopy will be used as a highly specific indicator of the excited states formed. This work is expected to have a major impact in the vast field of laser/matter interactions. Moreover, these experiments require state-of-the-art laser systems, giving students hands-on experience with the latest laser and optical technologies and, thus, will prepare them for a variety of research-based or high-tech career paths.

This project will follow up work that established and experimentally verified a theory of strong resonance multiphoton coupling in diatomic molecules. The goals of the project are to use this phenomenon in molecular hydrogen ions to populate vibrationally-bound excited states that have never before been studied. This effort will likely lead to the creation of population inversions on transitions around 9 eV. The project will also involve a study of the dynamics of strong-field ionization to evaluate an important assumption that is typically made in strong field physics, namely, the least bound electron is ionized by the strong laser field. It has been difficult to determine which electron is removed, or equivalently, in which electronic state the molecular ion is left. The final state of molecular ions produced by strong laser fields will be determined by measuring their unique vibrational motion's signature through pump-probe spectroscopy. This vibrational motion can also reveal information about electronic states of molecules that have not been studied before.

Beyond the specific field of study, strong multiphoton coupling in molecular systems may lead to a novel and possibly efficient way of producing amplification in the vacuum ultraviolet spectral region and it may also lead to a new source of metastable hydrogen atoms. After many years of research, basic questions about the behavior of diatomic molecules in strong laser fields remain, especially the extent of vibrational and electronic excitation. Until these questions are answered applications such as quantum tomography and high-harmonic generation will not be precisely understood. While addressing these questions, the project will determine new information about the electronic structure of molecular ions, which is important for physical chemistry. Finally, these experiments require state-of-the-art laser systems, giving students hands on experience building and refining the latest laser and optical technologies, preparing them for a variety of research-based academic or high-tech career paths.

Coherent control of chemical reactions has been a driving force in laser physics since the invention of the laser itself. The group will use this full arsenal of control techniques on molecules to answer basic questions about the interaction of molecules with intense laser fields. The broadest goal is to understand the transition from molecular to atomic behavior in molecules as a function of internuclear separation. In addition, over the past several years, an entirely new concept has evolved from studies of molecules in intense laser fields that we call "incoherent" or "dissipative" control. Unlike traditional coherent control, this new method actually works better at higher temperatures and may lead to methods for controlling chemical reactions at room temperature.

Interest in high-harmonic generation (HHG), attosecond physics, and short-pulse x-ray free electron lasers has exploded over the past decade. However, research in these fields requires huge investments of time and money. We can study and test many aspects underlying the physics of these processes more quickly and inexpensively at a small University-based lab to support projects at large facilities. In addition, the experiments require state-of-the-art laser systems, giving students hands-on experience with the latest laser and optical technologies and preparing them for a variety of research-based academic or high-tech industrial career paths.

Intellectual Merit

Using high-intensity ultra-fast laser pulses we can now control both the bond length and orientation of diatomic molecules. This gives us a unique opportunity to directly probe the structure of molecular orbitals and ask fundamental questions; such as, is there coherence between the different orbitals and how do relativistic orbitals differ from regular orbitals? In addition to probing molecular structure, we will also use our ability to manipulate molecules in order to generate highly excited states of the molecular ions with the ultimate goal of creating population inversions in the vacuum-ultraviolet spectral region. Finally, we will test our hypothesis that certain configurations of the molecule may be highly susceptible to generating harmonic radiation driven by an intense fundamental laser. Moreover, we have developed a new phase-matching technique that greatly enhances the efficiency of harmonic generation, in general. Coupling these two techniques together may provide a new source of intense short-pulse vacuum-ultraviolet radiation.

Broader Impact

The new phase-matching techniques may lead to a new class of coherent, ultra short pulse VUV radiation sources. The strong field techniques developed in this award may lead to important insights into molecular structure useful for physical chemistry. More broadly, interest in high-harmonic generation, attosecond physics, and short-pulse x-ray free electron lasers has exploded over the past decade, and this award contributes towards understanding important aspects such as inner-orbital ionization, excitation, and vibrational coherence. Finally, this award provides hands on research training in state-of-the-art laser and optical technologies and prepares them for academic or high-tech career paths.

Understanding how light interacts with matter is important in scientific research and applications. For example, one of the most vital processes in nature is the absorption of solar energy by plants in photosynthesis. Scientists use lasers to make forms of light with special properties that are used in research and technologies such as optical communications. The research supported in this project will investigate how extremely intense laser light interacts with small molecules. Insight will be gained into three main areas. 1) When molecules interact with intense laser light, the molecules can, in turn, generate new types of radiation with unique properties. In this case, the generated light can have very short wavelengths and very short durations in time. These properties can be used to study very fast processes such as chemical reactions. 2) Intense laser light can drive processes in molecules in a way that allows more to be learned about the structure of molecules. 3) Intense laser light can be used to control the vibrational motions of molecules, which is one of the properties of molecules used for storing and transferring energy. Energy transfer in molecules will be better understood at the fundamental level by controlling molecular vibrations. 4) The techniques and instrumentation used in these experiments provide excellent training for students to acquire skills used in many industries and research laboratories.

This project focuses specifically on strong field processes that give rise to excitation in molecules. Although the excitation of molecules by strong fields is ubiquitous, there is not nearly as much experimental and theoretical work on this compared to other areas of strong field physics. Moreover, the range of possible mechanisms for excitation remains largely unexplored. However, there is increasing interest in this field, as one pathway to excitation involves inner orbital ionization. It turns out that excitation can be so significant that it can lead to lasing in the atmosphere by intense laser beam filaments. Nevertheless, the exact process by which inversions are formed in nitrogen is still not fully understood. The common theme to all of these goals is to understand the excitation of molecules by strong laser fields and the dependence of this excitation on internuclear separation. Successful outcomes of these experiments should greatly expand the understanding of how intense light interacts with matter, particularly regarding molecular structure and dynamics.