Unusual and challenging ionization processes take place when an atom or molecule is placed in the presence of a super intense, ultra short laser field. One such process is the ionization and subsequent return of an electron to the ionic core. The electron carries with it the energy it has absorbed from the laser field and this energy drives different atomic phenomena such as high harmonic generation or multiple ionization. The mechanism of the electron return is often referred to as the “three-step” model. In this model, an electron is first ionized at the peak amplitude of the laser field. Once ionized, a change in the direction of the laser field forces the electron to return to the parent ion and causes a subsequent recollision. The purpose of this thesis is to examine in great detail the recollision process, its mechanisms, and its dependence on physical parameters (such as laser intensity and ellipticity) for a number of physically interesting scenarios.

The General theory of Relativity (GR) brought gravity into accord with the principles of locality and relativity. Since its discovery it has been preeminent, recognized as the most accurate description of gravity on the many scales where it has been tested. During this period, seemingly radical predictions like the existence of black holes and the expansion of the Universe have been verified and testify to the great leap of insight that GR represented in our understanding of space and time. However not all precision observations of astrophysical systems have yielded easily to interpretation within GR, and with the discovery of cosmic acceleration, there is genuine concern that General Relativity may be incomplete when describing the Universe on the largest sizes imaginable. In this uncertainty, many theoretical models have been proposed. In this thesis we shall first outline the motivation behind a certain subset of these models and the known issues that arise in interpreting these models as alternative theories of gravity. Then focus on one variety of theory the f(R) modifications to gravity. Demonstrating that many of the known instabilities have a common origin and that they are avoided when treating these theories via perturbative constraints. In the second part of this work we examine the astrophysical impact of modifications to gravity, first in the case of high mass neutron stars, then subsequently on corrections to the line profile of neutral hydrogen from violations of the equivalence principle. Finally we explore the phenomenology of modifications to gravity that produce late-Universe acceleration. In particular, what solutions are allowed and what range of accelerations are predicted as a result. Furthermore we explore how a correction to gravity at large scales would impact the growth and evolution of cosmological perturbations.

We consider electron dynamics in strong electromagnetic fields, such as those expected from the next generation of high-intensity laser facilities. Beginning with a review of constant classical fields, we demonstrate that the electron motion (as given by the Lorentz force equation) can be divided into one of four Lorentz invariant cases. Parameterising the field tensor in terms of a null tetrad, we calculate the radiative energy spectrum for an electron in crossed fields. Progressing to an infinite plane wave, we demonstrate how the electron orbit in the average rest frame changes from figure-of-eight to circular as the polarisation changes from linear to circular. To move beyond a plane wave one must resort to numerics. We therefore present a novel numerical formulation for solving the Lorentz equation. Our scheme is manifestly covariant and valid for arbitrary electromagnetic field configurations. Finally, we reconsider the case of an infinite plane wave from a strong field QED perspective. At high intensities we predict a substantial redshift of the usual kinematic Compton edge of the photon emission spectrum, caused by the large, intensity dependent effective mass of the electrons inside the laser beam. In addition, we find that the notion of a centre-of-mass frame for a given harmonic becomes intensity dependent.

Doctor of Philosophy / Department of Physics / Brett D. Esry / Strong-field ionization plays a central role in molecules interacting with an intense laser field since it is an essential step in high-order harmonic generation thus in attosecond pulse generation and serving as a probe for molecular dynamics through either the sensitivity of ionization to the internuclear separation or the laser-induced electron scattering. Strong-field molecular ionization has been studied both theoretically and experimentally, dominantly through the Born-Oppenheimer approximation and at equilibrium or small reaction distances. We have extended the theoretical studies of molecular ionization to a much broader extent. Specifically, due to the difficulty of treating ionization in Born-Oppenheimer representation especially for molecular dynamics involving strongly-correlated electron-nuclear motion, we have investigated an alternative time-independent--adiabatic hyperspherical--picture for a one-dimensional model of the hydrogen molecule. In the adiabatic hyperspherical representation, all the reaction channels--including ionization--for the hydrogen molecule have been identified in a single set of potential curves, showing the advantage of studying molecular dynamics involving multiple breakup channels coupled with each other. We have thus proposed a good candidate to study strongly-correlated molecular dynamics, such as autoionization and dissociative recombination. Moving to a time-dependent picture by numerically solving the time-dependent Schrödinger equation (TDSE), we have explored two extreme classes of strong-field ionization of hydrogen molecule ion: at large internuclear distances (R>30 a.u.) and for long-wavelength laser fields. Remarkably, we have found strong-field two-center effects in molecular ionization beyond the long-standing one-photon two-center interference as a manifestation of the double-slit interference. In particular, the total ionization probability at large internuclear distances shows strongly symmetry-dependent two-center dynamics in homonuclear diatomic molecules and two-center induced carrier-envelope phase effect in heteronuclear diatomic molecules. Such two-center effects are expected to generalize to other diatomic systems and could potentially be used to explain phenomena in multi-center strong-field physics. Moreover, we have theoretically confirmed, for the first time, the existence of low energy structure in molecular ionization in long-wavelength laser fields by solving the three-dimensional TDSE. Finally, we have performed a pump-probe study of the hydrogen molecular ion where a pump pulse first dissociates the molecule followed by a probe pulse which ionizes the dissociating wave packet, and surprisingly found a pronounced broad ionization peak at large R or large pump-probe delay (~150 fs). Numerically, we have developed and implemented new theoretical frameworks to more accurately and efficiently calculate quantum mechanical processes for small molecules--hydrogen molecule and its ion--which could readily be adapted to heavier diatomic systems.

Doctor of Philosophy / Department of Physics / Vinod Kumarappan / In general, molecules in the gas phase are free to rotate, and measurements made on such samples are averaged over a randomly oriented distribution of molecules. Any orientation dependent information is lost in such measurements. The goal of the work presented here is to a) mitigate or completely do away with orientational averaging, and b) make fully resolved orientation dependent measurements. In pursuance of similar goals, over the past 50 years chemists and physicists have developed techniques to align molecules, or to measure their orientation and tag other quantities of interest with the orientation. We focus on laser induced alignment of asymmetric top molecules. The first major contribution of our work is the development of an effective method to align all molecular axes under field-free conditions. The method employs a sequence of nonresonant, impulsive laser pulses with varied ellipticities. The efficacy of the method is first demonstrated by solution of the time dependent Schr\"{o}dinger equation for iodobenzene, and then experimentally implemented to three dimensionally align 3,5 difluoroiodobenzene. Measurement from molecules aligned in this manner greatly reduces orientational averaging. The technique was developed via a thorough understanding and extensive computations of the dynamics of rotationally excited asymmetric top molecules. The second, and perhaps more important, contribution of our work is the development of a new measurement technique to extract the complete orientation dependence of a variety of molecular processes initiated by ultrashort laser pulses. The technique involves pump-probe measurements of the process of interest from a rotational wavepacket generated by impulsive excitation of asymmetric top molecules. We apply it to make the first measurement of the single ionization probability of an asymmetric top molecule in a strong field as a function of all relevant alignment angles. The measurement and associated calculations help identify the orbital from which the electron is ionized. We expect that this technique will be widely applicable to ultrafast-laser driven processes in molecules and provide unique insight into molecular physics and chemistry.

We demonstrate experimentally that the multiphoton ionization rate in gallium arsenide depends on the alignment of the laser polarization with respect to the crystal axis. We show real-time modulation of 1900nm laser ionization rate, through viewing transmission, which mimics the symmetry of the semiconductor crystal. We propose that the modulation in the ionization rate arises because the varying reduced effective carrier mass, as predicted by Keldysh theory. We show direct comparison of the experimental transmission modulation depth with that predicted by Keldysh theory. This opens up a novel method for real-time non-invasive crystallography of crystalline materials.

We employ an extreme ultraviolet (XUV) pulse to impulsively excite dipole polarization in atoms or molecules, which corresponds to coherently prepared superposition of excited states. A delayed near infrared (NIR) pulse then perturbs the fast evolving polarization, and the resultant absorbance change is monitored in dilute helium, dense helium, and sulfur hexafluoride (SF6) molecules. We observe and quantify the time-dependence of various transient phenomena in helium atoms, including laser-induced phase (LIP), time-varying (AC) Stark shift, quantum path interference, and laser-induced continuum structure. In the case of dense helium targets, we discuss nonlinear macroscopic propagation effects pertaining to LIP and resonant pulse propagation, which account for the appearance of new spectral features in transient lineshapes. We then use tunable NIR photons to demonstrate the wavelength dependence of the transient laser induced effects. In the case of molecular polarization experiment in SF6, we show suppression of XUV photoabsorption corresponding to inter-valence transitions in the presence of a strong NIR field. In each case, the temporal evolution of transient absorption spectra allows us to observe and understand the transient laser induced modifications of the electronic structure of atoms and molecules.

Quantum mechanical motion of electrons in atoms and molecules is at the heart of many photophysical and photochemical processes. As the natural timescale of electron dynamics is in the range of femtoseconds or shorter, ultrashort pulses are required to study such phenomena. The ultrashort pulse light-matter interaction at high intensity regime can however dramatically alter the atomic and molecular structures. Our current understanding of such transient electronic modification is far from complete, especially when complicated light-induced couplings are involved. In this dissertation, we investigated how a femtosecond strong-field pulse can control or modify the evolution of atomic or molecular polarization, representing electric dipole excitation in various systems. Extreme ultraviolet (XUV) attosecond pulse trains are used to coherently prepare superposition of excited states in various atomic and molecular systems. A subsequent phase-locked infrared (IR) femtosecond pulse is applied to perturb the dipoles, and transient changes in the transmitted XUV spectra are measured. This scheme is termed as XUV attosecond transient absorption spectroscopy. In the first study, we applied this technique to study the modification of Rydberg states in dilute helium gas. We observed several transient changes to the atomic structure, including the ac Stark shift, laser-induced quantum phase, laser-induced continuum structure, and quantum path interference. When the experiments were extended to the study of a dense helium gas sample, new spectral features in the absorption spectra emerged which cannot be explained by linear optical response models. We found that these absorption features arise from the interplay between the XUV resonant pulse propagation and the IR-imposed phase shift. A unified physical model was also developed to account for various scenarios. Extending our work to argon atoms, we studied how an external infrared field can be used to impulsively control different photo-excitation pathways and the transient absorption lineshape of an otherwise isolated autoionizing state. It is found that by controlling the field polarization of the IR pulse, we can modify the transient absorption line shape from Fano-like to Lorentzian-like profiles. Unlike atoms, in our study of autoionizing states of the oxygen molecule, we observed both positive and negative optical density changes for states with different electronic symmetries. The predictions of two distinct and simplified dipole perturbation models were compared against both the experimental results and a full theoretical calculation in order to understand the origin of the sign of absorption change. We relate this symmetry-dependent sign change to the Fano parameters of static photoabsorption. The same approach was applied to study molecular nitrogen, in which we observed the decay dynamics of IR perturbed doubly-excited Rydberg states with many vibrational progressions. In addition, we also conducted experiments to investigate Rydberg state dynamics of other molecular systems such as carbon dioxide. In summary, we experimentally explored the ephemeral light-induced phenomena associated with excited states of atoms and molecules. These studies provide real-time information on ultrafast electronic processes and provide strategies for direct time-domain control of the light-matter interaction.

Master of Science / Department of Physics / Itzhak Ben-Itzhak / Ultrafast lasers allow us to study molecular dynamics on their natural timescale. The electronic dynamics can be studied using attosecond pulses, while the vibrational and rotational dynamics can be probed using tens of femtosecond and picosecond laser pulses, respectively. This capability has led to a broad understanding of the electronic dynamics in atoms and molecules as well as vibrational and rotational dynamics of molecules, which is one of the important goals in basic science. Moreover, it is possible to control quantum mechanical processes using ultrafast intense lasers. In this thesis, we focus on a couple of experiments. The first involves quantum control of the formation of neutral molecular fragments while the second focuses on three-body fragmentation of molecules employing the native-frames analysis method, which was recently introduced by our group [J. Rajput et al., Phys. Rev. Lett. 120, 103001 (2018)]. Experimental studies focused on the formation of excited neutral D fragments from D2 molecules are presented. We show that by manipulating the chirp of the intense laser pulses, i.e. the “time order” of the frequency components within the pulse, the formation of these fragments is controlled. To achieve this control we implement a single-prism compressor to manipulate the chirp of the laser pulses. Three-body fragmentation of CO₂ resulting in C+ + O+ + O+ is also studied. We show that if the two bonds break in a two-step process, i.e. a sequential breakup, the pathways from which the two identical O+ fragments originate can be separated using the native-frames analysis method. In contrast, the two O+ fragments cannot be distinguished if the two C-O bonds break simultaneously.

We demonstrate experimentally that the multiphoton ionization rate in gallium arsenide depends on the alignment of the laser polarization with respect to the crystal axis. We show real-time modulation of 1900nm laser ionization rate, through viewing transmission, which mimics the symmetry of the semiconductor crystal. We propose that the modulation in the ionization rate arises because the varying reduced effective carrier mass, as predicted by Keldysh theory. We show direct comparison of the experimental transmission modulation depth with that predicted by Keldysh theory. This opens up a novel method for real-time non-invasive crystallography of crystalline materials.