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.

No description available.

Master of Science / Physics / Vinod Kumarappan / This report includes the experimental details and results of two experiments. The first experiment addresses carrier envelope phase (CEP) effects in higher order harmonic generation (HHG), and the second experiment is a pump-probe experiment on CO₂ molecules using ultrashort laser pulses. Ultrashort laser pulses that are only a few optical cycles long are of interest for studying different atomic and molecular processes. The CEP of such a pulse is an important parameter that can affect the experimental results. Because the laser pulses we used in the HHG experiment have random CEP, we tagged a given harmonic spectrum with the CEP of the fundamental laser pulse that generated it by measuring both shot-by-shot. The first chapter of this report is about the experimental details and the results we got from our CEP-tagged HHG experiment that enabled us to observe the interference of different quantum pathways. In the second experiment, discussed in the second chapter of this report, we tried to study the structure of the CO₂⁺ ion created by strong field ionization in a pump-probe experiment. For this experiment, we used an ultrashort laser pulse to ionize CO₂ molecules, and after various time delays we probed the ionic wave packet by ionizing CO₂⁺ with another ultrashort laser pulse. By performing Fourier analysis on the delay-dependent CO₂⁺⁺ yield, we were able to identify the populated states of CO₂⁺.

Doctor of Philosophy / Department of Physics / Carlos A. Trallero / Ultra-fast spectroscopy has become a common tool for understanding the structure and dynamics of atoms and molecules, as evidenced by the award of the 1999 Nobel Prize in Chemistry to Ahmed H. Zewail for his pioneering work in femtochemistry. The use of shorter and more energetic laser pulses have given rise to high intensity table-top light sources in the visible and infrared which have pushed spectroscopic measurements of atomic and molecular systems into the strong-field limit. Within this limit, there are unique phenomena that are still not well understood. Many of such phenomena involve a photoionization step. For three decades, there has been a steady investigation of the single ionization of atomic systems in the strong-field regime both experimentally and theoretically. The investigation of the ionization of more complex molecular systems is of great interest presently and will help with the understanding of ultra-fast spectroscopy as a whole. In this thesis, we explore the single ionization of molecules in the presence of a strong electric field. In particular, we study molecular isomer pairs, molecules that are the same elementally, but different structurally. The main goal of this work is to compare the ionization yields of these similar molecular pairs as a function of intensity and gain some insight into what differences caused by their structure contribute to how they ionize in the strong-field limit. Through our studies we explore a wavelength dependence of the photoionization yield in order to move from the multi-photon regime of ionization to the tunneling regime with increasing wavelength. Also, in contrast to our strong-field studies, we investigate isomeric molecules in the weak-field limit through single photon absorption by measuring the total ionization yield as a function of photon energy. Our findings shed light on the complexities of photoionization in both the strong- and weak-field limits and will serve as examples for the continued understanding of single ionization both experimentally and theoretically.

This dissertation investigates the electronic spectroscopy of a series of alkyl phenyl ketone radical cations and the dynamics of selective launch states in the strong field regime with tunable near infrared ultrashort laser pulses from 790 nm - 1550 nm coupled to mass spectrometric detection. Our method relies on tunable strong field laser pulses in the range from 1150 nm - 1550 nm to adiabatically ioinized gas phase molecules and prepare ions in the ground ionic state that serve as a launch state for future excitation and control. Adiabatic ionization is capable of transferring little energy to the molecule and producing a majority of a parent molecular ion in comparison to nonadiabatic ionization wherein multiple ionic states can be populated with an accompanying high degree of molecular fragmentation. We measure a dynamic resonance in the low lying electronic states of the acetopheone radical cation via preparation of a launch state with adiabatic ionization followed by a one photon transition within a single pulse duration which facilitates bond dissociation to produce the benzoyl ion. Experiments on acetophenone homologues and derivatives elucidate the structural dependence of the electronic resonance and supporting ab initio calculations identify the dynamic resonance along the molecular torsional coordinate between the ground ionic state, D0, and second excited state, D2. Post ionization excitation within the pulse duration transfers the ground state wavepacket to the D2 surface where the wavepacket encounters a three state conical intersection that facilitates the preferred bond dissociation. Time resolved photodissociation experiments measure the dynamics of the launch state, large amplitude oscillations and extended coherence times support the notion that adiabatic ionization populates a majority of the ground ionic surface. Control of the dissociation products is initiated from the launch state by varying the pump wavelength and probe intensity. Elimination of the D0 wavepacket with a 1370 nm reveals additional secondary dynamics that are attributed to wavepacket motion on the D2 surface. Finally, the effect of para substitution on the acetophenone radical cation is explored as a strategy to control the launch state wavepacket dynamics. Suppresion of the wavepacket dynamics are observed with the addition of alkoxy groups whereas extended coherence of the launch state dynamics approaching ~5 ps is observed upon trifluoromethyl substitution. A possible mechanism for the extended coherenece based on coupled torsional rotors is proposed. / Chemistry

La génération d'harmoniques d'ordre élevé (HHG) est un processus non linéaire extrême qui peut être compris intuitivement par la séquence de trois étapes: i) ionisation tunnel de la cible atome/ molécule et création d'un paquet d'ondes électronique (EWP) dans le continuum, ii) accélération de l'EWP par le champ laser intense et iii) recombinaison avec le cœur ionique et émission d’une impulsion attoseconde de lumière cohérente dans l’extrême UV (XUV). La HHG fournit ainsi une source ultracourte accordable dans l’XUV/ rayons X mous à l'échelle de temps attoseconde pour les applications (schéma «direct»). Dans le même temps, elle encode de manière cohérente dans le rayonnement XUV émis la structure et la dynamique de réarrangement de charge des atomes/molécules qui rayonnent (schéma «auto-sonde» ou Spectroscopie d'harmoniques d'ordre élevé). Cette thèse est consacrée à ces deux schémas d'application en attophysique basés sur une caractérisation et un contrôle avancés de l'émission attoseconde. Dans ce qu'on appelle le schème "auto-sonde", la dernière étape de la HHG, la recombinaison électron-ion peut être considérée comme un procédé de sonde et l'émission peut coder des informations fructueuses sur le système se recombinant, telles que la structure moléculaire et la dynamique. Dans la première partie, nous avons effectué la spectroscopie harmonique de molécules N₂O et CO₂ qui sont alignées par rapport à la polarisation du laser générateur. Nous avons implémenté deux méthodes basées respectivement sur l'interférométrie optique et quantique afin de caractériser l'amplitude et la phase de l'émission attoseconde en fonction à la fois de l'énergie des photons et de l'angle d'alignement. Nous avons découvert de nouveaux effets dans la génération d'harmoniques qui ne peuvent pas être expliqués par la structure de l'orbitale moléculaire la plus haute occupée (HOMO). Au lieu de cela, nous avons trouvé que pendant l'interaction avec le champ laser, deux états électroniques sont excitées de manière cohérente dans l'ion moléculaire, formant un paquet d'ondes de «trou» se déplaçant à une échelle de temps attoseconde dans la molécule après l’ionisation tunnel. Nous nous sommes concentrés sur l'exploration de ce mouvement électronique cohérent à l'intérieur de la molécule, et comparé les mesures de N₂O et CO₂. La différence frappante dans la phase harmonique nous a conduits à l'élaboration d'un modèle multi-canal permettant l'extraction de l’amplitude et de la phase relative des deux canaux impliqués dans l'émission. Un déphasage inattendu de pi/4 entre les deux canaux est obtenu. En outre, nous avons étudié le profil des impulsions attosecondes émises par ces deux molécules, et nous avons proposé un moyen simple mais flexible pour la réalisation de la mise en forme d’impulsions attosecondes. Dans la deuxième partie, la spectroscopie harmonique a été étendue à d'autres systèmes moléculaires, y compris certaines molécules relativement complexes, par exemple, SF₆ et petits hydrocarbures (méthane, éthane, éthylène, acétylène). Elle a révélé de nombreux résultats intéressants tels que des distorsions de phase observées pour la première fois. Dans le schéma «direct», nous avons photoionisé des atomes de gaz rares en utilisant des impulsions attosecondes bien caractérisées combinées avec un laser infrarouge d’habillage avec un délai contrôlé, stabilisé à environ ± 60 as. Nous avons mesuré des différences marquées dans les distributions angulaires des photoélectrons, en fonction du nombre de photons IR échangés. Jointes à notre interprétation théorique, ces observations apportent de nouvelles connaissances sur la dynamique de cette classe de processus de photo-ionisation multi-couleurs qui sont une étape clé vers l'étude de la photo-ionisation dans le domaine temporel avec une résolution attoseconde. / High-order Harmonic Generation (HHG) is an extreme nonlinear process that can be intuitively understood as the sequence of 3 steps: i) tunnel ionization of the target atom/molecule, creating an electronic wave packet (EWP) in the continuum, ii) acceleration of the EWP by the strong laser field and iii) recombination to the core with emission of an attosecond burst of XUV coherent light. HHG thus provides a tunable ultrashort tabletop source of XUV/Soft X-ray radiation on attosecond time scale for applications (‘direct’ scheme). At the same time, it encodes coherently in the XUV radiation the structure and dynamical charge rearrangement of the radiating atoms/molecules (‘self-probing’ scheme or High Harmonic Spectroscopy). This thesis is dedicated to both application schemes in attophysics based on advanced characterization and control of the attosecond emission. In the so-called ‘self-probing’ scheme, the last step of HHG, the electron-ion re-collision can be considered as a probe process and the emission may encode fruitful information on the recombining system, including molecular structure and dynamics. In the first part, we performed high harmonic spectroscopy of N₂O and CO₂ molecules that are (laser-)aligned with respect to the polarization of the driving laser. We implemented two methods based on optical and quantum interferometry respectively in order to characterize the amplitude and phase of the attosecond emission as a function of both photon energy and alignment angle. We discovered new effects in the high harmonic generation, which could not be explained by the structure of the highest occupied molecular orbital (HOMO). Instead, we found that during the interaction with the laser field, two electronic states are coherently excited in the molecular ion and form a hole wave packet moving on an attosecond timescale in the molecule after tunnel ionization. We focused on exploring this coherent electronic motion inside the molecule, and compared the measurements in N₂O and CO₂. The striking difference in the harmonic phase behavior led us to the development of a multi-channel model allowing the extraction of the relative weight and phase of the two channels involved in the emission. An unexpected pi/4 phase shift between the two channels is obtained. Moreover, we studied the attosecond profile of the pulses emitted by these two molecules, and we proposed a simple but flexible way for performing attosecond pulse shaping. In the second part, high harmonic spectroscopy was extended to other molecular systems, including some relatively complex molecules, e.g., SF₆ and small hydrocarbons (methane, ethane, ethylene, acetylene). It revealed many interesting results such as phase distortions not previously reported. For the ‘direct’ scheme, we photoionized rare gas atoms using well characterized attosecond pulses of XUV coherent radiation combined with an infrared (IR) laser ”dressing” field with controlled time delay, stabilized down to about ± 60 as. We evidenced marked differences in the measured angular distributions of the photoelectrons, depending on the number of IR photons exchanged. Joined to a theoretical interpretation, these observations bring new insights into the dynamics of this class of multi-color photoionization processes that are a key step towards studying photoionization in the time domain, with attosecond time resolution.

High-order Harmonic Generation (HHG) is an extreme nonlinear process that can be intuitively understood as the sequence of 3 steps: i) tunnel ionization of the target atom/molecule, creating an electronic wave packet (EWP) in the continuum, ii) acceleration of the EWP by the strong laser field and iii) recombination to the core with emission of an attosecond burst of XUV coherent light. HHG thus provides a tunable ultrashort tabletop source of XUV/Soft X-ray radiation on attosecond time scale for applications ('direct' scheme). At the same time, it encodes coherently in the XUV radiation the structure and dynamical charge rearrangement of the radiating atoms/molecules ('self-probing' scheme or High Harmonic Spectroscopy). This thesis is dedicated to both application schemes in attophysics based on advanced characterization and control of the attosecond emission. In the so-called 'self-probing' scheme, the last step of HHG, the electron-ion re-collision can be considered as a probe process and the emission may encode fruitful information on the recombining system, including molecular structure and dynamics. In the first part, we performed high harmonic spectroscopy of N₂O and CO₂ molecules that are (laser-)aligned with respect to the polarization of the driving laser. We implemented two methods based on optical and quantum interferometry respectively in order to characterize the amplitude and phase of the attosecond emission as a function of both photon energy and alignment angle. We discovered new effects in the high harmonic generation, which could not be explained by the structure of the highest occupied molecular orbital (HOMO). Instead, we found that during the interaction with the laser field, two electronic states are coherently excited in the molecular ion and form a hole wave packet moving on an attosecond timescale in the molecule after tunnel ionization. We focused on exploring this coherent electronic motion inside the molecule, and compared the measurements in N₂O and CO₂. The striking difference in the harmonic phase behavior led us to the development of a multi-channel model allowing the extraction of the relative weight and phase of the two channels involved in the emission. An unexpected pi/4 phase shift between the two channels is obtained. Moreover, we studied the attosecond profile of the pulses emitted by these two molecules, and we proposed a simple but flexible way for performing attosecond pulse shaping. In the second part, high harmonic spectroscopy was extended to other molecular systems, including some relatively complex molecules, e.g., SF₆ and small hydrocarbons (methane, ethane, ethylene, acetylene). It revealed many interesting results such as phase distortions not previously reported. For the 'direct' scheme, we photoionized rare gas atoms using well characterized attosecond pulses of XUV coherent radiation combined with an infrared (IR) laser "dressing" field with controlled time delay, stabilized down to about ± 60 as. We evidenced marked differences in the measured angular distributions of the photoelectrons, depending on the number of IR photons exchanged. Joined to a theoretical interpretation, these observations bring new insights into the dynamics of this class of multi-color photoionization processes that are a key step towards studying photoionization in the time domain, with attosecond time resolution.

La microfabrication 3D par laser dans les matériaux à faible bande interdite néces- sitera l’utilisation d’impulsions intenses dans l’infrarouge proche et moyen. Cette étude expérimentale se concentre sur les spécificités de la physique d’ionisation nonlinéaire dans la gamme de longueur d’onde de 1300-2200nm. Contrairement aux semiconducteurs, l’ab- sorption nonlinéaire mesurée dans les diélectriques est indépendante de la longueur d’onde révélant ainsi l’importance accrue de l’ionisation par effet tunnel avec ces longueurs d’onde. Nous étudions également les rendements et les seuils d’ionisation multiphotonique et ava- lanche dans le silicium intrinsèque et dopé N. Les résultats couplés à l’observation des ma- tériaux irradiés montrent que les propriétés intrinsèques des semiconducteurs empêchent un dépôt d’énergie suffisamment confiné pour viser directement des applications de modifica- tion locale. Ce travail illustre les possibilités de micro-usinage laser 3D dans les diélectriques et les défis de l’extension de cette technique aux semiconducteurs. / 3D laser microfabrication inside narrow gap solids like silicon will require the use of long wavelength intense pulses. This experimental study concentrates on the specificity of the nonlinear ionization physics with tightly focused femtosecond laser beams over a wa- velength range of 1300-2200nm. The measured nonlinear absorption is independent of the wavelength in dielectrics revealing the increased importance of tunnel ionization with long wavelength. This can open up an alternative to pulse shortening toward ultraprecision op- tical breakdown in dielectrics. Using n-doped silicon, we study the multiphoton-avalanche absorption yields and thresholds inside semiconductors. Also observations of the irradia- ted materials reveal that the intrinsic properties of semiconductors prevent efficient direct energy deposition in the bulk for applications. This work illustrates opportunities for 3D laser micromachining in dielectrics and challenges for its extension to semiconductors.