Thermal Effects in Atomic and Molecular Polarizabilities with Path Integral Monte Carlo

Väitöskirja käsittelee polarisoituvuutta ja erilaisia keinoja sen laskemiseksi polkuintegraali–Monte Carlo -menetelmällä (PIMC). Polarisoituvuus on kvanttimekaaninen suure, joka vastaa sähköistä suskeptibiliteettiä: se kuvaa atomien ja molekyylien vastetta sähkökenttään. Staattiset ja dynaamiset multipoli-polarisoituvuudet ovatkin yksiä tärkeimmistä elektronien vasteominaisuuksista ja näin ollen monikäyttöisiä parametrejä fysikaalisessa mallinnuksessa. Polarisoituvuuksien äärimmäisen tarkka laskeminen on kuitenkin haasteellista. Väitöskirjassa keskitytään siksi muutamaan erityiseen ongelmaan: tarkkaan monen kappaleen korrelaatiokuvaukseen, ei-adiabaattisiin efekteihin sekä lämpötilan vaikutuksiin.Tässä työssä polarisoituvuuksien laskemista tarkastellaan ei-relativistisesti Feynmanin polkuintegraalien ja termisten tiheysmatriisien avulla. Sähkökentän ja sähköisten multipolien välinen vuorovaikutus kytketään kausaalisiin korrelaatiofunktioihin sekä epälineaarisen vasteen teoriaan. Uusi tieteellinen ansio muodostuu muutamasta erilaisesta keinosta määrittää polarisoituvuus PIMC-laskuista: äärellisen kentän simulointi, staattiset kenttä-derivaatan estimaattorit, sekä imaginääriajan korrelaatiofunktioiden analyyttinen jatkaminen. Vaadittu Matsubara-taajuuksien analyyttinen jatkaminen on yleisesti esiintyvä mutta huonosti määritelty numeerinen ongelma, jota lähestytään tässä työssä maksimientropiamenetelmällä.Tärkeimmät laskennalliset tulokset ovat seuraavien yhden tai kahden elektronin systeemien polarisoituvuudet ja hyperpolarisoituvuudet: H, H2+, H2, H3+, HD+, He, He+, HeH+, Li+, Be2+, Ps, PsH, ja Ps2. Born–Oppenheimer-approksimaatiossa (BO) lasketut referenssitulokset vastaavat tunnettuja kirjallisuuden arvoja ja monessa tapauksessa myös täydentävät niitä. BO-approksimaation ulkopuolelta voidaan osoittaa mm. rovibraatiosta johtuvia heikkoja sekä voimakkaita lämpötilaefektejä. Muut tulokset käsittävät multipoli-spektrejä, dynaamisia polarisoituvuuksia sekä van der Waals-vakioita. Simulaatioiden kvanttimekaaninen kuvaus monen kappaleen korrelaatioista sekä elektronien ja ytimien ei-ediabaattisesta kytkennästä on poikkeuksellisen tarkka.This Thesis is a review of polarizability and different means to estimate it from pathintegral Monte Carlo (PIMC) simulations. Polarizability is the quantum mechanical equivalent of electric susceptibility: it describes the electric field response of atoms and molecules. The static and dynamic multipole polarizabilies are, arguably, the most important electronic response properties and multipurpose parameters for physical modeling. Computing them from first principles is challenging in many ways, and in this Thesis we focus on a few particular aspects: exact many-body correlations, nonadiabatic effects and thermal coupling. The Thesis contains an introduction to polarizability in the framework of nonrelativistic Feynman path integrals and thermal density matrices. The electric field interactions due to electric multipoles is associated with causal time-correlation functions and nonlinear response theory. The original scientific contribution manifests in various strategies to obtain the polarizabilities from PIMC simulations: we demonstrate finite-field simulations, static field-derivative estimators, and analytic continuation of imaginarytime correlation functions. The required analytic continuation of Matsubara frequencies is a common but ill-posed numerical challenge, which we approach with the Maximum Entropy method. For data, we provide the most important polarizabilities and hyperpolarizabilities of several one- or two-electron systems: H, H2+, H2, H3+, HD+, He, He+, HeH+, Li+, Be2+, Ps, PsH, and Ps2. Our benchmark simulations within the Born–Oppenheimer approximation (BO) agree with the available literature and complement it in many cases. Beyond BO, we are able to demonstrate weak and strong thermal effects due to, e.g., rovibrational coupling. We also estimate the first-order multipole spectra, dynamic polarizabilities and van der Waals coefficients. The simulations show unprecedented accuracy in terms of exact many-body correlations and fully nonadiabatic coupling of the electronic and nuclear quantum effects

Nonlinear optical molecular switches for alkali ion identification

This work demonstrates by means of DFT and ab initio calculations that recognition of alkali cations can be achieved by probing the variations of the second-order nonlinear optical properties along the commutation process in spiropyran/merocyanine systems. Due to the ability of the merocyanine isomer to complex metal cations, the switching between the two forms is accompanied by large contrasts in the quadratic hyperpolarizability that strongly depend on the size of the cation in presence. Exploiting the nonlinear optical responses of molecular switches should therefore provide powerful analytical tools for detecting and identifying metal cations in solution

An experimental and computational research on the optical properties of ionic liquids

Este proxecto céntrase no estudo do efecto que a dopaxe química ou física exerce sobre as propiedades ópticas dun conxunto de líquidos iónicos. A dopaxe física con determinadas sales de metais dos grupos IA e IIA da lugar á formación de microestruturas similares ás observadas en cristais líquidos desordeados e facilmente detectables mediante microscopía de polarización. Mediante aliñamento mecánico ou inducido por campo eléctrico pódense ordear. Estas estruturas ordeadas presentan índices de refracción que dependen da dirección que poden ser caracterizados. A birrefrinxencia e o intervalo de temperatura no que o material se comporta como un cristal líquido iónico depende do metal do sal engadido. A adición de metais de transición tanto en proporción estequiométrica como non estequiométrica, pode dar lugar a líquidos iónicos con altos índices de refracción, termocromáticos, luminiscentes ou cunha resposta non lineal elevada ante campos electromagnéticos intensos

Pathways Towards a Second Generation 88Sr2 Molecular Clock

For years, frequency standards have been the cornerstone of precision measurement. Among these frequency standards, atomic clocks have set records in both precision and accuracy, and have redefined the second. There is growing interest in more complex molecular systems to complement precision measurements with atoms. The rich internal structure of even the simplest diatomic molecules could provide new avenues for fundamental physics research, including searches for extensions to the Standard Model, dark matter candidates, novel forces or corrections to gravity at short distances, and tests of the variation of fundamental constants. In this thesis, we discuss the fundamental architecture for a precise molecular system based on a strongly forbidden weakly-bound to deeply-bound vibrational transition in 88Sr dimers. We discuss early studies to characterise our system and gain technical and quantum control over the experiment in anticipation of a precise metrological measurement. We, then, demonstrate a record-breaking precision for our 88Sr2 molecular clock ushering in a new era for precision measurement with clocks. Borrowing techniques from previous atomic clock architecture, we measure a ∼32 THz clock transition between two vibrational levels in the electronic ground state, achieving a fractional uncertainty of 4.6 × 10−14 in a new frequency regime. In this current iteration, our molecular clock is fundamentally limited by two-body loss lifetimes of 200 ms and light scattering induced by our high-intensity lattice. Given these limitations, we suggest improvements to combat the effects from both the lattice and two-body collisions in our 1D trap. These include technical improvements to our experiment and strategic choices of particular clock states in our ground electronic potential. We describe in-depth studies of the chemistry and polarizability behaviour of our molecule, which elucidates preferential future directions for a second generation clock system. These empirical results are substantiated by an improved theoretical picture. Ultimately, our molecular system is built in order to probe new physics and as a tool for precision measurement. Leveraging our record-precision clock and our new-found understanding of our molecule, we predict the capacity for our system to place meaningful, competitive constraints on new physics, in particular on Yukawa-type extensions to gravity. These predictions motivate improvements to our current generation clock and set the stage for future measurements with this system

Recent Advances in Linear and Nonlinear Optics

Sight is the dominant sense of mankind to apprehend the world at the earth scale and beyond the frontiers of the infinite, from the nanometer to the incommensurable. Primarily based on sunlight and natural and artificial light sources, optics has been the major companion of spectroscopy since scientific observation began. The invention of the laser in the early sixties has boosted optical spectroscopy through the intrinsic or specific symmetry electronic properties of materials at the multiscale (birefringence, nonlinear and photonic crystals), revealed by the ability to monitor light polarization inside or on the surface of designed objects. This Special Issue of Symmetry features articles and reviews that are of tremendous interest to scientists who study linear and nonlinear optics, all oriented around the common axis of symmetry. Contributions transverse the entire breadth of this field, including those concerning polarization and anisotropy within colloids of chromophores and metal/semiconducting nanoparticles probed by UV-visible and fluorescence spectroscopies; microscopic structures of liquid–liquid, liquid–gas, and liquid–solid interfaces; surface- and symmetry-specific optical techniques and simulations, including second-harmonic and sum-frequency generations, and surface-enhanced and coherent anti-Stokes Raman spectroscopies; orientation and chirality of bio-molecular interfaces; symmetry breaking in photochemistry; symmetric multipolar molecules; reversible electronic energy transfer within supramolecular systems; plasmonics; and light polarization effects in materials

Computational Studies of Dispersion Interactions in Coinage and Volatile Metal Clusters

Intermolecular interactions are ubiquitous, and their intricate network plays a decisive role in most of the phenomena encountered in our everyday lives. The focus of this thesis is on the London dispersion forces, a component present in all interactions between atoms and molecules, and often the most important one at long intermolecular distances. The quantum-mechanical origin of these forces can be traced to the correlated fluctuations of the molecular charge distributions, which however render the dispersion interactions challenging to calculate accurately, due to the high-level electronic structure methods required. The aim of the research presented in this thesis is to investigate the dispersion interactions, and to develop a viable method for modeling them. The systems studied in the accompanying research articles mainly encompass small clusters of coinage (Cu, Ag, and Au) and volatile (Zn, Cd, and Hg) metals. The long-range forces present in these clusters are calculated by means of highly correlated electronic structure methods, and the interaction potentials are used to develop a simple but effective model, capable of accurately describing the dispersion interactions in a variety of systems. Some original theoretical considerations are also elaborated. A novel formula is derived for the tensor describing all intermolecular interactions, and it is applied to investigate the long-range interaction potential of coinage metal hydrogen clusters. The method developed to account for the dispersion energy is a pair-potential model, where the total intermolecular London forces are calculated by means of atomic dispersion coefficients describing the magnitude and orientation dependence of the interaction. The coefficients are calculated based on small model systems, and they are used to compute the dispersion energy in larger clusters at no additional cost. Encouraging results are also obtained for the computed orientation averaged interaction potentials. All things considered, the publications included in this thesis indicate that the methods proposed and implemented to analyze the studied systems are capable of accurately modeling the non-covalent forces in a straightforward fashion.Molekyylien väliset voimat vaikuttavat kaikkialla elinympäristössämme ja ne ovat avainasemassa useimmissa päivittäin kohtaamissamme tuotteissa, materiaaleissa ja kemiallisissa prosesseissa. Tässä väitöskirjassa tarkastellaan erityisesti niin kutsuttua Londonin dispersiovoimaa, joka on osallisena kaikissa atomien ja molekyylien välisissä vuorovaikutuksissa. Tällä täysin kvanttimekaanisella ilmiöllä ei ole vastinetta klassisessa fysiikassa, mutta sen voidaan ajatella saavan alkunsa molekyylien varausjakaumien tahdistuneista heilahteluista. Pitkillä molekyylien välisillä etäisyyksillä dispersio on usein tärkein vuorovaikutusenergiaan vaikuttava tekijä, mutta sen tarkka laskennallinen mallintaminen on haastavaa. Tässä väitöskirjassa kehitetyllä laskennallisella menetelmällä dispersiovuorovaikutuksia voidaan tutkituissa systeemeissä kuvata yksinkertaisesti ja tarkasti. Tähän väitöskirjaan kuuluvissa tutkimusartikkeleissa keskitytään lähinnä pieniin metalliryppäisiin, jotka koostuvat rahametalleista (Cu, Ag ja Au) sekä sinkkiryhmän (Zn, Cd, Hg) metalleista. Molekyylien välinen vuorovaikutusenergia määritetään korkeatasoisten elektroniverholaskujen avulla, ja tuloksena saatavista energiakäyristä lasketaan parametreja, jotka kuvaavat dispersioenergiaa kvantitatiivisesti useissa eri systeemeissä. Tässä väitöskirjassa esitetään myös molekyylien välisten voimien matemaattista käsittelyä yksinkertaistavia teoreettisia tuloksia. Voimien orientaatioriippuvuutta kuvaavalle vuorovaikutustensorille johdetaan uudenlainen kaava, jota käytetään rahametalli- ja vetydimeerien välisten pitkän kantaman vuorovaikutusten tutkimiseen. Dispersiovoimien kuvaamiseen käytetyssä menetelmässä molekyylien välinen vuorovaikutusenergia voidaan laskea eri atomipareille määritetyistä kertoimista. Nämä kertoimet lasketaan pienten metalliryppäiden avulla ja niiden osoitetaan kuvaavan dispersioenergiaa myös suuremmissa systeemeissä ilman ylimääräisiä elektroniverholaskuja. Käytettyä mallia sovelletaan hyvin tuloksin myös orientaatiokeskiarvoistettujen dispersiopotentiaalien laskemiseen sinkkiryhmän metallidimeerien välillä. Kaiken kaikkiaan tähän väitöskirjaan sisältyvät tutkimusartikkelit osoittavat, että käytetyillä laskennallisilla malleilla ja menetelmillä voidaan pitkän kantaman vuorovaikutuksia mallintaa luotettavasti ja suoraviivaisesti tutkittujen molekyylien välillä

Photophysics of fluorescent silver nanoclusters

Fluorescence imaging has been increasingly relied upon as the method of choice for many biological and medical applications. As demands for more sensitive and higher resolution imaging are ever-increasing, it is critical that photostable, and robust fluorophores capable of delivering high emission rates are available. Fluorescent silver nanoclusters offer an attractive compromise between the photostability and brightness of quantum dots and the compact versatility of organic chromophores. They have been shown to be superior in many roles, including as single molecule fluorophores and bulk multiphoton biological staining agents. The two-photon absorption cross sections are several orders of magnitude larger than commercially-available dyes, and they have demonstrated superior photostability under high intensity irradiation. In addition to the endogenous effects of the cluster, its small size of only a few atoms renders it highly susceptible to surface and environmental effects, which manifests, for example, in the observed photoinduced charge transfer between the silver cluster and oligonucleotide. This state has been shown to be highly advantageous in imaging applications, as control of this state enables better control over the time-averaged emission rate of the molecule. The mechanism of charge transfer, and the possible means by which this state can be controlled will be also be investigated in this work.Ph.D.Committee Chair: Dickson, Robert; Committee Member: Brown, Ken; Committee Member: Curtis, Jennifer; Committee Member: Payne, Christine; Committee Member: Perry, Josep

Molecular Engineering of Dipolar and Octupolar Non-Linear Optical Materials for Next-Generation Telecommunications

In an age where next-generation all-optical circuitry and optical data storage are at the forefront of the telecommunications industry, the molecular engineering and design of new organic materials continues apace. Such materials are particularly attractive on account of their fast optical response times, and superior non-linear optical susceptibilities, relative to their traditional inorganic counterparts. While dipolar molecules dominate the field of organic non-linear optical (NLO) materials, octupolar molecules have the potential to produce far greater NLO effects; moreover, they have the capacity to produce 3-D sensitive NLO phenomena. This PhD explores new classes of dipolar organic and octupolar organometallic materials, where computations have predicted them to serve with superior NLO properties. To this end, concerted experimental and theoretical data are employed to characterise the electronic structure of these materials and elucidate their NLO properties. Data for electronic structures in this thesis were secured via in-house and synchrotron-based X-ray diffraction experiments (by proxy), which the author employed for charge density analyses. Multipolar modelling of experimental charge densities of the subject NLO materials forms an integral part of this thesis. Topological analysis is applied to these electronic structures, using the quantum theory of atoms in molecules (QTAIM), from which the structural and chemical origins of their NLO properties are assessed. Complementary theoretical methods were also used in this work, including calculations undertaken via density functional theory, as well as the relatively new technique of X-ray constrained wave-function refinement, which especially complements multipolar modelling methods, providing direct corroboratory topological analysis. An array of complementary experimental and computational methods is employed to evaluate the NLO properties of these materials in the gas-, solution-, and solid state-phase. The organometallic complexes presented in this thesis were also synthesised by the author. Chapter 1 of this work begins by presenting some of the main principles behind NLO phenomena, before providing a review of some of the most salient organic and organometallic NLO materials investigated, to date. Chapter 2 provides details pertaining to the experimental and computational methods used within this work to evaluate the molecular origins of the NLO properties of the materials investigated herein. Chapter 3 explores the molecular origins of the NLO properties of a new class of dipolar organic chromophores via structural analysis, experimental charge density analyses, hyper-Rayleigh scattering and density functional theory. Chapter 4 similarly investigates a new class of dipolar organic NLO chromophores via structural analysis, hyper-Rayleigh scattering and density functional theory. However, topological analysis herein was undertaken solely via the X-ray constrained wave-function fitting method, due to the absence of high-resolution X-ray diffraction data for experimental multipolar modelling. Chapter 5 investigates two ionic organic chromophores and the implications of their intermolecular interactions on their respective NLO responses by building up the ionic system using a ‘molecular lego’ approach. Chapters 6-7 detail investigations of newly identified octupolar NLO organometallic complexes, and feature several rare examples of charge-density studies of materials containing heavy elements, such as the transition metal, zinc, and bromine These heavy elements are particularly challenging even for state-of-the-art experimental and computational materials characterisation methods. Chapter 8 concludes this work, and identifies possible future directions for investigations of NLO materials for next-generation telecommunications.EPSR

The strontium molecular lattice clock: Vibrational spectroscopy with hertz-level accuracy

The immaculate control of atoms and molecules with light is the defining trait of modern experiments in ultracold physics. The rich internal degrees of freedom afforded by molecules enrich the toolbox of precision spectroscopy for fundamental physics, and hold great promise for applications in quantum simulation and quantum information science. A vibrational molecular lattice clock with systematic fractional uncertainty at the 14th decimal place is demonstrated for the first time, matching the performance of the earliest optical atomic clocks. Van der Waals dimers of strontium are created at ultracold temperatures and levitated by an optical standing wave, whose wavelength is finely tuned to preserve the delicate molecular vibrational coherence. Guided by quantum chemistry theory refined by highly accurate frequency-comb-assisted laser spectroscopy, record-long Rabi oscillations were demonstrated between vibrational molecular states that span the entire depth of the ground molecular potential. Enabled by the narrow molecular clock linewidth, hertz-level frequency shifts were resolved, facilitating the first characterization of molecular hyperpolarizability in this context. In a parallel effort, deeply bound strontium dimers are coherently created using the technique of stimulated Raman adiabatic passage. Ultracold collisions of alkaline-earth metal molecules in the absolute ground state are studied for the first time, revealing inelastic losses at the universal rate. This thesis reports one of the most accurate measurement of a molecule's vibrational transition frequency to date, which may potentially serve as a secondary representation of the SI unit of time in the terahertz (THz) band where standards are scarce. The prototypical molecular clock lays the important groundwork for future explorations into THz metrology, quantum chemistry, and fundamental interactions at atomic length scales

Decomposition of Intermolecular Interactions in Ab Initio Spectroscopy

Spectroscopy, the molecular response to electromagnetic radiation of different wavelengths, is one of the most powerful experimental tools for interrogating a molecule's structure and dynamics as it interacts with its environment. However, relating a spectroscopic signature to a molecular picture relies on sophisticated computational approaches, which offer a wealth of methods for identifying structures, intermolecular interactions, and their correlation with spectroscopic response. This thesis focuses on the how to correlate a molecule's structure and interactions with its environment via ab initio calculation of spectroscopic parameters. To build a molecular picture of carbon dioxide dynamics in ionic liquids (ILs), quantum chemical calculations on small clusters qualitatively reproduced the experimental ordering for carbon dioxide's asymmetric vibrational stretch peak position which shifts when dissolved in a series of ILs with varying anions. To uncover the physical origin of the shift, the language of decomposition analysis based on absolutely localized molecular orbitals (ALMO-EDA) was translated from energies to vibrational frequencies. Geometric distortion of carbon dioxide, as a result of charge transfer (CT) from the anion into the carbon dioxide, is the driving force for differentiating the carbon dioxide asymmetric stretch shift in different IL anions. After validating these simple models, we further decomposed the CT contribution into geometry and curvature mechanisms, finding that CT is a significant contributor in both the geometry optimization and frequency calculation steps. A comparison between ALMO-EDA and symmetry-adapted perturbation theory (SAPT) showed that while dispersion dominates the binding energy, excellent correlation between both total interaction energies and individual components for ALMO-EDA and SAPT validates the use of DFT, enabling the construction of a semiempirical spectroscopic map. This decomposition presented the first application of an EDA outside the energy realm into molecular properties; however, it is not generally applicable to arbitrary perturbations. A reformulation of the canonical linear response equations for use with ALMOs provides a direct connection between EDA terms and their corresponding contribution to spectra. Results for argon-lithium cation dimer polarizabilities show that allowing CT is equally important in both the underlying ground-state wavefunction and the response calculation, and should not be confused with basis set superposition error