The radical SeCl: A theoretical contribution to the characterization of its low-lying electronic states

All doublet and quartet electronic states correlating with the first dissociation channel of SeCl and some Rydberg states are investigated theoretically at the CASSCF/MRCI level of theory using extended basis sets, including the contribution of spin-orbit effects. The similarity of the potential energy curves with those of SeF suggests that spectroscopic constants for the ground (X (2)Pi) and the first excited quartet (a(4)Sigma) of SeCl could also be determined via an emission resulting from the reaction of selenium with atomic chlorine. The coupling constant of the ground state at R-e is estimated as -1610 cm (1). The potential energy curves calculated and the derived spectroscopic constants do not support the interpretation and assignment of the scarce transitions recorded experimentally as due to (2)Pi-(2)Pi emissions. That the few observed lines might arise from transitions from the state b(4)Sigma(-)(1/2) to a very high vibrational level of the state a(4)Sigma(-)(1/2) is an open possibility, however, the number of vibrational states and the calculated Delta G(1/2) differ significantly from the reported ones. (C) 2012 Elsevier B. V. All rights reserved.

Circularly Polarized Photoluminescence as a Probe of Density of States in GaAs/AlGaAs Quantum Hall Bilayers

Polarized magnetophotoluminescence is employed to study the energies and occupancies of four lowest Landau levels in a couple quantum Hall GaAs/AlGaAs double quantum well. As a result, a magnetic field-induced redistribution of charge over the Landau levels manifesting to the continuous formation of the charge density wave and direct evidence for the symmetric-antisymmetric gap shrinkage at v = 3 are found. The observed interlayer charge exchange causes depolarization of the ferromagnetic ground state.

Ground state properties of neutron-rich Mg isotopes

Studies in regions of the nuclear chart in which the model predictions of properties of nuclei fail can bring a better understanding of the strong interaction in the nuclear medium. To such regions belongs the so called "island of inversion" centered around Ne, Na and Mg isotopes with 20 neutrons in which unexpected ground-state spins, large deformations and dense low-energy spectra appear. This is a strong argument that the magic N = 20 is not a closed shell in this area. In this thesis investigations of isotope shifts of stable 24,25,26Mg, as well as spins and magnetic moments of short-lived 29,31Mg are presented. The successful studies were performed at the ISOLDE facility at CERN using collinear laser and beta-NMR spectroscopy techniques. The isotopes were investigated as single-charged ions in the 280-nm transition from the atomic ground state 2S1/2 to one of the two lowest excited states 2P1/2,3/2 using continuous wave laser beams. The isotope-shift measurements with fluorescence detection for the three stable isotopes show that it is feasible to perform the same studies on radioactive Mg isotopes up to the "island of inversion". This will allow to determine differences in the mean charge square radii and interpret them in terms of deformation. The high detection efficiency for beta particles and optical pumping close to saturation allowed to obtain very good beta-asymmetry signals for 29Mg and 31Mg with half-lives around 1 s and production yields about 10^5 ions/s. For this purpose the ions were implanted into a host crystal lattice. Such detection of the atomic resonances revealed their hyperfine structure, which gives the sign and a first estimate of the value of the magnetic moment. The nuclear magnetic resonance gave also their g-factors with the relative uncertainty smaller than 0.2 %. By combining the two techniques also the nuclear spin of both isotopes could be unambiguously determined. The measured spins and g-factors show that 29Mg with 17 neutrons lies outside the "island of inversion". On the other hand, 31Mg with 19 neutrons has an unexpected ground-state spin which can be explained only by promoting at least two neutrons across the N = 20 shell gap. This places the above nucleus inside the "island". However, modern shell-model approaches cannot predict this level as the ground state but only as one of the low-lying states, even though they reproduce very well the experimental g-factor. This indicates that modifications to the available interactions are required. Future measurements include isotope shift measurements on radioactive Mg isotopes and beta-NMR studies on 33Mg.

Constructing correlated spin states with neutral atoms in optical lattices

This thesis reports on the experimental investigation of controlled spin dependent interactions in a sample of ultracold Rubidium atoms trapped in a periodic optical potential. In such a situation, the most basic interaction between only two atoms at one common potential well, forming a micro laboratory for this atom pair, can be investigated. Spin dependent interactions between the atoms can lead to an intriguing time evolution of the system. In this work, we present two examples of such spin interaction induced dynamics. First, we have been able to observe and control a coherent spin changing interaction. Second, we have achieved to examine and manipulate an interaction induced time evolution of the relative phase of a spin 1/2-system, both in the case of particle pairs and in the more general case of N interacting particles. The first part of this thesis elucidates the spin-changing interaction mechanism underlying many fascinating effects resulting from interacting spins at ultracold temperatures. This process changes the spin states of two colliding particles, while preserving total magnetization. If initial and final states have almost equal energy, this process is resonant and leads to large amplitude oscillations between different spin states. The measured coupling parameters of such a process allow to precisely infer atomic scattering length differences, that e.g. determine the nature of the magnetic ground state of the hyperfine states in Rubidium. Moreover, a method to tune the spin oscillations at will based on the AC-Zeeman effect has been implemented. This allowed us to use resonant spin changing collisions as a quantitative and non-destructive particle pair probe in the optical lattice. This led to a series of experiments shedding light on the Bosonic superfluid to Mott insulator transition. In a second series of experiments we have been able to coherently manipulate the interaction induced time evolution of the relative phase in an ensemble of spin 1/2-systems. For two particles, interactions can lead to an entanglement oscillation of the particle pair. For the general case of N interacting particles, the ideal time evolution leads to the creation of spin squeezed states and even Schrödinger cat states. In the experiment we have been able to control the underlying interactions by a Feshbach resonance. For particle pairs we could directly observe the entanglement oscillations. For the many particle case we have been able to observe and reverse the interaction induced dispersion of the relative phase. The presented results demonstrate how correlated spin states can be engineered through control of atomic interactions. Moreover, the results point towards the possibility to simulate quantum magnetism phenomena with ultracold atoms in optical traps, and to realize and analyze many novel quantum spin states which have not been experimentally realized so far.

Spatio-temporal Associations Between GOES Aerosol Optical Depth Retrievals and Ground-Level PM2.5

We assess the strength of association between aerosol optical depth (AOD) retrievals from the GOES Aerosol/Smoke Product (GASP) and ground-level fine particulate matter (PM2.5) to assess AOD as a proxy for PM2.5 in the United States. GASP AOD is retrieved from a geostationary platform and therefore provides dense temporal coverage with half-hourly observations every day, in contrast to once per day snapshots from polar-orbiting satellites. However, GASP AOD is based on a less-sophisticated instrument and retrieval algorithm. We find that correlations between GASP AOD and PM2.5 over time at fixed locations are reasonably high, except in the winter and in the western U.S. Correlations over space at fixed times are lower. Simple averaging over time actually reduces correlations over space dramatically, but statistical calibration allows averaging over time that produces strong correlations. These results and the data density of GASP AOD highlight its potential to help improve exposure estimates for epidemiological analyses. On average 40% of days in a month have a GASP AOD retrieval compared to 14% for MODIS and 4% for MISR. Furthermore, GASP AOD has been retrieved since November 1994, providing the possibility of a long-term record that pre-dates the availability of most PM2.5 monitoring data and other satellite instruments.

Probing Charge Transfer in Benzodifuran-C-60 Dumbbell-Type Electron Donor-Acceptor Conjugates: Ground- and Excited-State Assays

Rigid electron donor-acceptor conjugates (1-3) that combine -extended benzodifurans as electron donors and C-60 molecules as electron acceptors with different linkers have been synthesized and investigated with respect to intramolecular charge-transfer events. Electrochemistry, fluorescence, and transient absorption measurements revealed tunable and structure-dependent charge-transfer processes in the ground and excited states. Our experimental findings are underpinned by density-functional theory calculations.

Optical Light Curve Observations to Determine Attitude States of Space Debris

The currently proposed space debris remediation measures include the active removal of large objects and “just in time” collision avoidance by deviating the objects using, e.g., ground-based lasers. Both techniques require precise knowledge of the attitude state and state changes of the target objects. In the former case, to devise methods to grapple the target by a tug spacecraft, in the latter, to precisely propagate the orbits of potential collision partners as disturbing forces like air drag and solar radiation pressure depend on the attitude of the objects. Non-resolving optical observations of the magnitude variations, so-called light curves, are a promising technique to determine rotation or tumbling rates and the orientations of the actual rotation axis of objects, as well as their temporal changes. The 1-meter telescope ZIMLAT of the Astronomical Institute of the University of Bern has been used to collect light curves of MEO and GEO objects for a considerable period of time. Recently, light curves of Low Earth Orbit (LEO) targets were acquired as well. We present different observation methods, including active tracking using a CCD subframe readout technique, and the use of a high-speed scientific CMOS camera. Technical challenges when tracking objects with poor orbit redictions, as well as different data reduction methods are addressed. Results from a survey of abandoned rocket upper stages in LEO, examples of abandoned payloads and observations of high area-to-mass ratio debris will be resented. Eventually, first results of the analysis of these light curves are provided.

Zeeman effect in atmospheric O₂ measured by ground-based microwave radiometry

In this work we study the Zeeman effect on stratospheric O₂ using ground-based microwave radiometer measurements. The interaction of the Earth magnetic field with the oxygen dipole leads to a splitting of O₂ energy states, which polarizes the emission spectra. A special campaign was carried out in order to measure this effect in the oxygen emission line centered at 53.07 GHz. Both a fixed and a rotating mirror were incorporated into the TEMPERA (TEMPERature RAdiometer) in order to be able to measure under different observational angles. This new configuration allowed us to change the angle between the observational path and the Earth magnetic field direction. Moreover, a high-resolution spectrometer (1 kHz) was used in order to measure for the first time the polarization state of the radiation due to the Zeeman effect in the main isotopologue of oxygen from ground-based microwave measurements. The measured spectra showed a clear polarized signature when the observational angles were changed, evidencing the Zeeman effect in the oxygen molecule. In addition, simulations carried out with the Atmospheric Radiative Transfer Simulator (ARTS) allowed us to verify the microwave measurements showing a very good agreement between model and measurements. The results suggest some interesting new aspects for research of the upper atmosphere.

The elusive S2 state, the S1/S2 splitting, and the excimer states of the benzene dimer

We observe the weak S 0 → S 2 transitions of the T-shaped benzene dimers (Bz)2 and (Bz-d 6)2 about 250 cm−1 and 220 cm−1 above their respective S 0 → S 1 electronic origins using two-color resonant two-photon ionization spectroscopy. Spin-component scaled (SCS) second-order approximate coupled-cluster (CC2) calculations predict that for the tipped T-shaped geometry, the S 0 → S 2 electronic oscillator strength f el (S 2) is ∼10 times smaller than f el (S 1) and the S 2 state lies ∼240 cm−1 above S 1, in excellent agreement with experiment. The S 0 → S 1 (ππ ∗) transition is mainly localized on the “stem” benzene, with a minor stem → cap charge-transfer contribution; the S 0 → S 2 transition is mainly localized on the “cap” benzene. The orbitals, electronic oscillator strengths f el (S 1) and f el (S 2), and transition frequencies depend strongly on the tipping angle ω between the two Bz moieties. The SCS-CC2 calculated S 1 and S 2 excitation energies at different T-shaped, stacked-parallel and parallel-displaced stationary points of the (Bz)2 ground-state surface allow to construct approximate S 1 and S 2 potential energy surfaces and reveal their relation to the “excimer” states at the stacked-parallel geometry. The f el (S 1) and f el (S 2) transition dipole moments at the C 2v -symmetric T-shape, parallel-displaced and stacked-parallel geometries are either zero or ∼10 times smaller than at the tipped T-shaped geometry. This unusual property of the S 0 → S 1 and S 0 → S 2 transition-dipole moment surfaces of (Bz)2 restricts its observation by electronic spectroscopy to the tipped and tilted T-shaped geometries; the other ground-state geometries are impossible or extremely difficult to observe. The S 0 → S 1/S 2 spectra of (Bz)2 are compared to those of imidazole ⋅ (Bz)2, which has a rigid triangular structure with a tilted (Bz)2 subunit. The S 0 → S 1/ S 2 transitions of imidazole-(benzene)2 lie at similar energies as those of (Bz)2, confirming our assignment of the (Bz)2 S 0 → S 2 transition.

Current United States stem cell policy considerations and a recommendation for an alternative policy based on guidelines from existing international policies

Embryonic stem cell research is a widely debated topic in modern politics and religion. Differing views on the fetal rights conflict with the rights of an embryo. Those who believe an embryo has the same human qualities as a fetus accordingly believe embryonic stem cell research is unethical because it destroys a potential human life. However, scientists advocate the embryo does not have human qualities and should be used for valuable research in the stem cell field. Stem cell research may lead to vast developments in medical treatments, including cancer and brain conditions and injuries that are currently incurable. ^ The current stem cell policy introduced by President Bush in 2001 in an attempt to balance the moral issues with the need for scientific research has broad negative implications on the furthering of stem cell research. There is a limited diversity of available stem cell lines, there may be constitutional issues, there is an increasing disparity between the public and private research spheres, and the U.S. is struggling to maintain its scientific community. The U.S. must develop a new stem cell research policy that will balance the interest of science and public health with the moral issues that concern the public. ^ The United Kingdom allows researchers great liberty in conducting research, permitting the creation of embryos for the sole purpose of research, while Germany is equally conservative in their laws, as their policies support the philosophy that all embryos deserve the protection of full life. The United States should adopt a policy that takes the "middle ground" approach and permit research on excess embryos created for IVF purposes, rather than simply discarding those potentially valuable research tools. ^

Expectations from EMET and OESEE ground listening of possible EM events generated by the TSS 1R

On 22nd February '96, the space mission STS 75 started ,from the NASA facilities at Cape Canaveral. Such a mission consists in the launch of the shuttle Columbia in order to carry out two experiments in the space: the TSS 1R (Tethered Satellite Sistem 1 Refliight) and the USMP (United States Microgravity Payload). The TSS 1R is a replica of a similar mission TSS 1 '92. The TSS space programme is a bilateral scientific cooperation between the USA space agency NASA (National Aeronautics and Space Agency) and the ASI (Italian Space Agency. The TSS 1R system consists on the shuttle Columbia which deploys, up-ward, by means a conducting tether 20 km long, a spherical satellite (1.5 mt diameter) containing scientific instrumentation. This system, orbiting at about 300 km from the Earth's surface, represents, presently, the largest experimental space structure, Due to its dimensions, flexibility and conducting properties of the tether, the system interacts, in a quite complex manner, wih the earth magnetic field and the ionospheric plasma, in a way that the total system behaves as an electromagnetic radiating antenna as well as an electric power generator. Twelve scientific experiments have been assessed by US and Italian scientists in order to study the electro dynamic behaviour of the structure orbiting in the ionos phere. Two experiments have been prepared in the attempt to receive on the Earth's surface possible electromagnetic events radiated by the TSS 1R. The project EMET (Electro Magnetic Emissions from Tether),USA and the project OESEE (Observations on the Earth Surface of Electromagnetic Emissions) Italy, consist in a coordinated programme of passive detection of such possible EM emissions. This detection will supply the verification of some thoretical hypotheses on the electrodynamic interactions between the orbiting system, the Earth's magnetic field and the ionospheric plasma with two principal aims as the technological assesment of the system concept as well as a deeper knowledge of the ionosphere properties for future space applications. A theoretical model that keeps the peculiarities of tether emissionsis being developed for signal prediction at constant tether current. As a step previous to the calculation of the expected ground signal , the Alfven-wave signature left by the tether far back in the ionosphere has been determined. The scientific expectations from the combined effort to measure the entity of those perturbations will be outlined taking in to account the used ground track sensor systems.

Shedding light on the dark and weakly fluorescent states of green fluorescent proteins

Recent experiments on various similar green fluorescent protein (GFP) mutants at the single-molecule level and in solution provide evidence of previously unknown short- and long-lived “dark” states and of related excited-state decay channels. Here, we present quantum chemical calculations on cis-trans photoisomerization paths of neutral, anionic, and zwitterionic GFP chromophores in their ground and first singlet excited states that explain the observed behaviors from a common perspective. The results suggest that favorable radiationless decay channels can exist for the different protonation states along these isomerizations, which apparently proceed via conical intersections. These channels are suggested to rationalize the observed dramatic reduction of fluorescence in solution. The observed single-molecule fast blinking is attributed to conversions between the fluorescent anionic and the dark zwitterionic forms whereas slow switching is attributed to conversions between the anionic and the neutral forms. The predicted nonadiabatic crossings are seen to rationalize the origins of a variety of experimental observations on a common basis and may have broad implications for photobiophysical mechanisms in GFP.

The relaxation dynamics of the excited electronic states of retinal in bacteriorhodopsin by two-pump-probe femtosecond studies

We present the results of two-pump and probe femtosecond experiments designed to follow the relaxation dynamics of the lowest excited state (S1) populated by different modes. In the first mode, a direct (S0 → S1) radiative excitation of the ground state is used. In the second mode, an indirect excitation is used where the S1 state is populated by the use of two femtosecond laser pulses with different colors and delay times between them. The first pulse excites the S0 → S1 transition whereas the second pulse excites the S1 → Sn transition. The nonradiative relaxation from the Sn state populates the lowest excited state. Our results suggest that the S1 state relaxes faster when populated nonradiatively from the Sn state than when pumped directly by the S0 → S1 excitation. Additionally, the Sn → S1 nonradiative relaxation time is found to change by varying the delay time between the two pump pulses. The observed dependence of the lowest excited state population as well as its dependence on the delay between the two pump pulses are found to fit a kinetic model in which the Sn state populates a different surface (called S′1) than the one being directly excited (S1). The possible involvement of the Ag type states, the J intermediate, and the conical intersection leading to the S0 or to the isomerization product (K intermediate) are discussed in the framework of the proposed model.

Ultra-fast excited state dynamics in green fluorescent protein: multiple states and proton transfer.

The green fluorescent protein (GFP) of the jellyfish Aequorea Victoria has attracted widespread interest since the discovery that its chromophore is generated by the autocatalytic, posttranslational cyclization and oxidation of a hexapeptide unit. This permits fusion of the DNA sequence of GFP with that of any protein whose expression or transport can then be readily monitored by sensitive fluorescence methods without the need to add exogenous fluorescent dyes. The excited state dynamics of GFP were studied following photo-excitation of each of its two strong absorption bands in the visible using fluorescence upconversion spectroscopy (about 100 fs time resolution). It is shown that excitation of the higher energy feature leads very rapidly to a form of the lower energy species, and that the excited state interconversion rate can be markedly slowed by replacing exchangeable protons with deuterons. This observation and others lead to a model in which the two visible absorption bands correspond to GFP in two ground-state conformations. These conformations can be slowly interconverted in the ground state, but the process is much faster in the excited state. The observed isotope effect suggests that the initial excited state process involves a proton transfer reaction that is followed by additional structural changes. These observations may help to rationalize and motivate mutations that alter the absorption properties and improve the photo stability of GFP.

Exponential decay of spin-spin correlation between distant defect states produced by contour hydrogenation of polycyclic aromatic hydrocarbon molecules

The first few low-lying spin states of alternant polycyclic aromatic hydrocarbon (PAH) molecules of several shapes showing defect states induced by contour hydrogenation have been studied both by ab initio methods and by a precise numerical solution of Pariser-Parr-Pople (PPP) interacting model. In accordance with Lieb's theorem, the ground state shows a spin multiplicity equal to one for balanced molecules, and it gets larger values for imbalanced molecules (that is, when the number of π electrons on both subsets is not equal). Furthermore, we find a systematic decrease of the singlet-triplet splitting as a function of the distance between defects, regardless of whether the ground state is singlet or triplet. For example, a splitting smaller than 0.001 eV is obtained for a medium size C46H28 PAH molecule (di-hydrogenated [11]phenacene) showing a singlet ground state. We conclude that π electrons unbound by lattice defects tend to remain localized and unpaired even when long-range Coulomb interaction is taken into account. Therefore they show a biradical character (polyradical character for more than two defects) and should be studied as two or more local doublets. The implications for electron transport are potentially important since these unpaired electrons can trap traveling electrons or simply flip their spin at a very small energy cost.