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We briefly review the arguments for a multicomponent dark matter scenario which contains both the higgsino of natural supersymmetry (susy) and one or both of two other well-motivated particles—the axion and the higgson. This scenario has become increasingly compelling because of increasingly strong constraints from experiment and observation: (1) The predicted density of natural neutralinos (higgsinos with masses of $$\sim $$ 100–300 GeV/c $$^2$$ ) is $$\sim $$ 5–25 times smaller than the observed dark matter density, because the cross section for annihilation of light higgsinos in the early universe is large. (2) The coupling to nuclei through Higgs exchange (with a natural admixture of gaugino components in a particle that is dominantly higgsino) makes it difficult to evade the very low upper limits on the cross sections for scattering off nuclei in direct detection experiments. (3) Indirect detection experiments and Planck have imposed retrictive upper limits on the annihilation cross section. (4) CMS and ATLAS have obtained lower limits on the masses of some susy particles that are relatively high, up to about 2 TeV. Since the axion and axion dark matter experiments have already been extensively reviewed, here we emphasize a candidate which is similar to the neutralino in some respects—with spin 1/2 and R-parity $$=-1$$ —but which has a substantially smaller cross section for both annihilation and scattering, and which, therefore, has the potential to be consistent with (1) observations of the dark matter abundance, (2) spin-independent and spin-dependent scattering in direct detection experiments, (3) observations of gamma rays, antiprotons, etc. in indirect detection experiments, and (4) the LHC limits on masses of susy particles. The present theory also contains susy, with both the lightest neutralino and lightest higgson being stable dark matter particles (since neither can decay into a set of particles containing the other). This theory predicts a doubly rich variety of new particles whose existence can be tested in the foreseeable future.

Relatively simple but apparently novel ways are proposed for viewing three related subjects: black hole entropy, the black hole information paradox, and time travel paradoxes. (1) Gibbons and Hawking have completely explained the origin of the entropy of all black holes, including physical black holes -- nonextremal and in 3-dimensional space -- if one can identify their Euclidean path integral with a true thermodynamic partition function (ultimately based on microstates). An example is provided of a theory containing this feature. (2) There is unitary quantum evolution with no loss of information if the detection of Hawking radiation is regarded as a measurement process within the Everett interpretation of quantum mechanics. (3) The paradoxes of time travel evaporate when exposed to the light of quantum physics (again within the Everett interpretation), with quantum fields properly described by a path integral over a topologically nontrivial but smooth manifold., 18 pages, Journal of Modern Optics

Cavalleri and coworkers have discovered evidence of light-induced superconductivity and related phenomena in several different materials. Here, we suggest that some features may be naturally interpreted using a time-dependent Ginzburg–Landau model. In particular, we focus on the lifetime of the transient state in La1.675Eu0.2Sr0.125CuO4 (LESCO1∕8), which is remarkably long below about 25 K, but exhibits different behavior at higher temperature.

This minireview is meant to provide a broad overview of the most basic interpretations of consciousness and of the potential links to fundamental physics, as a complement to the large number of more detailed experimental and theoretical studies. In the spirit of previous ideas in the neuroscience community, but with a more physics-oriented perspective, we begin with the interpretation that consciousness is the collective excitation of a brainwide web of neural cells (where the phrases of other authors have been combined). This picture is inspired by the fact that, in all major areas of physics, a collective excitation has just as much physical reality as a particle or other localized object. The brainwide web extends into those regions (neuronal and glial networks) where processed information is received from the senses, memories, etc. (emerging out of unconscious processes in prior networks). It unifies those regions (plus motor control regions) via the vast complexity of the neural interactions that it spans. At the most fundamental level, all physical phenomena result from excitation of quantum fields (since, in current physics, these fields are the bedrock of reality). It follows that, in the present picture, quantum physics solves the old combination (or binding) problem of consciousness, since the experience of consciousness requires coherent excitation of only a single hybrid electron-electromagnetic field.

The WIMP proposed here yields the observed abundance of dark matter, and is consistent with the current limits from direct detection, indirect detection, and collider experiments, if its mass is $\sim 72$ GeV/$c^2$. It is also consistent with analyses of the gamma rays observed by Fermi-LAT from the Galactic center (and other sources), and of the antiprotons observed by AMS-02, in which the excesses are attributed to dark matter annihilation. These successes are shared by the inert doublet model (IDM), but the phenomenology is very different: The dark matter candidate of the IDM has first-order gauge couplings to other new particles, whereas the present candidate does not. In addition to indirect detection through annihilation products, it appears that the present particle can be observed in the most sensitive direct-detection and collider experiments currently being planned., 7 pages, to be published in EPL (Europhysics Letters)

We review a dark matter scenario with a number of favorable aspects: (1) all of the well-known successes of supersymmetry are preserved, (2) the parameters can satisfy naturalness, (3) the addition of an extended Higgs sector implies a doubly rich plethora of new particles and new physics to be discovered in the near or foreseeable future, (4) the mass of the dominant dark matter WIMP is $\le 125$ GeV/c$^2$, (5) the gauge couplings of this particle are precisely defined, and (6) naturalness implies that its Higgs-mediated couplings are also comparable to those of a natural neutralino. Recent (and earlier) analyses of the data from Planck, Fermi-LAT, AMS-02, and other experiments indicate that (i) the positron excess at $\sim 800$ GeV or above is not evidence of high-mass dark matter particles (which would have disconfirmed the present theory with a rigorous upper limit of $125$ GeV), (ii) the Galactic center excess of gamma rays observed by Fermi is evidence for dark matter particles with a mass below or near $100$ GeV, (iii) the gamma-ray excess from Omega Centauri is similar evidence of annihilation of such relatively low-mass particles, and (iv) the antiproton excess observed by AMS is again evidence of $\lesssim 100$ GeV dark matter particles. The present scenario, with two stable spin 1/2 WIMPs (a high-mass neutralino and a more abundant ``Higgson'' with a mass of $\le 125$ GeV/c$^2$) is consistent with these results (as well as all others which have been verified), and it also suggests that detection should be near in a variety of experiments for direct, indirect, and collider detection.

This paper is a celebration of the frontiers of science. Goodenough, the maestro who transformed energy usage and technology through the invention of the lithium ion battery, opens the programme, reflecting on the ultimate limits of battery technology. This applied theme continues through the subsequent pieces on energy related topics (the sodium ion battery and artificial fuels, by Mansson) and the ultimate challenge for 3 dimensional printing the eventual production of life, by Atala. A passage by Alexander follows, reflecting on a related issue: How might an artificially produced human being behave? Next comes a consideration of consiousness and free will by Allen and Lidstrom. Further voices and new instruments enter as Bowen, Mauranyapin and Madsen discuss whether dynamical processes of single molecules might be observed in their native state. The exploitation of chaos in science and technology, applications of Bose Einstein condensates and a consideration of the significance of entropy follow in pieces by Reichl, Rasel and Allen, respectively. Katsnelson and Koonin then discuss the potential generalisation of thermodynamic concepts in the context of biological evolution. Entering with the music of the cosmos, Yasskin discusses whether we might be able to observe torsion in the geometry of the universe. The crescendo comes with the crisis of singularities, their nature and whether they can be resolved through quantum effects, in the composition of Coley. The climax is Krenn, Melvin and Zeilinger consideration of how computer code can be autonomously surprising and creative. In a harmonious counterpoint, Yampolskiy concludes that such code is not yet able to take responsibility for coauthoring a paper., Please send comments and questions to Suzy Lidstrom

We describe two natural scenarios in which both dark matter WIMPs (weakly interacting massive particles) and a variety of supersymmetric partners should be discovered in the foreseeable future. In the first scenario, the WIMPs are neutralinos, but they are only one component of the dark matter, which is dominantly composed of other relic particles such as axions. (This is the multicomponent model of Baer, Barger, Sengupta, and Tata.) In the second scenario, the WIMPs result from an extended Higgs sector and may be the only dark matter component. In either scenario, both the dark matter WIMP and a plethora of other neutral and charged particles await discovery at many experimental facilities. The new particles in the second scenario have far weaker cross-sections for direct and indirect detection via their gauge interactions, which are either momentum-dependent or second-order. However, as we point out here, they should have much stronger interactions via the Higgs. We estimate that their interactions with fermions will then be comparable to (although not equal to) those of neutralinos with a corresponding Higgs interaction. It follows that these newly proposed dark matter particles should be within reach of emerging and proposed facilities for direct, indirect, and collider-based detection., 7 pages, to be published in Modern Physics Letters A

Since neither supersymmetry nor dark matter WIMPs have yet been observed, pessimism about their reality has been growing. Here we discuss a new supersymmetric theory and a new dark matter candidate which are naturally consistent with current experimental results, but which imply a plethora of new phenomena awaiting discovery within the foreseeable future., 9 pages, accepted by Physica Scripta

The Winter Colloquium on the Physics of Quantum Electronics (PQE) has been a seminal force in quantum optics and related areas since 1971. It is rather mindboggling to recognize how the concepts presented at these conferences have transformed scientific understanding and human society. In January, 2017, the participants of PQE were asked to consider the equally important prospects for the future, and to formulate a set of questions representing some of the greatest aspirations in this broad field. The result is this multi-authored paper, in which many of the world's leading experts address the following fundamental questions: (1) What is the future of gravitational wave astronomy? (2) Are there new quantum phases of matter away from equilibrium that can be found and exploited - such as the time crystal? (3) Quantum theory in uncharted territory: What can we learn? (4) What are the ultimate limits for laser photon energies? (5) What are the ultimate limits to temporal, spatial, and optical resolution? (6) What novel roles will atoms play in technology? (7) What applications lie ahead for nitrogen-vacancy centers in diamond? (8) What is the future of quantum coherence, squeezing, and entanglement for enhanced superresolution and sensing? (9) How can we solve (some of) humanity's biggest problems through new quantum technologies? (10) What new understanding of materials and biological molecules will result from their dynamical characterization with free electron lasers? (11) What new technologies and fundamental discoveries might quantum optics achieve by the end of this century? (12) What novel topological structures can be created and employed in quantum optics?, Comment: A review of the leading topics in quantum optics and related areas. Accepted for publication in J. Modern Optics (42 figures, 74 pages)

We propose a Higgs-related but spin $1/2$ dark matter candidate with a mass that is comparable to that of the Higgs. This particle is a WIMP with an R-parity of $-1$, but it can be distinguished from a neutralino by its unconventional couplings to W and Z bosons. Charged spin 1/2 particles of a new kind are also predicted at higher energy., 9 pages

In The Hitchhiker's Guide to the Galaxy, by Douglas Adams, the Answer to the Ultimate Question of Life, the Universe, and Everything is found to be 42 -- but the meaning of this is left open to interpretation. We take it to mean that there are 42 fundamental questions which must be answered on the road to full enlightenment, and we attempt a first draft (or personal selection) of these ultimate questions, on topics ranging from the cosmological constant and origin of the universe to the origin of life and consciousness., 54 pages; downloaded more than 11,000 times from journal site

String theory has many compelling features, including higher dimensions, supersymmetry, and topological defects of central importance. Here we describe a theory which also contains these features, but which is much more ambitious than string theory and also much closer to experiment. Since the mathematical details are given in a much longer paper, we summarize only the most important results, which include SO(N) gauge unification, vanishing of the usual cosmological constant, and a credible dark matter candidate.

In the spirit of previous ideas in the neuroscience community, but with a more physics-oriented perspective, we propose that consciousness can be described as the collective excitation of a brainwide web of neurones. This picture is inspired by the fact that, in all major areas of physics, a collective excitation has just as much physical reality as a particle or other localized object. The brainwide web extends into those regions (neuronal networks) where processed information is received from the senses, memories, etc. (emerging out of unconscious processes in prior networks). It unifies those regions (plus motor control regions) via the vast complexity of the neuronal interactions that it spans. A crude analogy is the worldwide web, which extends into servers which have been prepared by external agents. Other crude analogies are the many collective excitations in physics. True understanding of consciousness must rely mostly on experiment, but since probes of living brains have limited precision, there is an important role for large-scale simulations in revealing details and complementing experiment. Petascale computational facilities should make it possible to perform simulations with realistic complexity.

Our density-functional-based simulations demonstrate that atomic hydrogen can be encapsulated in C 60 via collisions of either H or H 2 with a C 60 molecule whose breathing mode has been properly excited by an optimized femtosecond-scale laser pulse. The basic mechanism is a ‘breathing trap’, with the C 60 first expanding, to admit the H, and then contracting, to contain it.

This is a very brief discussion of some experimental and theoretical studies of materials responding to fast intense laser pulses, with emphasis on those cases where the electronic response and structural response are both potentially important (and ordinarily coupled). Examples are nonthermal insulator-to-metal transitions and light-induced superconductivity in cuprates, fullerenes, and an organic Mott insulator.

A bridged azobenzene was recently found by Siewertsen et al. to exhibit highly efficient photoisomerization through nπ ∗ absorption. In the work of the present Letter, cis -to- trans photoisomerization of this molecule in the gas phase was examined in detail via density-functional-based dynamical simulations. Our results indicate that rotation around the central NN bond is the dominant mechanism when this reaction proceeds via nπ ∗ excitation, and the bridging feature was found not to hinder the rotation. Nonadiabatic de-excitation occurred at an avoided crossing between the ground state and lowest singlet excited state near the midpoint of the rotational pathway.

For nonresonant femtosecond-scale laser pulses, we showed earlier that the optimum FWHM pulse duration is 0.42 T if one wishes to achieve maximum relative excitation of a specific vibrational mode with period T . Here we show that much greater enhancement of a specific mode can be achieved with a sequence of laser pulses which have both this optimized duration and optimized delays between pulses. One can thus enhance a selected set of modes that are particularly useful in identifying or characterizing a chemical system, to lift them out of the background of less informative modes.

Using density-functional-based molecular dynamics simulations, we have performed comparative studies of the trans-cis isomerizations of azobenzene and bridged azobenzene (B-Ab) 5,6-dihydrodibenzo[c,g][1,2]diazocine induced by nπ* electronic excitation. The quantum yields found in our calculations, 45% for the bridged azobenzene versus 25% for azobenzene, are consistent with the experiment. Both isomerization processes involve two steps: (1) Starting from the trans structure, each molecule moves on its S(1) excited-state potential energy surface, via rotation around the NN bond, to an avoided crossing near the S(1)/S(0) conical intersection, where de-excitation occurs. (2) Subsequently, in the electronic ground state, there is further rotation around the NN bond, accompanied by twisting of the phenyl rings around their CN bonds, until the cis geometry is achieved. Because of its lower symmetry and smaller initial CNNC dihedral angle, the bridged azobenzene has a much shorter lifetime for the S(1) excited state, about 30 fs, as compared to about 400 fs for azobenzene. However, we find that the complete isomerizations have approximately the same time scales. Although the bridging feature in trans-B-Ab does not hinder rotation around the NN bond in step 1, it makes twisting of the two phenyl rings around the CN bonds much slower in step 2.

We predict that photocyclization of trans-stilbene to 4a,4b-dihydrophenanthrene (DHP) can be achieved with a femtosecond-scale laser pulse. Specifically, we have performed dynamical simulations in which the original trans-stilbene molecule is observed first to isomerize to cis-stilbene (as an excited-state intermediate) after more than one picosecond, and then to form a new bond to become DHP, after a few hundred additional femtoseconds. The process is initiated by excitation from the S 0 electronic state to S 1 , with subsequent de-excitation at avoided crossings.

The major chemical trends in the changes of Frenkel core-exciton energies caused by (1) a neighboring defect, (2) a second exciton (biexciton), or (3) a surface are predicted, using an extension of the impurity model of Hjalmarson et al. Specific results are given for Ga core excitons in GaP.

Dynamical simulations are reported for trans-to-cis photoisomerization of stilbene induced by a femtosecond-scale laser pulse. The results imply that the basic mechanism is the simplest imaginable: Electronic excitation by the laser pulse weakens the central C–C bond, and also excites vibrations of this bond. Vibrational energy redistribution then leads to rotation about the bond, resulting in isomerization. Finally, electronic de-excitation near a conical intersection restores the molecule to the electronic ground state. On the other hand, these dynamical simulations demonstrate complexity in the details and reveal supporting roles for other degrees of freedom, such as torsional motion about the vinyl-phenyl side bonds.

A realistic dynamical simulation is reported for ultrafast laser excitation and rotational de-excitation of cis -stilbene. Upon irradiation by a laser pulse with a FWHM of 100 fs, the molecule first rotates around its vinyl bond to about 90°, where an avoided crossing leads to electronic de-excitation. The molecule then immediately twists back to about −60° within only about 80 fs, avoiding isomerization. The other principal dihedral angles, within the phenyl rings, lag behind in time, so there is significant large strain between the phenyl rings and vinyl bond.

When a molecule is subjected to a short intense laser pulse, the ensuing dynamical processes depend qualitatively on the pulse parameters, including duration, frequency, and fluence. Here we report studies of cis to trans photoisomerization of azobenzene following femtosecond-scale laser pulses which are relatively short (10 fs) or long (100 fs) and which have a central frequency matched to either the first excited state (S1, or HOMO to LUMO in a molecular orbital picture) or the second (S2, or HOMO-1 to LUMO). The results presented here demonstrate that photoisomerization involves a rather intricate sequence of connected steps, with the nuclear and electronic degrees of freedom inextricably coupled. One important feature is the de-excitation required for the molecule to achieve its new ground-state after isomerization. If the primary excitation is to S1, then we find that only a single HOMO/LUMO avoided crossing is required and that this crossing occurs halfway along a rotational pathway involving the central CNNC dihedral angle. If the primary excitation is to S2, then the same HOMO/LUMO avoided crossing is observed, but it must be preceded by another avoided crossing that permits transfer of holes from the HOMO-1 to the HOMO, so that the HOMO is then able to accept electrons from the LUMO. We find that this earlier crossing can occur in either of two geometries, one near the cis configuration and the other near the trans. The fact that S2 (pi pi*) isomerization requires two steps may be related to the fact that isomerization yields are smaller for this (UV) excitation than for the S1 (n pi*, visible-light) excitation.

Detailed understanding of femtosecond-scale photochemical processes requires dynamical simulations that are complementary to interpretations based on transitions between energy surfaces. For cis to trans photoisomerization of azobenzene following a 100 fs laser pulse, we find that the mechanism is rotation about the central NN bond, and the process is complete in less than 1 ps. The initial excitation and subsequent de-excitation are each achieved via multiple steps, with the molecule always in a superposition of electronic states, as the 3N nuclear degrees of freedom are excited by the laser pulse.

The effect of C–C–C bond bending on the photodissociation of cyclobutane to form two ethylene molecules was investigated by performing semiclassical electron-radiation-ion dynamics simulations and also by examining the potential energy surfaces of the electronic ground state and lowest excited states. These potential energy surfaces, calculated at the CASSCF/MRPT2 level with 6-31G* basis sets along a reaction path determined by the semiclassical dynamics simulations, show well-defined energy minima and maxima in the intermediate state region. It is found that in addition to rotation of the molecule around the central C–C bond, C–C–C bond bending plays an important role in determining the features of the potential energy surfaces for the intermediate species. # 2007 Elsevier B.V. All rights reserved.

Simulations of the photoconversion of 4a,4b-dihydrophenanthrene (DHP) to stilbene, using semiclassical electron-radiation-ion dynamics, are reported. The carbon single bond connecting the phenyl rings breaks approximately 200 fs after the laser pulse is turned off. In the results shown here, the excited cis -stilbene molecule then further isomerizes to the trans conformation after about 1 ps. Each of these two changes of molecular geometry involves a HOMO–LUMO avoided crossing, which produces a transfer of electronic population from LUMO to HOMO. These detailed dynamical simulations are consistent with the interpretation that ring-opening in stilbene is a single photon process.

Semiclassical electron-radiation-ion dynamics simulations are reported for the photodissociation of cyclobutane into two molecules of ethylene. The results clearly show the formation of the tetramethylene intermediate diradical, with dissociation completed in approximately 400 fs. In addition, the potential energy surfaces of the electronic ground state and lowest excited-state were calculated at the complete-active-space self-consistent-field/multireference second-order perturbation theory (CASSCF/MRPT2) level with 6-31G* basis sets, along the reaction path determined by the dynamics simulations. There are well-defined energy minima and maxima in the intermediate state region. It is found that both C-C-C bond bending and rotation of the molecule (around the central C-C bond) have important roles in determining the features of the potential energy surfaces for the intermediate species. Finally, the simulations and potential energy surface calculations are applied together in a discussion of the full mechanism for cyclobutane photodissociation.

We have constructed a semiquantitative model which can be used to treat the response of molecules containing carbon, nitrogen, oxygen and hydrogen to ultrafast laser pulses. The parameters for the pairwise interaction of both nitrogen and oxygen are obtained from a simple scaling of carbon and hydrogen parameters calculated by Frauenheim and co-workers. For the initial application of this model, we have chosen dipicolinic acid (DPA), an important constituent of biological spores. The equilibrium bond lengths of all of the test molecules, including DPA, are within 5% of the experimental bond lengths. The calculated molecular orbital structure of DPA near the HOMO–LUMO gap region shows three nearly degenerate occupied molecular orbitals and two nearly degenerate unoccupied orbitals. Photoexcitation with an ultrashort 5 fs FWHM laser pulse matched to the HOMO–LUMO gap energy produced transitions from the lowest of the the three nearly degenerate occupied orbitals to both of the nearly degenerate unoccupied m...

If robust spectroscopic techniques are to be developed for the detection and identification of pathogens, one must understand the relevant spectroscopic properties of the target molecules. In this paper, we employ density functional theory (DFT) to study the structural, electronic, vibrational, optical, and magnetic properties of dipicolinic acid (DPA) and its dianion DPA−2. Our full geometrical optimization and Mulliken charge analysis show that DFT does not lead to the significant discrepancies between charges on symmetric carbon, hydrogen, and oxygen atoms that are found in less accurate calculations based on the complete active space MCSCF method. Our calculated vibrational frequencies, Raman spectra, and infrared spectra for ground-state DPA and DPA−2 are in good agreement with experiment, and this is also true of the four calculated 13C NMR spectral lines (for α, β, γ, and carboxyl sites). Our time-dependent DFT study of the optical excitation and absorption of both DPA and DPA−2 provides the first interpretation of the observed near ultraviolet absorption and fluorescence spectra. Finally, we discuss for the first time the effect of a solvent on the spectral properties of DPA.

Semiclassical electron-radiation-ion dynamics (SERID) has been used to calculate the bond lengths, HOMO–LUMO energy gap and vibrational modes of benzene, and to examine the photodissociation of benzene molecules subjected to fast intense laser pulses. The calculated ground-state properties are in good agreement with experiment, confirming that density-functional-based SERID simulations provide a reliable treatment of bonding. We show results for representative simulations of the response of benzene to femtosecond-scale laser pulses, at various fluences corresponding to no dissociation, partial dissociation and complete dissociation into atomic constituents.

Semiclassical simulations are reported for the dynamics of the photocyclization of cis-stilbene, leading to the formation of 4a,4b-dihydrophenanthrene. Photo-excited cis-stilbene rotates about its vinyl and vinyl-phenyl bonds simultaneously. The structural changes result in a series of strong couplings between the highest occupied molecular orbital and the lowest unoccupied molecular orbital. These couplings cause the formation of a new chemical bond between the two phenyl rings of stilbene. The length changes of different C─C bonds, corresponding to the formation of the new molecule, are presented and discussed in detail.

We present simulations of the response of Si and InSb to femtosecond-scale laser pulses of various intensities. In agreement with the experiments by various groups on various materials, there is a nonthermal phase transition for each of these semiconductors above a threshold intensity. Our simulations employ semiclassical electron-radiation-ion dynamics (SERID), a technique which is briefly described in the text. We also introduce a new addition to the technique, which provides a simple treatment of the correction due to motion of the atomic-orbital basis functions. We find that this correction is small in the present context, but it may be substantial in situations with more rapid atomic motion. Our expression for this correction is remarkably simple to employ because it amounts to nothing more than a generalized Peierls substitution.

The intricate dynamical processes in photochemical reactions are not fully accessible to either experiment or conventional theory. Here, we outline a technique for simulations in photochemistry, which employs classical trajectories for the nuclei moving in a mean field, with the electrons coupled to a laser pulse by the time-dependent Peierls substitution. We demonstrate that it provides an illuminating description of photoisomerization. One observes a nontrivial sequence of events which include multiple electronic excitations, conversion of double bonds to single bonds (and vice versa), nonadiabatic depopulation of excited levels at avoided crossings, vibrational energy redistribution, and an elegant interdependence of the various electronic and vibrational degrees of freedom.

The usual building blocks of molecular orbital theory are one-center atomic orbitals. If we instead use two-center orbitals derived from the exact solution for H + 2 , a simple calculation of the H 2 binding energy yields 4.5 eV, with a Hylleraas correlation factor having no adjustable parameters. Variational minimization with respect to a couple of parameters in our trial function then shifts the binding energy to 4.7 eV, very near the experimental value. The approach is also applicable to heteronuclear molecules; e.g., HeH + , whose ground state energy we calculate to be −2.96 hartree with no adjustable parameters. This is to be compared with −2.98 hartree previously calculated using a wavefunction with 83 terms.

Detailed simulations are reported for the dynamics of electrons and nuclei during the cis to trans photoisomerization of stilbene. Our method, which employs a semiclassical description of both the nuclear motion and the radiation field, is described in the text. After excitation of electrons from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) by a femtosecond-scale laser pulse, two principal avoided crossings are observed between the HOMO and LUMO levels, each of which leads to substantial depopulation of the LUMO. Based on our results and those of other groups, we propose that the first such HOMO–LUMO coupling can lead to the formation of 4a,4b-dihydrophenanthrene (DHP). The second coupling, on the other hand, leads to the formation of trans-stilbene. It is found that pyramidalization of the two carbon atoms of the vinyl group is involved significantly in both couplings, and that rotation of the two phenyl rings, together with their interaction, plays an important role in the first coupling. The occurrence along the same trajectory of two couplings, one leading to DHP and the other leading to isomerization, is a significant observation, and one which indicates that further theoretical and experimental investigations would be of considerable interest.

Detailed simulations are reported for the dynamics of electrons and nuclei during the cis—trans isomerization of butadiene following a femtosecond-scale laser pulse. Our technique, semiclassical electron-radiationion dynamics (SERID), is fully described in the text. The one-electron Hamiltonian and ion—ion interactions employed in the present work are density-functional based. Following excitation of electrons by the laser pulse, all three C—C bonds in the butadiene molecule become longer, as electrons are promoted from the HOMO to both the LUMO and LUMO+1 levels. In the excited electronic state, the molecule rotates about all of its three C—C bonds. There are then non-adiabatic events near avoided crossings, with electronic transitions to the ground state via creation of vibrational excitations. The molecule continues to twist around the central bond and one of the terminal C—C bonds, until the trans-conformation is achieved. Various features in the behaviour of the vibrational modes can also be...

The detailed dynamical processes involved in trans → cis photoisomerization of butadiene have been studied in realistic simulations, employing a technique that is described in the text. Many interesting features are observed, including the following sequence of events: (i) The initial electronic excitation converts the central single bond to a double bond and the terminal double bonds to single bonds, so the molecule at first rotates about only these end bonds. (ii) There is then a series of rapid nonadiabatic transfers of population among electronic states near the HOMO−LUMO gap, which ultimately result in depopulation of the excited states. (iii) The bonds consequently revert to their original ground-state character, permitting a continuous rotation about the central single bond. At the end, the molecule is essentially in the ground electronic state for the new conformation. The simulation results clearly demonstrate the couplings of C−C−C bending vibrations to nonadiabatic electronic transitions and i...

Detailed simulations are reported for the coupled dynamics of electrons and nuclei in the isomerization reaction of cis-stilbene stimulated by laser excitation. The results demonstrate that, in addition to the traditional vinyl and vinyl– phenyl torsions, the HCCH torsional coordinate of the vinyl group also makes a significant contribution to the HOMO and LUMO couplings that yield nonradiative electronic transitions to the ground state. Moreover, variations in the CCH angles of the vinyl group in the vicinity of the HOMO–LUMO couplings indicate that pyramidalization of the vinyl-group carbon atoms is strongly involved in these molecular orbital couplings. 2003 Published by Elsevier B.V.

Using the technique of tight-binding electron–ion dynamics, we have calculated the response of crystalline GaAs when a femtosecond laser pulse excites 1–20% of the valence electrons. Above a threshold fluence, which corresponds to promotion of about 12% of the valence electrons to the conduction band, the lattice is destabilized and the band gap collapses to zero. This result supports the conclusion that structural changes on a subpicosecond time scale observed in pump-probe experiments are of a nonthermal nature.

During the past several years, a new technique for realistic simulations of the interaction of light with matter has been developed and employed. Recent simulations of laser pulses interacting with molecules clearly demonstrate the potential for control of chemical reactions through various mechanisms, which include the following: (i) excitation of electrons to states that have different bonding properties; (ii) control of electron populations through a coherent pump-pulse, control-pulse sequence; and (iii) control of molecular vibrations through a pump-control sequence. Significant chemical insights are gained when one can watch a realistic animation of species interacting and reacting. One can monitor the time evolution of electronic states and their occupancy, as well as the motion of the atoms. One can also observe the evolution from reactants to products through transition states. Finally, one can determine how this evolution is affected by the various properties of the laser pulses, including intens...

We review a conceptual design for a moderate resolution optical spectrograph for the Giant Magellan Telescope (GMT). The spectrograph is designed to make use of the large field-of-view of the GMT and be suitable for observations of very faint objects across a wide range of wavelengths. We also review the status of the instrument and on-going trade studies designed to update the instrument science objectives and technical requirements.

The Visible Integral-Field Replicable Unit Spectrograph (VIRUS) instrument will be installed at the Hobby-Eberly Telescope† in the near future. The instrument will be housed in two enclosures that are mounted adjacent to the telescope, via the VIRUS Support Structure (VSS). We have designed the enclosures to support and protect the instrument, to enable servicing of the instrument, and to cool the instrument appropriately while not adversely affecting the dome environment. The system uses simple HVAC air handling techniques in conjunction with thermoelectric and standard glycol heat exchangers to provide efficient heat removal. The enclosures also provide power and data transfer to and from each VIRUS unit, liquid nitrogen cooling to the detectors, and environmental monitoring of the instrument and dome environments. In this paper, we describe the design and fabrication of the VIRUS enclosures and their subsystems.

The interaction of materials with ultrafast and ultraintense laser pulses is a current frontier of science both experimentally and theoretically. In this review, we briefly discuss some recent theoretical studies by the present authors with our method of semiclassical electron-radiation-ion dynamics (SERID). In particular, Zhou et al. and Jiang et al. respectively, determined the optimal duration and optimal timing for a series of femtosecond scale laser pulses to excite a specific vibrational mode in a general chemical system. A set of such modes can be used as a "fingerprint" for characterizing a particular molecule or a complex in a solid. One can therefore envision many applications, ranging from fundamental studies to detection of chemical or biological agents. Allen et al. proved that dimers are preferentially emitted during photofragmentation of C60 under an ultrafast and ultraintense laser pulse. For interactions between laser pulses and semiconductors, e.g., GaAs, Si and InSb, besides experimentally accessible optical properties--epsilon(omega) and chi(2)-Allen et al. offered many other indicators to confirm the nonthermal nature of structural changes driven by electronic excitations and occurring during the first few hundred femtoseconds. Lin et al. found that, after the application of a femtosecond laser pulse, excited electrons in materials automatically equilibrate to a Fermi-Dirac distribution within roughly 100 fs, solely because of their coupling to the nuclear motion, even though the resulting electronic temperature is one to two orders of magnitude higher than the kinetic temperature defined by the nuclear motion.

Using the technique of tight-binding electron–ion dynamics, we have calculated the evolution of the non-linear susceptibility χ(2)(ω) in GaAs during the first few hundred femtoseconds following an ultrafast and ultra-intense laser pulse. Above a threshold fluence, our simulations show that χ(2)(ω) drops to zero, in agreement with the experimental measurements. The results indicate a rapid non-thermal transition from the original tetrahedral structure to a disordered structure, and support the conclusion that structural changes following ultrashort pulses are a direct consequence of bond destabilization.

Using a new formalism that modifies a tight-binding Hamiltonian to include interaction with a timedependent electromagnetic field, we have obtained an analytical expression for the second-order susceptibility. This expression has been used to calculate the energy dependence of x (v) for GaAs. The results are in agreement with previous calculations and with available experimental data. @S0163-1829~98!01848-7#