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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

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...

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.

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...

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...