Volume 117, Issue 3, 15 July 2002
Index of content:
- Theoretical Methods and Algorithms
117(2002); http://dx.doi.org/10.1063/1.1484385View Description Hide Description
The quantum theory of stress is developed within the atoms in molecules (AIM) framework. The complete local stress field is introduced and integrated within atomic basins, and it is shown that the kinetic term gives rise to the atomic virial theorem. The role of the potential part of the stress field in the AIM theory is discussed, and its necessary consideration in order to define atomic pressures presented. These atomic pressures are shown to tend to the thermodynamic limit as the size of the system grows. A link between the AIM theory and the theory of electronic separability has also been found. A set of simple examples illustrates our results.
117(2002); http://dx.doi.org/10.1063/1.1483856View Description Hide Description
A state-selective multireference coupled-cluster algorithm is presented which is capable of describing single, double (or higher) excitations from an arbitrary complete model space. One of the active space determinants is chosen as a formal Fermi-vacuum and single, double (or higher) excitations from the other reference functions are considered as higher excitations from this determinant as it has been previously proposed by Oliphant and Adamowicz [J. Chem. Phys. 94, 1229 (1991)]. Coupled-cluster equations are generated in terms of antisymmetrized diagrams and restrictions are imposed on these diagrams to eliminate those cluster amplitudes which carry undesirable number of inactive indices. The corresponding algebraic expressions are factorized and contractions between cluster amplitudes and intermediates are evaluated by our recent string-based algorithm [J. Chem. Phys. 115, 2945 (2001)]. The method can be easily modified to solve multireference configuration interaction problems. Performance of the method is demonstrated by several test calculations on systems which require a multireference description. The problem related to the choice of the Fermi-vacuum has also been investigated.
117(2002); http://dx.doi.org/10.1063/1.1483854View Description Hide Description
The three ab initiononadiabatic coupling terms related to the three strongly coupled states of the molecule, i.e., and were studied applying the line integral technique [M. Baer, Chem. Phys. Lett. 35, 112 (1975)]. The following was verified: (1) Due to the close proximity of the conical intersections between these three states, two-state quantization cannot always be satisfied between two successive states. (2) It is shown that in those cases where the two-state quantization fails a three-state quantization is satisfied. This three-state quantization is achieved by applying the nonadiabatic coupling matrix that contains the three relevant nonadiabatic coupling terms. The quantization is shown to be satisfied along four different contours (in positions and sizes) surrounding the relevant conical intersections.
117(2002); http://dx.doi.org/10.1063/1.1485072View Description Hide Description
We present a set of second-order, time-reversible algorithms for the isothermal (NVT) molecular-dynamics (MD) simulation of systems with mixed hard-core/continuous potentials. The methods are generated by combining real-time Nosé thermostats with our previously developed Collision Verlet algorithm [Mol. Phys. 98, 309 (1999)] for constant energy MD simulation. In all we present five methods, one based on the Nosé–Hoover [Phys. Rev. A 31, 1695 (1985)] equations of motion and four based on the Nosé–Poincaré [J. Comput. Phys. 151, 114 (1999)] real-time formulation of Nosé dynamics. The methods are tested using a system of hard spheres with attractive tails and all correctly reproduce a canonical distribution of instantaneous temperature. The Nosé–Hoover based method and two of the Nosé–Poincaré methods are shown to have good energy conservation in long simulations.
A linear response approach to second-order electronic transition intensities for multiconfigurational self-consistent field wave functions117(2002); http://dx.doi.org/10.1063/1.1485724View Description Hide Description
A new theoretical approach to two-photon transition intensities at the multiconfigurational self-consistent field (MCSCF) level of theory, is described in detail. The fundamental property of an MCSCF wave function, that it is possible to define the response equations for an excited state, is a prerequisite. The method requires solely first-order multiconfigurational response calculations, because the equations involve the response of both the initial and final state. However, the method is approximate as the coupling between the +ω and −ω parts of the linear response is disregarded. The complete active space state interaction (CASSI) method is applied in the evaluation of the involved matrix elements. To illustrate the performance and the requirements of this method, it was used to determine TP transitions in trans-1,3-butadiene and trans-stilbene.
117(2002); http://dx.doi.org/10.1063/1.1483851View Description Hide Description
A simple algebraic model is used to show that Hirshfeld surfaces in condensed phases may be understood as approximations to the interatomic surfaces of the theory of atoms in molecules. The conditions under which this similarity is valid are explored, and both kinds of surfaces are calculated in the LiF and crystals to illustrate the main results. The link between Hirshfeld and interatomic surfaces provides a physical ground to understand the usage of the former to visualize intermolecular interactions.
117(2002); http://dx.doi.org/10.1063/1.1485068View Description Hide Description
This paper investigates the ability of closed loop quantum learning control techniques to meet a posed physical objective while simultaneously steering the dynamics to lie in a specified subspace. Achievement of successful control with reduced space dynamics can have a number of benefits including a more easily understood control mechanism. Judicious choices for the cost functional may be introduced such that the closed loop optimal control experiments can steer the dynamics to lie within a subspace of the system eigenstates without requiring any prior detailed knowledge about the system Hamiltonian. Learning control with reduced space dynamics takes advantage of the expected existence of a multiplicity of fields that can all give acceptable quality control outcomes. The procedure eliminates the hard demands of following a specific dynamical path by only asking that the dynamics reside in a subspace. Additional measurements characterizing the subspace are necessary to monitor the system evolution during the control field learning process. This procedure is simulated for optimally controlled population transfer experiments in systems of one and two degrees of freedom. The results demonstrate that optimal control fields can be found that successfully derive the system to the target state while staying within the desired subspace.
A new theoretical insight into the nature of intermolecular interactions in the molecular crystal of urea117(2002); http://dx.doi.org/10.1063/1.1482069View Description Hide Description
The nature of interactions in the molecular crystal of urea is analyzed in terms of the interaction energy decomposition. The influence of the electron correlation effects was estimated on the basis of the calculated second order Møller–Plesset corrections and their analysis. In the crystal, the urea molecules form infinite ribbons which reveal strong cooperative effects. The hydrogen-bonded interactions of the orthogonal ribbons do stabilize the crystal, whereas interactions between parallel tapes are repulsive. The stability of the crystal structure is determined by a subtle balance between these two types of interactions. Although, the electron correlation effects are stabilizing, their contribution is rather small in comparison with the total interaction energy.
- Gas Phase Dynamics and Structure: Spectroscopy, Molecular Interactions, Scattering, and Photochemistry
Photofragment emission yield spectroscopy of acetylene in the and states by vacuum ultraviolet and infrared vacuum ultraviolet double-resonance laser excitations117(2002); http://dx.doi.org/10.1063/1.1485064View Description Hide Description
The excitation spectra of the and states of in the 135.3–130.8 nm range are measured under jet-cooled conditions by detecting fluorescence emitted from or photofragments. In the photofragment emission yield spectra, the origin bands of the and transitions are observed with Lorentzian profiles with bandwidth (Γ) of 58.9(4) and 66.7(2) cm−1, respectively. By identifying the bending progressions of the transition appearing with narrower Lorentzian profiles with, the band previously considered to be the origin band of the transition is assigned to the transition to the second overtone level in the near-cis bending mode. The transitions to the C–H stretch excited levels in the and states are observed using the infrared-VUV double resonance excitation scheme. The and bands are identified at 74 334(3), 74 121(5), 74 522(3), and 74 388(3) cm−1, respectively, with much broader bandwidth than the and origin bands, indicating that the dissociation is accelerated significantly in both of the and states when the antisymmetric C–H stretch mode in the and states is excited.
117(2002); http://dx.doi.org/10.1063/1.1484386View Description Hide Description
The two lowest-lying states, the first and excited states of the AgCl molecule have been studied through extensive complete active space coupled pair functional calculations, using a 19-active-electron relativistic effective core potential (RECP) for Ag, a 7-active electron RECP for chlorine and large optimized Gaussian basis sets for both atoms. The and excited states present shallow relative minima very near the equilibrium geometry of the ground state, while the lowest states were found to be totally repulsive in the internuclear range studied. The states present very shallow minima around 5.2 a.u. The calculated spectroscopic constants for the ground- and excited states are compared with the available experimental data and have been found in good agreement. Even though the state is repulsive, it lies very close in energy to the one near the equilibrium geometry of the ground state; thus, a strong mixture through the spin–orbit interaction is predicted to occur that will lead to the fine-structure B state responsible for the recently revised transitions in AgCl. The transition observed at 43 500 cm−1, appears now to arise from a higher-lying root of the or Π manifolds, perhaps the third root, while the system seems to arise from the transition around 49 000 cm−1.
117(2002); http://dx.doi.org/10.1063/1.1484383View Description Hide Description
Two-photon excitation was used to excite a number of vibronic bands within the state of ammonia, including a number of combination bands involving the umbrella mode, and the asymmetric bend, Photoelectron spectra following single-photon ionization of these levels by the same laser provide insight into the character of the intermediate levels, and resolve questions raised by previous photoelectron studies via the state. Two-color double-resonance spectra were also recorded via selected rotational levels of the state. These spectra show complex series of resonances that appear to converge to the state of the ion. Photoelectron spectra for a large number of these resonances show that and processes dominate, but that the branching between the two processes depends strongly on the specific resonance.
117(2002); http://dx.doi.org/10.1063/1.1485770View Description Hide Description
Decays of individual rovibronic levels of trans-glyoxal in its first excited singlet state are investigated in supersonic-jet conditions. Several rotational levels display oscillatory decay from coherent excitation of superposition of singlet and triplet states. Analysis yields a lower bound for the state-dependent coupling matrix elements for the interaction. These matrix elements of the lowest singlet and triplet states show no simple systematic dependence on vibrational states, but the singlet state at greater K quantum number couples to a few triplet states with larger than for the state at lower K. The complex beat pattern is explored on varying the polarization of the laser beam relative to an external magnetic field. For state at excitation energy 25 254.36 cm−1, the Landé g factor of its coupling triplet hyperfine levels is estimated. From comparison with the theoretical g value for the Hund’s case (b), we found that for this state rotational angular momentum is conserved during singlet–triplet coupling. Irregular splittings and correlated beating frequencies resulting from resonance coupling between the singlet and triplet states are observed in the Fourier-transform spectra under the weak magnetic field condition.
117(2002); http://dx.doi.org/10.1063/1.1484387View Description Hide Description
Ab initio calculations are employed to understand the photoisomerization process in small clusters. This process is the first example of a photoinduced isomerization observed in an anion cluster gas-phase system. Potential energy surfaces for the ground state and the excited state and are explored by means of B3LYP, MP2, CI-singles, and CASSCF methods. We demonstrate that the isomerization process occurs between the global minimum singlet state Book structure and the triplet state Ring structure The calculated vertical excitation energy is 3.62 eV at the CASSCF level of approximation, in good agreement with the experimental value (3.49 eV). A nonplanar conical intersection, which hosts the intersystem crossing between the and surfaces is identified at the region of around Beyond the experimental results, we predict, that this isomerization is reversible upon absorption of a phonon with energy of 1.92 eV. Our results describe a unique system, whose structure depends on its spin multiplicity; it exists as the Book structure on singlet states and as the Ring structure on triplet states.
117(2002); http://dx.doi.org/10.1063/1.1484388View Description Hide Description
The ionization and double-ionization spectra of fluorinated carbanions of various chain lengths are compared with those of their corresponding acids. For the acidic systems we find a dramatic relative shift of the double-ionization spectra to higher energies due to the presence of just one additional proton. The impact of the proton on the ionization spectra is also important, but results in only half of the double-ionization spectra’s shift. A molecular electronic decay mechanism is found to be operative in the valence region of the molecules under investigation. The impact of this decay is more substantial for the anions. The threshold for electronic decay (i.e., the first double-ionization potential) is at much lower energy for the anions than for the acids. Interestingly, the localization pattern of the holes in the decay channels is, in contrast to the decay threshold, only a little affected by protonation. We also compare the impact of electron correlation effects on the ionization and double-ionization spectra of the series of fluorinated carbanions and of their acids.
117(2002); http://dx.doi.org/10.1063/1.1485066View Description Hide Description
The double photoionization of HBr molecules, by synchrotron radiation in the energy range between 25 and 55 eV, has been studied in a mass spectrometric experiment. The and product ions have been detected by a photoelectron-photoion-coincidence technique, while the formation, which follows the double ionization of HBr, has been studied by photoelectron-photoion-photoion-coincidence technique. ions are produced with a threshold of while the dissociative channel leading to shows a threshold around 33 eV. The production of occurs with a threshold of These results appear to be in a fairly good agreement with earlier nonrelativistic calculations of potential energy curves and also with values indirectly obtained from experimental Auger spectra.
117(2002); http://dx.doi.org/10.1063/1.1485726View Description Hide Description
Collisions of protons with ground state acetylene molecules at 30 eV are studied using the electron nuclear dynamics (END) theory. This time-dependent methodology for the study of molecular processes is a nonadiabatic approach to direct dynamics, which has been successfully applied to ion–atom and ion–molecule reactive collisions. Using the minimal END theory, we calculate the direct and charge-transfer differential cross sections. Different initial conditions lead to diverse product channels, such as charge transfer,proton exchange, and collision induced dissociation. Projectile energy loss is analyzed in terms of transfer into target electronic, translational, and rovibrational excitations. The comparison of the computed results with time-of-flightmeasurements is discussed.
117(2002); http://dx.doi.org/10.1063/1.1483259View Description Hide Description
The growth of water clusters in liquid heliumdroplets results in the formation of cyclic structures up to and including the hexamer. In view of the sequential nature of the molecular pick-up process, the formation of water rings involves the insertion of water monomers into preformed cyclic water clusters. The implication of this observation is that the barriers to the ring insertion process are low enough to be overcome during the experiment. This paper presents a combined experimental and theoretical effort to explore the insertion process in detail. Our results provide important new insights into the dynamics of hydrogen-bonded networks. We map out the cluster potential energy surfaces and visualize them using disconnectivity graphs. Nonequilibrium walks on these surfaces show that ring water clusters can be formed during sequential addition of water molecules by surmounting small barriers that are thermally accessible even at the low temperature of the experiment. We find that the effects of zero-point energy are significant in making these processes feasible.
Photodissociation of at 235 nm: Kinetic energy distributions and branching ratios of Cl atoms and CSCl radicals117(2002); http://dx.doi.org/10.1063/1.1480272View Description Hide Description
The photodissociation dynamics of thiophosgene and the respective branching ratios of both dissociation products Cl and CSCl have been studied by 3D imaging of the photodissociation product chlorine in its ground state and excited spin–orbit state employing the resonance enhanced multiphoton ionization and time-of-flight technique at a dissociation wavelength of about 235 nm. A novel technique is applied where the complete three-dimensional (3D) momentum vector of a reaction product is directly determined. The kinetic energy distribution (KED) for is observed for the first time. The obtained KEDs of Cl and are different in the low kinetic energy range due to the correlating state of the partner fragment CSCl. In the case of ground state Cl the CSCl partner radical is produced in the ground and states with a contribution of 4±0.5%, 60±5%, and 36±3%, respectively. In the case of the corresponding CSCl is produced with a contribution of 7.5±0.5% in the ground 71.5±5.5% in state and 21±1.5% in state The yield of was found to be 0.47. No significant velocity dependence of the anisotropy parameter β could be observed. The mean value +0.03 suggests a decay on the surface.
117(2002); http://dx.doi.org/10.1063/1.1484106View Description Hide Description
The speed and angular distributions of ions, produced when ICl molecules were exposed to both ultraviolet and visible radiation at 304+608 nm, 355+608 nm, and 304+532 nm, were measured by velocity map imaging. An intense central feature in the images was observed to be very sensitive to the polarization of the ultraviolet light and is attributed to a dissociativeionization mechanism involving three-body fragmentation: (visible)+3hv (ultraviolet The effect of varying the delay between the visible and ultraviolet radiation on the images suggests that an intermediate gateway state of ICl reached by absorption of one photon of visible light mediates the transition to the superexcited dissociativeionization state.
- Condensed Phase Dynamics, Structure, and Thermodynamics: Spectroscopy, Reactions, and Relaxation
Isotropic and anisotropic Raman scattering from molecular liquids measured by spatially masked optical Kerr effect spectroscopy117(2002); http://dx.doi.org/10.1063/1.1485070View Description Hide Description
Spatially masked optical Kerr effect (SM-OKE) spectroscopy is a nonresonant femtosecond pump–probe technique capable of measuring isotropic contributions to the transient birefringence of molecular liquids. In conjunction with traditional optical-heterodyne-detected optical Kerr effectspectroscopy, polarization-selective SM-OKE measurements are used to experimentally measure the anisotropic and isotropic third-order nonlinear response of acetonitrile, methanol, and water. These two responses, which allow the intermolecular dynamics to be separated by symmetry, form a complete and independent basis for describing the polarization dependence of nonresonant third-order experiments. The Fourier transform spectral densities of these responses are presented for each liquid and are interpreted in terms of the molecular and interaction-induced contributions to the many-body polarizability. The molecular contributions are suppressed in the isotropic response for all liquids, while the line shape in the interaction-induced portion of the spectra varies with the liquid. For the non-hydrogen-bonding liquids, the isotropic line shape is similar (albeit suppressed) as compared with that of the anisotropic spectrum, but the high-frequency wing of the isotropic spectrum exhibits completely new features in methanol and water. The isotropic water response is especially notable, since it is exceedingly fast and distinct from the anisotropic response.