Stark manifolds of barium Rydberg states

In a CW laser-atomic beam experiment Stark manifolds in barium originating from the Rydberg states 6s40f ¹ F ₃, 6s40g ^1,3 G ₄,³ G ₅ and 6s40h ^1,3 H ₅ have been studied. Accurate quantum defect values for higher orbital angular momentum states (l=6, 7) have been determined. The Stark manifolds are also calculated by diagonalization of the energy matrix in the presence of an external electric field. Good agreement between experiment and calculations is obtained.     

Vibronic dressed electronic states and nonadiabatic effects in polyacetylene

Conditions for low and high T_c superconductors following from the nonadiabatic electron-vibrational theory at ab initio level have been obtained. According to the presented results, the supercurrent is realized by the motion of the ground-state electronic-charge distribution of the fully occupied band. The motion of the electronic-charge distribution is conditioned by the newly arisen, nondissipative degrees of freedom of nuclear, Jahn–Teller-like, microflows. The Meissner effect is also interpreted.     

Approach to nonadiabatic transitions by density matrix evolution and molecular dynamics simulations

Many biological processes are characterized by an essentially quantum dynamical event, such as electron or proton transfer, in a complex classical environment. To treat such processes properly by computer simulation, allowing nonadiabatic transitions involving excited states, we recently developed a density matrix evolution (DME ) method [H. J. C. Berendsen and J. Mavri, J. Phys. Chem, 97 , 13464 (1993)] which simulates the dynamics of quantum systems embedded in a classical environment. The formalism of the method is presented and an overview of the applications ranging from collisions of a quantum harmonic oscillator with noble gas atoms to proton tunneling in a double-well hydrogen bond is given. The methodology for treatment of proton-transfer processes with inclusion of excited states is presented. Future application of the method on biologically interesting processes, such as proton transfer in enzymatic reactions, is discussed. © 1996 John Wiley & Sons, Inc.     

Theory of nonadiabatic electron transitions in symmetrical two-center tunnel electrochemical contact

Passage of current through two-center bridge contact is studied theoretically for a nonadiabatic mechanism of electron transfer. General expressions for steady-state current as a function of overvoltage and bias voltage are obtained. For a symmetrical contact, a simple approximate expression is derived for current in the limit when electron transfer between bridge redox-groups is the slow stage. It is shown that the current as a function of overvoltage passes through a maximum near the equilibrium potential. The shape of the maximum does not depend on the medium reorganization energy.     

Combining fixed- and moving-grid methods to study direct dissociation processes involving nonadiabatic transitions

We present a novel quantum-dynamics approach suitable for computing direct dissociation processes, including electronic transitions. This approach combines quantum trajectories in the Lagrangian reference frame with standard fixed-grid wave packets in order to overcome the limitations and difficulties of both techniques. As a model application, we consider the ultrafast photodissociation of H2 excited by a femtosecond extreme UV laser pulse.     

Assessing the options for identifying critically important potential surface regions: Applications to nonadiabatic transitions

Results from close coupling sensitivity density-based analyses are compared for some representative nonadiabatic collisions with those from Landau-Zener-Stueckelberg (LZS) and adiabatic analyses. These results seem to indicate that while for simple systems modeled by potential energy curves devoid of competing features LZS and adiabatic approaches also offer qualitatively correct insights these can be misleading for dynamics evolving on potential energy curves with competing features. The close-coupling sensitivity densities seem to offer a more reliable guide for detailed understanding of the impact of structure in potential energy curves and coupling matrix elements in the collisional outcome. © 1997 John Wiley & Sons, Inc.     

Sum and density of states of polyatomic systems with hindered rotors

The density or sum of states for a collection of independent oscillators, free rotors, and one-dimensional hindered rotors is obtained with good accuracy by numerical inversion of the corresponding total partition function by the method of steepest descents. The hindered-rotor partition functions are used in both classical and quantum forms, the latter in the approximation proposed by Truhlar [J. Comput. Chem., 12, 266 (1991)]. The numerical inversion compares well with analytical results obtained in a simple artificial case and also with an exact count of states in a large ethane-like system. Inversion of the hindered-rotor classical partition function is shown to lead to a somewhat different energy dependence of the sum or density of states, relative to the quantum counterpart, which is considered to be a more realistic representation. The routines presented are simple and fast enough to be of use in microcanonical rate calculations. © 1996 by John Wiley & Sons, Inc.     

Modeling droplets on superhydrophobic surfaces: equilibrium states and transitions

We present a lattice Boltzmann solution of the equations of motion describing the spreading of droplets on topologically patterned substrates. We apply it to model superhydrophobic behavior on surfaces covered by an array of micrometer-scale posts. We find that the patterning results in a substantial increase in contact angle, from 110 degrees to 156 degrees. The dynamics of the transition from drops suspended on top of the posts to drops collapsed in the grooves is described.     

Electronic states and radiative transitions in LiAr

Ab initio MRD-CI calculations have been carried out on the ground and the eight lowest excited electronic states of LiAr, correlating with excited Li atom states up to 3d ²D. The ground (X²∑⁺ (2s)) and 2 ²Σ⁺ (2p) electronic states are repulsive while the higher excited states show shallow Rydberg minima. Rates of radiative bound-bound and bound-free transitions have been also calculated.     

Stark and zeeman effects on the lower electronic states of s-triazine

A detailed optical study of the lower electronic states of s-triazine in a pure crystal at 1.8°K is presented. Stark and Zeeman experiments on these states give no support to previous assignments. The experiments indicate that the lowest triplet state observed in s-triazine corresponds to either a polar A″₁ state or to an E″ state in which the orbital angular momentum is completely quenched. Optical measurements on a very weak triplet state absorption at 28930.7 cm^?1 in s-triazine are also reported. The premilinary assignment of this line as a crystal field induced Davydov component is shown not to be inconsistent with its Zeeman effect.     

Stark deceleration of CaF molecules in strong- and weak-field seeking states

We report the Stark deceleration of CaF molecules in the strong-field seeking ground state and in a weak-field seeking component of a rotationally-excited state. We use two types of decelerator, a conventional Stark decelerator for the weak-field seekers and an alternating gradient decelerator for the strong-field seekers, and we compare their relative merits. We also consider the application of laser cooling to increase the phase-space density of decelerated molecules.     

Wetting transitions of asymmetric binary polymer mixtures

We examine the effect on surface-wetting phase transitions in polymer mixtures when the degrees of polymerization of the two components are different. It has been demonstrated by Schmidt and Binder (J. Phys. (Paris) 46 , 1631 (1985)) that in a symmetric polymer mixture a second-order wetting transition occurs if the wall-polymer interaction f(ϕ₀) has a negative curvature f″(ϕ₀), where ϕ₀ is the surface volume fraction of the polymer component preferred by the wall. We found that in an asymmetric mixture this is not necessarily the case.     

Collision-induced nonadiabatic transitions in the second-tier ion-pair states of iodine molecule: experimental and theoretical study of the I2(f0g+) collisions with rare gas atoms

Nonadiabatic transitions induced by collisions with He, Ar, Kr, and Xe atoms in the I(2) molecule excited to the f0(g)(+) second-tier ion-pair state are investigated by means of the optical-optical double resonance spectroscopy. Fluorescence spectra reveal that the transition to the F0(u)(+) state is a dominant nonradiative decay channel for f state in He, Ar, and Kr, whereas the reactive quenching is more efficient for collisions with Xe atom. Total rate constants and vibrational product state distributions for the f-->F electronic energy transfer are determined and analyzed in terms of energy gaps and Franck-Condon factors for the combining vibronic levels at initial vibrational excitations v(f)=8, 10, 14, and 17. Quantum scattering calculations are performed for collisions with He and Ar atoms, implementing a combination of the diatomics-in-molecule and long-range perturbation theories to evaluate diabatic PESs and coupling matrix elements. Calculated rate constants and vibrational product state distributions agree well with the measured ones, especially in case of Ar. Qualitative comparison is made with the previous results for the second-tier f0(g)(+)-->F0(u)(+) transition in collisions with I(2)(X) molecule and the first-tier E0(g)(+)-->D0(u)(+) transition induced by collisions with the rare gas atoms.     

Second order Stark shift of zero-field transitions in protonated and deuterated p-benzoquinones

The second order Stark effect on the zero-field transitions of the lowest triplet state of p-benzoquinone (-h₄ and -d₄) in a host of p-dibromobenzene have been measured and compared. The electric field shifts of the microwave transitions of the perdeuterated compound are 1.9 ± 0.3 times larger than for the protonated molecule. This result indicates that the principal mechanism of the Stark effects is the mixing of the closeby inversion doublet components and not the mixing with a higher electronic state as assumed in previous work.     

Stark coefficients for highly excited rovibrational states of H2O

Quantum beat spectroscopy is combined with triple-resonance vibrational overtone excitation to measure the Stark coefficients (SCs) of the water molecule for 28 rovibrational levels lying from 27,600 to 41,000 cm(-1). These data provide a stringent test for assessing the accuracy of the available potential energy surfaces (PESs) and dipole moment surfaces (DMSs) of this benchmark molecule in this energy region, which is inaccessible by direct absorption. SCs, calculated using the combination of a high accuracy, spectroscopically determined PES and a recent ab initio DMS, are within the 1% accuracy of available experimental data for levels below 25,000 cm(-1), and within 4.5% for coefficients associated with levels up to 35,000 cm(-1). However, the error in the computed coefficients is over 60% for the very high rovibrational states lying just below the lowest dissociation threshold, due, it seems, to lack of a high accuracy PES in this region. The comparative analysis suggests further steps, which may bring the theoretical predictions closer to the experimental accuracy.     

Simulation of first- and second-order transitions in asymmetric polymer mixtures

The critical properties of dense asymmetric binary polymer mixtures are studied by grand canonical simulations within the framework of the 3-dimensional bond fluctuation lattice model. The monomers interact with each other via a potential ranging over the entire first peak of the pair distribution. An asymmetry is realized by giving the ratio of interactions λ = ∈_AA/∈_BB between monomers of the A-species and of the B-species a value different from 1. Using multiple histogram extrapolation techniques for the data analysis, the two phase region, which is a line of first-order transitions driven by the chemical potential difference, and the critical point are determined for a mixture of chains with 32 monomers each. At a critical potential difference Δμ_c unmixing occurs below a critical temperature T_c. It is found that Δμ_c is proportional to the asymmetry (1 - λ) and that the quantity 4k_BT_c/(3 + λ)∈ is independent of the asymmetry, consistent with the prediction of the Flory theory.     

Theoretical study of the electronic nonadiabatic transitions in the photoelectron spectroscopy of F2O

The photoelectron spectrum of F2O pertaining to ionizations to the ground (X2B1) and low-lying excited electronic states (A2B2, B2A1, and C2A2) of F2O+ is investigated theoretically. The near equilibrium potential energy surfaces of the ground electronic state (X2B1) of F2O and the mentioned ground and excited electronic states of F2O+ reported by Wang et al. ( J. Chem. Phys. 2001, 114, 10682) for the C2v configuration are extended for the Cs geometry assuming a harmonic vibration along the asymmetric stretching mode. The vibronic interactions between the A2B2 and B2A1 electronic states of F2O+ are treated within a linear coupling approach, and the strength of the vibronic coupling parameter is calculated by an ab initio method. The nuclear dynamics is simulated by both time-independent quantum mechanical and time-dependent wave packet approaches. Although the first photoelectron band exhibits resolved vibrational progression along the symmetric stretching mode, the second one is highly overlapping. The latter is attributed to the nonadiabatic interactions among the energetically close A2B2, B2A1, and C2A2 electronic states of F2O+. The theoretical findings are in good accord with the available experimental results.     

Resolution and structural transitions of elongated states of ubiquitin

Electrospray ionization, combined with two-dimensional ion mobility spectrometry and mass spectrometry, is used to produce, select, and activate distributions of elongated ions, [M + 11H]11+ to [M + 13H]13+, of ubiquitin. The analysis makes it possible to examine state-to-state transitions for structural types, and transition diagrams associated with the efficiencies of structural changes are presented. The +11 and +12 charge states can form four resolvable states while only one state is formed for [M + 13H]13+. Some conformations, which appear to belong to the same family based on mobility analysis of different charge states, undergo similar transitions, others do not. Activation of ions that exist in low-abundance conformations, having mobilities that fall in between sharp peaks associated with higher abundances species, shows that the low-abundance forms undergo efficient (approximately 90 to 100%) conversion into states associated with well-defined peaks. This efficiency is significantly higher than the approximately 10 to 60% efficiency of transitions of structures associated with well-defined peaks. The formation of sharp features from a range of low-intensity species with different cross sections indicates that large regions of conformation space must be unfavorable or inaccessible in the gas phase. These results are compared with several previous IMS measurements of this system as well as information about gas-phase structure provided by other techniques.