784 research outputs found
Full dimensional (15D) quantum-dynamical simulation of the protonated water-dimer II: infrared spectrum and vibrational dynamics
The infrared absorption spectrum of the protonated water dimer (H5O2+) is
simulated in full dimensionality (15D) in the spectral range 0-4000 cm-1. The
calculations are performed using the Multiconfiguration Time-Dependent Hartree
(MCTDH) method for propagation of wavepackets. All the fundamentals and several
overtones of the vibrational motion are computed. The spectrum of H5O2+ is
shaped to a large extent by couplings of the proton-transfer motion to large
amplitude fluxional motions of the water molecules, water bending and
water-water stretch motions. These couplings are identified and discussed, and
the corresponding spectral lines assigned. The large couplings featured by
H5O2+ do not hinder, however, to describe the coupled vibrational motion by
well defined simple types of vibration (stretching, bending, etc.) based on
well defined modes of vibration, in terms of which the spectral lines are
assigned. Comparison of our results to recent experiments and calculations on
the system is given. The reported MCTDH IR-spectrum is in very good agreement
to the recently measured spectrum by Hammer et al. [JCP, 122, 244301, (2005)].Comment: 30 pages, 6 figures, submitted to J. Chem. Phy
Correlations in Ultracold Trapped Few-Boson Systems: Transition from Condensation to Fermionization
We study the correlation properties of the ground states of few ultracold
bosons, trapped in double wells of varying barrier height in one dimension.
Extending previous results on the signature of the transition from a
Bose-condensed state via fragmentation to the hard-core limit, we provide a
deeper understanding of that transition by relating it to the loss of coherence
in the one-body density matrix and to the emerging long-range tail in the
momentum spectrum. These are accounted for in detail by discussing the natural
orbitals and their occupations. Our discussion is complemented by an analysis
of the two-body correlation function.Comment: 22 pages, 7 figure
Excitations of Few-Boson Systems in 1-D Harmonic and Double Wells
We examine the lowest excitations of one-dimensional few-boson systems
trapped in double wells of variable barrier height. Based on a numerically
exact multi-configurational method, we follow the whole pathway from the
non-interacting to the fermionization limit. It is shown how, in a purely
harmonic trap, the initially equidistant, degenerate levels are split up due to
interactions, but merge again for strong enough coupling. In a double well, the
low-lying spectrum is largely rearranged in the course of fermionization,
exhibiting level adhesion and (anti-)crossings. The evolution of the underlying
states is explained in analogy to the ground-state behavior. Our discussion is
complemented by illuminating the crossover from a single to a double well.Comment: 11 pages, 10 figure
Tunneling dynamics of few bosons in a double well
We study few-boson tunneling in a one-dimensional double well. As we pass
from weak interactions to the fermionization limit, the Rabi oscillations first
give way to highly delayed pair tunneling (for medium coupling), whereas for
very strong correlations multi-band Rabi oscillations emerge. All this is
explained on the basis of the exact few-body spectrum and without recourse to
the conventional two-mode approximation. Two-body correlations are found
essential to the understanding of the different tunnel mechanisms. The
investigation is complemented by discussing the effect of skewing the double
well, which offers the possibility to access specific tunnel resonancesComment: 10 pages, 8 figure
Full dimensional (15D) quantum-dynamical simulation of the protonated water-dimer I: Hamiltonian setup and analysis of the ground vibrational state
Quantum-dynamical full-dimensional (15D) calculations are reported for the
protonated water dimer (H5O2+) using the multiconfiguration time-dependent
Hartree (MCTDH) method. The dynamics is described by curvilinear coordinates.
The expression of the kinetic energy operator in this set of coordinates is
given and its derivation, following the polyspherical method, is discussed. The
PES employed is that of Huang et al. [JCP, 122, 044308, (2005)]. A scheme for
the representation of the potential energy surface (PES) is discussed which is
based on a high dimensional model representation scheme (cut-HDMR), but
modified to take advantage of the mode-combination representation of the
vibrational wavefunction used in MCTDH. The convergence of the PES expansion
used is quantified and evidence is provided that it correctly reproduces the
reference PES at least for the range of energies of interest. The reported zero
point energy of the system is converged with respect to the MCTDH expansion and
in excellent agreement (16.7 cm-1 below) with the diffusion Monte Carlo result
on the PES of Huang et al. The highly fluxional nature of the cation is
accounted for through use of curvilinear coordinates. The system is found to
interconvert between equivalent minima through wagging and internal rotation
motions already when in the ground vibrational-state, i.e., T=0. It is shown
that a converged quantum-dynamical description of such a flexible, multi-minima
system is possible.Comment: 46 pages, 5 figures, submitted to J. Chem. Phy
Quantum dynamics of two bosons in an anharmonic trap: Collective vs internal excitations
This work deals with the effects of an anharmonic trap on an interacting
two-boson system in one dimension. Our primary focus is on the role of the
induced coupling between the center of mass and the relative motion as both
anharmonicity and the (repulsive) interaction strength are varied. The ground
state reveals a strong localization in the relative coordinate, counteracting
the tendency to fragment for stronger repulsion. To explore the quantum
dynamics, we study the system's response upon (i) exciting the harmonic ground
state by continuously switching on an additional anharmonicity, and (ii)
displacing the center of mass, this way triggering collective oscillations. The
interplay between collective and internal dynamics materializes in the collapse
of oscillations, which are explained in terms of few-mode models.Comment: 8 pages, 7 figure
Ultracold Few-Boson Systems in a Double-Well Trap
We investigate the transition of a quasi-one-dimensional few-boson system
from a weakly correlated to a fragmented and finally a fermionized ground
state. Our numerically exact analysis, based on a multi-configurational method,
explores the interplay between different shapes of external and inter-particle
forces. Specifically, we demonstrate that the addition of a central barrier to
an otherwise harmonic trap may supports the system's fragmentation, with a
symmetry-induced distinction between even and odd atom numbers. Moreover, the
impact of inhomogeneous interactions is studied, where the effective coupling
strength is spatially modulated. It is laid out how the ground state can be
displaced in a controlled way depending on the trap and the degree of
modulation. We present the one- and two-body densities and, beyond that,
highlight the role of correlations on the basis of the natural occupations
The coupling of the hydrated proton to its first solvation shell
The transfer of a hydrated proton between water molecules in aqueous solution
is accompanied by the large-scale structural reorganization of the environment
as the proton relocates, giving rise to the Grotthus mechanism. The Zundel
(H5O2+) and Eigen (H9O4+) cations are the main intermediate structures in this
process. They exhibit radically different gas-phase infrared (IR) spectra,
indicating fundamentally different environments of the solvated proton in its
first solvation shell. The question arises: is there a least common denominator
structure that explains the IR spectra of the Zundel and Eigen cations, and
hence of the solvated proton? Full dimensional quantum simulations of these
protonated cations demonstrate that two dynamical water molecules embedded in
the static environment of the parent Eigen cation constitute this fundamental
subunit. It is sufficient to explain the spectral signatures and anharmonic
couplings of the solvated proton in its first solvation shell. In particular,
we identify the anharmonic vibrational modes that explain the large broadening
of the proton transfer peak in the experimental IR spectrum of the Eigen
cation, of which the origin remained so far unclear. Our findings about the
quantum mechanical structure of the first solvation shell provide a starting
point for further investigations of the larger protonated water clusters with
second and additional solvation shells.Comment: main article with 4 figures, methods, and supporting informatio
Composite fermionization of 1-D Bose-Bose mixtures
We study the ground states of one-dimensional Bose-Bose mixtures under
harmonic confinement. As we vary the inter-species coupling strength up to the
limit of infinite repulsion, we observe a generalized, composite-fermionization
crossover. The initially coexisting phases demix as a whole (for weak
intra-species interactions) and separate on an atomic level (for strong
intra-species repulsion). By symmetry, the two components end up with strongly
overlapping profiles, albeit sensitive to symmetry-breaking perturbations.
Different pathways emerge in case the two components have different atom
numbers, different intra-species interactions, or different masses and/or trap
frequencies.Comment: 9 pages, 9 figure
- …