178 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
Diffusion Rates for Hydrogen on Pd(111) from Molecular Quantum Dynamics Calculations
Diffusion rates are calculated on the basis of van Hove's formula for the
dynamical structure factor (DSF) related to particle scattering at mobile
adsorbates. The formula is evaluated quantum mechanically using eigenfunctions
obtained from three dimensional realistic models for H/Pd(111) derived from
first principle calculations. Results are compatible with experimental data for
H/Ru(0001) and H/Pt(111), if one assumes that the total rate obtained from the
DSF is the sum of a diffusion and a friction rate. A simple kinetic model to
support this assumption is presented.Comment: 11 pages including 4 figures and 1 tabl
Monitoring the Birth of an Electronic Wavepacket in a Neutral Molecule with Attosecond Time-Resolved Photoelectron Spectroscopy
Numerical simulations are presented to validate the possible use of
cutting-edge attosecond time- resolved photoelectron spectroscopy to observe in
real time the creation of an electronic wavepacket and subsequent electronic
motion in a neutral molecule photoexcited by a UV pump pulse within a few
femtoseconds
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
Coherence revival during the attosecond electronic and nuclear quantum photodynamics of the ozone molecule
A coherent superposition of two electronic states of ozone (ground and
Hartley B) is prepared with a UV pump pulse. Using the multiconfiguration
time-dependent Hartree approach, we calculate the subsequent time evolution of
the two corresponding nuclear wave packets and the coherence between them. The
resulting wave packet shows an oscillation between the two chemical bonds. Even
more interesting, the coherence between the two electronics states reappears
after the laser pulse is switched off, which could be observed experimentally
with an attosecond probe pulse
State-To-State Inelastic Rotational Cross Sections in Five-Atom Systems with the Multiconfiguration Time Dependent Hartree Method
We present a MultiConfiguration Time Dependent Hartree (MCTDH) method as an attractive alternative approach to the usual quantum close-coupling method that approaches some computational limits in the calculation of rotational excitation (and de-excitation) between polyatomic molecules (here collisions between triatomic and diatomic rigid molecules). We have performed a computational investigation of the rotational (de-)excitation of the benchmark rigid rotor H2O-H2 system on a recently developed Potential Energy Surface of the complex using the MCTDH method. We focus here on excitations and de-excitations from the 000, 111, and 110 states of H2O with H2 in its ground rotational state, looking at all the potential transitions in the energy range 1-200 cm-1. This work follows a recently completed study on the H2O-H2 cluster where we characterized its spectroscopy and more generally serves a broader goal to describe inelastic collision processes of high dimensional systems using the MCTDH method. We find that the cross sections obtained from the MCTDH calculations are in excellent agreement with time independent calculations from previous studies but does become challenging for the lower kinetic energy range of the de-excitation process: that is, below approximately 20 cm-1 of collision energy, calculations with a relative modest basis become unreliable. The MCTDH method therefore appears to be a useful complement to standard approaches to study inelastic collision for various collision partners, even at low energy, though performing better for rotational excitation than for de-excitation
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
Intermolecular rovibrational bound states of H2OâH2 dimer from a multiconfiguration time dependent Hartree approach
We compute the rovibrational eigenstates of the H2OâH2 Van der Waals complex using the accurate rigid-rotor potential energy surface of Valiron et al. (2008) with the MultiConfiguration Time Dependent Hartree (MCTDH) method. The J=0â2 rovibrational bound states calculations are done with the Block Improved Relaxation procedure of MCTDH and the subsequent assignment of the states is achieved by inspection of the wavefunctionsâ properties. The results of this work are found to be in close agreement with previous time independent calculations reported for this complex and therefore supports the use of the MCTDH approach for the rovibrational spectroscopic study of such weakly bound complexes
Charge transfer of polyatomic molecules in ion-atom hybrid traps: Stereodynamics in the millikelvin regime
Rate constants for the charge transfer reaction between NH
and Rb in the mK regime are measured in an ion-atom hybrid trap and are found
to be lower than the Langevin capture limit. Multireference ab initio
computation of the potential energy surfaces involved in the reaction reveals
that the low-temperature charge transfer is hindered by short-range features
highly dependent on the collision angle and is promoted by a deformation of the
molecular frame. The present study highlights the importance of polyatomic
effects and of stereodynamics in cold molecular ion-neutral collisions.Comment: See supplemental material for details on electronic structure
methods, estimates of transition probabilities and timescales, and other PES
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