Andreas Heidenreich

Theoretical exploration of femtosecond multi-state nuclear dynamics of small clusters

We investigate ultrafast multi-state nuclear dynamics in a triatomic cluster. In particular, we explore how the intracluster nuclear dynamics of the Ag3−/Ag3/Ag3+ system is reflected in the femtosecond pump-probe negative ion-to neutral-to positive ion (NENEPO) signals. The nuclear dynamics is based on classical trajectories on the ground electronic adiabatic state potential hypersurfaces obtained from accurate ab initio quantum chemistry calculations. The nuclear dynamics of Ag3 initiated from the linear transition state involves distinct sequential processes of configurational relaxation to the triangular configuration, intracluster collisions, and the onset of IVR, resonant, and dissipative IVR, and vibrational equilibration. We determined the timescales for these processes and discussed their dependence on the initial cluster temperature. The Wigner representation of the density matrix was utilized to simulate the NENEPO-zero kinetic energy (NENEPO-ZEKE) signal and the total (integrated over the photo...

Time‐resolved dynamics of cluster isomerization

In this paper we report on the time‐resolved dynamics of the cube → ring isomerization of the Na4Cl4 cluster, which was interrogated by constant energy molecular dynamics simulations. The isomerization was induced by several excitation modes of the nuclear motion, i.e., nonselective, bond selective, ion selective, and normal‐mode selective vibrational excitations. The nuclear excitation was conducted from a cluster equilibrated state at 600 K (total vibrational energy Ev=7930 cm−1 ) to total energies in the range Ev=10 610 cm−1 (cube temperature T=800 K) to Ev=30 730 cm−1 (T=2300 K). The reaction rates for isomerization were initially obtained from the mean first passage times for the ring formation. Concurrently, we have simulated the time evolution of the concentrations of the cube, ladder, and ring isomers by the thermal quenching method. From the time‐dependent concentrations for nonselective excitation, we have obtained the Ev dependent four rate constants for the isomerization scheme cube ⇄ ladder ⇄...

Nonrigidity, delocalization, spatial confinement and electronic-vibrational spectroscopy of anthracene–helium clusters

In this paper we present quantum mechanical calculations for the energetics, nuclear dynamics, spectral shifts, and vibrational level structure of anthracene⋅Hen (n=1,2) clusters in the ground (S0) and in the first spin-allowed excited (S1) electronic states. The anthracene–He potential in the S0 state was described in terms of a sum of Lennard-Jones atom–atom potentials, while the potential in the S1 state also included changes in dispersive energy and in repulsive interactions. Variational calculations were conducted for anthracene⋅He1. For anthracene⋅He2 we carried out configuration interaction calculations with the wave functions consisting of Hartree products, accounting for boson permutation symmetry. Extensive, anisotropic, one-dimensional spatial delocalization of the He atoms on the anthracene microsurface, which originates from large-scale confinement by the aromatic molecule, is exhibited, being further enhanced by repulsive interactions in the S1 state and by the He–He repulsion. The anomalous...

Extreme ionization of Xe clusters driven by ultraintense laser fields

We applied theoretical models and molecular dynamics simulations to explore extreme multielectron ionization in Xe(n) clusters (n=2-2171, initial cluster radius R(0)=2.16-31.0 A) driven by ultraintense infrared Gaussian laser fields (peak intensity I(M)=10(15)-10(20) W cm(-2), temporal pulse length tau=10-100 fs, and frequency nu=0.35 fs(-1)). Cluster compound ionization was described by three processes of inner ionization, nanoplasma formation, and outer ionization. Inner ionization gives rise to high ionization levels (with the formation of [Xe(q+)](n) with q=2-36), which are amenable to experimental observation. The cluster size and laser intensity dependence of the inner ionization levels are induced by a superposition of barrier suppression ionization (BSI) and electron impact ionization (EII). The BSI was induced by a composite field involving the laser field and an inner field of the ions and electrons, which manifests ignition enhancement and screening retardation effects. EII was treated using experimental cross sections, with a proper account of sequential impact ionization. At the highest intensities (I(M)=10(18)-10(20) W cm(-2)) inner ionization is dominated by BSI. At lower intensities (I(M)=10(15)-10(16) W cm(-2)), where the nanoplasma is persistent, the EII contribution to the inner ionization yield is substantial. It increases with increasing the cluster size, exerts a marked effect on the increase of the [Xe(q+)](n) ionization level, is most pronounced in the cluster center, and manifests a marked increase with increasing the pulse length (i.e., becoming the dominant ionization channel (56%) for Xe(2171) at tau=100 fs). The EII yield and the ionization level enhancement decrease with increasing the laser intensity. The pulse length dependence of the EII yield at I(M)=10(15)-10(16) W cm(-2) establishes an ultraintense laser pulse length control mechanism of extreme ionization products.

Perylene–rare‐gas heteroclusters. I. Electronic spectroscopy

In this paper we report on the electronic two‐photon two‐color near threshold spectroscopy of mass‐resolved perylene⋅Arn (n=1–45), perylene⋅Krn (n=1–35), perylene⋅(N2)n (n=1–12), and perylene⋅(CH4)n (n=1–10) heteroclusters. The S0→S1 inhomogeneously broadened spectra of perylene⋅Arn (n=1–6) and perylene⋅Krn (n=1–4) exhibit resolved spectral features, which were assigned on the basis of experimental combination rules and polarizability relations to the electronic origins of distinct two‐sided and one‐sided structural isomers. Larger perylene⋅An (A=Ar, Kr; n=6–10) heteroclusters exhibit an ‘‘abnormal’’ specific size dependence of the red spectral shifts, which decrease with increasing n and reach a local minimum at n=8. Similar characteristics of the red spectral shifts are exhibited for perylene⋅(N2)n and perylene⋅(CH4)n (n=4–8) heteroclusters. This abnormal size dependence of the spectral shifts is attributed to the dominance of one‐sided single‐layered and double‐layered structural isomers in this cluste...

Formation, stability, and structures of antimony oxide cluster ions

The formation of positively charged antimony oxide clusters has been investigated as a function of oxygen partial pressure using time of flight mass spectrometry. With increasing oxygen partial pressure magic number patterns are observed, which can be attributed to the clusters of (Sb2O3)n+ and (Sb2O3)n(SbO)+ with 2⩽n<7 and 1⩽n<7, respectively. Oxygen rich clusters, i.e., clusters formed by the addition of one or more oxygen atoms to the above observed series, gain in intensity with increasing cluster size and increasing oxygen partial pressure. To obtain information about structures and general building principles of these clusters we have performed ab initio quantum chemistry calculations for the series (Sb2O3)n, (Sb2O3)n+, (Sb2O3)n(SbO)+, (Sb2O3)n(SbO2)+, and (Sb2O3)n(SbO3)+. Except from a defect center in the cationic series, antimony atoms are trivalent and oxygen atoms divalent. For the cationic series (Sb2O3)n(SbO)+, an open valence at one antimony atom is avoided by the formation of an additional ...

Perylene–rare‐gas heteroclusters. II. Nuclear dynamics and spectral line shapes

In this paper we report on molecular dynamics simulations of the line shapes of the absorption spectra of perylene⋅Arn heteroclusters (n=1–45), which rest on the spectral density method. Inhomogeneous semiclassical absorption line shapes were calculated by averaging of microcanonical spectra over the accessible phase space region. We utilized a potential modeling scheme for the excited‐state atom–atom potentials, which accounts well for the spectral shifts of small (n=1–6) clusters. The size dependence and the isomer specificity of the spectral shifts and spectral linewidths were elucidated. The analysis of the power spectra of the energy gap correlation function established that the dominating contribution to the line shape and line broadening of the electronic origin at finite temperatures (T≥10 K) originates from the low‐frequency diffusive soft mode. For these heteroclusters the soft mode contribution can be described within the stochastic slow modulation limit, being determined by the dispersion of t...

Permutational symmetry, isotope effects, side crossing, and singlet-triplet splitting in anthracene.HeN (n=1, 2) clusters

We present quantum-mechanical calculations for the vibrational states of anthracene⋅3HeN and anthracene⋅4HeN (N=1, 2) clusters in the ground (S0) and first excited singlet state (S1) of the anthracene molecule. The anthracene-He potential in the S0 state was described in terms of a sum of Lennard-Jones atom-atom potentials, while the potential in the S1 state also included changes in the dispersive energy and in the repulsive interactions. Variational calculations were carried out for anthracene⋅He1. For anthracene⋅He2, configuration interaction calculations were performed, accounting for the boson and fermion permutation symmetry. For both helium isotopes of the N=1 cluster, tunneling splitting is negligible (<0.01 cm−1), as an appreciable interaction of the densities was only found for highly excited states above the potential-energy barrier of side crossing (for energy eigenvalues ⩾−22 cm−1 below the dissociation limit). The two-boson anthracene⋅4He2 system assumes a singlet 1A1 ground state due to zer...

On the interrelation between nuclear dynamics and spectral line shapes in clusters

We analyze spectral absorption line shapes simulated using the molecular dynamics spectral density method. We explore three classes of line shapes: (1) the region of the 0–0 S0→S1(ππ*) transition of perylene⋅ArN clusters, (2) the Xe1S0→3P1 transition of XeArN clusters, and (3) the photoelectron spectrum of the Li4F4 cluster in the valence region. These spectra represent examples for weak, unresolved, and extensive vibrational progressions, which have been analyzed and assigned. Employing a simplified model for the energy gap autocorrelation function allows for an understanding of the different behaviors and for a classification of the interrelation between nuclear dynamics and spectral line shapes. With decreasing the characteristic decay time of the transition dipole autocorrelation function, the line shape passes the limiting cases of the model in the order fast modulation limit→vibrational progression limit→slow modulation limit, with the vibrational progression limit extending the limiting cases of th...

Temperature jump induction of isomerization dynamics of alkali halide clusters

The time resolved dynamics of diffusionless cube → ring isomerization of the Na4Cl4 cluster was interrogated by constant energy molecular dynamics simulations, utilizing the first passage time method. The nonreactive isomerization induced by nonselective vibrational excitation is well accounted for in terms of the statistical RRK theory, opening avenues for experimental exploration of time-resolved cluster isomerization dynamics.