TY - JOUR A1 - Hartmann, Michael A1 - Pittner, Jiřı́ A1 - Bonačić-Koutecký, Vlasta A1 - Heidenreich, Andreas A1 - Jortner, Joshua T1 - Theoretical exploration of femtosecond multi-state nuclear dynamics of small clusters JF - The Journal of Chemical Physics N2 - We investigate ultrafast multi-state nuclear dynamics in a triatomic cluster. In particular, we explore how the intracluster nuclear dynamics of the Ag−3/Ag3/Ag+3 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 photoelectron energy) NENEPO signal. We show how geometrical change, completion of IVR and vibrational coherenceeffects can be identified in the NENEPO signals. A comparison of the calculated NENEPO signals with the available experimental data is presented. KW - Cluster dynamics KW - Femtosecond dynamics Y1 - 1998 N1 - Volltext aus urheberrechtlichen Gründen nicht verfügbar. Fulltext due to copyright not available. VL - 108 IS - 8 SP - 3096 EP - 3113 ER - TY - JOUR A1 - Bonačić-Koutecký, Vlasta A1 - Pittner, Jiřı́ A1 - Pou-Amérigo, Rosendo A1 - Hartmann, Michael T1 - Ab-initio study of structural and optical properties of nonstoichiometric alkalimetal- oxides JF - Zeitschrift für Physik D N2 - Structural and optical properties of nonstoichiometric LinO and NanO (n = 3, 4) clusters containing one and two excess electrons are studied using ab-initio methods accounting for electron correlation. We show that calculated absorption patterns are excellent fingerprints of structural and bonding properties. The optical response of Li4O and Na4O clusters with the most stable tetrahedral type structures is characterized by a common feature, that is the appearance of a dominant intense transition in infrared regime although excess of electrons are not localized, as it is the case for small alkali-halide clusters, with cuboid corner vacancy (surface F-center in finite systems). KW - Inorganic Chemistry KW - Physical chemistry Y1 - 1997 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:0298-opus4-5748 VL - 40 IS - 1 SP - 445 EP - 447 ER - TY - JOUR A1 - Mitrić, Roland A1 - Hartmann, Michael A1 - Stanca, B. A1 - Bonačić-Koutecký, Vlasta A1 - Fantucci, P. T1 - Ab Initio Adiabatic Dynamics Combined with Wigner Distribution Approach to Femtosecond Pump−Probe Negative Ion to Neutral to Positive Ion (NeNePo) Spectroscopy of Ag2Au, Ag4, and Au4 Clusters JF - Journal of Physical Chemistry A N2 - Ultrafast ground state nuclear dynamics of small coinage metal clusters is theoretically explored in the framework of negative ion to neutral to positive ion (NeNePo) femtosecond pump−probe spectroscopy with the aim to determine the scope and perspective of this technique. This approach involves the preparation of an initial ensemble in the anionic ground state of the cluster, one-photon detachment by the pump pulse, the propagation of the system on the neutral electronic ground state, and detection via the cationic ground state by a time-delayed ionizing probe pulse. The calculations of the NeNePo-ZEKE signals under the condition of zero kinetic energy electrons are based on the combination of the Wigner distribution approach and ab initio molecular dynamics (MD) “on the fly” in the framework of gradient-corrected DFT for the propagation of an ensemble of classical trajectories and involves the average over the entire phase space. We have chosen examples of Ag2Au and Au4 to investigate time scales and the character of isomerization processes that are influenced by a local minimum corresponding to an energetically high-lying isomer with the structure related to the initial anionic structure. Capture of the nuclei within the local minimum takes place with dephased (Ag2Au) or nondephased (Au4) vibrational relaxation. Structural relaxations leading to isomerization processes are in both cases of delocalized nature and take place after 1 ps. In contrast, the Ag4 cluster represents an example for which isomerization processes do not occur, since rhombic structures with slightly different bond lengths are global minima of the anionic and neutral ground states. Therefore, the relaxation dynamics is characterized by regular oscillations, which serve as fingerprints of structural properties. On the basis of the analysis of pump−probe signals and underlying dynamics, this contribution provides information about conditions under which structural properties of gas-phase clusters (global and local minima) and isomerization processes can be observed in the framework of the NeNePo-ZEKE type of spectroscopy. The latter is important in the context of applicability of this technique to the investigation of cluster reactivity. KW - Physical chemistry Y1 - 2001 N1 - Volltext aus urheberrechtlichen Gründen nicht verfügbar. Fulltext due to copyright not available. VL - 105 IS - 39 SP - 8892 EP - 8905 ER - TY - JOUR A1 - Bonačić-Koutecký, Vlasta A1 - Hartmann, Michael A1 - Pittner, Jiřı́ T1 - Theoretical exploration of ultrafast spectroscopy of small clusters JF - The European Physical Journal D N2 - We present the ultrafast multistate nuclear dynamics involving adiabatic and nonadiabatic excited states of non-stoichiometric halide deficient clusters (NanF) characterized by strong ionic bonding and one-excess electron for which the "frozen ionic bonds" approximation has been justified allowing to consider the optical response of the single excess electron in the effective field of the other electrons. We combined the Wigner-Moyal representation of the vibronic density matrix with the ab initio multi state molecular dynamics in the ground and excited electronic states including the nonadiabatic couplings calculated "on the fly" at low computational demand. This method allows the simulation of femtosecond pump-probe and pump-dump signals based on an analytical formulation, which utilizes temperature dependent ground state initial conditions, an ensemble of trajectories carried out on the electronic excited state as well as on the ground state after the passage through the conical intersection in the case of nonadiabatic dynamics and for probing either in the cationic state or in the ground state. The choice of the systems we presented has been made in order to determine the timescales of the fast geometric relaxation leaving the bonding frame intact as during the dynamics in the first excited state of Na4F3, and of the bond breaking processes leading to conical intersection between the first excited state and the ground state as in Na3F2. The former is the smallest finite system prototype for an surface F-center of bulk color centers. The latter allows to study the photo isomerization in full complexity taking into account all degrees of freedom. In the case of Na4F3 after the fast geometric relaxation in the excited state leading to deformed cuboidal structure without breaking of bonds, different types of internal vibrational redistribution (IVR) processes have been identified in pump-dump signals by tuning the dump laser. In contrast, from the analysis of the pump-probe signals of Na3F2 cluster, the timescales for the metallic and the ionic bond breaking, as well as for the passage through conical intersection have been determined. Finally the conditions under which these processes can be experimentally observed have been identified. KW - Optical Physics KW - Molecular Physics Y1 - 2001 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:0298-opus4-5536 VL - 16 SP - 133 EP - 138 ER - TY - JOUR A1 - Hartmann, Michael A1 - Mitrić, Roland A1 - Stanca, B. A1 - Bonačić-Koutecký, Vlasta T1 - Theoretical investigation of the ultrafast NeNePo spectroscopy of Au4 and Ag4 Clusters JF - The European Physical Journal D N2 - Ultrafast ground state nuclear dynamics of Au4 and Ag4 is theoretically explored in the framework of negative ion - to neutral - to positive ion (NeNePo) pump-probe spectroscopy based on the ab initio Wigner distribution approach. This involves the preparation of a nonequilibrium neutral ensemble by pump induced photodetachment of a thermal anionic ground state distribution, gradient corrected DFT classical trajectory simulations "on the fly" on the neutral ground state, and detection of the relaxation process of the ensemble in the cationic ground state by a time-delayed probe pulse. In Au4, the initially prepared linear structure is close to a local minimum of the neutral state giving rise to characteristic vibrations in the signals for probe wavelength near the initial Franck-Condon transition. A timescale of ~1 ps for the structural relaxation towards the stable rhombic D neutral isomer was determined by the increase of the signal for probe wavelength in vicinity of the vertical ionization energy of the rhombic structure. In contrast, the relaxation dynamics in Ag4 is characterized by normal mode vibrations since both the initially prepared anionic ground state and the neutral ground state have rhombic minimum geometries. Thus, time-resolved oscillations of pump-probe signals are fingerprints of structural behaviour which can be used experimentally for the identification of particular isomers in the framework of NeNePo spectroscopy. KW - Molecular physics KW - Optical physics Y1 - 2001 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:0298-opus4-5542 VL - 16 SP - 151 EP - 155 ER - TY - JOUR A1 - Hartmann, Michael A1 - Pittner, Jiřı́ A1 - Bonačić-Koutecký, Vlasta T1 - Ab initio nonadiabatic dynamics involving conical intersection combined with Wigner distribution approach to ultrafast spectroscopy illustrated on Na3F2 cluster JF - The Journal of Chemical Physics N2 - We present a theoretical study of a femtosecond photo isomerization process due to a nonadiabatic radiationless decay from the first excited state through a conical intersection occurring in one of the nonstoichiometric halide-deficient clusters with one excess electron (Na3F2). This is an extension of the adiabatic dynamics study presented in the accompanying paper [J. Chem. Phys. 114, 2106 (2001)] for other members of the NanFn−1 series characterized by a strong ionic bonding for which the “frozen ionic bonds” approximation has been justified, allowing consideration of the optical response of the single excess electron in the effective field of the other electrons. In this contribution we outline the extension of the ab initio Wigner-distribution approach to nonadiabatic molecular dynamics which combines the Wigner–Moyal representation of the vibronic density matrix with the ab initio multistate molecular dynamics in the ground- and excited electronic states including the nonadiabatic coupling computed “on the fly” in connection with the fewest-switches hopping algorithm. This scheme allows accounting for temperature-dependent initial conditions, for the propagation in the excited state and in the ground state after the passage through the conical intersection, and for probing in the cationic ground state as well as for deriving analytic expressions for the pump–probe signals which utilize an ensemble of classical trajectories obtained at low computational demand. Our approach permits investigation of the photo isomerization through the conical intersection due to the long amplitude motion in the Na3F2 system in full complexity, taking into account all degrees of freedom. After breaking of one metallic and of one ionic bond the conical intersection occurs at the linear geometry and involves states of different symmetry which differ in the translocation of the one excess electron or positive charge localized at the Na atom from one end to the other of the system and separates two isomers with Cs and C2v structures. From the analysis of the nonadiabaticdynamics, the time scales for the metallic bond breaking of ∼90 fs and for the ionic bond breaking of ∼220 fs, for the passage through the conical intersection after ∼0.4 ps and for the internal vibrational energy redistribution (IVR) of more than 0.9 ps for the individual isomers, have been determined. The simulated fs pump–probe signals confirm the above results and provide the information about the experimental conditions such as laser frequencies and pulse duration under which bond breaking of different type as well as the population of each of the two isomers after the passage through the conical intersection can be identified. In this contribution we show that the mechanism of the photo isomerization at a conical intersection due to a long amplitude motion can occur in atomic clusters and is not necessarily limited to organic photochemistry. KW - Wigner distribution KW - Chemical physics Y1 - 2001 N1 - Volltext aus urheberrechtlichen Gründen nicht verfügbar. Fulltext due to copyright not available. VL - 114 IS - 5 SP - 2123 EP - 2136 ER - TY - JOUR A1 - Hartmann, Michael A1 - Pittner, Jiřı́ A1 - Bonačić-Koutecký, Vlasta T1 - Ab initio adiabatic dynamics involving excited states combined with Wigner distribution approach to ultrafast spectroscopy illustrated on alkali halide clusters JF - The Journal of Chemical Physics N2 - We investigate the ultrafast multistate nuclear dynamics involving adiabatic electronic excited states of nonstoichiometric halide deficient clusters (NanFn−1) characterized by strong ionic bonding and one excess electron, which is localized either in the halide vacancy or on the alkali atom attached to the ionic subunit depending on the cluster size. For this purpose we developed an ab initio adiabatic nuclear dynamics approach in electronic excited and ground states “on the fly” at low computational demand by introducing the “frozen ionic bonds” approximation, which yields an accurate description of excited states considering the excitation of the one excess electron in the effective field of the other n−1 valence electrons involved in the ionic bonding. We combined this multistate dynamics approach with the Wigner–Moyal representation of the vibronic density matrix forming the ab initio Wigner distribution approach to adiabatic dynamics. This method allows the simulation of femtosecondNeExPo-pump–probe and NeExNe-pump–dump signals based on an analytic formulation which utilizes temperature-dependent ground-state initial conditions (Ne), an ensemble of trajectories carried out on the electronic excited state (Ex) for the investigation of the dynamics of the system, and either the cationic (Po) or the ground state (Ne) for the probing step. The choice of the systems has been made in order to determine the time scales of processes involving (i) metallic bond breaking such as during the dynamics in the first excited state of Na2F, and (ii) fast geometric relaxation leaving the bonding frame intact as during the dynamics in the first excited state of Na4F3. The bond-breaking process via a conical intersection involving nonadiabatic dynamics will be presented in the accompanying paper [Hartmann et al., J. Chem. Phys. 114, 2123 (2001)]. The dynamics in the first excited state of Na2F from triangular-to linear-to triangular structure gives rise to fast geometric relaxation due to Na–Na bond breaking at the time scale of ∼90 fs but no signature of internal vibrational energy redistribution (IVR) is present in NeExNe-pump–dump signals since the broken metallic bond prevents the coupling between stretching and bending modes. Instead, anharmonicities of the bending periodic motion have been identified. In contrast, in the case of Na4F3, which is the smallest finite system for a surface F-center prototype of bulk color centers, after the geometric relaxation in the excited state of ∼100 fs leading to the deformed cuboidal type of structure without breaking of bonds, different types of IVR have been identified in NeExNe signals by tuning the dump laser: one-mode selective energy leaving IVR, resonant, and restricted energy arriving IVR corresponding to the selection of different parts of the phase space. Dissipative IVR could not be identified in NeExNe signals of Na4F3 at low initial temperature on the time scale up to 2 ps in spite of 15 degrees of freedom. Due to similar structural and electronic properties such as F centers in bulk, these findings can serve as guidance for establishing the time scales for geometric relaxation and IVR in excited states of larger systems. KW - Wigner distribution KW - Chemical physics Y1 - 2001 N1 - Volltext aus urheberrechtlichen Gründen nicht verfügbar. Fulltext due to copyright not available. VL - 114 IS - 5 SP - 2106 EP - 2122 ER -