Nonadiabatic Mechanism Research Articles

Breakdown of a Mott Insulator: A Nonadiabatic Tunneling Mechanism

Time-dependent nonequilibrium properties of a strongly correlated electron system driven by large electric fields is obtained by means of solving the time-dependent Schrödinger equation for the many-body wave function numerically in one dimension. While the insulator-to-metal transition depends on the electric field and the interaction, the metallization is found to be described in terms of a universal Landau-Zener quantum tunneling among the many-body levels. These processes induce current oscillation for small systems, while giving rise to finite resistivity through dissipation for larger systems/on longer time scales.

Global imaging of O+from IMAGE/HENA

The magnetospheric O+ population in the 52–180 keV range during storms is investigated through the analysis of energetic neutral atom (ENA) images. The images are obtained from the high energy neutral atom (HENA) imager onboard the IMAGE satellite. At each substorm onset following the commencement of a geomagnetic storm the oxygen ENA display ~30 min intense bursts. Only very weak corresponding features in the 60–119 keV hydrogen ENA can be occasionally seen. The dominating fraction of the oxygen ENA emissions are produced when O+ ions mirror/precipitate at low altitudes, where the number density of the neutral atmosphere is high. During the storm we observed several bursts of oxygen ENA, but it is still not clear how much the O+ content of the ring current increases during the storm main phase. Our observations suggest that the responsible injection mechanism is mass-dependent and scatters the pitch angles. This leads us to favor a non-adiabatic mechanism proposed by (Delcourt, 2002).

Ultrafast Charge Separation from the S2Excited State of Directly Linked Porphyrin−Imide Dyads: First Unequivocal Observation of the Whole Bell-Shaped Energy-Gap Law and Its Solvent Dependencies

Photoinduced charge separation (CS) due to electron transfer from the S2 state of zinc porphyrin (ZP) to a directly linked imide (I) has been studied by means of femtosecond fluorescence up-conversion and femtosecond−picosecond transient absorption spectral measurements. S2 fluorescence decay curves of ZP−I dyads in various solvents were all single-exponential, from which accurate rate constants for the CS reaction were obtained. The whole bell-shaped energy-gap law (EGL) has been confirmed unambiguously in polar solvents tetrahydrofurane, acetonitrile, and triacetin as the first example for CS, which can be reproduced on the basis of a nonadiabatic mechanism considering solvent reorganization and intrachromophore high-frequency vibrations (hfm) approximated by an averaged single mode. Mainly the top and inverted regions have been observed in nonpolar solvents cyclohexane and toluene. This result may be ascribed to the large decrease of the solvent reorganization energy in nonpolar solvents, which causes a large shift in the reaction coordinate for CS, making the S2-state surface partially embedded in that of the CS state and transforming the EGL for the CS reaction into one that is somewhat analogous to that of the radiationless transition in the weak-coupling limit. The single-exponential fluorescence decay in 100 fs and the EGL with both normal and inverted regions observed in triacetin indicate strongly that very fast solvent relaxation should take place despite the very long τL of 125 ps at 20 °C (see the text). Thus, by using specifically designed ZP−I linked systems, we have directly demonstrated that both the intramolecular hfm and ultrafast solvent relaxation play very important roles in the ultrafast CS from the S2 state and that we can regulate its EGL by controlling the nature of the solvent.

Non-adiabatic small polaron hopping conduction in Gd 1/3Sr 2/3FeO 3

The transport properties of Gd 1/3Sr 2/3FeO 3 have been studied carefully by using resistivity and thermoelectric power. Interestingly, a small polaron hopping model of Emin and Holstein is found to fit the resistivity data above 200 K, and the resistivity data of the high temperature region followed a non-adiabatic hopping conduction mechanism. At high temperatures, a significant difference between the activation energy deduced from the electrical resistivity ρ( T) and thermoelectric power α( T), a characteristic of a small polaron, is observed.

Tilted wave emission in optical parametric oscillators induced by a bichromatic pumping.

Tilted wave emission is found in a mean-field model of a degenerate optical parametric oscillator pumped by a bichromatic field with a frequency offset much smaller than the cavity free-spectral range. Tilted wave emission arises due to a nonadiabatic mechanism induced by the slow pump modulation, and disappears when either one of the two pump waves is blocked.

Anomalous isotopic predissociation in the F 3Πu(v=1) state of O2

Using a tunable, narrow-bandwidth vacuum-ultraviolet source based on third-harmonic generation from excimer-pumped dye-laser radiation, the F 3Πu←X 3Σg−(1,0) photoabsorption cross sections of O216 and O218 have been recorded in high resolution. Rotational analyses have been performed and the resultant F(v=1) term values fitted to the Π3 Hamiltonian of Brown and Merer [J. Mol. Spectrosc. 74, 488 (1979)]. A large rotationless isotope effect is observed in the F(v=1) predissociation, wherein the Lorentzian linewidth component for O218 is a factor of ∼50 smaller than the corresponding O216 linewidth. This effect, a consequence of the nonadiabatic rotationless predissociation mechanism, is described using a coupled-channel treatment of the strongly Rydberg-valence-mixed Πu3 states. Significant J, e/f-parity, and sublevel dependencies observed in the isotopic F(v=1) rotational widths are found to derive from an indirect predissociation mechanism involving an accidental degeneracy with the E 3Σu−(v=3) level, itself strongly predissociated by Σu−3 Rydberg-valence interactions, together with L-uncoupling (rotational) interactions between the Rydberg components of the F and E states. Transitions into the E(v=3) level are observed directly for the first time, specifically in the O218 spectrum.

Density functional studies of the ground- and excited-state potential-energy curves of stilbene cis-trans isomerization.

Using spin-unrestricted density functional theory (the VWN Becke-Perdew potential), including broken-symmetry and spin-projection methods, we have obtained the potential-energy curves as a function of the central torsional angle of stilbene in the ground (S0), the first excited triplet (T1), the first excited singlet (S1), and the doubly excited singlet (S2) states. The thermal trans-->cis isomerization of stilbene passes through a diradical broken-symmetry electronic structure around the twisted conformation (90 degrees central torsional angle) in the ground state. Our calculations support the proposed triplet mechanism for sensitized cis [symbol: see text] trans photoisomerization and the nonadiabatic singlet mechanism proposed by Orlandi and Siebrand. On the T1 potential-energy curve, the rotation of the C=C bond for both trans- and cis-stilbene will lead stilbene to the twisted conformation, from which the twisted stilbene will decay to the ground-state surface that is nearly isoenergetic with the T1 surface and has diradical electronic structure in the twisted region. On the S1 potential-energy curve, the energy increases in the direction from trans- to the twisted stilbene, and crosses with the neutral doubly excited S2 potential-energy curve, which has a minimum at the twisted structure and is lower in energy than the zwitterionic doubly excited state. The twisted stilbene around the energy minimum of the neutral doubly excited S2-state will decay onto the ground-state surface from where the rotation of the C=C bond leads the twisted stilbene to either the trans or cis configuration and the isomerization of stilbene is then completed. Similar studies have also been performed on a stilbene derivative with a substituent group, NHCOCH3.

Adiabatic and non-adiabatic small-polaron hopping conduction in La1−xPbxMnO3+δ (0.0 ≤ x ≤ 0.5)-type oxides above the metal–semiconductor transition

Above the semiconductor-to-metallic transition (SMT) temperature (Tp), transport properties of the La1−xPbxMnO3+δ (0 ≤ x ≤ 0.5)-type mixed valence oxides with Tp between 230 and 275 K (depending on x) have been thoroughly examined for a small-polaron hopping conduction mechanism of the carriers. Although the variable range hopping (VRH) model was used earlier to fit the entire conductivity data above SMT, we noticed two distinct regions (above and below θD/2; θD is the Debye temperature) where different types of conduction mechanisms are followed. The high temperature (T > θD/2) conductivity data of all the Pb-doped samples follow the adiabatic hopping conduction mechanism, while those of LaMnO3 (x = 0) showing no SMT follow the non-adiabatic hopping conduction mechanism of Mott or Emin with reasonable values of polaron radius, hopping distance, polaron binding energy, activation energy, etc being different for different systems. The VRH model, however, fits the corresponding low temperature (T < θD/2) data of all the samples. Both resistivity ρ(T) and thermoelectric power S(T) follow a similar microscopic theory above Tp supporting the small-polaron hopping mechanism. Thermoelectric power also showed appreciable magnetic field dependence around SMT.

Nature of small-polaron hopping conduction and the effect of Cr doping on the transport properties of rare-earth manganite La0.5Pb0.5Mn1−xCrxO3

The conductivity and magnetoresistance of La0.5Pb0.5Mn1−xCrxO3 (0.0⩽x⩽0.45) measured at 0.0 and 1.5 T magnetic field have been reported. All the oxide samples except x=0.45, showed metal insulator transition (MIT) between 158–276 K, depending on x. In contrast to the behavior of a similar sample La0.7Ca0.3Mn1−xCrxO3 showing no (MIT) for x⩾0.3, the Pb doped samples showed MIT even with x=0.35. The MIT peak temperature (Tp) shifts towards lower temperature with increasing x while magnetic field shifts Tp to the high temperature regime. The metallic (ferromagnetic) part of the temperature dependent resistivity (ρ) curve (below Tp) is well fitted with ρ(T)=ρ0+ρ2.5T2.5 indicating the importance of electron–magnon interaction (second term). We have successfully fitted the high temperature (T&amp;gt;θD/2, θD is Debye temperature) conductivity data, both in presence and in absence of magnetic field, with small polaron hopping conduction mechanism. Adiabatic small polaron hopping conduction mechanism is followed by the samples showing MIT while nonadiabatic hopping conduction mechanism is obeyed by the samples showing no MIT. The lower temperature (between Tp and θD/2) conductivity data of all the samples can be well fitted to the variable range hopping (VRH) model similar to the case of many semiconducting transition metal oxides. Temperature dependent Seebeck coefficient data also support the small polaron hopping conduction mechanism above Tp.

Polaron hopping conduction and thermoelectric power in LaMnO3+δ

Two different phases of LaMnO3+δ [one showing a metal–insulator transition (MIT), referred to as LaMn–C, and the other not showing a MIT, referred to as LaMn-S] have been clearly observed to follow two different conduction mechanisms. Interestingly, small polaron hopping models of Mott, Schnakenberg, and Emin are found to fit the conductivity data of all the samples above the corresponding MIT temperature. The conductivity data of the insulating (semiconducting) LaMn–S followed a nonadiabatic hopping conduction mechanism while LaMn–C and the Pb doped samples viz. La1−xPbxMnO3 (x=0.05–0.5) showed a similar type of MIT and followed an adiabatic small polaron hopping conduction mechanism in the high temperature paramagnetic phase (above the respective MIT temperature). Activation energy (W), density of states at the Fermi level N(EF), Debye temperature (θD), electron–phonon interaction constant (γP), etc. of LaMn–S showed appreciable differences from those of LaMn–C and La1−xPbxMnO3, which show a MIT. Polaron hopping conduction is also supported by thermoelectric power (TEP) measurements. An observed small but appreciable magnetic field dependence of the TEP data (measured at B=1.5 T) is considered to be associated with magnetic polarons.

An ab initio study of the photochemical decomposition of 3, 3-dimethyldiazirine

Photochemical decomposition of 3,3-dimethyldiazirine (DMD) has been computationally investigated by using high-level ab initio calculations in conjunction with the 6-31G and cc-pvdz basis sets. The geometries of minima and transition states, as well as conical intersection points in the seam of crossing of two surfaces, have been optimized with the complete active space self-consistent field (CAS-SCF) method, and their energies, recalculated with second-order multireference perturbation (CAS/MP2) theory. The reaction path starting at the excited n-pi state of DMD is predicted to occur via a nonadiabatic mechanism, giving carbene and molecular dinitrogen (both in their singlet ground states) as the main products; the computed barrier height (1.0 kcal mol(-)(1)) agrees well with the experimental estimate of the activation energy in the singlet excited state (0.0-1.5 kcal mol(-)(1)). Ground state of dimethylcarbene is the only species where a 1,2-hydrogen shift takes place, being the only source of propene. The calculated potential energy barrier height for dimethylcarbene to propene isomerization (2.6 kcal mol(-)(1)) agrees well with the observed activation energy (2.56 kcal mol(-)(1)). No evidence for rearrangement in the first singlet excited state of DMD has been found; such a process would lead to a higher activation energy than the observed one. Consequently, 1,2-hydrogen migration concurrent with N(2) extrusion in the excited state has been ruled out.

Nonadiabatic pattern formation in optical parametric oscillators.

A nonadiabatic mechanism for pattern formation in parametrically forced systems subjected to a slow periodic modulation of the excitation frequency is proposed and discussed in detail for the case of an optical parametric oscillator. It is demonstrated that nonautonomous dynamics may induce nonadiabatic off-axis emission of down-converted photons even when the signal field is blueshifted from the nearby cavity resonance.

On nonadiabatic effects in H−+H2 collisions

Abstract Nonadiabatic couplings between the low-lying singlet states of H 3 − are calculated by means of the diatomics-in-molecule method. The couplings between the ground state and the first excited singlet state are described in detail as being the most important for H − +H 2 collisional processes. The nonadiabatic regions, including the simultaneously intersecting four seams of conical intersection, are specified in the three-dimensional space. It is shown that the nonadiabatic couplings influence remarkably the dynamics of all collisional processes in H − +H 2 collisions. In particular, it is shown that the nonadiabatic transition mechanism dominates over the direct one for the electron detachment process. The electron detachment probabilities in collinear H − +H 2 ( v ) collisions are calculated by means of the classical trajectory method combined with the Landau–Zener model for nonadiabatic transitions for different initial vibrational quantum numbers v .

Photodissociation spectroscopy and dynamics of MgC2H4+

The weakly bound ion–molecule complex MgC2H4+ has been studied by photodissociation spectroscopy in a reflectron time-of-flight mass spectrometer over the spectral range 218–510 nm. Mg+ is the major photofragment throughout this range, although for λ&amp;lt;270 nm, charge-transfer dissociation to C2H4+ is observed as a minor channel. We have identified five absorption bands of MgC2H4+. The spectral assignment is facilitated by results from ab initio calculations for the ground and low-lying excited states of MgC2H4+. Three of the bands, 1 2B2←1 2A1, 1 2B1←1 2A1, and 2 2A1←1 2A1, are based primarily in the metal-centered Mg+(3p 2P←3s 2S) atomic transition. One of the remaining bands is assigned as 2 2B2←1 2A1, a transition correlating with the a 3B1u←X 1Ag forbidden band of C2H4, with mixed charge-transfer character. The final band, 3 2A1←1 2A1, is assigned to a metal-to-ligand charge-transfer transition, enhanced by coupling with the nearby 2 2A1 state. The 1 2B2←1 2A1 band is a broad continuum, indicative of fast predissociation in the upper state. A nonadiabatic dissociation mechanism involving C=C π-bond activation by Mg+(3p) is suggested by ab initio calculations. The 1 2B1←1 2A1 band shows pronounced vibrational structure with a strong progression in the Mg+–CH4 intermolecular stretch (ν2), and weaker progressions assigned to combination bands built on the intermolecular out-of-plane wag (ν3), and a CH2–CH2 wag (ν7). The observed vibrational constants are ω2=329, x22=−2.3, ω3=439, and ω7=1024 cm−1. Measurement of the photofragment kinetic energy release determines the bond dissociation energies for the ground state [D0″(Mg+–C2H4)=0.7±0.2 eV], and for the 1 2B1 excited state, [D0′(Mg+–C2H4)=1.8±0.2 eV]. Spectroscopic constants are in good agreement with ab initio predictions.

Adiabatic and non-adiabatic cluster collisions

Adiabatic collisions (fusion, deep inelastic scattering) between two fullerenes are investigated on the basis ofQuantum Molecular Dynamics. As a first application of the so-calledNon-adiabatic Quantum Molecular Dynamics, developed recently, the non-adiabatic mechanism of collision induced dissociation in metallic clusterion-atom collisions is studied.

One-atom cage effect in collinear I 2 Ar(B) complexes: a quasiclassical trajectory investigation

Quasiclassical trajectory calculations were carried out to investigate the one-atom cage effect in I 2Ar(B) van der Waals complex. To compare with a recent exact wave packet calculation, the complex is restricted to a collinear configuration and the same potential energy surfaces were used. Very good agreement with the exact results demonstrate the applicability of the quasiclassical method to this problem. Without invoking any non-adiabatic mechanism, our theoretical results agree well with experimental observations at all wavelengths considered. This work demonstrates the importance of mechanical impact mechanism and the insignificance of a non-adiabatic mechanism.

Charge carrier transport in Cu(COOH) 2 organic molecular single crystal

The temperature dependences of the d.c. electrical conductivity (σ) and thermoelectric power (TEP) of low-dimensional Cu(COOH) 2 single crystal have been studied. The electrical conductivity is highly anisotropic (σ ‡/σ ⊥ = 3.0 × 10 2) and the crystal undergoes a Peierls transition at 208 K and two polymorphous phase transitions, namely α → β and β → α at 327 and 377 K respectively. The conductivity in this crystal is found to be mainly due to a non-adiabatic hopping mechanism. TEP measurements carried out at various temperatures show that the conductivity is due to the mobility of holes.

Temperature dependence of nitrogen dissociation on metal surfaces

The effect of crystal temperature on the dissociation dynamics of nitrogen on a catalytic metal surface is studied. The framework is a nonadiabatic mechanism where the nitrogen crosses from the physisorption potential energy surface to a dissociative chemisorption potential. Within this framework the quantum dynamics is solved in three degrees of freedom including surface vibrational excitation. In general, surface vibrations promote the dissociation. However, if the nonadiabatic coupling potential is peaked at a restricted geometry, exciting the surface vibrations can hinder dissociation. This effect is maximized for the N2/Fe mass ratio which leads to a negative temperature effect on the dissociation. For higher surface metal masses this effect disappears (N2/Ru) and even reverses to a positive temperature effect for the N2/Re mass ratio.

Non-adiabatic polaron hopping conduction in semiconducting V2O5-Bi2O3 oxide glasses doped with BaTiO3

BaTiO3-doped (5–40 wt %) 90V2O5-10Bi2O3 (VB) glasses have been prepared by a quick quenching technique. The d.c. electrical conductivities, σd.c., of these glasses have been reported in the temperature range 80–450 K. The electrical conductivity of these glasses, which arises due to the presence of V4+ and V5+ ions, has been analysed in the light of the small-polaron hopping conduction mechanism. The adiabatic hopping conduction valid for the undoped VB glasses (with 80–95 mol % V2O5), in the high-temperature region, is changed to a non-adiabatic hopping mechanism in the BaTiO3-doped VB glasses. At lower temperatures, however, a variable range hopping (VRH) mechanism dominates the conduction mechanism in both the glass systems. Such a change-over from adiabatic to non-adiabatic conduction mechanism is a new feature in transition metal oxide glasses. Various parameters, such as density of states at the Fermi level N(EF), electron wave-function decay constant, α, polaron radius, r p, and its effective mass, m p * , etc., have been obtained for all the glass samples from a critical analysis of the electrical conductivity data satisfying the theory of polaron hopping conduction.

Study of the a.c. conductivity of BaTiO3-doped V2O5-Bi2O3 glasses using Long's overlapping large-polaron tunnelling model

Abstract In the present paper we report the results of a study of the frequency-dependent a.c. conductivity σa.c. of the BaTiO3-doped (5–40 wt%) 90V2O5-10 Bi2O3 (VB) glassy semiconductor system over a wide frequency range (500-10 000Hz) and wide temperature range (80–450 K). The semiconducting behaviour of these glasses is due to the presence of V4+ and V5+ valence states. While the correlated barrier hopping of bipolarons is responsible for the a.c. conductivity in the undoped glasses, Long's overlapping large polaron tunnelling mechanism is found to be valid for all these BaTiO3-doped VB glass compositions. Such a change in conduction mechanism is not usually observed in transition-metal oxide glasses and it is due to the change in network structure of the VB glass arising from the addition of BaTiO3. The doped glasses, however, show phase separation even in the glassy phase in contrast with the pure VB glass. The d.c. conductivity of doped glasses showed a non-adiabatic conduction mechanism in contrast...