Vibronic relaxation processes of the FA center in RbCl:Li

The vibronic relaxation process of F_A centers in RbCl:Li, which have Type I emission at low temperatures and Type II emission at high temperatures, has been studied at 10 and 77 K by measuring the resonant secondary radiation (RSR) spectrum from optically excited F_A centers with pico-second time-resolved spectroscopy. This study has been prompted by the quest to improve our knowledge on the de-excitation process in the F_A centers. The adiabatic potential energy curves are deduced from a theoretical analysis of the measured RSR spectra and compared with those already known. It is shown that Type II relaxation observed at 77 K occurs after Type I relaxation, supporting in turn a model proposed by Baldacchini et al. based on photoluminescence measurements.     

The model of ultrafast light-induced insulator-metal phase transition in VO2

Ultrafast light-induced insulator-metal phase transitions (PT) in VO₂ thin films was studied with use of a pump-probe technique. The theoretical and experimental study of PT kinetics shows that the PT could be realized via an intermediate state. The relaxation processes after optical pumping are dependent on pump energy. The excitonic controlled model for such type of PT is proposed. The main channel for the ultrafast light-induced PT is the resonant transition between excited states of correlated vibronic Wannier-Mott excitons (WME) in insulator phase and the unoccupied excited states in metallic phase. During this process an equilibrium local distortion occurred. According to the proposed model the experimental observation of the drastic temperature- and pump power- dependent relaxation processes could be interpreted.     

Theoretical exploration of ultrafast spectroscopy of small clusters

We present the ultrafast multistate nuclear dynamics involving adiabatic and nonadiabatic excited states of non-stoichiometric halide deficient clusters (Na_nF_n-1) 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. Received 22 December 2000     

Molecular picture of excited states and fragmentation paths of the Na 5 F 4 cluster

The stability against fragmentation and possible relaxation of the lowest excited states of the Na₅F₄ cluster (representative of cubic non stoechiometric clusters with an excess sodium atom, also called sodium-tail) is investigated by means of one-electron pseudopotential calculations with particular reference to photoabsorption processes from the ground state. Whereas the equilibrium configuration of the ground state has C_3v symmetry, the doubly degenerate 1²E excited state is affected by a conical intersection and a Jahn-Teller effect associated with the rotation of the sodium tail around the C₃-axis. This yields a “Mexican hat" topology for the lowest sheet with three equivalent C_s minima. Alternatively the 2²A₁ state has a minimum retaining the C_3v symmetry. The dissociation paths of the cluster along the C₃-axis into respectively Na₄F_{4 +} Na and Na₄F_{3 +} NaF are also investigated. Among the former paths, the excited states are found adiabatically stable with respect to the products. However in the A₁ symmetry, fragmentation into NaF exhibits an interesting avoided crossing between configurations correlated respectively with Na₄F₃ ^{+ +} NaF^- and Na₄F_{3 +} NaF. Such interaction, similar to the well-known charge exchange processes in elementary molecules might induce non adiabatic predissociation of the 2²A₁ state. This mechanism is invoked to explain the differences between R2PI and depletion spectra, correlated with the dissociation or relaxation of the excited states. Received 24 March 2000 and Received in final form 11 July 2000     

Spin-selective electron tunneling from excited FA centers to F+ centers in CaO

From pulsed phosphorescence microwave double resonance experiments it is established that electron tunneling between F_A and F⁺ centers in CaO occurs from F_A centers in the excited ³A₁ state. Only the radiative sub-levels within this ³A₁ state act as precursor states in the electron transfer process. The tunneling rate at 1.2 K is determined to be 1.5±0.2s^-1.     

Donor-like excited states of exciton bound to neutral acceptor in ZnTe

Photoexcitation spectroscopy has been used to study the excited states of the neutral c-acceptor bound exciton complex A^c₁ in ZnTe. We have detected four excited states at ～ 11.2 meV above the bound exciton ground state. Zeeman effects on these excited states have also been studied. The results show that they correspond to excitations of the bound electron to donor-like 2p and 2s orbital states. This represents an unambiguous experimental evidence of the pseudo-donor model previously suggested by Rühle and Bimberg for acceptor bound exciton complexes when m_e ? m_h.     

Electron transfer from excited F centres to F+, F+A and F+AA centres in CaO studied by optically detected magnetic resonance (ODMR)

ODMR measurements via the magnetic circular polarization of the emission (MCPE) of F_A(Mg) centres in CaO at 1.6 K showed besides the known F_A-ODMR lines a non zero MCPE, which vanishes at g ? 2. From the dynamical behaviour of this MCPE and the F_A luminescence is shown that an electron tunneling takes place from the excited T_1u F centre state to F⁺_A centres forming excited F_A centres. The ground state back tunneling rate is 10⁷ times smaller and cannot explain the observed electron transfer cycle. The back tunneling is also going via excited states. The results can be understood assuming a random centre distribution. A new luminescence band at 785 nm could be identified by ODMR as being due to tetragonal F_AA centres. The electron transfer is also observed between F and F⁺ and F⁺_AA centres.     

Vibronic problem in the higher excited states of F-center in alkali halides

Vibronic problem in the higher excited states of F-center in alkali halidesA new approach to the vibronic problems of bound polaron is proposed, taking advantage of the spherical symmetry of the system. By applying the obtained formula to the analysis of the transient absorption spectra due to the transition from the relaxed excited state to higher excited states of F-center in alkali halides, a good fit between the theory and the experiments is obtained and the final states of the transition are assigned to be 3p state and its phonon side bands perturbed through vibronic coupling.     

Metastable F2 center states in LiF crystals

Using time-resolved pulsed absorption and luminescence spectroscopy we have studied the creation ofF ₂ centers during electron pulses of nanosecond duration in the temperature range 80 to 300 K. We show that the formation ofF ₂ centers in long-lived metastable states is thermally activated and competes withF ₂ center relaxation to the singlet state. It is shown that the S-T crossing in the set of states which describe theFC₂ center lies above the¹ _u level. We propose that when the crystal is excited with the lowest energy quanta which produce a photocurrent, the formation of tripletF ₂ centers takes place by localization of a conduction electron on anF ₂ ⁺ center.State Academy of Architecture and Construction, Tomsk. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 17–22, November, 1994.     

Formation time of F centers in alkali halides

It is argued that in most alkali halides an appreciable fraction of the self-trapped excitons may undergo non-radiative transitions from vibrationally excited states of B_3u to the A_1g state during the relaxation of the self-trapped excitons created by ionizing radiation. Numerical calculations show that the non-radiative transition probabilities, from vibrational levels of B_3u near or above the crossing point of the A_1g and B_3u potential curves to the A_1g state, are consistent with the observed formation times of F centers in various alkali halides. The exceptional case of KI is also discussed.     

Motional averaging in an excited state of ZnS: Cu2+

The single isotropic g factor found in the ²E excited state of ZnS:Cu²⁺ is hard to reconcile with the properties of this state, in particular with the moderate strength of the E ? ? vibronic coupling. A vibronic coupling between this state and the vibronic levels of the ground ²T₂ state due to τ₂ modes can bring about motional averaging between strain split states of ²E through a two phonon non-resonant process. In general, excited state mechanisms will be effective between states of the same spin multiplicity and which have large Stoke shifts.     

Temperature dependence of the stress-induced polarization of F center luminescence

The temperature dependence of the stress-induced polarization of F center luminescence is calculated for the simplest vibronic model for the relaxed excited state, in which 2s and 2p electronic states interact in cubic symmetry via a triply degenerate odd-parity vibrational model. The 2s state is assumed lower in energy than 2p by |E_sp|. A lower bound for |E_sp| can be determined for those alkali halides for which this polarization has been found experimentally to be independent of temperature.     

Two-photon vibronic spectroscopy of allene at 7.0–10.5 eV: experiment and theory

2?+?1 resonance-enhanced multiphoton ionization (REMPI) spectra of allene at 7.0–10.5?eV have been observed. The excited vibronic symmetry has been determined from polarization-ratio measurements. Based on the vibronic energies and peak intensities calculated using ab initio MO and time-dependent density functional theory, the very congested REMPI spectra have been assigned as due to π*?←?π, 3p?←?π, 4s?←?π, 4p?←?π, and 4d?←?π transitions. Vibrational progressions related to the CH₂ twisting (ν₄ ～770?cm^?1) have been observed for several excited electronic states. Calculated Franck–Condon factors also confirm that CH₂ twisting is the most active mode in the vibronic spectra of allene. In this study, theoretical calculations of two-photon intensities and polarization ratios have been made through the ab initio computed one-photon transition dipole moments to various electronic states as intermediates. As a starting point to interpret the complicated vibronic spectrum of allene, the theoretical approach, without vibronic couplings, has been applied to predict the peak positions, spectral intensities, and polarization ratios of Rydberg states, and qualitatively shows a considerable agreement with experimental observations.     

Microwave optical double resonance used as an assignment tool in optical spectroscopy

Microwave transitions between excited rovibronic states were used to assign transitions in the optical spectrum of NH₂ using microwave optical double-resonance spectroscopy. The microwave frequency was set so as to pump one particular hyperfine component of the previously observed 1₁₀, $\text{J =}\text{1}\text{2}$, Π(0, 10, 0), $\text{A}\text{?}{}^2\text{A}{}_1\text{? u}$, F₂ transition around 6.3 GHz, the frequency of the dye laser was then tuned over a wide spectral range. Double-resonance signals occurred in four level systems whenever the optically excited level was collisionally coupled to either of the microwave pumped levels. This method, when used in conjunction with the results of a conventional absorption spectrum of the molecule, yields information on perturbing states around the Π(0, 10, 0) region. In addition it has proved possible to detect further levels of similar vibronic character to the u state, in particular the spin rotation partners of u and u′. Some discussion on a possible vibronic assignment for these levels is included.     

Faraday Laser Magnetic Resonance Spectroscopy of Vibrationally Excited C2H

The gas phase spectrum of the C₂H radical in the region between 3191 and 3342 cm⁻¹has been studied using a Faraday LMR spectrometer with a CO overtone laser. The C₂H molecules were produced in an electric dc glow discharge of normal type containing a mixture of helium, acetylene, and hydrogen. We observed two bands of theX²Σ⁺→A²Π electronic transition with origins at 3321 and 3229 cm⁻¹. The lower level of both bands was the first excited bending level of the electronic ground stateX. The upper levels were assigned to two²Π vibronic levels with term values of 3693 and 3600 cm⁻¹, respectively. They correspond to mixtures of vibrationally excited levels of theXelectronic state with the lowest vibronic level of the first excited electronic stateA. From the analysis of the spectra we could determine the orbitalgfactors of the upper levels. These parameters are a very sensitive measure for the mixing between theXandAelectronic states. The experimentally derived values were compared with theoretical values, obtained byab initiocalculations, and could be explained by the theoretical model using an improved energy distance between theXandAstates.     

Electronic structure of F,-center in MgO

The embedded-cluster numerical variational method has been developed to calculate the electronic structure of perfect MgO, F and F⁺-centers in MgO. The energy band, bulk density of states has been calculated by cluster Mg₁₄O₁₃, Mg₁₄O₁₂F⁺ and Mg₁₄O₁₂F. The calculated absorption energy for F⁺ and F centers is in good agreement with experimental data. In our calculated defect energy levels, that the first excited state of F⁺-center is at CB-3.46 eV, indicates the necessity of a large photoelectron yielding energy. We also calculate the radius of color center electron, and plot the map of charge-density distribution of valence electrons in which the structure of the color center is shown directly. Received 22 May 1998     

Type I FA (Rb, Cs) and II FA (Li, Na) tunable laser activities and donor-acceptor properties of O and O at KCl (001) surface: first principle calculations

Type I F_A (Rb⁺, Cs⁺) and II F_A (Li⁺, Na⁺) tunable laser activities, adsorptivity and donor-acceptor properties of O and O⁻ adsorbates at the flat surface of KCl crystal were investigated using an embedded cluster model and ab initio methods of molecular electronic structure calculations. Ion clusters were embedded in a simulated Coulomb field that closely approximates the Madelung field of the host surface, and the nearest neighbor ions to the defect site were allowed to relax to equilibrium. Based on the calculated Stokes shifted optical transition bands, F_A tunable laser activities were found to be inversely proportional to the size of the dopant cation (Li⁺, Na⁺, Rb⁺, Cs⁺) relative to the host cation (K⁺). This relation was explained in terms of the axial perturbation of the impurity cation. The probability of orientational bleaching attributed to the RES saddle point ion configuration along the 〈110〉 axis was found to be inversely proportional to the size of the dopant cation, with activation energy barriers of ca. 0.44-3.34 eV. Surface relaxation energies of type II F_A centers were more important than those of type I F_A centers. In terms of defect formation energies, the products of type II F_A center imperfection were more stable than those of type I F_A. The difference between F or F_A band energies and exciton bands depended almost exclusively on the size of the positive ion species. As far as the adsorptivity of O and O⁻ is concerned, the results confirm that surface imperfection enhances the adsorption energies by ca. 4.38-16.37 eV. O and O⁻ penetrate through the defect-containing surface. The energy gap between the adsorbate and the defect containing surface and the donor-acceptor property of adsorbate play the dominant role in the course of adsorbate substrate interactions and the results were explained in terms of electrostatic potential curves and Mulliken population analysis.     

Electron trapping and untrapping during the dark decay of flash-produced KC1 F′ and F′ > A(Na) bands between 290–350°K

Both the thermal lifetimes of the F′(F′_A) centers and the ratios of the electron trapping cross-sections of (F_A) to α(α_A) centers were derived from the decay kinetics of flash-produced F′(F′_A) bands. In particular, the ratios dropped steeply with the temperature possibly due to a thermally activated de-excitation following excited state pre-trapping of the electron by the anion vacancy.     

Tailoring the chemical reactivity and optical properties of clusters by size, structures and lasers

We present results of theoretical investigations in three areas. I. Reactivity of anionic noble metal oxide clusters (Ag, Au) relevant for catalyst design: It will be shown that the cooperative effects are needed to activate clusters in order to invoke strongly size selective reactions with O₂ and CO. These results obtained with DFT method elucidated fully experimental findings. II. Stationary optical properties of silver clusters: ab initio results on absorption spectra of small silver clusters and on geometric relaxation of their excited states leading to the observed fluorescence are presented and compared with experimental data. III. Real-time investigation of ultrafast processes and their control by tailored laser fields: nonstoichiometric Na_nF_n-1 clusters are suitable prototypes to study dynamics in excited electronic states. For this purpose we use our combination of the Wigner distribution method and MD “on the fly” allowing to treat all degrees of freedom. Analysis of simulated pump-probe signals will be shown for Na₂F for which experimental data are available. Pump-dump control of the photoisomerization in Na₃F₂ avoiding conical intersection will be presented using our new strategy for obtaining tailored laser fields based on the intermediate target in excited state which (if available) guarantees the controllability in the complex systems.     

Study of ground state and some low-lying excited states of formaldehyde molecule by CNDO/VN−1 method

The semiempirical CNDO/V^N–1 method presented for calculating the excited states of molecules permits both the ground state and excited states to be considered with one parametrization. The equilibrium geometry is calculated for the ground state¹A₁ and excited states³A₂,¹A₂, and³A₁ of the formaldehyde molecule. The results of calculation are in good agreement with experimental data. The frequencies of electronic transitions are calculated for radiation and absorption.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 6, pp. 43–48, June, 1978.