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The ultrafast nonadiabatic dynamics of a two-electronic-state four-vibrational-mode conical intersection coupled to a finite bath with up to 20 harmonic oscillators has been investigated by employing the multiple Davydov D2ansatz. It is demonstrated, using the multi-configuration time-dependent Hartree method as a benchmark, that this approach provides an efficient and robust description of the internal conversion process at multimode conical intersections. Thanks to the Gaussian nature of the Davydov ansatz, it allows for numerically accurate simulations of time-dependent diabatic and (for the first time for a 24-mode system) adiabatic populations of the electronic states and reduced probability densities of the tuning and coupling modes. The obtained adiabatic populations and wave packets can be used as benchmarks for the testing of various simulation methods, in particular, surface-hopping methods. [ABSTRACT FROM AUTHOR]

Internal conversion is an inherently quantum mechanical process. To date, 'ab initio' computation of internal conversion rates was limited to harmonic based approximations. These are questionable since the typical transition to the ground electronic state occurs at energies which are far from the harmonic limit. It is thus of interest to study the applicability of the Semiclassical Initial Value Representation (SCIVR) approach which is in principle amenable to 'on the fly' studies even with 'many' degrees of freedom. In this work we apply the Herman-Kluk-SCIVR methodology to compute the internal conversion rates for formaldehyde for a variety of initial vibronic states. The SCIVR computation gives reasonable agreement with experiment, while the harmonic approximation typically gives rates that are too high. [ABSTRACT FROM AUTHOR]

Two species that possess different absorption and emission properties have been observed in the low resolution fluorescence excitation spectrum of 5-fluorosalicylic acid (FSA) in the gas phase. The two species were identified as monomer and dimer species using high resolution techniques. Studies of these spectra in the presence of an applied electric field, together with ab initio quantum chemistry calculations, show that the monomer is a 'closed' form of FSA exhibiting an intramolecular C = O···H-O-C hydrogen bond in the ground state. Absorption of light at ∼344 nm transforms this species into the tautomeric form C-O-H···O = C via a barrierless proton transfer process. The large charge rearrangement that accompanies this process results in a significantly red-shifted emission spectrum. The (FSA)2 dimer exhibits two intermolecular C=O...H-O-C hydrogen bonds but in this case the double proton transfer leads to a conical intersection with the ground state and rapid nonradiative decay. The onset of this process and the time scale on which it occurs are revealed by a homogeneous broadening of the dimer's high resolution spectrum. [ABSTRACT FROM AUTHOR]

Spectator resonant KL23L23 Auger electron spectra have been measured in the Si 1s photoexcitation region of Si(CH3)4 using monochromatized undulator radiation combined with a hemispherical electron spectrometer. The broad peak with high intensity in a total ion yield spectrum, coming mainly from excitation of a 1s electron into the 6t2 vacant orbital, induces a spectator Auger decay in which the excited electron remains in its excited orbital. The component on the higher energy side of this peak through 1s excitation into a Rydberg orbital produces resonant Auger decays in which the excited Rydberg electron moves into a slightly higher Rydberg orbital, or is partly shaken up to a significantly higher Rydberg orbital. These findings of Si(CH3)4 indicate a clear contrast to those for SiF4, in which the 1s excitation into a Rydberg orbital induces a shake-down phenomenon as well as a shake-up one. The results of these molecules exhibit a clear splitting effect among excited orbitals which are smeared out by overlapping due to lifetime widths and due to densely populated levels in the 1s electron excitation spectrum. This is consistent with the calculation on photoexcitation within the framework of density functional theory. [ABSTRACT FROM AUTHOR]

In this work single and double ionization spectra of the homo- and heteronuclear argon/krypton dimers and trimers are calculated by means of propagator methods where a four-component implementation was employed for the single ionizations. Scalar relativistic effects play only a minor role for the outer valence spectral structure, whereas spin-orbit coupling and electron correlation have to be treated adequately in order to reproduce the features correctly. Nonradiative decay mechanisms of subvalence vacancies in the argon and krypton dimers and trimers are discussed both for the interatomic Coulombic decay and the electron transfer mediated decay (ETMD). In the heteronuclear triatomic system which serves as a model for larger clusters, a possible ETMD process of the Ar 3s vacancy is found for the linear arrangement of the atoms. In the bent configuration the ETMD channel is closed. [ABSTRACT FROM AUTHOR]

A path integral Monte Carlo method based on the fast-Fourier transform technique combined with the important sampling method is proposed to calculate the decay rate of a metastable quantum system with an arbitrary shape of a potential barrier. The contribution of all fluctuation actions is included which can be used to check the accuracy of the usual steepest-descent approximation, namely, the perturbation expansion of potential. The analytical approximation is found to produce the decay rate of a particle in a cubic potential being about 20% larger than the Monte Carlo data at the crossover temperature. This disagreement increases with increasing complexity of the potential shape. We also demonstrate via Langevin simulation that the postsaddle potential influences strongly upon the classical escape rate. [ABSTRACT FROM AUTHOR]

The photodissociation of DNCN following excitation of the C 2A″←X 2A″ electronic transition was studied using fast beam photofragment translational spectroscopy. Analysis of the time-of-flight distributions reveals a photodissociation channel leading to D+NCN competitive with the previously observed CD+N2 product channel. The translational energy distributions describing the D+NCN channel are peaked at low energy, consistent with internal conversion to the ground state followed by statistical decay and the absence of an exit barrier. The results suggest a relatively facile pathway for the reaction CH+N2→H+NCN that proceeds through the HNCN intermediate and support a recently proposed mechanism for prompt NO production in flames. [ABSTRACT FROM AUTHOR]

Singly ionized systems in high-lying energetic final states can stabilize themselves via various electronic decay mechanisms. With increasing system size interatomic and intermolecular processes dominate over intra-atomic (Auger) decay channels. For the small (HI)2 and (LiI)2 clusters fully relativistic ionization spectra are calculated and the subsequent electronic decay of the cations is investigated. Due to the presence of the iodine atom a fully relativistic description is mandatory and was performed by the algebraic diagrammatic construction technique in its four-component form. The lifetimes of the singly ionized final states are estimated by the application of Weisskopf-Wigner [Z. Phys. 63, 54 (1930)] theory. [ABSTRACT FROM AUTHOR]

A femtosecond pump-probe technique was employed to study the dissociation dynamics of sulfur dioxide and sulfur dioxide clusters in real time. Dissociation is initiated by a multiphoton scheme that populates the E state. The SO2+ transient is fit to a biexponential decay comprising a fast and a slow component of 230 fs and 8 ps, respectively. The SO+ transient consists of a growth component of 225 fs as well as a subsequent decay of 373 fs. The pump-probe response obtained from the monomer clearly shows the predissociative cleavage of a S–O bond. Upon cluster formation, a sequential increase in the fast decay component is observed for increasing cluster size, extending to 435 fs for (SO2)4+. The transient response of cluster dissociation products SO(SO2)n+, where n=1–3, reflects no growth component indicating that formation proceeds through the ion state. Therefore, cluster formation results in a caging effect, which impedes the dissociation process. Further direct evidence for our proposed mechanism is obtained by a technique that employs a comparison of the amplitude coefficients of each respective component of the fit. This method makes possible the determination of branching ratios of competing relaxation processes and thereby the influence of cluster formation on each can be resolved. The caging effect is attributed to a steric hindrance placed on the SO2 chromophore, preventing it from attaining a linear geometry necessary for dissociation. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

In this paper, I consider theoretical models of the decay via photobleaching of a sample of surface-immobilized fluorescent molecules excited by a spatially varying laser intensity profile. I show that, with mild restrictions on the photobleaching mechanism, the fluorescence decay measured in a nonuniform excitation profile is always nonexponential. Under the same conditions, the fluorescence decay can always be approximated by a discrete sum of exponentials. A particular example is given in which a homogeneous population of fluorophores with a single (intensity-dependent) photobleaching lifetime, when illuminated by a Gaussian laser, exhibits power law fluorescence decay at long times. These results indicate that the observation of multiple exponentials in single molecule or ensemble photobleaching lifetime measurements can arise solely as an artifact of a spatially varying laser profile and is not necessarily indicative of heterogeneity in molecular internal states, conformation, or local environment. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The doubly degenerate core-excited Π state of N[sub 2]O splits into two due to the static Renner–Teller effect. The lower state, A[sub 1], has a bent stable geometry and the molecule excited to this state starts to deform itself toward this bent geometry. To probe the effect of the potential energy surfaces of the core-excited A[sub 1] states on the nuclear motion, we measure the momenta of the three atomic ions in coincidence by means of the ion momentum imaging technique. We find that the potential energy surface affects the molecular deformation significantly. N[sub 2]O in the terminal N 1s[sup -1]3πA[sub 1] excited state is observed to be bent more than that in the central N 1s[sup -1]3πA[sub 1] excited state. This means that N[sub 2]O in the terminal N 1s[sup -1]3πA[sub 1] excited state bends faster than that in the central N 1s[sup -1]3πA[sub 1] excited state. When the excitation energy is decreased within the 1s[sup -1]3π resonances, the nuclear motion in the A[sub 1] states becomes faster. This is interpreted by the notion that the excitation occurs onto the steeper slope part of the potential energy surface of the excited state for the lower excitation energy. The branching ratio of the A[sub 1] excitation increases with the decrease in the excitation energy. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Ab initio calculations on the heterodimer C[sub 8]H[sub 6]NH...NH[sub 3] are carried out for its ground, the excited πσ[sup *], and the ground cationic electronic states, enabling the description of hydrogen or proton transfer, respectively. Two-dimensional quantum-dynamical computations on the πσ[sup *] potential surface help one to understand the mechanism and the time scale of the hydrogen transfer. Subsequent decay processes are discussed depending on the vibrational excitation of the ammonium constituent. Finally, the theoretical results obtained are used for the interpretation of the time-dependent signals observed in femtosecond pump–probe experiments. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The feasibility of efficient population transfer between an initial state and a decaying target state of the same parity without populating an intermediate state, in the presence of large-amplitude stochastic energy level fluctuations that mimic the dephasing in a solute molecule due to the influence of a solvent, is demonstrated theoretically. In particular, it is shown that a decaying target state, whose decay rate constant is large compared with the band width of picosecond laser pulses but small compared with the associated peak Rabi frequencies, can dramatically suppress the dephasing-induced nonadiabaticity associated with the dynamics of population transfer, resulting in, irrespective of the correlation time of stochastic energy level fluctuations, negligible population in the intermediate state and complete population transfer to the decaying target state. These results should further motivate experimental studies of optical control of molecular dynamics in a liquid. An interesting connection between our results and the quantum Zeno and anti-Zeno effects is also discussed. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Transport properties including collective and tracer diffusivities of nitrogen, modeled as a diatomic molecule, in single walled carbon nanotubes have been studied by equilibrium molecular dynamics at different temperatures and as a function of pressure. It is shown that while the asymptotic decay of the translational and rotational velocity autocorrelation function is algebraic, the collective velocity decays exponentially with the relaxation time related to the interfacial friction. The tracer diffusivity in the nanochannel, which is comparable in magnitude with diffusivity in the equilibrium bulk phase, depends only weakly on the conditions at the fluid–solid interface, whereas the collective diffusivity is a strong function of the hydrodynamic boundary conditions and is found to be three orders of magnitude higher than self-diffusivity in carbon nanotubes and for the comparatively rough surface of the rare-gas tube it is one order of magnitude greater. A relationship between the collective diffusivity and the Maxwell coefficient describing wall collisions is obtained. The transport coefficients appear to be insensitive to the long-range details of the potential function. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A hole in a 4d orbital of atomic xenon relaxes through Auger decay after a lifetime of 3 fs. Adding electronegative fluorine ligands to form xenon fluoride molecules, results in withdrawal of valence-electron density from Xe. Thus, within the one-center picture of Auger decay, a lowered Xe 4d Auger width would be expected, in contradiction, however, with experiment. Employing extensive ab initio calculations within the framework of many-body Green’s functions, we determine all available decay channels in XeF[sub n] and characterize these channels by means of a two-hole population analysis. We derive a relation between two-hole population numbers and partial Auger widths. On this basis, interatomic electronic decay processes are demonstrated to be so strong in the xenon fluorides that they overcompensate the reduction in intra-atomic Auger width and lead to the experimentally observed trend. The nature of the relevant processes is discussed. These processes presumably underlie Auger decay in a variety of systems. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The vacuum ultraviolet pulsed field ionization (PFI)-photoelectron (PFI-PE) spectrum of CO[sub 2] has been measured in the energy region of 19.0-20.0 eV. The PFI-PE vibrational bands resolved for CO[sup +, sub 2] are overwhelmingly dominated by the origin band along with weak vibrational bands corresponding to excitations of the v[sup +, sub 1] (symmetric stretching), v[sup +, sub 2] (bending), and v[sup +, sub 3] (antisymmetric stretching) modes. The simulation of the rotational contour resolved in the origin PFI-PE band yields a value of 19.3911±0.0005 eV for the ionization energy of CO[sub 2] to form CO[sup +, sub 2](C [sup 2]&Zigma;[sup +, sub g])./ A PFI-PE peak is found to coincide with each of the 0 K dissociation thresholds for the formation of O[sup +]([sup 4]S) + CO(X [sup 1]&Zigma;[sup +]) and CO[sup +] (X ²&Zigma;[sup +] ) +O(³P). This observation is tentatively interpreted to result from the lifetime switching effect, arising from the prompt dissociation of excited CO[sub 2] in high-n (n ≥ 100) Rydberg states prior to PFI. We have also examined the decay pathways for state-selected CO[sup +, sub 2] in the internal energy range of 5.2-6.2 eV using the PFI-PE-photoion coincidence scheme. The coincidence TOF data show unambiguously the formation of O[sup +]([sup 4]S) + CO(X 1&Zigma;[sup +]; v″ = 0,1) and CO[sup +](X ²&Zigma;[sup +] ; v[sup +] = 0,1) + O(³P). Analysis of the kinetic energy releases of fragment ions suggests that the dissociation of excited CO[sup +, sub 2] involved is nonstatistical and proceeds with an impulsive mechanism. Potential energy functions (PEFs) for the CO[sup +, sub 2] (C ²&Zigma;[sup +, sub g]) state and the lowest quartet states of CO[sup +, sub 2], together with their spin-orbit interactions, have been calculated using the complete active space self-consistent field and internal contracted multireference configuration... [ABSTRACT FROM AUTHOR]

Dipolar effects in solution nuclear magnetic resonance lead to additional peaks in two-dimensional experiments. These peaks, which have the experimental properties of intermolecular multiple-quantum coherences, have been used in a variety of applications. Most efforts have focused on intermolecular zero-quantum or double-quantum coherences, which originate in two-spin terms from the equilibrium density matrix. In this paper, we characterize the “third-order experiments” (Hahn echo decay and triple-quantum CRAZED, which both originate in the three-spin terms in the equilibrium density matrix) both theoretically and experimentally. For example, in the coupled-spin picture, Hahn echo decays in concentrated solutions arise initially from intermolecular, 3-spin, -1-quantum coherences, which are partially converted to 3-spin, +1-quantum coherences by the second pulse, and hence survive the 1:1 coherence transfer echo. Such terms require two dipolar couplings to become observable. We discuss the general properties of both of these sequences, and show that they only give information that is already present in the “second-order” double-quantum and zero-quantum experiments. Finally, we also show that relaxation and diffusion can be introduced into the coupled-spin picture in a straightforward manner. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Studies the spin-forbidden predissociation reaction of the ground state N[sub 2]O by quantum dynamics calculations. Obtaining of the potential energy surfaces of the singlet ground state of the N[sub 2]O and three triplet ones; Estimation of the decay rate of individual singlet vibrational state.

Examines the stability and decay of hydrogen, deuterium, and nitrogen atoms in frozen ammonia. Factor contributing to the residual differences in the atomic spectra; Establishment of the presence of hydrogen and deuterium atoms by observation of resonances at magnetic field strengths; Details on the splitting of the nitrogen atom doublet.

Examines the time-resolved fluorescence spectroscopy of Zinc-tetraphenylporphyrin (ZnTPP) in high-lying electronic states. Role of up-conversion technique in measuring ZnTPP; Details on depopulation using the vibrational states; Relevance of rotational diffusion in anisotropy decay.

Discusses dipole-dipole interaction and anisotropy effects in the itinerant oscillator model. How decay modes of the tagged molecule and its surroundings may be decoupled for rotation in space; Possibility in the electric polarization of the itinerant oscillator; General separation procedure for inertial response; Inertial effects in the motion of a rigid body in space.

Discusses the application of electronic structure methods and nonperturbative resonance theory to study the radiative and radiationless decay mechanisms of the magnesium bromide vibrational levels. Characterization of the adiabatic electronic states; Modification of the nonrelativistic potential energy curves in the first order degenerate perturbation theory.

The description of resonances originating from several coupled electronic states in a diabatic or approximate diabatic basis can offer both conceptual insights and computational challenges. In a three-state problem, two bound electronic states strongly coupled to a single dissociative continuum, large resonance energy shifts (thousands of cm-1), and linewidths varying over 4 orders of magnitude can be encountered. In this work a nonperturbative computational approach is developed to treat this class of resonances. Expressions for both the radiative and radiationless decay rates are developed. Although the approach is nonperturbative, the linewidth is expressed in a Golden-Rule-type formula. The resonance energy is obtained from the iterative solution of an eigenvalue problem in the bound state space. These attributes enable efficient determination both narrow and broad linewidths and large resonance energy shifts. The approach is used to characterize both radiative and radiationless decay of the 2,3 3Πg states of Al2 using a rigorous three-state diabatic basis. Lifetimes ranging from tenths of picoseconds to nanoseconds are determined. The corresponding resonance energy shifts are on the order of 4000 cm-1. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The decay of the excited singlet state of an aromatic molecule such as naphthalene adsorbed to the surface of silica is nonexponential. This nonexponential decay is shown to result from a multipolar interaction between the excited adsorbate and the intrinsic defects of silica. We present experimental evidence for direct energy transfer between the donor molecules, naphthalene and 2-methoxynaphthalene, and acceptor defects sites of a crystalline silica zeloite (silicalite). The principal condition for direct energy transfer, spectral overlap between the optical absorption of the defects and the fluorescence emission of the adsorbate is demonstrated for this system, and the nonexponential relaxation of the donor is shown to be described by a Forster-type stretched exponential. Based on the spectral overlap and energy transfer rate determined, the intrinsic defect density of silicalite is calculated to be 3±2×1020 cm-3. [ABSTRACT FROM AUTHOR]

Calculations of the total and partial decay widths are presented for the vibrational predissociation of H2HF. The calculation is done for the total angular momentum J=0 of the molecule and for the dissociation process H2HF(v=1)→H2(v=0)+HF(v=0). A time-dependent golden rule wave packet method is employed in the numerical calculation for the decay widths. The lifetime of the complex is determined from our calculation to be about 600 ns for para- and 1600 ns for ortho-H2HF. These values are much larger than the experimentally measured value of 27 ns for ortho-H2HF. The large discrepancy in lifetime for H2HF is in sharp contrast to the excellent agreement in lifetime for D2HF between theory and experiment, though the same potential energy surface is used in both calculations. We also present the rotational state distribution of the fragments H2 and HF following the vibrational predissociation of H2HF. It is found that about 58% of the final rotational population is in j=4 states of H2 for para-H2HF and about 48% is in j=5 states of H2 for ortho-H2HF. [ABSTRACT FROM AUTHOR]

We have measured the decay rates of optically excited triplet states of pyrazine and methylpyrazine in a supersonic jet. The excess vibrational energy dependence of the radiationless rate constants in the energy range between the T1 and the S1 electronic origins of the isolated molecules was explored. Decay rates between 5×102 and 105 s-1 were found in the 1800 cm-1 range of excess vibrational energy above the origin of the T1 state in pyrazine. In methylpyrazine the decay rates increase from 8×102 to 3.3×104 in the first 600 cm-1 excess energy range above the T1 origin. The decay rates are free of mode specificity and rotational effects. The wide dynamic range of the T1→S0 radiationless rates of pyrazine is substantially enhanced by methyl substitution. The results support a model which suggests that certain vibrational modes, those which undergo large frequency decreases in the excited state, control the strong vibrational energy dependence of the T1→S0 intersystem crossing of pyrazine. These large frequency changes result from the interaction of the near lying 3nπ* and 3ππ* states (the ‘‘proximity effect’’). The enhancement of the excess energy dependence of the decay rates by methyl substitution indicates a further decrease in the frequency of these modes. This effect of the methyl group is consistent with the ‘‘proximity effect.’’ The frequency reduction of these modes in the S1 manifold of methylpyrazine also support this mechanism. [ABSTRACT FROM AUTHOR]

Fluorescence upconversion was used to measure the time evolution of the fluorescence anisotropy of aniline in three hydrocarbon, alcohol, and aprotic polar solvents. The deconvoluted anisotropy decay time of 0.94±0.05 ps in aniline/isopentane solution is only 54% longer than calculated for free rotation of collision-free molecules, and increasing the solvent viscosity by a factor of 15 by going from isopentane to hexadecane increases the rotational relaxation time by only another 70% to 1.6 ps. The extended diffusion theory applied to the aniline/isopentane anisotropy decay resulted in a collisional time of 205±15 fs, implying that electronically excited aniline in this hydrocarbon rotates on average 19.5±1.5 deg between collisions. These results indicate that the orientational motion of electronically excited aniline in isopentane is significantly nondiffusive. The anisotropy decayed much more slowly in methanol and acetonitrile than in the hydrocarbons, presumably due to dipole–dipole interactions in acetonitrile and/or hydrogen bonding interactions in the hydroxylic solvent. The anisotropy decay in methanol presented double exponential behavior, probably due either to the coexistence of two slowly interconverting aniline species (hydrogen and nonhydrogen bonded) in solution or to a fast contribution from the hydrogen-bonded aniline molecules rotating around their long axes. Free rotation, hydrodynamic models, extended diffusion, and Langevin calculations were used to evaluate the results and comparisons were made among the various approaches. [ABSTRACT FROM AUTHOR]

The nonradiative decay channels of the valence electronic states of the gas-phase tetrahedral ions CF+4, SiF+4, CCl+4, SiCl+4, and GeCl+4 have been studied in the range 35–100 nm by a novel form of photoionization mass spectrometry. Tunable vacuum UV radiation from a synchrotron source ionizes the parent neutral molecule, and electrons and ions are detected by the photoelectron–photoion coincidence technique. The experiment is repeated continuously as a function of photon energy, and a three-dimensional histogram of photon energy versus ion time of flight versus coincidence count rate is produced. By taking cuts through this histogram, photoionization curves for the different fragment ions can be extracted. The appearance energies of the fragment ions (e.g., CF+2 from CF4, CCl+ from CCl4) occur at the adiabatic ionization potential of an electronic state of the parent ion, and not at the thermodynamic appearance energy of that ion. Attempts to measure the kinetic-energy releases in the fragmentation pathways have only been partially successful. The results are complementary to those of recent experiments to probe the radiative decay of these electronic states of MX+4 [M=C, Si, Ge; X=F, Cl] [J. Chem. Phys. 89, 2675 (1988); 89, 2683 (1988)], where the C 2T2 and D 2A1 third and fourth excited electronic states can show radiative decay to a surprising degree. The decay dynamics of the C and D states of MX+4 are reviewed. In general, the fluorides show different behavior to the analogous chlorides, and the carbon species behaves differently to the corresponding silicon or germanium species. [ABSTRACT FROM AUTHOR]

The decay fractions of metastable ammonia cluster ions which undergo unimolecular (evaporative) dissociation in a time window of 1–40 μs were measured by using a reflection time-of-flight mass spectrometer. Corrections concerning instrumental artifacts and ion trajectory of parents and daughters are made to imporve the precision of the measurements. The data are used to derive the Gspann parameter and heat capacity of clusters as described in evaporative ensemble model of metastable dissociation. Using the dissociation fractions measured in the present studies, in conjunction with kinetic energy release values previously measured in our laboratory, we apply Klots’ evaporative ensemble model to obtain binding energies of ammonia cluster ions (NH3)nH+, 4≤n≤17. The deduced binding energy values are found to be in very good agreement with both thermochemical data and Engelking’s modified statistical theory. [ABSTRACT FROM AUTHOR]

The decay rates of optically excited triplet states of pyrazine in supersonic expansion were measured by using three different methods. The excess energy dependence of the radiationless rate constants in the energy range between the T1 and the S1 electronic origins of the isolated molecule was explored. Decay rates between 7×102 –2.5×104 s-1 were found in the 1500 cm-1 range of excess vibrational energy from the origin of the T1 state. The decay rates are free of mode specificity and rotational effects. The pure radiative lifetime in the measured range is rovibronic independent. The results support a model which suggests that certain vibrational modes, those which undergo large frequency changes in the excited state, control the strong vibrational energy dependence of the T1 →S0 intersystem crossing of pyrazine. [ABSTRACT FROM AUTHOR]

Polarization-detected transient gain spectroscopy (PTGS), a time-resolved PUMP/PROBE scheme with single rotation–vibration level selectivity in both preparative and monitoring stages, has been used to study total depopulation and rotational state-to-state rates in the H2CO A 1A2 v4=1 vibrational level. For H2CO(A)/H2CO(X) collisions, single rovibronic level depopulation rates in the range 88±2 to 127±6 μs-1 Torr-1 (∼10 times faster than the hard sphere gas kinetic rate) were obtained from Stern–Volmer plots which were found to be linear over at least the 0.02–0.2 Torr pressure region. Rotational relaxation between the 50,5 and 60,6 eigenstates reflects a-dipole propensity rules with the sum of first order ΔJ=+1 and ΔJ=-1 processes accounting for well over 50% of the total removal cross section measured for a single rovibronic level. The agreement between collisional decay rates for the 132,12 eigenstate measured previously by the TGS technique and here by PTGS, 110±3 and 106±4 μs-1 Torr-1, respectively, suggests that elastic reorientation processes (ΔJ=ΔKa =ΔKc =0, ΔM≠0) occur with much less efficiency than inelastic energy transfer. Collisional depopulation from single rotational levels of H2CO A 1A2 v4=1 is a remarkably efficient process, but completely understandable in the small molecule, isolated binary collision limit. None of the multiexponential decay and nonlinear Stern–Volmer effects manifest in previous undispersed fluorescence studies remain when PTGS is used to populate and monitor a single rovibronic level. [ABSTRACT FROM AUTHOR]

The nonradiative decay dynamics of crystal violet, and other triphenyl-methane dyes, dissolved in a variety of solvents, are studied as a function of temperature. A linear viscosity dependence of the excited state absorption decay time in n-alcohol solvents is found at several constant temperatures. The temperature dependence at constant viscosity is anomalously negative over the entire viscosity range (0.6 to 8.0 cP) of these experiments. Various possible mechanisms for the observed behavior are critically discussed. Two color excite-and-probe studies reveal ground state bleach recovery times which are independent of excitation wavelength but strongly dependent on probe wavelength. The faster decay on the red side of the ground state absorption is shown to be the result of stimulated emission rather than the influence of a second state. [ABSTRACT FROM AUTHOR]

Using data from alpha excitation single photon counting experiments at 100–1200 mbar and from literature reports in other pressure ranges a general consistent model is proposed for the formation and decay of the rare gas excimers in the pressure range 10-2–10+5 mbar. Special attention has been paid to the study of the vibrational relaxation processes. In this model two atomic excited states (3P2,1) and six molecular excimer states (1,3Σ+g, 1,3Σ[ATOTHER]@B|[/ATOTHER]u, 1,3Σ0u) are interconnected by a number of processes, excimer formation, atomic and molecular excitation transfer, vibrational relaxation, diffusion, resonance radiation, collision induced radiation, and excimer radiation. As a result the loss rates of the relevant atomic and molecular states of krypton are determined. [ABSTRACT FROM AUTHOR]

A recently developed methodology based on the Breit–Pauli approximation and the symbolic matrix element method is used to consider the spin-forbidden radiative decay processes, b 1Σ+→X 3Σ- and a 1Δ→X 3Σ- in NF. We report nonrelativistic potential energy curves for the b 1Σ+, a 1Δ, and X 3Σ- states and geometry dependent spin-forbidden transition moments, μ|| ≡μ0(b 1Σ+0+, X 3Σ-0+), μ⊥ ≡-μ-1(b1Σ+0+, X3Σ-1), and μ1(a 1Δ2, X 3Σ-1) based on large scale MCSCF/CI wave functions (∼5–9×104 CSF’s). Mean values for μ||, μ⊥, and μ1(a 1Δ2, X 3Σ-1) corresponding to v=0 harmonic oscillator functions are found to be 0.181×10-2, 0.467×10-3, and 0.278×10-3 a.u., respectively. The mean value of the ratio μ||/ μ⊥ is found to be, 〈μ||〉0,0/ <μ⊥>0,0 ∼3.87. These results are in good agreement with the available experimental data. [ABSTRACT FROM AUTHOR]

Electron pulse irradiated samples of high purity, crystalline H2O ice at 88 K showed three kinetically distinguishable regions of luminescence emission at 280–340 nm (band I); 320–600 nm (band II); and 450–600 nm (band III). Band II emission was assigned to the A 2∑+→X 2Π transition of OH, the gas phase peak being shifted from 306.4 to ∼385 nm by the ice lattice. The decay half-life of the band II emission resulting from a single, ∼0.05 Mrad electron pulse, was 25±3 ns and increased steeply to 210±10 ns for the second pulse and then steadily decreased to 140±10 ns after 20 pulses. Band II emission from the second or later pulses was resolved into a short lived component with a decay half-life of ∼30 ns and a longer lived component with a half-life of ∼400 ns. The latter decay fitted a second order homogeneous rate equation in which the initial concentrations of the two reactants were in the ratio (2.6±0.1):1 and was attributed to the formation of excited OH by electron–ion recombination in the bulk ice. The short lived band II emission was also attributed to excited OH and probably arose from a mixture of a fast intraspur recombination reaction with some other process of different reaction order. Dose accumulation (memory) effects were attributed to the accumulation of OH radicals and lattice vacancies in the irradiated ice. The band III emission had a half-life of 25±5 ns and its decay kinetics were consistent with emission from species such as excited OH- or H3O produced when electrons tunnel from a trapping site to a geminate partner. [ABSTRACT FROM AUTHOR]

Ion–ion coincidence measurements on H2O, HDO, and D2O ionized by Heii light show that the main ion-pair dissociations from low-lying states of H2O2+ lead to OH++H+ and H++H++O. Distinct states of H2O2+ are deduced to be at 36.5±1 and at 42±1 eV above the H2O ground state. Double charge transfer measurements show two sets of singlet states of H2O2+ at 41.4 and 45 eV in agreement with the Auger spectrum. New ab initio calculations predict a vertical appearance energy of 39.2 eV for the H2O2+ ion in its 3B1 ground state, but show a very strong geometry dependence, and give an adiabatic value near 36.3 eV, in agreement with experiment. [ABSTRACT FROM AUTHOR]

Fluorescence decay rates were measured for individual rovibronic spin levels of the B 3Σ-u and B‘3Πu states of diatomic sulfur (S2) in a supersonic free jet. The radiative lifetimes of the B 3Σ-u and B‘3Πu states were found to be strongly dependent on the spin and vibrational quantum numbers, and fall in the range of 34.2 to 49.3 and 108 to 3888 ns, respectively. The marked variation of the lifetimes was interpreted satisfactorily in terms of the homogeneous interactions between the B 3Σ-u and B‘3Πu states. A quantitative deperturbation treatment of both the spectroscopic and lifetime data leads to the evaluation of interaction strengths between the B 3Σ-u and B‘ 3Πu states. [ABSTRACT FROM AUTHOR]

Comments on an article by N. Correia and colleagues on the theory of band shape formation in Auger and autoionization spectra of molecules. Interface effects on Auger and autoionization decay spectra that arise when the core-hole lifetime is comparable to the characteristics vibrational time; Presentation of a number of computed spectra to show the effects and importance of interference on line shape; Analysis of such effects on the nitrogen autoionization spectrum of nitric oxide.

We consider the relation between observed pump-probe signals in the femtosecond regime and the kinetics of unimolecular reactions, that is, the exponential decay of reactants and the exponential rise of the product population, respectively. It is shown that the signals cannot be fully accounted for within standard approaches of unimolecular decay, conventionally used in the past, since interference effects between the quasi-bound vibrational states within the bandwidth of the pump laser cannot be neglected. When these effects are included, all features of the signals can be accounted for. We apply this theoretical treatment of coherent interference to examine the dynamics and kinetics of the quasi-bound transition configurations, and relate them to the decay rates of individual quasi-bound resonance states. The signals show multi-exponential behavior, reflecting the different decay rates of the resonance states, with an average rate constant (within the bandwidth of the pump laser) which can be extracted directly from the signals. The persistence of coherence is evident in the observed signals. The predissociation of NaI is used as a prototype for numerical illustration. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Time-resolved multiphoton photoelectron spectroscopy is employed to study collective excitations and their decay dynamics in silver nanoparticles on highly oriented pyrolytic graphite. Resonant excitation of the surface plasmon in the silver nanoparticles with 400 nm femtosecond radiation allows to distinguish between photoemission from the nanoparticles and the substrate. This extends the method of time-resolved two-photon photoemission spectroscopy to inhomogeneous surfaces and permits to probe the dynamics of a confined electron gas. The multiphoton photoelectron spectra, the polarization dependence of the photoelectron yield and the time-resolved measurements reveal the double excitation of the surface plasmon and allow the identification of two different decay channels of the collective excitation. The multiply excited plasmon transfers its total excitation energy to a single photoelectron or decays into at least two single-particle excitations which share the total energy. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]