Decay rates of inner-valence excitations in noble gas atoms

A Fano - algebraic diagrammatic construction - Stieltjes method has been recently developed for ab initio calculations of nonradiative decay rates [V. Averbukh and L. S. Cederbaum, J. Chem. Phys. 123, 204107 (2005)] of singly ionized states. In the present work this method is generalized for the case of electronic decay of excited states. The decay widths of autoionizing inner-valence-excited states of Ne, Ar, and Kr are calculated. Apart from the lowest excitation of Kr, they are found to be in good to excellent agreement with the experimental values. Comparison with the other theoretical studies shows that in many cases the new method performs better than the previously available techniques.     

An excited state paired interacting orbital method

A new method for analyzing and visualizing the molecular excited states, named "excited state paired interacting orbital (EPIO)," is proposed. The method is based both on the paired interacting orbital (PIO) proposed by Fujimoto and Fukui [J. Chem. Phys. 60, 572 (1974)] and the natural transition orbital (NTO) by Martin [J. Chem. Phys. 118, 4775 (2003)]. Within the PIO method, orbital interactions between the two fragmented molecules are represented practically only by a few pairs of fragment orbitals. The NTO method is a means of finding a compact orbital representation for the electronic transitions in the excited states. With the method, electronic transitions are expressed by a few particle-hole orbital pairs and a clear picture on the electronic transitions is obtained. EPIO method is designed to have both properties of the preceding two methods: electronic transitions in composite molecular systems can be expressed with a few pairs of EPIOs which are constructed with fragmented molecular orbitals (MOs). Excited state characters, such as charge transfer and local excitations, are analyzed by using EPIOs with their generation probabilities. Thus, the present method gives us clear information on the composition of MOs which play an important role in the molecular excitation processes, e.g., optical processes.     

Nonadiabatic effects in C-Br bond scission in the photodissociation of bromoacetyl chloride

Bromoacetyl chloride photodissociation has been interpreted as a paradigmatic example of a process in which nonadiabatic effects play a major role. In molecular beam experiments by Butler and co-workers [J. Chem. Phys. 95, 3848 (1991); J. Chem. Phys. 97, 355 (1992)], BrCH2C(O)Cl was prepared in its ground electronic state (S0) and excited with a laser at 248 nm to its first excited singlet state (S1). The two main ensuing photoreactions are the ruptures of the C-Cl bond and of the C-Br bond. A nonadiabatic model was proposed in which the C-Br scission is strongly suppressed due to nonadiabatic recrossing at the barrier formed by the avoided crossing between the S1 and S2 states. Recent reduced-dimensional dynamical studies lend support to this model. However, another interpretation that has been given for the experimental results is that the reduced probability of C-Br scission is a consequence of incomplete intramolecular energy redistribution. To provide further insight into this problem, we have studied the energetically lowest six singlet electronic states of bromoacetyl chloride by using an ab initio multiconfigurational perturbative electronic structure method. Stationary points (minima and saddle points) and minimum energy paths have been characterized on the S0 and S1 potential energy surfaces. The fourfold way diabatization method has been applied to transform five adiabatic excited electronic states to a diabatic representation. The diabatic potential energy matrix of the first five excited singlet states has been constructed along several cuts of the potential energy hypersurfaces. The thermochemistry of the photodissociation reactions and a comparison with experimental translational energy distributions strongly suggest that nonadiabatic effects dominate the C-Br scission, but that the reaction proceeds along the energetically allowed diabatic pathway to excited-state products instead of being nonadiabatically suppressed. This conclusion is also supported by the low values of the diabatic couplings on the C-Br scission reaction path. The methodology established in the present study will be used for the construction of global potential energy surfaces suitable for multidimensional dynamics simulations to test these preliminary interpretations.     

Simulation of vibrational dephasing of I(2) in solid Kr using the semiclassical Liouville method

In this paper, we present simulations of the decay of quantum coherence between vibrational states of I(2) in its ground (X) electronic state embedded in a cryogenic Kr matrix. We employ a numerical method based on the semiclassical limit of the quantum Liouville equation, which allows the simulation of the evolution and decay of quantum vibrational coherence using classical trajectories and ensemble averaging. The vibrational level-dependent interaction of the I(2)(X) oscillator with the rare-gas environment is modeled using a recently developed method for constructing state-dependent many-body potentials for quantum vibrations in a many-body classical environment [J. M. Riga, E. Fredj, and C. C. Martens, J. Chem. Phys. 122, 174107 (2005)]. The vibrational dephasing rates gamma(0n) for coherences prepared between the ground vibrational state mid R:0 and excited vibrational state mid R:n are calculated as a function of n and lattice temperature T. Excellent agreement with recent experiments performed by Karavitis et al. [Phys. Chem. Chem. Phys. 7, 791 (2005)] is obtained.     

The role of the excited electronic states in the C+ + H2O reaction

The electronic excited states of the [COH2]+ system have been studied in order to establish their role in the dynamics of the C+ + H2O-->[COH]+ +H reaction, which is a prototypical ion-molecule reaction. The most relevant minima and saddle points of the lowest excited state have been determined and energy profiles for the lowest excited doublet and quartet electronic states have been computed along the fragmentation and isomerization coordinates. Also, nonadiabatic coupling strengths between the ground and the first excited state have been computed where they can be large. Our analysis suggests that the first excited state could play an important role in the generation of the formyl isomer, which has been detected in crossed beam experiments [D. M. Sonnenfroh et al., J. Chem. Phys. 83, 3985 (1985)], but could not be explained in quasiclassical trajectory computations [Y. Ishikawa et al., Chem. Phys. Lett. 370, 490 (2003); J. R. Flores, J. Chem. Phys. 125, 164309 (2006)].     

Electronic excited-state energies from a linear response theory based on the ground-state two-electron reduced density matrix

Ground-state two-particle reduced density matrices (2-RDMs) are used to calculate excited-state energy spectra. Solving the Schrodinger equation for excited states dominated by single excitations from the ground-state wavefunction requires the ground-state 2- and 3-RDMs. The excited states, however, can be obtained without a knowledge of the ground-state 3-RDM by two methods: (i) cumulant expansion methods which build the 3-RDM from the 2-RDM, and (ii) double commutator methods which eliminate the 3-RDM. Previous work [Mazziotti, Phys. Rev. A 68, 052501 (2003)] examined the accuracy of excited states extracted from ground-state 2-RDMs, which were calculated by full configuration interaction or the variational 2-RDM method. In this work we employ (i) advances in semidefinite programming to treat the excited states of water and hydrogen fluoride and chains of hydrogen atoms, and (ii) the addition of partial three-particle N-representability conditions to compute more accurate ground-state 2-RDMs. With the hydrogen chains we examine the metal-to-insulator transition as measured by the band gap (the difference between the ground-state and the first excited-state energies), which is difficult for excited-state methods to capture.     

The vibrational Stokes shift of water (HOD in D2O)

The vibrational Stokes shift of the OH stretching transition nu(OH) of water is the shift between the ground-state absorption and the excited-state (v=1) emission. A recent measurement on HOD in D(2)O solvent [S. Woutersen and H. J. Bakker, Phys. Rev. Lett. 83, 2077 (1999)] of a 70 cm(-1) redshift, and a subsequent calculation of a 57 cm(-1) redshift using equilibrium molecular dynamics simulations [C. P. Lawrence and J. L. Skinner, J. Chem. Phys. 117, 8847 (2002)] were in good agreement. We now report extensive measurements of the vibrational Stokes shift in HOD/D(2)O using an ultrafast IR pump, Raman probe method. The vibrational Stokes shift is seen to depend on the pump pulse frequency and on time delay; by varying these parameters it can be made to range from 112 to -32 cm(-1) (negative values indicate a blueshift in the excited state). The equilibrium vibrational Stokes shift is actually a negative rather than a positive quantity. Possible reasons for the disagreement between experiment and theory are briefly discussed.     

On the accuracy of computed excited-state dipole moments

The dipole moments of furan and pyrrole in many electronically excited singlet states have been determined using coupled cluster theory including large one-electron basis sets. The inclusion of connected triple excitations is shown to uniformly decrease the equation-of-motion coupled-cluster singles and doubles (EOM-CCSD) excitation energies by 0.04-0.24 eV, with an average reduction of 0.08 eV. Using a basis set larger than DZP (++)D (double-zeta plus polarization augmented with atom- and molecule-centered diffuse functions) uniformly increases the computed EOM-CCSD excitation energies by 0.03-0.29 eV, with an average increase of 0.20 eV. The corresponding shifts in excited-state dipole moments are more erratic. Including connected triple excitations changes the computed dipole moments by an rms amount of 0.17 au. More importantly, using a larger basis set shifts the dipole moments by an rms amount of 0.52 au, with an increase or a decrease being equally likely. The CC dipole moments are compared to those from time-dependent density functional theory (TD-DFT) computed by Burcl, Amos, and Handy [ Chem. Phys. Lett. 2002, 355, 8]. For 29 excited states of furan and pyrrole, the predicted TD-DFT dipole moments differ from the CC results by rms amounts of 1.6 au (HCTH functional) and 1.5 au (B97-1 functional). Including the asymptotic correction to TD-DFT developed by Tozer and Handy [ J. Chem. Phys. 1998, 109, 10180; J. Comput. Chem. 1999, 20, 106] reduces the rms differences for both functionals to 1.2 au. If those Rydberg excited states with very large polarizabilities are excluded, the rms differences from the CC results for the remaining 17 excited states become 1.31 au (HCTH) and 0.88 au (B97-1). For asymptotically corrected functionals and this subset of states, the rms differences from the CC results are only 0.54 au (HCTHc) and 0.34 au (B97-1c). Thus, the Tozer-Handy asymptotic correction for TD-DFT significantly improves the predictions of excited-state dipole moments. For excited states without very large polarizabilities, good agreement is achieved between excited-state dipole moments computed by coupled cluster theory and by the asymptotically corrected B97-1c density functional.     

Theoretical study of the two-photon absorption properties of several asymmetrically substituted stilbenoid molecules

Two-photon absorption (TPA) properties of noncentrosymmetric pi-conjugated stilbenoid molecules with D-pi-A structures, TPA spectra of which have been reported [L. Antonov et al., Phys. Chem. Chem. Phys. 5, 1193 (2003)], have been investigated theoretically by ab initio molecular orbital methods. The difference in the observed one-photon absorption and TPA spectra among compounds with the same donor (D) and acceptor (A) units is well reproduced by the present calculations, although the calculated excitation energies are overestimated by the configuration interaction with single excitation method used. It was found that the spectral differences among the compounds were mainly due to the deviation from the planar structure by intramolecular rotation around the N[Single Bond]C (phenyl) bond of the N-benzilideneanilines having the C[Double Bond]N linkage as the central pi bridge. Substitution of the end donor or acceptor groups with weaker ones leads to a decrease in the TPA intensity of the lowest pi-pi(*) TPA states, resulting mainly from the decrease in the dipole moment of the excited states. The total TPA cross section spectra have been separated into contributions of the dipolar term, which appear only in noncentrosymmetric systems, and the three-state term, which appear in any systems irrespective of symmetry. The dipolar term predominates only for the lowest pi-pi(*) state, while for the higher excited states the three-state terms become predominant. An analysis employing the index R(f) defined with the transition polarizability shows that the TPA properties of the higher excited states are well described by the three-state approximation mediated by the lowest pi-pi(*) state. The differences found between the centrosymmetric and dipolar molecules in the enhancement mechanism of the TPA intensity by substituting the end groups with strong donors are discussed by comparison with the TPA properties of azobenzenes symmetrically substituted with the same donors.     

Symmetry-adapted formulation of the combined G-particle-hole hypervirial equation and Hermitian operator method

High accuracy energies of low-lying excited states, in molecular systems, have been determined by means of a procedure which combines the G-particle-hole hypervirial (GHV) equation method (Alcoba et al. in Int J Quantum Chem 109:3178, 2009) and the Hermitian operator (HO) one (Bouten et al. in Nucl Phys A 202:127, 1973). This work reports a suitable strategy to introduce the point group symmetry within the framework of the combined GHV-HO method, which leads to an improvement of the computational efficiency. The resulting symmetry-adapted formulation has been applied to illustrate the computer timings and the hardware requirements in selected chemical systems of several geometries. The new formulation is used to study the low-lying excited states torsional potentials in the ethylene molecule.     

Elastic and charge transfer processes in H+ + CO collisions

Proton and hydrogen atom time-of-flight spectra in collision energy range of E(trans) = 9.5-30 eV show that the endoergic charge transfer process in the H+ + CO system is almost an order of magnitude less probable than the elastic scattering [G. Niedner-Schatteburg and J. P. Toennies, Adv. Chem. Phys. LXXXII, 553 (1992)]. Ab initio computations at the multireference configuration interaction level have been performed to obtain the ground- and several low-lying excited electronic state potential energy curves in three different molecular orientations namely, H+ approaching the O-end and the C-end (collinear), and H+ approaching the CO molecule in perpendicular configuration with fixed CO internuclear distance. Nonadiabatic coupling terms between the ground electronic state (H+ + CO) and the three low-lying excited electronic states (H + CO+) have been computed and the corresponding diabatic potentials have been obtained. A time-dependent wavepacket dynamics study is modeled first involving only the ground and the first excited states and then involving the ground and the three lowest excited states at the collision energy of 9.5 eV. The overall charge transfer probability have been found to be approximately 20%-30% which is in qualitative agreement with the experimental findings.     

Long-time and unitary properties of semiclassical initial value representations

We numerically compare the semiclassical "frozen Gaussian" Herman-Kluk propagator [Chem. Phys. 91, 27 (1984)] and the "thawed Gaussian" propagator put forward recently by Baranger et al. [J. Phys. A 34, 7227 (2001)] by studying the quantum dynamics in some nonlinear one-dimensional potentials. The reasons for the lack of long-time accuracy and norm conservation in the latter method are uncovered. We amend the thawed Gaussian propagator with a global harmonic approximation for the stability of the trajectories and demonstrate that this revised propagator is a true alternative to the Herman-Kluk propagator with similar accuracy.     

Geometric constraints in semiclassical initial value representation calculations in Cartesian coordinates: excited states

The authors show that a recently proposed approach [J. Chem. Phys. 123, 084103 (2005)] for the inclusion of geometric constraints in semiclassical initial value representation calculations can be used to obtain excited states of weakly bound complexes. Sample calculations are performed for free and constrained rare gas clusters. The results show that the proposed approach allows the evaluation of excited states with reasonable accuracy when compared to exact basis set calculations.     

Two new double Rydberg anions plus access to excited states of neutral Rydberg radicals via anion photoelectron spectroscopy

We have observed and characterized two new double Rydberg anions N6H19- and N7H22- through their anion photoelectron spectra. The vertical detachment energies of these anions were found to be 0.443 and 0.438 eV, respectively. In addition, for three of the seven double Rydberg anions now known, we measured photodetachment transitions not only to the ground electronic states of their corresponding neutral Rydberg radicals but also to their first electronically excited states. In each spectrum, the energy spacing between the resulting peaks provided the ground-to-first electronically excited-state transition energy for the double Rydberg anion's corresponding neutral Rydberg radical. For the radicals, N4H13, N5H16, and N6H19, the spacings were found to be 0.83, 0.70, and 0.67 eV, respectively. These values are in excellent agreement with ground-to-first excited-state transition energies measured in absorption for the same neutral Rydberg radicals by Fuke and co-workers [Eur. Phys. J. D 9, 309 (1999); J. Phys. Chem. A 106, 5242 (2002).] The duplication of this neutral Rydberg property by photodetachment of double Rydberg anions further confirms that double Rydberg anions are indeed the negative ions of their corresponding neutral Rydberg molecules and cluster-like systems.     

Approach to nonadiabatic transitions by density matrix evolution and molecular dynamics simulations

Many biological processes are characterized by an essentially quantum dynamical event, such as electron or proton transfer, in a complex classical environment. To treat such processes properly by computer simulation, allowing nonadiabatic transitions involving excited states, we recently developed a density matrix evolution (DME ) method [H. J. C. Berendsen and J. Mavri, J. Phys. Chem, 97 , 13464 (1993)] which simulates the dynamics of quantum systems embedded in a classical environment. The formalism of the method is presented and an overview of the applications ranging from collisions of a quantum harmonic oscillator with noble gas atoms to proton tunneling in a double-well hydrogen bond is given. The methodology for treatment of proton-transfer processes with inclusion of excited states is presented. Future application of the method on biologically interesting processes, such as proton transfer in enzymatic reactions, is discussed. © 1996 John Wiley & Sons, Inc.     

Direct observation of competitive ultrafast CO dissociation and relaxation of an MLCT excited state: picosecond time-resolved infrared spectroscopic study of [Cr(CO)(4)(2,2'-bipyridine)

Early excited-state dynamics of [Cr(CO)(4)(bpy)] were studied in a CH(2)Cl(2) solution by picosecond time-resolved IR spectroscopy, which made it possible to characterize structurally the individual species involved and to follow separately the temporal evolution of the IR bands due to the bleached ground-state absorption, the fac-[Cr(CO)(3)(Sol)(bpy)] photoproduct, and two (3)MLCT states. It was found that the fac-[Cr(CO)(3)(Sol)(bpy)] photoproduct is formed alongside population of two (3)MLCT states during the first picosecond after excitation at 400 or 500 nm by a branched evolution of the optically populated excited state. Vibrationally relaxed (3)MLCT excited states are unreactive, decaying directly to the ground state on a picosecond time scale. The photoproduct is long-lived, persistent into the nanosecond time domain. Changing the excitation wavelength from 400 to 500 nm strongly increases the extent of the bleach recovery and decreases the yield of the photoproduct formation relative to the initial yield of the population of the unreactive (3)MLCT states. The photochemical quantum yield of CO dissociation also decreases with increasing excitation wavelength (Víchová, J.; Hartl, F.; Vlcek, A., Jr. J. Am. Chem. Soc. 1992, 114, 10903). These observations demonstrate the relationship between the early dynamics of optically populated excited states and the overall outcome of a photochemical reaction and identify the limiting role of the branching of the initial excited-state evolution between reactive and relaxation pathways as a more general principle of organometallic photochemistry.     

Ultrafast singlet excited-state polarization in electronically asymmetric ethyne-bridged bis[(porphinato)zinc(II)] complexes

The excited-state dynamics of two conjugated bis[(porphinato)zinc(II)] (bis[PZn]) species, bis[(5,5'-10,20-bis[3,5-bis(3,3-dimethyl-1-butyloxy)phenyl]porphinato)zinc(II)]ethyne (DD) and [(5,-10,20-bis[3,5-bis(3,3-dimethyl-1-butyloxy)phenyl]porphinato)zinc(II)]-[(5',-15'-ethynyl-10',20'-bis(heptafluoropropyl)porphinato)zinc(II)]ethyne (DA), were studied by pump-probe transient absorption spectroscopy and hole burning techniques. Both of these meso-to-meso ethyne-bridged bis[PZn] compounds display intense near-infrared (NIR) transient S(1)-->S(n) absorptions and fast relaxation of their initially prepared, electronically excited Q states. Solvational and conformational relaxation play key roles in both DD and DA ground- and excited-state dynamics; in addition to these processes that drive spectral diffusion, electronically excited DA manifests a 3-fold diminution of S(1)-->S(0) oscillator strength on a 2-20 ps time scale. Both DD and DA display ground-state and time-dependent excited-state conformational heterogeneity; hole burning experiments show that this conformational heterogeneity is reflected largely by the extent of porphyrin-porphyrin conjugation, which varies as a function of the pigment-pigment dihedral angle distribution. While spectral diffusion can be seen for both compounds, rotational dynamics driving configurational averaging (tau approximately 30 ps), along with a small solvational contribution, account for essentially all of the spectral changes observed for electronically excited DD. For DA, supplementary relaxation processes play key roles in the excited-state dynamics. Two fast solvational components (0.27 and 1.7 ps) increase the DA excited-state dipole moment and reduce concomitantly the corresponding S(1)-->S(0) transition oscillator strength; these data show that these effects derive from a time-dependent change of the degree of DA S(1)-state polarization, which is stimulated by solvation and enhanced excited-state inner-sphere structural relaxation.     

One- and two-photon photochemistry and photophysics of poly(arylenevinylene)s containing a biphenyl moiety.

Photochemical and photophysical properties were investigated for poly(arylenevinylene)s containing a flexible biphenyl "hinge" unit by applying one-photon (OP) and two-photon (TP) excitation to explore excited-state properties. The poly(arylenevinylene)s were poly[(2,5-dihexyloxy-p-phenylenevinylene)-alt-(4,4'-dihexyloxy-3,3'-biphenylenevinylene)] (1), poly[(2,5-dihexyloxy-p-phenylenevinylene)-alt-(2,2'-dihexyloxy-3,3'-biphenylenevinylene)] (2), and poly[(2,5-dihexyloxy-p-phenylenevinylene)-alt-(2,2'-biphenylenevinylene)] (3). Effective emission quantum yields and related photonic properties were evaluated on a realistic per-chromophore basis using effective conjugation lengths based on the Strickler-Berg relationship. Intramolecular photocyclization was deduced to occur in the one case where the biphenyl molecular connectivity permitted the reaction, based on matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF), heteronuclear multiple-quantum coherence (HMQC)-NMR, and gel-permeation chromatography (GPC) results. The various photoprocesses could be induced by either OP or TP excitation, though the first excited singlet state is the photoactive state. The higher excitation energy of the TP excited state favors indirect population of the S, state by electronic coupling between the TP and OP excited states [lambda(max)TPE (nm): 726; delta (GM): 1=229, 2=215, 3= 109). Photochemical processes occurring from the lowest OP excited state (S1) could therefore also be indirectly induced by TP excitation.