Showing papers on "Excited state published in 1993"

The equation of motion coupled‐cluster method. A systematic biorthogonal approach to molecular excitation energies, transition probabilities, and excited state properties

Abstract: A comprehensive overview of the equation of motion coupled‐cluster (EOM‐CC) method and its application to molecular systems is presented. By exploiting the biorthogonal nature of the theory, it is shown that excited state properties and transition strengths can be evaluated via a generalized expectation value approach that incorporates both the bra and ket state wave functions. Reduced density matrices defined by this procedure are given by closed form expressions. For the root of the EOM‐CC effective Hamiltonian that corresponds to the ground state, the resulting equations are equivalent to the usual expressions for normal single‐reference CC density matrices. Thus, the method described in this paper provides a universal definition of coupled‐cluster density matrices, providing a link between EOM‐CC and traditional ground state CC theory.Excitation energy,oscillator strength, and property calculations are illustrated by means of several numerical examples, including comparisons with full configuration interaction calculations and a detailed study of the ten lowest electronically excited states of the cyclic isomer of C4.

Quantum Yields for the Photosensitized Formation of the Lowest Electronically Excited Singlet State of Molecular Oxygen in Solution

Abstract: Quenching of excited singlet and triplet states of many substances by ground state molecular oxygen produces singlet oxygen, the lowest electronically excited singlet state of molecular oxygen, O2(1Δg). The fractions of singlet and triplet states quenched which produce singlet oxygen and the quantum yields of formation of singlet oxgyen in fluid solutions have been critically compiled. Methods for determination yield parameters have been reviewed. Data have been compiled from the literature through 1991. Photosensitizers such as aromatic hydrocarbons, aromatic ketones and thiones, quinones, coumarins, fluoresceins, transition metal complexes, and heterocyclics are included in Table 1. Porphyrins and phthalocyanines are included in Table 2. Other materials which have been investigated for singlet oxygen production, such as dyes and drugs, are collected in Table 3 along with heterogeneous systems such as polymer‐bound photosensitizers.

Excited-state proton transfer reactions I. Fundamentals and intermolecular reactions

Abstract: Theoretical models that have been proposed and applied to proton transfer reactions are reviewed in this work. Simple models, like the Eigen model, Marcus theory and the intersecting state model, are applied to excited-state intermolecular proton transfers. The kinetics and thermodynamics of proton transfers occuring in the singlet states of aromatic molecules with OH, NH3+, NH2 and CO substituents are reviewed.

Factors affecting lifetimes and resolution of Rydberg states observed in zero-electron-kinetic-energy spectroscopy

Abstract: It is shown that under the usual conditions of zero‐electron‐kinetic‐energy, pulsed field ionization (ZEKE–PFI) spectroscopy the lifetimes of very high‐lying Rydberg states are increased by at least approximately the factor n (in addition to the expected factor of n3), the principal quantum number, due to strong l mixing by the Stark effect. Additional factors may increase lifetimes by still another factor of approximately n. Pulsed field ionization under typical conditions is shown as likely to be predominantly diabatic and the effect on resolution is assessed. Factors affecting rotational line intensities are also discussed.

Towards an accurate molecular orbital theory for excited states: Ethene, butadiene, and hexatriene

Abstract: A newly proposed quantum chemical approach for ab initio calculations of electronic spectra of molecular systems is applied to the molecules ethene, trans‐1,3‐butadiene, and trans‐trans‐1,3,5‐hexatriene. The method has the aim of being accurate to better than 0.5 eV for excitation energies and is expected to provide structural and physical data for the excited states with good reliability. The approach is based on the complete active space (CAS) SCF method, which gives a proper description of the major features in the electronic structure of the excited state, independent of its complexity, accounts for all near degeneracy effects, and includes full orbital relaxation. Remaining dynamic electron correlation effects are in a subsequent step added using second order perturbation theory with the CASSCF wave function as the reference state. The approach is here tested in a calculation of the valence and Rydberg excited singlet and triplet states of the title molecules, using extended atomic natural orbital (ANO) basis sets. The ethene calculations comprised the two valence states plus all singlet and triplet Rydberg states of 3s, 3p, and 3d character, with errors in computed excitation energies smaller than 0.13 eV in all cases except the V state, for which the vertical excitation energy was about 0.4 eV too large. The two lowest triplet states and nine singlet states were studied in butadiene. The largest error (0.37 eV) was found for the 2 1Bu state. The two lowest triplet and seven lowest singlet states in hexatriene had excitation energies in error with less than 0.17 eV.

Identification of radiative transitions in highly porous silicon

Abstract: The authors report time-resolved photoluminescence spectroscopy of highly porous silicon. Their results show that the luminescence is due to localized quantum-confined excitons in undulating crystalline silicon wires. The resonantly excited photoluminescence spectrum exhibits satellite structure due to momentum-conserving phonons of crystalline silicon. This provides a clear signature of the crystalline-silicon electronic band structure. The spin states of the localized exciton are split by the electron-hole exchange interaction. This splitting is manifested both in the strong dependence of the luminescence lifetime on temperature, and as an energy gap in the resonantly excited photoluminescence spectrum. The experimental splitting is in good agreement with the value calculated for a localized exciton in crystalline silicon.

Evidence for strong mixing between the LC and MLCT excited states in bis(2-phenylpyridinato-C2,N')(2,2'-bipyridine)iridium(III)

Abstract: The well-resolved absorption, excitation, and luminescence spectra of [Ir(ppy)2bpy]+ (ppyH = 2-phenylpyridine, bpy = 2,2'-bipyridine) in different media at cryogenic temperatures are presented. In solutions and glasses at ambient temperature the lowest energy excited state corresponds to an Ir - bpy charge-transfer excitation whereas in the crystalline host lattice [Rh(ppy)2bpy]PF6 the lowest excited state at 21 450 cm-1 is assigned to a 37r-r* excitation localized on the cyclometalating ppy- ligands. The next higher excited Ir - bpy charge-transfer state has shifted to 21 820 cm-', only 300 cm-I above the 3LC excited state. The close proximity of the 3LC and 3MLCT excited states and the large spin-orbit coupling constant of Ir3+ induce a strong mixing of charge-transfer character into the 3LC lowest excited states, resulting in increased oscillator strengths, reduced lifetimes, short axis polarized transitions, and a large zero-field splitting of 10-15 cm-1.

Bare transition metal atoms in the gas phase: reactions of M, M+, and M2+ with hydrocarbons

Abstract: This Account focuses on gas-phase measurements and high quality ab initio calculations that are beginning to explain how metal atom electronic structure determines chemical reactivity The authors have an enormous body of basic chemical reactivity data for M[sup +] and a growing body of data for M[sup 2+] and neutral M Sophisticated experiments can control the kinetic energy and the electronic state of M[sup +] reactants One can study the reactivity of Fe[sup +] in the 3d[sup 6]4s, high-spin ground state, the 3d[sup 7], high-spin first excited state, or the 3d[sup 6]4s, low-spin second excited state The authors have learned to follow the elimination of H[sub 2] and C[sub 2]H[sub 6] from bimolecular Ni[sup +](n-butane) complexes in real time, on a 50-ns time scale In M[sup +] reactions, the authors can control the kinetic energy and the electronic state of the reactants A key advantage in interpreting these results is that one understands the electronic structure of the bare metal atom reactants very well Solution-phase chemists might well question the relevance of atomic species with genuine 1+ or 2+ charges and no ligands or solvent to the [open quotes]real world[close quotes] of organometallic chemistry Yet connections surely exist, as witnessedmore » by the fact that Rh and Ir atoms are unusually reactive in all phases Theoretical chemists are beginning to provide a conceptual framework that will unify seemingly diverse branches of experimental chemistry Of necessity, the ab initio quantum chemist works on model transition metal systems, draws experimental evidence from all available sources, and tries to abstract from the calculations what is robust and common to all phases Gas-phase metal atoms are idealized model systems well matched to the capabilities of modern theory Many new conceptual insights in the next 10 years will come from careful analysis of ab initio wave functions informed by incisive gas-phase experiments 30 refs, 5 figs, 1 tab« less

Electronic spectra and photophysics of platinum(II) complexes with α-diimine ligands. Mixed complexes with halide ligands

Abstract: Emission properties have been studied for a series of compounds of the formula (L_2)PtC1_2, where L_2 is N,N,N',N'-tetramethylethylenediamine, 2,2'-bipyridine (bpy), 4,4'-Me_2bpy, 5,5'-Me_2bpy, 4,4'-(t-Bu)_2bpy, 3,3'-(CH_3OCO)_2bpy, and 1,10-phenanthroline, and also for the compound Pt(bpy)I_2. Most of them exhibit orange to red luminescence from a triplet ligand-field (^3LF) state, both as solids and in glassy solution. These emissions are very broad (fwhm 2300-3400 cm^(-1) at 10 K) and structureless and are strongly Stokes-shifted from absorption. The two exceptions are the solid "red" form of Pt(bpy)Cl_2, which exhibits a relatively narrow (fwhm 1050 cm^(-1) at 10 K), vibronically structured (Δν ~ 1500 cm^-1)) red emission, and Pt(3,3'-(CH_3OCO)_2bpy)Cl_2, which exhibits a broad (fwhm 2500 cm^(-1) at 10 K) but structured (Δν ~1300 cm^(-1)) orange emission. Both of these emissions are assigned to triplet metal-to-ligand charge-transfer (^3MLCT) excited states. For the former compound, a linear-chain structure has destabilized a dσ*(d_(z^2)) level, yielding a dσ* → π*(bpy) state as the lowest energy excited state, while for the latter, the strongly electron-withdrawing substituents have stabilized a bpy π* level, yielding a dσ* → π*(bpy) state as the lowest energy excited state. The relative energies of the various types of excited states, including ligand 3_(ππ*) states, are discussed in detail. The crystal structures of Pt(5,5'-Me_2bpy)Cl_2 (monoclinic Cc, Z = 4, a = 13.413(7) A, b = 9.063(4) A, c = 12.261(9) A, 0 = 121.71(6)') and Pt(3,3'-(CH_3OCO)_2bpy)Cl_2 (triclinic P1, Z = 2, a = 7.288(2) A, b = 9.932(3) A, c = 11.881(5) A, α = 98.04(3)°, β = 103.56(3)°, γ = 106.54(3)°) are reported.

Electron‐impact dissociation of nitrogen

Abstract: The electron‐impact dissociation of N2 to form two nitrogen atoms is observed in a crossed beam experiment at electron energies between 18.5 and 148.5 eV. Detection of the correlated dissociation fragments with a time and position sensitive detector permits detection of both ground and excited state fragments, but excludes interference from dissociative ionization products. The observed translational energy releases in the N2 dissociation are consistent with predissociation to N(2D)+N(4S) fragments as the primary dissociation mechanism. Absolute cross sections for the electron impact dissociation are measured and compared with previous measurements. Recommended values of this cross section are given for electron‐impact energies between 10 and 200 eV.

Diabatic surfaces and the pathway for primary electron transfer in a photosynthetic reaction center

Abstract: We have performed molecular dynamics simulations for two different models of a photosynthetic reaction center (Rps.viridis) to examine the diabatic surfaces governing the primary charge separation after photoexcitation. We include the electrostatic energy of the entire proteic complex and also account for the energies of the electronic states of chromophores as computed by semiempirical quantum theory. The statistics we have acquired from our dynamics trajectories is sufficient to contrast the behaviors of the two models, to deduce the effect of crystallization water, and to measure the size of nonlinear response on the pertinent diabetic surfaces. Further, with the perspective we develop, we are able to juxtapose the active and inactive branches of the reaction center. By renormalizing our computed diabetic surfaces with a physically reasonable value for the high-frequency dielectric response of the system, the simulation results can be brought into accord with experimental observations of the thermodynamic driving force for the primary electron transfer. With no further adjustment, we find that the diabatic surfaces for the excited special pair state, SP*, and the charge-separated state SP[sup +]-BPL intersect with essentially no activation barrier. Here, BPL refers to the bacteriopheophytin on the L branch. In contrast, the SP* and SP[supmore » +]-BPM-surfaces intersect in the normal region with an activation barrier and an endothermic thermodynamic driving force. A related observation is that we find fluctuations in the pertinent energy gaps to be significantly smaller on the active L branch than they are on the inactive M branch. We also examine the surfaces associated with moving charge to the accessory bacteriochlorophylls, BCL and BCM. We find that these surfaces lies at energies far above SP*. 41 refs., 11 figs., 3 tabs.« less

Long-lived, highly luminescent rhenium(I) complexes as molecular probes: intra- and intermolecular excited-state interactions

Abstract: A series of isonitrile complexes of the form fac-LRe(CO) 3 CNR + , where L is an α-diimine and R=t-Bu or n-alkyl, have been synthesized and characterized. These complexes can have extraordinarily high quantum yields (>0.7) and long excited-state lifetimes (>100 μs) in fluid solutions at room temperature. The lowest excited state (charge transfer or ligand localized) can be controlled by varying either L or the temperature. Intramolecular foldback can affect the luminescence properties. These complexes are bihydriphobic because they have two spacially separated hydrophobic binding sites (i.e. L and R). By suitably engineering the complex, one can control which group will bind to hydrophobic targets such as cyclodextrins

Resonance Raman studies of the ground and lowest electronic excited state in CdS nanocrystals

Abstract: The size dependence of the resonance Raman spectrum of CdS nanocrystals ranging in size from 10 to 70 A radius has been studied. We find that while the lowest electronic excited state is coupled strongly to the lattice, this coupling decreases as the nanocrystal size is decreased. We demonstrate that the lifetime of the initially prepared excited state can influence the apparent strength of electron‐vibration coupling. Absolute resonance Raman cross section measurements can be used to determine the value of the excited state lifetime, thus removing this parameter. The coupling to the lattice, while less in nanocrystals than in the bulk, is still greater than what is predicted assuming an infinite confining potential. The width of the observed LO mode broadens with decreasing size, indicating that the resonance Raman process is intrinsically multimode in its nature. The frequency of the observed longitudinal optic (LO) mode has a very weak dependence on size, in contrast to results obtained from multiple quantum well systems. The temperature dependence of the frequency and linewidth of the observed LO mode is similar to the bulk and indicates that the LO mode decays into acoustic vibrations in 2.5 ps.

Silver atoms and clusters in aqueous solution: absorption spectra and the particle growth in the absence of stabilizing Ag+ ions

Abstract: The absorption spectra of Ag 0 and Ag 2+ , the first products of silver ion reduction in aqueous solution, are redetermined using pulse radiolysis. Ag 0 absorbs at 360 nm (1.6×10 4 M -1 cm -1 ), i.e., at a longer wavelength than the free atom in vacuo. The great width of 0.91 eV of the absorption band is explained by a strong interaction of the atom in the excited state with the aqueous solvent. The absorption spectrum of Ag 2+ has two peaks (at 310 and 265 nm). The equilibrium Ag 0 +Ag + ⇄Ag 2+ has a free enthalpy <-0.36 eV, and from this value the hydration free energy of Ag 2 + is calculated to be more negative than -3.6 eV

Transition-state spectroscopy via negative ion photodetachment

Abstract: Experiments which directly probe this transition states can provide detailed insight into the most important part of the reaction potential energy surface, enabling us to learn about the microscopic, interatomic forces that control chemical reactivity. One can improve on the resolution obtained in photoelectron spectroscopy by using a different technique, threshold photodetachment spectroscopy. Here, the anions are photodetached with a tunable laesr, and only those electrons produced with nearly zero kinetic energy are collected as a function of laser frequency. this, in principle, yields the same information as photoelectron spectroscopy, namely, the energies of vibrational and electronic levels of the neutral formed by photodetachment, but at considerably higher resolution (0.3-0.4 meV). A comparison of results from the two methods is presented in the following section. 40 refs., 9 figs.

Potential curves for the ground and excited states of the Na2 molecule up to the (3s+5p) dissociation limit: Results of two different effective potential calculations

Abstract: Theoretical calculations for the ground state and for 83 excited states of the Na2 molecule are presented in the framework of two independent approaches. The electron–core interaction is represented either by a pseudopotential or by a model potential, and a core polarization potential is introduced in both cases. The basis set contains either Gaussian orbitals or two‐center generalized Slater orbitals. The two methods appear to give similar results, one being more accurate for the ground and first excited states, the other being better adapted to the intermediate Rydberg states. A very good agreement is obtained with the experimental spectroscopic constants determined for 26 states, the mean deviation being ΔRe=0.05a0, Δωe=0.86 cm−1, and ΔDe=57 cm−1.

Cavity Quantum Electrodynamics

Abstract: Ever since Einstein demonstrated that spontaneous emission must occur if matter and radiation are to achieve thermal equilibrium, physicists have generally believed that excited atoms inevitably radiate.1 Spontaneous emission is so fundamental that it is usually regarded as an inherent property of matter. This view, however, overlooks the fact that spontaneous emission is not a property of an isolated atom but of an atom-vacuum system. The most distinctive feature of such emission, irreversibility, comes about because an infinity of vacuum states is available to the radiated photon. If these states are modified—for instance, by placing the excited atom between mirrors or in a cavity—spontaneous emission can be greatly inhibited or enhanced.

A conical intersection mechanism for the photochemistry of butadiene. A MC-SCF study

Abstract: The excited state (2 1 Ag) reaction paths involved in the photochemical transformations of butadiene have been studied via ab initio MC-SCF methods. It is demonstrated that the reaction funnel assumes the form of a conical intersection region where the ground (1 1 Ag) and first excited (2 1 Ag) potential energy surfaces are degenerate. This mechanism is consistent with experimental results for the photochemical isomerization and is also consistent with the observed absence of fluorescence from the 2 1 Ag state. Thus the currently accepted mechanisms for butadiene photochemistry which involve radiationless decay at avoided crossing minima need to be replaced with a model that involves fully efficient return to the ground state via a conical intersection

Competing channels in single-electron tunneling through a quantum dot.

Abstract: Coulomb blockade effects are investigated in lateral transport through a quantum dot defined in a two-dimensional electron gas. Tunneling through excited states of the quantum dot is observed for various tunneling barriers. It is shown that transport occurring via transitions between ground states with different numbers of electrons can be suppressed by the occupation of excited states. Measurements in a magnetic field parallel to the current give evidence for tunneling processes involving states with different spin.

Low-J rotational spectra, internal rotation, and structures of several benzene-water dimers

Abstract: Low J (0–4) rotational transitions have been observed for the benzene–water dimer of which high J (≥4) transitions were reported recently by Blake [Science 257, 942 (1992)]. Our experiments used a modified Balle/Flygare Fourier transform microwave spectrometer, with a pulsed supersonic nozzle as the sample source, and examined a variety of isotopic species in the ground and first excited internal rotor states (m=0 and 1). The dimers of the parent C6H6 benzene with H2O, HDO, D2O, and H218O have symmetric top spectra characteristic of two coaxial rotors with a symmetric top frame and a very low effective V6 barrier. The dimers of H2O and D2O with the 13C and D monosubstituted benzenes have asymmetric top spectra of which only the m=0 state was assigned. However, doublets in the m=1, J=0→1 transitions show that there is a V2 term of ∼0.5 MHz in their barriers. A substitution analysis was made of the rotational constants found for the m=0 state of the dimers with H218O, D2O, and the 13C and D monosubstituted ...

Many‐body methods for excited state potential energy surfaces. I. General theory of energy gradients for the equation‐of‐motion coupled‐cluster method

Abstract: The formal theory is presented for calculating the analytic first derivative of the energy with respect to arbitrary perturbations within the equation‐of‐motion coupled‐cluster (EOM‐CC) approximation. Through use of the Dalgarno–Stewart interchange theorem (Z‐vector method), terms involving derivatives of the ground state cluster amplitudes are eliminated, leading to the definition of a new quasiparticle de‐excitation operator which simplifies the theory and significantly reduces the expected cost associated with studying potential energy surfaces for excited electronic states. For both illustrative and pragmatic reasons, the final equations are cast in a form similar to that developed for ground state CC energy derivatives, involving contraction of effective one‐ and two‐particle density matrices with matrix elements of the differentiated Hamiltonian. Some aspects regarding calculation of the gradient are discussed with particular attention devoted to similarities between the structure of the present formulas and those which have been previously implemented for the ground state problem.

Nitrogen donors in 4H‐silicon carbide

Abstract: Hall‐effect and infrared‐absorption measurements are performed on n‐type 4H‐SiC samples to investigate the energy positions of the ground state and the excited states of the nitrogen donor in the 4H polytype of silicon carbide. Two electrically active levels (Hall effect) and three series of absorption lines (infrared spectra) are assigned to two nitrogen donor species which substitute on the two inequivalent lattice sites (h,k) in 4H‐SiC. Valley‐orbit splitting of the ground‐state level of the nitrogen donors on hexagonal sites (h) is found to be equal to ΔEvo(h)=7.6 meV. It is shown that the energy position of excited states of both nitrogen donors can be calculated by the effective‐mass approximation by assuming anisotropic effective masses m⊥=0.18m0 and m∥=0.22m0. The influence of the two inequivalent lattice sites on the values of ionization energy and valley orbit splitting of the nitrogen donor ground‐state levels is discussed.

Ab initio calculation of a global potential, vibrational energies, and wave functions for HCN/HNC, and a simulation of the (A-tilde)-(X-tilde) emission spectrum

Abstract: We present a potential energy surface for the HCN/HNC system which is a fit to extensive, high quality ab initio, coupled‐cluster calculations. The new surface is an improved version of one that was reported previously by us [J. A. Bentley, J. M. Bowman, B. Gazdy, T. J. Lee, and C. E. Dateo, Chem. Phys. Lett. 198, 563 (1992)]. Exact vibrational calculations of energies and wave functions of HCN, HNC, and delocalized states are done with the new potential using a new method, which combines a truncation/recoupling method in a finite basis representation procedure with a moveable basis to describe the significant bend–CH stretch correlation. All HCN and HNC states with energies below the energy of the first delocalized state are reported and characterized. All delocalized states up to 18 347 cm−1 above the HCN zero‐point energy and higher energy localized HCN states are also reported and characterized. Vibrational transition energies are compared with all available experimental data on HCN and HNC, including high CH‐overtone states up to 23 063 cm−1. We also report a simulation of the A–X stimulated emission pumping (SEP) spectrum, and compare the results to experiment. The simulation is performed within the Franck–Condon approximation, and makes use of 400 even‐bend wave functions for the ground electronic state, and a realistic vibrational wave function for the first excited bend state in the excited A state. The potential for the A state is slightly modified, relative to one implied by a previously reported force field, to improve agreement with the experimental fundamentals for the A state. In addition, the A‐state wave function is adjusted slightly to improve agreement with the SEP spectrum. We also report Franck–Condon factors for odd bending states of HCN, with one quantum of vibrational angular momentum, in order to compare with the recent assignment by Jonas, Yang, and Wodtke [J. Chem. Phys. 97, 2284 (1992)], based on axis‐switching arguments of a number of previously unassigned states in the SEP spectrum.