Ground State Products Research Articles

Energy-Resolved Photoionization of Alkylperoxy Radicals and the Stability of Their Cations

The photoionization of alkylperoxy radicals has been investigated using a newly developed experimental apparatus that combines the tunability of the vacuum ultraviolet radiation of the Advanced Light Source at Lawrence Berkeley National Laboratory with time-resolved mass spectrometry. Methylperoxy (CH(3)OO) and ethylperoxy (C(2)H(5)OO) radicals are produced by the reaction of pulsed, photolytically produced alkyl radicals with molecular oxygen, and the mass spectrum of the reacting mixture is monitored in time by using synchrotron-photoionization with a double-focusing mass spectrometer. The kinetics of product formation is used to confirm the origins and assignments of ionized species. The photoionization efficiency curve for CH(3)OO has been measured, and an adiabatic ionization energy of (10.33 +/- 0.05) eV was determined with the aid of Franck-Condon spectral simulations, including ionization to the lowest triplet and singlet cation states. Using the appearance energy of CH(3)(+) from CH(3)OO, an enthalpy of formation for CH(3)OO of Delta(f) (CH(3)OO) = (22.4 +/- 5) kJ mol(-1) is derived. The enthalpy of formation of CH(3)OO(+) is derived as Delta(f) = (1019 +/- 7) kJ mol(-1) and the CH(3)(+)-OO bond energy as (CH(3)(+) - O(2)) = (80 +/- 7) kJ mol(-1). The C(2)H(5)OO(+) signal is not detectable; however, the time profile of the ethyl cation signal suggests its formation from dissociative ionization of C(2)H(5)OO. Electronic structure calculations suggest that hyperconjugation reduces the stability of the ethylperoxy cation, making the C(2)H(5)OO(+) ground state only slightly bound with respect to the ground-state products, C(2)H(5)(+) and O(2). The value of the measured appearance energy of C(2)H(5)(+) is consistent with dissociative ionization of C(2)H(5)OO via the Franck-Condon favored ionization to the ã (1)A' state of C(2)H(5)OO(+).

Conical Intersection Control of Heterocyclic Photochemical Bond Scission1,2

The photochemistry of the heterocycle 5,6-dihydro-1-methyl-5,5-diphenylpyridin-2(1H)-one (compound 1 in the text) leads to two competitive reactions (the reactions are depicted in the Introduction to the article). These arise from fission of bond a, between the nitrogen (N-6) and C-1, and bond b, between C-4, with the two phenyl substituents, and C-5, adjacent to the nitrogen. Scission of bond a alone leads to a zwitterionic intermediate which can be trapped by nucleophiles, while cleavage of bonds a and b together affords two fragments--a ketene and an imine. The ketene could be intercepted with nucleophiles and the imine trimerized. Computation reveals little weakening of bonds a and b. But as stretching begins, conical intersections are encountered, leading to ground-state products.

An ab initio study of the low-lying electronic states of S3

Accurate calculations of the low-lying singlet and triplet electronic states of thiozone, S(3), have been carried out using large multireference configuration interaction wave functions. Cuts of the full potential energy surfaces along the stretching and bending coordinates have been presented, together with the vertical excitation spectra. The strong experimentally observed absorption around 395 nm is assigned to the 1 (1)B(2) state, which correlates to ground state products. Absorption at wavelengths shorter than 260 nm is predicted to lead to singlet excited state products, S(2) (a (1)Delta(g))+S((1)D). The spectroscopic properties of the X (3)Sigma(g) (-), a (1)Delta(g), and b (1)Sigma(g) (+) electronic states of the S(2) radical have also been accurately characterized in this work. The investigations of the low-lying electronic states were accompanied by accurate ground state coupled cluster calculations of the thermochemistry of both S(2) and S(3) using large correlation consistent basis sets with corrections for core-valence correlation, scalar relativity, and atomic spin-orbit effects. Resulting values for D(0)(S(2)+S) and SigmaD(0) for S(3) are predicted to be 61.3 and 162.7 kcal/mol, respectively, with conservative uncertainties of +/-1 kcal/mol. Analogous calculations predict the C(2v)-D(3h) (open-cyclic) isomerization energy of S(3) to be 4.4+/-0.5 kcal/mol.

Toward Exceeding the Shockley−Queisser Limit: Photoinduced Interfacial Charge Transfer Processes that Store Energy in Excess of the Equilibrated Excited State

Nanocrystalline (anatase), mesoporous TiO2 thin films were functionalized with [Ru(bpy)2(deebq)](PF6)2, [Ru(bq)2(deeb)](PF6)2, [Ru(deebq)2(bpy)](PF6)2, [Ru(bpy)(deebq)(NCS)2], or [Os(bpy)2(deebq)](PF6)2, where bpy is 2,2'-bipyridine, bq is 2,2'-biquinoline, and deeb and deebq are 4,4'-diethylester derivatives. These compounds bind to the nanocrystalline TiO2 films in their carboxylate forms with limiting surface coverages of 8 (+/- 2) x 10(-8) mol/cm2. Electrochemical measurements show that the first reduction of these compounds (-0.70 V vs SCE) occurs prior to TiO2 reduction. Steady state illumination in the presence of the sacrificial electron donor triethylamine leads to the appearance of the reduced sensitizer. The thermally equilibrated metal-to-ligand charge-transfer excited state and the reduced form of these compounds do not inject electrons into TiO2. Nanosecond transient absorption measurements demonstrate the formation of an extremely long-lived charge separated state based on equal concentrations of the reduced and oxidized compounds. The results are consistent with a mechanism of ultrafast excited-state injection into TiO2 followed by interfacial electron transfer to a ground-state compound. The quantum yield for this process was found to increase with excitation energy, a behavior attributed to stronger overlap between the excited sensitizer and the semiconductor acceptor states. For example, the quantum yields for [Os(bpy)2(dcbq)]/TiO2 were phi(417 nm) = 0.18 +/- 0.02, phi(532.5 nm) = 0.08 +/- 0.02, and phi(683 nm) = 0.05 +/- 0.01. Electron transfer to yield ground-state products occurs by lateral intermolecular charge transfer. The driving force for charge recombination was in excess of that stored in the photoluminescent excited state. Chronoabsorption measurements indicate that ligand-based intermolecular electron transfer was an order of magnitude faster than metal-centered intermolecular hole transfer. Charge recombination was quantified with the Kohlrausch-Williams-Watts model.

A computational study on the reaction of yttrium with ketene

The reaction of yttrium ( 2D) with ketene (H 2CCO) examined recently in crossed molecular beam experiments has been studied at B3LYP and CCSD (T) level. After the formations of different π-complexes with Y binding to C C O, C C and C O sites, respectively, several reaction channels are open. Two possible H-elimination pathways have been identified and they were found to correlate with singlet ground state and triplet excited state product, 1 YCHCO and 3 YCHCO, respectively. Three possible H 2-elimination pathways have been identified, and the calculation results indicate that the two concerted pathways can be competing processes at low energy, while the step-wise mechanism needs higher collision energy. The experimentally observed products YCCO were found to exist in two different forms (cyclic and linear). The calculated barrier of Y insertion into C C bond leading to the dominant product YCH 2 at low collision energy in CMB study is only 2 kcal/mol and the potential energy surface of this reaction is flat. In addition, the insertion of Y into C O bond was also identified, but this reaction is correlated to the product (vinylidene H 2CC) with excited state, agreeing with the fact that the formation of YO+H 2CC was not observed in experiment.

Probing spin-orbit quenching in Cl (2P) + H2 via crossed molecular beam scattering

In our previous work we investigated electronically non-adiabatic effects in \(\rm Cl~(^{2}P_{3/2,1/2})+D_{2}\) using crossed molecular beam scattering coupled with velocity mapped ion imaging. The prior experiments placed limits on the cross-section for electronically non-adiabatic spin-orbit excitation \(\rm Cl~(^{2}P_{3/2})+D_{2} \to Cl^*~(^{2}P_{1/2})+D_{2}\) and electronically non-adiabatic spin-orbit quenching \(\rm Cl^*~(^{2}P_{1/2})+D_{2}\to Cl~(^{2}P_{3/2})+D_{2}\). In the present work, we investigate electronically non-adiabatic spin-orbit quenching for \(\rm Cl^*~(^{2}P_{1/2})+H_{2}\) which is the required first step for the reaction of Cl* to produce ground state HCl+H products. In these experiments we collide Cl (2P) with H2 at a series of fixed collision energies using a crossed molecular beam machine with velocity mapped ion imaging detection. Through an analysis of our ion images, we determine the fraction of electronically adiabatic scattering in Cl* +H2, which allows us to place limits on the cross-section for electronically non-adiabatic scattering or quenching. We determine the following quenching cross-sections σ quench(2.1 kcal/mol) = 26 ± 21 A2, σ quench(4.0 kcal/mol) = 21 ± 49 A2, and σ quench(5.6 kcal/mol) = 14 ± 41 A2.

Dissociative recombination of the weakly bound NO-dimer cation: Cross sections and three-body dynamics

Dissociative recombination (DR) of the dimer ion (NO)(2) (+) has been studied at the heavy-ion storage ring CRYRING at the Manne Siegbahn Laboratory, Stockholm. The experiments were aimed at determining details on the strongly enhanced thermal rate coefficient for the dimer, interpreting the dissociation dynamics of the dimer ion, and studying the degree of similarity to the behavior in the monomer. The DR rate reveals that the very large efficiency of the dimer rate with respect to the monomer is limited to electron energies below 0.2 eV. The fragmentation products reveal that the breakup into the three-body channel NO+O+N dominates with a probability of 0.69+/-0.02. The second most important channel yields NO+NO fragments with a probability of 0.23+/-0.03. Furthermore, the dominant three-body breakup yields electronic and vibrational ground-state products, NO(upsilon=0)+N((4)S)+O((3)P), in about 45% of the cases. The internal product-state distribution of the NO fragment shows a similarity with the product-state distribution as predicted by the Franck-Condon overlap between a NO moiety of the dimer ion and a free NO. The dissociation dynamics seem to be independent of the NO internal energy. Finally, the dissociation dynamics reveal a correlation between the kinetic energy of the NO fragment and the degree of conservation of linear momentum between the O and N product atoms. The observations support a mechanism in which the recoil takes place along one of the NO bonds in the dimer.

Recombination and separation of photochemically created radical–ion pairs subjected to incoherent spin-conversion

The recombination/separation of the photochemically created singlet radical pair to the ground state and excited triplet products is studied. The spin-conversion in a pair is considered as a stochastic (incoherent) process, assuming that the recombination of both singlet and triplet radical pairs is contact. The quantum yields of recombination products are calculated for any initial separation of radicals in a pair. A special attention is paid to the dissipation of contact born pairs from both singlet and triplet states or via one of them. It is shown that the viscosity dependence of the product yields of the double-channel recombination differs qualitatively from the single-channel one whatever channel is open for the recombination. The validity range of incoherent description of spin-conversion is clarified by a straightforward comparison of the recombination efficiencies with 1those obtained within the Hamiltonian description of spin-conversion. In addition the role of the radical pair distribution over initial (starting) distance is discussed as well as contact approximation for recombination.

Nonlocal model of dissociative electron attachment and vibrational excitation of NO

We present the results of a study of elastic scattering and vibrational excitation of NO by electron impact in the low-energy $(0--2\phantom{\rule{0.3em}{0ex}}\mathrm{eV})$ region where the cross sections are dominated by resonance contributions. The $^{3}\ensuremath{\Sigma}^{\ensuremath{-}}$, $^{1}\ensuremath{\Delta}$, and $^{1}\ensuremath{\Sigma}^{+}$ $\mathrm{N}{\mathrm{O}}^{\ensuremath{-}}$ resonance lifetimes are taken from our earlier study [Phys. Rev. A 69, 062711 (2004)], but the resonance energies used here are obtained from different configuration-interaction studies. Here we employ a more elaborate nonlocal treatment of the nuclear dynamics, which is found to remedy the principal deficiencies of the local complex potential model we employed in our earlier study, and gives cross sections in better agreement with the most recent experiments. We also present cross sections for dissociative electron attachment to NO leading to ground-state products, ${\mathrm{O}}^{\ensuremath{-}}(^{2}\mathrm{P})+\mathrm{N}(^{4}\mathrm{S})$. The calculations show that, while the peak cross sections starting from NO in its ground vibrational state are very small $(\ensuremath{\sim}{10}^{\ensuremath{-}20}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{2})$, the cross sections are extremely sensitive to vibrational excitation of the target and should be readily observable for target NO molecules excited to $\ensuremath{\nu}=10$ and above.

Theoretical study of the reactivity of X( 3P) (X=Ge, Sn, Pb) with N 2O(X 1Σ)

The reactivity of X( 3P) (X=Ge, Sn, Pb) with N 2O(X 1Σ) on both singlet and triplet potential energy surfaces have been investigated at the B3LYP level of theory. To accurately evaluate the activation barrier and reaction energy, the coupled cluster single point calculations using the B3LYP structures is performed. The calculated results are in good agreement with experimental observations. The X ( P 3 ) + N 2 O ( X 1 Σ ) → XO ( X 1 Σ ) + N 2 ( X 1 Σ g + ) (X=Ge, Sn, and Pb) reactions that Arthur Fontijn has been observed are spin-forbidden for formation of ground state products in the experiments. The present paper is discussed with the aid of the direct abstraction (DA) mechanism (also called the surface crossing model), a correlation argument based on separated fragments X+O+N 2 can explain the inefficiency of the X ( P 3 ) + N 2 O ( X 1 Σ ) → XO ( X 1 Σ ) + N 2 ( X 1 Σ g + ) (X=Ge, Sn, and Pb) reactions at 298 K by the need to switch electron configurations along low-energy adiabatic paths from ground-state reactants to energetically accessible XO ( X 1 Σ ) + N 2 ( X 1 Σ g + ) product states.

A new mechanism for the production of highly vibrationally excited OH in the mesosphere: An ab initio study of the reactions of O2(AΣu+3andA′Δu3)+H

In an attempt to explain the observed nightglow emission from OH(v=10) in the mesosphere that has the energy greater than the exothermicity of the H+O(3) reaction, potential energy surfaces were calculated for reactions of high lying electronic states of O(2)(A (3)Sigma(u) (+) and A' (3)Delta(u)) with atomic hydrogen H((2)S) to produce the ground state products OH((2)Pi)+O((3)P). From collinear two-dimensional scans, several adiabatic and nonadiabatic pathways have been identified. Multiconfigurational single and double excitation configuration interaction calculations show that the adiabatic pathways on a (4)Delta potential surface from O(2)(A' (3)Delta)+H and a (4)Sigma(+) potential surface from O(2)(A (3)Sigma(u) (+))+H are the most favorable, with the zero-point corrected barrier heights of as low as 0.191 and 0.182 eV, respectively, and the reactions are fast. The transition states for these pathways are collinear and early, and the reaction coordinate suggests that the potential energy release of ca. 3.8 eV (larger than the energy required to excite OH to v=10) is likely to favor high vibrational excitation.

High-level ab initio studies of unimolecular dissociation of the ground-state N3 radical

A comprehensive study of the unimolecular dissociation of the N(3) radical on the ground doublet and excited quartet potential energy surfaces has been carried out with multireference single and double excitation configuration interaction and second-order multireference perturbation methods. Two forms of the N(3) radical have been located in the linear and cyclic region of the lowest doublet potential energy surface with an isomerization barrier of 62.2 kcal/mol above the linear N(3). Three equivalent C(2v) minima of cyclic N(3) are connected by low barrier, meaning the molecule is free to undergo pseudorotation. The cyclic N(3) is metastable with respect to ground state products, N((4)S)+N(2), and dissociation must occur via intersystem crossing to a quartet potential energy surface. Minima on the seams of crossing between the doublet and quartet potential surfaces are found to lie substantially higher in energy than the cyclic N(3) minima. This strongly suggests that cyclic N(3) possesses a long collision-free lifetime even if formed with substantial internal excitation.

Examination of photoproducts in the ablation plume of doped PMMA

Optical emission and laser-induced fluorescence spectroscopies (OES and LIF) are employed to examine the plume ejected into vacuum upon UV pulsed laser ablation (248 nm, 20 ns, and 266 nm, 5 ns) of poly(methyl methacrylate) PMMA films doped with photostable compounds naphthalene (NapH) and phenanthrene (PhenH), and their photolabile iodide derivatives iodonaphthalene (NapI) and iodophenanthrene (PhenI). Spontaneous emissions observed in the irradiation of NapI and PhenI sensitised films can be assigned to excited products resulting from photodissociation of the dopants and to excited C2, CH, and CN radicals. The presence in the plume of ground state products is revealed by LIF upon excitation at 266 nm. Measurements of the dependence of the spontaneous and LIF emissions on distance to the surface, ablation fluence, and flight velocities of species are discussed with reference to the mechanistic implications derived from the measurements.

The reaction of F with N 3 studied using multiconfiguration wave functions

CASSCF (complete active space self-consistent field) and CASPT2 (multiconfiguration second-order perturbation theory) calculations with the cc-pVTZ and cc-pVQZ basis sets were carried out for exploring the mechanisms of the N 3 + F → NF( 1 Δ ) + N 2 (i) and N 3 + F → NF( 3 Σ − ) + N 2 (ii) reactions. The CASSCF and CASPT2 calculations indicate that channel (i) occurs in the 1 A ′ potential energy surface (PES) and along the reaction path there exists an intermediate [IM ( 1 A ′ )] followed by a transition state and that channel (i) is energetically feasible. The CASSCF calculations indicate that channel (ii) occurs in the 3 A″ PES and it has a single step with a transition state [TS( 3 A″ )], but the CASPT2 single-point calculations imply that TS( 3 A″ ) may not exist. The 1 A ′ and 3 A″ PES crossing was explored by performing conical intersection and spin–orbit coupling calculations at the CASSCF level and the calculations imply that the crossing may not cause predissociation of IM( 1 A ′ ) into the triplet ground state product.

Excited state interfacial electron transfer from a compound with a single pyridine ligand.

The coordination compound Ru(NH(3))(5)(eina)(PF(6))(2), where eina is ethyl isonicotinate, was synthesized and attached to optically transparent nanocrystalline (anatase) TiO(2) films, abbreviated Ru(NH(3))(5)(eina)/TiO(2). The metal-to-ligand-charge-transfer (MLCT) absorption was found to shift in wavelength with solvent. The absorption maximum of the low energy MLCT band was observed at 486 nm in acetonitrile and 528 nm in dimethylformamide for Ru(NH(3))(5)(eina)(PF(6))(2) and at 512 and 555 nm for Ru(NH(3))(5)(eina)/TiO(2), respectively. The compound was found to be nonemissive with an excited state lifetime <10 ns under all conditions studied. Light excitation in fluid solution and when attached to insulating ZrO(2) films resulted in a loss of the MLCT absorption, consistent with ligand field photochemistry. Pulsed light excitation of Ru(NH(3))(5)(eina)/TiO(2) yields an absorption difference spectrum consistent with an interfacial charge separated state, Ru(III)(NH(3))(5)(eina)/TiO(2)(e(-)). This state forms within 10 ns and returns cleanly to ground state product within milliseconds. The injection quantum yields were determined by comparative actinometry and were found to be excitation wavelength dependent: phi(inj)(417 nm) = 0.30 +/- 0.05 and phi(inj)(532.5 nm) = 0.15 +/- 0.03. Regenerative solar cells based on Ru(NH(3))(5)(eina)/TiO(2) with 0.5 M TBAI, where TBA is tetrabutylammonium, and 0.05 M I(2) in acetonitrile were very inefficient. Sluggish iodide oxidation is expected, on the basis of the negative E degrees (Ru(III/II)) = +0.17 (V vs Ag/AgCl) reduction potential, and this presumably allows a greater fraction of the injected electrons to recombine with the oxidized compound thereby lowering the solar cell efficiency.

Velocity-map imaging study of the O(3P)+N2 product channel following 193 nm photolysis of N2O

Velocity-map imaging has been used to characterize the velocity and angular momentum alignment distributions of the O(3PJ) products of N2O photolysis at 193 nm. The measured velocity and spatial anisotropy distributions indicate that around 60% of the available energy appears in product translation, with the remainder being released into internal excitation of the N2 cofragment. The measured O(3PJ) alignment parameters have been interpreted in terms of an instantaneous dissociation model, which suggests preferential population of ML=0 states and an electron density distribution peaking perpendicular to the direction of the breaking bond. This is consistent with dissociation on a surface of Σ− symmetry, the most likely candidate correlating with ground state products being the asymptotic Σ1 −Σ−(3A″) surface. There are several possible routes to this surface, and these are considered in light of the measured velocity distributions and velocity-dependent spatial anisotropy parameters of the O(3P) products relative to the dominant O(1D) dissociation channel.

O–D bond dissociation from the 3s state of deuterated hydroxymethyl radical (CH2OD)

The photodissociation of the deuterated hydroxymethyl radical CH2OD is investigated on the lowest excited state, the 3s Rydberg state, in the wavelength region 365–318 nm where the D atom is the only significant product. The translational energy distribution and kinetic energy-dependent anisotropy parameter of the D channel are determined by the core-sampling time-of-flight technique at 352.5 nm. The negative recoil anisotropy parameter βeff=−0.7±0.1 is consistent with the perpendicular nature of the transition from the ground state. The fraction of the available energy partitioned into the translational degree of freedom is 0.69. Ground state products CH2O (1 1A1)+D constitute the main photodissociation channel, and no significant H product is detected at these wavelengths. Comparison with the conical intersection calculations of Hoffman and Yarkony suggests that O–D bond rupture involves crossing from the 3s potential energy surface to a repulsive region of the ground state surface at a large O–D bond distance. Isomerization of CH2OD to CH2DO is not competitive with the fast O–D dissociation.

Reaction of Cl with N 3: a CAS study

The CAS methods were used for exploring mechanisms of the Cl + N 3(X 2 Π ) → NCl( 1 Δ ) + N 2 (i) and Cl + N 3(X 2 Π ) → NCl(X 3 Σ − ) + N 2 (ii) reactions. The CASSCF/cc- pVTZ path calculations indicate that channel (i) occurs in the 1A ′ potential energy surface (PES) and there exists an intermediate (IM( 1A ′)) followed by a transition state along the reaction path and that channel (ii) occurs in the 3A ′′ PES and it has a single step with a transition state. The CASPT2/cc-pVTZ energetic results indicate that channel (i) is feasible while channel (ii) is unfeasible. Preliminary exploration for the 1A ′– 3A ′′ PES crossing at the CASSCF/cc-pVTZ level implies that the crossing may not cause predissociation of IM( 1A ′) into the ground state product (NCl ( 3 Σ − ) + N 2).

Dissociative recombination of NO+: Dynamics of the X 1Σ+ and a 3Σ+ electronic states

We have studied the dissociation dynamics of NO+ ions in their ground, X 1Σ+, and first excited metastable, a 3Σ+ states, induced by the capture of electrons of variable collision energy in the dissociative recombination (DR) process. The branching over the different dissociation channels has been measured in a merged-beam experiment on the heavy-ion storage ring, CRYRING. In accord with previous observations, NO+ (X 1Σ+,v=0) ions dissociate dominantly to the N(2D)+O(3P) product limit at 0 and 1.2 eV collision energies. In contrast to earlier reports, the spin-forbidden N(4S)+O(1D) dissociation limit contributes 0(±2)% at 0 eV. At 5.6 eV a new channel coupled to the production of ground-state atoms becomes more important, but no increase in the production of ground-state product atoms was observed. All observed branching fractions compare very favorably with predictions from a simple statistical model, which is based on the multiplicity of each dissociation limit in combination with spin conservation during the dissociation and the initial electron capture. We also report the distribution of fragment pairs from the DR reaction involving the metastable a 3Σ+ state. This state is found to dissociate to nearly all of the energetically allowed product pairs. The lifetime of the a 3Σ+ state is found to be 730(±50) ms, in agreement with earlier, sometimes indirect, observations. The experimental observations have been complemented with ab initio calculations on the different radiative decay processes both for the X 1Σ+ and the a 3Σ+ states. It is found that vibrational relaxation via infrared radiation is faster for NO+ (a 3Σ+,v&amp;gt;0) ions than the electronic decay of these metastable-state ions to the ground state.

Local variational principle

A generalization of the Gibbs-Bogoliubov-Feynman inequality for spinless particles is proven and then illustrated for the simple model of a symmetric double-well quartic potential. The method gives a pointwise lower bound for the finite-temperature density matrix and it can be systematically improved by the Trotter composition rule. It is also shown to produce ground state energies better than the ones given by the Rayleigh-Ritz principle as applied to the ground state eigenfunctions of the reference potentials. Based on this observation, it is argued that the local variational principle performs better than the equivalent methods based on the centroid path idea and on the Gibbs-Bogoliubov-Feynman variational principle, especially in the range of low temperatures.