Lowest Triplet Electronic States Research Articles

Magnetic Shielding, Aromaticity, Antiaromaticity and Bonding in the Low-Lying Electronic States of S2 N2.

Aromaticity, antiaromaticity and chemical bonding in the ground (S0 ), first singlet excited (S1 ) and lowest triplet (T1 ) electronic states of disulfur dinitride, S2 N2 , were investigated by analysing the isotropic magnetic shielding, σiso (r), in the space surrounding the molecule for each electronic state. The σiso (r) values were calculated by state-optimized CASSCF/cc-pVTZ wave functions with 22 electrons in 16 orbitals constructed from gauge-including atomic orbitals (GIAOs). The S1 and T1 electronic states were confirmed as 11 Au and 13 B3u , respectively, through linear response CC3/aug-cc-pVTZ calculations of the vertical excitation energies for eight singlet (S1 -S8 ) and eight triplet (T1 -T8 ) electronic states. The aromaticities of S0 , S1 and T1 were also assessed using additional magnetic criteria including nucleus-independent chemical shifts (NICS) and magnetic susceptibilities calculated at several levels of theory, the highest of which were CCSDT-GIAO/cc-pVTZ for S0 and CASSCF(22,16)-GIAO/aug-cc-pVQZ for S1 and T1 . The results strongly suggest that: 1) the S0 electronic ground state of S2 N2 is aromatic but less so than the electronic ground state of benzene; 2) S1 is profoundly antiaromatic, to an extent that removes any bonding interactions that would keep the atoms together; and 3) T1 is also antiaromatic, but its antiaromaticity is more moderate and similar to that observed in the electronic ground state of square cyclobutadiene. S2 N2 is the first example of an inorganic ring for which theory predicts substantial changes in aromaticity upon vertical transition from the ground state to the first singlet excited or lowest triplet electronic states.

Anharmonicity Effects in IR Spectra of [Re(X)(CO)3(α-diimine)] (α-diimine = 2,2'-bipyridine or pyridylimidazo[1,5-a]pyridine; X = Cl or NCS) Complexes in Ground and Excited Electronic States.

Infrared spectra of [Re(X)(CO)(3)(α-diimine)] (α-diimine = 2,2'-bipyridine, X = Cl, NCS, or pyridylimidazo[1,5-a]pyridine, X = Cl) in the ground and the lowest triplet electronic states were calculated by a global hybrid density functional going beyond the harmonic level by means of second-order vibrational perturbation theory (VPT2) and including bulk solvent effects by the polarizable continuum model (PCM). The full-dimensionality (FD) VPT2 is compared with the reduced-dimensionality (RD) model, where only selected vibrational modes are calculated anharmonically. The simulated difference IR spectra (excited state minus ground state) in the ν(CO) region closely match experimental time-resolved infrared (TRIR) spectra. Very good agreement was also obtained for ground-state spectra in the fingerprint region. In comparison with the harmonic simulated spectra, the calculated anharmonic frequencies are closer to experimental values and do not require scaling when the B3LYP functional is used. Several spectral features due to combination bands have been identified by VPT2 simulations in the ν(CO) spectral region, which are of importance for a correct interpretation of TRIR experiments.

Reactive scattering calculations for (87)Rb+(87)RbHe→Rb2((3)Σ(u)(+),v)+He from ultralow to intermediate energies.

We investigate atom-diatom reactive collisions, as a preliminary step,in order to assess the possibility of forming Rb(2) molecules in their lowest triplet electronic state by cold collisions of rubidium atoms on the surface of helium nanodroplets [corrected]. A simple model related to the well-known Rosen treatment of linear triatomic molecules [N. Rosen, J. Chem. Phys. 1, 319 (1933)] in relative coordinates is used, allowing to estimate reactive probabilities for different values of the total angular momentum. The best available full dimensional potential energy surface [Guillon et al., J. Chem. Phys. 136, 174307 (2012)] is employed through the calculations. Noticeable values of the probabilities in the ultracold regime, which numerically fulfill the Wigner threshold law, support the feasibility of the process. The rubidium dimer is mainly produced at high vibrational states, and the reactivity is more efficient for a bosonic helium partner than when the fermion species is considered.

Formation of ultracold (7)Li(85)Rb molecules in the lowest triplet electronic state by photoassociation and their detection by ionization spectroscopy.

We report the formation of ultracold (7)Li(85)Rb molecules in the a(3)Σ(+) electronic state by photoassociation (PA) and their detection via resonantly enhanced multiphoton ionization (REMPI). With our dual-species Li and Rb magneto-optical trap apparatus, we detect PA resonances with binding energies up to ∼62 cm(-1) below the (7)Li 2s (2)S1/2 + (85)Rb 5p (2)P1/2 asymptote. In addition, we use REMPI spectroscopy to probe the a(3)Σ(+) state and excited electronic 3(3)Π and 4(3)Σ(+) states and identify a(3)Σ(+) (v″ = 7-13), 3(3)Π (vΠ' = 0-10), and 4(3)Σ(+) (vΣ' = 0-5) vibrational levels. Our line assignments agree well with ab initio calculations. These preliminary spectroscopic studies on previously unobserved electronic states are crucial to discovering transition pathways for transferring ultracold LiRb molecules created via PA to deeply bound rovibrational levels of the electronic ground state.

DEPOPULATION OF METASTABLE HELIUM BY RADIATIVE ASSOCIATION WITH HYDROGEN AND LITHIUM IONS

Depopulation of metastable He(23S) by radiative association with hydrogen and lithium ions is investigated using a fully quantal approach. Rate coefficients for spontaneous and stimulated radiative association of the HeH+, HeD+, and LiHe+ molecular ions on the spin-triplet manifold are presented as functions of temperature considering the association to rotational-vibrational states of the lowest triplet electronic states a 3Σ+ and b 3Σ+ from the continuum states of the b 3Σ+ electronic state. Evaluation of the rate coefficients is based on highly accurate quantum calculations, taking into account all possible state-to-state transitions at thermal energies (for spontaneous association) or at higher background energies (stimulated association). As expected, calculations show that the rate coefficients for radiative association to the a state are several orders of magnitude larger than the one for the b state formation. A noticeable effect by blackbody background radiation on the radiative association is only obtained for the b → b process. Aspects of the formation and abundance of the metastable HeH+(a 3Σ+) in astrophysical environments are briefly discussed.

Electronic Spectroscopy of Toluene in Helium Nanodroplets: Evidence for a Long-Lived Excited State

Optical excitation of toluene to the S1 electronic state in helium nanodroplets is found to alter the rate of production of the fragment ions C7H7(+) and C5H5(+) when the droplets are subjected to subsequent electron ionization. The optical excitation process reduces the abundance of C7H7(+) ions delivered into the gas phase, whereas C5H5(+) ions become more abundant beyond a minimum droplet size. This process contrasts with normal optical depletion spectroscopy, where the optical absorption of a molecular dopant in a helium nanodroplet shrinks the helium droplet, and thus, the electron impact cross-sections because of dissipation of the absorbed energy by evaporative loss of helium atoms. The observations here are interpreted in terms of formation of an excited state in the neutral molecule, which survives for several hundred μs. This long-lived excited state, which is assumed to be the lowest triplet electronic state, shows different cross-sections for production of C7H7(+) and C5H5(+) relative to the S0 state.

Formation of ultracold metastable RbCs molecules by short-range photoassociation

Ultracold metastable RbCs molecules are observed in a double species magneto-optical trap through photoassociation near the Rb(5S(1/2)) + Cs(6P(3/2)) dissociation limit followed by radiative stabilization. The molecules are formed in their lowest triplet electronic state and are detected by resonance enhanced two-photon ionization through the previously unobserved (3)(3)Π ← a (3)Σ(+) band. The large rotational structure of the observed photoassociation lines is assigned to the lowest vibrational levels of the 0(+) or 0(-) excited states correlated to the Rb(5P(1/2)) + Cs(6S(1/2)) dissociation limit. This demonstrates the possibility of inducing direct photoassociation in heteronuclear alkali-metal molecules at a short internuclear distance, as pointed out earlier [J. Deiglmayr et al., Phys. Rev. Lett., 2008, 101, 13304].

Chemical stereodynamics of the [formula omitted] reaction on the two lowest triplet electronic states

In this article, we apply the quasi-classical trajectory (QCT) method to study the reaction O ( 3 P ) + H 2 ( ν = 0 , j = 0 ) → OH + H on the 3A′ and 3A″ potential energy surfaces (PESs). Information of the vector correlations revealing the product alignment and orientation on the two different triplet PESs has been provided. The results of the calculations show that the products not only align strongly along the direction perpendicular to reagent initial relative vector k , but also orient along the direction of the negative y-axis, with stronger product polarizations being observed on the 3A″ PES at the collision energy smaller than 24 kcal/mol and on the 3A′ PES at the collision energy bigger than 24 kcal/mol. On both PESs, especially on the 3A″ PES, the products show preference for backward scattering which becomes weaker as collision energy increases.

Formation of ultracold polar molecules in a single quantum state

We compute the formation rate of a polar molecule, LiH, into the lowest triplet electronic state, a {sup 3{Sigma}+}, via population of the intermediate excited electronic state, b {sup 3{Pi}}, followed by radiative decay. We find large formation rates into the single rovibrational bound state (v=0,J=0) of the a {sup 3{Sigma}+}, which can be explained by the unusually large overlap of its wave function with those of the two upper-most bound levels of the b {sup 3{Pi}}. With conservative parameters, we estimate that over 10{sup 4} molecules/s could be produced in the single rovibrational level of the a {sup 3{Sigma}+} state. We also discuss scattering properties of LiH triplet molecules and their relevance to ultracold chemical reactions.

Study of cis–trans isomerization mechanism of 3,3′-azobenzene disulphonate in the lowest singlet and triplet electronic states by density functional theory

The cis–trans isomerization pathways of 3,3′-azobenzene disulphonate in the S0 and T1 states are studied by DFT method at the B3LYP/6-31G(d,p) level. In the S0 state, the cis–trans isomerization concerns the complex pathway that is characterized by the inversion of one NNC angle combined with rotation around the NC bond, and the three sequential transition states are also found on the potential energy profile. Therefore, the cis–trans isomerization of 3,3′-azobenzene disulphonate can be understood in terms of a pathway involving successive rotation, inversion, and rotation processes. The energy barrier of the S0 state is 22.79 kcal mol−1. In the T1 state, the isomerization mainly concerns the rotational pathway around the NN double bond, and the two isomers are connected through only one transition state. The isomerization of the T1 state is related to a lower energy barrier, 5.02 kcal mol−1, but requires a change in spin-multiplicity.

Geometric phase effects in resonance-mediated scattering: H+H2+ on its lowest triplet electronic state

We report a quantum dynamics study of H + H(2)(+) (v(0) = 0, j(0) = 0) scattering on its lowest triplet state, for J = 0 total angular momentum and total energies up to 1.85 eV. This provides a benchmark example of indirect resonance-mediated reaction in presence of a conical intersection (CI). Visualization of time-dependent wave packets shows significant "looping" around the CI, which is facilitated by long-lived H(3)(+) scattering resonances, predominant at low energies. State-to-state inelastic transition probabilities exhibit a highly oscillatory structure and pronounced geometric phase effects, which, in contrast to direct reactions, are more strongly marked at lower energies.

Rotational Transitions of CO+Induced by Atomic Hydrogen

The CO+ molecular ion has been observed in photon-dominated regions. Recent modeling has not been able to explain the abundances of CO+ in these regions. The most abundant collision partners are believed to be hydrogen atoms, hydrogen molecules, and electrons. The reactions of these species with CO+ have been studied previously and found to be fast. The only inelastic processes studied before were collisions of CO+ with electrons. Here we investigate the inelastic collisions of CO+ with hydrogen atoms. We argue that this can be done on the lowest triplet electronic state. This implies that CO+ ions, in a hydrogen atom dominated surrounding, experiences a few inelastic collisions before reacting. Rotationally inelastic state-to-state cross sections (j ≤ 8) are presented together with state-resolved thermal rate coefficients. The results suggest that the rotationally inelastic collision cross sections can be of comparable magnitude to the reactive ones. Implications for the rotational temperature of CO+ in PDRs are discussed. To perform the calculations, a potential energy surface is fitted using the reproducing-kernel Hilbert space method based on multireference configuration-interaction calculations employing the aug-cc-pVQZ basis set.

Low-energy electron scattering cross section for the production of CO within solid films of carbon dioxide.

We report absolute electron scattering cross sections sigma(p) for the production of CO within thin solid film of carbon dioxide (CO(2)) condensed on a solid Ar substrate. The CO fragments, which remain trapped within the bulk of the carbon dioxide film, are detected in situ by recording energy losses to their lowest triplet electronic state a (3)Pi using high-resolution electron-energy-loss spectroscopy. The production of CO is studied as a function of the electron exposure, film thickness, and incident electron energy between 2 and 30 eV, a range within which most of the secondary electrons are created in systems irradiated by high-energy particles. The energy dependence is characterized by a feature around 4 eV with sigma(p)=(7.0+/-4.0)x10(-18) cm(2), a minimum around 7 eV, a strong rise up to a large and broad maximum around 15 eV with sigma(p)=(5.4+/-2.5)x10(-17) cm(2), a decrease to a minimum around 18.5 eV, and finally a monotonous increase up to 30 eV. The CO production is discussed in terms of the formation of electron resonances or transient anion states, which may lead directly to the fragmentation of the molecule via dissociative electron attachment or indirectly by decaying into an entirely repulsive part of the corresponding excited neutral and positive ion states.

Optimal control of photoassociation of cold atoms and photodissociation of long-range molecules: Characteristic times for wave-packet propagation

This paper aims at studying the time-dependent effects involved in the photoassociation reaction for a sample of cold alkali-metal atoms, within a two-channel model where the vibrational motion in the excited state is coupled by laser light to the continuum state describing two colliding atoms in the lowest triplet electronic state $(a{}^{3}{\ensuremath{\Sigma}}_{u}^{+}).$ Both photodissociation and photoassociation processes are considered at a time scale shorter than the radiative lifetime, so that spontaneous emission does not have to be considered. The characteristic times are then the vibrational period in the excited state, which for alkali-metal dimers can be estimated of the order of a few hundreds of picoseconds, and the Rabi period, depending upon the laser intensity. Numerical calculations using wave-packet propagation are performed for the coupling of the vibrational motion in the ${\mathrm{Cs}}_{2}{1}_{g}{(6s+6p}_{3/2})$ and ${a}^{3}{\ensuremath{\Sigma}}_{u}^{+}(6s+6s)$ channels by a cw laser slightly red detuned relative to the ${\mathrm{D}}_{2}$ resonance line. The results show Rabi oscillations in the populations of the two channels during time intervals when the vibrational motion is stopped at the outer turning point. At intensities of $\ensuremath{\approx}250\mathrm{kW}{\mathrm{cm}}^{\ensuremath{-}2},$ a new characteristic time appears, a factor of 2 larger than the classical vibrational period, which corresponds to vibrational motion in the upper adiabatic potential created by the coupling. Such an effect modifies the scattering length for collisions in the lower state, and it clearly opens a possibility of control by tuning the laser intensity.

Ab Initio MO Study of the Global Potential Energy Surface of C4H4 in Triplet Electronic State and the Reactions of C(3Pj) with C3H4 (Allene and Propyne) and C2(A3Πu) with C2H4(X1A1g+)

The global potential energy surface of C4H4 in the lowest triplet electronic state have been studied at the G2M(RCC,MP2) level. Of 28 distinct isomers the most stable are aromatic cyclobutene q3 (3A1g,D4h) and linear butyne c1 (3E,D2d), and 66 transition states for various isomerization and dissociation pathways have been found. The information about the global PES is applied to describe the potential energy surfaces for the C(3Pj) + H2CCCH2, C(3Pj) + H3CCCH, and C2(3Πu) + C2H4 reactions, recently studied experimentally in crossed molecular beams. The reaction of the carbon atom with allene is shown to occur by a barrierless addition of C to the CC bond to yield the three-member ring structure t1 and/or to the central carbon atom of allene to form the branching structure b1 which isomerizes to t1 with a low barrier. t1 undergoes ring opening to c1 with a barrier of 9.4 kcal/mol, and the latter emits a H atom to give the major reaction product n-C4H3 with an exit barrier of 2.2 kcal/mol. The minor reaction product, i-C4H3, is formed through a 1,2-H shift in c1 leading to c3, followed by the hydrogen loss. The reaction of the carbon atom with methylacetylene starts with a barrierless C addition to the β-C-atom of H3CCCH to form c6, to the α-C-atom to give b2 or b2‘, or to the acetylenic C⋮C bond to yield the cyclic isomers t4 or t4‘. c6, b2, and b2‘ are metastable and would rapidly rearrange to the linear isomer c5 or to the cyclic t4 and then to t4‘ with low barriers. The latter ring opens to c5 with a barrier of 14.7 kcal/mol. At high collision energies, the major reaction product n-C4H3 is formed through the methyl hydrogen emission in c5 with an exit barrier of 5.8 kcal/mol. The second product i-C4H3 can be reached via a H-shift from c5 to c4 and subsequent hydrogen elimination. Cyclic isomers C4H3 p3 and p4 can originate from t4, t4‘, and t5. The reaction of C2(3Πu) with ethylene proceeds through a C2 addition to a carbon atom of C2H4 to yield the chain isomer c2 with an entrance barrier of ∼4 kcal/mol. c2 rearranges to linear c1 in several steps via the four-member ring intermediate q1 or the three-membered ring structures t2 and t1. c1 loses a H atom to yield n-C4H3 or undergoes a hydrogen migration to c3 followed by a H emission producing n- or i-C4H3. Similarly to C(3Pj) + allene, n-C4H3 is expected to be the major product of C2(3Πu) + C2H4, while i-C4H3 to give a minor contribution.

Photoassociation intensities and radiative trap loss in lithium

We interpret measurements of photoassociative spectroscopy in a gas of lithium atoms at ultralow temperatures in which photons are absorbed into high vibrational levels of excited electronic singlet and triplet states of ${\mathrm{Li}}_{2}.$ The excited vibrational levels decay by spontaneous emission into the vibrational continuum and the discrete vibrational levels of the ground singlet and lowest triplet electronic states. Spontaneous emission into discrete levels produces molecules that are no longer trapped, and decay into the vibrational continuum can produce atoms with sufficient kinetic energy to escape the trap. We present values of the efficiencies of trap loss by radiative excitation as a function of the trap depth for individual excited vibrational levels of the singlet and triplet states of the ${}^{6}{\mathrm{Li}}_{2}$ and ${}^{7}{\mathrm{Li}}_{2}$ molecules.

Vibrational relaxation of T1 pyrazine: Results from the refined competitive radiationless decay method

Gas phase collisional vibrational relaxation of pyrazine in its lowest triplet electronic state has been further investigated using a refined version of the competitive radiationless decay (CRD) method. Improvements to the experimental apparatus now provide primary kinetic data of much greater accuracy and precision, allowing the use of sample pressures low enough to ensure negligible self-relaxation. A major refinement of the data handling procedure permits the sample’s energy-dependent triplet–triplet molar absorptivity to be determined through an iterative analysis. To validate the refined CRD method, average energy loss per collision has been deduced as a function of donor energy under conditions of varying sample pressure, relaxer pressure, excitation beam energy, and excitation wavelength. Consistent results have been obtained for all of these variations, suggesting the absence of major systematic errors. The two main findings of earlier pyrazine triplet relaxation studies are confirmed by the new measurements: the presence of a threshold donor energy above which relaxation becomes much more efficient, and relatively large values of average energy loss per collision. Comparing to results from a recent ground state study [L. A. Miller and J. R. Barker, J. Chem. Phys. 105, 1383 (1996)], it is found that pyrazine containing 5000 cm−1 of vibrational energy is relaxed by a variety of monatomic, diatomic, and polyatomic gases approximately seven times more efficiently when the donor is in its triplet rather than its ground electronic state. The order of relaxer efficiencies toward triplet pyrazine is found to be He<H2<Ne<D2<N2<Kr<Ar<Xe<CH4<CO<CO2<CH3F<H2O. Energy loss efficiencies correlate rather well with relaxer boiling points.

A multireference coupled-cluster study of the ground state and lowest excited states of cyclobutadiene

The electronic structure of the ground state and several low-lying excited states of cyclobutadiene are studied using the new state-universal multireference coupled-cluster method with single and double excitations (MR-CCSD) augmented by a noniterative inclusion of the triple excitations [MR-CCSD(T)]. Two possible ground state configurations are examined, namely the square and the distorted rectangular geometries, and the multireference coupled-cluster energy barrier for the interconversion between the two rectangular ground state structures is estimated to be 6.6 kcal mol−1 compared with the best theoretical value, 6.4 kcal mol−1 obtained using the highly accurate coupled-cluster method with full inclusion of the triple excitations (CCSDT). The ordering of electronic states for the square geometry is determined, with the ground state singlet being located 6.9 kcal mol−1 below the lowest triplet electronic state. We also examine the potential energy surface for the interconversion between the two equivalent second-order Jahn–Teller rhombic structures for the first excited singlet state. When comparing the MRCC energies with the results provided by various single- and multireference correlation methods, the critical importance of including both the dynamic and nondynamic correlation for a qualitatively correct description of the electronic structure of cyclobutadiene is emphasized. We also address the invariance properties of the present MRCC methods with respect to the alternative selections of reference orbital spaces.

Triplet electronic states of acetylene: c i s and t r a n s structures and energetics

Molecular electronic structure theory has been used to predict the equilibrium geometries and energies of acetylene in its ground state and lowest triplet electronic states. Both double zeta (DZ) and double zeta plus polarization (DZ+P) basis sets of Gaussian functions were used, in conjunction with self-consistent-field (SCF) and large scale configuration interaction (CI) wavefunctions. The predicted 1Σ+g ground state structure is re(C–C) =1.213 Å, re(C–H) =1.066 Å, in good agreement with experiment, re(C–C) =1.203 Å, re(C–H) =1.060 Å. The first excited state of acetylene is the cis 3B2 state, predicted to lie 3.43 eV above the 1Σ+g ground state. The predicted equilibrium geometry of this cis state is re(C–C) =1.343 Å, ϑe(HCC) =127.8°, re(C–H) =1.090 Å. The three other lowest triplet states are trans 3Bu (Te=3.78 eV), trans 3Au (4.13) eV, and cis 3A2 (4.50 eV). The predicted ordering and CCH bond angles for these states are not entirely consistent with Walsh’s rules.

Vibrational populations of the excited states of N2 under auroral conditions

The equations of statistical equilibrium for the population and depopulation of the electronic states of N2 have been solved for steady state normal auroral conditions. All vibrational levels for the seven lowest triplet electronic states (A, B, W, B′, C, E, D) and the three lowest excited singlet electronic states (a, a′, w) of N2 were included in the calculations. Recently measured auroral secondary electron spectra were employed to calculate the rates for electron impact excitation. The transition probabilities, electron impact excitation cross sections, and vibrationally dependent quenching rates used were a combination of the best available experimental and theoretical data. The results of these calculations are the absolute vibrational populations of all the electronic states. Intrasystem cascading and vibrationally dependent quenching were found to be important in determining the vibrational populations in the lower singlet and triplet excited electronic states. The predicted vibrational populations for the A and B triplet states agree well with the available experimental data. These calculations predict that a fairly large density of N2 in excited metastable states is produced under auroral conditions.