Structure and photoabsorption properties of cationic alkali dimers solvated in neon clusters

We present a theoretical investigation of the structure and optical absorption of M(2)(+) alkali dimers (M=Li,Na,K) solvated in Ne(n) clusters for n=1 to a few tens Ne atoms. For all these alkali, the lowest-energy isomers are obtained by aggregation of the first Ne atoms at the extremity of the alkali molecule. This particular geometry, common to other M(2)(+)-rare gas clusters, is intimately related to the shape of the electronic density of the X (2)Σ(g)(+) ground state of the bare M(2)(+) molecules. The structure of the first solvation shell presents equilateral Ne(3) and capped pentagonal Ne(6) motifs, which are characteristic of pure rare gas clusters. The size and geometry of the complete solvation shell depend on the alkali and were obtained at n=22 with a D(4h) symmetry for Li and at n=27 with a D(5h) symmetry for Na. For K, our study suggests that the closure of the first solvation shell occurs well beyond n=36. We show that the atomic arrangement of these clusters has a profound influence on their optical absorption spectrum. In particular, the XΣ transition from the X (2)Σ(g)(+) ground state to the first excited (2)Σ(u)(+) state is strongly blueshifted in the Frank-Condon area.     

Decomposition of excited electronic state s-tetrazine and its energetic derivatives

Decomposition of excited electronic state s-tetrazine and its energetic derivatives, such as 3-amino-6-chloro-1,2,4,5-tetrazine-2,4-dioxide (ACTO), and 3,3(')-azobis (6-amino-1,2,4,5-tetrazine)-mixed N-oxides (DAATO(3.5)), is investigated through laser excitation and resonance enhanced multi photon ionization techniques. The N(2) molecule is detected as an initial product of the s-tetrazine decomposition reaction, through its two photon, resonance absorption transitions [a(") (1)Σ(g)(+) (v(') = 0) ← X (1)Σ(g)(+) (v(") = 0)]. The suggested mechanism for this reaction is a concerted triple dissociation yielding rotationally cold (～20 K) ground electronic state N(2) and 2 HCN molecules. The comparable decomposition of excited electronic state ACTO and DAATO(3.5) yields an NO product with a cold rotational (～20 K) but a hot vibrational (～1200 K) distribution. Thus, tetrazine and its substituted energetic materials ACTO and DAATO(3.5) evidence different decomposition mechanisms upon electronic excitation. N(2)O is excluded as a potential intermediate precursor of the NO product observed from these two s-tetrazine derivatives through direct determination of its decomposition behavior. Calculations at the CASMP2∕CASSCF level of theory predict a concerted triple dissociation mechanism for generation of the N(2) product from s-tetrazine, and a ring contraction mechanism for the generation of the NO product from the energetic s-tetrazine derivatives. Relaxation from S(n) evolves through a series of conical intersections to S(0), upon which surface the dissociation occurs in both mechanisms. This work demonstrates that the substituents on the tetrazine ring change the characteristics of the potential energy surfaces of the derivatives, and lead to a completely different decomposition pathway from s-tetrazine itself. Moreover, the N(2) molecule can be excluded as an initial product from decomposition of these excited electronic state energetic materials.     

1 + 1] photodissociation of CS(2)(+)(X̃2Π(g)) via the vibrationally mediated B̃2Σ(u)+ state: multichannels exhibiting and mode specific dynamics

Dissociation dynamics of CS(2)(+) vibrationally mediated via its B?(2)Σ(u)(+) state, was studied using the time-sliced velocity map imaging technique. The parent CS(2)(+) cation was prepared in its X?(2)Π(g) ground state through a [3 + 1] resonance enhanced multiphoton ionization process, via the 4pσ(3)Π(u) intermediate Rydberg state of neutral CS(2) molecule at 483.14 nm. CS(2)(+)(X?(2)Π(g)) was dissociated by a [1?+?1] photoexcitation mediated via the vibrationally selected B? state over a wavelength range of 267-283 nm. At these wavelengths the C?(2)Σ(g)(+) and D?(2)Σ(u)(+) states are excited, followed by numerous S(+) and CS(+) dissociation channels. The S(+) channels specified as three distinct regions were shown with vibrationally resolved structures, in contrast to the less-resolved structures being presented in the CS(+) channels. The average translational energy releases were obtained, and the S(+)∕CS(+) branching ratios with mode specificity were measured. Two types of dissociation mechanisms are proposed. One mechanism is the direct coupling of the C? and D? states with the repulsive satellite states leading to the fast photofragmentation. The other mechanism is the internal conversion of the C? and D? states to the B? state, followed by the slow fragmentation occurred via the coupling with the repulsive satellite states.     

Photodissociation dynamics of D2O via the B̃(1A1) electronic state

Photodissociation dynamics of D(2)O in the B?((1)A(1)) state at different photolysis wavelengths have been investigated using the D-atom Rydberg "tagging" time-of-flight (TOF) technique, in combination with a tunable vacuum ultraviolet photolysis light source. TOF spectra of the D-atom product from the D(2)O photodissociation in both parallel and perpendicular polarizations have been measured. Product kinetic energy distributions and angular distributions have been derived from these TOF spectra. From these distributions, internal state distributions of the OD product as well as the OD quantum state specific angular anisotropy parameters have been derived. Two product channels governed by distinct dissociation dynamics have been clearly observed in the B?((1)A(1)) state photodissociation: ground electronic state radical product OD(X (2)Π) + D and excited electronic state OD(A (2)Σ(+)) + D. The OD(A) + D channel proceeds via adiabatic pathway on the B?((1)A(1)) state surface, producing rovibrational excitation in the OD(A) product, while the OD(X) + D channel is generated through nonadiabatic pathway mainly via conical intersections between the B?((1)A(1)) and the X?((1)A(1)) state surfaces. Due to strong angular force induced by the conical intersections, the OD(X) product is extremely hot in the rotational excitation close to the energy limit (N ～ 50 for v = 0). However, the vibrational excitation is cold in the OD(X) product with dominant population in the ground vibrational state v = 0. Detailed experimental results at different photolysis wavelengths show that at higher energy the unstable periodic orbit, from which dissociation starts, on the B? state has stronger excitation degree of the OD internal state. The negative angular anisotropy parameters of the OD(A) products suggest that the angular forces in this adiabatic dissociation pathway from these periodic orbits have changed the original angular distribution of the D(2)O molecule excited by the B?((1)A(1))←X?((1)A(1)) parallel transition.     

Ultrafast internal conversion in ethylene. II. Mechanisms and pathways for quenching and hydrogen elimination

Through a combined experimental and theoretical approach, we study the nonadiabatic dynamics of the prototypical ethylene (C(2)H(4)) molecule upon π → π(?) excitation with 161 nm light. Using a novel experimental apparatus, we combine femtosecond pulses of vacuum ultraviolet and extreme ultraviolet (XUV) radiation with variable delay to perform time resolved photo-ion fragment spectroscopy. In this second part of a two part series, the XUV (17 eV < hν < 23 eV) probe pulses are sufficiently energetic to break the C-C bond in photoionization, or to photoionize the dissociation products of the vibrationally hot ground state. The experimental data is directly compared to excited state ab initio molecular dynamics simulations explicitly accounting for the probe step. Enhancements of the CH(2)(+) and CH(3)(+) photo-ion fragment yields, corresponding to molecules photoionized in ethylene (CH(2)CH(2)) and ethylidene (CH(3)CH) like geometries are observed within 100 fs after π → π(?) excitation. Quantitative agreement between theory and experiment on the relative CH(2)(+) and CH(3)(+) yields provides experimental confirmation of the theoretical prediction of two distinct conical intersections and their branching ratio [H. Tao, B. G. Levine, and T. J. Martinez, J. Phys. Chem. A. 113, 13656 (2009)]. Evidence for fast, non-statistical, elimination of H(2) molecules and H atoms is observed in the time resolved H(2)(+) and H(+) signals.     

Collisional quenching of OD A 2Σ+ by H2: experimental and theoretical studies of the state-resolved OD X 2Π product distribution and branching fraction

We report joint experimental and theoretical studies of outcomes resulting from the nonreactive quenching of electronically excited OD?A (2)Σ(+) by H(2). The experiments utilize a pump-probe technique to detect the OD?X (2)Π product state distribution under single collision conditions. The OD?X (2)Π products are observed primarily in their lowest vibrational state (v(") = 0) with substantially less population in v(") = 1. The OD?X (2)Π products are generated with a high degree of rotational excitation, peaking at N(") = 21 with an average rotational energy of 4600 cm(-1), and a strong propensity for populating the Π(A(')) Λ-doublet component indicative of alignment of the half-filled pπ orbital in the plane of OD rotation. Branching fraction measurements show that the nonreactive channel accounts for less than 20% of quenching outcomes. Complementary classical trajectory calculations of the postquenching dynamics are initiated from representative points along seams of conical intersections between the ground and excited-state potentials of OD(A (2)Σ(+),X (2)Π) + H(2). Diabatic modeling of the initial momenta in the dynamical calculations captures the key experimental trends: OD?X (2)Π products released primarily in their ground vibrational state with extensive rotational excitation and a branching ratio that strongly favors reactive quenching. The OD?A (2)Σ(+) + H(2) results are also compared with previous studies on the quenching of OH?A (2)Σ(+) + H(2); the two experimental studies show remarkably similar rotational energy distributions for the OH and OD?X (2)Π radical products.     

Experimental and theoretical studies of the quenching of Li(3p,4p) by N2

Quenching mechanisms of the Li3p and Li4p states in collision with the nitrogen molecule are studied by laser-induced fluorescence spectroscopy and by a quantum chemical calculation. The Li3p state is observed to be efficiently quenched to the Li3s state detected as intense 3s-->2p emission. The Li4p state is efficiently quenched to the Li4s and Li3d states detected as 4s-2p and 3d-2p emissions, respectively. The potential-energy surfaces for the Li(2s-4p)N2 states show a large number of conical intersections and avoided crossings resulting from the couplings between the ionic [Li+(N2)-] and covalent configurations. There are a large number of stable excited states, and we give here the spectroscopic constants for the lowest two stable isomers correlating to Li2p+N2.     

Decomposition of pentaerythritol tetranitrate [C(CH2ONO2)4] following electronic excitation

We report the experimental and theoretical study of the decomposition of gas phase pentaerythritol tetranitrate (PETN) [C(CH(2)ONO(2))(4)] following electronic state excitation. PETN has received major attention as an insensitive, high energy explosive; however, the mechanism and dynamics of the decomposition of this material are not clear yet. The initial decomposition mechanism of PETN is explored with nanosecond energy resolved spectroscopy and quantum chemical theory employing the ONIOM algorithm at the complete active space self-consistent field (CASSCF) level. The nitric oxide (NO) molecule is observed as an initial decomposition product from PETN at three UV excitation wavelengths (226, 236, and 248 nm) with a pulse duration of 8 ns. Energies of the three excitation wavelengths coincide with the (0-0), (0-1), and (0-2) vibronic bands of the NO A (2)Σ(+) ← X (2)Π electronic transition, respectively. A unique excitation wavelength independent dissociation channel is observed for PETN, which generates the NO product with a rotationally cold (~20 K) and a vibrationally hot (~1300 K) distribution. Potential energy surface calculations at the ONIOM(CASSCF:UFF) level of theory illustrate that conical intersections play an important role in the decomposition mechanism. Electronically excited S(1) PETN returns to the ground state through the (S(1)/S(0))(CI) conical intersection, and undergoes a nitro-nitrite isomerization to generate the NO product.     

MRSDCI STUDIES OF LOW-LYING ELECTRONIC STATES OF THE N_2F~+ MOLECULE

Ab initio electronic structure calculations are reported for low-lying electronic states,X ~1Σ~+andA ~1Π of the N_2F~+ molecule.Geometric parameters for the ground state X ~1Σ~+ are predicted by means of mul-tireference single and double excitation configuration interaction(MRSDCI)calculations with a double zeta pluspolarization(DZ+P)basis set.Vertical excitation energy for these two electronic states is determined usingMRSDCI/DZ+P calculations at the ground state equilibrium geometry.The oscillator strength for the X~1Σ+→A ~1Π transition and the radiative lifetime for the A~1Π state are calculated based on the MRSDCI wavefunc-tions.     

Photodissociation of hydrogen iodide on the surface of large argon clusters: the orientation of the librational wave function and the scattering from the cluster cage

A set of photodissociation experiments and simulations of hydrogen iodide (HI) on Arn clusters, with an average size n = 139, has been carried out for different laser polarizations. The doped clusters are prepared by a pick-up process. The HI molecule is then photodissociated by a UV laser pulse and the outgoing H fragment is ionized by resonance enhanced multiphoton ionization in a (2 + 1) excitation scheme within the same laser pulse at the wavelength of 243 nm. The measured time-of-flight spectra are transformed into hydrogen kinetic energy distributions. They exhibit a strong fraction of caged H atoms at zero-kinetic energy and peaks at the unperturbed cage exit for both spin-orbit channels nearly independent of the polarization. At this dissociation wavelength, the bare HI molecule exhibits a strict state separation, with a parallel transition to the spin-orbit excited state and perpendicular transitions to the ground state. The experimental results have been reproduced using molecular simulation techniques. Classical molecular dynamics was used to estimate the HI dopant distribution after the pick-up procedure. Subsequently, quasi-classical molecular dynamics (Wigner trajectories approach) has been applied for the photodissociation dynamics. The following main results have been obtained: (i) The HI dopant lands on the surface of the argon cluster during the pick-up process, (ii) zero-point energy plays a dominant role for the hydrogen orientation in the ground state of HI-Arn surface clusters, qualitatively changing the result of the photodissociation experiment upon increasing the number of argon atoms, and, finally, (iii) the scattering of hydrogen atoms from the cage which originate from different dissociation states seriously affects the experimentally measured kinetic energy distributions.     

Conical intersections in thymine

The mechanisms which are responsible for the radiationless deactivation of the npi* and pipi* excited singlet states of thymine have been investigated with multireference ab initio methods (the complete-active-space self-consistent-field (CASSCF) method and second-order perturbation theory with respect to the CASSCF reference (CASPT2)) as well as with the CC2 (approximated singles and doubles coupled-cluster) method. The vertical excitation energies, the equilibrium geometries of the 1npi*and 1pipi* states, as well as their adiabatic excitation energies have been determined. Three conical intersections of the S1 and S0 energy surfaces have been located. The energy profiles of the excited states and the ground state have been calculated with the CASSCF method along straight-line reaction paths leading from the ground-state equilibrium geometry to the conical intersections. All three conical intersections are characterized by strongly out-of-plane distorted geometries. The lowest-energy conical intersection (CI1) arises from a crossing of the lowest 1pipi* state with the electronic ground state. It is found to be accessible in a barrierless manner from the minimum of the 1pipi* state, providing a direct and fast pathway for the quenching of the population of the lowest optically allowed excited states of thymine. This result explains the complete diffuseness of the absorption spectrum of thymine in supersonic jets. The lowest vibronic levels of the optically nearly dark 1npi* state are predicted to lie below CI1, explaining the experimental observation of a long-lived population of dark excited states in gas-phase thymine.     

Electronic states and spin-orbit splitting of lanthanum dimer

Lanthanum dimer (La(2)) was studied by mass-analyzed threshold ionization (MATI) spectroscopy and a series of multi-configuration ab initio calculations. The MATI spectrum exhibits three band systems originating from ionization of the neutral ground electronic state, and each system shows vibrational frequencies of the neutral molecule and singly charged cation. The three ionization processes are La(2)(+) (a(2)∑(g)(+)) ← La(2) (X(1)∑(g)(+)), La(2)(+) (b(2)Π(3/2, u)) ← La(2) (X(1)∑(g)(+)), and La(2)(+) (b(2)Π(1/2, u)) ← La(2) (X(1)∑(g)(+)), with the ionization energies of 39,046, 40,314, and 40,864 cm(-1), respectively. The vibrational frequency of the X(1)Σ(g)(+) state is 207 cm(-1), and those of the a(2)Σ(g)(+), b(2)Π(3/2, u) and b(2)Π(1/2, u) are 235.7, 242.2, and 240 cm(-1). While X(1)Σ(g)(+) is the ground state of the neutral molecule, a(2)Σ(g (+) and b(2)Π(u) are calculated to be the excited states of the cation. The spin-orbit splitting in the b(2)Π(u) ion is 550 cm(-1). An X(4)Σ(g)(-) state of La(2)(+) was predicted by theory, but not observed by the experiment. The determination of a singlet ground state of La(2) shows that lanthanum behaves differently from scandium and yttrium.     

Communication: multistate quantum dynamics of photodissociation of carbon dioxide between 120 nm and 160 nm

UV absorption cross section of CO(2) is studied using high level ab initio quantum chemistry for electrons and iterative quantum dynamics for nuclear motion on interacting global full dimensional potential energy surfaces. Six electronic states-1, 2, 3(1)A(') and 1, 2, 3(1)A(")-are considered. At linearity, they correspond to the ground electronic state X?(1)Σ(g) (+) and the optically forbidden but vibronically allowed valence states 1(1)Δ(u), 1(1)Σ(u) (-), and 1(1)Π(g). In the Franck-Condon region, these states interact via Renner-Teller and conical intersections and are simultaneously involved in an intricate network of non-adiabatic couplings. The absorption spectrum, calculated for many rotational states, reproduces the distinct two-band shape of the experimental spectrum measured at 190 K and the characteristic patterns of the diffuse structures in each band. Quantum dynamics unravel the relative importance of different vibronic mechanisms, while metastable resonance states, underlying the diffuse structures, provide dynamically based vibronic assignments of individual lines.     

A three-state model for the photophysics of guanine

The nonadiabatic photochemistry of the guanine molecule (2-amino-6-oxopurine) and some of its tautomers has been studied by means of the high-level theoretical ab initio quantum chemistry methods CASSCF and CASPT2. Accurate computations, based by the first time on minimum energy reaction paths, states minima, transition states, reaction barriers, and conical intersections on the potential energy hypersurfaces of the molecules lead to interpret the photochemistry of guanine and derivatives within a three-state model. As in the other purine DNA nucleobase, adenine, the ultrafast subpicosecond fluorescence decay measured in guanine is attributed to the barrierless character of the path leading from the initially populated 1(pi pi* L(a)) spectroscopic state of the molecule toward the low-lying methanamine-like conical intersection (gs/pi pi* L(a))CI. On the contrary, other tautomers are shown to have a reaction energy barrier along the main relaxation profile. A second, slower decay is attributed to a path involving switches toward two other states, 1(pi pi* L(b)) and, in particular, 1(n(O) pi*), ultimately leading to conical intersections with the ground state. A common framework for the ultrafast relaxation of the natural nucleobases is obtained in which the predominant role of a pi pi*-type state is confirmed.     

Vibrationally mediated photodissociation of ethylene cation by reflectron multimass velocity map imaging

A new imaging technique, reflectron multimass velocity map ion imaging, is used to study the vibrationally mediated photodissociation dynamics in the ethylene cation. The cation ground electronic state is prepared in specific vibrational levels by two-photon resonant, three-photon ionization via vibronic bands of (pi, nf) Rydberg states in the vicinity of the ionization potential of ethylene, then photodissociated through the (B 2A(g)) excited state. We simultaneously record spatially resolved images of parent C2H4+ ions as well as photofragment C2H3+ and C2H2+ ions originating in dissociation from the vibronic excitations in two distinct bands, 7f 4(0)2 and 8f 0(0)0, at roughly the same total energy. By analyzing the images, we directly obtain the total translation energy distributions for the two dissociation channels and the branching between them. The results show that there exist differences for competitive dissociation pathways between H and H2 elimination from C2H4+ depending on the vibronic preparation used, i.e., on the vibrational excitation in the ground state of the cation prior to photodissociation. Our findings are discussed in terms of the possible influence of the torsional excitation on competition between direct dissociation, isomerization, and radiationless transitions through conical intersections among the numerous electronic states that participate in the dissociation.     

Theoretical investigation of the character of the electronic states of H2O along a linear dissociation path leading to OH + H

A series of MRD CI calculations is presented for the linear asymmetric dissociation of singlet states of the water molecule. The present calculations complement previous work on H₂O, providing further information on conical intersections in the higher excited states and on the nature and dissociation limits of the third ¹A' state in C_s symmetry, which is shown to correlate with the ion-pair limit (OH⁻ +H⁺). At large distances the ion-pair state is no longer the third ¹A' species as other states, corresponding to the OH(X ²Π) + H(2s,2p) dissociation limit, are then lower in energy.     

LiH分子X 1Σ+ 、A 1Σ+和B 1Π态的势能函数

The energies, equilibrium geometries and harmonic frequencies of the three electronic states (the ground state X 1Σ+, the first excitation state A 1Σ+ and the second excitation degenerate state B 1Π) of LiH molecule have been calculated by using the GSUM (Group Sum of Operators) method of SAC/ SAC-CI with the basis sets D95(d), 6-311G**, and cc-PVTZ. Comparing with the above-mentioned three basis sets, the conclusion is gained that the basis set D95(d) is the most suitable for the energy calculation of LiH molecule. The whole potential curves for these three electronic states are further scanned, using SAC/D95(d) method for the ground state and SAC-CI/D95(d) methods for the excited states. Murrell-Sorbie function were fitted using a least square and then the spectroscopy constants are calculated, which are in good agreement with the experimental data.     

Luminescence measurements of Xe(+) + N2 and Xe(2+) + N2 hyperthermal charge transfer collisions

Luminescence spectra are recorded for collisions between Xe(+)/Xe(2+) and molecular nitrogen at energies ranging from 4.5 to 316 eV in the center-of-mass frame. In the Xe(+) + N(2) collision system, evidence for luminescent charge-transfer products is only found through Xe I emission lines. The most intense features of the luminescence spectra are attributed to atomic N emissions observed above ～20 eV. Intense N(2)(+) A (2)Π(u) - X(2)Σ(g)(+) and B(2)Σ(u)(+) - X(2)Σ(g)(+) radiance is observed from Xe(2+) + N(2) collisions. The B state formation cross section decreases with collision energy until 20 eV, after which it becomes independent of impact energy with an approximate value of 3 ?(2). The cross section for N(2) (+) A (ν > 0) formation increases with energy until 20 eV, after which it remains nearly constant at ～1 ?(2). The N(2)(+) product vibrational distributions extracted from the spectra are non-Franck-Condon for both electronic product states at low collision energies. The distributions resemble a Franck-Condon distribution at the highest energies investigated in this work.     

Three-state conical intersections in nucleic acid bases

The involvement of three-state conical intersections in the photophysics and radiationless decay processes of the nucleobases has been investigated using multireference configuration interaction methods. Three-state conical intersections have been located for the pyrimidine base, uracil, and the purine base, adenine. In uracil, a three-state degeneracy between the S(0), S(1), and S(2) states has been located at 6.2 eV above the ground-state minimum energy. This energy is 0.4 eV higher than vertical excitation to S(2) and at least 1.3 eV higher than the two-state conical intersections found previously. In adenine, two different three-state degeneracies between the S(1), S(2), and S(3) states have been located at energies close to the vertical excitation energies. The energetics of these three-state conical intersections suggest they can play a role in a radiationless decay pathway present in adenine. The existence of two different seams of three-state conical intersections indicates that these features are common and complicate the potential energy surfaces of adenine and possibly many other aromatic molecules.     

Theoretical study of the photodissociation of Li(2)+ in one-color intense laser fields

A theoretical treatment of the photodissociation of the molecular ion Li(2) (+) in one-color intense laser fields, using the time-dependent wave packet approach in a Floquet Born-Oppenheimer representation, is presented. Six electronic states 1,2?(2)Σ(g)(+), 1,2?(2)Σ(u)(+), 1?(2)Π(g), and 1?(2)Π(u) are of relevance in this simulation and have been included. The dependences of the fragmental dissociation probabilities and kinetic energy release (KER) spectra on pulse width, peak intensity, polarization angle, wavelength, and initial vibrational level are analyzed to interpret the influence of control parameters of the external field. Three main dissociation channels, 1?(2)Σ(g)(+) (m = -1), 2?(2)Σ(g)(+) (m = -2), and 2?(2)Σ(u)(+) (m = -3), are seen to dominate the dissociation processes under a wide variety of laser conditions and give rise to well separated groups of KER features. Different dissociation mechanisms for the involved Floquet channels are discussed.