Ultrafast dynamics and coherent oscillations in ethylene and ethylene-d4 excited at 162 nm

The fifth harmonic (162 nm, 11 fs), generated in a short argon cell from 12 fs Ti-sapphire laser pulses, was used to excite C2H4 and C2D4 in the maximum of the first pi pi* transition. Around 10% of the molecules were excited to the pi3s Rydberg state instead. The subsequent motion of the wave packet, moving over the potentials from the Franck-Condon region down to the ground state, was monitored by nonresonant ionization at 810 nm with mass-selective detection of the ion yield. Five time constants (from approximately 20 fs in excited states to 0.6-11 ps in the hot ground state) and four coherent oscillations (CC stretch and torsion vibrations or hindered free rotation) were determined for each isotopomer. The initial relaxation follows a superposition of CC twist and stretch coordinates; this explains a surprisingly small deuterium isotope effect of the initial time constant (21 versus 24 fs). Also the vibrations in the Franck-Condon region have such a mixed character and a correspondingly small isotope shift. From the perpendicular minimum the wave packet reaches (within 17 or 21 fs for the two isotopomers) a conical intersection via a direction that also involves partial hydrogen migration. This is concluded from the detection of ethylidene (CH3CH), formed simultaneously with ground-state ethylene. This carbene isomerizes in the ground state within 0.6 ps (1.6 ps for CD3CD) to ethylene. Two time constants for dissociation (4.5 and 11 ps) in the hot ground state were also identified. The small yields of bimolecular reactions (photodimerization, addition reactions involving a "suddenly polarized" excited state, carbene reactions) are interpreted in terms of the short lifetimes. It is pointed out that the relaxation path starting from the Rydberg state merges into that from the pi pi* state; nevertheless, there is a wavelength dependence in the photochemistry of olefins, because due to a momentum effect the wave packet remembers from which state it came.     

MRD CI calculations of excited states of the SiH3 radical,including potential curves for the inversion mode and the dissociation SiH3 → SiH2 + H

Using the MRD CI method and large basis sets the vertical spectrum of silyl radical (SiH₃) has been calculated. The lowest excited state is the 4s Rydberg state, 41000 cm⁻¹ (5.2 eV) above the ground state. Only one excited valence state (2²E) was encountered, all other states are of Rydberg type. From potential curves for the inversion mode (symmetric bending motion) it was inferred that all Rydberg states are planar, whereas the valence excited state is highly pyramidalized. The investigation of the dissociation reaction SiH₃ → SiH₂ + H leads to the conclusion that the first excited state is dissociative.     

Wave packet simulation of nonadiabatic dynamics in highly excited 1,3-dibromopropane

We have conducted wave packet simulations of excited-state dynamics of 1,3-dibromopropane (DBP) with the aim of reproducing the experimental results of the gas-phase pump-probe experiment by Kotting et al. [ Kotting, C. ; Diau, E. W.-G. ; S?lling, T. I. ; Zewail, A. H. J. Phys. Chem. A 2002, 106, 7530 ]. In the experiment, DBP is excited to a Rydberg state 8 eV above the ground state. The interpretation of the results is that a torsional motion of the bromomethylene groups with a vibrational period of 680 fs is activated upon excitation. The Rydberg state decays to a valence state, causing a dissociation of one of the carbon bromine bonds on a time scale of 2.5 ps. Building the theoretical framework for the wave packet propagation around this model of the reaction dynamics, the simulations reproduce, to a good extent, the time scales observed in the experiment. Furthermore, the simulations provide insight into how the torsion motion influences the bond breakage, and we can conclude that the mechanism that delays the dissociation is solely the electronic transition from the Rydberg state to the valence state and does not involve, for example, intramolecular vibrational energy redistribution (IVR).     

Vibrational spectroscopy of the G...C base pair: experiment, harmonic and anharmonic calculations, and the nature of the anharmonic couplings

The results of harmonic and anharmonic frequency calculations on a guanine-cytosine complex with an enolic structure (a tautomeric form with cytosine in the enol form and with a hydrogen at the 7-position on guanine) are presented and compared to gas-phase IR-UV double resonance spectral data. Harmonic frequencies were obtained at the RI-MP2/cc-pVDZ, RI-MP2/TZVPP, and semiempirical PM3 levels of electronic structure theory. Anharmonic frequencies were obtained by the CC-VSCF method with improved PM3 potential surfaces; the improved PM3 potential surfaces are obtained from standard PM3 theory by coordinate scaling such that the improved PM3 harmonic frequencies are the same as those computed at the RI-MP2/cc-pVDZ level. Comparison of the data with experimental results indicates that the average absolute percentage deviation for the methods is 2.6% for harmonic RI-MP2/cc-pVDZ (3.0% with the inclusion of a 0.956 scaling factor that compensates for anharmonicity), 2.5% for harmonic RI-MP2/TZVPP (2.9% with a 0.956 anharmonicity factor included), and 2.3% for adapted PM3 CC-VSCF; the empirical scaling factor for the ab initio harmonic calculations improves the stretching frequencies but decreases the accuracy of the other mode frequencies. The agreement with experiment supports the adequacy of the improved PM3 potentials for describing the anharmonic force field of the G...C base pair in the spectroscopically probed region. These results may be useful for the prediction of the pathways of vibrational energy flow upon excitation of this system. The anharmonic calculations indicate that anharmonicity along single mode coordinates can be significant for simple stretching modes. For several other cases, coupling between different vibrational modes provides the main contribution to anharmonicity. Examples of strongly anharmonically coupled modes are the symmetric stretch and group torsion of the hydrogen-bonded NH2 group on guanine, the OH stretch and torsion of the enol group on cytosine, and the NH stretch and NH out-of-plane bend of the non-hydrogen-bonded NH group on guanine.     

Ab initio study of the VUV-induced multistate photodynamics of formaldehyde

Although formaldehyde, H?CO, has been extensively studied there are still several issues not-well understood, specially regarding its dynamics in the VUV energy range, mainly due to the amount of nonadiabatic effects governing its dynamics. Most of the theoretical work on this molecule has focused on vertical excitation energies of Rydberg and valence states. In contrast to photodissociation processes involving the lowest-lying electronic states below 4.0 eV, there is little known about the photodynamics of the high-lying electronic states of formaldehyde (7-10 eV). One question of particular interest is why the (π, π*) electronic state is invisible experimentally even though it corresponds to a strongly dipole-allowed transition. In this work we present a coupled multisurface 2D photodynamics study of formaldehyde along the CO stretching and the symmetric HCH bending motion, using a quantum time-dependent approach. Potential energy curves along all the vibrational normal modes of formaldehyde have been computed using equation-of-motion coupled cluster including single and double excitations with a quadruply augmented basis set. In the case of the CO stretching coordinate, state-averaged complete active space self-consistent field followed by multireference configuration interaction was used for large values of this coordinate. 2D (for the CO stretching coordinate and the HCH angle) and 3D (including the out-of-plane distortion) potential energy surfaces have been computed for several Rydberg and valence states. Several conical intersections (crossings between potential energy surfaces of the same multiplicity) have been characterized and analyzed and a 2D 5 × 5 diabatic model Hamiltonian has been constructed. Based on this Hamiltonian, electronic absorption spectra, adiabatic and diabatic electronic populations and vibrational densities have been obtained and analyzed. The experimental VUV absorption spectrum in the 7-10 eV energy range is well reproduced, including the vibrational structure and the high irregularity in the regime of strong interaction between the (π, π*) electronic state and neighboring Rydberg states.     

The VUV electronic spectroscopy of acetone studied by synchrotron radiation

The electronic state spectroscopy of acetone (CH3)2CO has been investigated using high-resolution VUV photoabsorption spectroscopy in the energy range 3.7-10.8 eV. New vibronic structure has been observed, notably in the low energy absorption band assigned to the 1(1)A(1) --> 1(1)A2 (ny --> pi*) transition. The local absorption maximum at 7.85 eV has been tentatively attributed to the 4(1)A1 (pi --> pi*) transition. Six Rydberg series converging to the lowest ionisation energy (9.708 eV) have been assigned as well as a newly-resolved ns Rydberg series converging to the first ionic excited state (12.590 eV). Rydberg orbitals of each series have been classified according to the magnitude of the quantum defect (delta) and are extended to higher quantum numbers than in the previous analyses.     

Ion-pair formation observed in a pulsed-field ionization photoelectron spectroscopic study of HF

The pulsed-field ionization (PFI) photoelectron (PE) spectrum of HF has been recorded at the chemical dynamics beamline of the advanced light source over the photon energy range 15.9-16.5 eV using a time-of-flight selection scheme at a resolution of 0.6 meV. Rotationally-resolved structure in the HF+(X 2 pi 3/2, 1/2, v+ = 0, 1) band systems are assigned. The spectral appearance of these systems agrees with a previous VUV laser PFI-PE study. Importantly, extensive rotationally-resolved structure between these two vibrational band systems is also observed. This is attributed to ion-pair formation via Rydberg states converging on the v+ = 1 vibrational levels of the HF+(X 2 pi 3/2, 1/2) spin-orbit states. These Rydberg states are assigned to the 1 sigma+ part of the nd-complexes (sigma, pi, and delta). Ion-pair formation is observed in this study by the detection of F- ions. Some partially rotationally-resolved structure in a previously published threshold photoelectron spectrum is similarly attributed to ion-pair formation (F- detection) through a combination of the v+ = 17 level of the (A 2 sigma+) 3s sigma Rydberg state and the (X 2 pi 3/2, 1/2, v+ = 1) 7d Rydberg states. On the basis of the present study, an accurate experimental value for the dissociation energy of the ground state of HF has been obtained, D0(HF) = 5.8650(5) eV.     

Electronic structure of the 1,3-butadiyne anion studied by electron transmission and vibrational excitation spectra in the gas phase

The electron transmission and vibrational excitation spectra of the title compound are reported. Two bands, at 1.0 and 5.6 eV are found in the ETS spectrum. The 1.0 eV resonance excites mainly the CC stretch vibration ν₂ and the CH bend vibrations ν_6,8 in even quanta. It has been assigned to the ²Π_u (in linear geometry) ground state of the anion. This state is found to be bent in the equilibrium geometry and the minimum of the anion potential surface lies outside of the Franck-Condon region. The band around 5.6 eV is interpreted as the ²Π_g resonance overlapping with a σ^* resonance which excites mainly the CH stretch vibration ν₁. A remarkable feature is the excitation of high levels of the CH bend vibration near threshold, with E_r ? 0.3 eV.     

The electronic states of 1,2,3-triazole studied by vacuum ultraviolet photoabsorption and ultraviolet photoelectron spectroscopy, and a comparison with ab initio configuration interaction methods

The Rydberg states in the vacuum ultraviolet photoabsorption spectrum of 1,2,3-triazole have been measured and analyzed with the aid of comparison to the UV valence photoelectron ionizations and the results of ab initio configuration interaction (CI) calculations. Calculated electronic ionization and excitation energies for singlet, triplet valence, and Rydberg states were obtained using multireference multiroot CI procedures with an aug-cc-pVTZ [5s3p3d1f] basis set and a set of Rydberg [4s3p3d3f] functions. Adiabatic excitation energies obtained for several electronic states using coupled-cluster (singles, doubles, and triples) and complete active space self-consistent field procedures agree well with experimental values. Variations in bond lengths with the electronic state are discussed. The lowest energy UV band (～5.5-6.5 eV) is assigned to three electronically excited states and demonstrates the occurrence of a nonplanar upper state on the low energy side. A UV photoelectron spectrum with an improved resolution yielded adiabatic and vertical ionization energies and reorganization energies for several of the lowest cationic states. As well as excitations to the s, p, d-Rydberg states are the excitations consistent with an f-series.     

Calculation of Franck-Condon factors including anharmonicity: simulation of the C2H4+X2B3u<--C2H4X1A(g) band in the photoelectron spectrum of ethylene

Our new simple method for calculating accurate Franck-Condon factors including nondiagonal (i.e., mode-mode) anharmonic coupling is used to simulate the C2H4+X2B3u<--C2H4X1A(g) band in the photoelectron spectrum. An improved vibrational basis set truncation algorithm, which permits very efficient computations, is employed. Because the torsional mode is highly anharmonic it is separated from the other modes and treated exactly. All other modes are treated through the second-order perturbation theory. The perturbation-theory corrections are significant and lead to a good agreement with experiment, although the separability assumption for torsion causes the C2D4 results to be not as good as those for C2H4. A variational formulation to overcome this circumstance, and deal with large anharmonicities in general, is suggested.     

Quantum chemical investigation of the electronic spectra of the keto, enol, and keto-imine tautomers of cytosine

The low-lying excited singlet states of the keto, enol, and keto-imine tautomers of cytosine have been investigated employing a combined density functional/multireference configuration interaction (DFT/MRCI) method. Unconstrained geometry optimizations have yielded out-of-plain distorted structures of the pi --> pi and n --> pi excited states of all cytosine forms. For the keto tautomer, the DFT/MRCI adiabatic excitation energy of the pi --> pi state (4.06 eV including zero-point vibrational energy corrections) supports the resonant two-photon ionization (R2PI) spectrum (Nir et al. Phys. Chem. Chem. Phys. 2002, 5, 4780). On its S1 potential energy surface, a conical intersection between the 1pipi state and the electronic ground state has been identified. The barrier height of the reaction along a constrained minimum energy path amounts to merely 0.2 eV above the origin and explains the break-off of the R2PI spectrum. The 1pipi minimum of the enol tautomer is found at considerably higher excitation energies (4.50 eV). Because of significant geometry shifts with respect to the ground state, long vibrational progressions are expected, in accord with experimental observations. For the keto-imine tautomer, a crossing of the 1pipi potential energy surface with the ground-state surface has been found, too. Its n --> pi minimum (3.27 eV) is located well below the conical intersection between the pi --> pi and S0 states, but it will be difficult to observe because of its small transition moment. The identified conical intersections of the pi --> pi excited states of the keto cytosine tautomers are made responsible for the ultrafast decay to the electronic ground states and thus may explain their subpicoseconds lifetimes.     

Photoelectron imaging spectroscopy and theoretical investigation of ZrSi

The photoelectron spectrum of ZrSi(-) has been measured at two different photon energies: 2.33 eV and 3.49 eV, providing electron binding energy and photoelectron angular distribution information. The obtained vertical detachment energy of ZrSi(-) is 1.584(14) eV. The neutral ground and excited state terms are assigned based on experimental and theoretical results. The ground state of ZrSi is tentatively assigned as a (3)Σ(+) state with a configuration of 1σ(2) 1π(4) 1δ(0) 2σ(1) 3σ(1). A low lying (3)Π(i) neutral excited state is identified to be 0.238 eV (1919 cm(-1)) above the ground state. The anion ground state is designated as a (2)Σ(+) state with a 1σ(2) 1π(4) 1δ(0) 2σ(2) 3σ(1) valence electron configuration. A Franck-Condon (FC) simulation of the photoelectron spectrum has been carried out. For the (3)Σ(+) ← (2)Σ(+) band, theoretically calculated bond lengths and frequencies are used in the FC calculation which give good agreement with experiment, while for the (3)Π(i) ← (2)Σ(+) band, the ZrSi bond length is estimated from the FC spectrum. Comparisons are made with previously published theoretical studies and inconsistencies are pointed out. To the best of our knowledge, this study provides the first spectroscopic information on the transition metal-silicon diatomic, ZrSi.     

A MS-CASPT2 study of the low-lying electronic excited states of CH2BrCl

The low-lying singlet excited states of CH₂BrCl have been calculated using multiconfigurational CASSCF, second-order perturbation theory CASPT2 and its multistate extension MS-CASPT2. The CASSCF method shows spurious valence–Rydberg mixing and a wrong order of states. Inclusion of dynamical correlation by single root CASPT2 lowers dramatically the energy of the valences states but does not lead to a complete separation between valence and Rydberg states. This situation is improved by the MS-CASPT2 calculations, which gives two valence states for both A^′ and A″ symmetries below the lowest Rydberg state, corresponding to n(Br)→σ^*(C–Br) and n(Cl)→σ^*(C–Cl) transitions at 6.1 eV (203 nm) and 7.2 eV (173 nm), and being repulsive along C–Br and C–Cl coordinates.     

Excited-state potential energy surface for the photophysics of adenine

The decay paths on the singlet excited-state surface of 9H-adenine and the associated energy barriers have been calculated at the CAS-PT2//CASSCF level. There are three fundamental paths for the photophysics: two paths for the (1)L(b) state which are virtually barrierless at the present level of theory and correspond to formation of the (n,pi) intermediate and direct decay to the ground state and a third path for ground-state decay of the (n,pi) state with an activation barrier of approximately 0.1 eV. The (1)L(a) state, which has the largest oscillator strength, either decays directly to the ground state or contributes indirectly to the excited-state lifetime by populating the two other states. The results are used to interpret the photophysics in terms of an excited-state plateau for the (1)L(b) state that corresponds to the short-lived excited-state component (approximately 0.1 ps) and a well (i.e., a proper minimum) for the (n,pi) state that gives rise to the long component (1 ps or more). The direct decay to the ground state of the (1)L(b) state is probably the decay channel invoked to explain the experimental wavelength dependence of the relative amplitudes of the two components. In addition to that, the excited-state component in the nanosecond range detected in the time-resolved photoelectron spectrum is proposed to be a triplet (pi,pi) state formed after intersystem crossing from the singlet (n,pi) state.     

Effects of anharmonicity on nonadiabatic electron transfer: a model

The effect of anharmonicity in the intramolecular modes of a model system for exothermic intramolecular nonadiabatic electron transfer is probed by examining the dependence of the transition probability on the exoergicity. The Franck-Condon factor for the Morse potential is written in terms of the Gauss hypergeometric function both for a ground initial state and for the general case, and comparisons are made between the first-order perturbation theory results for transition probability for harmonic and Morse oscillators. These results are verified with quantum dynamical simulations using wave-packet propagations on a numerical grid. The transition-probability expression incorporating a high-frequency quantum mode and low-frequency medium mode is compared for Morse and harmonic oscillators in different temperature ranges and with various coarse-graining treatments of the delta function from the Fermi golden rule expression. We find that significant deviations from the harmonic approximation are expected for even moderately anharmonic quantum modes at large values of exoergicity. The addition of a second quantum mode of opposite displacement negates the anharmonic effect at small energy change, but in the inverted regime a significantly flatter dependence on exoergicity is predicted for anharmonic modes.     

Experimental matrix isolation study and quantum-mechanics-based normal-coordinate analysis of the anharmonic infrared spectrum of picolinic acid N-oxide

This work is, according to our knowledge, the first experimental matrix isolation study of a molecular system with a very short and strong intramolecular OH...O hydrogen bond. It also includes a satisfying interpretation of its entire infrared spectrum. The interpretation relies on the calculation at the DFT/B3LYP/6-31G(d,p) level of the harmonic spectrum and of the anharmonic relaxed potential energy for the stretching motion of the hydrogen-bonded proton, used with our recently modified quantum-mechanics-based normal-coordinate analysis. An important observation about the anharmonic spectrum obtained from this procedure is that the OH stretch coordinate contributes to several normal modes, mixing extensively with other in-plane internal coordinates, in particular OH-bending and C=O-stretching. The two intense normal modes with the largest contributions from the OH-stretching coordinate to the potential energy distribution and to the intensity are located near 1700 and 1500 cm(-1). A calculated anharmonic spectrum obtained from this procedure agrees with the experimental spectrum (frequencies and intensity distribution), within the limits of the estimated uncertainties for the calculation and experiment, allowing the interpretation of the latter. The agreement for the frequencies is about 1-3%. The anharmonic spectrum calculated using the anharmonic keyword in Gaussian 03w is not in satisfactory agreement with experiment insofar as the OH-stretching mode is concerned.     

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

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

Electronic spectra and photodissociation of vinyl chloride: a symmetry-adapted cluster configuration interaction study

The vertical absorption spectrum and photodissociation mechanism of vinyl chloride (VC) were studied by using symmetry-adapted cluster configuration interaction theory. The important vertical pi --> pi* excitation was intensively examined with various basis sets up to aug-cc-pVTZ augmented with appropriate Rydberg functions. The excitation energy for pi --> pi* transition obtained in the present study, 6.96 eV, agrees well with the experimental value, 6.7-6.9 eV. Calculated excitation energies along with the oscillator strengths clarify that the main excitation in VC is the pi --> pi* excitation. Contrary to the earlier theoretical reports, the results obtained here support that the C-Cl bond dissociation takes place through the n(Cl-)sigma(C-Cl)* state.     

The fluorescence mechanism of 5-methyl-2-pyrimidinone: an ab initio study of a fluorescent pyrimidine analog

The photophysically important potential energy surfaces of the fluorescent pyrimidine analog 5-methyl-2-pyrimidinone have been explored using multireference configuration-interaction ab initio methods at three levels of dynamical correlation, all of which support a fluorescence mechanism. At vertical excitation S1 (dark, n(N)pi*) and S2 (bright, pipi*) are almost degenerate at 4.4 eV, with S3 (dark, n(O)pi*) at 5.1 eV. The excited system can follow the S1-S2 seam of conical intersections, accessible from the Franck-Condon region, to its minimum and then evolve from this conical intersection on the S1 (pipi*) surface to a global minimum. At lower levels of correlation, the S1 surface shows two minima separated by a barrier of up to 0.18 eV. The secondary minimum found at the lower levels of correlation becomes the global minimum with higher correlation. The S1 population at this minimum can be trapped from accessing the lowest energy S0-S1 (pipi*/gs) conical intersection by an energy gap at least 0.3-0.4 eV higher than the S1 minimum. The calculated emission energy from this minimum is 2.80 eV. Gradient pathways connecting important S1 geometries are presented, as well as other excited state conical intersections.     

6-N,N-二甲基腺嘌呤激发态结构动力学的共振拉曼光谱

采用共振拉曼光谱技术和密度泛函理论方法研究了6-N,N-二甲基腺嘌呤(DMA)的A带和B带电子激发和Franck-Condon 区域结构动力学. π_H→π_L^*跃迁是A带吸收的主体, 其振子强度约占整个A带吸收的79%.由弥散轨道参与的n→Ryd 和π_H→Ryd 跃迁在B带跃迁中扮演重要角色, 其振子强度约占B带吸收的62%,而在A带吸收中占主导的π_H→π_L^*跃迁的振子强度在B带吸收中仅占33%. 嘌呤环变形伸缩+C8H/N9H面内弯曲振动ν₂₃和五元环变形伸缩+C8H弯曲振动ν₁₃的基频、泛频和合频占据了A带共振拉曼光谱强度的绝大部分, 说明¹π_Hπ_L^*激发态结构动力学主要沿嘌呤环的变形伸缩振动, N9H/C8H/C2H弯曲振动等反应坐标展开, 而ν₁₀, ν₂₉, ν₂₁, ν₂₆和ν₄₀的基频、泛频和合频占据了B带共振拉曼光谱强度的主体部分, 它们决定了B带激发态的结构动力学. A带共振拉曼光谱中ν₂₆和ν₁₂被认为与1nπ*/1ππ*势能面锥型交叉有关. B带共振拉曼光谱中ν₂₁的激活与¹ππ^*/¹πσ_N9H^*势能面锥型交叉相关.