Electronic states of F2CO as studied by electron energy-loss spectroscopy and ab initio calculations

This paper reports on the first measurements of the electron impact electronic excitation cross-sections for carbonyl fluoride, F(2)CO, measured at 30 eV, 10° and 100 eV, 5° scattering angle, while sweeping the energy loss over the range 5.0-18.0 eV. The electronic-state spectroscopy has been investigated and the assignments are supported by quantum chemical calculations. The energy bands above 9.0 eV and the vibrational progressions superimposed upon it have been observed for the first time. Vibronic coupling has been shown to play an important role dictating the nature of the observed excited states, especially for the low-lying energy region (6.0-8.0 eV). New experimental evidence for the 6(1)B(2) state proposed to have its maximum at 12.75 eV according to the vibrational excitation reported in this energy region (11.6-14.0 eV). The n = 3 members of the Rydberg series have been assigned converging to the lowest ionization energy limits, 13.02 eV ((2)B(2)), 14.09 eV ((2)B(1)), 16.10 ((2)B(2)), and 19.15 eV ((2)A(1)) reported for the first time and classified according to the magnitude of the quantum defects (δ).     

Multiconfigurational perturbation theory (CASPT2) applied to the study of the low-lying singlet and triplet excited states of cyclopropene

The electronic spectrum of cyclopropene has been studied using multiconfigurational second-order perturbation theory (CASPT2) with extended ANO-type basis sets. The calculation comprises two valence states and the 3s, 3p, 3d members of the Rydberg series converging to the π and σ ionization limits. A total of twenty singlet and twenty triplet excited states have been analyzed. The results confirm the valence nature of the lowest energy singlet-singlet band and yield a conclusive assignment: the first dipole-allowed transition in cyclcopropene is due to absorption to a (σ → π*) state. The (π → π*) (V) state is interleaved among a number of Rydberg states in the most intense band of the system. The remaining spectral bands are due to Rydberg transitions of higher energy. The two lowest singlet-triplet transitions involve the same valence states. The results are in agreement with available experimental data and provide a number of new assignments of the experimental spectra.     

The benzophenone S1(n,pi*) --> T1(n,pi*) states intersystem crossing reinvestigated by ultrafast absorption spectroscopy and multivariate curve resolution

The well-known benzophenone intersystem crossing from S(1)(n,pi*) to T(1)(n,pi*) states, for which direct transition is forbidden by El-Sayed rules, is reinvestigated by subpicosecond time-resolved absorption spectroscopy and effective data analysis for various excitation wavelengths and solvents. Multivariate curve resolution alternating least-squares analysis is used to perform bilinear decomposition of the time-resolved spectra into pure spectra of overlapping transient species and their associated time-dependent concentrations. The results suggest the implication of an intermediate (IS) in the relaxation process of the S(1) state. Therefore, a two step kinetic model, S(1) --> IS --> T(1), is successfully implemented as an additional constraint in the soft-modeling algorithm. Although this intermediate, which has a spectrum similar to the one of T(1)(n,pi*) state, could be artificially induced by vibrational relaxation, it is tentatively assigned to a hot T(1)(n,pi*) triplet state. Two characteristic times are reported for the transition S(1) --> IS and IS --> T(1), approximately 6.5 ps and approximately 10 ps respectively, without any influence of the solvent. Moreover, an excitation wavelength effect is discovered suggesting the participation of unrelaxed singlet states in the overall process. To go further discussing the spectroscopic relevancy of IS and to rationalize the expected involvement of the T(2)(pi,pi*) state, we also investigate 4-methoxybenzophenone. For this neighboring molecule, triplet energy level is tunable through solvent polarity and a clear correlation is established between the intermediate resolved by multivariate data analysis and the presence of a T(2)(pi,pi*) above the T(1)(n,pi*) triplet. It is therefore proposed that the benzophenone intermediate species is a T(1)(n,pi*) high vibrational level in interaction with T(2)(pi,pi*) state.     

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 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.     

Autoionization and neutral dissociation of superexcited HI studied by two-dimensional photoelectron spectroscopy

Two-dimensional photoelectron spectroscopy of hydrogen iodide (HI) has been performed in the photon energy region of 11.10-14.85 eV, in order to investigate dynamical properties on autoionization and neutral dissociation of Rydberg states HI*(RA) converging to HI+(A 2Sigma1/2(+)). A two-dimensional photoelectron spectrum exhibits strong vibrational excitation of HI+(X 2Pi) over a photon energy region from approximately 12 to 13.7 eV, which is attributable to the autoionizing feature of the 5 dpi HI*(RA) state. A noticeable set of stripes in the photon energy region of 13.5-14.5 eV is assigned as resulting from autoionization of the atomic Rydberg states of I* converging to I+ (3P0 or 3P1). The formation of I* is understood in terms of predissociation of multiple HI*(RA) states by way of the repulsive Rydberg potential curves converging to HI+(4Pi1/2).     

Experiments and quantum-chemical calculations on Rydberg states of H2CS in the region 5.6-9.5 eV

Absorption spectrum of H(2)CS in the region 5.6-9.5 eV was recorded with a continuously tunable light source of synchrotron radiation. After we subtracted absorption bands of CS(2), our spectrum clearly shows vibrational progressions associated with transitions (1)A(1)(pi,pi*)-X (1)A(1) and (1)B(2)(n,4s)-X (1)A(1) in the region 5.6-6.7 eV. A spectrum from which absorption of C(2)H(4) and CS(2) are subtracted shows several discrete bands in the region 6.9-9.5 eV. A Rydberg state (1)B(2)(n,4p(z)) lying below Rydberg state (1)A(1)(n,4p(y)) is confirmed, and the C-H symmetric stretching (nu(1)) and CH out-of-plane bending (nu(4)) modes for a transition (1)B(2)(n,4s)-X (1)A(1) are identified. New transitions to Rydberg states associated with excitation to 5s-11s, 5p(z)-7p(z), 5p(y)-7p(y), and 3d-6d are identified based on quantum defects and comparison with vertical excitation energies predicted with time-dependent density-functional theory (TD-DFT) and outer-valence Green's-function (OVGF) methods. For lower excited states predictions from these TD-DFT6-31+G calculations agree satisfactorily with experimental values, but for higher Rydberg states the OVGF method using aug-cc-pVTZ basis set augmented with extra diffuse functions yields more accurate predictions of excitation energies.     

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.     

Absolute electronic excitation cross sections for low-energy electron (5-12 eV) scattering from condensed thymine

The absolute cross sections for electronic excitations of thymine by electron impact between 5 and 12 eV are determined by means of electron-energy loss (EEL) spectroscopy for the molecule deposited at submonolayer coverage on an inert Ar substrate. The lowest EEL features at 3.7 and 4.0 eV are attributed to the excitation of the triplet 1 3A'(pi --> pi*) and 1 3A'(n --> pi*) valence states of the molecule. The higher EEL features located at 4.9, 6.3, 7.3, and 9 eV with a weak shoulder around 6 eV are ascribed mostly to triplet valence (pi --> pi*) excitation manifold of the molecule. The energy dependence of the cross section for both the lowest triplet valence excitations shows essentially a peak at about 5 eV reaching a value of 2.9 x 10(-17) cm2. The cross sections for the higher EEL features are generally characterized by a common broad maximum around 8 eV. The latter reaches a value of 1.36 x 10(-16) cm2 for the combined 6 and 6.3 eV excitation region. The maxima in the present cross sections are found to correspond to the resonances that have been reported at about the same energies in the O- yield from electron impact on thymine in the gas phase.     

Electronic and cationic states of 2-methylpropene (isobutene) studied by VUV absorption,scattered electron spectroscopy and AB initio multireference configuration interaction calculations

VUV (6.2–9 eV) and electron scattering spectra (1–9 eV) have been recorded for 2-methylpropene (isobutene). Also, electronic states of the molecule, including the ground state and cationic states, have been investigated using ab initio multi-reference configuration interaction calculations. Some Koopmans-type in the UV photoelectron spectrum are reassigned and a number of shake-up states computed. In the electronic spectrum, Rydberg excited have been assigned and a second valence excited state (σ π*) located within about 1 eV of the V(ππ*) state. The experiments show, and theory confirms, that the Rydberg R(π3s) state has a positive electron affinity. Some interesting correlations between ionisation energies, energies of shake-up state electronic excitation energies are identified.     

Photophysical processes in quinoxaline vapor at low pressure

Excitation and pressure dependence of fluorescence and phosphorescence quantum yields has been reinvestigated in detail for quinoxaline in the static vapor phase at pressure range from 10(-3) to 10(-1) Torr. It is shown that the ratio of the nonradiative rate from T(1)(pi, pi*) to the rate of the S(1)(n, pi*) approximately -->T(1)(pi, pi*) intersystem crossing decreases with increasing the excitation energy in the S(0)-->S(1) excitation region. The phosphorescence quantum yield measured as a function of the excitation energy at low pressure shows an abrupt decrease on going the excitation from S(0)-->S(1) to S(0)-->S(2), indicating the slow vibrational energy redistribution between the S(1) levels optically populated and those populated through the internal conversion from S(2) to S(1).     

Rydberg series in the absorption spectra of H2O and D2O in the vacuum ultraviolet

The photoabsorption cross section of molecular H₂O and D₂O has been determined in the range from hν = 10eV to 20 eV with 0.03 Å resolution. A refined analysis of the Rydberg series including the rotational line shapes of several bands to locate the band origins and a comparison with recent ab initio calculations is given. In the region of continuous absorption we have assigned a p-type and an s-type Rydberg series leading to the ²A₁ and the ²B₂ state respectively with quantum defects δ ≈ 0.75 and δ ≈ 1.36.     

Ab initio calculations of triplet excited states and potential-energy surfaces of vinyl chloride: insights into spectroscopy and photodissociation dynamics

The equilibrium geometries and harmonic vibrational frequencies of three low-lying triplet excited states of vinyl chloride have been calculated using the state-averaged complete active space self-consistent field (CASSCF) method with the 6-311++G(d,p) basis set and an active space of four electrons distributed in 13 orbitals. Both adiabatic and vertical excitation energies have been obtained using the state-averaged CASSCF and the multireference configuration-interaction methods. The potential-energy surfaces of six low-lying singlet states have also been calculated. While the 3(pi, pi*) state has a nonplanar equilibrium structure, the 3(pi, 3s) and 3(pi, sigma*) states are planar. The calculated vertical excitation energy of the 3(pi, pi*) state is in agreement with the experiment. The singlet excited states are found to be multiconfigurational, in particular, the first excited state is of (pi, 3s) character at the planar equilibrium structure, of (pi, sigma*) as the C-Cl bond elongates, and of (pi, pi*) for highly twisted geometries. Avoided crossings are observed between the potential-energy surfaces of the first three singlet excited states. The absorption spectra of vinyl chloride at 5.5-6.5 eV can be unambiguously assigned to the transitions from the ground state to the first singlet excited state. The dissociation of Cl atoms following 193-nm excitation is concluded to take place via two pathways: one is through (pi, sigma*) at planar or nearly planar structures leading to fast Cl atoms and the other through (pi, pi*) at twisted geometries from which internal conversion to the ground state and subsequent dissociation produces slow Cl atoms.     

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.     

Ion-pair dissociation dynamics of Cl2: adiabatic state correlation

The ion-pair dissociation dynamics of Cl2 -->(XUV) Cl(-)((1)S0) + Cl(+)((3P(2,1,0)) in the range 12.41-12.74 eV have been studied employing coherent extreme ultraviolet (XUV) radiation and the velocity map imaging) method. The ion-pair yield spectrum has been measured, and 72 velocity map images of Cl(-)((1)S0) have been recorded for the peaks in the spectrum. From the images, the branching ratios among the three spin-orbit components Cl(+)((3)P2), Cl(+)((3)P1) and Cl(+)((3)P0) and their corresponding anisotropic parameters beta have been determined. The ion-pair dissociation mechanism is explained by predissociation of Rydberg states converging to ion-core Cl2(+)(A(2)Pi(u)). The Cl(-)((1)S0) ion-pair yield spectrum has been assigned based on the symmetric properties of Rydberg states determined in the imaging experiments. The parallel and perpendicular transitions correspond to the excitation to two major Rydberg series, [A(2)Pi(u)]3d pi(g), (1)Sigma(u)(+) and [A(2)Pi(u)]5s sigma(g), (1)Pi(u), respectively. For the production of Cl(+)((3)P0), it is found that all of them are from parallel transitions. But for Cl(+)((3)P1), most of them are from perpendicular transitions. The production of Cl(+)((3)P2) is the major channel in this energy region, and they come from both parallel and perpendicular transitions. It is found that for most of the predissociations the projection of the total electronic angular momentum on the molecular axis (Omega) is conserved. The ion-pair dissociation may be regarded as a probe for the symmetric properties of Rydberg states.     

The lowest excited triplet (T1) energies of p-methoxybenzaldehyde and p-cyanobenzaldehyde vapors estimated from the temperature dependence of the T2(n,pi*) phosphorescence and the S1(n,pi*) fluorescence spectra

Invisible energy levels of the T1(pi, pi*) state of p-methoxybenzaldehyde (anisaldehyde) and p-cyanobenzaldehyde vapors have been estimated through the temperature dependence of the T2(n, pi*) --> S0 phosphorescence and the S1(n, pi*) --> S0 delayed fluorescence spectra. It is shown that the T1(pi, pi*) levels are located at 900 +/- 100 and 300 +/- 100 cm(-1) below the T2(n, pi*) levels, respectively, for p-methoxybenzaldehyde and p-cyanobenzaldehyde vapors. The estimated T1 energy levels are in good agreement with the phosphorescence origins in rigid glass at 77 K.     

Vibronic absorption, fluorescence, and phosphorescence spectra of psoralen: a quantum chemical investigation

Excited state potential energy hypersurfaces of 7H-furo[3,2-g][1]benzopyran-7-one (psoralen) have been explored employing (time-dependent) Kohn-Sham density functional theory. At selected points, we have determined electronic excitation energies and electric dipole (transition) moments utilizing a combined density functional/multireference configuration interaction method. Spin-orbit coupling has been taken into account employing an efficient, non-empirical spin-orbit mean-field Hamiltonian. Franck-Condon factors have been computed for vibrational modes with large displacements in the respective Dushinsky transformations. The simulated band spectra closely resemble experimental band shapes and thus validate the theoretically determined nuclear structures at the S(0), S(1), and T(1) minima. In the S(1) (pi(HOMO)-->pi*(LUMO)) state, the lactone bond of the pyrone ring is significantly elongated. From excited vibrational levels of the S(1) state a conical intersection between a (pi-->sigma*) excited state and the electronic ground state may be energetically accessible. Fast non-radiative decay via this relaxation pathway could explain the low fluorescence quantum yield of psoralen. The T(1) (pi(HOMO-1)-->pi*(LUMO)) exhibits a diradicaloid electronic structure with a broken C(5)-C(6) double bond in the pyrone ring. A variational multireference spin-orbit configuration interaction procedure yields a phosphorescence lifetime of 3 s, in excellent agreement with experimental estimates.     

First principles simulation of the UV absorption spectrum of ethylene using the vertical Franck-Condon approach

A new method which we refer to as vertical Franck-Condon is proposed to calculate electronic absorption spectra of polyatomic molecules. In accord with the short-time picture of spectroscopy, the excited-state potential energy surface is expanded at the ground-state equilibrium geometry and the focus of the approach is more on the overall shape of the spectrum and the positions of the band maxima, rather than the precise position of the 0-0 lines. The Born-Oppenheimer approximation and the separability of the excited-state potential energy surface along the excited-state normal mode coordinates are assumed. However, the potential surface is not necessarily approximated as harmonic oscillator potentials along the individual normal modes. Instead, depending upon the nature of the potential surface along a particular normal mode, it is treated either in the harmonic approximation or the full one-dimensional potential is considered along this mode. The vertical Franck-Condon approach is applicable therefore even in cases where the excited state potential energy surface is highly anharmonic and the conventional harmonic Franck-Condon approach is inadequate. As an application of the method, the ultraviolet spectrum of ethylene between 6.2 eV (50,000 cm(-1)) and 8.7 eV (70,000 cm(-1)) is simulated, using the Similarity Transformed Equation of Motion Coupled-Cluster method to describe the required features of the potential energy surfaces. The spectrum is shown to be a result of sharp doublet structures stemming from the pi --> 3s (Rydberg) state superimposed on top of a broad band resulting from the pi --> pi* (valence) state. For the Rydberg state, the symmetric C=C stretch and the torsion mode contribute to the spectrum, while the broad valence band results from excitation into the C=C stretch, CH2 scissors, and the torsion mode. For both states, the potential along the torsion mode is highly anharmonic and the full treatment of the potential along this mode in the vertical Franck-Condon method is required.     

A complete active space self-consistent field study of the photochemistry of nitrosamine

Photodissociation mechanisms of nitrosamine (NH2NO) have been studied at the complete active space self-consistent field level of theory in conjunction with atomic-natural-orbital-type basis sets. In addition, the energies of all the critical points and the potential energy curves connecting them have been recomputed with the multiconfigurational second-order perturbation method. Ground state minimum of nitrosamine has a C1 nonplanar structure with the hydrogen atoms of the amino moiety out of the plane defined by the N-N-O bonds. Electronic transitions to the three lowest states are allowed by selection rules: (i) S0-->S3 (7.41 eV) has an oscillator strength of f=0.0006 and it is assigned as an (npO)0-->(piNO*)2 transition, (ii) S0-->S2 (5.86 eV) has an oscillator strength of f=0.14 and it is assigned as an npN-->piNO* transition, and (iii) S0-->S1 (2.98 eV) has an oscillator strength of f=0.002 and it is assigned as an npO-->piNO* transition. It is found that N-N bond cleavage is the most likely process in all the photochemical relevant states, namely, S1 (1 1A"), S2 (2 1A'), and T1 (1 3A"). While S1 and T1 yield exclusively homolytic dissociation: NH2NO-->NH2 (1 2B1)+NO(X 2Pi), on S2 the latter process constitutes the major path, but two additional minor channels are also available: adiabatic homolytic dissociation: NH2NO-->NH2 (1 2A1)+NO(X 2Pi), and adiabatic oxygen extrusion: NH2NO-->NH2N (1 3A1)+O(3P). The excited species NH2 (1 2A1) experiences a subsequent ultrafast decay to the ground state, the final products in all cases the fragments being in their lowest electronic state. We have not found a unimolecular mechanism connecting excited states with the ground state. In addition, homolytic dissociation in the ground state, tautomerizations to NHNOH and NHNHO, and intersystem crossings to T1 are considered. The most favorable process on this state is the isomerization to NHNOH.