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We present a comprehensive revisited experimental high-resolution vacuum ultraviolet (VUV) photoabsorption spectrum of ammonia, NH3, covering for the first time the full 5.4–10.8 eV energy-range, with absolute cross sections determined. The calculations on the vertical excitation energies and oscillator strengths were performed using the equation-of-motion coupled cluster method restricted to single and double excitation levels and used to help reanalyze the observed Rydberg structures in the photoabsorption spectrum. The VUV spectrum reveals several new features that are not previously reported in the literature, with particular reference to the vibrational progressions of the ( D ̃ 1 E ′ ← X ̃ 1 A 1 ′ ), the ( F ̃ 1 E ′ ← X ̃ 1 A 1 ′ ), and the ( G ̃ 1 A 2 ″ ← X ̃ 1 A 1 ′ ) absorption bands. In addition, new Rydberg members have been identified in n d a 1 ′ ← 1 a 2 ″ D ̃ ′ ′ 1 A 2 ″ ← X ̃ 1 A 1 ′ , where n > 3 has not been reported before as well as in n d e ″ ← 1 a 2 ″ F ̃ 1 E ′ ← X ̃ 1 A 1 ′ and in n s a 1 ′ ← 1 a 2 ″ G ̃ 1 A 2 ″ ← X ̃ 1 A 1 ′ . The measured absolute photoabsorption cross sections have been used to calculate the photolysis lifetime of ammonia in the Earth's atmosphere (0–50 km). [ABSTRACT FROM AUTHOR]

Electron energy loss spectra of carbon tetrafluoride, silicon tetrafluoride, and germanium tetrafluoride molecules (CF4, SiF4, and GeF4) have been measured for incident electron energies of 50-360 eV at 1.5°-15.5° and for 30 eV and 30° scattering angle, while sweeping the energy loss over the range 9.0-20.0 eV. Low-lying valence excited triplet and singlet states are investigated by quantum chemical ab initio calculations. The Rydberg series converging to the (lowest) ionisation energy limits of XF4 (X = C, Si, Ge) are also identified and classified using the systematic behaviour according to the magnitude of the quantum defects. A generalized oscillator strength analysis is employed to derive oscillator strength f 0 value and the apparent Born integral cross sections from the corresponding differential cross sections by using the Vriens formula for the optically allowed transitions. The f 0 value is compared with the optical oscillator strength of the photoabsorption, pseudo-photon measurements, and theoretical values. The binary-encounter and f -scaled Born cross sections of the most intense optically allowed transitions have been also derived from the excitation threshold to the high energy region where the Born approximation is valid. Potential energy curves were obtained along the XF3 + F coordinate with two different basis sets to lend support on electron impact dissociation processes yielding radical formation. We found that in CF4, the lowest-lying dissociative character is due to intramolecular conversion from Rydberg 3s to valence character (σ*(C-F)), whereas in SiF4 and GeF4, an antibonding behaviour prevails. [ABSTRACT FROM AUTHOR]

We present the experimental high-resolution vacuum ultraviolet (VUV) photoabsorption spectra of phenol covering for the first time the full 4.3-10.8 eV energy-range, with absolute cross sections determined. Theoretical calculations on the vertical excitation energies and oscillator strengths were performed using time-dependent density functional theory and the equation-of-motion coupled cluster method restricted to single and double excitations level. These have been used in the assignment of valence and Rydberg transitions of the phenol molecule. The VUV spectrum reveals several new features not previously reported in the literature, with particular reference to the 6.401 eV transition, which is here assigned to the 3sσ/σ*(OH) ← 3π(3a") transition. The measured absolute photoabsorption cross sections have been used to calculate the photolysis lifetime of phenol in the earth's atmosphere (0-50 km). [ABSTRACT FROM AUTHOR]

The highest resolution vacuum ultraviolet photoabsorption spectrum of ethyl formate, C2H5OCHO, yet reported is presented over the wavelength range 115.0-215.5 nm (10.15-4.5 eV) revealing several new spectral features. Valence and Rydberg transitions and their associated vibronic series, observed in the photoabsorption spectrum, have been assigned in accordance with new ab initio calculations of the vertical excitation energies and oscillator strengths. Calculations have also been carried out to determine the ionization energies and fine structure of the lowest ionic state of ethyl formate and are compared with a newly recorded He(I) photoelectron spectrum (from 10.1 to 16.1 eV). New vibrational structure is observed in the first photoelectron band. The photoabsorption cross sections have been used to calculate the photolysis lifetime of ethyl formate in the upper stratosphere (20-50 km). [ABSTRACT FROM AUTHOR]

We report on measurements of differential cross sections (DCSs) for electron impact excitation of a series of Rydberg electronic-states in α-tetrahydrofurfuryl alcohol (THFA). The energy range of these experiments was 20-50 eV, while the scattered electron was detected in the 10°-90° angular range. There are currently no other experimental data or theoretical computations against which we can directly compare the present measured results. Nonetheless, we are able to compare our THFA DCSs with earlier cross section measurements for Rydberg-state electronic excitation for tetrahydrofuran, a similar cyclic ether, from Do et al. [J. Chem. Phys. 134, 144302 (2011)]. In addition, "rotationally averaged" elastic DCSs, calculated using our independent atom model with screened additivity rule correction approach are also reported. Those latter results give integral cross sections consistent with the optical theorem, and supercede those from the only previous study of Milosavljević et al. [ABSTRACT FROM AUTHOR]

We present a detailed study of the potential energy surfaces of the water dication correlating asymptotically with O(3P) and O(1D). Using ab initio multireference configuration interaction method, we computed a large ensemble of data, which was used to generate a fit of each potential energy surface for bending angles θ≥80° degrees and OH distances ROH≥1.0 a.u. The fit is used to investigate the dissociation dynamics along each potential energy surface for several initial geometries corresponding to Franck–Condon transition from neutral or singly ionized water molecule. For each case, we determine the dissociation channels and we compute the kinetic energy release and angular momentum distribution of the final arrangements. Among the eight potential energy surfaces investigated here, only the lowest triplet and the three lowest singlet can lead to the formation of bound residual fragment. The dissociation of HOD2+ presents a strong preference for OH rather than OD bond breakage. It is characterized by the isotopic ratio, defined as the number of OD+ over the number of OH+ residual fragments. This ratio depends strongly on the shape of each potential energy surface and on the initial conditions. [ABSTRACT FROM AUTHOR]

The photodissociation dynamics of a HX (X=Cl,F) molecule adsorbed on a hexagonal ice surface at T=0 K is studied using time-dependent quantum wave packets and quasiclassical trajectories. The relevant potential energy surfaces are calculated using high-level ab initio methods. We present here two dimensional calculations for the dynamics of the hydrogen photofragment for both HCl and HF molecules. The purpose of this paper is to compare the photodissociation dynamics of the two molecules which are adsorbed on the ice surface with different equilibrium geometries. The total photodissociation cross section and the angular distribution are calculated. The comparison with classical trajectory calculations provides evidence for typical quantum effects and reveals rainbow structures. [ABSTRACT FROM AUTHOR]

The K-shell spectra of gaseous furan have been measured using the inner-shell electron energy loss spectroscopy (ISEELS) method at the carbon and oxygen thresholds. Large-scale ab initio configuration interaction calculations have been carried out in order to help in the assignments of the observed bands. The spectra are close to previous low resolution ones obtained using ISEELS in the gas phase and photoabsorption with the synchrotron radiation in gaseous and condensed phases. The presence of a new feature located at 287.3 eV in the C1s spectrum and recently detected by photoabsorption with synchrotron radiation is confirmed. At both edges, the calculations confirm the assignments proposed in earlier experimental works, with the exception of several C1s pre-edge features for which a new interpretation is given. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The carbon and oxygen K-shell spectra of gaseous 2-propenal (acrolein) have been measured using the inner-shell electron energy loss spectroscopy method. Large scale ab initio configuration interaction calculations have been carried out to enable firm assignments of the observed bands. The overall shapes of the spectra are similar to previous low resolution monolayer and multilayer phases NEXAFS spectra recorded by photoabsorption of synchrotron radiation, but the spectral bands are much better resolved than the earlier ones. The spectra are dominated by excitation of π[sup *] type states and by interaction between the C = C and C = O π[sup *] orbitals. [ABSTRACT FROM AUTHOR]

The electronic transitions and resonance-enhanced vibrational excitations of octafluorocyclopentene (c-[C.sub.5][F.sub.8]) are examined by using high-resolution photoabsorption spectroscopy. The photoabsorption cross sections are used for deriving the local lifetime in the Earth's atmosphere, indicating that octafluorocyclopentene might have a short residence time in the upper atmosphere but a longer residential time in the lower stratosphere, leading to a significant global warming potential (GWP).

Studies the core excitation of pyridine and pyridazine. Inner-shell electron-energy loss spectra obtained with a vibrating sample magnetometer spectrometer; Computational method used in the study; Electronic configuration of pyridine in its neutral ground; Carbon K-shell electron-energy loss spectrum; Nitrogen K-shell electron-energy loss spectrum.

The chloromethane cation and dication have been studied using the complete active space self-consistent field method followed by a multireference perturbative configuration interaction. The 2E ground state of CH3Cl+ presents a weak Jahn–Teller distortion, the 2A′ and 2A″ components being separated by only 3.4 kcal/mol with very similar geometries. Both states are equilibrium structures. Contrary to previous calculations, the lowest singlet and triplet states of CH3Cl++ are calculated to be stable. While the 3A2 state keeps the neutral C3v symmetry, the 1E singlet state shows a large Jahn–Teller effect, with a splitting of 32.7 kcal/mol in favor of the 1A′ state, which is the ground state of the molecule. The planar 1A′ state of CH2ClH++ is found to be the most stable isomer, lying 41.4 kcal/mol below the corresponding state of CH3Cl++. The 3A″ state, which lies 22.59 kcal/mol above the 1A′ state, has a nonplanar Cs-symmetry geometry. Finally, the CH3Cl++ (1A′)→CH2ClH++ (1A′) isomerization takes place via a C1-symmetry transition state, with an energy barrier of 32.85 kcal/mol. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The C2H++2 fragmentation processes have been studied using the complete active space self-consistent field method followed by a multireference perturbative configuration interaction, in order to interpret recent charge separation spectroscopy experiments. For two-body processes, the calculated appearance thresholds of the C2H+/H+ and CH+/CH+ fragment pairs are in good agreement with the experimental data. It is shown that the C2H++2→CH++CH+ dissociation occurs with an important rotation of the CH+ ions. The presence of the CH+2 ion is explained by a preliminary isomerization of acetylene to vinylidene dication. This reaction has been studied for the lowest lying states of C2H++2 (3Σ-g and 1Δg) and compared with other acetylenic ions isomerizations (C2H2, C2H+2, C2H-2). For three-body processes, the calculations are consistent with the mechanisms proposed by the experimentalists. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The C2H photodissociation processes have been studied using ab initio multireference perturbative configuration interaction methods. The calculated structural parameters of the two linear lowest lying states X 2Σ+ and A 2Π were found to be in good agreement with previous theoretical and experimental works. Construction of the correlation diagram indicates that C2H cannot photodissociate into C+CH. Therefore, two-dimensional potential energy surfaces of the six lowest lying A’ and A‘ doublet states have been calculated as functions of the bond angle and the C–H bond distance, keeping the C–C distance at the equilibrium C2H value. It appears that the 1 2A’, 2 2A’, and 1 2A‘ states correlating to the X 2Σ+ and A 2Π states in linear geometry remain bound at all angles, whereas the 3 2A’, 4 2A’, and 4 2A‘ are dissociative. However, the structure of the energy surfaces is complicated by the presence of numerous avoided crossings between these states. Transition dipole moments connecting the excited states with the X 2Σ+ and A 2Π states have been also obtained in linear geometry. They were calculated to be very weak, leading to the conclusion that the photodissociation rate of C2H will be negligible below 7 eV. Higher lying states, including Rydberg states, were not considered here, since they cannot dissociate directly into C2+H. [ABSTRACT FROM AUTHOR]

Examines the spectroscopy of molecular orbitals of gaseous pyrrole and pyrazole using inner-shell electron energy loss method. Resolution used in observing transitions from carbon atoms with different chemical environment; Demonstration of ab initio calculations using the equivalent core model.

Dissociations of the ethyne dication following its production by photoionization in the photon energy range of 35–65 eV have been investigated by the photoelectron–ion–ion coincidence technique using both synchrotron radiation and laboratory light sources. New quantum mechanical calculations identify and locate the electronic states of the molecular dication in this energy range and show that the dissociation products are formed in their ground states by heterogeneous processes. Five reaction channels leading to three molecular fragments have been identified and are interpreted as sequential processes, several faster than fragment rotation and one possibly involving dissociation of CH+ to H+ with a lifetime of the order of 25 fs. [ABSTRACT FROM AUTHOR]