Revisiting the photoabsorption spectrum of NH3 in the 5.4-10.8 eV energy region.

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 (D̃1E'←X̃1A1 '), the (F̃1E'←X̃1A1 '), and the (G̃1A2 ″←X̃1A1 ') absorption bands. In addition, new Rydberg members have been identified in nda1 '←1a2 ″D̃''1A2 ″←X̃1A1 ', where n > 3 has not been reported before as well as in nde″←1a2 ″F̃1E'←X̃1A1 ' and in nsa1 '←1a2 ″G̃1A2 ″←X̃1A1 '. The measured absolute photoabsorption cross sections have been used to calculate the photolysis lifetime of ammonia in the Earth's atmosphere (0-50 km).

Experimental scaling of plane-Born cross sections and ab initio assignments for electron-impact excitation and dissociation of XF4 (X = C, Si, and Ge) molecules.

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 f0 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 f0 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.

Studies on GeF4 Valence and Rydberg States by Electron Impact Spectroscopy and Ab Initio Calculations.

Electron energy loss (EEL) spectra of GeF4 have been measured with incident electrons at 100 eV for 5° scattering angle and at 30 eV for 30° scattering angle, while sweeping the energy loss over the range 7.0-18.0 eV. Low-lying excited triplet, singlet, valence, and Rydberg states are investigated and the assignments supported by quantum chemical ab initio calculations. This provides the first comprehensive investigation of all singlet and triplet excited electronic states of germanium tetrafluoride up to the first ionization energy. The Rydberg series converging to the (lowest) ionization energy limits of GeF4 are also identified according to the magnitude of the quantum defects (δ).

Valence and lowest Rydberg electronic states of phenol investigated by synchrotron radiation and theoretical methods.

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).

Toluene Valence and Rydberg Excitations as Studied by ab initio Calculations and Vacuum Ultraviolet (VUV) Synchrotron Radiation.

The electronic spectroscopy of isolated toluene in the gas phase has been investigated using high-resolution photoabsorption spectroscopy in the 4.0-10.8 eV energy range, with absolute cross-section measurements derived. We present the first set of ab initio calculations (vertical energies and oscillator strengths), which we use in the assignment of valence and Rydberg transitions of the toluene molecule. The spectrum reveals several new features not previously reported in the literature, with particular relevance to 7.989 and 8.958 eV, which are here tentatively assigned to the π*(17a') ← σ(15a') and 1π*(10a″) ← 1π(14a') transitions, respectively. The measured absolute photoabsorption cross sections have been used to calculate the photolysis lifetime of toluene in the upper stratosphere (20-50 km).

Electronic State Spectroscopy of Halothane As Studied by ab Initio Calculations, Vacuum Ultraviolet Synchrotron Radiation, and Electron Scattering Methods.

We present the first set of ab initio calculations (vertical energies and oscillator strengths) of the valence and Rydberg transitions of the anaesthetic compound halothane (CF3CHBrCl). These results are complemented by high-resolution vacuum ultraviolet photoabsorption measurements over the wavelength range 115-310 nm (10.8-4.0 eV). The spectrum reveals several new features that were not previously reported in the literature. Spin-orbit effects have been considered in the calculations for the lowest-lying states, allowing us to explain the broad nature of the 6.1 and 7.5 eV absorption bands assigned to σ*(C-Br) ← nBr and σ*(C-Cl) ← n(Cl) transitions. Novel absolute photoabsorption cross sections from electron scattering data were derived in the 4.0-40.0 eV range. The measured absolute photoabsorption cross sections have been used to calculate the photolysis lifetime of halothane in the upper stratosphere (20-50 km).

Fourier transform microwave and millimeter-wave spectroscopy of bromoiodomethane, CH2BrI.

Bromoiodomethane, CH2BrI, is a molecule of natural origin emitted in significant amount into the marine boundary layer. It can easily be decomposed by solar radiation, releasing Br and I atoms in the troposphere, which in turn impacts the atmospheric chemistry. Spectroscopy is an invaluable tool to monitor species present in the atmosphere. Since no high-resolution spectroscopic studies are available for this dihalomethane, we have investigated the rotational spectra of the two bromine isotopologues of CH2BrI in its vibrational ground state in the microwave and millimeter-wave regions. Transitions of b-type have been recorded by Fourier transform microwave spectroscopy below 25 GHz while both a- and b-type spectral lines have been measured below 230 GHz. Observed transitions correspond to energy levels with J ≤ 132 and Ka ≤ 14. Molecular constants including those describing the nuclear quadrupole coupling tensors for (79)Br, (81)Br, and (127)I were accurately determined from the least-squares analysis of a total of 1873 distinct transition frequencies (of which 943 belong to the CH2(79)BrI isotopologue). An experimental (r0) structure of the title species has been derived from the two sets of rotational constants.

BF3 valence and Rydberg states as probed by electron energy loss spectroscopy and ab initio calculations.

In this contribution we probe BF3 low-lying excited singlet states measured at 100 eV, 2.8° scattering angle and triplet states at 40 eV, 40° scattering angle, while sweeping the energy loss over the range 10.0-20.0 eV. The electronic state spectroscopy has been investigated and the assignments supported by quantum chemical calculations. This provides the first comprehensive investigation of all singlet and triplet excited electronic states of boron trifluoride up to the first ionization potential. A generalized oscillator strength analysis is employed to derive oscillator strength f0 value and integral cross sections (ICSs) from the corresponding differential cross sections (DCSs). The f0 value is compared with the optical oscillator strength (OOS) from photoabsorption, and the unscaled Born ICSs are then compared with relevant energy and binary-encounter and f-scaled Born cross section (BEf-scaling) results determined as a part of this investigation. The lowest n members of the Rydberg series have been assigned as converging to the lowest ionization energy limits of boron trifluoride and classified according to the magnitude of the quantum defects (δ).

Valence and ionic lowest-lying electronic states of ethyl formate as studied by high-resolution vacuum ultraviolet photoabsorption, He(I) photoelectron spectroscopy, and ab initio calculations.

The highest resolution vacuum ultraviolet photoabsorption spectrum of ethyl formate, C2H5OCHO, yet reported is presented over the wavelength range 115.0-275.5 nm (10.75-4.5 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).

Differential cross sections for intermediate-energy electron scattering from α-tetrahydrofurfuryl alcohol: excitation of electronic-states.

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 Milosavljevic et al. [Eur. Phys. J. D 40, 107 (2006)].

Electronic states of tetrahydrofurfuryl alcohol (THFA) as studied by VUV spectroscopy and ab initio calculations.

The electronic spectroscopy of isolated tetrahydrofurfuryl alcohol (THFA) in the gas phase has been investigated using high-resolution photoabsorption spectroscopy in the 5.0-10.8 eV energy-range, with absolute cross-section measurements derived. The He(I) photoelectron spectrum was also collected to quantify ionization energies in the 9-16 eV spectral region. These experiments are supported by the first high-level ab initio calculations performed on the excited states of the neutral molecule and on the ground and excited state of the positive ion. The good agreement between the theoretical results and the measurements allows us to quantify for the first time the electronic-state spectroscopy of THFA. The present work also considers the question of the lowest energy conformers of the molecule and its population distribution at room temperature.

Electronic excitation to singlet states of 1,3-C4F6, c-C4F6 and 2-C4F6 by electron impact--electron energy-loss spectroscopy and ab initio calculations.

We report on the first measurements of the electron impact electronic excitation cross sections for C(4)F(6) isomers, hexafluoro-1,3-butadiene (1,3-C(4)F(6)), hexafluorocyclobutene (c-C(4)F(6)), and hexafluoro-2-butyne (2-C(4)F(6)), measured at 100 eV, 3° scattering angle, while sweeping the energy loss over the range 2.0-15.0 eV. Under these experimental conditions, optically allowed transitions are favored. The electronic state spectroscopy has been investigated and the assignments supported by quantum chemical calculations. The n = 3 members of the Rydberg series have been assigned converging to the lowest ionization energy limits of the C(4)F(6) isomers and classified according to the magnitude of the quantum defects (δ).

Electronic state spectroscopy of 1,4-pentadiene as studied by VUV photoabsorption spectroscopy and ab initio calculations.

We present high resolution VUV photoabsorption spectra of 1,4-pentadiene, C(5)H(8), over the wavelength range 115-247 nm (10.8-5.0 eV). These spectra reveal several new features not previously reported in the literature. These measurements are complemented by the first ab initio calculations for the three most abundant conformational isomers of 1,4-pentadiene, C(5)H(8), which we then use in the assignment of valence and Rydberg transitions. Calculations of the two lowest energy ionic states of 1,4-pentadiene are also presented and compared with the experimental data available in the literature. The measured absolute photoabsorption cross sections have been used to calculate the photolysis lifetime of 1,4-pentadiene in the upper stratosphere (20-50 km).

Limonene: electronic state spectroscopy by high-resolution vacuum ultraviolet photoabsorption, electron scattering, He(I) photoelectron spectroscopy and ab initio calculations.

Electronic state spectroscopy of limonene has been investigated using vacuum ultraviolet photoabsorption spectroscopy in the energy range 5.0-10.8 eV. The availability of a high resolution photon beam (~0.075 nm) enabled detailed analysis of the vibrational progressions and allowed us to propose, for the first time, new assignments for several Rydberg series. Excited states located in the 7.5-8.4 eV region have been studied for the first time. A He(I) photoelectron spectrum has also been recorded from 8.2 to 9.5 eV and compared to previous low resolution works. A new value of 8.521 ± 0.002 eV for the ground ionic state adiabatic ionisation energy is proposed. Absolute photoabsorption cross sections were derived in the 10-26 eV range from electron scattering data. All spectra presented in this paper represent the highest resolution data yet reported for limonene. These experiments are complemented by new ab initio calculations performed for the three most abundant conformational isomers of limonene, which we then used in the assignment of the spectral bands.

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).

Electronic state spectroscopy of methyl formate probed by high resolution VUV photoabsorption, He(i) photoelectron spectroscopy and ab initio calculations.

The first ab initio calculations of the vertical excitation energies and oscillator strengths are presented for the neutral electronic transitions of methyl formate, C(2)H(4)O(2). The highest resolution VUV photoabsorption spectrum of the molecule yet reported is presented over the wavelength range 115 to 310 nm (10.8 to 4.0 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 theoretical results. The calculations have been carried out to determine the excitation energies of the lowest energy ionic states of methyl formate and are compared with a newly recorded He(i) photoelectron spectrum (10.4 to 17.0 eV). New vibrational structure is observed in the first photoelectron band. The photoabsorption cross-sections have been used to calculate the photolysis lifetime of methyl formate in the upper stratosphere (20-50 km).

Valence shell electronic spectroscopy of isoprene studied by theoretical calculations and by electron scattering, photoelectron, and absolute photoabsorption measurements.

The first ab initio calculations (vertical energies and oscillator strengths) are reported for the neutral electronic transitions of isoprene (2-methyl-1,3-butadiene), CH(2)CHC(CH(3))CH(2). The VUV photoabsorption spectroscopy of the molecule is presented in the energy range 4.6 to 10.8 eV (270-125 nm) with the highest resolution yet reported above 6.05 eV, revealing new spectral features. Valence and Rydberg transitions have been assigned in accordance with the theoretical results and the associated vibronic series have been analysed. The absolute photoabsorption cross sections at energies below 6.89 eV have been used to calculate the photolysis lifetime of isoprene in the upper stratosphere (20-50 km). Electron energy loss spectroscopy (EELS) measurements have enabled further photoabsorption cross sections to be derived in the range 9-28 eV. The first ab initio calculations have been carried out to determine excitation energies to the lowest energy ionic states of isoprene. The calculations are compared with the He(i) photoelectron spectrum (8 to 17 eV) and new vibrational structure is observed in the first photoelectron band.

Photoabsorption measurements and theoretical calculations of the electronic state spectroscopy of propionic, butyric, and valeric acids.

Absolute photoabsorption cross sections of propionic (C2H5COOH), butyric (C3H7COOH), and valeric (C4H9COOH) acids have been measured from the dissociative pi* <-- n(o) transition (beginning around 5.0 eV) up to 10.7 eV. This constitutes the first study of the neutral electronic states of propionic and butyric acids at energies above the pi* <-- n(o) band, while no previous spectroscopic data is available for valeric acid in the present range. The present assignments are supported by the first theoretical calculations of electronic transition energies and oscillator strengths for these organic acids. In addition, the excitation energies of the vibrational modes of propionic acid in its neutral electronic ground state and the vertical ionisation energies of all three molecules have been calculated for the first time. The He(I) photoelectron spectroscopy of propionic acid has been measured from 10 to 16 eV, revealing new fine structure in the first ionic band.

The H2O(2+) potential energy surfaces dissociating into H(+)/OH(+): theoretical analysis of the isotopic effect.

We present a detailed study of the potential energy surfaces of the water dication correlating asymptotically with O((3)P) and O((1)D). 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 theta > or = 80 degrees degrees and OH distances R(OH) > or = 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 HOD(2+) 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.

Electronic state spectroscopy of c-C5F8 explored by photoabsorption, electron impact, photoelectron spectroscopies and ab initio calculations.

The electronic transitions and resonance-enhanced vibrational excitations of octafluorocyclopentene (c-C5F8) have been investigated using high-resolution photoabsorption spectroscopy in the energy range 6-11 eV. In addition, the high-resolution electron energy loss spectrum (HREELS) was recorded under the electric dipolar excitation conditions (100 eV incident energy, approximately 0 degrees scattering angle) over the 5-14 eV energy loss range. A He(I) photoelectron spectrum (PES) has also been recorded between 11 and 20 eV, allowing us to derive a more precise value of (11.288 +/- 0.002) eV for the ground neutral state adiabatic ionization energy. All spectra presented in this paper represent the first and highest resolution data yet reported for octafluorocyclopentene. Ab initio calculations have been performed for helping in the assignment of the spectral bands for both neutral excited states and ionic states.