Electron spectroscopy using excited atoms and photons IX. Penning ionization of CO2, CS2, COS,N2O,H2S,SO2, and NO2

The electron spectra resulting from thermal collisions of He* (predominantly 2³S) metastable atoms with the seven triatomic molecules, CO₂, COS, CS², N₂O H₂S, SO₂ and NO₂, are compared with their respective 584-Å photoelectron spectra using a transmission-corrected electron spectrometer. The normalised relative electronic-state transition probabilities for production of ionic states in Penning ionization and photoionization are reported together with energy shifts (ΔE values) for He*(2³S) Penning ionization. The cross-section for Penning ionization to lower states of NO₂⁺ is extremely low as has been observed in other open shell molecules such as NO and O₂.     

The photoelectron spectrum of CSe2

The He I photoelectron spectrum of CSe₂ has been recorded and four of the bands observed assigned to ionization from valence shell levels that correlate well with those found for CO₂ and CS₂. Weak bands in the spectra of CS₂ and CSe₂ are assigned to shake-up processes in which electronic excitation of the ion accompanies ionization.     

Photoelectron angular distribution of simple molecules at 30.4 nm photons

The asymmetry parameter (β) of angular distribution of photoelectrons has been obtained with He(I) (58.4 nm) and He(II) (30.4 nm) resonance radiation for several simple compounds (N₂, O₂, H₂O, CO₂ and CS₂), in order to examine how the β value depends on the excitation energy. It has been found that on going from He(I) to He(II) radiation the energy dependence of β for the nonbonding orbitals of triatomic molecules is similar to that obtained theoretically by Manson for the corresponding atomic p shells. For N₂ it has also been found that there is essentially no difference in β values between two vibrational peaks (υ′= 0 and 1) of the first ionisation band in the He(II) spectrum, whereas in the He(I) spectrum an anomalous variation is observed in the β values between the two peaks as originally indicated by Carlson. These results support the interpretation of the autoionisation mechanism suggested by Mintz and Kuppermann.     

Vibrationally inelastic scattering of H∼ ions from H2, N2, O2 and CO2

State-resolved measurements are presented for vibrational excitation of H₂, N₂, O₂ and CO₂ by H^? impact in the collision energy rangeE _cm=20–180 eV and for scattering in the forward direction (0±0.5°). The data obtained from the measurements are the relative intensities and differential cross sections for vibrational excitation up toν′=4, the transition probabilitiesP _0→ν′ and the vibrational energy transferΔE _vib. For the systems H^?-H₂, N₂, O₂ the vibrational inelasticity increases in the order H₂-N₂-O₂. The mechanism for vibrational excitation in these systems is due to transient charge transfer from the H^? ion into antibonding orbitals of the target molecule which provides a bond stretching force during the collision. For H₂ and N₂, the results are compared with corresponding measurements for H⁺ scattering where the interaction mechanism is quite different. In the case of CO₂, vibrational excitation in forward scattering is caused primarily by the long-range dipole interaction. The spectra are very similar for H^?-CO₂ and H⁺-CO₂. Finer details are attributed to the influence of transient charge transfer and valence interactions.     

ESCA studies of CO2, CS2 and COS

The core level electron spectra of CO₂, CS₂ and COS excited by Mg Kα radiation have been studied to identify shake-up satellite lines associated with ionization from these levels. A number of such lines have been seen and possible assignments have been suggested using the excited states of the molecule as a guide. The valence spectra have also been recorded and they too were found to be rich in shake-up structure. The observed variation of the valence line intensities is discussed and compared with predictions made from an intensity model. The validity of distinguishing between π and σ symmetries in linear molecules in applying the intensity model is confirmed.     

Etude par spectrometrie de photoelectrons de la structure electronique des enynes conjugues

We have measured by UV excited photoelectron spectroscopy the ionization potentials of 22 conjugated alkynes, in particular, those associated with three π orbitals (π_, π_, π₊. The corresponding bands appear in this order in the spectrum. We have also calculated the orbital energies of these compounds by two semi-empirical quantum mechanical methods (INDO and SPINDO).Discussion of the ionization potential variations in terms of polar effects confirms the spatial distribution of the orbitals predicted by calculation.The vibrational structure of the photoelectron bands confirms their attribution and the role predicted for the orbitals associated with the chemical bonds of these compounds.     

Angle-resolved photoelectron spectroscopy of benzene and hexafluorobenzene as a function of photon energy

Angle-resolved photoelectron spectra of benzene have been recorded using synchrotron radiation as the photon source. From these results the angular dis the first three orbitals; 1e_1g(π), 3e_2g (σ) and 1a_2u(π). Calculations of β employing the MS Xα method have also b and theory for the first two orbitals is excellent. Poor agreement in the core of the third orbital is believed to be due to overlap of the second and has been verified by data obtained for perfluorobenzene. The universality of the energy dependence of β for π orbitals, noted previously for unsatu aromatic systems. The importance of the results for benzene for the future application of angle-resolved photoelectron spectroscopy to complex molecule     

Quasi-molecular angle dependence of photoemission from thin films of polystyrene

Angle-resolved photoelectron spectra from thin films of randomly oriented polystyrene show a pronounced angular dependence of the emission for several peaks derived from benzene molecular orbitals in the valence band region. The analysis yields a clear molecular behaviour for the angular distribution of the uppermost π-band (1e_1g) which can be described by a β parameter of β = 0.6 for hν = 31 eV. These results support, for polystyrene, the notion of weak intermolecular interaction in a randomly oriented “condensed gas”.     

Photoelectron spectra of substituted benzenes: II. Seven valence electron substituents

The He I photoelectron spectra of the isoelectronic series of substituted benzenes toluene, aniline, phenol, and fluorobenzene have been measured and the ionization bands in the low binding energy region of the spectra have been identified and assigned. The spectrum of pentafluoroaniline has also been obtained in order to use the “perfluoro effect” to identify the lowest energy σ ionization band. The photoelectron bands were assigned by using a “composite molecule” approach in which the known levels of benzene and the various substituents are correlated with those of the molecule, by using information supplied by vibrational analysis of the bands, and by reference to CNDO/2 and INDO MO calculations. The spectra of these composite molecules are consistent with the … 1a_2u(π), 3e_2g(σ), 1e_1g(π) MO configuration of benzene. The e_1g(π) orbital is split into two components in all of these substituted benzenes. Vibrational progressions in the ring CC stretching and the X-sensitive modes have been identified in these π ionization bands. Excitation of these two modes is evidence for delocalization of the π electrons over the composite molecule. The n (nonbonding) orbital of the substituents is shown to occur in the penultimate region along with the 1a_2u(π) and 3e_2g(σ) orbitals. The shifts in the n and π orbitals between the composite molecules and their constituent molecules are discussed in terms of resonance and inductive effects.     

Electron spectroscopy using excited atoms and photons: II. Penning ionization of diatomic molecules

A high resolution electrostatic electron analyser has been used to study Penning ionization electron spectra of H₂, HD, D₂, N₂, CO, NO and O₂ using helium metastable atoms (2¹S, 2³S). Results for H₂, N₂ and CO are in good agreement with other work. New data are presented for HD, D₂, NO and O₂. The Penning electron spectra are also compared to the 584 Å photoelectron spectra obtained in the same apparatus. The relative vibrational intensifies for the given electronic bands indicate that in most cases Franck—Condon factors for Penning ionization and photoionization are very similar. However for the O₂⁺(X²Π_g) band, the (2³S) Penning electron and photoelectron spectra show significant differences in the Franck—Condon envelopes This perturbation of the envelope for the Penning ionization may be explained by a competing autoionization process. The relative electronic transition probabilities are in many cases found to be different for Penning ionization and photoionization.     

K- and LII,III-Shell excitations in CS2 and COS by 2.5 keV electron impact

Energy loss spectra of 2.5 keV electrons, scattered by CS₂ and COS through small angles, have been studied in the regions of the respective oxygen K, carbon K and sulfur L_{II, III}(2p) edges. The carbon and oxygen K-shell spectra are dominated by the first discrete transition similar to the K-shell spectra of the “isoelectronic” molecules CO₂ and N₂O. However, the carbon K-shell spectra of CS₂ and COS have features which may indicate the possible existence of an effective potential barrier in these molecules.     

Angle-resolved photoelectron spectra of ethylene as a function of photon energy using synchrotron radiation

As a test of the applicability of multiple-scattering theory to an unsaturated organic system, angle-resolved photoelectron spectra of the first four bands of ethylene have been measured from 1.5 eV above threshold to a photon energy of 28 eV. The results are compared to the multiple-scattering theory calculations of Grimm. The comparisons are excellent except for the 1 b_3u orbital below 14 eV, and the disagreement is attributed to the occurrence of autoionization. It is found that the π orbital has a much higher β value than the σ orbitals, but the β value for the π orbital varies more rapidly than that for the σ orbitals from threshold to ～0.5 Rydberg (～7 eV).     

Photoelectron spectra of fluorotribromomethane and fluorotrichloromethane

High resolution He(I) photoelectron spectra are reported for fluorotribromomethane CFBr₃ and fluorotrichloromethane CFCl₃. The assignments are based on CNDO/2 calculations, symmetry arguments, the fine structures of the bands, and comparison with the photoelectron spectra of related compounds. Linear relationships have been found between the Pauling electronegativity values for the halogen atoms and the observed vertical ionization energies corresponding to a₂ of a₂′ orbitals of CFX₃ and related compounds CHX₃, OPX₃, BX₃ and PX₃ (X = F, Cl, Br, I).     

Molecular photoelectron spectroscopy at 132.3 eV: N2, CO,C2H4 and O2

The molecular photoelectron spectra of gaseous N₂, CO, C₂H₄ and O₂ were obtained using yttrium Mζ X-rays (132.3 eV). Comparison with spectra taken with MgKα₁₂ X-rays (1253.6 eV) showed the molecular orbitals derived from atomic 2p orbitals to be emphasized in the YMζ spectra. Orbital compositions were confirmed in N₂ and CO, and the presence of several peaks was either better established or detected for the first time (e.g., a ²Π_u state at 23.5 eV in O₂⁺) in C₂H₄ and O₂. The relative cross-section predictions of Rabalais et al. were tested by these spectra. The theoretical values, which were based on ground-state wavefunctions and plane-wave (PW) or OPW continuum states, were found to agree qualitatively with experiment, establishing that this level of theory has diagnostic value. Quantitative agreement is lacking, however. The potential application of 132.3 eV X-rays to the study of photoemission from adsorbed molecules on surfaces is emphasized.     

Quenching of infrared emission from CO2 by near-resonant vibrational interactions with SF6, BCl3, and PF5

The transfer of substantial amounts of vibrational energy from CO₂ to specific complex molecules has been observed in flow-tube experiments by monitoring the spectra from the 4·3 μm fundamental bands and the 2·7 μm mixed-mode Fermi-resonance bands of CO₂. Small amounts of SF₆, BCl₃, and PF₅ were found to reduce the intensities of these emissions significantly. Quenching cross-sections were calculated using a non-equilibrium analysis.     

Penning ionization electron spectroscopy of H2O,D2O,H2S and SO2

Electron energy peak shifts and peak shapes were determined in the ionization of H₂O, D₂O, H₂S and SO₂ by Ne(³P₂) and He(2¹S, 2³S) metastable atoms. The shifts are large, especially in ionization of H₂O and D₂O into the ionic ground state and are probably mostly due to chemical interaction during the collision.In a previous paper the electron energy distribution curves for ionization of CO, HCl, HBr, N₂O, NO₂, CO₂, COS and CS₂ by helium, neon and argon metastables and the characteristics of this ionization were described¹. In this paper the series of triatomic molecules was extended to the molecules H₂O, D₂O, H₂S and SO₂. Because all these molecules have considerable dipole moments it could be expected that the peak shifts might be enhanced as compared with other triatomic molecules.     

Synchrotron radiation VUV double photoionization of some small molecules

The VUV double photoionizations of small molecules (NO,CO,CO2 ,CS2 ,OSC and NH3 ) were investigated with photoionization mass spectroscopy using synchrotron radiation. The double ionization energies of molecules were determined with photoionization efficiency spectroscopy. The total energies of these molecules and their parent dications (NO2+ ,CO2+ ,CO2+2 ,CS2+2 ,OSC2+ and NH2+3 ) were calculated using the Gaussian 03 program and Gaussian 2 calculations. Then,the adiabatic double ionization energies of the molecules were predicated by using high accuracy energy mode. The experimental double ionization energies of these small molecules were all in reasonable agreement with their respective calculated adiabatic double ionization energies. The mechanisms of double photoionization of these molecules were discussed based on a comparison of our experimental results with those predicted theoretically. The equilibrium geometries and harmonic vibrational frequencies of molecules and their parent dications were calculated by using the MP2 (full) method. The differences in configurations between these molecules and their parent dications were also discussed on the basis of theoretical calculations.     

Detection of vibrationally excited O2* by variable temperature photoelectron spectroscopy

Hot bands have been observed in the photoelectron spectra of the heavier diatomics Cl₂, Br₂, and J₂, at room temperature ^2–4. We report here the detection of hot bands in the photoelectron spectrum of O₂ at elevated temperatures using the technique of variable temperature photoelectron spectroscope ^{5, 6}.     

Atomic orbitals and photoelectron intensity angular distribution patterns of MoS2 valence band

The ratio of atomic orbitals contributing to the valence band can be determined from the photoelectron intensity angular distribution (PIAD) by using linearly polarized light and display-type spherical mirror analyzer. The experiment was done for MoS₂ using a linearly polarized light at the photon energy of 45 eV perpendicularly incident to the sample surface. Atomic orbitals contributing to the bands near the Fermi level were investigated. The PIAD patterns around the Γ point showed splitting of intensity. The intensity at the top and bottom K points was strong, while the intensity was weak at the left and right side K points. The PIAD patterns from various kinds of atomic orbitals were calculated. By comparing the experimental PIAD patterns to the simulated ones, we concluded that at the Γ point Mo 4d_z2 and S 3p_z atomic orbitals are the main components and at the K points the Mo 4d_xy atomic orbital is dominant. The atomic orbital Mo 4d_x2−y2 also gives contribution to the PIAD pattern. These results were in good agreement with the coefficients of the atomic orbitals derived using ab initio band calculation.     

Some anomalies in the Ne(I)(744/736 Å) photoelectron spectra of N2O

The Ne(I) 774/736 Å photoelectron spectra of N₂O are reported for the X?²Π state of N₂O⁺. The spectra in general do not show any autoionization behavior to the extent reported for CO₂ and CS₂. There is an apparent “enhancement” of the 101 level by the 744 Å line. In contrast to the He(I) 584 Å PES, the intensity ratio for the 100 and 001 levels are reversed when excited by Ne(I) 736 Å radiation.The spectra also show excitation to higher vibrational levels of N₂O⁺X²Π. This can be explained within the framework of autoionization of a Rydberg state whose core is similar to that of the B? state of N₂O⁺.