High Resolution HeI Research Articles

Electronic structure of hexafluorobenzene by high-resolution vacuum ultraviolet photo-absorption and He(I) photoelectron spectroscopy

The VUV photo-absorption spectrum of hexafluorobenzene, C6F6, recorded using the synchrotron radiation in the 4–10.75eV photon range exhibits broad bands between 4 and 8eV, and fine features above 8eV. The broad bands are likely to be due to valence states dissociating into neutral fragments. They have been assigned to a π–π∗ transition, two π–σ∗ transitions and one σ–π∗ transition. A small shoulder observed at 7.36eV, confirms earlier observations and is evidence of a further valence excited electronic state. Above 8.3eV numerous fine structures are observed and the features are assigned by comparison with the high-resolution HeI photoelectron spectrum to a nd Rydberg series converging to the ground ionic state and a ns series associated with the first electronic excited state limit. The first two nd terms show vibrational structure. The origin of the vibrational structure is discussed in terms of Jahn–Teller distortion. For the 3s term, excitation of the vibration normal mode ν2′ is reported for the first time with a value of 0.066eV, a value consistent with that of the ion. The HeI photoelectron spectrum of C6F6 has also been revisited and a new interpretation of the vibrational structure for the first and the fourth electronic bands is proposed. Finally, using a previously derived solar flux model, the atmospheric lifetime relative to photolysis is estimated and the consequences for atmospheric chemistry briefly discussed.

Threshold photoelectron spectroscopy of cyanogen bromide

A high-resolution (2–15 meV) threshold photoelectron (TPE) spectroscopic study of BrCN was performed using synchrotron radiation and a penetrating-field electron spectrometer over the outer valence ionization region of the molecule. Significant differences were found between the TPE spectrum and the published high-resolution (∼6 meV) HeI photoelectron spectrum of BrCN [Chem. Phys. 212 (1996) 457]. Autoionization of Rydberg states and the 6σ valence state of BrCN were found to be primarily responsible for the formation of the observed band systems of BrCN + in this study.

High-resolution photoelectron spectroscopy of HI and DI: Experimental and theoretical analysis of the A 2Σ+ ion system

A combined high-resolution (6 meV) HeI photoelectron (PE) and ab initio theoretical study of the A 2Σ+ ion system of HI and DI has been conducted to elucidate the origin of a peculiar “holelike” feature in the vibrational distribution found in the high-resolution (6 meV) threshold photoelectron (TPE) spectrum of HI. The PE and TPE spectra were found to yield essentially identical results. Ab initio potential energy curves for the low-lying cationic states of HI have been calculated for the first time with and without spin–orbit contributions included. It has been found that the diabatic A 2Σ+ state of HI+ is strongly predissociated due to spin–orbit coupling with the Σ−4, Σ−2, and Π4 repulsive states leading to a complex set of adiabatic curves. It is shown that the adiabatic A 2Σ1/2+ state is only slightly bound (by 260 cm−1 after suitable adjustments of the positions of the various repulsive potentials relative to that of the A state are made based on observed atomic spectral data) and should support at most one vibrational level in both HI+ and DI+. However, using the complex rotational method, it was possible to calculate the energies, predissociation linewidths, and rotational constants of a number of nonstationary vibrational levels (or resonances) of the A 2Σ1/2+ state. Reasonably good agreement has been found between experiment and theory. The observed “hole” in the TPE and PE spectra is attributed to the fact that the stability of the v+=1 and 2 levels is notably less than for v+=0 in the A 2Σ1/2+ state of both HI+ and DI+.

Thiazyl chloride: an experimental and theoretical study of the valence shell HeI photoelectron spectrum

Abstract High level (CASSCF-MRCI) ab initio calculations are used to investigate the structural, electronic and vibrational properties of the electronic ground state of thiazyl chloride (NSCl) and of the low-lying electronic states of NSCl + . A new high resolution HeI photoelectron spectrum of NSCl has been recorded in the 10–16 eV energy region. From the results of the calculations, the first band is assigned to the (1) 2 A ′ state of NSCl + . The second one corresponds to the (2) 2 A ′ and (1) 2 A ″ states which are quasi-degenerate. Despite the high resolution, the two first bands show no vibrational fine structure. For the first one, Franck–Condon analysis shows that it is due to the overlapping of two vibrational progressions involving the S–Cl stretching and the NSCl bending modes. In the case of the second band, it is explained by the highly repulsive character of the potential energy surfaces of two states of NSCl + in the Franck–Condon region of the neutral molecule. For the third band, which shows vibrational peaks, the calculation of Franck–Condon factors permits the determination of the adiabatic ionisation energy of the (3) 2 A ′ electronic state of NSCl + at 13.798 eV. Finally, the fourth band, which is due to three different ionic states with vibrational progressions, is too complicated to be assigned quantitatively.

High-resolution photoemission spectroscopy for the layered antiferromagnetic (La(1-z)Nd(z))(0.46)Sr(0.54)MnO(3).

High-resolution HeI photoemission spectroscopy (UPS), Mn 2p-3d resonant photoemission spectroscopy (RPES) and Mn 2p X-ray absorption spectroscopy (XAS) have been performed to investigate the electronic structure and its effect on the electrical resistivity in (La(1-z)Nd(z))(0.46)Sr(0.54)MnO(3) (z = 0, 0, 2, 0.6 and 1.0). It was found that in the UPS and RPES spectra the Fermi edge persisted over the temperature range 15 < or = T < or = 340 K regardless of the magnetic structure or the composition of the samples. The density of states at the Fermi level [N(E(F))] in the samples where 0 < or = z < or = 0.6 was increased drastically at the Curie temperature (T(C)) with decreasing temperature, but essentially kept unchanged across the Néel temperature (T(N)). A fairly large reduction at T(C) and a small increase at T(N) in the electrical resistivity with decreasing temperature are found to be well accounted for in terms of the temperature dependence of N(E(F)). The presence of a finite N(E(F)) in the insulating Nd(0.46)Sr(0.54)MnO(3) was also found. Thus the origin of the insulating behavior in this sample can be regarded as the Anderson localization associated with the small density of states and the chemical disorder between Nd and Sr.

A reinvestigation of the vibrational structure and the orbital assignments in the photoelectron bands of cyclopropane

The valence shell photoelectron spectrum of cyclopropane has been studied using synchrotron and HeI radiation. The photoelectron asymmetry parameters and branching ratios have been determined using monochromated synchrotron radiation in the photon energy range 15–120 eV. The spectral behaviour of the asymmetry parameters associated with the outer valence orbitals has been used to propose an assignment for the valence shell electronic configuration. The experimental features observed in the inner valence energy region of the photoelectron spectrum have been compared with theoretical predictions. The high resolution HeI excited spectrum has allowed vibrational structure to be observed in the outer valence photoelectron bands, and in two of the bands these features can be assigned to progressions involving the ν 2 (a 1′) and ν 11 (e′) modes. The X̃ 2E′ photoelectron band, which exhibits a doublet structure due to Jahn–Teller interaction, displays an extended vibrational progression in the doubly degenerate ν 11 (e′) mode.

On the high-resolution HeI photoelectron spectrum of Cl 2O

The high-resolution HeI (58.4 nm) photoelectron spectrum of dichlorine monoxide, Cl 2O, has been recorded in the region of the four lowest-energy ionic electronic states. Formation of the ion in its ground and excited electronic states is accompanied in each case by vibrational excitation. In particular, the vibrational structure of the first and second excited states of Cl 2O + is resolved. Analysis of the vibrational progressions associated with formation of the various ionic states has been completed, allowing confirmation of the symmetry and bonding characteristics of the four highest-energy occupied molecular orbitals of Cl 2O.

An experimental and theoretical study of the valence shell photoelectron spectrum of carbon disulphide

The complete valence shell photoelectron spectrum of carbon disulphide has been studied using HeI, HeII and synchrotron radiation. In addition to the photoelectron bands associated with the X 2Πg, A 2Πu, B 2∑u+ and C 2∑g+ single-hole ionic states, several satellite features have been observed which are due to multielectron processes. The high resolution HeI and HeII excited spectra have allowed a detailed analysis to be made of the vibrational structure exhibited in the photoelectron bands corresponding to the single-hole and satellite states. The photoelectron spectra recorded with synchrotron radiation demonstrate that the inner valence region contains much complicated structure, and that distinct features are discernible up to a binding energy of at least 34 eV. Many-body Green's function calculations have been performed to evaluate the ionization energies and pole strengths associated with the main lines and satellite lines distributed throughout the valence shell region, and an interpretation of some of the experimental features is proposed, based upon these predictions.

An experimental and theoretical study of the valence shell photoelectron spectrum of sulphur hexafluoride

Abstract The complete valence shell photoelectron spectrum of sulphur dioxide has been studied using HeI, HeII and synchrotron radiation. The high resolution HeI and HeII excited spectra have allowed a detailed analysis to be made of the vibrational structure exhibited in the photoelectron bands associated with ionisation from the outer valence orbitais. A new satellite band containing vibrational structure has been observed around 19 eV. The photoelectron spectra recorded with synchrotron radiation demonstrate that the inner valence region contains several complicated structures and is dominated by a prominent feature centred at a binding energy around 34 eV. Many-body Green's function calculations have been performed to evaluate the energies and pole strengths associated with the main lines and satellite states distributed throughout the valence shell region, and an interpretation of some of the experimental features is proposed, based upon these predictions.

An experimental and theoretical study of the valence shell photoelectron spectrum of sulphur dioxide

Abstract The complete valence shell photoelectron spectrum of sulphur dioxide has been studied using HeI, HeII and synchrotron radiation. The high resolution HeI and HeII excited spectra have allowed a detailed analysis to be made of the vibrational structure exhibited in the photoelectron bands associated with ionisation from the outer valence orbitais. A new satellite band containing vibrational structure has been observed around 19 eV. The photoelectron spectra recorded with synchrotron radiation demonstrate that the inner valence region contains several complicated structures and is dominated by a prominent feature centred at a binding energy around 34 eV. Many-body Green's function calculations have been performed to evaluate the energies and pole strengths associated with the main lines and satellite states distributed throughout the valence shell region, and an interpretation of some of the experimental features is proposed, based upon these predictions.

High-resolution HeI photoelectron spectrum of acetonitrile

The photoelectron spectrum of acetonitrile has been revisited under high resolution and high sensitivity using the HeI resonance line. Ionization of the three outermost valence molecular orbitals has been discussed in terms of ionization energies and vibrational frequencies. Additional vibration terms have been reported. Ionization of the 2e molecular orbital is accompanied by a Jahn—Teller effect involving the ν 8 degenerate mode. The second band of the spectrum shows weak features assigned to the excitation of the degenerate modes ν 6 and ν 8. The second excited ionic state has been suggested to be of E symmetry.

Photoelectron spectroscopy and electronic structure of clusters of the group V elements. II. Tetramers: Strong Jahn–Teller coupling in the tetrahedral 2E ground states of P+4, As+4, and Sb+4

High resolution HeI (584 Å) photoelectron spectra have been obtained for the tetrameric clusters of the group V elements: P4, As4, and Sb4. The spectra establish that the ground 2E states of tetrahedral P+4, As+4, and Sb+4 are unstable with respect to distortion in the ν2(e) vibrational coordinate. The E⊗e Jahn–Teller problem has been treated in detail, yielding simulated spectra to compare with experimental ones. Vibronic calculations, extended to second order (quadratic coupling) for P+4, account for vibrational structure which is partially resolved in its photoelectron spectrum. A Jahn–Teller stabilization energy of 0.65 eV is derived for P+4, which can be characterized in its ground vibronic state as being highly distorted, and highly fluxional. Linear-only Jahn–Teller coupling calculations performed for As+4 and Sb+4, show good qualitative agreement with experimental spectra, yielding stabilization energies of 0.84 and 1.4 eV, respectively.

Photoelectron spectroscopy and electronic structure of clusters of the group V elements. III. Tetramers: The 2T2 and 2A1 excited states of P+4, As+4, and Sb+4

Methods employing high resolution HeI (584 Å) photoelectron spectroscopy have been applied to the tetrameric clusters of the group V elements, to resolve details of vibronic and spin–orbit structure in the first three electronic states of P+4, As+4, and Sb+4. Measured spacings of distinct vibrational progressions in the ν1 mode for the 2A1 states of P+4 and As+4, yield vibrational frequencies of 577 (5) cm−1 for P+4 and 350 (6) cm−1 for As+4. Franck–Condon factor calculations suggest bond length changes for the ions in the 2A1 states of 0.054 (3) Å for P+4 and 0.060 (3) Å for As+4. Strong Jahn–Teller distortions in the ν2(e) vibrational mode dominate the structure of the 2E ground states of the tetrameric ions. Both Jahn–Teller and spin–orbit effects appear in the spectra of the 2T2 states of the tetrameric ions, with the spin–orbit effect being dominant in Sb+4 and the Jahn–Teller effect dominant in P+4. Vibrational structure is resolved in the P+4 spectrum, and the ν3(t2) mode is found to be the one principally active in the Jahn–Teller coupling. A classical metal-droplet model is found to fit well with trends in the IPs of the clusters as a function of size.

Molecular beam photoelectron spectroscopy of allene

High-resolution HeI (584 Å) photoelectron spectra of allene rotationally cooled in a seeded supersonic molecular beam were obtained. Vibrational structure was completely resolved in the first band (X 2E), which involved a strong Jahn—Teller effect. A series of ab initio double-zeta multiconfiguration self-consistent field (DZ MCSCF) calculations were carried out for the ground state, and a twist angle of 52° was derived for the ground ionic state. Vibrational fine structure was resolved for the first excited ionic state (A 2E), and assigned to combinations of two symmetric vibrations, ν 2(a 1) and ν 3(a 1), with vibrational frequencies of 1320(6) and 1030(6) cm −1, respectively.

High resolution photoelectron spectroscopy of clusters of group V elements

High resolution HeI (580 Å) photoelectron spectra of As2, As4, and P4 were obtained with a newly-built high temperature molecular beam source. Vibrational structure was resolved in the photoelectron spectra of the three cluster species. The Jahn-Teller effect is discussed for the 2E and 2T2 states of P4+ and As4+. As a result of the Jahn-Teller effect, the 2E state splits into two bands, and the 2T2 state splits into three bands, in combination with the spin-orbit effect. It was observed that the ν2 normal vibrational mode was involved in the vibronic interaction of the 2E state, while both the ν2 and ν3 modes were active in the 2T2 state.

High resolution UV photoelectron spectroscopy of CO +2, COS + and CS +2 using supersonic molecular beams

Very high resolution HeI (584 Å) photoelectron spectra of CO + 2, COS +, and CS + 2 are presented. The spectra were obtained by using supersonic molecular beams to eliminate the rotational and Doppler broadenings. The spin-orbit split components in the Π bands are all explicitly resolved. The removal of the inelastic scattering peaks, due to the use of molecular beams, allowed new features to be observed in the B̃ and C̃ bands of CS + 2, and new spectroscopic constants were derived.

High-resolution HeI and HeII photoelectron spectra of the Zn 3d and Cd 4d orbitals in the zinc and cadmium dihalides: Ligand field splittings and intensity variations

We have recorded the high-resolution HeI and HeII photoelectron spectra of the Zn 3d and Cd 4d levels in gas-phase MX 2 molecules (M = Zn, Cd; X = Cl, Br, I). The d level spectra split into five peaks due to the combined effect of spin-orbit splitting and ligand field splitting on the d 9 hole state, and the spectra have been fitted to a crystal field hamiltonian involving the cubic ( C 4 0) and non-cubic ( C 2 0) parts of the field from the halide ligands. Additional peaks in some spectra are due to vibrational splitting and configuration interaction. The ¦ C 2 0¦ value increases substantially from the chloride to the iodide for both Zn and Cd. Calculations of both the crystal field ( C 2 CF 0) and valence ( C 2 val 0) parts of C 2 0 show that the increase in observed C 2 0 is due to the C 2 val 0 term attributed to the increase in covalency from the chlorides to the iodides. Shifts in the peak position due to the 2Σ 1 2 g and 1Π 3 2 g states from those expected on the ligand field basis. are attributed to slight bonding effects. These effecs cause a large discrepancy between calculated and observed C 4 0 values. The intensities of the five Zn 3d peaks change markedly from HeI- to HeII-excited spectra. The Xα SW method has been used to calculate the intensities of the σ, π and δ 3d orbitals as a function of photon energy. These calculations show dramatic changes in intensity due. for example. to shape resonances. There is usually qualitative agreement between calculated and observed intensities.

High-resolution HeI and HeII photoelectron spectra of the zinc and cadmium dihalide valence bands

We have recorded high-resolution HeI and HeII photoelectron spectra of the valence bands of six MX 2 compounds (M = Zn, Cd: X = Cl, Br, I) in the gas phase. Transition-state Xα SW calculations on all six compounds confirm the assignments. For the first time, we have clearly resolved the spin-orbit splitting of the II g and II u states in the bromide and iodide spectra: and calculated Π u splittings, using a simple MO LCAO model and the D xh double group, are in very good agreement with the experimental values. The relative intensities of the photoelectron peaks change markedly from HeI to HeII spectra, and Xα SW cross section calculations on ZnCl 2 predict these intensity changes rather well.

Effects of vibronic interaction and autoionization on the photoelectron spectrum of N2O

High-resolution and high-sensitivity HeI photoelectron spectra (PES) are reported for the first four valence levels of N2O. The vibrational structure, including many new peaks, is completely assigned for the X 2Π, A 2Σ+, and C 2Σ+ electronic states, and in all three cases excitation of a single quantum of the bending vibration is observed. The bending mode appears as a result of vibronic coupling within and between different electronic states. The Renner–Teller splitting is resolved in the (0,1,0) band of the transition to the X 2Π state. The relative intensities of the vibrational bands in the X 2Π and A 2Σ+ states are compared to the intensities determined from the threshold photoelectron spectra (TPES) of Frey et al. [Chem. Phys. Lett. 54, 411 (1978)]. In both states, autoionization of intermediate neutral states that are nearly degenerate with the molecular ion state to which they decay increases the relative intensities of the higher vibrational bands in the TPES. This resonant autoionization process can enhance bands selectively, and, in particular, the higher quanta of the symmetric stretch are systematically more intense in the TPES than in the HeI PES. New relative photoionization cross sections for N2O and its fragment ions are presented which show that even weak autoionization features can have a substantial effect on the intensities in the TPES.

High resolution photoelectron spectra of deuterated ethylenes

The high resolution Hei photoelectron spectra of ethylene and five of its deuterated derivatives (CH2CHD, CH2CD2, cis- and trans-CHDCHD, and CD2CD2) have been recorded and the vibrational bands in the first systems of the molecular ions have been assigned. The anomalous deuterium shifts have been explained by Fermi resonance between ν2 and ν3 levels in CH2CH2 and cis-CHDCHD. The PE spectra of cis- and trans-CHDCHD were found to be different supporting the D2 symmetry of the molecular ion.