Threshold Photoelectron Spectroscopy Research Articles

Threshold photoelectron spectroscopy on inner-valence ionic states of NO

The NO(+) states lying in the ionization region of 20-40 eV have been investigated by high-resolution threshold photoelectron spectroscopy and a configuration interaction calculation. Substantial agreement between the structures on the present experimental and theoretical spectra in the 21-27 eV range enables us to assign the relevant inner-valence ionic states unambiguously. The dissociation products from the ion states are measured with threshold photoelectron-photoion coincidence spectroscopy, and the dissociation processes are discussed with reference to the potential energy curves calculated. Sharp peaks are observed in the ionization region of 27.5-35 eV, which are allocated to ionic Rydberg states converging to NO(2+).

Satellite structures of Kr and Xe studied by high-resolution threshold photoelectron spectroscopy

A high-resolution threshold photoelectron spectrum of Xe was measured in the photon energy region of the 5s25p4nl states. The peak width of the measured Xe 5s−1 main line was close to 5 meV. A previously measured spectrum of Kr was also analysed. Rich satellite peaks were clearly observed, including many peaks not observed in previous optical, photoelectron and threshold photoelectron studies. The satellite peak energies and their assignments for Kr and Xe are listed. The observed Rydberg series converging to the Xe2+ ground state was assigned to the (3P2)ng series by detailed analysis. The energy level of the Xe2+ 3P2 state as the converging limit of the observed Rydberg series was obtained as 38.355 eV.

Threshold photoelectron–photoion coincidence study of the fragmentation of valence states of CH2F–CH3+

We have carried out a comprehensive study of the decay dynamics of the valence electronic states of CH2F–CH3+, by using threshold photoelectron–photoion coincidence (TPEPICO) spectroscopy and a tunable vacuum ultraviolet synchrotron radiation source. Threshold photoelectron spectroscopy (TPES) has been recorded for the first time in the photon energy range of ∼12–24 eV. Ion yield curves and breakdown diagrams have been determined in the energy range of ∼ 11.5–18 eV. The dissociation process and fragmental mechanism of the valence states of the parent ion are discussed. The mean translational kinetic energy releases for fragmentation of CH2F–CH3+, via a single bond cleavage have been measured and compared with the predictions of statistical and impulsive mechanisms. For dissociation CH2F–CH3+→CHF–CH3+ + H+e−, a non-statistical decay is assumed, and there seems to be a transition to statistical behaviour with increasing photon energy. For cleavage of the C–C bond to form CH2F+, there seems again to be a transition to statistical decay. The ionization threshold measured from the TPES and the appearance energies (AEs) identified from the ion yield curves are presented. An upper limit for the ionization threshold of CH2F–CH3 (11.66 ± 0.03 eV) and the enthalpy of formation of CH2F–CH3+ at 298 K (679 ± 3 kJ mol−1) are determined.

Rovibrational photoionization dynamics of methyl and its isotopomers studied by high-resolution photoionization and photoelectron spectroscopy

High-resolution photoionization and pulsed-field-ionization zero-kinetic-energy photoelectron spectra of CH(3), CH(2)D, CHD(2), and CD(3) have been recorded in the vicinity of the first adiabatic ionization threshold following single-photon excitation from the ground neutral state using a narrow-bandwidth vacuum-ultraviolet laser. The radicals were produced from the precursor molecules methyl-bromide, methyl-iodide, dimethyl-thioether, acetone, and nitromethane by 193 nm excimer photolysis in a quartz capillary and were subsequently cooled to a rotational temperature T(rot) approximately equal to 30 K in a supersonic expansion. Nitromethane was identified as a particularly suitable photolytic precursor of methyl for studies by photoionization and threshold photoelectron spectroscopy. Thanks to the cold rotational temperature reached in the supersonic expansion, the rotational structure of the threshold ionization spectra could be resolved, and the photoionization dynamics investigated. Rydberg series converging on excited rotational levels of CH(3) (+) could be observed in the range of principal quantum number n=30-50, and both rotational autoionization and predissociation were identified as decay processes in the threshold region. The observed photoionization transitions can be understood in the realm of an orbital model for direct ionization but the intensity distributions can only be fully accounted for if the rotational channel interactions mediated by the quadrupole of the cation are considered. Improved values of the adiabatic ionization thresholds were derived for all isotopomers [CH(3): 79 356.2(15) cm(-1), CH(2)D: 79 338.8(15) cm(-1), CHD(2): 79 319.1(15) cm(-1), and CD(3): 79 296.4(15) cm(-1)].

Inner-valence states of N2+ and the dissociation dynamics studied by threshold photoelectron spectroscopy and configuration interaction calculation

The N2(+) states lying in the ionization region of 26-45 eV and the dissociation dynamics are investigated by high-resolution threshold photoelectron spectroscopy and threshold photoelectron-photoion coincidence spectroscopy. The threshold photoelectron spectrum exhibits several broad bands as well as sharp peaks. The band features are assigned to the N2(+) states associated with the removal of an inner-valence electron, by a comparison with a configuration interaction calculation. In contrast, most of the sharp peaks on the threshold photoelectron spectrum are allocated to ionic Rydberg states converging to N2(2+). Dissociation products formed from the inner-valence N2(+) states are determined by threshold photoelectron-photoion coincidence spectroscopy. The dissociation dynamics of the inner-valence ionic states is discussed with reference to the potential energy curves calculated.

Development of a threshold positron spectrometer

We introduce a new positron spectroscopy experimental technique that we call threshold positron spectroscopy, by analogy with its similar threshold photoelectron spectroscopy. An apparatus that is being developed to explore this technique is described in this work.

On the ionization energy of CF 3SF 5 in the valence region measured by angle-resolved photoelectron spectroscopy

Ionization energies for valence electrons of CF 3SF 5 were measured in the 12–35 eV region by means of angle-resolved photoelectron spectroscopy. The observed ionization energies were basically consistent with previous ones measured by threshold photoelectron and HeI photoelectron spectroscopy, and the ionization levels at 22.60, 26.92 and 27.86 eV were newly observed. Asymmetry parameter derived at hν = 684.7 eV was a constant of β = 1.02 ± 0.04 in the ionization energy lower than 20 eV, suggesting that lone pair electron of F atom is probably released through 2p → ɛd like electron emission. Characters of the 22 valence bands were elucidated with the help of ab initio calculations and of the nature of valence orbitals for CF 4 and SF 6.

Threshold photoelectron spectroscopy of benzene up to 26.5 eV

The threshold photoelectron spectrum of benzene has been recorded up to 26.5 eV photon energy under high-resolution conditions using synchrotron radiation and employing the penetrating-field threshold electron collection method. By means of a direct comparative study with a recent HeI photoelectron spectrum of benzene of equally high resolution, numerous autoionization effects are observed in the formation of the ionic band systems of benzene in the outer valence ionization region in the threshold photoelectron spectrum. The Rydberg states responsible for these effects are identified. Autoionization does not appear to play a role in the formation of the two lowest-energy, inner-valence bands of C6H6+.Key words: threshold photoelectron spectroscopy, photoelectron spectroscopy, benzene, ionization, autoionization, Rydberg states.

希ガスダイマーイオンの振動準位と解離過程

Threshold photoelectron spectroscopy is a useful tool for measuring vibrational levels of rare gas dimer ions. In this short review, features of threshold photoelectron spectroscopy are introduced. After that, the observed vibra-tional levels of Ar2+ are described in detail. Explanations for the vibrational levels of KrXe+ measured by threshold photoelectron-photoion coincidence technique are also given. In the subsequent study, it is revealed that the II (1/2g) states of Kr2+ and Xe2+ completely dissociate via optical transitions to the repulsive I (1/2g) states, while that of Ail does not dissociate. Precise analysis of the time-of-flight spectrum for the Kr2+ fragment from the Kr2+ II (1/2u) state is also shown. The potential energy curve of the I (1/2g) state and the lifetime of the optical transition from the II (1/2g) state to the I (1/2g) state are obtained from this analysis.

Inner-valence states of O2+ and dissociation dynamics studied by threshold photoelectron spectroscopy and a configuration interaction calculation

High-resolution threshold photoelectron spectroscopy has been used to study O2 in the 24–50 eV photon energy region, where several broad bands with superimposed fine peaks are observed. Theoretical calculations on the basis of the state-averaged complete-active-space self-consistent-field method followed by the second order configuration interaction method reproduce well the general band features of the threshold photoelectron spectrum, enabling us to assign the bands to inner-valence O2+ states. Dissociation products formed from the inner-valence O2+ states have been investigated by threshold photoelectron–photoion coincidence spectroscopy and their dissociation dynamics discussed with reference to the theoretical potential energy curves.

Threshold processes in the photoionisation of atoms and molecules

The technique of threshold photoelectron spectroscopy using synchrotron radiation is described and recent technical developments are discussed. The sorts of information that such studies can provide about atoms and molecules are considered. Examples of recent experimental studies of both single and double photoionisation are given in the context of the high photon resolution now provided by synchrotron radiation sources.

The lowest electronic states of Ne2 +, Ar2 + and Kr2 +: comparison of theory and experiment

Non-relativistic configuration interaction (CI) ab initio calculations using large basis sets have been carried out to determine the potential curves of the first electronic states of Ne2 +, Ar2 + and Kr2 +. The spin—orbit interaction was treated assuming that the spin—orbit coupling constant is independent of the internuclear separation (R). For Ar2 +, calculated dissociation energies and equilibrium separations are in good agreement with experimental results. The calculations for Ne2 + suggest that the lowest vibrational level of the I(1/2u) ground state observed by threshold photoelectron spectroscopy by Hall et al. [1995, J. Phys. B: At. molec. opt. Phys., 28, 2435] and assigned to either ν = 0 or ν = 2 actually corresponds to ν = 4. The calculations also predict the I(1/2g) state of Ne2 + and Ar2 + to possess a double-well potential and that of Kr2 + to be repulsive at short range and to only possess a single shallow well at large internuclear separation. The ab initio calculations provide an explanation for the observation made by Yoshii et al. [2002, J. chem. Phys., 117, 1517] that Kr2 + and Xe2 + dissociate after photoemission from the II(1/2u) state to the I(1/2g) state whereas Ar2 + does not.

Threshold photoelectron spectroscopy with velocity focusing: an ideal match for coincidence studies

Velocity focusing of low energy electrons has been used to collect threshold photoelectron spectra with a continuous tunable vacuum UV light source. Velocity focusing permits focusing the photoionization image from 5 mm×8 mm to 1 mm×1 mm, and thus to collect threshold electrons (less than 10 meV) with a collection efficiency close to 50%. The overall resolution for Xe + of 13 meV is limited primarily by the 12 meV band pass of the photon monochromator. The collection efficiency for energetic electrons of 1 eV is reduced 0.07%. This method of detecting threshold electrons has improved the resolution by a factor of 4 and increased the signal by a factor of 10. It is an ideal approach for photoelectron photoion coincidence (PEPICO) studies because it permits the collection of electrons and ions with relatively high electric fields of 20 V/cm, which are necessary to have narrow time of flight peaks for the coincident ion signals.

Threshold photoelectron spectroscopy using synchrotron radiation

Recent developments in the techniques of threshold photoelectron spectroscopy using synchrotron radiation are discussed. The spectroscopic information that such studies can provide about molecules are considered. Examples of current experimental techniques of threshold spectroscopy are given in the context of the high photon resolution now provided by synchrotron radiation sources. These techniques are illustrated by recent experimental results.

Threshold photoelectron spectroscopy in the inner-valence ionization region and photo-double ionization of SF6

The threshold photoelectron (TPE) spectrum of SF6 has been recorded over the photon energy range 25-140 eV using synchrotron radiation and a penetrating-field electron spectrometer. In addition, the photo-double ionization spectrum of SF6 has been obtained over the energy range 30.7-49.3 eV using the threshold photoelectrons coincidence technique. The TPE spectrum in the inner-valence ionization region is found to be significantly different to the conventional photoelectron spectrum in the same binding energy region. Indirect autoionization of neutral Rydberg and shape-resonance states of SF6 dominate the formation of the TPE spectrum. Inner-valence SF6+ ion states appear to play a major role in the formation of the SF62+ ion states. The onset of double ionization in SF6 was found to occur at 31.98±0.02 eV.

Observation and characterization of the fluorescence decay of the 2s2p(6)np 1P(o) excited states of Ne

Fluorescence decay of the 2s2p(6)np (1)P(o) excited states of neon has been observed directly for the first time. This process has also been observed indirectly by threshold photoelectron spectroscopy and in electron/photon coincidences. The measurements have allowed this weak decay channel to be characterized and branching ratios derived. The fluorescence excitation spectrum is well described by a simple Rydberg model that highlights the interplay and competition between the decay channels of autoionization and fluorescence.

Valence-level photofragmentation of gaseous Si(CH3)Cl3 and solid-state analogs studied with synchrotron radiation

The dissociative photoionization channels of gaseous Si(CH3)Cl3 and ion desorption mechanisms of solid-state analogs following valence-level excitation have been investigated by means of photoionization mass spectroscopy, threshold photoelectron spectroscopy (TPES), and photon-stimulated ion desorption (PSID) using synchroton radiation. The adiabatic ionization threshold of the parent molecular ion was determined to be 11.18 eV, consistent with the value of 11.16 eV obtained from the TPES spectrum. An energy shift ∼0.8 eV toward lower binding energies for the orbitals of solid Si(CH3)Cl3 with respect to the gas phase values was observed. Two thresholds at 14.97 and 17.51 eV in the CH3+ photoionization efficiency spectrum are probably associated with the ionization of 2e″ and 11a1 orbitals, respectively. The H+ desorption threshold at 20.1 eV in the PSID spectrum may be attributed to the excitation of C 2s electron correlation states to the unoccupied states. The Cl+ desorption threshold at 19.9 eV is likely initiated by an Auger-stimulated desorption process.

Observation of excited states of H 2+ by threshold photoelectron spectroscopy

The excited states of H 2 + have been studied by threshold photoelectron spectroscopy. The relative intensities of these states with respect to that of the H 2 + ground state have been determined and compared with previous measurements obtained. The mechanisms that lead to the formation of the excited ionic states and to their appearance in the threshold spectra are discussed.

Threshold photoelectron spectroscopy of HF and DF in the outer valence ionization region

We have recorded threshold photoelectron spectra of HF and DF over the outer valence ionization region under good resolution conditions (3-6 meV) using synchrotron radiation and employing the penetrating-field electron detection technique. The spectra show extensive vibrational structure in the X(2i) system in the first Franck-Condon gap region that is attributed to resonance autoionization of Rydberg states lying in this energy range. The Rydberg states responsible for these effects are identified as [A(2+)]ns 1+, with n = 4-6. Analyses of all the vibrational data contained in these spectra for the X(2i) state using a modified isotopic vibrational Dunham equation have led to greatly improved spectroscopic constants for this state in HF+ and DF+. The same method was also used to determine improved spectroscopic constants for the A(2+) state. Autoionization is found not to be important in the formation of the A(2+) state due to the absence of Rydberg states in the energy region of the A state. Based on the observations and interpretations of the present work, combined with literature data, we have constructed a potential-energy diagram of the relevant states of HF and HF+. Partially rotationally resolved threshold photoelectron spectra of HF+ and DF+ over the v+ = 0 vibrational band of the X 2i system show pronounced effects due to autoionization processes.

Threshold photoelectron spectroscopy of HCl and DCl

We have measured the threshold photoelectron spectra of HCl and DCl with varying resolution (3–40 meV) using tunable synchrotron radiation and employing the penetrating-field electron-spectroscopic technique over a wide photon energy range (up to 34 eV) that includes the inner-valence ionization region in the molecules. The observed spectra display features produced by both direct ionization and autoionization processes. Autoionization is found to play a particularly important role in the outer-valence ionization region of these molecules. For example, in the formation of the v +=0 band of the X ( 2 Π 3/2) subsystem, in the populating of high vibrational levels of the X ( 2 Π i) system in the Franck–Condon gap region between the X ( 2 Π i) and A ( 2 Σ +) states, and in the predissociated levels of the A ( 2 Σ +) state. The production of extended vibrational structure in the X ( 2 Π i) system is attributed to resonance autoionization of 4sσ, 5sσ and 6sσ 1 Σ + Rydberg states. These and other threshold photoelectron results suggest a propensity for threshold electron production from sσ Rydberg states. Photoionization of HCl and DCl in the inner-valence ionization region between 25 and 30 eV yields nine vibrationally resolved band systems that include the `main line' 3sσ −1 band system and four sets of pairs of vibrationally very similar satellite ion systems. Absolute vibrational numbering in these systems was greatly facilitated by having the isotopomer threshold photoelectron spectra.