Threshold Photoelectrons Coincidence Research Articles

Threshold photoelectron and electron-ion coincidence spectroscopies: Past, present and future

Threshold photoelectron and electron-ion coincidence spectroscopies: Past, present and future

Photodissociation mechanisms of the CO22+ dication studied using multi-state multiconfiguration second-order perturbation theory

Employing the multi-state multiconfiguration second-order perturbation theory (MS-CASPT2) and complete active space self-consistent field (CASSCF) methods, the geometries, relative energies (T(v)') to the ground state (X(3)Σg(-)), adiabatic excited energies, and photodissociation mechanisms and corresponding kinetic energy releases for the lower-lying 14 electronic states of the CO2 (2+) ion are studied. The T(v)' values are calculated at the experimental geometry of the ground state CO2 molecule using MS-CASPT2 method and highly close to the latest threshold photoelectrons coincidence and time-of-flight photoelectron photoelectron coincidence spectrum observations. The O-loss dissociation potential energy curves (PECs) for these 14 states are drawn using MS-CASPT2 partial optimization method at C(∞v) symmetry with one C-O bond length ranging from 1.05 to 8.0 Å. Those 14 states are confirmed to be correlated to the lowest four dissociation limits [CO(+)(X(2)Σ(+)) + O(+)((4)S(u)), CO(+)(A(2)Π) + O(+)((4)S(u)), CO(+)(X(2)Σ(+)) + O(+)((2)D(u)), and CO(+)(X(2)Σ(+)) + O(+)((2)P(u))] by analyzing Coulomb interaction energies, charges, spin densities, and bond lengths for the geometries at the C-O bond length of 8.0 Å. On the basis of these 14 MS-CASPT2 PECs, several state/state pairs are selected to optimize the minimum energy crossing points (MECPs) at the CASSCF level. And then the CASSCF spin-orbit couplings and CASPT2 state/state energies are calculated at these located MECPs. Based on all of the computational results, the photodissociation mechanisms of CO2(2+) are proposed. The relationships between the present theoretical studies and the previous experiments are discussed.

Photo-double ionization of water studied by threshold photoelectrons coincidence spectroscopy

Photo-double ionization of gaseous water has been investigated in the 30–53 eV energy range using synchrotron radiation and the threshold photoelectrons coincidence (TPEsCO) method. The TPEsCO spectrum of H2O, encompassing the formation of the X˜3B1, 1 1A1, b˜1B1 and 2 1A1 states of H2O2+, was recorded. The TPEsCO spectrum is very broad with no discernable vibrational structure. In addition, the TPE spectrum of H2O for the formation of H2O+ was recorded simultaneously. Indirect double ionization of H2O by dissociative autoionization of H2O+ is found to play a major role in the TPEsCO spectrum well below the lowest vertical double IP.

Spectroscopy and metastability of CO22+ molecular ions

The spectroscopy and metastability of the carbon dioxide doubly charged ion, the CO(2) (2+) dication, have been studied with photoionization experiments: time-of-flight photoelectron photoelectron coincidence (TOF-PEPECO), threshold photoelectrons coincidence (TPEsCO), and threshold photoelectrons and ion coincidence (TPEsCO ion coincidence) spectroscopies. Vibrational structure is observed in TOF-PEPECO and TPEsCO spectra of the ground and first two excited states. The vibrational structure is dominated by the symmetric stretch except in the TPEsCO spectrum of the ground state where an antisymmetric stretch progression is observed. All three vibrational frequencies are deduced for the ground state and symmetric stretch and bending frequencies are deduced for the first two excited states. Some vibrational structure of higher electronic states is also observed. The threshold for double ionization of carbon dioxide is reported as 37.340+/-0.010 eV. The fragmentation of energy selected CO(2) (2+) ions has been investigated with TPEsCO ion coincidence spectroscopy. A band of metastable states from approximately 38.7 to approximately 41 eV above the ground state of neutral CO(2) has been observed in the experimental time window of approximately 0.1-2.3 mus with a tendency towards shorter lifetimes at higher energies. It is proposed that the metastability is due to slow spin forbidden conversion from bound excited singlet states to unbound continuum states of the triplet ground state. Another result of this investigation is the observation of CO(+)+O(+) formation in indirect dissociative double photoionization below the threshold for formation of CO(2) (2+). The threshold for CO(+)+O(+) formation is found to be 35.56+/-0.10 eV or lower, which is more than 2 eV lower than previous measurements.

Photo-double ionization of hydrogen iodide: experiment and theory

Abstract Photo-double ionization of hydrogen iodide has been investigated over the photon energy range 29.0–32.5 eV by threshold photoelectrons coincidence (TPEsCO) spectroscopy and by ab initio calculations that included spin–orbit interaction. Good agreement is found between experiment and theory for the adiabatic ionization potentials for the formation of the X 3 Σ − , a 1 Δ and b 1 Σ + states of HI2+ and for the vibrational separations and relative vibrational intensities within the states. The spin–orbit splitting of the X 3 Σ − state of HI2+ is calculated to be 0.17924 eV for ν2+=0. Because this splitting is close to the calculated vibrational separations within the spin–orbit components of this state [e.g., for ν2+=0–1, 0.22382 eV in 3 Σ 0 − and 0.22163 eV in 3 Σ 1 − ], there is an overlapping of vibrational structure within the TPEsCO spectrum at the resolution used that is confirmed by the simulation spectrum.

Photo-double ionization of HBr and DBr studied by threshold photoelectrons coincidence spectroscopy

Photo-double ionization of HBr and DBr has been investigated in the 32–37 eV energy range using synchrotron radiation and the threshold photoelectrons coincidence (TPEsCO) method. The TPEsCO spectra of HBr and DBr, encompassing the formation of the X 3Σ −, a 1Δ and b 1Σ + states of HBr 2+ and DBr 2+, were recorded using a pair of penetrating-field electron spectrometers. All electronic band systems exhibited vibrational structure. In addition, the threshold photoelectron spectra of HBr and DBr for the formation of singly charged HBr and DBr were recorded simultaneously.

Doubly Charged Molecular Ions Studied by Double Charge Transfer Spectroscopy

AbstractWe have developed a double charge transfer (DCT) spectroscope to study doubly charged molecular ions (dications). In the course of the development, DCT spectra of N2, CO, and NO has been ob served with energy resolution of about 0.5 eV. The spectral profiles observed have shown the Franck‐Condon (FC) behavior. We have made an effort to in crease the resolution up to about 140 meV, which is much better than those (1–1.5 eV) reported in previous works. In the present measurement, the vibrationally resolved DCT spectrum for N2++ could be obtained for the first time. The observed peak structures agree well with those calculated using FC factors, differing meaningfully from the spectra taken with Auger electron spectroscopy (AES) and thresh old photoelectrons coincidence (TPEsCO) spectroscopy. The DCT spectroscopy might be a powerful method to study the potential curves of short‐lived dications.

Direct observation of theKr (3d−14p−1)and Xe(4d−15p−1)doubly charged ion states by threshold-photoelectron coincidence spectroscopy

Threshold-photoelectron coincidence spectra and threshold-photoelectron spectra are measured for Kr and Xe in the energy regions 105--127 and 75--100 eV, respectively. ${\mathrm{Kr}}^{2+}{3d}^{\ensuremath{-}1}{4p}^{\ensuremath{-}1}$ and ${\mathrm{Xe}}^{2+}{4d}^{\ensuremath{-}1}{5p}^{\ensuremath{-}1}$ doubly charged ion states are observed, and also ${\mathrm{Kr}}^{+}{3d}^{\ensuremath{-}1}{4p}^{\ensuremath{-}1}\mathrm{nl}$ and Xe ${4d}^{\ensuremath{-}1}{5p}^{\ensuremath{-}1}\mathrm{nl}$ satellite states. To our knowledge, this is the first direct observation in electron spectroscopy of doubly charged ion states where one hole occurs in an inner shell and another in a valence shell. The results are compared with theoretical calculations performed with an algebraic Green's-function method, and also with previous experimental information from Auger electron spectroscopy.

Investigation of the CS 22+ dication using threshold photoelectrons coincidence spectroscopy

A threshold photoelectrons coincidence (TPEsCO) spectrum of CS 2 has been recorded in the 26.8–29.8 eV range with a resolution of 40 meV using a synchrotron radiation source. The X 3 Σ − g , a 1 Δ g , b 1 Σ + g and c 1 Σ − u states of CS 2 2+ have been observed in the spectrum and their double ionisation potentials determined. Vibrational structure of these states was also observed for the first time and comparison with recent theoretical results allowed its tentative assignment to vibrational progressions associated with the symmetric stretching mode. The experimental spectrum also revealed important contributions from indirect photoionisation processes in the formation of CS 2 2+ from the neutral CS 2 molecule.

A study of and satellite states observed by `threshold photoelectrons coincidence' spectroscopy

The and spectra have been studied from their respective thresholds up to their respective triple ionization potentials by threshold photoelectrons coincidence (TPEsCO) spectroscopy. The and (n = 2, 3 in Ne and Ar, respectively) states of the dications as well as the satellite states, where two electrons are ejected and one is promoted into an unfilled orbital, have been observed. This study is the first extensive observation of the satellite states of dications by electron emission spectroscopy and extends the previous observations by optical emission spectroscopy to the region of the triple ionization threshold. New features have been discerned in the spectra and an assignment is proposed based on previous results from optical spectroscopy and on calculations with an atomic multiconfigurational Dirac - Fock code.

Threshold photoelectrons coincidence spectroscopy of N22+ and CO2+ ions

Threshold photoelectrons coincidence (TPEsCO) spectra of the nitrogen and carbon monoxide dications have been recorded with improved resolution (≈ 40 meV) allowing us to clearly resolve the lowert three vibrational progressions in each species. For the nitrogen dication we have identified, for the first time, the 3Σg− state predicted by theory and determined its constants. In the case of the CO dication, we have determined that the ν = 0 level of the 11Σ+ state is almost degenerate with X3Π, ν = 1 and is thus one vibrational quantum lower than previously believed.

Observation of doubly charged atomic and molecular states by threshold PhotoElectrons Coincidence spectroscopy

States of doubly charged atomic and molecular ions have been studied by Threshold PhotoElectrons COincidence (TPEsCO) spectroscopy. This technique involves the detection, in coincidence, of two zero energy electrons ejected in the photodouble ionisation process. The application of the TPEsCO technique to rare gases and diatomic molecules is discussed. In addition the first observation of the Ar 2+(3s 2+3p 3 nl) satellite states at threshold is reported.

Observation of the Ar2+(3s23p3nl) satellite states by threshold photoelectrons coincidence (TPEsCO) spectroscopy.

The ${\mathrm{Ar}}^{2+}$(3${\mathit{s}}^{2}$3${\mathit{p}}^{3}$nl) satellite states between 73 and 85 eV have been observed directly by threshold photoelectrons coincidence (TPEsCO) spectroscopy. The present study extends the previous observations of these states by optical emission spectroscopies to the region close to the ${\mathrm{Ar}}^{3+}$ ionization. Different features have been discerned in the TPESCO spectrum. A preliminary assignment of these features using the Rydberg formula and values of the quantum defect that are deduced from lower-lying states is proposed.

A threshold-photoelectron coincidence spectrometer using a single detector and time-of-flight analysis

A simple instrument is described for the coincidence measurement of two near-zero energy (threshold) electrons, which can be used for the study of double-photo-ionization processes. The threshold electrons are collected by means of the penetrating field technique and the coincidence measurements are performed using a single detector (channel plates). The application of time-of-flight techniques leads to much improved spectral resolution and yields information on the double-photo-ionization process.

Double photoionization of CO 2, OCS, C 2H 2, CF 4, and C 6H 6 studied by threshold photoelectrons coincidence (TPEsCO) spectroscopy

Double photoionization of CO 2, OCS, C 2H 2, CF 4 and C 6H 6 molecules has been studied in an electron-electron coincidence experiments, where nearly zero kinetic energy (⩽20 meV) electrons are detected. This first application of TPEsCO spectroscopy to polyatomic molecules confirms the ability of this experimental technique to determine accurate double ionization potentials and locate excited electronic states. The spectrum of benzene reveals what appears to be the first observation of vibrational structure in a polyatomic dication.

Threshold photoelectrons coincidence spectroscopy of doubly-charged ions of nitrogen, carbon monoxide, nitric oxide and oxygen

Single photon double ionization of nitrogen (N2), carbon monoxide (CO), nitric oxide (NO) and oxygen (O2) has been studied in an electron-electron coincidence experiment. Threshold photoelectrons coincidence (TPEsCO) spectra have been obtained using a dual penetrating field technique, in which only electrons with nearly zero kinetic energy (<20 meV) are detected. The high sensitivity and energy resolution attainable have allowed observation of several states of N22+, CO2+, NO2+ and O22+, excited essentially at their respective thresholds, resolving vibrational energy levels for nearly all states where they exist. Accurate energies of the features observed in the TPEsCO spectra, and the molecular parameters derived from them, are presented, and the possible mechanisms of formation are discussed.

Threshold photoelectrons coincidence spectroscopy of doubly charged ions of hydrogen chloride and chlorine

Single photon double ionization of hydrogen chloride (HCl) and chlorine (Cl2) has been studied in an electron-electron coincidence experiment. Threshold PhotoElectrons COincidence (TPEsCO) spectra have been obtained by using a penetrating field technique, in which only electrons with nearly zero kinetic energy (<20 meV) are detected. The high sensitivity and energy resolution attainable has allowed observation of several states of HCl2+ and Cl22+, excited essentially at their respective thresholds, including vibrational energy levels for some states. Accurate energies of the dicationic (doubly charged ion) features observed in the TPEsCO spectra are presented, and the possible mechanisms of formation are discussed.

Vibrational structure of the O22+ ground state observed by threshold photoelectron coincidence spectroscopy.

Double ionization of O 2 induced by photons from a synchrotron radiation source has been studied in an electron-electron coincidence experiment. A new technique has been developed wherein only electrons with near zero energy are detected by use of the penetrating field method. In this way high sensitivity and energy resolution have been attained allowing nineteen vibrational levels of the O 2 2+ ground state to be observed and molecular parameters of this state to be obtained

Transmission characteristics of collimated hole threshold photoelectron detectors

The threshold photoelectron-coincidence photoion mass spectrometer is a powerful tool for the study of state-selected ions. Threshold photoelectron detectors used in this apparatus transmit small but significant numbers of electrons at energies up to two orders of magnitude higher than the energy resolution of this detector. It is frequently important to know the fraction of the total coincidence count which results from non-threshold electrons, and to correct for them. This requires measurement of the transmission characteristics of the threshold photoelectron detector over a wide range of energies. We have developed a simple experimental method for checking the transmission performance of such a detector, as well as a mathematical formalism for calculating the transmission function in closed form.