High-resolution Threshold Photoelectron Research Articles

High-resolution threshold photoelectron study of the Kr 4s satellite states

High-resolution threshold photoelectron spectra of Kr were measured in the photon energy region between the Kr + 4s −1 main line and the Kr 2+ 1D 2 state. The measured peak width was close to 7 meV. Rich satellite structures, 178 peaks including 48 newly observed peaks, were found in these spectra. Assignments of these levels were performed by comparison with previous optical, photoelectron, and threshold photoelectron measurements. The Rydberg series converging to the Kr 2+ 3P 2 states, which had not been observed, as well as three Rydberg series converging to the Kr 2+ 1D 2 states, some levels of which had been observed, were observed. The energy levels of the Kr 2+ 3P 2 and 1D 2 states as converging limits of the observed Rydberg series were obtained as 38.355 and 40.180 eV, respectively.

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.

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.

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+.

Threshold photoelectron spectroscopy of iodine monobromide

A high-resolution (1–7 meV) threshold photoelectron spectroscopic study of IBr was performed using synchrotron radiation and a penetrating-field electron spectrometer over the valence ionization region of the molecule. Extensive vibrational structure was found in all three electronic-state band systems ( X 2 Π i, A 2 Π i and B 2 Σ + ) of IBr +. In the ( X 2 Π i ) band system both spin–orbit components exhibited extended vibrational structure in the Franck–Condon gap regions that is attributed to resonance autoionization of neutral Rydberg states lying in these energy regions. Analysis of this vibrational structure yielded accurate spectroscopic constants.

Vibrationally resolved threshold photoelectron–photoion coincidence spectra of ArXe

High-resolution threshold photoelectron–photoion coincidence spectra of ArXe were measured in the photon energy region 11.9–15.9 eV using the penetrating field technique and synchrotron radiation. Six vibrational progressions were observed and ascribed to the production of the six electronic states of ArXe+ ions (X 1/2, A1 3/2, A2 1/2, C1 3/2, B 1/2, and C2 1/2) via photoionization of the ground state ArXe dimer. The lowest vibrational level ν=0 of the ground X 1/2 state and some of the other vibrational levels were observed for the first time in the present measurements. The vibrational constants were obtained from observed vibrational energy levels. The dissociation energy of the ground state (X 1/2) was obtained as D00=1432±8 cm−1.

Threshold photoelectron spectroscopy of iodine monochloride

A high-resolution (1–11 meV) threshold photoelectron spectroscopic study of ICl has been performed using synchrotron radiation and a penetrating-field electron spectrometer over the valence ionization region of the molecule. Accurate vibrational constants have been obtained for both spin–orbit components of the ICl + (X 2 Π i ) state through the observation of extended vibrational bands in the Franck–Condon gap regions of the molecular ion. The appearance of these vibrational bands is attributed to resonant autoionization of Rydberg states residing in this energy region of the ionic state. The adiabatic ( v +=0) ionization potentials for formation of the ground state of the ICl + ion are found to be 10.076±0.002 eV for (X 2 Π 3/2 ) and 10.655±0.002 eV for (X 2 Π 1/2 ) yielding a spin–orbit splitting in this state of 0.579±0.002 eV. The adiabatic ( v +=0) ionization potential for formation of the (A 2 Π i ) state of ICl + is estimated to be at ∼12.5 eV. Additional autoionization effects are noted in the formation of the (A 2 Π i ) and (B 2 Σ + ) band systems in ICl +.

Vibrationally resolved threshold photoelectron–photoion coincidence spectra of KrXe

High resolution threshold photoelectron–photoion coincidence spectra of KrXe were measured in the photon energy region 11.7–14.7 eV using the penetrating field technique and synchrotron radiation. Six vibrational progressions were observed and ascribed to the production of the six electronic states of KrXe+ ions (X 1/2, A1 3/2, A2 1/2, C1 3/2, B 1/2, and C2 1/2) via photoionization of the ground state KrXe dimer. The vibrational structures for the X 1/2, A2 1/2, B 1/2, and C2 1/2 states were observed for the first time in the present measurements. The molecular constants for vibration were obtained from the analysis of the deduced vibrational energy levels. The potential curves of each state were drawn by using the Morse function.

Two-color zero kinetic energy–pulsed field ionization spectroscopy of the acetone n-radical cation: the a 2 torsional vibration

Two-color, (2+1′) resonance multiphoton ionization via the 1A 2 3p x Rydberg state is used in combination with high-resolution threshold photoelectron spectroscopy (ZEKE–PFI) to measure the a 2 ( ν 12) anti-gearing torsional frequencies in the n-radical cation of acetone (-h 6 and -d 6). The photoelectron band intensities and measured torsional frequencies show that the anti-gearing torsional potentials for the cation and the 3p x Rydberg state are very similar. The cation torsional frequencies also indicate that, despite the localization of positive charge in the carbonyl group, the barrier for synchronous methyl rotation in the cation is higher than that for the neutral acetone ground state.

Threshold Photoelectron−Photoion Coincidence Spectroscopy of Perfluorocarbons. 2. Unsaturated and Cyclic Perfluorocarbons C2F4, C3F6, 2-C4F8, and c-C4F8

Threshold photoelectron−photoion coincidence (TPEPICO) spectroscopy of three unsaturated (C2F4, C3F6, 2-C4F8) and one cyclic (c-C4F8) perfluorocarbons has been performed using vacuum-ultraviolet radiation from a synchrotron source in the energy range 10−27 eV. Electrons and ions are detected by threshold electron analysis and time-of-flight mass spectrometry, respectively, allowing breakdown diagrams that show the formation probability of fragment/parent ions as a function of the internal energy of the parent ion to be obtained. Fixed-energy TPEPICO spectra with improved time resolution were performed on some of the fragment ions observed, from which the mean kinetic energy released in fragmentation was determined. High-resolution (ca. 0.01 eV at 10 eV) threshold photoelectron spectra of the three unsaturated species have been recorded, and assignments have been made of the type of electron removed by photoionization to the observed states of the parent ion. Unlike the saturated perfluorocarbons studied i...

High-resolution threshold photoelectron spectra of molecular oxygen in the 18–24 eV region

A high-resolution threshold photoelectron spectrum of molecular oxygen has been studied between 18 and 24 eV using the penetrating field technique and a synchrotron radiation source. Nine ionic states are observed, and three of them for the first time. Higher vibrational levels of the b 4Σg− state and the 2Φu state are also studied. Furthermore, the doublet structure of the 2Φu state is resolved for the first time. Two new states with quartet multiplicity are assigned to the 4Πg and the 4Πu states which dissociate to the limit, [O(3P)+O+(2D)], at 22.059 eV. Also we tentatively assign another newly observed band to the 2Πu state. The long vibrational progression of the 3 2Πu state is clearly observed in the region 22.3–23.8 eV with doublet splitting which increases with increasing of the vibrational quantum number.

High resolution threshold photoelectron spectrum of oxygen in the 12–19 eV region

The threshold photoelectron spectrum of oxygen has been measured with high resolution in the photon energy range of 12–19 eV using a penetrating field technique and a synchrotron radiation source. The long vibrational progression of the X2Πg state of O2+ was observed to v′=45. An extrapolation in a Birge–Sponer plot (ΔG versus v′+1/2) gives a maximum vibrational level vmax′=56. The A2Πu state was also measured in detail. Most vibrational levels of the A2Πu state of O2+ (from v′=0 to 24) were observed for the first time. A Birge–Sponer plot of the A2Πu state gives a maximum vibrational level vmax′=31. The dissociation limit of the A2Πu state is slightly higher (76 meV) than the well-known dissociation limit O(3P2)+O+(3S0) 18.733 eV. Therefore the A2Πu state may be considered to have a hump. We observed new states which were assigned to Σg+4 and Σu+2 according to a previous theoretical investigation [Beebe et al., J. Chem. Phys. 64, 2080 (1976)]. Potential curves of the X2Πg and A2Πu states were calculated by the Rydberg–Klein–Rees method using both the rotational constants obtained previously by others [K. P. Huber and G. Herzberg, Molecular Spectra and Molecular Structure, Constants of Diatomic Molecules Volume IV (Van Nostrand, New York, 1974)] and the vibrational energy values observed in the present experiment. A Le Roy–Bernstein plot as well as a Birge–Sponer plot were used to determine the dissociation energy of the X2Πg state. However, the plot gave an unreasonably large dissociation limit.

High-resolution threshold photoelectron spectroscopy of hydrogen iodide

The threshold photoelectron spectrum of hydrogen iodide has been recorded at high resolution (4–30 meV) using synchrotron radiation and a penetrating-field electron spectrometer over the photon energy range 10.2–31.0 eV. Extended vibrational structure in the first Franck-Condon gap region between the X ( 2Π i) and A ( 2Σ +) states of HI + has been observed and analyzed to yield improved spectroscopic constants for the X ( 2Π i) state. The Rydberg states responsible for the production of this structure, through resonance autoionization, are identified as [ A ( 2 Σ +]nsσ 1 Σ + for n = 6–8. A vibrational progression in the A ( 2Σ +) band system in HI + leading up to the dissociation continuum of this state has been identified for the first time, yielding spectroscopic constants for the state. The A ( 2Σ +) state of HI + appears to be formed by an avoided crossing between the diabatic A ( 2 Σ 1 2 + ) and 4 Π 1 2 potentials. Spectroscopic constants are derived for the ‘main’ band system for 5s σ −1 inner-valence ionization of HI.

High-resolution threshold photoelectron spectroscopy of molecular fluorine

The threshold photoelectron spectrum of molecular fluorine has been recorded in the 5.6–21.9 eV photon energy range, at resolutions ranging from 3 to 12 meV, using synchrotron radiation and a penetrating-field electron spectrometer. In addition to observing the three known band systems of F 2 + at higher resolution than previously achieved with conventional photoelectron spectroscopy, extensive vibrational structure is found in the Franck-Condon gaps between the main electronic systems of F 2 +. This extended vibrational structure is attributed to resonance autoionization of neutral Rydberg states.

Vibrational structure of the N+2 ground state observed by threshold photoelectron spectroscopy

The long vibrational progression of the ground state of N+2 was observed in a high resolution threshold photoelectron spectrum obtained using the penetrating field technique and synchrotron radiation. Vibrational states were observed up to v′=67 for the first time and the final vibrational level was deduced to be v′=77 from extrapolation. The complete molecular constants of this state were obtained and the entire potential curve was then drawn.

Threshold photoelectron spectrum of O 2 in 31–34 eV region

High resolution threshold photoelectron spectra of O 2 were obtained in the region of 31–34 eV. Three vibrational progressions were observed. These structures are assigned to two ionic Rydberg states converging to the O 2 2+ ground state and to a valence state.

High resolution threshold photoelectron spectroscopy of rare gas dimers

Information on the structure of the ions of neon, argon, krypton and xenon dimers has been obtained using threshold photoelectron spectroscopy and synchrotron radiation under conditions of high resolution. Vibrational structure has been well resolved for the ground state of all these species thus allowing accurate values for the spectroscopic constants to be derived. Structure corresponding to excited states of the dimer ions dissociating to the 2P3/2,1/2 levels of the atomic ion have also been identified and their dissociation energies determined.

Vibronic coupling in the X̃ 2Π and à 2Σ+ states of HCN+

The vibronic structure associated with the X̃ 2Π and à 2Σ+ states of HCN+ have been examined by high resolution threshold photoelectron spectroscopy using coherent vacuum ultraviolet radiation and pulsed field ionization techniques. Rotationally resolved spectra for 12 vibronic bands are presented from which vibronic symmetry assignments and cation spectroscopic constants are derived. The latter are compared to the ab initio calculations of Köppel et al. [Chem. Phys. 37, 303 (1979)] who calculated the vibronic structure of HCN+ based on strong X̃–à interstate coupling. Finally, the observed rotational branch structure is used to infer the ionization dynamics of strongly coupled vibronic levels for which the Born–Oppenheimer approximation is not valid.

Ground and excited states of Xe+2 observed by high resolution threshold photoelectron spectroscopy of Xe2

Threshold photoelectron spectra of the xenon dimer have been observed with a resolution of 2 meV in the wavelength range 920–945 Å and 1022–1112 Å using the penetrating field technique and synchrotron radiation. Threshold photoelectron bands associated with transitions to the A2 Σ+1/2u, B2 Π3/2g, C2 Π3/2u, C2 Π1/2u, and D2 Σ+1/2g states of Xe+2 have been identified. Vibrational structure associated with the C2 Π1/2u state has been observed for the first time and a new value of the D2 Σ+1/2g state ionization potential is reported.

Satellite and resonance states in the near-threshold photoionization of argon

High resolution threshold photoelectron spectroscopy, in conjunction with synchrotron radiation has been employed to investigate single and double photoionization of argon in the 32-51 eV photon energy range. We have been able to distinguish between satellite and resonance states observed in our spectrum and thereby correct previous misassignments. Furthermore, we have developed a method to extract partial cross sections for satellite states from threshold to 150 meV. The excitation functions of these electron-correlated processes generally show a strong threshold peak, which we attribute to dynamic effects within the excitation complex, as well as other peaks due to the nearby neutral resonances. In addition, the apparent asymmetry of the Ar2+1D threshold cusp is discussed in terms of a possible pressure effect.