Ion-pair Channel Research Articles

Ionization of Dichloromethane by a Vacuum Ultraviolet Krypton Lamp: Competition Between Photoinduced Ion-Pair and Photodissociation-Assisted Photoionization.

The photodissociation and photoionization behaviors of haloalkanes in the VUV regime are important to fully understand the mechanism of ozone depletion in the stratosphere. The ionization of dichloromethane (CH2Cl2) under the irradiation of 10.0 and 10.6 eV light was investigated. CH2Cl+ was observed at 10 Pa, while both CH2Cl+ and CHCl2+ were observed at higher pressure. The production efficiency of CH2Cl+ decreased with the increasing number density of CH2Cl2, while that of CHCl2+ increased. A kinetic model was successfully derived to quantitatively describe the variation trends of CH2Cl+ and CHCl2+, in which the competition between photoinduced ion-pair and photodissociation-assisted photoionization (PD-PI) were included. The ion-pair channel was quenched efficiently at higher pressure or concentration, which reduced its contribution. Our study proposed new insights into the complicated photoexcitation behaviors of CH2Cl2 in the VUV regime and revealed the important role of photodissociation in photoionization at low photon flux.

Investigating core-excited states of nitrosyl chloride (ClNO) and their break-up dynamics following Auger decay.

The fragmentation of ClNO upon resonant core-electron excitation to the LUMO and LUMO+1 orbitals at the N and O K-edges is investigated. The produced fragment ions were detected in coincidence with a position sensitive ion time-of-flight detector which enables deduction of the angular distribution of the ions. This facilitates a comparison between the two resonances and the two K-edges with respect to fragmentation time, transition dipole moment orientation, fragment yield of single-ion and ion-pair channels, and fragmentation mechanisms. We observe significant correlations between the core-excited site and the location of the bonds that are broken, as well as the dissociation time. Moreover, we observe preferential cleavage of specific bonds upon excitation to the LUMO and LUMO+1 states which can be attributed to their orbital character.

Inelastic processes in collisions of lithium positive ions with hydrogen anions and atoms

Inelastic processes in the low-energy collisions Li3+ + H−, Li2+ + H, Li2+ + H- and Li+ + H are investigated for all collisional channels with the excited ionic lithium states Li2+ (nl) and Li+ (1s nl) up to and including the corresponding ion-pair states for the temperature range 1000–20 000 K. For all possible processes in the Li3+ + H- and Li2+ + H collisions inelastic cross sections and rate coefficients are calculated for the transitions between the ion-pair channel Li3+ + H- and the 35 below lying contributing Li2+ (nl) + H channels. It is found that the highest values of cross sections and rate coefficients are obtained for the recombination processes and their inverse, the ion-pair formation processes, involving the Li2+ (3l), Li2+ (4l), and Li2+ (5l) states. For the processes in the Li2+ + H- and Li+ + H collisions, cross sections and rate coefficients are calculated for all transitions between 34 Li+ (1s nl) + H channels lying below Li2+ + H- plus this ion-pair channel. In this case the highest rate coefficients correspond to the recombination processes with the Li+(1s3l1,3L) and Li+(1s4l1,3L) final states, as well as their inverse processes of ion-pair production. Rate coefficient values for these most efficient processes are rather high, of the order of 10−8 cm3/s. This leads to total recombination rate coefficients in Li3+ + H- and Li2+ + H- collisions with values larger than 10−7 cm3/s.Graphical abstract

A model for charge transfer in ultracold Rydberg ground-state atomic collisions

In excited molecules, the interaction between the covalent Rydberg and ion-pair channels forms a unique class of excited states, in which the infinite manifold of vibrational levels are the equivalent of atomic Rydberg states with a heavy electron mass. Production of the ion-pair states usually requires excitation through one or several interacting Rydberg states; these interacting channels lead to loss of flux, diminishing the rate of ion-pair production. Here, we develop an analytical, asymptotic charge-transfer model for the interaction between ultracold Rydberg molecular states, and employ this method to demonstrate the utility of off-resonant field control over the ion-pair formation, with near unity efficiency.

Double-arm three-dimensional ion imaging apparatus for the study of ion pair channels in resonance enhanced multiphoton ionization.

We present a novel experimental configuration for the full quantitative characterization of the multichannel resonance enhanced multiphoton ionization (REMPI) of small molecules in cases when the ion-pair dissociation channel is important. For this purpose, a double-arm time-of-flight mass spectrometer with three-dimensional (3D) ion imaging detectors at both arms is constructed. The REMPI of HCl molecules is used to examine the constructed setup. The apparatus allows us to perform simultaneous measurements of the 3D velocity vector distributions of positive (H(+), HCl(+), and Cl(+)) and negative (Cl(-)) photoions. The characterization consists of the determination of "two-photon absorption cross sections" for the process HCl(X)+2hν → HCl*, one-photon absorption cross sections for subsequent processes HCl* + hν → HCl*, and the probability of the subsequent non-adiabatic transition HCl* → HCl(B) → H(+) + Cl(-), which leads to ionic pairs. All these data should be obtained from the analysis of the dependencies of the number of ions on the laser energy. The full characterization of the laser beam and the knowledge of the ion detection probability are necessary parts of the analysis. Detailed knowledge of losses of produced ions in the mass spectrometer before detection requires understanding and characterization of such processes like electron emission from metallic grids under ion bombardment or charge transfer between positive ions and the metal surface of the grids, like Cl(+) + (grid) → Cl(-). These important phenomena from surface science are rarely discussed in the imaging literature, and here, we try to compensate for this shortcoming.

On the reactivity of ion pairs into different diatomic systems

SynopsisIon pair collisions are an important process in many astrophysical environments. We present ab initio calculations of highly excited states of C2+ and identify the ion pair channel C2+/C−. We use these results to interpret recent experiments on carbon cluster dissociation.

Core photoionization of the argon dimer in the photon-energy range of 255–340 eV studied by a photoelectron-photoion-photoion coincidence technique

Single-photon multiple ionization of the argon dimer van der Waals complex, ${\mathrm{Ar}}_{2}$, is studied by the photoelectron-photoion-photoion coincidence technique using synchrotron radiation in the photon-energy range of 255--340 eV, which covers the Ar 2$p$ and Ar 2$s$ ionization continua. Dissociation processes into ${\mathrm{Ar}}^{+}+{\mathrm{Ar}}^{+}$, ${\mathrm{Ar}}^{+}+{\mathrm{Ar}}^{2+}$, ${\mathrm{Ar}}^{+}+{\mathrm{Ar}}^{3+}$, and ${\mathrm{Ar}}^{2+}+{\mathrm{Ar}}^{2+}$ ion pair channels are observed. The ${\mathrm{Ar}}^{+}+{\mathrm{Ar}}^{+}$ and ${\mathrm{Ar}}^{+}+{\mathrm{Ar}}^{2+}$ channels show the most intense ion-ion coincidences, compared to other observed dissociative channels. For the four observed channels the intensities are integrated and compared as functions of photon energy.

Photoionization of CH3N3 Produces 3B2 N3–: A Theoretical and Experimental Study of the Ion-Pair Channel

We observe two ion formation thresholds when monochromatized synchrotron radiation is tuned through the onset of the methyl azide ion-pair channel (CH3N3 + hν → CH3+ + N3–). We assign these to production of the two lowest electronic states of the azide anion: the singlet linear (1Σg+) and the triplet bent (3B2) forms of N3–. This finding is supported by extensive quantum chemical calculations on the N3– singlet and triplet potential energy surfaces. Quantum chemical calculations are also used to rule out alternative ionization channels that exhibit the same m/z ratios: NH+ + isomers of CH2N2–. A value of 292.9 kJ/mol for the D0(CH3–N3) is suggested.

Dissociation dynamics of ion-pair states of Cl2at principal quantum numbers beyond 1500

Long-lived ion-pair states of Cl{sub 2} have been observed by delayed pulsed-field ionization in the vicinity of the Cl{sup -}({sup 1}S{sub 0})+Cl{sup +}({sup 3}P{sub 2}) dissociation threshold following single-photon excitation from the X {sup 1}{Sigma}{sub g}{sup +}(v=0) ground state with a tunable vacuum-ultraviolet laser. The field-ionization spectra reveal a series of resonances corresponding to ion-pair states with effective principal quantum number n* between 1858 and 1876 belonging to a series converging to the Cl{sup -}({sup 1}S{sub 0})+Cl{sup +}({sup 3}P{sub 0}) dissociation threshold. These states are observed by forced predissociation into the Cl{sup -}({sup 1}S{sub 0})+Cl{sup +}({sup 3}P{sub 2}) ion-pair channel. This process is the ion-pair analog of the process of forced autoionization observed in Rydberg states. The analysis of the spectra and of the field-ionization behavior provides information on the couplings between the relevant ionization and dissociation channels and has enabled the determination of the ion-pair dissociation threshold [E{sub IPD}(Cl{sup -}({sup 1}S{sub 0})+Cl{sup +}({sup 3}P{sub 2}))=95 449.8{+-}1.0 cm{sup -1}] and of the dissociation energies of Cl{sub 2} [D{sub 0}(X {sup 1}{Sigma}{sub g}{sup +})=19 998.4{+-}1.1 cm{sup -1}] and Cl{sub 2}{sup +} [D{sub 0}(X{sup +} {sup 2}{Pi}{sub u})=31 942.1{+-}1.5 cm{sup -1}].

Velocity Imaging Studies on Ion-Pair Dissociation of CH3Br + hνVUV → CH3+ + Br- as a Function of Wavelength

The ion-pair dissociation dynamics of CH3Br + hνVUV → CH3+ + Br- as a function of vacuum ultraviolet (VUV) wavelength have been studied by means of velocity imaging. Photofragment excitation (PHOFEX) spectra as well as images for both the cation and the anion products have been collected, which confirm the dissociation occurs through the ion-pair channel from the momentum matching of both products. The wavelength region for the PHOFEX covers a small portion (from 121.4 to 122.7 nm) of the absorption spectrum where transitions from the ground state to both ns and np Rydberg states as well as to the underlying continuum states are all present. Images were taken at selected wavelengths both on the peaks and in the valleys of the PHOFEX spectrum, as well as at 118.22 nm. The PHOFEX spectra closely resemble the absorption spectra of CH3Br, and the images taken at peaks and continuum wavelengths in PHOFEX show no marked differences with respect to the product energy and angular distributions. These observations prompt us to conclude that the ion-pair state is directly populated by the VUV photons and the ion-pair dissociation is one of the major exit channels for the excited states in the energy region just below the ionization energy of the parent molecule, although large uncertainties in wavelength in the absorption spectra make the proposed mechanisms less conclusive.

Ion-pair formation and product branching ratios in dissociative recombination ofHD+

The cross section for ion-pair formation produced by dissociative recombination of low-energy (≤10 eV) electrons with HD + is calculated using a time-dependent wave-packet method. The source of the experimentally observedoscillations as a function of energy in this cross section is confirmed to be an interference effect arising from the existence of multiple pathways to the final product. The total cross section for dissociative recombination as well as the branching ratios into the various product channels has also been calculated. These cross sections, including the ion-pair channel, are in good agreement with the available experimental data.

A Determination of the Bond Dissociation Energy (D0(H−SH)): Threshold Ion-Pair Production Spectroscopy (TIPPS) of a Triatomic Molecule

We present the first threshold ion-pair production spectrum of a triatomic molecule. We have recorded the ion-pair yield spectrum and TIPP spectrum for the H2S → H++SH- ion-pair channel using single-photon excitation. From the TIPP spectrum, we have determined the H−SH bond energy (31451 ± 4 cm-1) to unprecedented accuracy and demonstrated the formation of weakly bound H+−SH- (J‘) ion-pair states, with rotational excitation of the SH- anion up to J‘ = 4. The bound nature of these states, and the assigned spectrum that results from their field dissociation suggests that this technique can be applied to many other triatomic and larger polyatomic molecules in the future, leading to energetic, spectroscopic, and dynamical information about these species.

Vibrationally resolved cross sections for single-photon ionization of LiH

Ab initio studies of the single-photon ionization of LiH(X 1Σ+) leading to LiH+(X 2Σ+) are reported. The process is studied as function of the internuclear distance (2 a.u.–6 a.u.) and the kinetic energy of the electron (0.05 eV–10 eV). The calculations are based on the so-called iterative Schwinger approach which is implemented within the frozen-core single center approximation. By comparing vibrationally resolved cross sections calculated beyond and within the Franck–Condon principle we find, that the Franck–Condon principle applies relatively well to the present photoionization process. The vibrational wave functions needed for these calculations are obtained from very accurate CI-potentials, also reported in this paper. For LiH the first 10 vibrational spacings are found to agree with the experimental data within 0.2%. Photoelectron spectra (PES) calculated under simulated experimental conditions suggest that photoionization experiments on LiH may yield information on the vibrational structure of LiH+. The calculated permanent dipole moment of the LiH ground state shows strong influence of the ion-pair channel (Li+H−).

Electron capture and fragmentation in Ar11+ + CO collisions

Collisions between 32.2, 130.5 and 570.5 keV Ar11+ ions and CO molecules have been studied using the Macdonald Laboratory CRYEBIS. Coincidence time of flight was used to detect all recoil ions originating from each molecule and a position sensitive detector was used to determine final projectile charge states. Single-and double-electron capture cross-sections are much larger than those for ionization at these collision energies. The dominant recoil channel associated with the Ar10+ final charge state is the CO+ molecular ion. The main ion-pair channel is the C+ + O+ dissociation of CO2+ while the relative yields of higher charge states of the transient COq+ fall off rapidly. The dissociated ions corresponding to charge states up to CO4+ were detected in coincidence with Ar10+ (and Ar9+), indicating that multielectron capture followed by autoionization occurs.

Negative-ion mass spectrometric study of ion-pair formation in the vacuum ultraviolet. III. SF6→F−+SF+5

Ion-pair formation from photoexcitation of SF6 has been studied by negative-ion mass spectrometry using synchrotron radiation in the 11.27–31.0 eV photon energy range. Negative ions F−, SF−6, and SF−5 have been observed. The appearance energy of the F− ion is about 1 eV higher than the thermochemical threshold for the formation of the pair of the ground state ions F−(1Sg) and SF+5(X̃1A1). The peak features observed in the F− efficiency curve are interpreted as resulting from transitions to neutral excited states with the 1T1u symmetry which effectively couple with ion-pair states through avoided potential surface crossings. The peaks assigned to diffuse Rydberg states are distinctively enhanced in the F− efficiency curve, probably because of large transition probabilities from the dissociative Rydberg states to the ion-pair states. In contrast, the excited states of valence type autoionize in a short period and have quite small branching to the ion-pair channel. Consequently, the corresponding peaks are markedly suppressed in the F− spectrum. Assignments of the peak features in the previous photoabsorption spectra are also performed by using the term values for related Rydberg and virtual valence orbitals. Other negative ions observed, SF−6 and SF−5, are produced by resonance capture of low energy electrons emitted by photoionization of the parent molecules, and are not of major concern of the present study.

Excitation transfer into bound and continuum states investigated by optical and electron spectroscopy

We have investigated the reaction of metastable helium He(21 S, 23 S) with the homogeneous halogens Br2 and I2 and with the heterogeneous molecules ICl and IBr. Further, we have taken fluorescence spectra from these reactions. For reasons of comparison, we have taken the same set of data with HeI photons as projectiles. This rather complete set of data reveals several general trends concerning the reaction of metastable helium atoms with halogen molecules. One result consists in very direct support for the previously published statement that the process of Penning ionization is very stereospecific with respect to the involved target orbital. Further, we find interesting hints as for the dynamics of the reaction which proceeds via formation of an intermediate ion-pair channel.

Hyperthermal molecular beam scattering: K–O2 ion/neutral product angular, energy, and branching ratio analysis

Angular distributions of K+ ions and K atoms from collisions of a beam of hyperthermal K atoms with a cross beam of thermal O2 molecules were determined in the range from 18 to 38 eV (lab). A pronounced rainbow was observed in the ion-pair channel at a reduced angle of 240 eV deg. A smaller rainbow was also found in the neutral channel at 220 eV deg. Energy loss distributions for both product species were also determined at 28 eV (lab) by time-of-flight measurements. Both K+ ions and K atoms from K+O2 were detected concurrently. A pseudorandom pulsing method was used to increase the duty cycle. The TOF spectra indicated overlapping energy-loss distributions corresponding to ground and excited state formation for both the neutral and ion products. Excited state to ground state branching ratios for both the neutral and ion products were determined as a function of reduced angle. The neutral branching ratio shows a pronounced peak at ∼180 eV deg., while the ion ratio increases rapidly above 200 eV deg. From these ratios the differential cross sections were calculated for neutral K in which either K or O2 is electronically excited. These cross sections are similar to the differential cross sections of K+ ions. The neutral to ion branching ratios, also determined, exhibit a broad minimum in the vicinity of the rainbow angle. Differential cross sections for neutral and ion scattering were integrated to give the total neutral/ion branching ratio of ∼4. A preliminary analysis of the experimental differential cross sections has been performed via an atom–atom model. Although the model has substantial and expected deficiencies, the analysis strongly indicates that the quartet surfaces emanating from the neutral ground electronic asymptote must be substantially more repulsive than the corresponding doublet surface. An analysis of the low velocity features in the TOF measurements of K indicates production of the lowest electronically excited state of O2 (i.e., 1Δg) as well as electronically excited states of K. A similar analysis of the TOF spectra of K+ suggests the occurrence of autoionization from the neutral ground state surfaces to the K+(1S0)+O2(1Δg) surface.