State-selected Ion-molecule Reactions Research Articles

State-selected ion-molecule reactions with VUV synchrotron radiation : the O2 + + C3H6 case

SynopsisO2 + molecular ion is produced by photoionization close to threshold at SOLEIL, DESIRS beam line. By means of coincidence methods, ions are prepared in selected vibrational and electronic states (X, v=0,3,15 and a, v=3). Their reactivity with the propene in single collision regime is studied in the guided ion beam in-strument CERISES, adding ability to vary the collision energy. Absolute reaction cross section and branching ratio are recorded for the 5 most intense reactive channels, described as dissociative charge transfer. Contrasted effects of internal and collision energy are observed.

Comparison of experimental and theoretical quantum-state-selected integral cross-sections for the H2O(+) + H2 (D2) reactions in the collision energy range of 0.04-10.00 eV.

To understand the dynamics of H3O(+) formation, we report a combined experimental-theoretical study of the rovibrationally state-selected ion-molecule reactions H2O(+)(X(2)B1; v1(+)v2(+)v3(+); NKa(+)Kc(+)(+)) + H2 (D2) → H3O(+) (H2DO(+)) + H (D), where (v1(+)v2(+)v3(+)) = (000), (020), and (100) and NKa(+)Kc(+)(+) = 000, 111, and 211. Both quantum dynamics and quasi-classical trajectory calculations were carried out on an accurate full-dimensional ab initio global potential energy surface, which involves nine degrees of freedom. The theoretical results are in good agreement with experimental measurements of the initial state specific integral cross-sections for the formation of H3O(+) (H2DO(+)) and thus provide valuable insights into the surprising rotational enhancement and vibrational inhibition effects in these prototypical ion-molecule reactions that play a key role in the interstellar generation of OH and H2O species.

State-selected ion–molecule reactions with Coulomb-crystallized molecular ions in traps

State-selected Coulomb-crystallized molecular ions were employed for the first time in ion–molecule reaction studies using the prototypical charge-transfer process N2++N2→N2+N2+ as an example. By preparing the reactant ions in a well-defined rovibrational state and localizing them in space by sympathetic cooling to milliKelvin temperatures in an ion trap, state- and energy-controlled reaction experiments with sensitivities on the level of single ions were performed. The experimental results were interpreted with quasi-classical trajectory simulations on a six-dimensional potential-energy surface which provided detailed insight into translation-to-rotation energy transfer occurring during charge transfer between N2 and N2+.

The study of state-selected ion-molecule reactions using the vacuum ultraviolet pulsed field ionization-photoion technique

This paper presents the methodology to generate beams of ions in single quantum states for bimolecular ion-molecule reaction dynamics studies using pulsed field ionization (PFI) of atoms or molecules in high-n Rydberg states produced by vacuum ultraviolet (VUV) synchrotron or laser photoexcitation. Employing the pseudocontinuum high-resolution VUV synchrotron radiation at the Advanced Light Source as the photoionization source, PFI photoions (PFI-PIs) in selected rovibrational states have been generated for ion-molecule reaction studies using a fast-ion gate to pass the PFI-PIs at a fixed delay with respect to the detection of the PFI photoelectrons (PFI-PEs). The fast ion gate provided by a novel interleaved comb wire gate lens is the key for achieving the optimal signal-to-noise ratio in state-selected ion-molecule collision studies using the VUV synchrotron based PFI-PE secondary ion coincidence (PFI-PESICO) method. The most recent development of the VUV laser PFI-PI scheme for state-selected ion-molecule collision studies is also described. Absolute integral cross sections for state-selected H2+ ions ranging from v+ = 0 to 17 in collisions with Ar, Ne, and He at controlled translational energies have been obtained by employing the VUV synchrotron based PFI-PESICO scheme. The comparison between PFI-PESICO cross sections for the H2+(HD+)+Ne and H2+(HD+)+He proton-transfer reactions and theoretical cross sections based on quasiclassical trajectory (QCT) calculations and three-dimensional quantum scattering calculations performed on the most recently available ab initio potential energy surfaces is highlighted. In both reaction systems, quantum scattering resonances enhance the integral cross sections significantly above QCT predictions at low translational and vibrational energies. At higher energies, the agreement between experiment and quasiclassical theory is very good. The profile and magnitude of the kinetic energy dependence of the absolute integral cross sections for the H2+(v+ = 0-2,N+ = 1)+He proton-transfer reaction unambiguously show that the inclusion of Coriolis coupling is important in quantum dynamics scattering calculations of ion-molecule collisions.

Fundamental Aspects of Gas Phase Ion Chemistry Studied Using the Selected Ion Flow Tube Technique

Gas phase ion chemistry using the selected ion flow tube (SIFT) technique is discussed with the focus on fundamental studies on ion structure, energetics, reactivity, and dynamics: Laser-induced fluorescence (LIF) kinetics/dynamics of N2+(v) ions (vibrationally state-selected ion-molecule reactions; energy transfer processes), mechanisms of prototypical organic ion reactions (SN2 nucleophilic substitution and kinetic isotope effects; hydrogen/deuterium exchange; isotope effects in termolecular association), and ion-molecule reactions with biological implications (SN2 reactions at a nitrogen center; ECO2 elimination reactions of alkyl hydroperoxides). Anion chemistry of small nitroalkanes is also presented. A powerful combination of SIFT and negative ion photoelectron spectroscopy is used to study thermochemistry and structure of energetic and exotic ions (alkyl peroxides; anionic derivatives of diazo and azole compounds, cyclooctatetraene, and cyclopentanone). Finally, some of the new directions in SIFT research are discussed, i.e., reactions of ions with polyatomic organic radicals, and chemical ionization mass spectrometry with new detection schemes.

Rovibrational state-selected study of H2+ (X,ν+=0–17, N+=1)+Ar using the pulsed field ionization-photoelectron-secondary ion coincidence scheme

A high-resolution pulsed field ionization photoelectron-secondary ion coincidence (PFI-PESICO) method has been developed for absolute cross section measurements of state-selected ion–molecule reactions. Employing this new PFI-PESICO scheme, we have measured rovibrational-state-selected absolute cross sections for the reaction of H2+(X,ν+=0–17, N+=1)+Ar at the center-of-mass collision energies of 1.1 and 2.4 eV. The ν+=17 state is the second to the last vibrational level of H2+(X), which lies a mere 0.03 eV below the H2+(X) dissociation limit. The absolute cross sections observed for the formation of Ar+ and ArH+ [σv+(Ar++ArH+)] from H2+(X,ν+=0–4, N+=1) are consistent with previous experiments, exhibiting strong enhancement of the reaction cross section for ν+=2. However, the σv+(Ar++ArH+) values for H2+(X,ν+=5–14, N+=1)+Ar are found to be nearly constant. The drop in σv+(Ar++ArH+) observed for H2+(X,ν+=15–17, N+=1)+Ar is attributed to the further dissociation of product ArH+.

State-Selected Ion-Molecule Reactions Studied by the Threshold Electron-Secondary Ion Coincidence (TESICO) Technique, Ar+(2P3/2, 2P1/2) + HCl

Abstract The reactions of Ar+ with HCl were studied for the two spin-orbit states of Ar+, 2P3/2 and 2P1/2, by the threshold electron-secondary ion coincidence (TESICO) technique. State selected relative reaction cross sections of the two product channels, hydrogen abstraction (HA) to form ArH+, and charge transfer (CT) to form HCl+, were determined at five collision energies in the range of 0.71–3.5 eV. At low collision energies, the cross sections of both channels show no substantial difference between the two spin-orbit states. At high collision energies, the cross sections for the HA channel also show no difference between the two states, whereas those for the CT channel show significant dependence on the spin-orbit state, the cross sections for Ar+(2P1/2) being always larger than those for Ar+(2P3/2). Combining these results with the coincidence time-of-flight (TOF) spectra, a direct reaction mechanism is suggested for both channels. The importance of the nonadiabatic transitions between the surfaces correlating to the Ar+ + HCl and Ar + HCl+(A2Σ) states in leading the reaction is proposed. Proton transfer follows the nonadiabatic transition for the HA channel and radiative or nonradiative transition follows for the CT channel.

State-selected ion-molecule reactions: The charge-transfer N2+(X,v+)+O2→O2++N2

The charge-transfer process, which is the subject of this paper, is an important link in the chain of reactive events which occur in the E- and F-regions of the earth’s atmosphere. The reaction is suspected to proceed via the formation of a short-lived intermediate complex (N2⋅O2)+. The transient existence of this van der Waals molecule has been inferred from molecular orbital and ab initio arguments. To the present day, however, none of the numerous studies focusing on this system has provided proof of its existence. The positive evidence obtained from this experiment is of an indirect nature, based on measurements of the kinetic energy of the released O2+ as a function of the expendable center-of-mass collision energy. The results point to an indirect reaction (i.e., via an intermediate) in the low energy regime, and a direct process once the center-of-mass energy exceeds the bond energy of the complex. A model description which contracts both pictures matches the complete set of energy-dependent cross section data in a very persuasive manner. A competing reactive channel which has also been given attention results in the formation of NO+. The initiation of this reaction requires a cleavage of the N–N bond, which should be eased by vibrational excitation of the reagent ion. The cross section data, however, turn out to be unaffected by such details of reagent state preparation.

Molecules in high Rydberg states.

The unusual properties of high molecular Rydberg states with principal quantum number n>>100 have enabled the development of new research tools for the study of molecules and ions in the gas phase. These tools range from spectroscopic techniques such as zero kinetic energy (ZEKE) photoelectron spectroscopy and mass-analyzed threshold ionization (MATI) spectroscopy to techniques that are suited to the investigation of photodissociation processes, bimolecular reactions, and state-selected ion-molecule reactions. This review summarizes recent progress in the understanding of the properties of high molecular Rydberg states and gives an overview of recent chemical applications of these states.

With Courage: The U.S. Army Air Forces in World War II.

Abstract : Resonance-Enhanced Multiphoton Ionization-Photoelectron Spectroscopy. A single time-of-flight (TOF) photo-electron spectrometer is to be used in conjunction with the MPI of jet-cooled molecules. This apparatus will measure energy and single-resolved photoelectron spectra. Such information has been of direct used in measuring the internal state distribution of REMPI-formed ions used in ion-molecule reaction studies. REMPI Spectroscopy of HBr and DBr. The laser induced fluorescence (LIF) technique has been developed to probe ion species. The ions produced directly from REMPI can have their rotational propensity roles in photoionization processes. The ions produced from a reaction between the state selected ions generated by REMPI and other molecules can be observed for studying state to state ion molecule reaction dynamics. State Selected Ion Molecule Reactions. The reactions of ammonia cations with neutral molecule, were conducted using a tandem quadropole mass spectrometer. A new tapole ion trap was proposed to increase vastly our control over and understanding of biomolecular ion-molecular reactions. To make the quadropole/ octapole/quadropole ion trap operational it was necessary to solve the technically challenging problem of interfacing three dissimilar radio frequency (rf) devices.

State-selected ion-molecule reactions: Statistical calculations with constraints

For the two reactive systems, NH3+(Eint)+N H3→NH4++NH2 and H2+(Eint)+H2→H3++H, for which the relative cross sections were measured earlier in our group for Ec.m.≊40 meV we calculated the relative cross section as a function of internal energy using the statistical Rice–Ramsperger–Kassel–Marcus (RRKM) theory that implicitly conserves total energy and total angular momentum. We found satisfactory agreement between theory and experiment by imposing rather mild constraints upon the loose transition state configuration. These constraints involve inactive vibrations and steric hindrance. The steric hindrance imposed in case of the (NH3–NH3)+ system is interpreted as being due to the anisotropic interaction of the ionic charge with the permanent electric dipole of the respective neutral collision partner in the two dissociation channels. We cannot be absolutely sure that the specific combination of modifications we propose for each of the two systems is the only one that agrees well with experiment. However, we find it striking that an agreement can be obtained by such weak and physically meaningful modifications, and we take this as a strong indication that the two studied systems do behave statistical.

New experimental method for studying rotationally state-selected ion-molecule reactions

A new apparatus is described in which a beam of molecular ions in a selected vibration–rotation state is prepared by field ionization of high Rydberg states, in an adaptation of the zero-kinetic-energy photoelectron technique. The state-selected ions undergo low energy reactive collisions within a molecular beam and the ionic products are detected in a quadrupole mass filter. The Rydberg states are populated by two-color stepwise multiphoton excitation, and by tuning to the pseudocontinuum of high-Rydberg states associated with different vibration–rotation states of the ion core, different states of the ion are selected and the effect on reactivity determined. Some preliminary results for the H2+H+2→H+3+H reaction are reported.

ZEKE threshold photoelectron spectroscopy

New methods for the precise (state-detailed) resolution of ionization thresholds open new vistas in photoionization dynamics, ion spectroscopy and state-selected ion-molecule reactions.

Performance of a dodecapole collision chamber for the study of ion–molecule reactions using synchrotron radiation

For the study of state selected ion–molecule reactions using synchrotron radiation, an apparatus incorporating a dodecapole collision chamber has been constructed. Its performance has been examined utilizing the Ne+ ions produced by photoionization of Ne at the 2P3/2 threshold. It has been confirmed that both the transmission of the primary ions and the collection efficiency for the secondary ions remain unchanged when the ion kinetic energy is varied over a wide range of interest. The ion–molecule reaction Ne++OCS has been studied using this apparatus, selecting the spin-orbit states of the reactant ion by the time-of-flight coincidence technique.

State selected ion–molecule reactions by a coincidence technique. XV. Hydrogen atom abstraction as an electron jump followed by proton transfer in the ND+3 (v)+NH3 and NH+3 (v)+ND3 reactions

The effects of the vibrational excitation of the ν2 mode of ND+3 (v) and NH+3 (v) on the three channels of their reaction with NH3 and ND3 , respectively, are investigated up to v=12 in the center-of-mass kinetic energy range from 0.9 to 4.5 eV by use of the TESICO technique. The ratio (γ) of the hydrogen/deuterium atom abstraction cross section over the competing deuteron/proton transfer cross section has a maximum systematically at a vibrational level (vmax ) slightly higher than that giving maximum Franck–Condon factor (vFC ) for the neutralization of ND+3 (v)/NH+3 (v). A new reaction model based on a nonadiabatic transition theory and the potential energy surface calculated for the ammonia dimer cation is proposed to explain the experimental results. The hydrogen/deuterium atom abstraction reaction is interpreted as a near resonant electron jump at larger intermolecular separation followed by proton/deuteron transfer which proceeds on the ‘‘electron (charge)–transferred’’ potential surface.

State-selected ion-molecule reactions: H +2(ν) + He → HeH + + H and He + H + + H

Total cross sections for production of HeH + and H + in the reaction of state-selected H + 2 ( v = 0 to 6) with He at 3.1 eV c.m. collision energy are measured by means of the threshold-photoelectron/photoion coincidence method, using pulsed synchrotron radiation. Both reaction cross sections are observed to rise with vibrational energy. The H +/HeH + branching ratio, which is determined directly, remains approximately constant at about 0.3 for v ⩽ 3 and rises gradually for higher levels to reach the value 1.3 for v = 6. For v ⩽ 3 both reactions involve hard-type collisions and result in large-angle scattering. In contrast, at higher v levels, the HeH + becomes essentially forward scattered with respect to the incident He direction, but with a velocity greater than that expected from the spectator stripping model. The H + products are backward scattered with respect to the incident H + 2 for v ⩽ 1 and receed faster from the He atom than the H products. This observation directly leads to the conclusion that collision-induced dissociation from v = 0 and 1 involves transitions to the first excited potential-energy surface.

State selected ion–molecule reactions by the TESICO technique. XIII. Vibrational state dependence of the cross sections in the reaction C2D+2(ν2)+H2

Vibrational-state selected reaction cross sections have been measured for three product channels of the reaction C2D+2(ν2)+H2, by use of the threshold electron–secondary ion coincidence (TESICO) technique. The ν2 vibrational states of the C2D+2 ion were selected up to v=2 and the collision energies were changed from 0.1 to 2.0 eV. At low collision energies up to 0.2 eV, considerable enhancement of the cross sections for the channel producing C2D2H+ was observed when the vibrational quantum number was increased successively. As the collision energy was increased, the extent of this enhancement diminished gradually and the cross section became almost independent of v at 2.0 eV. The cross section for the H/D exchange channel, on the other hand, was found to decrease with increasing vibrational quantum number at low collision energies. The latter cross section also became independent of v at high collision energies. These results have been explained in terms of the formation of an intermediate complex and the statistical decomposition of this complex to products.

State selected ion–molecule reactions by a TESICO technique. XII. Internal energy dependence of the relative cross section and mechanism branching of the reaction CH+4(v) +CH4 →CH+5 +CH3 and its isotopic variants

Total cross sections of the title reaction studied at collision energies 0.6–3 eV by the TESICO method were found to decrease by about 1.6 times as the internal energy of the reactant ion is increased from 0.1 to 1 eV. Branching ratios of collisional mechanisms were derived from isotope labeling studies: with decreasing total initial energy (sum of collision energy and reactant ion internal energy) from 4 to 0.9 eV the fraction of the dominant proton transfer mechanism slightly decreases, the fraction of H-atom transfer is almost constant, and the fraction of intermediate complex formation increases from about 5% to 15%. Significant isotope effects were observed in the impulsive proton and H-atom transfer mechanisms depending on whether the transferred particle was H or D. The charge transfer channel represents about 10%–50% of the chemical reaction channel.

State selected ion–molecule reactions by a TESICO technique. X. O+2(v)+CH4

Vibrational state selected (relative) reaction cross sections have been determined for v=0–3 of the O+2 ion, for each of the three product channels of the reaction O+2(v)+CH4, viz. O+2(v)+CH4→CH3O+2+H (1) →CH+3+HO2 (2) →CH+4+O2 , (3) using the TESICO (threshold electron–secondary ion coincidence) technique. At a fixed collision energy of 0.27 eV, it has been found that the cross section of exoergic channel (1) increases most prominently with increasing vibrational quantum number v in the range v=0–2, but decreasees sharply in going from v=2 to v=3. The cross sections of endoergic channels (2) and (3) also increase with increasing v but their rates of increase are much smaller than that of channel (1) in the range v=0–2. When v is increased to 3, however, charge transfer channel (3) is enhanced dramatically and the CH+4 ion becomes the most abundant product ion. The cross section of channel (2) also increases more sharply in going from v=2 to v=3 than in the range v=0–2, but the CH+3 ion still remains the least abundant of the three product ions. As a result of these variations in the individual cross sections, the overall cross section for the O+2+CH4 reaction increases monotonically with increasing v throughout the range studied (v=0–3). The results are compared with that of the collision energy dependence as obtained in drift and flow-drift experiments and the implications are discussed in conjunction with the structure of the CH3O+2 ion and the relevant potential energy surfaces.

State selected ion–molecule reactions by a TESICO technique. IX. Vibrational state dependence of the cross sections in the reaction C2H+2(ν2)+D2(H2)

Vibrational-state selected reaction cross sections have been measured for four product channels of the reaction C2H+2+D2, by use of the threshold electron–secondary ion coincidence (TESICO) technique. The ν2 vibrational states of the C2H+2 ion were selected up to v=2 and the collision energies were changed from 0.2 to 3.7 eV. At low collision energies close to 0.2 eV, considerable enhancement of the cross sections for the channels producing C2H2D+ and C2HD+2 was observed when the vibrational quantum number was increased successively. As the collision energy was increased, the extent of this enhancement diminished gradually and the cross sections finally became almost independent of v at about 2 eV. The cross section for the H/D exchange channel, on the other hand, was found to decrease with increasing vibrational quantum number at low collision energies. However, the v=0 and v=1 cross sections of this channel, especially the former, decayed very quickly as the collision energy was increased, reaching essentially zero at ∼2 eV, while the cross section for v=2 remained substantial even at this and higher collision energies. These results have been discussed in terms of two concurrent reaction mechanisms, i.e., a complex mechanism predominating at low collision energies and a direct one predominating at high collision energies. The ν2 state selected cross sections for the reaction C2H+2+H2 →C2H+3+H have also been studied in a similar collision energy range for comparison.