Ion Momentum Research Articles

Angular distribution in two-body dissociation of

SynopsisThe two-body dissociation of multiply charged SO2 molecules is studied using a recoil ion momentum spectrometer. These molecular ions were generated in collision of the neutral parent with an energetic highly charged ion. From the measured momenta of the recoil ions, angular distributions are generated with respect to the incoming beam direction and are found to vary strongly with kinetic energy release.

Three body dissociation of

SynopsisMultiply charged ion impact induced three body fragmentation pathways of (q=3, 4) molecular ions have been studied using the technique of recoil ion momentum spectroscopy. An attempt has been made to distinguish the three-body breakup based on mechanism of dissociation, whether sequential or concerted.

Methods of calibrating kinetic energy release in dissociation process of molecular dications

In the studies of fragmentation processes of molecules induced by extreme ultraviolet photons, intense laser fields, or charged particles, kinetic energy release (KER) is a key physical parameter. It can reveal the electronic states of the parent molecular ion, and provide an insight into the molecular structures and the dissociation dynamics. Therefore, it is essential to obtain the accurate KER spectrum for studying the fragmentation process of molecules. However, in the experiments using reaction microscope, experimental parameters such as the time-of-flight (TOF), the voltage of the TOF spectrometer and the detector image of the fragments have significant influence on the accuracy of KER determination. In this work, by taking the two-body fragmentation process of CO<sup>2+</sup> → C<sup>+</sup> + O<sup>+</sup> induced by 108 keV/u Ne<sup>8+</sup> impact on CO molecules as a prototype, we introduce two methods to accurately calibrate the reconstructed KER spectrum. The first method is to employ two-dimensional momentum spectra of C<sup>+</sup> ions obtained by slicing the momentum sphere. The parameters are correctly calibrated when the circular distribution of the two-dimensional ion momentum image is restored. The second method is to use the correlation spectra of the KER as a function of the emission angle of the C<sup>+</sup> ions to calibrate the experimental parameters, the calibration meets the required level only when the linear dependence of the emission angle on the KER is fulfilled. Then, calibrated KER spectrum is obtained for the dissociation process. By fitting the peak dissociated from the <inline-formula><tex-math id="M1">\begin{document}$^{3}\Sigma^{+}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="20-20200901_M1.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="20-20200901_M1.png"/></alternatives></inline-formula> state of CO<sup>2+</sup> in the KER spectrum, the energy resolution is estimated at 0.24 eV under these experimental conditions. Although these two methods can be used to accurately calibrate the reconstructed KER spectrum, the second calibration method does not require particularly high data statistics, and is suitable for analyzing the processes with lower reaction cross section. Furthermore, this method is convenient for debugging the parameters. Both methods are reliable for parameter calibration and guarantee high accuracy KER for molecular fragmentation experiments in future.

Signatures of Light-Induced Potential Energy Surfaces in

SynopsisUsing theory and Cold Target Recoil Ion Momentum Spectroscopy we find signatures of light-induced molecular potential energy surfaces in the 3-dimensional proton momentum distributions of dissociating .

Coincident detection of recoil ions and photoelectrons after multiphoton-ionisation of cold 6Li atoms trapped in a dipole trap

SynopsisWe report on performing high resolution recoil ion momentum spectroscopy in a reaction microscope (ReMi) on a target of dipole-force trapped cold 6Li atoms in a non-interacting |1〉-|2〉 spin mixture and coincident detection of recoil ions and photoelectrons. Photo-ionization occurs from the electronic ground state of the 6Li atoms and IR photons significantly contribute to the ion yield via 3-photon resonances. High resolution time-of-flight spectra are obtained by modulating the trap depth synced with the arrival of the fs laser pulses.

Investigating the CO trimer geometry using Coulomb imaging technique

SynopsisThe Coulombic explosion of multi-charged (CO)3q+ molecular ions produced in low energy collisions with Ar9+ projectiles was investigated using recoil ion momentum spectrometry (RIMS). A preliminary analysis of the data clearly shows a dominant triangular cyclic structure. Events that may correspond to a linear chain are also observed, but the latter could as well originate from false coincidences, associated to the fragmentation of dimers. A careful analysis of this source of background is proposed to disentangle trimer fragmentation events from false coincidences.

Fragmentation kinematics of SF6 upon photo–excitation of S([formula omitted]) core shell and subsequent Auger decays

The fragmentation kinematics of SF6 upon S(2p) core excitation is studied by a combination of Auger electron-multi-ion coincidence technique and ion momentum spectroscopy. Fragmentation of SF6 is studied for excitation at S 2p1/2 threshold and 4eg shape resonance transition for Auger decays resulting into molecular ions with different electronic configurations. Changes in the branching ratios of fragmentation channels with the photo–excitation energies imply activation of different Auger decay pathways. Experimentally measured kinetic energies of fragment ions indicate that the energy gained by the excited molecular ions is mostly converted to internal energy of neutral/ionic fragments or kinetic energy of neutral fragments.

Two-body fragmentation dynamics of C2H22+ by electron impact: Disentangling vinylidene decarbonation from symmetric breakup

We report experimental investigations of two-body fragmentation of ${\mathrm{C}}_{2}{{\mathrm{H}}_{2}}^{2+}$ induced by 1 keV electron collision utilizing an ion momentum imaging spectrometer. With the ion-ion coincidence measurement, dissociation channels ${\mathrm{C}}_{2}{{\mathrm{H}}_{2}}^{2+}\ensuremath{\rightarrow}{\mathrm{H}}^{+}+{\mathrm{C}}_{2}{\mathrm{H}}^{+}$ (deprotonation) and ${\mathrm{C}}_{2}{{\mathrm{H}}_{2}}^{2+}\ensuremath{\rightarrow}{{\mathrm{H}}_{2}}^{+}+{{\mathrm{C}}_{2}}^{+}$ (${{\mathrm{H}}_{2}}^{+}$ formation) are directly identified, while the symmetric breakup ${\mathrm{C}}_{2}{{\mathrm{H}}_{2}}^{2+}\ensuremath{\rightarrow}\mathrm{C}{\mathrm{H}}^{+}+\mathrm{C}{\mathrm{H}}^{+}$ channel and vinylidene decarbonation ${\mathrm{C}}_{2}{{\mathrm{H}}_{2}}^{2+}\ensuremath{\rightarrow}{\mathrm{C}}^{+}+\mathrm{C}{{\mathrm{H}}_{2}}^{+}$ channel are not well separated in the measured time-of-flight (TOF) correlation map. In this work, by taking advantage of the independence of kinetic energy release (KER) on the dissociation angle, we are able to disentangle the events from the TOF map. Consequently, KER distributions for all four fragmentation channels are deduced, and the relative branching ratios are precisely determined from the measurements. By comparing the measured KER values with the previous calculated potential energy surfaces, pathways for the fragmentation channels are assigned.

Relative phase effect of nonsequential double ionization in Ar by two-color elliptically polarized laser field**Project supported by the National Natural Science Foundation of China (Grant No. 61575077) and the Natural Science Foundation of Jilin Province of China (Grant No. 20180101225JC).

We investigated the nonsequential double ionization (NSDI) in Ar by two-color elliptically polarized laser field with a three-dimensional (3D) classical ensemble method. We study the relative phase effect of NSDI and distinguish two particular recollision channels in NSDI, which are recollision–impact ionization (RII) and recollision-induced excitation with subsequent ionization (RESI), according to the delay-time between the recollision and the final double ionization. The numerical results indicate that the ion momentum distribution is changed and the triangle structure is more obvious with the decrease of the relative phase. We also demonstrate that the RESI process always dominates in the whole double ionization process and the ratio of RESI and RII channels can be influenced by the relative phase.

Angle dependence of dissociative tunneling ionization of NO in asymmetric two-color intense laser fields

Dissociative tunneling ionization of nitric oxide (NO) in linearly polarized phase-locked two-color femtosecond intense laser fields (45 fs, $\ensuremath{\lambda}=800$ and 400 nm, total field intensity $I=1\ifmmode\times\else\texttimes\fi{}{10}^{14}\phantom{\rule{0.16em}{0ex}}\mathrm{W}/\mathrm{c}{\mathrm{m}}^{2}$) has been studied by three-dimensional ion momentum imaging. The ${\mathrm{N}}^{+}$ fragment produced by the dissociative ionization, $\mathrm{NO}\ensuremath{\rightarrow}\mathrm{N}{\mathrm{O}}^{+}+{e}^{\ensuremath{-}}\ensuremath{\rightarrow}{\mathrm{N}}^{+}+\mathrm{O}+{e}^{\ensuremath{-}}$, exhibits a butterflylike momentum distribution peaked at finite angles with respect to the laser polarization direction. In addition, a clear dependence on the relative phase between the two laser fields is observed, showing that the tunneling ionization occurs efficiently when the electric field points from N to O. For the highest kinetic energy component, the observed orientation dependence is well explained with theoretical calculations by the weak-field asymptotic theory for the 2\ensuremath{\pi} highest occupied molecular orbital (HOMO). On the other hand, the peak angle shifts toward the laser polarization direction as the kinetic energy decreases, indicating that pathways other than direct ionization from the HOMO contribute to the dissociative ionization.

Formation of H3+ from hydrocarbon dications induced by collisions with charged particles

Formation mechanism of ${\mathrm{H}}_{3}^{+}$ ions from doubly charged hydrocarbons ($\mathrm{C}{\mathrm{H}}_{4}$ and ${\mathrm{C}}_{2}{\mathrm{H}}_{4}$) is investigated by combining charged particle (300 eV electrons and 3 keV/u ${\mathrm{Ar}}^{8+}$ ions) collision experiments and quantum chemistry calculations. The kinetic energy release (KER) distribution for each ${\mathrm{H}}_{3}^{+}$ loss process was measured with the cold target recoil ion momentum spectroscopy. The good agreement between the mean KER and corresponding theoretical reverse activation energy, related to a transition state and the asymptote of the dissociation products, provides information on the ${\mathrm{H}}_{3}^{+}$ formation dynamics. A ${\mathrm{H}}_{2}$ roaming mechanism is proposed for the formation of ${\mathrm{H}}_{3}^{+}$. Since the hydrocarbons in the interaction with charged particles have direct interests in astronomical environments, the finding of this work would be helpful in the understanding of the energetics and dynamics of ${\mathrm{H}}_{3}^{+}$ formation in space.

Threshold photodissociation dynamics of NO2 studied by time-resolved cold target recoil ion momentum spectroscopy.

We study the near-threshold photodissociation dynamics of NO2 by a kinematically complete femtosecond pump-probe scheme using a cold target recoil ion momentum spectrometer. We excite NO2 to the optically bright Ã2B2 state with a 400 nm pulse and probe the ensuing dynamics via strong field single and double ionization with a 25 fs, 800 nm pulse. The pump spectrum spans the NO(X2Π) + O(3P) dissociation channel threshold, and therefore, following internal conversion, excited NO2 is energetically prepared both "above threshold" (dissociating) and "below threshold" (nondissociating). Experimentally, we can clearly discriminate a weak two-photon pump channel from the dominant single-photon data. In the single ionization channel, we observe NO+ fragments with nonzero momentum at 200 fs delay and an increasing yield of NO+ fragments with near-zero momentum at 3.0 ps delay. For double ionization events, we observe a time-varying Coulombic kinetic energy release between the NO+ and O+ fragments impulsively created from the evolving "hot" neutral ground state. Supported by classical trajectory calculations, we assign the decreasing Coulombic kinetic energy release at longer time delays to the increasing average NO-O distances in the ground electronic state during its large amplitude phase space evolution toward free products. The time-resolved kinetic energy release in the double ionization channel probes the large amplitude ground state evolution from a strongly coupled "inner region" to a loosely coupled "outer region" where one O atom is on average much further away from the NO. Both the time evolution of the kinetic energy release and the NO+ angular distributions support our assignments.

Paradigm shift in thin-film growth by magnetron sputtering: From gas-ion to metal-ion irradiation of the growing film

Ion irradiation is a key tool for controlling the nanostructure, phase content, and physical properties of refractory ceramic thin films grown at low temperatures by magnetron sputtering. However, in contrast to gas-ion bombardment, the effects of metal-ion irradiation on properties of refractory ceramic thin films have not been extensively studied due to (i) low metal-ion concentrations (a few percents) during standard direct-current magnetron sputtering (DCMS) and (ii) difficulties in separating metal-ion from gas-ion fluxes. Recently, the situation has changed dramatically, thanks to the development of high-power impulse magnetron sputtering (HiPIMS), which provides highly-ionized metal-ion plasmas. In addition, careful choice of sputtering conditions allows exploitation of gas-rarefaction effects such that the charge state, energy, and momentum of metal ions incident at the growing film surface can be tuned. This is possible via the use of pulsed substrate bias, synchronized to the metal-ion-rich portion of each HiPIMS pulse. In this review, the authors begin by summarizing the results of time-resolved mass spectrometry analyses performed at the substrate position during HiPIMS and HiPIMS/DCMS cosputtering of transition-metal (TM) targets in Ar and Ar/N2 atmospheres. Knowledge of the temporal evolution of metal- and gas-ion fluxes is essential for precise control of the incident metal-ion energy and for minimizing the role of gas-ion irradiation. Next, the authors review results on the growth of binary, pseudobinary, and pseudoternary TM nitride alloys by metal-ion-synchronized HiPIMS. In contrast to gas ions, a fraction of which are trapped at interstitial sites, metal ions are primarily incorporated at lattice sites resulting in much lower compressive stresses. In addition, the closer mass match with the film-forming species results in more efficient momentum transfer and provides the recoil density and energy necessary to eliminate film porosity at low deposition temperatures. Several novel film-growth pathways have been demonstrated: (i) nanostructured N-doped bcc-CrN0.05 films combining properties typically associated with both metals and ceramics, (ii) fully-dense, hard, and stress-free Ti0.39Al0.61N, (iii) single-phase cubic Ti1−xSixN with the highest reported SiN concentrations, (iv) unprecedented AlN supersaturation in single-phase NaCl-structure V1−xAlxN, and (v) a dramatic increase in the hardness, due to selective heavy-metal ion bombardment during growth, of dense Ti0.92Ta0.08N films deposited with no external heating.

Photoluminescence study of ZnO:Al thin films with different power plasma

ZnO doped Al (ZnO:Al ) thin film was deposited on corning glass substrate using DC magnetron sputtering method. Depositon process of the ZnO:Al thin films was kept constant at Argon pressure, deposition temperature and deposition time are 500 mTorr, 400°C and 2 hours, respectivelly. Furthermore, for deposition process has been done on the variation of power plasma are 33, 43, and 50 watt. For the optical properties of the ZnO:Al thin films using Photoluminescence spectroscopy. Different plasma power will affecting on ion energy and momentum pounder. It’s effect on the quality of thin films that influence to photoluminescence intensity was obtained.

Ion Momentum Imaging Study of the Ion-Molecule Charge Exchange Reaction of Ar+ + CO2.

Three-dimensional ion momentum imaging is developed in a combination of ion velocity map imaging technique and delay-line anode ion detection, and it is applied for the ion-molecule charge exchange reaction between Ar+ and CO2. In a center-of-mass collision energy range of 7.23-15.96 eV, CO2+ products are primarily populated at the ground state X2Πg and the single-electron excited states A2Πu, B2Σu+, and C2Σg+; the multielectron excited states of CO2+ are also found at the higher collision energies. The production efficiency profiles of CO2+ are distinctly different from the photoionization electron spectrum of CO2, implying that the charge transfer from Ar+ would be not fast as expected. The strong electron correlations in the short-lived intermediate (Ar-CO2)+ should be responsible for the CO2+ yields at the multielectron excited states.

Direct observation of interatomic Coulombic decay and subsequent ion-atom scattering in helium nanodroplets

We report on the experimental observation of interatomic Coulombic decay (ICD) in pure $^4$He nanoclusters of mean sizes between $N \approx$ 5000 and 30000 and the subsequent scattering of energetic He$^+$ fragments inside the neutral cluster by using cold target recoil ion momentum spectroscopy. ICD is induced in He clusters by using vacuum ultraviolet light of $h\nu =$ 67 eV from the BESSY II synchrotron. The electronic decay creates two neighboring ions in the cluster at a well-defined distance. The measured fragment energies and angular correlations show that a main energy loss mechanism of these ions inside the cluster is a single hard binary collision with one atom of the cluster.

Study of sheath properties in collisional plasma consisting of non-extensive electrons and thermal ions

Abstract The main purpose of the present work is to study the behavior of sheath properties in unmagnetized collisional plasma. The plasma consists of neutral atoms, thermal positive ions and non-extensive electrons. The ions are described by fluid model approach, while the electrons are considered following the q-non-extensive distribution and obey the Tsallis statistics. In this study, a steady state one-dimensional fluid model based on the ion continuity and momentum transport equations, in presence of source term, coupled to the Poisson’s equation and non-extensive electrons density equation is developed. The modified Bohm sheath criterion is also determined and studied. The numerical results obtained highlight the effect of the nonextensivity q-parameter and the positive ion temperature on the evolution of the quantities characterizing the plasma sheath, such as charged particles densities, ion velocity, electric potential and sheath thickness. It is seen that as the q-parameter increases, the charged particles densities decrease more rapidly in the sheath region while the normalized electric potential increases strongly, which leads to a significant decrease in sheath thickness. It is also shown that by increasing the ion temperature for constant q, or by increasing the value of q-parameter for constant ion temperature, the ion velocity increases significantly in the sheath.

Extending the dynamic range of electronics in a Time Projection Chamber

Abstract When Time Projection Chambers (TPCs) are used in low to intermediate heavy ion collisions, the mass and momentum range of the emitted particles cover a wide range in energy losses. Many TPC readout electronics currently only have a single gain output with a fixed dynamic range. In a recent set of experiments using the SAMURAI Pion-Reconstruction and Ion-Tracker (S π RIT) TPC, it was important to simultaneously measure relativistic pions and heavy ion tracks from the same collisions. As the ionization from a track’s energy loss is collected and multiplied by the anode wires, a distribution of image charges is induced on the TPC read-out pads. If the avalanche on a wire is large enough, the charge collected by pads directly underneath will saturate the readout electronics; pads farther away in the distribution will not be saturated. Using these unsaturated pads and the known pad distribution function, we can estimate the charge on saturated pads, increasing the dynamic range by a factor of 5.

Capture of an external anion beam into a linear Paul trap

We describe a linear Paul trap and high-voltage platform specifically designed for the trapping of a large and pure sample of anions from an external keV-energy beam. Capture and confinement in a static configuration of DC potentials was found to occur by transfer of axial to radial ion momentum due to elastic and inelastic collisions. Stability diagrams both in transmission and capture mode were recorded. The trapping efficiency was observed to be very sensitive to the axial potentials relative to the beam energy. Several 104 oxygen anions were loaded from a 2 keV primary beam and confined for several 100 s. Identification of trapped O− ions and contaminants was achieved by selective laser photodetachment.

Squeezing out higher precision.

Well-timed kicks to an ion's momentum enable better position measurements