Kinetic Energy Release Distributions Research Articles

Kinetic-energy release distributions and stability of niobium-carbon clusters sputtered from the surface of niobium carbide by xenon ions

The results of studying the emission and fragmentation of niobium-carbon clusters NbmCn+ synthesized upon sputtering a niobium carbide surface with Xe+ ions are presented. The fragmentation channels of NbmCn+ clusters are studied. We present the values of the dissociation energy of some NbmCn+ clusters (m = 1–7, n = 2–8) calculated within the context of the “evaporation ensemble” model and theory of unimolecular reactions based on performed measurements of the kinetic-energy spectra of fragment ions. The obtained results are compared with published data.

Dissociative-electron-attachment dynamics near the 8-eV Feshbach resonance of CO2

We present experimental results for dissociative electron attachment to carbon dioxide near the 8-eV Feshbach resonance. In particular, the dissociation channel leading to O${}^{\ensuremath{-}}$ production has been investigated with a momentum imaging technique that utilizes a supersonic gas jet to form a low-temperature, confined molecular target. Angular fragmentation and kinetic energy release distributions are compared to recent results by Slaughter et al. [J. Phys. B 44, 205203 (2012)] and Wu et al. [Phys. Rev. A 85, 052709 (2012)] using similar techniques. We show that careful attention to weighting of the O${}^{\ensuremath{-}}$ fragmentation momentum space is required to interpret the kinetic energy release observations and that there is no appreciable change in the angular distribution of O${}^{\ensuremath{-}}$ fragments at different energies around the resonance peak, as previously reported by Wu et al. The present O${}^{\ensuremath{-}}$ momentum distribution and kinetic energy release differ from previous results and provide alternative guidance for the theoretical consideration of potential energy surface dynamics that takes place during attachment.

Ejection of triatomic molecular ion [formula omitted] from methyl chloride in an intense femtosecond laser field

Ejection of H3+ and 35Cl+ from CH3Cl molecule is investigated and the Coulomb explosion channels are confirmed by their kinetic energy release and angular distributions. The fragment anisotropy of H3+ ejection can be attributed to the dynamic alignment of CH335Cl2+ induced by femtosecond laser field, the non-zero H3+ ejection angle with respect to the a-axis of CH335Cl2+ and the remarkable overall rotation of the precursor species [CCl+…H3+]. Lifetimes and geometries of precursor species [CCl+…H3+] at different laser intensities are investigated. Competition between Coulomb explosion channels with H3+ and 35Cl+ ejection is also discussed.

Vibrational Excitation and Product Branching Ratios in Dissociation of the Isotopologs of H3O: Experiment and Theory

The dissociation dynamics of the Rydberg radical H3O and the deuterated isotopologs have been studied by dissociative charge exchange of H3O(+) with Cs. Center-of-mass kinetic energy release distributions were measured with a fast-beam translational spectrometer and compared with direct dynamics quasiclassical trajectory calculations with initial conditions from an ab initio potential energy surface for H3O(+). The experimental branching fractions for dissociation of each isotopolog were obtained and compared with the calculated branching fractions. The dominant dissociation channel for all species is elimination of an H/D atom, and the water product was formed with a significant vibrational inversion in stretching vibrations that varies with the mass of the leaving atom in the dissociation. Branching fractions for the mixed isotopologs show that H atom elimination is favored over D atom elimination. Given the importance of H3O(+) in plasmas, astrochemistry, and in condensed phases, the striking energy partitioning found in this neutralization process is notable.

Kinematics and dissociation dynamics of a water molecule under the impact of 10 keV electrons

The kinematics and dissociation dynamics of a H2O molecule induced by 10 keV electrons are studied using a time-of-flight mass spectrometer in conjunction with a position-sensitive detector in multi-hit coincidence mode. Five dissociative channels arising from the complete as well as the incomplete Coulomb explosions of H2Oq+ (q = 2, 3) ions are observed and identified. The dissociation mechanisms (concerted and/or sequential) for these channels are examined. Further, the angular correlation of different fragment ions and the geometrical structure of the precursor ion are studied. The kinetic energy release distributions for the observed channels are also determined. It is found that the pure Coulomb explosion model is insufficient to explain the observed kinetic release distributions. The mean kinetic energy release for these channels is compared with the available data reported by earlier workers who have employed different charged projectiles and sources of photons.

Quantification of ions with identical mass-to-charge (m/z) ratios by velocity-map imaging mass spectrometry

By integrating a velocity-map imaging lens and position sensitive detector into an electron-impact time-of-flight mass spectrometer, it becomes possible to record ion kinetic energy release (KER) distributions for each fragment ion alongside the time-of-flight mass spectrum. The KER distributions allow ions of identical mass-to-charge ratio to be distinguished and quantified.

Momentum spectroscopy of fragment ions of a multiply charged N2O molecule under impact of 10-keV electrons

The dissociative ionization of a N${}_{2}$O molecule is studied at an electron energy of 10 keV using the multiple-ion-coincidence imaging technique. The complete as well as the incomplete Coulomb explosion pathways for N${}_{2}$O${}^{2+}$ and N${}_{2}$O${}^{3+}$ ions are examined and identified. The precursor-specific relative partial ionization cross sections for resulting fragment ions are obtained. It is found that about 81.8$%$ of single ionization, 17.8$%$ of double ionization, and about 0.4$%$ of triple ionization of the parent molecule contribute to the total fragment ion yield. Furthermore, the relative ionic fractions for ion-pair and ion-triple formation are also determined. The kinetic energy release distributions for different coincidence channels are obtained and compared with the available data at lower energies of electron and photon impacts and with that of high-energy ion impact. From the angular correlation studies of fragment ions, it is inferred that states with bent geometries are involved for most of the fragmentation channels of N${}_{2}$O${}^{2+}$ and N${}_{2}$O${}^{3+}$ ions. The concerted and/or sequential nature of the six dissociation pathways is also assigned. No other experimental or theoretical data exist in the literature to compare with the results obtained at the considered impact energy.

Charge-asymmetric dissociation of nitrogen molecules colliding with highly charged ions

Ion pair fragmentation of nitrogen molecules following Ar8+ ion collision was observed by using a position-sensitive time-of-flight method. Kinetic energy release (KER) distributions were derived from the double hit data. Higher-asymmetric fragmentation channels have peak positions of the KER higher than the Columbic value.

Intense laser-induced dissociation pathways of CD+

The dissociation of CD+ induced by intense 30 fs pulses of central wavelengths 385 nm and 790 nm, over a range of peak intensities (5 × 1013 − 1 × 1016 W/cm2), was measured using a coincidence 3D momentum imaging technique. The observed kinetic energy release and angular distributions of the different products are presented and the possible dissociation pathways are discussed.

Quantum control of photodissociation by manipulation of bond softening

We present a method to control photodissociation by manipulating the bond softening mechanism occurring in strong shaped laser fields, by varying the chirp sign and magnitude of an ultra-short laser pulse. Manipulation of bond-softening is experimentally demonstrated for strong field (795 nm, 10^12 - 10^13 W/cm^2) photodissociation of H2+, exhibiting substantial increase of dissociation by positively chirped pulses with respect to both negatively chirped and transform limited pulses. The measured kinetic energy release and angular distributions are used to quantify the degree of control of dissociation. The control mechanism is attributed to the interplay of dynamic alignment and chirped light induced potential curves.

Measurements of intense ultrafast laser-driven D3+fragmentation dynamics

Experiments on the triatomic hydrogen molecular ion in intense ultrashort laser pulses are important for understanding the fundamentals of polyatomic molecular dynamics and for providing a benchmark for theory. Here we extend our earlier measurements [Phys. Rev. Lett. 103, 103004 (2009)] to provide a comprehensive picture of D${}_{3}$${}^{+}$ fragmentation in 7- and 40-fs, 790-nm laser pulses at intensities up to 10${}^{16}$ W/cm${}^{2}$. Our measurements incorporate two- and three-body coincidence three-dimensional momentum imaging involving a crossed-beam setup. We provide details of the relative fragmentation rates of all the possible breakup channels as a function of intensity, as well as kinetic energy release and angular distributions.

Angular and energy distribution of fragment ions in dissociative double photoionization of acetylene molecules at 39 eV

The two-body dissociation reactions of the dication, C(2)H(2)(2+), produced by 39.0 eV double photoionization of acetylene molecules, have been studied by coupling photoelectron-photoion-photoion coincidence and ion imaging techniques. The results provide the kinetic energy and angular distributions of product ions. The analysis of the results indicates that the dissociation leading to C(2)H(+)+H(+) products occurs through a metastable dication with a lifetime of 108±22 ns, and a kinetic energy release (KER) distribution exhibiting a maximum at ∼4.3 eV with a full width at half maximum (FWHM) of about 60%. The reaction leading to CH(2)(+)+C(+) occurs in a time shorter than the typical rotational period of the acetylene molecules (of the order of 10(-12) s). The KER distribution of product ions for this reaction, exhibits a maximum at ∼4.5 eV with a FWHM of about 28%. The symmetric dissociation, leading to CH(+)+CH(+), exhibits a KER distribution with a maximum at ∼5.2 eV with a FWHM of 44%. For the first two reactions the angular distributions of ion products also indicate that the double photoionization of acetylene occurs when the neutral molecule is mainly oriented perpendicularly to the light polarization vector.

Fragmentation and kinetic energy release distribution of ions produced from methanol CH3OH

A double focusing mass spectrometer of reversed Nier-Johnson geometry was used in a research of ionization and fragmentation processes induced by the collision of methanol molecules CH3OH with an energetic electron beam. Two observed metastable fragmentation reactions were studied: decay of the molecular parent ion CH3OH+ into CHOH+ and neutral 2H, and decay of the fragment ion CH3+ into CH2+ and neutral hydrogen. For these reactions the mean values of the kinetic energy release were calculated using MIKE (Mass-Analyzed Ion Kinetic Energy) technique and the KERD (Kinetic Energy Release Distribution) were determined. We obtain the < Ɛ > values of 16 meV and 52 meV for CHOH+ and CH2+ ions, respectively. We suggest a metastable electronically-excited state as a reason for the CH3+ fragmentation reactions.

Investigating the dissociation energies of niobium and vanadium oxide clusters synthesized by ion sputtering

The mass distributions and fragmentation routes of NbnOm+and VnNm+ clusters sputtered from an metal surfaces with Xe+ ions under O2 pressure of P = 4−5 × 10−3 Pa in a bombardment chamber are studied by means of secondary ion mass spectrometry. The spectra of kinetic energy release distribution (KERD) are measured for the most probable routes of the fragmentation of NbnOm+and VnNm+ and clusters, on the basis of which the activation energies of the decomposition (dissociation) of NbnOm+ and VnNm+ clusters are determined within the model of evaporative ensemble and the theory of monomolecular reactions. The results show the closeness of the numerical values of dissociation energies derived using both models.

Formation, structure, and dissociation dynamics of CO2q+(q≤3) ions due to impact of 12-keV electrons

The electron-impact multiple ionization and subsequent dissociation of CO${}_{2}$ is studied for 12-keV electron energy using a linear time-of-flight mass spectrometer coupled with a multihit, position-sensitive detector. The complete as well as incomplete Coulomb explosion pathways for CO${}_{2}$${}^{2+}$ and CO${}_{2}$${}^{3+}$ ions are examined and identified. The kinetic energy release distributions for these precursor ions are obtained. The experimental kinetic energy release values for the complete Coulomb fragmentation channels are found to be overestimated by those calculated from the Coulomb explosion model. From the angular correlation studies, it is inferred that bent geometrical states are involved for most of the fragmentation channels of CO${}_{2}$${}^{2+}$ and CO${}_{2}$${}^{3+}$ ions. The concerted and/or sequential nature of all the dissociation pathways is also assigned. This study provides the first results on energetics associated with the charge separation in dissociative ionization of CO${}_{2}$ under the impact of electrons at a subrelativistic energy.

One- and Two-Dimensional Translational Energy Distributions in the Iodine-Loss Dissociation of 1,2-C2H4I2+ and 1,3-C3H6I2+: What Does This Mean?

Threshold photoelectron photoion coincidence (TPEPICO) has been used to study the sequential photodissociation reaction of internal energy selected 1,2-diiodoethane cations: C(2)H(4)I(2)(+) → C(2)H(4)I(+) + I → C(2)H(3)(+) + I + HI. In the first I-loss reaction, the excess energy is partitioned between the internal energy of the fragment ion C(2)H(4)I(+) and the translational energy. The breakdown diagram of C(2)H(4)I(+) to C(2)H(3)(+), i.e., the fractional ion abundances below and above the second dissociation barrier as a function of the photon energy, yields the internal energy distribution of the first daughter, whereas the time-of-flight peak widths yield the released translational energy in the laboratory frame directly. Both methods indicate that the kinetic energy release in the I-loss step is inconsistent with the phase space theory (PST) predicted two translational degrees of freedom, but is well-described assuming only one translational degree of freedom. Reaction path calculations partly confirm this and show that the reaction coordinate changes character in the dissociation, and it is, thus, highly anisotropic. For comparison, data for the dissociative photoionization of 1,3-diiodopropane are also presented and discussed. Here, the reaction coordinate is expected to be more isotropic, and indeed the two degrees of freedom assumption holds. Characterizing kinetic energy release distributions beyond PST is crucial in deriving accurate dissociative photoionization onset energies in sequential reactions. On the basis of both experimental and theoretical grounds, we also suggest a significant revision of the 298 K heat of formation of 1,2-C(2)H(4)I(2)(g) to 64.5 ± 2.5 kJ mol(-1) and that of CH(2)I(2)(g) to 113.5 ± 2 kJ mol(-1) at 298 K.

The ultraviolet photodissociation of CS2: The S(1D2) channel

The photodissociation of CS(2) has been investigated using velocity-map ion imaging of the S((1)D(2)) atomic photofragments following excitation at 193 nm and at longer wavelengths close to the S((1)D(2)) channel threshold. The experiments probe regions both above and below the energetic barrier to linearity on the (1)Σ(u) (+)((1)B(2)) potential energy surface. The imaging data in both regions indicate that the electronic angular momentum of the S((1)D(2)) atom products is unpolarized, but also reveal different dissociation dynamics in the two regions. Excitation above the barrier to linearity yields an inverted CS((1)Σ(+)) vibrational population distribution, whereas the long-wavelength state-to-state results following excitation below the barrier reveal CS((1)Σ(+))(v, J) coproduct state distributions which are consistent with a statistical partitioning of the energy. Below the barrier, photofragment excitation spectra point to an enhancement of the singlet channel for K = 1, relative to K = 0, where K is the projection of the angular momentum along the principal axis, in agreement with previous work. However, the CS cofragment product state distributions are found to be insensitive to K. It is proposed that dissociation below the barrier to linearity occurs primarily on a surface with a significant potential energy well and without an exit channel barrier, such as that for the ground electronic state. However, oscillatory structure is also observed in the kinetic energy release distributions, which is shown to be consistent with a mapping of parent molecule bending motion. This could indicate the operation of competing direct and indirect dissociation mechanisms below the barrier to linearity.

Fragmentation of sputtered vanadium and niobium cluster ions, kinetic release and dissociation energy

The generation and unimolecular fragmentation of Vn+ and Nbn+ clusters formed in sputtering vanadium and niobium surfaces by Xe+ ions has been studied. The method of measuring the kinetic energy of fragment ions (kinetic energy release distribution) has been used to determine the dissociation energy. Kinetic energy spectra have been measured in the field-free zone (corresponding to a time window of 10−5–10−4 sec after emission) of an ion microanalyzer with double focusing in reverse geometry. The results of spectra measurement were treated using the Rice-Ramsperge-Kassel theory of unimolecular reactions and the “evaporative ensemble”, which allowed us to calculate the dissociation energies of homonuclear Vn1 (n= 5–11) and Nbn1 (n = 3–8) clusters.

Interrelation between the Distributions of Kinetic Energy Release and Emitted Electron Energy following the Decay of Electronic States

In an electronic decay process followed by fragmentation the kinetic energy release and electron spectra can be measured. Classically they are the mirror image of each other, a fact which is often used in practice. Quantum expressions are derived for both spectra and analyzed. It is demonstrated that these spectra carry complementary quantum information and are related to the nuclear dynamics in different participating electronic states. Illustrative examples show that the classical picture of a mirror image can break down and shed light on the underlying physics.

Dissociative double ionization of 1-bromo-2-chloroethane irradiated by an intense femtosecond laser field

The dissociative double ionization and multi-photon ionization of 1-bromo-2-chloroethane (BCE) irradiated by the 800 nm femtosecond laser field have been investigated by dc-slice imaging technology. The charged parent ion ratio [BCE(2+)]/[BCE(+)] was measured, and the corresponding ionization process including non-sequential double ionization and sequential double ionization was analyzed. The sliced images of different photo-dissociated ions were detected, and the corresponding kinetic energy release (KER) distributions were calculated and extracted. Furthermore, the dissociative double ionization channels, attributed to the cleavage of the C-C, C-Br, and C-Cl bonds by the Coulombic repulsive forces, were discussed, and the revised equilibrium distance R(e)*, the energy ratio E(exp)/E(coul), and the value a=√(R(e)*)/(E(exp)/E(coul)) were calculated.