Fragmentation Threshold Research Articles

A comparison of the femto-, pico- and nano-second multiphoton ionization and dissociation processes of NO2 at 248 and 496 nm

The multiphoton ionization and dissociation of NO 2 at 248 nm ( B ̃ 2 B 2 ←X 2 A 1 ) and 496 nm has been carried out for a number of different laser pulsewidths from 15 ns to 300 fs by time-of-flight mass spectrometry in a molecular beam. At 248 nm the NO 2 /NO ion ratio (NO 2 + /NO + ) varies from about 1% using nanosecond laser pulses to about 60% for the shortest pulses used (300 fs). In contrast, at 496 nm the NO 2 + /NO + ratio remains very small for excitation with both 5 ps and 500 fs pulses. These results are interpreted in terms of the limited knowledge of the dissociation rates above the NO(X)+O( 1 D) fragmentation threshold at 40126 cm −1 using a simple rate equation model. The results can be described using dissociation rates which increase with decreasing pulsewidth, an idea which conforms with the distinction that has been drawn between time and frequency domain experiments. The implications of the results for the analytical potential of femtosecond mass spectrometry are discussed.

A comparison of the femto-, pico- and nano-second multiphoton ionization and dissociation processes of NO 2 at 248 and 496 nm

The multiphoton ionization and dissociation of NO 2 at 248 nm ( B ̃ 2B 2←X 2A 1 ) and 496 nm has been carried out for a number of different laser pulsewidths from 15 ns to 300 fs by time-of-flight mass spectrometry in a molecular beam. At 248 nm the NO 2/NO ion ratio (NO 2 +/NO +) varies from about 1% using nanosecond laser pulses to about 60% for the shortest pulses used (300 fs). In contrast, at 496 nm the NO 2 +/NO + ratio remains very small for excitation with both 5 ps and 500 fs pulses. These results are interpreted in terms of the limited knowledge of the dissociation rates above the NO(X)+O( 1D) fragmentation threshold at 40126 cm −1 using a simple rate equation model. The results can be described using dissociation rates which increase with decreasing pulsewidth, an idea which conforms with the distinction that has been drawn between time and frequency domain experiments. The implications of the results for the analytical potential of femtosecond mass spectrometry are discussed.

The fragmentation threshold in two-electron atoms: an illustration with classical orbits

Semiclassical S-matrix theory is applied to electron-hydrogen scattering near the fragmentation threshold E=0. The relation between resonant scattering (classically chaotic motion for E<0) and ionization (E>0) is worked out with emphasis on the qualitative behaviour of the different types of orbit.

Ionization of hydrogen by positron impact near the fragmentation threshold.

The semiclassical approximation of Feynman's path integral is used to calculate the S matrix for the positron-impact ionization of hydrogen. The formulation provides a full scattering amplitude, and more importantly does not require knowledge of the asymptotic three-body Coulomb state in the continuum. In the limit of vanishing excess energy, the results confirm Wannier's classical model for fragmentation [Phys. Rev. 90, 817 (1953)]. The experimentally observable ratio of fragmentation versus total ionization (including positronium formation) is predicted.

Size Distribution of Collisionally Evolved Asteroidal Populations: Analytical Solution for Self-Similar Collision Cascades

Collision cascades play a prominent role in processing the present asteroid belt, as well as formative planetary systems. As a first step toward understanding how this mechanism operates, an analytical solution has been obtained for the steady-state size distribution of a self-similar collisional fragmentation cascade. It is found that this corresponds to a power law for the differential mass distribution dn = Cm-αdm , where the exponent α is very nearly 11/6, equivalent to a differential radius distribution dn = C *r-3.5dr. This is in agreement with the earlier conclusion of J. S. Dohnanyi (1969, J. Geophys. Res. 74, 2531-2554). The work of Dohnanyi has been extended considerably to include a full treatment of the simultaneous occurrence of cratering and catastrophic fragmentation. A more physically realistic model of catastrophic fragmentation is used in which the size of the largest fragment decreases with projectile mass. The average steady-state value of the exponent α (1.8333) is found to be extremely insensitive to the assumed physical parameters of the fragmentation process, such as the strength of the target, the threshold for catastrophic fragmentation, the relative contribution of cratering and catastrophic fragmentation, the size and number of the largest fragments, and the size distribution of the fragments produced by individual fragmentation events. The robust nature of the result is a consequence of the steady-state solution being primarily the consequence of geometrical rather than mechanical considerations.

Ionization potentials and fragmentation thresholds of the heteroclusters benzene-(SF 6) m, m = 1–4

Applying mass selective two-color resonant two-photon ionization spectroscopy, we have measured the ionization and fragmentation thresholds of small benzene-(SF 6) m clusters. For the resonant excitation of the electronic S 1 state either the O 0 0 vibrationless transition or the 6 1 0 vibronic transition of the heteroclusters were used. Simple theoretical model estimations were considered to interpret the experimental results. As results well defined ionization potentials for the four clusters benzene-(SF 6 m , n = 1–4, were obtained while the measured fragmentation threshold could be identified as the appearance potential only for the two smallest heteroclusters ( m = 1,2). In this case the binding energies of the clusters in the ionic and the neutral ground states have been derived. For the larger clusters ( m = 3, 4) a kinetic shift has been observed causing higher fragmentation thresholds than the true appearance potentials.

Dissociation energies for carbon cluster ions (C+2–15): A system where photodissociation is misleading

This letter reports a collision-induced dissociation study of the stabilities and fragmentation patterns for carbon cluster ions C+n (n = 2–15). For clusters of six or more atoms, the primary fragmentation channel is loss of C3 neutral. Fragmentation thresholds are analyzed to yield dissociation energies, which are found to be substantially higher than the upper limits estimated from photodissociation experiments of Geusic et al. [Z. Phys. D 3, 309 (1986)], but in good agreement with theoretical estimates. The stabilities oscillate strongly, with some evidence of a change in behavior as size increases above 9 atoms. This is the size where the most stable structure is believed to change from linear to cyclic. Photodissociation measurements were also done, and we find that the clusters dissociate efficiently at photon energies far below the CID thresholds. Possible reasons for the anomalous photodissociation results are discussed.

Biliary calculi fragmentation by a 308 nm excimer laser: a preliminary study.

The use of a 308-nm XeCl excimer laser for biliary stone fragmentation is reported. The 130-nsec laser pulses are delivered via UV grade fused silica fibers to the target stones immersed in normal saline solution and placed in direct contact with the fiber. Sixty biliary calculi, 20 cholesterol and 40 pigment, were fragmented in vitro. The energy delivered per unit mass of the stone is kept constant at 50 mJ/mg. The effect of laser repetition rate, energy fluence, and fiber core size on stone fragmentation was studied. Fragmentation thresholds for a variety of biliary calculi of known composition were measured. It was found that higher fragmentation efficiency was obtained with larger fluence, lower repetition rate, and fiber of larger core. Our study indicates that the 308-nm excimer laser may be effective as a laser lithotriptor with low threshold and good efficiency for biliary stone fragmentation.

Experimental Cholelitholysis with the Pulsed Tunable Dye Laser

This study evaluates the pulsed tunable dye laser with wavelength 504 nm, frequency 10 Hz, and pulse width 1.2 microseconds for cholelitholysis. Power of 10-40 kW was directed through a 250-microns quartz fiber optic to ablate 55 gallstones (removed from 14 patients). The fiber was positioned in direct contact with the stones under saline. Power delivery was begun at 10 kW and increased in 10-kW increments until litholysis began. The range of power and energy necessary to fragment the gallstones was evaluated on four common bile ducts (fresh autopsy specimens). Following fragmentation, all stones were analyzed. There were 35 cholesterol stones (3 calcified) and 20 bilirubin stones (4 calcified). Size ranged from 0.012 to 7.56 cm3 (mean 0.96 +/- 1.41 cm3). Energy necessary for fragmentation ranged from 0.4 to 11.2 J (exposure time 1.0-28 s). Power necessary for fragmentation was 20 kW for 2/55 stones and 40 kW for 53/55 stones. At 40 kW (40 mJ/pulse), common bile duct perforation occurred within 1.1 +/- 0.1 s (0.44 +/- 0.04 J). The pulsed tunable dye laser can fragment gallstones of all compositions. The threshold for fragmentation is 40 kW, but common bile duct perforation occurs at this power. We conclude that laser radiation sufficient to fragment gallstones can injure the common bile duct.

Fragmentation of biliary stones with a 308 nm excimer laser

The use of a XeCl excimer laser (308 nm) for biliary stone fragmentation is reported. Laser energy is delivered via UV grade fused silica fibers to the target stones immersed in normal saline solution. Sixty biliary calculi--pigment (n = 40), and cholesterol (n = 20)--were fragmented in vitro. The total energy delivered per unit mass of the stone is kept constant. Two energy fluences (80 and 110 mJ/mm2) at two repetition rates (5 and 20 Hz) delivered through fibers of two core sizes (300 and 600 microns) are utilized to study the effect of different laser parameters on the fragmentation process. Although both pigment and cholesterol stones are susceptible to excimer laser fragmentation, higher fragmentation efficiency is obtained for the pigment stones than for the cholesterol stones. Our study suggests that higher energy fluence and larger fiber core size result in higher fragmentation efficiency for pigment stones. Fragmentation thresholds at stone surface for a variety of biliary calculi of known composition were measured. The threshold energy fluence is approximately 3 mJ/mm2 and 17 mJ/mm2 for pigment and cholesterol stones, respectively. Our study indicates that the 308 nm excimer laser may be effective as a laser lithotriptor with low threshold and good efficiency for biliary stone fragmentation.

ChemInform Abstract: Single and Double Photoionization Processes in Naphthalene Between 8 and 35 eV

The photostability of C{sub 10}D{sub 8}{sup +} and C{sub 10}D{sub 8}{sup 2+} is investigated between 8 and 35 eV by threshold photoelectron-photoion coincidence (T-PEPICO) measurements using monochromatized synchrotron radiation as excitation source for naphthalene. MNDO calculations were used to interpret photoelectron spectra. The threshold photoelectron spectrum shows evidence of extensive autoionization, considered to be mainly to high vibrational levels of the X{sup 2} A{sub u} state of C{sub 10}D{sub 8}{sup +}. The possibility of a kinetic isotope effect on dissociation is discussed. Fragmentation processes, thresholds, and cutoffs are interpreted. The dissociation rates of the process C{sub 10}D{sub 8}{sup +} {yields} C{sub 8}D{sub 6}{sup +} + C{sub 2}D{sub 2} are measured between 16 and 17.7 eV. The results of an RRKM calculation are fitted to the experimental results to give an activation energy of 3.50 eV and activation entropy {Delta}S{sup {double dagger}} = 14.1 J/mol{center dot}K at 1000 K. A mechanistic model of this dissociation is propose din terms of two successive C-C bond rupture processes. Appearance potentials determined for the dications C{sub 10}D{sub 8}{sup 2+} (21.5 {plus minus} 0.5 eV) and C{sub 8}D{sub 6}{sup 2+} (23.5 {plus minus} 0.5 eV) are about 1 eV lower than electron impact values. Applicationmore » of the results to interstellar polycyclic aromatic hydrocarbons is briefly discussed. Further work with improved mass resolution is proposed.« less

Single and double photoionization processes in naphthalene between 8 and 35 eV

The photostability of C{sub 10}D{sub 8}{sup +} and C{sub 10}D{sub 8}{sup 2+} is investigated between 8 and 35 eV by threshold photoelectron-photoion coincidence (T-PEPICO) measurements using monochromatized synchrotron radiation as excitation source for naphthalene. MNDO calculations were used to interpret photoelectron spectra. The threshold photoelectron spectrum shows evidence of extensive autoionization, considered to be mainly to high vibrational levels of the X{sup 2} A{sub u} state of C{sub 10}D{sub 8}{sup +}. The possibility of a kinetic isotope effect on dissociation is discussed. Fragmentation processes, thresholds, and cutoffs are interpreted. The dissociation rates of the process C{sub 10}D{sub 8}{sup +} {yields} C{sub 8}D{sub 6}{sup +} + C{sub 2}D{sub 2} are measured between 16 and 17.7 eV. The results of an RRKM calculation are fitted to the experimental results to give an activation energy of 3.50 eV and activation entropy {Delta}S{sup {double dagger}} = 14.1 J/mol{center dot}K at 1000 K. A mechanistic model of this dissociation is propose din terms of two successive C-C bond rupture processes. Appearance potentials determined for the dications C{sub 10}D{sub 8}{sup 2+} (21.5 {plus minus} 0.5 eV) and C{sub 8}D{sub 6}{sup 2+} (23.5 {plus minus} 0.5 eV) are about 1 eV lower than electron impact values. Applicationmore » of the results to interstellar polycyclic aromatic hydrocarbons is briefly discussed. Further work with improved mass resolution is proposed.« less

Scatter profiling: A potentially useful technique for isomeric ion distinction and stability determination

A method to determine the extent of angular scattering of fragment-ion products of keV-collision-activated decomposition (CAD) reactions and, in particular, the collisional scatter incurred by the parent ions prior to their dissociation, is outlined. Since the half-widths of the collisional scatter profiles correlate approximately with the mean reaction endothermicities for some ‘test’ reactions, the method may, in principle, be used to estimate the stabilities of isomeric ion structures relative to a common fragmentation threshold level. For single-proton-loss CAD reactions of some [H3, C, X]+ ions (X = F, Cl, OH) with either [H3CX]+ or ylidion, [H2CXH]+ structure, collisional scatter is found in each case to be greater for the isomeric ion with the more stable structure. The estimated magnitudes of the mean energy depositions occurring in the keV-collision-activation processes are generally much larger than the calculated minimum energy requirements, suggesting that survivable [M  H]+ products can be formed with up to several eV of internal energy.

Resonance formation in few (3) body Coulomb systems

The standard physical picture of resonance formation breaks down for three-body Coulomb systems close to the threshold for total fragmentation. Near this threshold the complex as a whole performs collective motions localised essentially along potential energy surface ridges. Above threshold these motions are nonperiodic and describe total fragmentation, below threshold unstable periodic motions lead to new resonances. A semiclassical theory which describes these phenomena for two-electron atoms in excellent agreement with experiments is generalised to the ppp system.

Effect of pulse duration on microsecond‐domain laser lithotripsy

The pulsed dye laser (wavelength 504 nm, pulse duration 1 microsecond) is widely used for fragmenting urinary and biliary calculi. In this study, the performance of this laser was compared with pulsed dye lasers producing pulse durations of 8 and 20 microseconds. Fragmentation thresholds and fragmentation rates were measured using a variety of urinary and biliary calculi. Effective fragmentation of urinary and biliary calculi was obtained with 1-microsecond and 8-microseconds pulse durations, but satisfactory fragmentation could not be achieved at 20 microseconds.

Internal resonances for energy flow in collisionally perturbed symmetric van der Waals complexes near fragmentation threshold

Classical trajectory calculations and analysis of collision-induced energy flow near the fragmentation threshold in a symmetric van der Waals complex of the type BL⋅⋅⋅X2⋅⋅⋅BR, in which two weak bonds are separated by a molecular unit, are presented. The collision of an incident atom with BL results in transfer of a large fraction of translational energy to the complex and most of the energy is found to be localized on the right-hand side bond X2⋅⋅⋅BR. Though the amount of energy transferred to the complex greatly exceeds the sum of van der Waals bond strengths, the total fragmentation does not occur until internal resonances between BL⋅⋅⋅X2 and X2⋅⋅⋅BR bonds cease. Between the fragmentation thresholds of X2⋅⋅⋅BR and BL⋅⋅⋅X2, there is a tendency for BL⋅⋅⋅X2 to stabilize through internal resonant exchange of energy.

Collision-induced intramolecular energy flow and fragmentation in van der Waals complexes: Atom+tetramer collisions

Intramolecular energy flow and fragmentation processes in a linear van der Waals complex consisting of four molecular units undergoing collision with an incident atom are studied by solving Hamilton’s equations of motion for positions and momenta of all molecular units and van der Waals bonds. Numerical results are obtained for an O2 tetramer, which is in interaction with an argon atom: Ar+(1–2⋅⋅⋅3–4⋅⋅⋅5–6⋅⋅⋅7–8). Energy flow is found to be very efficient, and above the fragmentation threshold most of the energy transferred to the complex through the collision of the first unit (1–2) of the complex with Ar is found to become localized at the terminal van der Waals bond 5–6⋅⋅⋅7–8, the dissociating mode, thus causing the end molecular unit (7–8) to break off. Short-time dynamics of energy buildup in each molecular unit and van der Waals bond is analyzed in detail.

Collisional energy flow in weakly bound complexes

Collisional energy transfer dynamics in weakly bound complexes is studied at collision energies above and below the fragmentation threshold in classical mechanics. The collinear collision of a linear complex C⋅⋅⋅B–A, where C is an atom or a diatomic molecule, with an incident atom is considered. For the collision energy range 0.01–10 eV, energy transfer to the complex is efficient, and essentially all of the energy transferred to the complex is localized in the weak bond. The energy initially stored in the molecular bond remains in it when the collision is over, and the bond gains essentially no energy from translation. The collision model is formulated such that the incident atom hits the outer atom A of the molecular unit B–A, so the efficient energy buildup in the weak bond indicates an easy flow of energy across the stiff molecular bond from the initial impact region. Energy flows from the molecular bond to the weak bond through a sequence of energy gain–loss steps on a subpicosecond time scale. A brief discussion of the collision of a diatom–diatom complex with an atom is presented to show the effects of vibration of the second molecular unit on energy flow.

Collision-induced dissociation of aluminum cluster ions: Fragmentation patterns, bond energies, and structures for Al+2–Al+7

Collision-induced dissociation (CID) of cooled, mass selected aluminum cluster ions (Al+2–7) by xenon, has been studied over an energy range of 0–10 eV (center of mass). These experiments were carried out in a new apparatus which is described in detail. From the product branching ratios and cross section magnitudes we derive qualitative structural information about the cluster ions. The fragmentation thresholds are analyzed to yield dissociation energies, approximate ionization potentials, and further structural information about the cluster ions and their neutral counterparts. Cluster stabilities range from 0.85±0.40 eV for Al+4 to 2.25±0.70 eV for Al+7. The results provide a stringent test for recent calculations on Al2–6.

A photoionization study of SeH and H2Se

The photoion yield curve of SeH, prepared by the reaction H+H2Se is presented. The adiabatic I.P. is 9.845±0.003 eV, and autoionization structure is observed, from which higher I.P.’s are inferred. The photoion yield curves of H2Se+, SeH+, and Se+ from H2Se are also measured. The fragmentation thresholds, together with I.P. (SeH), enable one to infer the bond energies D0(HSe−H)=78.99±0.18 kcal/mol and D0(SeH)=74.27±0.23 kcal/mol. The adiabatic I.P. for H2Se (X̃ 2B) is 9.886±0.003 eV.