Luminescence measurements of Xe(+) + N2 and Xe(2+) + N2 hyperthermal charge transfer collisions

Luminescence spectra are recorded for collisions between Xe(+)/Xe(2+) and molecular nitrogen at energies ranging from 4.5 to 316 eV in the center-of-mass frame. In the Xe(+) + N(2) collision system, evidence for luminescent charge-transfer products is only found through Xe I emission lines. The most intense features of the luminescence spectra are attributed to atomic N emissions observed above ～20 eV. Intense N(2)(+) A (2)Π(u) - X(2)Σ(g)(+) and B(2)Σ(u)(+) - X(2)Σ(g)(+) radiance is observed from Xe(2+) + N(2) collisions. The B state formation cross section decreases with collision energy until 20 eV, after which it becomes independent of impact energy with an approximate value of 3 ?(2). The cross section for N(2) (+) A (ν > 0) formation increases with energy until 20 eV, after which it remains nearly constant at ～1 ?(2). The N(2)(+) product vibrational distributions extracted from the spectra are non-Franck-Condon for both electronic product states at low collision energies. The distributions resemble a Franck-Condon distribution at the highest energies investigated in this work.     

A guided-ion beam study of the reactions of Xe(+) and Xe(2+) with NH(3) at hyperthermal collision energies

We have measured the absolute cross sections for reactions of Xe(+) and Xe(2+) with NH(3) at collision energies in the range from near-thermal to ～34 and ～69?eV, respectively. For Xe(+), the cross section for charge transfer, the only exothermic channel, decreases from ～200A?(2) below 0.1 eV to ～12A?(2) at the highest energies studied. The production of NH(3) (+) is the only channel observed below 5 eV, above which a small amount of NH(2) (+) is also formed. In Xe(2+) reactions, the main products observed are NH(3) (+) and NH(2) (+). The charge transfer cross section decreases monotonically from ～80 to ～6A?(2) over the studied energy range. The NH(2) (+) cross section is similar to the charge transfer cross section at the lowest energies, and exhibits a second component above 0.4 eV, with a maximum of 65A?(2) at 0.7 eV, above which the cross section decreases to ～30A?(2) at the highest energies studied. At energies above 10 eV, a small amount of NH(+) is also observed in Xe(2+) collisions. Product recoil velocity distributions were determined at selected collision energies, using guided-ion beam time-of-flight methods.     

Collision-induced dissociation of transition metal-oxide ions: dynamics of VO+ collision with Xe

The collision-induced dissociation of VO(+) by Xe has been studied by the use of classical dynamics procedures on London-Eyring-Polanyi-Sato potential-energy surfaces in the collision energy range of 5.0-30 eV. The dissociation threshold behavior and the dependence of reaction cross sections on the collision energy closely follow the observed data with the threshold energy of 6.00 eV. The principal reaction pathway is VO(+) + Xe --> V(+)+ O + Xe and the minor pathway is VO(+) + Xe--> VXe(+) + O. At higher collision energies (E > 8.0 eV), the former reaction preferentially occurs near the O-V(+)...Xe collinear and perpendicular alignments, but the latter only occurs near the perpendicular alignment. At lower energies close to the threshold, the reactions are found to occur near the collinear configuration. No reaction occurs in the collinear alignment V(+)-O...Xe. The high and low energy-transfer efficiencies of the collinear alignments O-V(+)...Xe and V(+)-O...Xe are attributed to the effects of mass distribution. The activation of the VO(+) bond toward the dissociation threshold occurs through a translation-to-vibration energy transfer in a strong collision on a time scale of about 50 fs.     

Formation of doubly positively charged diatomic ions of Mo2(2+) produced by Ar+ sputtering of an Mo metal surface

Long-lived metastable doubly positively charged diatomic ions of Mo2(2+) have been produced by Ar+ bombardment of a molybdenum metal surface. These exotic molecular dications, such as for example 92,95Mo2(2+) at m/z 93.5, could be observed in positive ion mass spectra for ion flight times of approximately 17 micros in a Cameca IMS-3f secondary ion mass spectrometer, when the ion extraction field was adjusted for detection of ions that are formed in the gas phase several micrometers in front of the sputtered surface. Mo2(2+) was observed at high primary current densities for projectile ions of Ar+, but could not be detected under very similar bombarding conditions for projectile ions of Xe+. Such a dependence of ion production by inert gas sputtering on the primary ion species [ionization energies: IP1(Ar) = 15.76 eV and IP1(Xe) = 12.13 eV] is unusual. It is shown that formation of Mo2(2+) dications takes place by resonant charge transfer in grazing gas-phase collisions between incoming projectile ions of Ar+ and sputtered molecular ions of Mo2+. The efficiency for such a resonant electron capture (Mo2+ + Ar+ --> Mo2(2+) + Ar) is of the order of 10(-5) for the bombarding conditions in our mass spectrometer and corresponds to a cross section of a few 10(-15) cm2.     

Polymerization of ethylene molecules chemisorbed on CrOH+ as a model system of chromium-containing catalyst

The reaction process of the production of CrOH(C2H4)2(+) was studied in connection with the ethylene polymerization on a silica-supported chromium oxide catalyst (the Phillips catalyst). Cluster ions CrOH(C2H4)2(+) and CrOH(C4H8)+ were produced by the reactions of CrOH+ with C2H4 (ethylene) and C4H8 (1-butene), respectively, and were allowed to collide with a Xe atom under single collision conditions. The cross section for dissociation of each parent cluster ion was measured as a function of the collision energy (collision-induced dissociation, or CID). It was found that (i) the CID cross section for the production of CrOH+ from CrOH(C2H4)2(+) increases sharply at the threshold energy of 3.16 +/- 0.22 eV and (ii) the CID cross section for the production of CrOH+ and C4H8 from CrOH(C4H8)+ also increases sharply at the threshold energy of 3.26 +/- 0.21 eV. In comparison with the calculations based on a B3LYP hybrid density functional method, it is concluded that two ethylene molecules in CrOH(C2H4)2(+) are polymerized to become 1-butene. The calculation also shows that the dimerization proceeds via CrOH(C2H4)+ (ethylene complex) and CrOH(C2H4)2(+) (ethylene complex), in which the ethylene molecules bind with CrOH+ through a pi-bonding.     

Classical trajectory study of the formation of XeH+ and XeCl+ in the Xe++HCl collision

The collision-induced reaction of Xe+ with HCl has been studied by use of classical dynamics procedures at collision energies 2-20 eV using empirical potential parameters. The principal reaction pathway on the potential energy surface is the formation of XeH+ with the maximum reaction cross section, 1.2 A2, occurring at E=9 eV. At lower energies, the cross section for the charge transfer process Xe++HCl-->Xe+HCl+ is comparable to that for XeH+ formation, but at higher energies, it is larger by a factor of 2. The cross section of the XeCl+ formation is an order of magnitude smaller than that of XeH+. For both XeH+ and XeCl+ formations, the reaction threshold is approximately 2 eV. The XeH+ formation takes place immediately following the turning point in a direct-mode mechanism, whereas an indirect-mode mechanism operates in the formation of XeCl+. Both XeH+ and XeCl+ formations come mainly from the perpendicular configuration, Xe+...HCl, at the turning point. Product vibrational excitation is found to be strong in both XeH+ and XeCl+.     

Fragmentation mechanisms of protonated benzylamines. Electrospray ionisation-tandem mass spectrometry study and ab initio molecular orbital calculations

Our research into neurotransmitters in a biological fluid presented an opportunity to investigate the fragmentations under low collision energy characterising benzyl-amines protonated under electrospray ionisation (ESI) conditions in a triple quadrupole mass spectrometer. In this work we present the breakdown graphs of protonated 3,4-dihydroxybenzylamine, DHBAH(+), and 3-methoxy, 4-hydroxybenzylamine, HMBAH(+), at various source temperatures and various pressures in the collision cell, the collision energy varying from 0 to 46 eV in the laboratory frame. Both parent ions eliminate first NH(3) at very low collision energy. The fragmentations of [MH - NH(3)](+) occur at high collision energy and are quite different for DHBAH(+) and HMBAH(+): formation of [MH - NH(3) - H(2)O - CO](+) for the former; formation of the radical cation [MH - NH(3) - CH(3)](+.) for the latter. These fragmentations are interpreted by means of ab initio calculations up to the B3LYP/6-311+G(2d,2p) level of theory. The successive losses of H(2)O and CO involve first the rearrangement in two steps of benzylic ions formed by loss of NH(3) into tropylium ions. The transition states associated with this rearrangement are very high in energy (about 400 kJ mol(-1) above MH(+)) explaining (i). the absence of an ion corresponding to [DHBAH - NH(3) - H(2)O](+). The determining steps associated with the losses of H(2)O and with H(2)O + CO are located lower in energy than the transition states associated with the isomerisation of benzylic ions into tropylium ions; explaining (ii). the formation of the radical cation [MH - NH(3) - CH(3)](+.). The homolytic cleavage of CH(3)-O requires less energy than does the rearrangement.     

Quasiclassical dynamics simulation of the collision-induced dissociation of Cr(CO)6 + with Xe

Quasiclassical trajectory calculations are employed to investigate the dynamics of collision-induced dissociation (CID) of Cr(CO)6 + with Xe atoms at collision energies ranging from 1.3 to 5.0 eV. The trajectory simulations show that direct elimination of CO ligands, during the collision, becomes increasingly important as the collision energy increases. In a significant number of cases, this shattering mechanism is accompanied with a concomitant formation of a transient Xe-Cr(CO)x +(x<6) complex. The calculated results are in very good agreement with the experimental results presented previously [F. Muntean and P. B. Armentrout, J. Chem. Phys. 115, 1213 (2001)]. In particular, the computed cross sections and scattering maps for the product ions Cr(CO)x +(x=3-5) compare very favorably with the reported experimental data. However, in contrast with the conclusions of the previous study, the present calculations suggest that CID dynamics for this system exhibits a significant impulsive character rather than proceeding via a complex surviving more than a rotational period.     

Rainbow scattering for Ar + Ar and Xe + Xe

Elastic differential scattering measurements have been performed on Ar⁺ + Ar and Xe⁺ + Xe. The rainbow scattering angle is found at τ = Eθ ≈ 115 eV deg for Ar⁺₂ and τ ≈ 93 eV deg for Xe⁺₂. These data are consistent with a potential well depth of 1.25 eV for Ar⁺₂ and 0.97 eV for Xe⁺₂.     

Ionization and dissociation of CH3I in intense laser field

The ionization-dissociation of methyl iodide in intense laser field has been studied using a reflection time-of-flight mass spectrometry (RTOF-MS), at a laser intensity of < or =6.6x10(14) W/cm(2), lambda=798 nm, and a pulse width of 180 fs. With the high resolution of RTOF-MS, the fragment ions with the same M/z but from different dissociation channels are resolved in the mass spectra, and the kinetic energy releases (KERs) of the fragment ions such as I(q+) (q=1-6), CH(m) (+) (m=0-3), C(2+), and C(3+) are measured. It is found that the KERs of the fragment ions are independent of the laser intensity. The fragments CH(3) (+) and I(+) with very low KERs (<1 eV for CH(3) (+) and <0.07 eV for I(+)) are assigned to be produced by the multiphoton dissociation of CH(3)I(+). For the fragments CH(3) (+) and I(+) from CH(3)I(2+), they are produced by the Coulomb explosion of CH(3)I(2+) with the interaction from the covalent force of the remaining valence electrons. The split of the KER of the fragments produced from CH(3)I(2+) dissociation is observed experimentally and explained with the energy split of I(+)((3)P(2)) and I(+)((3)P(0,1)). The dissociation CH(3)I(3+)-->CH(3) (+)+I(2+) is caused by Coulomb explosion. The valid charge distance R(c) between I(2+) and CH(3) (+), at which enhanced ionization of methyl iodide occurs, is obtained to be 3.7 A by the measurements of the KERs of the fragments CH(3) (+) and I(2+). For the CH(3)I(n+) (n> or =3), the KERs of the fragment ions CH(3) (p+) and I(q+) are attributed to the Coulomb repulsion between CH(3) (p+) and I(q+) from R(c) approximately 3.7 A. The dissociation of the fragment CH(3) (+) is also discussed. By the enhanced ionization mechanism and using the measured KER of I(q+), all the possible Coulomb explosion channels are identified. By comparing the abundance of fragment ions in mass spectrum, it is found that the asymmetric dissociation channels with more charges on iodine, q>p, are the dominant channels.     

Electron transfer and bond-forming reactions following collisions of Cl2+ and HCl2+ with CO

Collisions between Cl(2+) and CO have been investigated using time-of-flight mass spectrometry over a collision energy range between 2.2 eV and 7.1 eV in the centre-of-mass frame. The formation of Cl(+), CO(+) and C(+) in electron transfer reactions has been detected and an unusual bond-forming reaction which generates CCl(2+) has also been observed. The reactive cross-sections, in arbitrary units, for the electron transfer reactions have been evaluated. To extract these cross sections we employ a new method of analysing mass spectral intensities for crossed-beam experiments, an algorithm which allows inter-comparison of the fluxes of all the ionic products from the electron transfer reactions. The observed electron transfer reactivity has been rationalized by calculations based on Landau-Zener theory. To account for the observation of CCl(2+), we have calculated the relevant energetics showing that the lowest lying doublet state of this dication is bound and is energetically accessible at our collision energies. These energetic arguments indicate that electron transfer in the exit channel between the separating CCl(2+) and O atom probably forms C(+) ions via the dissociation of CCl(+). Additionally, collisions between HCl(2+) and CO have been studied at collision energies from 2.2 to 7.0 eV in the centre-of-mass frame. In this collision system, proton transfer to form HCO(+) is observed to compete efficiently with dissociative and non-dissociative electron transfer.     

The anomalous shape of the cross section for the formation of SF3+ fragment ions produced by electron impact on SF6 revisited

The partial ionization cross section for the formation of SF(3) (+) fragment ions following electron impact on SF(6) is known to have a pronounced structure in the cross section curve slightly above 40 eV. We used the mass-analyzed ion kinetic energy (MIKE) scan technique to demonstrate the presence of a channel contributing to the SF(3) (+) partial ionization cross section that we attribute to the Coulomb explosion of doubly charged metastable SF(4) (2+) ions into two singly charged ions SF(3) (+) and F(+), with a threshold energy of about 45.5 eV. Thus the observed unusual shape of the SF(3) (+) partial ionization cross section is the result of two contributions, (i) the direct formation of SF(3) (+) fragment ions via dissociative ionization of SF(6) with a threshold energy of 22 eV and (ii) the Coulomb explosion of metastable SF(4) (2+) ions with a threshold energy of about 45.5 eV. A detailed analysis of the MIKE spectrum reveals an average kinetic energy release of about 5 eV in the Coulomb explosion of the SF(4) (2+) ions with evidence of a second channel corresponding to an average kinetic energy release of about 1.1 eV.     

Reactions of O+ with CnH2n+2, n=2-4: a guided-ion beam study

We have measured absolute reaction cross sections for the interaction of O(+) with ethane, propane, and n-butane at collision energies in the range from near thermal to approximately 20 eV, using the guided-ion beam (GIB) technique. We have also measured product recoil velocity distributions using the GIB time-of-flight (TOF) technique for several product ions at a series of collision energies. The total cross sections for each alkane are in excess of 100 A(2) at energies below approximately 2 eV, and in each case several ionic products arise. The large cross sections suggest reactions that are dominated by large impact parameter collisions, as is consistent with a scenario in which the many products derive from a near-resonant, dissociative charge-transfer process that leads to several fragmentation pathways. The recoil velocities, which indicate product ions with largely thermal velocity distributions, support this picture. Several product ions, most notably the C(2)H(3) (+) fragment for each of the alkanes, exhibit enhanced reaction efficiency as collision energy increases, which can be largely attributed to endothermic channels within the dissociative charge-transfer mechanism.     

Potential energy curves of diatomic molecular ions from high-resolution photoelectron spectra. II. The first six electronic states of Xe2 +

The pulsed-field-ionization zero-kinetic-energy photoelectron spectrum of Xe(2) has been measured between 90 000 and 109 000 cm(-1) following single-photon excitation from the ground neutral state. Transitions to five of the six low-lying electronic states of Xe(2) (+) could be observed. Whereas extensive vibrational progressions were observed for the X0(g) (+)-->I(1/2u), I(3/2g), and II(1/2u) photoelectron transitions, only the lowest vibrational levels of the I(3/2u) and II(1/2g) states could be detected. Unambiguous assignments of the vibrational quantum numbers were derived from the analysis of the isotopic shifts of the vibrational bands and of the intensity distribution and from the modeling of the potential energy curves. Analytical potential energy curves of spectroscopic accuracy (i.e., approximately 1 meV) were determined for all six low-lying electronic states using a global model, which includes the first (charge-induced dipole, proportional to 1/R(4)) member of the long-range interaction series and treats the spin-orbit interaction explicitly. The assumption of an R-independent spin-orbit coupling constant was tested and found to be an excellent approximation.     

Study of luminescence properties of novel Er3+ single-doped and Er3+/Yb3+ co-doped tellurite glasses

The novel Er(3+) single-doped and Er(3+)/Yb(3+) co-doped tellurite glasses were prepared. The effect of Yb(2)O(3) concentration on absorption spectra, emission spectra and upconversion spectra of glasses were measured and investigated. The emission intensity, fluorescence full width at half maximum (FWHM) and upconversion luminescence of Er(3+) go up with the increasing concentration of Yb(3+) ions. The maximum FWHM of (4)I(13/2) --> (4)I(15/2) transition of Er(3+) is approximate 77 nm for 1.41 x 10(21)ions/cm(3) concentration of Yb(3+)-doped glass. The visible upconversion emissions at about 532, 546 and 659 nm, corresponding to the (2)H(11/2) --> (4)I(15/2), (4)S(3/2) --> (4)I(15/2) and (4)F(9/2) --> (4)I(15/2) transitions of Er(3+), respectively, were simultaneously observed under the excitation at 970 nm. Subsequently, the possible upconversion mechanisms and important role of Yb(3+) on the green and red emissions were discussed and compared. The results demonstrate that this kind of tellurite glass may be a potentially useful material for developing potential amplifiers and upconversion optical devices.     

Dissociative recombination of NH4+ and ND4+ ions: storage ring experiments and ab initio molecular dynamics

The dissociative recombination (DR) process of NH4+ and ND4+ molecular ions with free electrons has been studied at the heavy-ion storage ring CRYRING (Manne Siegbahn Laboratory, Stockholm University). The absolute cross sections for DR of NH4+ and ND4+ in the collision energy range 0.001-1 eV are reported, and thermal rate coefficients for the temperature interval from 10 to 2000 K are calculated from the experimental data. The absolute cross section for NH4+ agrees well with earlier work and is about a factor of 2 larger than the cross section for ND4+. The dissociative recombination of NH4+ is dominated by the product channels NH3+H (0.85+/-0.04) and NH2+2H (0.13+/-0.01), while the DR of ND4+ mainly results in ND3+D (0.94+/-0.03). Ab initio direct dynamics simulations, based on the assumption that the dissociation dynamics is governed by the neutral ground-state potential energy surface, suggest that the primary product formed in the DR process is NH3+H. The ejection of the H atom is direct and leaves the NH3 molecule highly vibrationally excited. A fraction of the excited ammonia molecules may subsequently undergo secondary fragmentation forming NH2+H. It is concluded that the model results are consistent with gross features of the experimental results, including the sensitivity of the branching ratio for the three-body channel NH2+2H to isotopic exchange.     

The bond-forming reactions of atomic dications with neutral molecules: formation of ArNH+ and ArN+ from collisions of Ar2+ with NH3

An experimental and computational study has been performed to investigate the bond-forming reactivity between Ar(2+) and NH(3). Experimentally, we detect two previously unobserved bond-forming reactions between Ar(2+) and NH(3) forming ArN(+) and ArNH(+). This is the first experimental observation of a triatomic product ion (ArNH(+)) following a chemical reaction of a rare gas dication with a neutral. The intensity of ArNH(+) was found to decrease with increasing collision energy, with a corresponding increase in the intensity of ArN(+), indicating that ArN(+) is formed by the dissociation of ArNH(+). Key features on the potential energy surface for the reaction were calculated quantum chemically using CASSCF and MRCI methods. The calculated reaction mechanism, which takes place on a singlet surface, involves the initial formation of an Ar-N bond to give Ar-NH(3)(2+). This complexation is followed by proton loss via a transition state, and then loss of the two remaining hydrogen atoms in two subsequent activationless steps to give the products (3)ArN(+) + H(+) + 2H. This calculated pathway supports the sequential formation of ArN(+) from ArNH(+), as suggested by the experimental data. The calculations also indicate that no bond-forming pathway exists on the ground triplet surface for this system.     

Quantum and quasiclassical state-to-state dynamics of the NH + H reaction: competition between abstraction and exchange channels

Quantum and quasiclassical state-to-state dynamics for the NH + H' reaction at high collision energies up to 1.6 eV was studied on an accurate ab initio potential energy surface. Both of the endothermic abstraction (NH + H' → N + HH') and thermoneutral exchange (NH + H' → H + NH') channels were investigated from the same set of wave packets using an efficient coordinate transformation method. It is found that the abstraction represents a minor reaction channel in the energy range studied, primarily due to endothermicity. The cross section for the abstraction reaction increases monotonically with the collision energy, while that for the exchange reaction is relatively energy insensitive. As a result, the thermal rate constant for the abstraction reaction follows the Arrhenius law, where that for the exchange reaction is nearly temperature independent. Finally, it is shown that the quantum mechanical results can be reasonably reproduced by the Gaussian-binning quasiclassical trajectory method and to a lesser extent by a quantum statistical model.     

A guided-ion beam study of the O+(4S) + NH3 system at hyperthermal energies

We have measured absolute cross section for the reaction of ground-state O(+) with ammonia at collision energies in the range from near-thermal to approximately 15 eV, using the guided-ion beam (GIB) method. Measurements were also performed using ammonia-d3 to aid in mass assignments. The reaction is dominated at low collision energies by charge transfer; however, the cross section for this exothermic channel is rather small, decreasing sharply with energy from approximately 40 A(2) for normal ammonia at near-thermal energies and leveling off at 3.7 A(2) above 6 eV; the cross section is slightly smaller for ammonia-d3. Other channels, corresponding to the production of NH2(+) and NO(+), and possibly OH(+), were detected. The NO(+) channel, although nominally exothermic, is very small and exhibits a threshold at approximately 7 eV. Product recoil velocity distributions were also determined at selected collision energies, using GIB time-of-flight methods.     

Spectroscopic properties and thermal stability of Er3+ -doped TeO2-B2O3-Nb2O5-ZnO glass for potential WDM amplifier

A series of novel 70TeO2-(15-x)B2O3-xNb2O5-15ZnO-1wt.% Er2O3 (TBN x=0, 3, 6, 9, 12 and 15 mol%) tellurite glasses were prepared. The thermal stability, absorption spectra, emission spectra, and the lifetime of the (4)I(13/2) level of Er(3+) ions were measured and investigated. Three Judd-Ofelt intensity parameters Omega(t) (t=2, 4 and 6) (Omega(2)=(5.42-6.76)x10(-20)cm(2); Omega(4)=(1.37-1.73)x10(-20)cm(2); Omega(6)=(0.70-0.94)x10(-20)cm(2)) of Er(3+) ions were calculated by Judd-Ofelt theory. It is found that the Omega(6) first increases with the increase of Nb2O5 content from 0 to 6 mol% and then decreases, which is mainly affected by the number of non-bridging oxygen ions of the glass network. The high peak of stimulated emission cross-section (sigma(e)(peak)=(0.77-0.91)x10(-20)cm(2)) of Er(3+): (4)I(13/2)-->(4)I(15/2) transition were obtained according to McCumber theory and broad full width at half maximum (FWHM=65-73 nm) of the (4)I(13/2)-->(4)I(15/2) transition of Er(3+) ions were measured. The results indicate that these new TBN glasses can be used as a candidate host material for potential broadband optical amplifiers.