A theoretical and experimental study of positive and neutral LiF clusters produced by fast ion impact on a polycrystalline LiF target.

The positive and neutral clusters produced by the impact of approximately 60 MeV (252)Cf fission fragments on a LiF polycrystalline target are analyzed. The positive ion spectrum is dominated by the (LiF)(n)Li(+) series, n = 0-7, exhibiting a total yield 2 orders of magnitude higher than that of the (LiF)(n)(+) series. The yield for the dominant (LiF)(n)Li(+) series decreases roughly as exp(-kn), where k approximately 0.9 for n = 0-3 and k approximately 0.6 for the heavier clusters (n = 4-9), while the yield of the (LiF)(n)(+) series also decreases exponentially as n increases with k approximately 0.6. Theoretical calculations were performed for the (LiF)(n)Li(0), (LiF)(n)Li(+), and (LiF)(n)(0) series for n up to 9. For the smaller clusters the structures first obtained with a genetic algorithm generator were further optimized at the DFT/B3LYP/6-311+G(3df), DFT/B3LYP/LACV3P*, and MP2/LACV3P* levels of theory. An energy criterion is used for a proper taxonomic description of the optimized cluster isomers. Cluster properties such as fragmentation energy and stability are discussed for the proposed configurations. The results show that for all three series the most stable isomers present a linear structure for small cluster size (n = 1-3), while cubic cells or polyhedral structures are preferred for larger cluster sizes (n = 4-9). Fragmentation energy results suggest that a desorbed excited (LiF)(n)Li(+) ion preferentially dissociates via a cascade of (LiF)(n)(0) units, in agreement with the slope modification in the exponential decay of the (LiF)(n)Li(+) ion abundances for n > or = 3.

Cluster emission and chemical reactions in oxygen and nitrogen ices induced by fast heavy-ion impact.

Two ices, O2 and a mixture of O2 and N2, are bombarded by 252Cf fission fragments (FF) (approximately 65 MeV at target surface); the emitted positive and negative secondary ions are analyzed by time-of-flight mass spectrometry (TOF-SIMS). These studies shall enlighten sputtering from planetary and interstellar ices. Three temperature regions in the 28-42-K range are analyzed: (1) before N2 sublimation, in which hybrid chemical species are formed, (2) before O2 sublimation, in which the TOF mass spectrum is dominated by low-mass (O2)p cluster ions and (3) after O2 sublimation, in which (N2)p or (O2)p cluster ions are practically inexistent. In the first region, four hybrid ion series are observed: NOn-1+, N2On-2(+/-), and N4On-4(-). In the second region, two positive and negative ion series are identified: (O2)pO(+/-) and (O2)pO2(+/-). Their yield distributions are fitted by the sum of two decreasing exponentials, whose decay constants are the same for all series. It is observed that the cluster ion desorption from solid oxygen is very similar to that of other frozen gases, but its yield distribution oscillates with a three- or six-atom periodicity, suggesting O3 or 3O2 units in the cluster structure, respectively.

UV laser-induced desorption mechanism analyzed through two-layer alkali halide samples.

Time of flight-mass spectrometry (TOF-MS) is used to analyze positive and negative desorbed ions generated by UV laser ablation of several alkali (X) halide (Y) salts. Most of the observed desorbed cluster ions have the structure (XY)(n)X(+) or (XY)(n)Y(-). Their desorption yields decrease as exp(-kn), where k approximately 2 for both series, suggesting that the neutral component (XY)(n) plays the dominant role in the desorption process. Mass spectrum measurements were performed for compound samples in which two salts (out of CsI, RbI, KBr, KCl and KI) are homogeneously mixed or disposed in two superposed layers. The detection of small new ion species and large cluster ions of the original salts supports the scenario that the uppermost layers are completely atomized while deep layers are emitted colder and fragmented: It is proposed that ns-pulsed laser induced desorption of ionic salts occurs via two sequential mechanisms: (1) ejection of cations and anions in the hot plume, followed by recombination into new cluster ions and (2) ejection of relatively cold preformed species originated from deep layers or from periphery of the irradiated region.

Secondary ion emission induced by fission fragment impact in CO-NH(3) and CO-NH(3)-H(2)O ices: modification in the CO-NH(3) ice structure.

CO-NH(3) and CO-NH(3)-H(2)O ices at 25-130 K were bombarded by (252)Cf fission fragments ( approximately 65 MeV at the target surface) and the emitted secondary ions were analyzed by time-of-flight mass spectrometry (TOF-SIMS). It is observed that the mass spectra obtained from both ices have similar patterns. The production of hybrid ions (formed from CO and NH(3) molecules) emitted from CO-NH(3) ice has already been reported by R. Martinez et al., Int. J. Mass. Spectrom. 262 (2006) 195; here, the secondary ion emission and the modifications of the CO--NH(3) ice structure during the temperature increase of the ice are addressed. These studies are expected to throw light on the sputtering from planetary and interstellar ices and the possible formation of new organic molecules in CO-NH(3)-H(2)O ice by megaelectronvolt ion bombardment. The presence of water in the CO-NH(3) ice mixture generates molecular ion series such as (NH(3))(p-q)(H(2)O)(q)CO(+) and replaces the cluster series (NH(3))(n)NH(4) (+) emission by the hybrid series (NH(3))(I-i)(H(2)O)(i=1, 2...I)H(+). The distribution of NH(3) and H(2)O molecules within the cluster groups indicates that ammonia and water mix homogeneously in the icy condensate at T = 25 K. The desorption yield distribution of the cluster series (NH(3))(n)NH(4) (+) is described by the sum of two exponential functions: one, slow-decreasing, attributed to the fragmentation of the solid target into clusters; and another, fast-decreasing, due to a local sublimation followed by recombination of ammonia molecules. The analysis of the time-temperature dependence of these two yield components gives information on the formation process of molecular ions, the transient composition of the ice target and structural changes of the ice. Data suggest that the amorphous and porous structure of the NH(3) ice, formed by the condensation of the CO--NH(3) gas at T = 25 K, survives CO sublimation until the occurrence of a phase transition around 80 K, which produces a more fragile ice structure.

Fragmentation of (LiF)(n)Li(+) clusters in the acceleration region of TOF spectrometers.

The binary decay of ionized clusters in the extraction region of time-of-flight (TOF) spectrometers is analyzed. The dynamics of the fragments is studied and an analytical expression for the TOF peak shape is deduced; simulations are performed for linear spectrometers of different configurations. The questions addressed refer to the design of TOF spectrometers to improve their accuracy in the determination of metastable-state mean lives, the identification of precursor masses and the investigation of desorption mechanisms. As an illustration, the method is applied to the decay of positive ion clusters (LiF)(n)Li(+) for both spontaneous and collision-induced fragmentation processes. No clear evidence of delayed emission is found. The bumps observed in the TOF spectrum are due to tertiary ions emitted by the LiF target sputtered by secondary ions produced in the grid, a process that increases with higher target bias. The main cluster fragmentation observed is (LiF)(3)Li(+*) decaying preferentially into (LiF)Li(+); the data are compatible with a spontaneous decay of metastable clusters with mean lives of 20-30 ns.

Effects of gamma radiation on phase behaviour and critical micelle concentration of Triton X-100 aqueous solutions.

Ionising radiation used for sterilization can have an effect on the physicochemical properties of pharmaceutically relevant excipient systems, affecting therefore the stability of the formulation. The effect of gamma irradiation on the phase behaviour (cloud point--CP) and critical micelle concentration (CMC) of aqueous solutions of Triton X-100, used as a model nonionic surfactant, is investigated in this paper. Micellar solutions were irradiated with gamma-rays in a dose range between 0 and 70 kGy, including the sterilization range of pharmaceutical preparations. The decreased observed in CP and CMC values of micellar solutions at all absorbed doses was explained in terms of changes in molecular mass distribution of ethoxylated surfactant and the formation of cross-linked species. These results were complemented by mass spectrometry, UV-vis and NMR spectroscopy. Although the findings indicate degradation of polyethoxylated chains by water radical attacks, there was no spectroscopic evidence of radiation damage to aromatic ring or hydrocarbon tail of surfactant. Models based on Flory-Huggins theory were employed to estimate CP from changes in mass distribution and to obtain cross-linking fractions. Surface tension measurements of non-irradiated and irradiated solutions were used for estimating the effectiveness and efficiency of surfactant in the formulation.

Theoretical and experimental analysis of ammonia ionic clusters produced by 252Cf fragment impact on an NH3 ice target.

Positive and negatively charged ammonia clusters produced by the impact of (252)Cf fission fragments (FF) on an NH(3) ice target have been examined theoretical and experimentally. The ammonia clusters generated by (252)Cf FF show an exponential dependence of the cluster population on its mass, and the desorption yields for the positive (NH(3))(n)NH(4)(+) clusters are 1 order of magnitude higher than those for the negative (NH(3))(n)NH(2)(-) clusters. The experimental population analysis of (NH(3))(n)NH(4)(+) (n = 0-18) and (NH(3))(n)NH(2)(-) (n = 0-8) cluster series show a special stability at n = 4 and 16 and n = 2, 4, and 6, respectively. DFT/B3LYP calculations of the (NH(3))(0)(-)(8)NH(4)(+) clusters show that the structures of the more stable conformers follow a clear pattern: each additional NH(3) group makes a new hydrogen bond with one of the hydrogen atoms of an NH(3) unit already bound to the NH(4)(+) core. For the (NH(3))(0)(-)(8)NH(2)(-) clusters, the DFT/B3LYP calculations show that, within the calculation error, the more stable conformers follow a clear pattern for n = 1-6: each additional NH(3) group makes a new hydrogen bond to the NH(2)(-) core. For n = 7 and 8, the additional NH(3) groups bind to other NH(3) groups, probably because of the saturation of the NH(2)(-) core. Similar results were obtained at the MP2 level of calculation. A stability analysis was performed using the commonly defined stability function E(n)(-)(1) + E(n)(+1) - 2E(n), where E is the total energy of the cluster, including the zero point correction energy (E = E(t) + ZPE). The trend on the relative stability of the clusters presents an excellent agreement with the distribution of experimental cluster abundances. Moreover, the stability analysis predicts that the (NH(3))(4)NH(4)(+) and the even negative clusters [(NH(3))(n)NH(2)(-), n = 2, 4, and 6] should be the most stable ones, in perfect agreement with the experimental results.

Electronic sputtering produced by fission fragments on condensed CO and CO2.

Condensed CO and CO2 are bombarded by approximately 65 MeV 252Cf fission fragments and the desorbed ions are analyzed by time-of-flight mass spectrometry as a function of target temperature, in the ranges 25-33 K and 75-91 K, respectively. Absolute desorption yields are measured up to complete ice sublimation. The mass spectra of both ice targets reveal the emission of: (1) low mass ions, produced by direct Coulomb interaction of the highly charged projectiles and delta-electrons with CO and CO2, and (2) pronounced series of cluster ions. The basic ice cluster structures (CO)n and (CO2)n are present in the emitted cluster series such as (CO)nCO+, (CO2)nCO2+, or (CO2)nCO3-. In the case of CO ice, however, the intense production of the series Cn+, Cn-, and (CO)mCn+ shows that Cn is the main cluster structure, consequence of a higher concentration of free carbon atoms in the nuclear track plasma of CO ice than in that of CO2 ice. Ion cluster abundance is observed to decrease exponentially with cluster mass. The decay constant is k(n) congruent with 0.13, about the same for series based on (CO)n and (CO2)n, but a factor 3.3 higher for the Cn series. The Cn clusters are formed by gas-phase condensation, but the (CO)n and (CO2)n clusters are produced by fracturing of the highly excited solid around the nuclear track. A dramatic reduction of the ion desorption yield is observed near T = 29 K for CO and near T = 85 K for CO2, when fast sublimation occurs and ice thickness vanishes. Close to sublimation temperature, the decay constant of the (CO)2Cn+ series increases due to a decreasing formation probability of large Cn clusters.

Laser induced formation of CsI ion clusters analyzed by delayed extraction time-of-flight mass spectrometry.

(CsI)nCs+ (n = 1,2) cluster ion formation from polycrystalline CsI irradiated by pulsed-UV laser (337 nm) is analyzed by delayed extraction time-of-flight mass spectrometry technique. Measurements were performed for different laser intensities and for several delayed extraction times. Experimental data show that CsI laser ablation produces the emission of (CsI),Cs+ ions (n = 0, 1, 2), whose yields decrease exponentially with n and increase exponentially with the laser pulse energy. A quasi equilibrium evolution of the clusters is proposed to extract a parameter characteristic of the cluster recombination process. The delayed extraction method of initial velocity determination was improved to take into account collisions in the high density plasma close to the target. The new parameterization helps to describe the dynamics of secondary ions of different masses for laser irradiances above the ion desorption threshold in a collision regime. The initial velocity of the secondary ions [(CsI)nCs+ (n = 0, 1, 2)] as function of the laser irradiance was determined. The distance to the target when the free expansion process starts is reported as function of the secondary ions mass and of the laser irradiance. The collision regime's influence on the secondary ion dynamics is discussed.