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.

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.

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.