ALBERT

Notes: We use 42 different charged nanoparticles generated by electrospray ionization and a differential mobility analyzer of unusual resolving power, as well as a condensation chamber of the turbulent-mixing type to study the dependence between the diameter, charge state, and critical supersaturation of embryos promoting heterogeneous nucleation. The nanoparticles investigated have diameters ranging from 0.43 to 6.51 nm, and positive charge states varying between 1 and 5 elementary units. We find that the critical supersaturation of small singly charged ions (mobility diameters bellow 1.01 nm) is independent of their size and its value coincides with the result anticipated by Thompson in his theory of ion-induced nucleation. On the other hand, the supersaturation required to activate multiply charged embryos is consistently higher than Thomson's prediction. In fact, the reduction of the critical supersaturation induced by electrification is much smaller than expected for large and multiply charged embryos, and their critical radius is estimated better by Kelvin's criterion. We speculate that this discrepancy is due to the geometrical differences between the actual nucleation sites and the idealized embryo considered in Thomson's model. © 2002 American Institute of Physics.

Notes: A combined computational and experimental study of the methyl-p-aminobenzoate(H2O)n, (n=2,3,4) complexes [MAB(H2O)n] is reported. Complexes potential energy surfaces were explored by ab initio density functional theory (DFT) methods, at the B3LYP/6-31G level, and the stable isomer structures and vibrational modes further computed at the B3LYP/6-31+G* level. A set of self-contained experimental techniques, including laser induced fluorescence (LIF), resonance enhanced multiphoton ionization mass-resolved spectroscopy (REMPI), two-color resonance enhanced multiphoton ionization mass-resolved spectroscopy (R2PI), "hole burning" spectroscopy (HB), and two-color ionization thresholds were used to study the spectra and other physical features of the complexes. Of the three title complexes only MAB(H2O)4 has been observed with our experimental methods, while the MAB(H2O)3 was formed by evaporation and MAB(H2O)2 was not detected at all. It has been shown that the observed MAB(H2O)4 complex has only one isomer with a hydrogen bonded water ring structure attached to the amino hydrogens and its low vibrational modes (up to 165 cm−1) have been assigned. A discussion of the results, including structures of stable isomers, isomer energies, ionization thresholds, and the difficulties in observing some solvated complexes is presented. © 2000 American Institute of Physics.

Notes: Methyl-p-aminobenzoate(NH3)1 complex, henceforth MAB(NH3)1, prepared in a pulsed supersonic expansion, has been examined by laser mass-selective spectroscopies and density functional theory calculations, aiming to ascertain its isomer number, structures, identification, ionization energies, and vibrational assignments. Resonance enhanced multiphoton ionization and hole burning spectra of the species in supersonic beams show two 000 transitions redshifted by −715 and −709 cm−1 from that of bare MAB band origin and are plausibly associated with two different isomers, whereas ab initio calculations indicate the likely existence of five stable isomer structures. Identification of the experimental isomer spectra with the calculated structures is reported and, in particular, several isomer vibrational bands are identified by contrast with the calculated modes. Properties and features of the MAB(NH3)1 are compared with those of the MAB/water complexes. © 2000 American Institute of Physics.

Notes: Weakly bound complexes of phenol (Ph) and fluoromethane (CH3F) formed in a supersonic expansion have been identified by one- and two-color mass-resolved and hole burning spectroscopies. Only one isomer has been observed for the 1:1 complex. Threshold fragmentation has been employed to determine the binding energies of the complex in its ground, S0, and first electronic, S1, states, as well as in the ion ground state, I0, yielding the following results: D0(S0)=1540±50 cm−1, D0(S1)=1713±50 cm−1, and D0(I0)=3932±50 cm−1, respectively. In a complementary study, calculations on the complex geometries and binding energies were conducted at the B3LYP/6-31+G* and the MP2/6-31+G* levels. It has been shown that the binding energies computed at the MP2/6-31+G* level are in excellent agreement with the experimental values, whilst those calculated at the B3LYP/6-31+G* level underestimate them by nearly 30%, probably due to the poor description of the dispersion forces. © 2001 American Institute of Physics.

Notes: When concentrated solutions of NaI in formamide with electrical conductivities K larger than 1.1 S/m are electrosprayed from a Taylor cone-jet in a vacuum, ions are evaporated at substantial rates from the surface of the meniscus and the drops. This constitutes a new source of ions and nanoparticles, where the relative importance of these two contributions is adjustable. The currents of ions are measured independently from those associated with drops by a combination of stopping voltage analysis and preferential scattering in a gas background. The magnitude E of the electric field at the surface of the drops and at the apex of the cone-jet is controlled through the electrical conductivity K of the liquid and its flow rate Q through the jet. E is related through available scaling laws for Taylor cone-jets to the ratios K/Q or I/Q, where I is the current of drops emitted by the jet. Ion currents are very small or null at typical K/Q values used in the past. A relatively small initial ion current is attributed to a few particularly sharp features present, perhaps associated with small satellite drops. At still higher K/Q this first ionization source saturates, and ion evaporation from the main drops begins to dominate (E∼1 V/nm). E can then be determined with little ambiguity, and the associated ion current is also measured over a broad enough range of electric fields to determine the ionization kinetics. At still higher K/Q the ion current from the drops approaches saturation, and ion evaporation directly from the meniscus becomes dominant. The total spray current then presents the anomaly of increasing rapidly at decreasing liquid flow rate. The ion current from the meniscus can also be measured in this regime over a broad range of K/Q, with qualitative agreement with the ionization measurements from the drops. But the relation established between K/Q and E becomes suspect because ion and drop currents are now comparable. A third approach to infer the ionization rate is based on the related disappearance of Coulomb explosions of the drops above a critical K/Q. These results are congruent with the model of Iribarne and Thomson, with an activation barrier for ion evaporation equal to 1.7 eV−(e3E/4πε0)1/2. © 2000 American Institute of Physics.

Notes: Weakly bound hydrogen bonded ethyl-p-aminobenzoate/water complexes, referred to henceforth both as EAB/(H2O)n (n=1–4) or by their stoichiometry 1:n, have been investigated with a combined approach of mass and light detector laser spectroscopic techniques and ab initio calculations. The experimental studies follow explorations with laser induced fluorescence (LIF), and include one-color resonant enhanced multiphoton ionization (REMPI), two-color REMPI (R2PI), pressure dependent R2PI and hole burning (HB) spectroscopies. Calculations were conducted at the B3LYP/6-31+G* level and for the 1:1 complex led to the existence of six stable isomers, identified as the experimental origin bands at +4, +6, +13, +89, +96, and +108 cm−1 above the bare EAB 000 transition. It has been shown that three of these bands originate in the EAB trans conformer, while the other three derive from the EAB gauche conformer. None of the experimental methods used lead us to observe the EAB(H2O)2 complex spectrum and the inspection of the EAB(H2O)3 REMPI and R2PI spectra has been shown to be a fragmentation from the EAB(H2O)4 complex. The structures and identification of the set of isomers are reported and a comparison with the results on the family complexes methyl-p-aminobenzoate/water is discussed. © 2000 American Institute of Physics.

Notes: A complementary laser spectroscopy and computational study of the MAB(NH3)2–4 complexes, hereafter referred to by its stoichiometry, i.e., 1:2, 1:3, and 1:4, prepared in a supersonic expansion, is reported. Experimental evidence shows the existence of abundant fragmentation cascades, the most notorious being the observation of the 1:4 complex spectrum in the 1:3 and to 1:2 mass channels, in fact, the observed spectra of the 1:2 and 1:3 complexes are not genuine but a consequence of fragmentation. The observed 1:4 complex resonance enhanced multiphoton ionization (REMPI) spectrum has a significant redshift of −1160 cm−1 from the bare MAB 000 transition and appears over a noisy background that decreases, although it does not disappear, in resonance enhanced two-color photo ionization (R2PI) studies. "Hole burning" spectroscopy corroborates the presence of only one 1:4 isomer. Calculations at the B3LYP/6−31+G* level conduct to a number of 1:2, 1:3, and 1:4 stable isomer structures, the most stable being the 1:4 with a four ammonia chain coordinated to the NH2 group. The good agreement between calculated and experimental vibrational frequencies confirms the ammonia ring structure and allows us to assign a number of MAB(NH3)4 inter- and intramolecular vibrational bands. © 2000 American Institute of Physics.

Notes: Methyl-p-aminobenzoate(H2O)1 complex, henceforth MAB(H2O)1, prepared by pulsed supersonic expansion, has been examined by a broad range of laser based spectroscopic, mass and isomer selective techniques and density functional theory (DFT) calculations, in order to identify its isomer structures, ionization energies, and vibrational frequencies. The experimental techniques used include laser induced fluorescence (LIF), mass resolved excitation spectroscopy (MRES) either with one (REMPI) or two laser colors (R2PI), laser excited dispersed emission (DE), high resolution MRES, pressure controlled R2PI, hole burning (HB) spectroscopy, and photoion fragmentation threshold (PIFT). Experimental results have been interpreted, rationalized and extended with density functional theory (DFT) computations at the B3LYP/6-31G and B3LYP/6-31+G* levels. Although bare MAB molecule have four possible solvation sites, prone to yielding hydrogen bonds with the water molecule, LIF, R2PI, and HB spectroscopy of MAB(H2O)1 only pick out the presence of three blue shifted isomers, each accompanied by a number of vibrational features extending to (approximate)500 cm−1. The high intensity bands have been demonstrated to originate in three isomers and their ionization energies, dispersed emission, vibrational spectra, and photoion fragmentation threshold have been measured and characterized. Isomer shifts and structures are discussed in the light of experimental and theoretical results. © 2000 American Institute of Physics.

Notes: Spectroscopic and laser properties have been characterized for Nd3+ in LaSc3(BO3)4. The Judd–Ofelt analysis has been applied to the measured room temperature absorption spectrum to determine the radiative decay rates and branching ratios of Nd3+ transitions from the 4F3/2 metastable state to the 4IJ lower-lying manifolds. The parameters Ω2, Ω4, and Ω6 are larger than those reported for Nd3+ in other laser host crystals. The value of Ω4/Ω6 is approximately 3.0 times larger than that of Nd3+ in yttrium–aluminum–garnet (YAG) and about 1.4 times larger than that of Nd3+ in the β phase of LaSc3(BO3) reported recently. The measured room temperature fluorescence lifetime of the 4F3/2→4I11/2 transition is 150 μs, while the Judd–Ofelt analysis predicts a radiative lifetime for the 4F3/2 state to be 249 μs, resulting in the fluorescence quantum efficiency of 60%. The emission cross sections of the 4F3/2→4I11/2 and 4F3/2→4I13/2 intermanifold transitions have been also determined at room temperature. Finally, these results are compared with those of the standard laser material Nd3+:YAG. © 2001 American Institute of Physics.

Notes: The collisional behavior of NCO[X˜(0,n,0)] in specific vibronic states in the gas phase has been investigated in the time-domain by laser-induced fluorescence (LIF) on transitions within the system NCO(A˜ 2Σ+–X˜ 2Π). The NCO radical was generated by the infrared multiphoton dissociation (IRMPD) of phenyl isocyanate (PhNCO) by means of a TEA-CO2 laser operating on the 9R24 line at λ=9.25 μm with subsequent monitoring of the vibronic levels of the X˜ state, characterized by Renner–Teller coupling, in the presence of N2, O2, NO, CO2, N2O, SO2, and PhNCO itself. The states probed were as follows: (0010)2Π3/2, (0010)2Π1/2, (0100)μ2Σ+, (0120)2Δ5/2, (0120)2Δ3/2, (0210)μ2Π3/2,1/2, (0230)2Φ7/2, and (0230)2Φ5/2. Various pairs of spin–orbit states were found to be tightly coupled kinetically. Thus, the time-evolution of the pairs of vibronic states (0010)2Π3/2 and (0010)2Π1/2; (0120)2Δ5/2 and (0120)2Δ3/2; (0230)2Φ7/2 and (0230)2Φ5/2 were found to be equal, yielding an effective local equilibrium within these spin–orbit components within experimental error. Further, states such as NCO(0100) and NCO(0120) were characterized by relatively long decay profiles in the presence of molecules such as CO2 and O2 where the contribution of rotational quenching to the overall decay process could be neglected. By contrast, NCO(0210) and NCO(0230) were removed on significantly faster time scales on collision with SO2. In the absence of extensive information required for solving the large set of coupled differential kinetic equations, albeit reduced in number of those states strongly coupled kinetically, such as a detailed knowledge of the nascent state distributions in NCO following IRMPD, not necessarily Boltzmann in character, the vibronic states were taken to behave independently as the most practical approach to this study. Absolute second-order rate data for the collisional quenching of NCO in the vibronic states (0010), (0100), (0120), (0210), and (0230) by the above molecular species are reported. No clear selection rules are apparent except for the low propensity rule ΔK=2 within the same vibronic state, i.e., μ 2Σ+(0100)–2Δ5/2(0120) and 2Π3/2,1/2(0210)–2Φ7/2(0230). This is presumed to reflect the role in the collisional interaction of the oscillating dipole in the vibronic state, facilitating ΔK=1, whereas ΔK=2 would involve the quadrupole which is smaller. It is found that the data for (V–V) and (V–T) energy transfer correlate best with the attractive part of the potential curves between the collision partners using the established Parmenter–Seaver plots, yielding well depths [(εMM/kB)1/2] for the vibronic states NCO[μ 2Σ+(0100), 2Δ5/2(0120), 2Π3/2,1/2(0210), and 2Φ7/2(0230)], significantly larger than those of the closed shell collision partners and equal within experimental error. The data are also considered in terms of a multipolar attractive force model involving a collision complex where a sensible correlation is found between the computed and observed collision cross sections for O2, N2, CO2, N2O, and SO2 assuming no change in the multipoles with vibrational state. © 1997 American Institute of Physics.