Size-dependent collisional incorporation of D2O into (H2O)−n around n = 15: implications on the origin of magic numbers in the hydrated electron cluster distribution
We report a rapid increase in the propensity of D2O molecules to incorporate into (H2O)−n clusters upon collisions between D2O and a distribution of (H2O−n, 6 <n< 25, clusters. We interpret this effect by invoking a size-dependent associative electron detachment cross section which increases rapidly for n < 15. The implications of this detachment process on the “magic numbers” in the (H2O)−n distribution are discussed.
Photodestruction spectra of the anionic water clusters, (H2O)−n, n=18 and 30: Absorption to the red of e−aq
We report absolute photodestruction cross sections of mass selected, anionic water clusters (H2O)−n, n=18 and 30, in order to compare the photoabsorption of the clusters with that of the bulk hydrated electron. The cluster absorptions are similar in magnitude to that of e−aq, but red shifted by about 0.75 eV relative to 298 K e−aq and are observed to rapidly increase in the vicinity of the vertical electron detachment energies of the clusters.
Photoabsorption of negative cluster ions near the electron detachment threshold: A study of the (O2)−n system
In order to better understand the propensity of negative cluster ion systems to photodissociate when excited near their photodetachment thresholds, we present a detailed photochemical study of the oxygen cluster negative ions, (O2)−n. Irradiation of the 3≤n≤6 parent ions at or slightly below their photodetachment thresholds in the near ir (1064 nm) is found to result in significant photofragmentation, even though the dimer is effectively photostable at this wavelength. The cross sections for n≥3 photofragmentation are approximately constant over the higher clusters and are about a factor of 3 larger than the O−2 photodetachment cross section. These observations suggest that photoabsorption of the higher clusters may result from a charge‐transfer process between the O−4 core ion and the ‘‘solvent’’ O2 ligands.
Isotopic fractionation in low temperature ion–molecule exchange reactions: Enrichment of 22Ne in Ne+n clusters formed by association in an ionized free jet
Cationic clusters of neon atoms (Ne+n ) formed by association of neutrals onto seed ions in an ionized supersonic expansion are found to favor incorporation of the heavier isotope (22Ne) by as much as a factor of 15 (in the dimer and trimer ions) when compared to a simple statistical distribution based on natural abundances. This enrichment is attributed to the small difference in zero‐point energies among species formed with the two major isotopes of neon (20Ne and 22Ne), which is of the same order as the collisional energy of particles in the expanding jet. This enrichment is anticipated by current models of isotope exchange which are invoked to explain the anomalous isotope abundance patterns in interstellar clouds.
The angular distribution of photoelectrons ejected from the hydrated electron cluster (H2O)−18
The angular distribution of the photoelectrons ejected from the hydrated electron cluster (H2O)−18 was measured to determine the spatial character of the orbital in which the excess electron resides. The asymmetry parameter β was determined to be 0.92±0.1, consistent with a roughly spherical orbital, similar to that of the hydrated electron in solution.
On the origin of the competition between photofragmentation and photodetachment in hydrated electron clusters, (H2O)−n
Photoexcitation of size‐selected hydrated electron clusters, (H2O)−n , in the near IR results in a competition between photofragmentation and electron photodetachment. To investigate the origin of this competition, the decay probability into ionic fragments for the n=25 cluster was measured as a function of photon energy from 0.91≤hν≤3.49 eV. The photofragmentation probability increases rapidly with decreasing excitation energy in the general vicinity of the vertical detachment energy of this cluster (1.4 eV) determined via photoelectron spectroscopy. This result suggests that fragmentation accompanies photoexcitation of the excess electron with near zero kinetic energy. Thus, photofragmentation appears to proceed through an optically prepared intermediate similar to that reached in electron scattering from neutral clusters, which displays an enhanced dissociative attachment pathway with near zero kinetic energy electrons.
Observation of a UV absorption band in Ar3+ near 300 nm
The characteristic 2Σg+ ← 2Σu+ absorption band of Ar2+ near 300 nm is found to remain essentially intact in Ar3+ as well as the larger argon clusters. This absorption is slightly blue-shifted and of about half the intensity of the Ar2+ absorption. We account for these observations with a model invoking an Ar2+·Ar double minimum ground state for Ar3+ and suggest that the visible spectrum of this species arises from an intracluster electron transfer excitation.
Preservation of rotational cooling following pulse compression TOF mass selection of N2O+ ions created in a supersonic plasma
Cold ions created in the dense region of an ionized free jet are mass selected by pulsing them into a tandem time-of-flight (TOF) photofragmentation spectrometer after the plasma breaks up in the 1/r2 expansion. These ions are then refocused for interaction with a pulsed laser to recover the high signal levels typical of ion spectroscopies carried out inside the plasma. The rotational cooling of the N2O+ ion in the source was found to be preserved even after transverse extraction through the unskimmed jet. Details of the cavity stabilized, single mode pulsed dye laser are also presented.
Reactions of hydrated electron clusters (H2O)n-: scavenging the excess electron
Reactions of negatively charged water clusters (H2O)n– with O2, CO2, NO, N2O, Br2, and CHBr3 result in the formations of O2–∙(H2O)n-7, CO2–∙(H2O)n-3, NO–∙(H2O)n-5, O–∙(H2O)n-5, Br–∙(H2O)n-5, and Br–∙(H2O)n-4 clusters, respectively. The evaporation of neutral water molecules accompanying these reactions is manifested by a shift in the unique (H2O)n– parent cluster ion distribution and is correlated with the calculated exothermicity for the processes of charge transfer of the loosely bound, hydrated electron to the reactant molecule followed by hydration of the resulting ion.
Photofragmentation of Cn−, 4⩽n⩽20: Loss of neutral C3
Size-selected photofragmentation of the negative carbon cluster ions Cn−, 4⪕n⪕20, has been surveyed when excited both above and below their electron binding energies (at 3.49, and 2.33 eV, respectively). For most clusters smaller than n=14, fragmentation occurs via loss of C3 neutrals, analogous to the behavior found in the carbon cations.
Photochemistry of hydrated electron clusters (H2O)−n (15≤n≤40) at 1064 nm: Size dependent competition between photofragmentation and photodetachment
Photofragmentation is found to be surprisingly efficient when hydrated electron clusters (H2O)−n, 15≤n≤40, are excited at 1064 nm (1.165 eV). The decay probability into ionic channels rises sharply from zero in the size range 15≤n≤20 before leveling off at a value of 0.56±0.10. The propensity of smaller clusters to detach an electron rather than fragment is correlated with the peculiar shape of the cluster ion distribution obtained by dissociative attachment of low energy electrons onto neutral water clusters, where the ionic clusters are only observed in abundance for n≥11.
Photoelectron spectroscopy of (CO2)−n clusters with 2≤n≤13: Cluster size dependence of the core molecular ion
Photoelectron spectra of the negatively charged clusters of CO2 are recorded with 3.49 eV photon energy and appear as bell-shaped, unresolved vibrational envelopes similar to that observed for the monomer ion. The maxima of the photoelectron spectra, found by fitting the envelopes to Gaussian profiles, correspond to the vertical electron detachment energies (VDE) of the clusters. These VDE values, when combined with the previously measured value for CO2-, display sharp discontinuities at cluster sizes n=2 and n=6. The magnitudes of these shifts are on the order of 1 eV and are in near quantitative agreement with the calculated difference in VDE between the monomer anion and the D2d form of the dimer anion. We infer from this agreement that the dimer ion is the core of clusters 2≤n≤5 while the monomer ion forms the core for n≥7. The hexamer is special in that both forms are evident in the photoelectron spectra. These structural changes are not manifested as ‘‘magic numbers’’ in the parent spectra, which have been previously observed at n=4, 7, 10, and 14.
Pulsed photoelectron spectroscopy of negative cluster ions: Isolation of three distinguishable forms of N2O2−
Three different ionic species with stoichiometry N2O2− are generated by varying the neutral precursors in an electron beam ionized free jet expansion. In each case, the ion is isolated by mass spectrometry and then probed using pulsed photoelectron spectroscopy (PES) at 532 and 355 nm (2.330 and 3.495 eV, respectively). The neutral starting materials used in the three preparations are (I) O2 seeded 5% in N2, (II) pure N2O, and (III) NO seeded 10% in Ar. Based on their PES and photofragmentation properties, the three species appear to be best described as (I) O−2 ⋅N2, (II) either O−⋅N2O or more likely a chemically bound species, and (III) NO−⋅NO. It is likely that two of these species are trapped intermediates in the O−+N2O→NO−+NO reaction, suggesting a double minimum potential energy surface. The formation mechanisms of these ions in our source are discussed in the context of previous preparation schemes.
Demonstration of a pulsed photoelectron spectrometer on mass-selected negative ions - O-, O2-, and O4-
A new type of negative ion photoelectron spectrometer which utilizes time-of-flight for both ion mass selection and photoelectron energy analysis is described and demonstrated on the 532 nm photodetachment of the well-known O− and O2- ions as well as the O4- cluster ion. Photoelectron spectra of O4- at 355 and 532 nm depend on laser power, suggesting that photodissociation competes with photodetachment at both wavelengths.