Infrared Predissociation Spectroscopy of Large Water Clusters: A Unique Probe of Cluster Surfaces (original) (raw)
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The Journal of Chemical Physics, 2011
Infrared photodissociation spectroscopy is reported for mass-selected H + (H 2 O) n complexes and their deuterated analogues with and without argon "tagging." H + (H 2 O) n Ar m and D + (D 2 O) n Ar m complexes are studied in the O-H (O-D) stretching region for clusters in the small size range (n ) 2-5). Upon infrared excitation, these clusters fragment by the loss of either argon atoms or one or more intact water molecules. Their excitation spectra show distinct bands in the region of the symmetric and asymmetric stretches of water and in the hydrogen bonding region. Experimental studies are complemented by computational work that explores the isomeric structures, their energetics and vibrational spectra. The addition of an argon atom is essential to obtain photodissociation for the n ) 2-3 complexes, and specific inclusion of the argon in calculations is necessary to reproduce the measured spectra. For n ) 3-5, spectra are obtained both with and without argon. The added argon atom allows selection of a subset of colder clusters and it increases the photodissociation yield. Although most of these clusters have more than one possible isomeric structure, the spectra measured correspond to a single isomer that is computed to be the most stable. Deuteration in these small cluster sizes leads to expected lowering of frequencies, but the spectra indicate the presence of the same single moststable isomer for each cluster size.
Infrared spectroscopy of negatively charged water clusters: Evidence for a linear network
Journal of Chemical Physics, 1999
We report autodetachment spectra of the mass-selected, anionic water clusters, (H 2 O) n Ϫ , nϭ2, 3, 5-9, 11 in the OH stretching region ͑3000-4000 cm Ϫ1 ͒, and interpret the spectra with the aid of ab initio calculations. For nу5, the spectra are structured and are generally dominated by an intense doublet, split by about 100 cm Ϫ1 , which gradually shifts toward lower energy with increasing cluster size. This behavior indicates that the nϭ5-11 clusters share a common structural motif. The strong bands appear in the frequency region usually associated with single-donor vibrations of water molecules embedded in extended networks, and theoretical calculations indicate that the observed spectra are consistent with linear ''chainlike'' (H 2 O) n Ϫ species. We test this assignment by recording the spectral pattern of the cooled ͑argon solvated͒ HDO•(D 2 O) 5 Ϫ isotopomer over the entire OH stretching frequency range.
A size resolved investigation of large water clusters
Physical Chemistry Chemical Physics, 2014
Size selected water clusters are generated by photoionizing sodium doped clusters close to the ionization threshold. This procedure is free of fragmentation. Upon infrared excitation, size-and isomerspecific OH-stretch spectra are obtained over a large range of cluster sizes. In one application of this method the infrared spectra of single water cluster sizes are investigated. A comparison with calculations, based on structures optimized by genetic algorithms, has been made to tentatively derive cluster structures which reproduce the experimental spectra. We identified a single all-surface structure for n = 25 and mixtures with one or two interior molecules for n = 24 and 32. In another application the sizes are determined at which the crystallization sets in. Surprisingly, this process strongly depends on the cluster temperature. The crystallization starts at sizes below n = 200 at higher temperatures and the onset is shifted to sizes above n = 400 at lower temperatures.
Far-infrared absorption of water clusters by first-principles molecular dynamics
Chemical Physics, 2008
Based on first-principle molecular dynamic simulations, we calculate the far-infrared spectra of small water clusters (H2O)n (n=2,4,6) at frequencies below 1000 cm-1 and at 80 K and at atmospheric temperature (T>200 K). We find that cluster size and temperature affect the spectra significantly. The effect of the cluster size is similar to the one reported for confined water. Temperature changes not only the shape of the spectra but also the total strength of the absorption, a consequence of the complete anharmonic nature of the classical dynamics at high temperature. In particular, we find that in the frequency region up to 320 cm-1, the absorption strength per molecule of the water dimer at 220 K is significantly larger than that of bulk liquid water, while tetramer and hexamer show bulklike strengths. However, the absorption strength of the dimer throughout the far-infrared region is too small to explain the measured vapor absorption continuum, which must therefore be dominated by other mechanisms.
Structure and vibrational spectra of small water clusters from first principles simulations
The Journal of Chemical Physics, 2010
The structure and vibrational spectra of ͑H 2 O͒ n ͑n =2-5͒ clusters have been studied based on first-principles molecular dynamics simulations. Trends of the cluster structures with the cluster size show that water molecules in cluster are bound more tightly. The vibrational spectra as a function of cluster size and temperature are obtained using Fourier transformation of the velocity autocorrelation function. Results of the clusters in ground state show that when the cluster size increases, the librational peaks shift to blue and the bonded intramolecular OH stretching bands shift to red due to the clusterization and hydrogen-bond strengthening. Meanwhile, there are no significant shifts in the intramolecular bending and free OH stretching modes, indicating that the free hydrogen atoms are insensitive to the local bonding environment. The temperature-dependent vibrational spectra, which exhibit similar behaviors from the dimer to pentamer, show that there are significant broadenings of the spectra with temperature caused by thermal motions. Moreover, different bands shift to different directions, where librational bands shift to red while bonded OH stretching bands shift to blue, although the blueshifts are quite small for the dimer and trimer.
Proceedings of the National Academy of Sciences, 2014
Theoretical models of proton hydration with tens of water molecules indicate that the excess proton is embedded on the surface of clathrate-like cage structures with one or two water molecules in the interior. The evidence for these structures has been indirect, however, because the experimental spectra in the critical H-bonding region of the OH stretching vibrations have been too diffuse to provide band patterns that distinguish between candidate structures predicted theoretically. Here we exploit the slow cooling afforded by cryogenic ion trapping, along with isotopic substitution, to quench water clusters attached to the H 3 O + and Cs + ions into structures that yield well-resolved vibrational bands over the entire 215-to 3,800-cm −1 range. The magic H 3 O + (H 2 O) 20 cluster yields particularly clear spectral signatures that can, with the aid of ab initio predictions, be traced to specific classes of network sites in the predicted pentagonal dodecahedron H-bonded cage with the hydronium ion residing on the surface.
Water cluster interpretation of IR absorption spectra in the 8–14-µm wavelength region
Applied Optics, 1979
Recently various investigators have suggested that the anomalous atmospheric absorption spectra in the window wavelength region of 8-14,um is due to the presence of the water dimer or other water clusters. This suggestion has been made on the grounds that the IR absorptivity has a dependency on the squared value of the partial water vapor pressure or nonlinear vapor pressure and negative temperature dependence, and that the effect of foreign broadening is negligible compared with that of self-broadening. Here we present a theoretical study of the IR continuous absorption by the water dimer in the path containing pure water vapor. In this paper we report our computed results of the intermolecular normal vibrational frequencies, concentrations, and absorption coefficients of the water dimer in the gas phase, focusing our primary attention to the IR absorption in the 8-14-,um wavelength region. Our analysis indicates that the dimer may be an important contributor to the IR absorption in the above mentioned spectral range, and that this region corresponds to the wing of the vibrational transition band associated with an intermolecular librational motion of the water dimer.
Infrared cavity ringdown spectroscopy of water clusters: O–D stretching bands
The Journal of Chemical Physics, 1998
The infrared O-D stretching spectrum of fully deuterated jet-cooled water clusters is reported. Sequential red-shifts in the single donor O-D stretches, which characterize the cooperative effects in the hydrogen bond network, were accurately measured for clusters up to (D 2 O) 8 . Detailed comparisons with corresponding data obtained for (H 2 O) n clusters are presented. Additionally, rotational analyses of two D 2 O dimer bands are presented. These measurements were made possible by the advent of infrared cavity ringdown laser absorption spectroscopy ͑IR-CRLAS͒ using Raman-shifted pulsed dye lasers, which creates many new opportunities for gas phase IR spectroscopy.
The Journal of Physical Chemistry A, 2006
We report vibrational predissociation spectra of water cluster anions, (H 2 O) n)3-24in the HOH bending region to explore whether the characteristic red-shifted feature associated with electron binding onto a double H-bond acceptor (AA) water molecule survives into the intermediate cluster size regime. The spectra of the "tagged" (H 2 O) n -‚Ar clusters indeed exhibit the signature AA band, but assignment of this motif to a particular isomer is complicated by the fact that argon attachment produces significant population of three isomeric forms (as evidenced by their photoelectron spectra). We therefore also investigated the bare clusters since they can be prepared exclusively in the high binding (isomer class I) form. Because the energy required to dissociate a water molecule from the bare complexes is much larger than the transition energies in the bending region, the resulting (linear) action spectroscopy selectively explores the properties of clusters with most internal energy content. The (H 2 O) 15predissociation spectrum obtained under these conditions displays a more intense AA feature than was found in the spectra of the Ar tagged species. This observation implies that not only is the AA motif present in the class I isomer, but also that it persists when the clusters contain considerable internal energy.