Formation of Exotic Networks of Water Clusters in Helium Droplets Facilitated by the Presence of Neon Atoms.

Water clusters are formed in helium droplets via the sequential capture of monomers. One or two neon atoms are added to each droplet prior to the addition of water. The infrared spectrum of the droplet ensemble reveals several signatures of polar, water tetramer clusters having dipole moments between 2D and 3D. Comparison with ab initio computations supports the assignment of the cluster networks to noncyclic "3 + 1" clusters, which are ∼5.3 kcal/mol less stable than the global minimum nonpolar cyclic tetramer. The (H2O)3Ne + H2O ring insertion barrier is sufficiently large, such that evaporative helium cooling is capable of kinetically quenching the nonequilibrium tetramer system prior to its rearrangement to the lower energy cyclic species. To this end, the reported process results in the formation of exotic water cluster networks that are either higher in energy than the most stable gas-phase analogs or not even stable in the gas phase.

Formation of cold ion-neutral clusters using superfluid helium nanodroplets.

A strategy for forming and detecting cold ion-neutral clusters using superfluid helium nanodroplets is described. Sodium cations generated via thermionic emission are directed toward a beam of helium droplets that can also pick up neutral molecules and form a cluster with the captured Na(+). The composition of the clusters is determined by mass spectrometric analysis following a desolvation step. It is shown that the polar molecules H(2)O and HCN are picked up and form ion-neutral clusters with sizes and relative abundances that are in good agreement with those predicted by the statistics used to describe neutral cluster formation in helium droplets. [Na(H(2)O)(n)](+) clusters containing six to 43 water molecules were observed, a size range of sodiated water clusters difficult to access in the gas phase. Clusters containing N(2) were in lower abundance than expected, suggesting that the desolvation process heats the clusters sufficiently to dissociate those containing nonpolar molecules.

(HCN)(m)-M(n) (M = K, Ca, Sr): vibrational excitation induced solvation and desolvation of dopants in and on helium nanodroplets.

Infrared (IR) laser spectroscopy is used to probe the rotational and vibrational dynamics of the (HCN)(m)-M(n) (M = K, Ca, Sr) complexes, either solvated within or bound to the surface of helium nanodroplets. The IR spectra of the (HCN)(m)-K (m = 1-3), HCN-Sr, and HCN-Ca complexes have the signature of a surface species, similar to the previously reported spectra of HCN-M (M = Na, K, Rb, Cs) [Douberly, G. E.; Miller, R. E. J. Phys. Chem. A 2007, 111, 7292.]. A second band in the HCN-Ca spectrum is assigned to a solvated complex. The relative intensities of the two HCN-Ca bands are droplet size dependent, with the solvated species being favored in larger droplets. IR-IR double resonance spectroscopy is used to probe the interconversion of the two distinct HCN-Ca populations. While only a surface-bound HCN-Sr species is initially produced, CH stretch vibrational excitation results in a population transfer to a solvated state. Complexes containing multiple HCN molecules and one Sr atom are surface-bound, while the nu(1) (HCN)(2)Ca spectrum has both the solvated and surface-bound signatures. All HCN-(Ca,Sr)(n) (n > or = 2) complexes are solvated following cluster formation in the droplet. Density-functional calculations of helium nanodroplets interacting with the HCN-M show surface binding for M = Na with a binding energy of 95 cm(-1). The calculations predict a fully solvated complex for M = Ca. For M = Sr, a 2.2 cm(-1) barrier is predicted between nearly isoenergetic surface binding and solvated states.

Study of the CH3 H2O radical complex stabilized in helium nanodroplets.

The weakly bound CH(3)H(2)O radical complex has been investigated by infrared laser spectroscopy. The complex is stabilized in helium nanodroplets and prepared by sequential pick up of a methyl radical and water molecule. Partially rotationally resolved spectra corresponding to the v = 1 <-- 0 excitation of the symmetric H(2)O stretching vibration within the complex show a significant red shift (25.06 cm(-1)) when compared with the symmetric stretch of H(2)O monomer, in agreement with the hydrogen bonded like structure derived by theory. Additional broad features were observed in the region predicted by theory for the antisymmetric stretch supporting our assignment. The B rotational constant is found to be 3.03 times smaller than predicted by ab initio calculations, with the reduction being attributed to the effects of helium solvation. The permanent electric dipole moment of the complex is experimentally determined to be 2.1 +/- 0.3 D using Stark spectroscopy. Ab initio calculations are also reported that provide support to the experimental results, as well as investigate the nature of large amplitude vibrational motion within the complex.

High-resolution infrared spectroscopy of Mg-HF and Mg-(HF)2 solvated in helium nanodroplets.

High-resolution infrared (IR) spectroscopy is used to investigate the Mg-HF and Mg-(HF)(2) van der Waals complexes. Both complexes are formed and probed within helium nanodroplets. Rotationally resolved zero-field and Stark spectra are assigned to a linear binary complex composed of a Mg atom bound to the hydrogen end of the HF molecule. Although high level ab initio calculations predict a fluorine bonded complex, none of the observed IR bands can be assigned to this complex. The collocation method is employed to determine the bound states on the two-dimensional intermolecular Mg-HF potential energy surface. The ground and first excited state wave functions for this potential surface have zero amplitude in the well corresponding to the fluorine bonded complex, consistent with experiment. The two HF stretching bands of the Mg-(HF)(2) complex are observed and assigned using a combination of the spectral symmetry, ab initio calculations, pick-up cell pressure dependencies, and dipole moment measurements. Comparisons with the helium solvated HF dimer show large changes to the HF stretching frequencies upon the addition of a single Mg atom to the hydrogen side of (HF)(2).

Ionization and fragmentation of isomeric van der Waals complexes embedded in helium nanodroplets.

The ionization and charge transfer processes, which occur when a doped helium droplet undergoes electron impact, are studied for droplets doped with van der Waals complexes with various structures and electrostatic moments. The mass spectra of the two isomers of hydrogen cyanide complexed with either cyanoacetylene or acetylene in helium droplets were obtained using optically selected mass spectrometry, and show that the structure of the complex has a large effect on the fragmentation pattern. The resulting fragmentation pattern is consistent with an ionization process in which charge steering strongly influences the site of initial ionization. The observed dissociation products may also be subject to caging by the helium matrix.

Nonplanarity of adenine: vibrational transition moment angle studies in helium nanodroplets.

Mid-infrared spectra are reported for adenine monomer in helium nanodroplets. We show that there is only one tautomer of adenine, the global minimum structure, observed in helium nanodroplets and characterized by using ab initio calculations and the measurement of vibrational transition moment angles (VTMAs) for the various vibrational modes of the adenine monomer. On the basis of the VTMA analysis on the amino group of the global minimum tautomer, which gives insights into its nonplanarity, a detailed VTMA study of three lowest-energy amino tautomers of adenine is discussed in this study.

Infrared spectroscopy of prereactive aluminum-, gallium-, and indium-HCN entrance channel complexes solvated in helium nanodroplets.

Prereactive metal atom-HCN entrance channel complexes [M-HCN (M=Al, Ga, In)] have been stabilized in helium nanodroplets. Rotationally resolved infrared spectra are reported for the CH stretching vibration of the linear nitrogen-bound HCN-Ga and HCN-In complexes that show significant perturbation due to spin-orbit coupling of the 2Pi1/2 ground state with the 2Sigma1/2 state which are degenerate at long range. Six unresolved bands are also observed and assigned to the linear hydrogen-bound isomers of Al-HCN, Ga-HCN, and In-HCN corresponding to the fundamental CH stretching vibration and a combination band involving the CH stretch plus intermolecular stretch for each isomer. A nitrogen-bound HCN-Al complex is not observed, which is attributed to reaction, even at 0.37 K. This conclusion is supported by the observation of a weakly bound complex containing two HCN's and one Al atom which, from the analysis of its rotationally resolved zero-field and Stark spectra is assigned to a weakly bound complex of a HCNAl reaction product and a second HCN molecule. Theoretical calculations are presented to elucidate the reaction mechanisms and energetics of these metal atom reactions with HCN.

Rotational dynamics of HCN-M (M = Na, K, Rb, Cs) van der Waals complexes formed on the surface of helium nanodroplets.

Infrared laser spectroscopy was used to probe the unique rotational dynamics of the HCN-M (M = Na, K, Rb, Cs) complexes formed on the surface of helium droplets. The nu1 CH stretch ro-vibrational spectra were measured revealing what appears to be the P and R contours of a nearly rigid linear rotor. To simulate the linear molecule spectra, given a rotational temperature of 0.37 K, effective moments of inertia, IB, were required to be 10(4)-10(5) amu.A2 larger than the ab initio predicted values. The large moments of inertia were found to be strongly dependent on both the mass of the complex and the size of the helium droplet, consistent with a model where the dopant is located in a dimple site on the surface of the droplet. In this model, the moment of inertia is representative of the rotational motion of the dopant on the surface about an inertial axis through the center of the droplet.

Rovibrational spectra for the HCCCN*HCN and HCN*HCCCN binary complexes in 4He droplets.

Rovibrational spectra are measured for the HCCCN*HCN and HCN*HCCCN binary complexes in helium droplets at low temperature. Though no Q-branch is observed in the infrared spectrum of the linear HCN*HCCCN dimer, which is consistent with previous experimental results obtained for other linear molecules, a prominent Q-branch is found in the corresponding infrared spectrum of the HCCCN*HCN complex. This Q-branch, which is reminiscent of the spectrum of a parallel band of a prolate symmetric top, implies that some component of the total angular momentum is parallel to the molecular axis. The appearance of this particular spectroscopic feature is analyzed here in terms of a nonsuperfluid helium density induced by the molecular interactions. Finite temperature path integral Monte Carlo simulations are performed using potential energy surfaces calculated with second-order Möller-Plesset perturbation theory, to investigate the structural and superfluid properties of both HCCCN*HCN(4He)N and HCN*HCCCN(4He)N clusters with N < or = 200. Explicit calculation of local and global nonsuperfluid densities demonstrates that this difference in the rovibrational spectra of the HCCCN*HCN and HCN*HCCCN binary complexes in helium can be accounted for by local differences in the superfluid response to rotations about the molecular axis, i.e., different parallel nonsuperfluid densities. The parallel and perpendicular nonsuperfluid densities are found to be correlated with the locations and strengths of extrema in the dimer interaction potentials with helium, differences between which derive from the variable extent of polarization of the CN bond in cyanoacetylene and the hydrogen-bonded CH unit in the two isomers. Calculation of the corresponding helium moments of inertia and effective rotational constants of the binary complexes yields overall good agreement with the experimental values.

High-resolution infrared spectroscopy of HCN-Agn (n = 1-4) complexes solvated in superfluid helium droplets.

High-resolution infrared spectroscopy has been used to determine the structures, C-H stretching frequencies, and dipole moments of the HCN-Agn (n = 1-3) complexes formed in superfluid helium droplets. The HCN-Ag4 cluster was tentatively assigned based upon pick-up cell pressure dependencies and harmonic vibrational shift calculations. Ab initio and density functional theory calculations were used in conjunction with the high-resolution spectra to analyze the bonding nature of each cluster. All monoligated species reported here are bound through the nitrogen end of the HCN molecule. The HCN-Agn complexes are structurally similar to the previously reported HCN-Cun clusters, with the exception of the HCN-Ag binary complex. Although the interaction between the HCN and the Agn clusters follows the same trends as the HCN-Cun clusters, the more diffuse nature of the electrons surrounding the silver atoms results in a much weaker interaction.

Infrared-infrared double resonance spectroscopy of the isomers of acetylene-HCN and cyanoacetylene-HCN in helium nanodroplets.

Infrared-infrared double resonance spectroscopy is used to probe the vibrational dynamics of molecular complexes solvated in helium nanodroplets. We report results for the acetylene-HCN and cyanoacetylene-HCN binary complexes, each having two stable isomers. We find that vibrational excitation of an acetylene-HCN complex results in a population transfer to the other isomer. Photoinduced isomerization is found to be dependent on both the initially excited vibrational mode and the identity of the acetylene-HCN isomer. However, population transfer is not observed for the cyanoacetylene-HCN complexes. The results are rationalized in terms of the ab initio intermolecular potential energy surfaces for the two systems with particular emphasis on the long-range barriers to rearrangement.

Infrared laser spectroscopy of uracil and thymine in helium nanodroplets: vibrational transition moment angle study.

Vibrational spectra are reported in the N-H stretching region for uracil and thymine monomers in helium nanodroplets. Each monomer shows only a single isomer, the global minimum, in agreement with previous experimental and theoretical studies. The assignment of the infrared vibrational bands in the spectra is aided by the measurement of the corresponding vibrational transition moment angles (VTMAs) and ab initio frequency calculations. The ambiguity in the VTMA assignment of the N3H band for the uracil monomer is explained by the presence of dimer bands, which are overlapped with the monomer band.

A high-resolution infrared spectroscopic investigation of the halogen atom-HCN entrance channel complexes solvated in superfluid helium droplets.

Rotationally resolved infrared spectra are reported for the X-HCN (X = Cl, Br, I) binary complexes solvated in helium nanodroplets. These results are directly compared with those obtained previously for the corresponding X-HF complexes [J. M. Merritt, J. Küpper and R. E. Miller, Phys. Chem. Chem. Phys., 2005, 7, 67]. For bromine and iodine atoms complexed with HCN, two linear structures are observed and assigned to the (2)Sigma(1/2) and (2)Pi(3/2) ground electronic states of the nitrogen and hydrogen bound geometries, respectively. Experiments for HCN + chlorine atoms give rise to only a single band which is attributed to the nitrogen bound isomer. That the hydrogen bound isomer is not stabilized is rationalized in terms of a lowering of the isomerization barrier by spin-orbit coupling. Theoretical calculations with and without spin-orbit coupling have also been performed and are compared with our experimental results. The possibility of stabilizing high-energy structures containing multiple radicals is discussed, motivated by preliminary spectroscopic evidence for the di-radical Br-HCCCN-Br complex. Spectra for the corresponding molecular halogen HCN-X(2) complexes are also presented.

Structures and bonding nature of small monoligated copper clusters (HCN-Cun, n = 1-3) through high-resolution infrared spectroscopy and theory.

The structures, C-H stretching frequencies, and dipole moments of HCN-Cun (n = 1-3) clusters are determined through high-resolution infrared spectroscopy. The complexes are formed and probed within superfluid helium droplets, whereby the helium droplet beam is passed over a resistively heated crucible containing copper shot and then through a gas HCN pickup cell. All complexes are found to be bound to the nitrogen end of the HCN molecule and on the "atop site" of the copper cluster. Through the experimental C-H vibrational shifts of HCN-Cun and ab initio calculations, it was found that the HCN-metal interaction changes from a strong van der Waals bond in n = 1 to a partially covalent bond in HCN-Cu3. Comparisons with existing infrared data on copper surfaces show that the HCN-Cun bond must begin to weaken at very large copper cluster sizes, eventually returning to a van der Waals bond in the bulk copper surface case.

Infrared laser spectroscopy of imidazole complexes in helium nanodroplets: monomer, dimer, and binary water complexes.

Infrared laser spectroscopy has been used to characterize imidazole (IM), imidazole dimer (IMD), and imidazole-water (IMW) binary systems formed in helium nanodroplets. The experimental results are compared with ab initio calculations reported here. Vibrational transition moment angles provide conclusive assignments for the various complexes studied here, including IM, one isomer of IMD, and two isomers of the IMW binary complexes.

Four tautomers of isolated guanine from infrared laser spectroscopy in helium nanodroplets.

Infrared laser spectroscopy is used to study the four lowest energy tautomers of guanine, isolated in helium nanodroplets. The large number of vibrational bands observed in the infrared spectrum are assigned by comparing the corresponding experimental vibrational transition moment angles with those obtained from ab initio theory. The result is the conclusive assignment of the spectrum to the N9H-Keto, N7H-Keto, N9Ha-Enol(trans), and N9Hb-Enol(cis) tautomers. The dipole moments of these tautomers are also experimentally determined and compared with ab initio theory.

High-resolution infrared spectroscopy of HCN-Znn (n = 1-4) clusters: structure determination and comparisons with theory.

High-resolution infrared laser spectroscopy has been used to obtain rotationally resolved spectra of HCN-Zn(n) (n = 1-4) complexes formed in helium nanodroplets. In the present study the droplets passed through a metal oven, where the zinc vapor pressure was adjusted until one or more atoms were captured by the droplets. A second pickup cell was then used to dope the droplets with a single HCN molecule. Rotationally resolved infrared spectra are obtained for all of these complexes, providing valuable information concerning their structures. Stark spectra are reported and used to determine the corresponding permanent electric dipole moments. Ab initio calculations are also reported for these complexes for comparison with the experimental results.

Infrared spectroscopy of HCN-salt complexes formed in liquid-helium nanodroplets.

Rotationally resolved infrared spectra are reported for the binary complexes of HCN and LiF, LiCl, NaF, and NaCl, formed in helium nanodroplets. Stark spectroscopy is used to determine the dipole moments for these complexes. Ab initio calculations are also reported for these complexes, revealing the existence of several different isomers of these binary systems. In the frequency region examined in this experimental study we only observe one of these, corresponding to the salt binding to the nitrogen end of the HCN molecule. The experimental rotational constants, dipole moments, and vibrational frequency shifts are all compared with the results from ab initio calculations for this isomer.

Laser ablation of imidazolium based ionic liquids.

Time-of-flight mass spectrometry has been used to investigate the IR ablation of several ionic liquid imidazolium salts of the form R(1)R(2)Iium X (R(1) = methyl; R(2) = methyl, ethyl, butyl, and hexyl; X = Cl(-), NO(3)(-), and CH(3)SO(4)(-)). The ablated ionic species were analyzed by time-of-flight mass spectrometry using pulsed extraction, and neutral species were detected using vacuum UV photoionization at 10.5 eV. The results demonstrate that at least 99% of the ablated material is removed in the form of nano- or microdroplets consisting of intact ionic liquid. Approximately 1% is ejected as imidazole molecules (R(1)R(2)Im) produced through the elimination of HCl, and about 0.1% of the material is ejected in the form of single salt molecules of R(1)R(2)Iium X. A chemical thermometer was used to measure the internal temperature (475 +/- 25 K) of the ablated vapor plume.