The hydrogen fluoride dimer in liquid helium: A prototype system for studying solvent effects on hydrogen bonding

Onset of Rotational Decoupling for a Molecular Ion Solvated in Helium: From Tags to Rings and Shells

Little is known about how rotating molecular ions interact with multiple ^{4}He atoms and how this relates to microscopic superfluidity. Here, we use infrared spectroscopy to investigate ^{4}He_{N}⋯H_{3}O^{+} complexes and find that H_{3}O^{+} undergoes dramatic changes in rotational behavior as ^{4}He atoms are added. We present evidence of clear rotational decoupling of the ion core from the surrounding helium for N>3, with sudden changes in rotational constants at N=6 and 12. In sharp contrast to studies on small neutral molecules microsolvated in helium, accompanying path integral simulations show that an incipient superfluid effect is not needed to account for these findings.

A comprehensive theoretical study of positron binding and annihilation properties of hydrogen bonded binary molecular clusters

Small hydrogen inorganic molecules such as water have no positron binding ability. We revealed that their hydrogen bonded binary molecular clusters exhibit greater positron affinities due to the increased dipole moments and polarization effect.

High resolution infrared spectroscopy of (HCl)2 and (DCl)2 isolated in solid parahydrogen: Interchange-tunneling in a quantum solid

Infrared spectroscopic studies of weakly bound clusters isolated in solid parahydrogen (pH₂) that exhibit large-amplitude tunneling motions are needed to probe how quantum solvation perturbs these types of coherent dynamics. We report high resolution Fourier transform infrared absorption spectra of (HCl)₂, HCl-DCl, and (DCl)₂ isolated in solid pH₂ in the 2.4-4.8 K temperature range. The (HCl)₂ spectra show a remarkable amount of fine structures that can be rigorously assigned to vibration-rotation-tunneling transitions of (HCl)₂ trapped in double substitution sites in the pH₂ matrix where end-over-end rotation of the cluster is quenched. The spectra are assigned using a combination of isotopically (H/D and ³⁵Cl/³⁷Cl) enriched samples, polarized IR absorption measurements, and four-line combination differences. The interchange-tunneling (IT) splitting in the ground vibrational state for in-plane and out-of-plane H³⁵Cl-H³⁷Cl dimers is 6.026(1) and 6.950(1) cm^-1, respectively, which are factors of 2.565 and 2.224 smaller than in the gas phase dimer. In contrast, the (DCl)₂ results show larger perturbations where the ground vibrational state IT splitting in D³⁵Cl-D³⁷Cl is 1.141(1) cm^-1, which is a factor of 5.223 smaller than in the gas phase, and the tunneling motion is quenched in excited intramolecular vibrational states. The results are compared to similar measurements on (HCl)₂ made in liquid helium nanodroplets to illustrate the similarities and differences in how both these quantum solvents interact with large amplitude tunneling motions of an embedded chromophore.

Graph theoretical enumeration of topology-distinct structures for hydrogen fluoride clusters (HF)n (n ≤ 6)

A graph theoretical procedure to generate all the possible topology-distinct structures for hydrogen fluoride (HF) clusters is presented in this work. The hydrogen bond matrix is defined and used to enumerate the topology-distinct structures of hydrogen fluoride (HF)n (n = 2-8) clusters. From close investigation of the structural patterns obtained, several restrictions that should be satisfied for a structure of the HF clusters to be stable are found. The corresponding digraphs of generated hydrogen bond matrices are used as the theoretical framework to obtain all the topology-distinct local minima for (HF)n (n ≤ 6), at the level of MP2/6-31G**(d, p) of ab initio MO method and B3LYP/6-31G**(d, p) of density functional theory method. For HF clusters up to tetramers, the local minimum structures that we generated are same as those in the literature. For HF pentamers and hexamers, we found some new local minima structures which had not been obtained previously.

Rotation of methane molecules in dimers and small clusters

This work reports on the study of the internal rotation of methane molecules in small clusters containing up to about five molecules. The clusters were assembled in helium droplets at T = 0.38 K by successive capture of single methane molecules and studied by infrared laser spectroscopy of the fundamental CH4 ν3 vibration around 3030 cm(-1). The spectra demonstrate well resolved structure due to internal rotation of the constituent molecules in the clusters. The most resolved spectrum for the dimers shows characteristic splitting of the lines due to anisotropic intermolecular interaction. The magnitude of the splitting is found to be in a good quantitative agreement with the recent theoretical anisotropic intermolecular potentials.

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).

Blueshift and intramolecular tunneling of NH3 umbrella mode in 4He n clusters

We present diffusion Monte Carlo calculations of the ground and first excited vibrational states of NH(3) (4)He(n) for n< or =40. We use the potential energy surface developed by one of us [M. P. Hodges and R. J. Wheatley, J. Chem. Phys. 114, 8836 (2001)], which includes the umbrella mode coordinate of NH(3). Using quantum Monte Carlo calculations of excited states, we show that this potential is able to reproduce qualitatively the experimentally observed effects of the helium environment, namely, a blueshift of the umbrella mode frequency and a reduction of the tunneling splittings in ground and first excited vibrational states of the molecule. These basic features are found to result regardless of whether dynamical approximations or exact calculations are employed.

Probing collective excitations in helium nanodroplets: Observation of phonon wings in the infrared spectrum of methane

The authors have recorded the nu(3) infrared spectrum of methane in helium nanodroplets using our cw infrared optical parametric oscillator. In a previous paper, Nauta and Miller [Chem. Phys. Lett. 350, 225 (2001)] reported the observation of the monomer rovibrational transitions of methane in helium nanodroplets. Here, they report the observation of additional absorption bands in the frequency range between 2990 and 3070 cm(-1) blueshifted compared to the monomer transitions. They attribute these absorption features to phonon wings of individual rovibrational transitions, i.e., the simultaneous excitation of collective excitation modes of the quantum fluid and the rovibrational excitation of the methane monomer in the helium nanodroplet.

Formation and properties of metal clusters isolated in helium droplets

The unique conditions forming atomic and molecular complexes and clusters using superfluid helium nanodroplets have opened up an innovative route for studying the physical and chemical properties of matter on the nanoscale. This review summarizes the specific characteristics of the formation of atomic clusters partly generated far from equilibrium in the helium environment. Special emphasis is on the optical response, electronic properties as well as dynamical processes which are mostly affected by the surrounding quantum matrix. Experiments include the optical induced response of isolated cluster systems in helium under quite different excitation conditions ranging from the linear regime up to the violent interaction with a strong laser field leading to Coulomb explosion and the generation of highly charged atomic fragments. The variety of results on the outstanding properties in the quantum size regime highlights the peculiar capabilities of helium nanodroplet isolation spectroscopy.

Multiple photon excitation and ionization of NO in and on helium droplets

The photoexcitation of NO embedded in superfluid Hen nanodroplets having n approximately 10(4) has been examined. Two-photon excitation prepares electronically excited states (NO(*)), most notably, the embedded analog of the A 2Sigma state of gas phase NO. Vertical excitation to this low Rydberg state is blueshifted and broadened relative to its gas phase counterpart because of the repulsive electron-helium interaction. Transport to the droplet surface is believed to be facile in the superfluid. For example, NO* prefers (energetically) to reside at the droplet surface rather than at the droplet center, in contrast to NO. Photoionization of surface-bound NO* occurs over a significant photon energy range. This yields small cluster ions NO+Hek) with approximately 90% of these clusters having k< or =10. The variation of ion yield with photon energy displays a precipitous change in the region of 24 300-24 400 cm(-1) for all values of k. Possible photoionization mechanisms are discussed and it is suggested that intermediate levels with high-n Rydberg character play a role. This work underscores the important role played by transport in the photophysics of species embedded in the superfluid host.

Stereographic projection path-integral simulations of (HF)n clusters

We perform several quantum canonical ensemble simulations of (HF)(n) clusters. The HF stretches are rigid, and the stereographic projection path-integral method is employed for the simulation in the resulting curved configuration space. We make use of the reweighted random series techniques to accelerate the convergence of the path-integral simulation with respect to the number of path coefficients. We develop and test estimators for the total energy and heat capacity based on a finite difference approach for non-Euclidean spaces. The quantum effects at temperatures below 400 K are substantial for all sizes. We observe interesting thermodynamic behaviors in the quantum simulations of the octamer and the heptamer.

Electron Impact Ionization of Haloalkanes in Helium Nanodroplets

Electron impact (EI) mass spectra of a selection of C1-C3 haloalkanes in helium nanodroplets have been recorded to determine if the helium solvent can significantly reduce molecular ion fragmentation. Haloalkanes were chosen for investigation because their EI mass spectra in the gas phase show extensive ion fragmentation. There is no evidence of any major softening effect in large helium droplets ( approximately 60 000 helium atoms), but some branching ratios are altered. In particular, channels requiring C-C bond fission or concerted processes leading to the ejection of hydrogen halide molecules are suppressed by helium solvation. Rapid cooling by the helium is not sufficient to account for all the differences between the helium droplet and gas phase mass spectra. It is also suggested that the formation of a solid "snowball" of helium around the molecular ion introduces a cage effect, which enhances those fragmentation channels that require minimal disruption to the helium cage for products to escape.

(HCl)2 and (HF)2 in small helium clusters: Quantum solvation of hydrogen-bonded dimers

We present a rigorous theoretical study of the solvation of (HCl)(2) and (HF)(2) by small ((4)He)(n) clusters, with n=1-14 and 30. Pairwise-additive potential-energy surfaces of He(n)(HX)(2) (X=Cl and F) clusters are constructed from highly accurate four-dimensional (rigid monomer) HX-HX and two-dimensional (rigid monomer) He-HX potentials and a one-dimensional He-He potential. The minimum-energy geometries of these clusters, for n=1-6 in the case of (HCl)(2) and n=1-5 for (HF)(2), correspond to the He atoms in a ring perpendicular to and bisecting the HX-HX axis. The quantum-mechanical ground-state energies and vibrationally averaged structures of He(n)(HCl)(2) (n=1-14 and 30) and He(n)(HF)(2) (n=1-10) clusters are calculated exactly using the diffusion Monte Carlo (DMC) method. In addition, the interchange-tunneling splittings of He(n)(HCl)(2) clusters with n=1-14 are determined using the fixed-node DMC approach, which was employed by us previously to calculate the tunneling splittings for He(n)(HF)(2) clusters, n=1-10 [A. Sarsa et al., Phys. Rev. Lett. 88, 123401 (2002)]. The vibrationally averaged structures of He(n)(HX)(2) clusters with n=1-6 for (HCl)(2) and n=1-5 for (HF)(2) have the helium density localized in an effectively one-dimensional ring, or doughnut, perpendicular to and at the midpoint of the HX-HX axis. The rigidity of the solvent ring varies with n and reaches its maximum for the cluster size at which the ring is filled, n=6 and n=5 for (HCl)(2) and (HF)(2), respectively. Once the equatorial ring is full, the helium density spreads along the HX-HX axis, eventually solvating the entire HX dimer. The interchange-tunneling splitting of He(n)(HCl)(2) clusters hardly varies at all over the cluster size range considered, n=1-14, and is virtually identical to that of the free HCl dimer. This absence of the solvent effect is in sharp contrast with our earlier results for He(n)(HF)(2) clusters, which show a approximately 30% reduction of the tunneling splitting for n=4. A tentative explanation for this difference is proposed. The implications of our results for the interchange-tunneling dynamics of (HCl)(2) in helium nanodroplets are discussed.

A new lattice-based theory for hydrogen-bonding liquids in uniform electric fields

We propose a new lattice-based, mean-field theory for predicting alignment of molecular dipoles and hydrogen bonds in liquids subject to uniform electric fields. The theory is presently restricted to liquids whose molecules possess one (proton) donor and one acceptor sites each, and wherein the H-bond axis is collinear with the dipole moments of the bonded molecules. The final expressions for hydrogen bond stoichiometry and polarization are free of lattice parameters, are interpretable using simple phenomenological arguments, and reduce to known limiting forms. The theory is applied to understand the internal structure of hydrogen cyanide in the liquid state at different electric fields.

Rotational and vibrational dynamics of ethylene in helium nanodroplets

Rotationally resolved infrared spectra are reported for the asymmetric C-H stretching fundamental bands of C(2)H(4) in helium nanodroplets, as well as two weak combination bands. The J=2 rotor levels are strongly shifted from the energies estimated from a rigid rotor calculation and can be accounted for with two centrifugal distortion constants. The excited states of the three bands with B(3u) symmetry are strongly coupled in the gas phase and exhibit lifetimes >100 ps in helium, with the upper member of the polyad exhibiting the shortest lifetime. In contrast, the nu(9) band (B(2u) symmetry) exhibits very broad, homogeneously broadened line profiles (full width at half maximum approximately 0.5 cm(-1)) corresponding to an excited state lifetime of approximately 10 ps. This short lifetime is presumed to be due to an efficient, solvent mediated vibration-to-vibration relaxation process. In addition, the absence of transitions to the 2(21) and 2(20) rotor levels in the nu(9) band suggests they form rotational resonances with the elementary modes of helium, resulting in very short excited state lifetimes of less than 2 ps.

The isomers of HF–HCN formed in helium nanodroplets: Infrared spectroscopy and ab initio calculations

Binary complexes containing hydrogen cyanide and hydrogen fluoride are formed in helium nanodroplets, and studied using high-resolution infrared laser spectroscopy. Rotationally resolved spectra are reported for the H-F and C-H stretches of the linear HCN-HF complex, a system that has been thoroughly studied in the gas phase. We report the high-resolution spectra of the higher energy, bent HF-HCN isomer, which is also formed in helium. Stark spectra are reported for both isomers, providing dipole moments of these complexes. The experimental results are compared with ab initio calculations, also reported here. Spectra are reported for several ternary complexes, including (HCN)2-HF, HCN-(HF)2, HF-(HCN)2, and HF-HCN-HF.

Raman spectra of (He)N-Br2(X) clusters: The role of boson/fermion statistics in a quantum solvent

The aim of this paper is to elucidate the role played by the bosonic/fermionic character of N He atoms solvating a Br2(X) molecule. To this end, an adiabatic model in the molecular stretching coordinate is assumed and the ground energy levels of the complexes are searched by means of Hartree (or Hartree-Fock) Quantum Chemistry calculations for 4He (or 3He) solvent atoms. Simulations of vib-rotational Raman spectra point at the spin multiplicity as the main feature responsible for the drastic difference in the rotational structures of molecules embedded in boson or fermion helium drops as already observed by the experiments of Grebenev et al. [S. Grebenev, J. P. Toennies, and A. F. Vilesov, Science 279 (1998) 2083].

Role of boson-fermion statistics on the Raman spectra of Br2(X) in helium clusters

The role played by the bosonic or fermionic character of He atoms surrounding a Br2(X) molecule is analyzed through vibrotational Raman spectra simulations. Quantum chemistry-type calculations reveal the spin multiplicity to be chiefly responsible for the drastic difference observed by Grebenev et al. [Science 279, 2083 (1998)]] in the rotational structure of molecules embedded in helium droplets.

Polar isomer of formic acid dimers formed in helium nanodroplets

The infrared spectrum of formic acid dimers in helium nanodroplets has been observed corresponding to excitation of the "free" OH and CH stretches. The experimental results are consistent with a polar acyclic structure for the dimer. The formation of this structure in helium, as opposed to the much more stable cyclic isomer with two O-H...O hydrogen bonds, is attributed to the unique growth conditions that exist in helium droplets, at a temperature of 0.37 K. Theoretical calculations are also reported to aid in the interpretation of the experimental results. At long range the intermolecular interaction between the two monomers is dominated by the dipole-dipole interaction, which favors the formation of a polar dimer. By following the minimum-energy path, the calculations predict the formation of an acyclic dimer having one O-H...O and one C-H...O contact. This structure corresponds to a local minimum on the potential energy surface and differs significantly from the structure observed in the gas phase.

Bias in the temperature of helium nanodroplets measured by an embedded rotor

The rovibrational spectra of molecules dissolved in liquid 4He nanodroplets display rotational structure. Where resolved, this structure has been used to determine a temperature that has been assumed to equal that of the intrinsic excitations of the helium droplets containing the molecules. Consideration of the density of states as a function of energy and total angular momentum demonstrates that there is a small but significant bias of the rotor populations that make the temperature extracted from a fit to its rotational level populations slightly higher than the temperature of the ripplons of the droplet. This bias grows with both the total angular momentum of the droplet and with the moment of inertia of the solute molecule.