Rotational spectrum of cyanoacetylene solvated with helium atoms

Unusual Dynamics and Vibrational Fingerprints of van der Waals Dimers Formed by Linear Molecules and Rare-Gas Atoms

Detailed structural, dynamical, and vibrational analyses have been performed for systems composed of linear triatomic molecules solvated by a single rare-gas atom, He, Ne, or Ar. Among the chromophores of these van der Waals (vdW) dimers, there are four neutral molecules (CO2, CS2, N2O, and OCS) and six molecular cations (HHe2+, HNe2+, HAr2+, HHeNe+, HHeAr+, and HNeAr+), both of apolar and polar nature. Following the exploration of bonding preferences, high-level four-dimensional (4D) potential energy surfaces (PESs) have been developed for 24 vdW dimers, keeping the two intramonomer bond lengths fixed. For these 24 complexes, over 1500 bound vibrational states have been obtained via quasi-variational nuclear-motion computations, employing exact kinetic-energy operators together with the accurate 4D PESs and their 2D/3D cuts. The reduced-dimensional (2D to 4D) dimer models have been compared with full-dimensional (6D) ones in the cases of the neutral CO2·Ar and charged HHe2+·He dimers, corroborating the high accuracy of the 2D to 4D vibrational energies. The reduced-dimensional models suggest that (a) while the equilibrium structures are T-shaped and planar, the effective ground-state structures are nonplanar, (b) certain bound states belong to collinear molecular structures, even when they are not minima, (c) the vdW vibrations are heavily mixed and many states have amplitudes corresponding to both the T-shaped and collinear structures, (d) there are a few dimers, for which even some of the vdW fundamentals lie above the first dissociation limit, and (e) the vdW vibrations are almost fully decoupled from the intramonomer bending motion.

Deciphering the Impact of Helium Tagging on Flexible Molecules: Probing Microsolvation Effects of Protonated Acetylene by Quantum Configurational Entropy

Helium, the lightest and most weakly interacting noble gas, is well-known for its unsurpassed chemical inertness. In many applications of helium in experimental techniques, such as tagging, messenger, or nanodroplet isolation action spectroscopy of molecules or complexes, it is assumed that the interaction of helium with the respective species, and thus the resulting interaction-induced perturbation, is small enough not to affect their structure and dynamics. Here, we probe the impact of one up to many attached helium atoms on protonated acetylene─an important nonclassical carbocation subject to three-center two-electron bonding in its ground state structure─using highly accurate interaction potentials in conjunction with entropy-based higher-order nonlinear correlation analysis. In particular, using neural network potentials at CCSD(T) accuracy, we disclose the specific structural perturbations due to the tagging of C₂H₃⁺ with up to 20 He atoms at a temperature of 1 K. Analysis reveals that microsolvation by helium influences the structure of C₂H₃⁺ noticeably, while our investigation of the quantum configurational information entropy additionally shows that correlations between individual orientational degrees of freedom are affected as a function of cluster size. In particular, it is found that the most probable bridge-like structure of the ro-vibrational quantum ground state of C₂H₃⁺, which is nonplanar and trans-bent in contrast to the perfectly planar equilibrium structure, becomes increasingly more localized upon adding helium atoms. The remarkably nonlinear behavior of the angular correlations as a function of cluster size is traced back to the buildup of the quantum microsolvation shell that enhances anisotropy up to N_He = 6 while more and more isotropic solvation takes over beyond six. Our approach is general and thus sets the stage to investigate the salient effects on the structure of flexible molecules due to tagging beyond the specific case.

Suppression of Parahydrogen Superfluidity in a Doped Nanoscale Bose Fluid Mixture

Helium (^{4}He) nanodroplets provide a unique environment to observe the microscopic origins of superfluidity. The search for another superfluid substance has been an ongoing quest in the field of quantum fluids. Nearly two decades ago, experiments on doped parahydrogen (p-H_{2}) clusters embedded in ^{4}He droplets displayed anomalous spectroscopic signatures that were interpreted as a sign of the superfluidity of p-H_{2} [S. Grebenev et al., Science 289, 1532 (2000)SCIEAS0036-807510.1126/science.289.5484.1532]. Here, we observe, using first-principles quantum Monte Carlo simulations, a phase separation between a symmetric and localized p-H_{2} core and ^{4}He shells. The p-H_{2} core has minimal superfluid response. These findings are consistent with the recorded spectra but not with their original interpretation, and lead us to conclude that doped p-H_{2} clusters form a nonsuperfluid core in ^{4}He droplets.

Quasiparticle Approach to Molecules Interacting with Quantum Solvents

Understanding the behavior of molecules interacting with superfluid helium represents a formidable challenge and, in general, requires approaches relying on large-scale numerical simulations. Here, we demonstrate that experimental data collected over the last 20 years provide evidence that molecules immersed in superfluid helium form recently predicted angulon quasiparticles [Phys. Rev. Lett. 114, 203001 (2015)PRLTAO0031-900710.1103/PhysRevLett.114.203001]. Most important, casting the many-body problem in terms of angulons amounts to a drastic simplification and yields effective molecular moments of inertia as straightforward analytic solutions of a simple microscopic Hamiltonian. The outcome of the angulon theory is in good agreement with experiment for a broad range of molecular impurities, from heavy to medium-mass to light species. These results pave the way to understanding molecular rotation in liquid and crystalline phases in terms of the angulon quasiparticle.

Persistence of dual free internal rotation in NH4(+)(H2O)·Hen=0-3 ion-molecule complexes: expanding the case for quantum delocalization in He tagging

To explore the extent of the molecular cation perturbation induced by complexation with He atoms required for the application of cryogenic ion vibrational predissociation (CIVP) spectroscopy, we compare the spectra of a bare NH4(+)(H2O) ion (obtained using infrared multiple photon dissociation (IRMPD)) with the one-photon CIVP spectra of the NH4(+)(H2O)·He1-3 clusters. Not only are the vibrational band origins minimally perturbed, but the rotational fine structures on the NH and OH asymmetric stretching vibrations, which arise from the free internal rotation of the -OH2 and -NH3 groups, also remain intact in the adducts. To establish the location and the quantum mechanical delocalization of the He atoms, we carried out diffusion Monte Carlo (DMC) calculations of the vibrational zero point wave function, which indicate that the barriers between the three equivalent minima for the He attachment are so small that the He atom wave function is delocalized over the entire -NH3 rotor, effectively restoring C3 symmetry for the embedded -NH3 group.

Higher order diffusion Monte Carlo propagators for linear rotors as diffusion on a sphere: development and application to O2@He(n)

Exploiting the theoretical treatment of particles diffusing on corrugated surfaces and the isomorphism between the "particle on a sphere" and a linear molecule rotation, a new diffusion kernel is introduced to increase the order of diffusion Monte Carlo (DMC) simulations involving linear rotors. Tests carried out on model systems indicate the superior performances of the new rotational diffusion kernel with respect to the simpler alternatives previously employed. In particular, it is evidenced a second order convergence toward exact results with respect to the time step of dynamical correlation functions, a fact that guarantees an identical order for the diffusion part of the DMC projector. The algorithmic advantages afforded by the latter are discussed, especially with respect to the "a posteriori" and "on the fly" extrapolation schemes. As a first application to the new algorithm, the structure and energetics of O(2)@He(n) (n = 1-40) clusters have been studied. This was done to investigate the possible cause of the quenching of the reaction between O(2) and Mg witnessed upon increasing the size of superfluid He droplets used as a solvent. With the simulations on O(2) indicating a strong localization in the cluster core, the behaviour as a function of n is ascribed to the extremely fluxional comportment of Mg@He(n), which dwells far from the droplet center, albeit being solvated, when n is large.

Theoretical and experimental study of weakly bound CO2-(pH2)2 trimers

The infrared spectrum of CO(2)-(pH(2))(2) trimers is predicted by performing exact basis-set calculations on a global potential energy surface defined as the sum of accurately known two-body pH(2)-CO(2) (J. Chem. Phys. 2010, 132, 214309) and pH(2)-pH(2) potentials (J. Chem. Phys. 2008, 129, 094304). These results are compared with new spectroscopic measurements for this species, for which 13 transitions are now assigned. A reduced-dimension treatment of the pH(2) rotation has been employed by applying the hindered-rotor averaging technique of Li, Roy, and Le Roy (J. Chem. Phys. 2010, 133, 104305). Three-body effects and the quality of the potential are discussed. A new technique for displaying the three-dimensional pH(2) density in the body-fixed frame is used, and shows that in the ground state the two pH(2) molecules are localized much more closely together than is the case for the two He atoms in the analogous CO(2)-(He)(2) species. A clear tunneling splitting is evident for the torsional motion of the two pH(2) molecules on a ring about the CO(2) molecular axis, in contrast to the case of CO(2)-(He)(2) where a more regular progression of vibrational levels reflects the much lower torsional barrier.

Theoretical study of the microwave spectrum of isotopologues of OCS–(He)2

The rovibrational energy levels (J = 0–3) and rotational spectra of seven isotopologues of the OCS–(He)2 complex have been determined by numerically exact basis set calculations. The interaction energy is represented as a sum of two-body terms consisting of the OCS–He potential, which Howson and Hutson (J. Chem. Phys. 2001, 115, 5059) obtained at the CCSD(T)/aug-cc-pVTZ level of theory, and the He–He potential that Jeziorska et al. (J. Chem. Phys. 2007, 127,124303) obtained with SAPT theory. Three-body effects and the quality of the potential are discussed. Comparison with experiment shows that microwave transitions can be predicted by this additive approach with an accuracy equal or better than 0.7% for all the observed spectral lines. A method for the three-dimensional representation of the helium density in the body-fixed frame is presented that highlights the highly delocalized nature of the helium subsystem.

Molecular superfluid: nonclassical rotations in doped para-hydrogen clusters

Clusters of para-hydrogen (pH₂) have been predicted to exhibit superfluid behavior, but direct observation of this phenomenon has been elusive. Combining experiments and theoretical simulations, we have determined the size evolution of the superfluid response of pH₂ clusters doped with carbon dioxide (CO₂). Reduction of the effective inertia is observed when the dopant is surrounded by the pH₂ solvent. This marks the onset of molecular superfluidity in pH₂. The fractional occupation of solvation rings around CO₂ correlates with enhanced superfluid response for certain cluster sizes.

Theoretical study of the rovibrational spectrum of He2–OCS

We report calculated microwave and infrared rovibrational transitions of the van der Waals complex He2–OCS. The calculations were done using a product basis, a Lanczos eigensolver, and potentials built from He–OCS, and He–He potential functions taken from the literature. All five of the large amplitude coordinates are treated exactly and calculations are done for J values up to five. All rovibrational levels are converged to 0.001 cm–1 by using basis sets with as many as 87 million funcions. Good agreement is found with previously reported experimental results. Although we assume that the dipole moment is along the OCS axis, we find transitions with appreciable intensity between different torsion states.

Rotational study of carbon monoxide solvated with helium atoms

High resolution microwave and millimeter-wave spectra of HeN-CO clusters with N up to 10, produced in a molecular expansion, were observed. Two series of J = 1-0 transitions were detected, which correspond to the a-type and b-type J = 1-0 transitions of He1-CO. The B rotational constant initially decreases with N and reaches a minimum at N = 3. Its subsequent rise indicates the transition from a molecular complex to a quantum solvated system already for N = 4. For N > or =6, the B value becomes larger than that of He1-CO, indicating an almost free rotation of CO within the helium environment.

Exact, Born-Oppenheimer, and quantum-chemistry-like calculations in helium clusters doped with light molecules: The He2N2(X) system

Helium clusters doped with diatomic molecules, He(N)-BC, have been recently studied by means of a quantum-chemistry-like approach. The model treats He atoms as "electrons" and dopants as "nuclei" in standard electronic structure calculations. Due to the large mass difference between He atoms and electrons, and to the replacement of Coulomb interactions by intermolecular potentials, it is worth assessing up to what extent are the approximations involved in this model, i.e., decoupling of the BC rotation from the He-atom orbital angular momenta and Born-Oppenheimer separation of the BC stretch versus the He motions, accurate enough. These issues have been previously tackled elsewhere for the (4)He(2)-Br(2)(X) system, which contains a heavy dopant [Roncero et al., Int. J. Quantum Chem. 107, 2756 (2007)]. Here, we consider a similar cluster but with a much lighter dopant such as N(2)(X). Although the model does not provide the correct energy levels for the cluster, positions and intensities of the main detectable lines of the vibrotational Raman spectrum at low temperature are accurately reproduced.

Microsolvation of Cationic Dimers in 4He Droplets: Geometries of (He)N (A = Li, Na, K) from Optimized Energies

Ab initio computed interaction forces are employed to describe the microsolvation of the A+2(2Sigma) (A=Li, Na, K) molecular ion in 4He clusters of small variable size. The minimum energy structures are obtained by performing energy minimization based on a genetic algorithm approach. The symmetry features of the collocation of solvent adatoms around the dimeric cation are analyzed in detail, showing that the selective growth of small clusters around the two sides of the ion during the solvation process is a feature common to all three dopants.

Infrared spectra of helium clusters seeded with nitrous oxide, HeN4–N2O, with N=1–80

High resolution infrared spectra of HeN-N2O clusters are studied in the 2200 cm(-1) region of the N2O nu1 fundamental band. The clusters are produced in a pulsed supersonic jet expansion from a cooled nozzle source and probed using a tunable diode laser operating in a rapid-scan mode. Three isotopic forms are used (14N14N16O, 15N14N16O, and 15N15N16O) in order to support the spectral analyses. For clusters up to N approximately 24, the individual spectra are resolved, assigned, and analyzed together with complementary microwave data. Assignments for larger clusters are uncertain due to overlapping transitions, but an approximate analysis is still possible for N approximately 25-80. Compared to helium clusters containing the related CO2 or OCS molecules, the rotational dynamics of HeN-N2O clusters show similarities but also important differences. In particular, HeN-N2O has more irregular behavior in the range of N=6-17, indicating that conventional molecular structure plays a greater role. In general terms, these differences can be attributed to a greater degree of angular anisotropy in the He-N2O intermolecular potential.

High-resolution microwave spectrum of the weakly bound helium-pyridine complex

High-resolution rotational spectra of the helium-pyridine dimer were obtained using a pulsed molecular beam Fourier transform microwave spectrometer. Thirty-nine R-branch (14)N nuclear quadrupole hyperfine components of a- and c-type dipole transitions were observed and assigned. The following spectroscopic parameters were obtained: rotational constants A=3875.2093(48) MHz, B=3753.2514(45) MHz, and C=2978.4366(81) MHz; quartic centrifugal distortion constants D(J)=0.124 08(55) MHz, D(JK)=0.1200(43) MHz, D(K)=-0.2451(25) MHz, d(1)=0.004 27(27) MHz, and d(2)=0.000 16(10) MHz; sextic centrifugal distortion constants H(J)=0.003 053(35) MHz, H(JK)=-0.006 598(47) MHz, and H(K)=0.004 11(59) MHz; (14)N nuclear quadrupole coupling constants chi(aa)((14)N)=-4.7886(76) MHz, chi(bb)((14)N)=1.4471(76) MHz, and chi(cc)((14)N)=3.3415(43) MHz. Our analyses of the rotational and (14)N quadrupole coupling constants show that the He atom binds perpendicularly to the aromatic plane of C(5)H(5)N with a displacement angle of approximately 7.0 degrees away from the c axis of the pyridine monomer, toward the nitrogen atom. Results from an ab initio structure optimization on the second order Moller-Plesset level are consistent with this geometry and gave an equilibrium well depth of 86.7 cm(-1).

Solvation Structure and Rotational Dynamics of LiH in 4He Clusters

We present results of path integral Monte Carlo simulations of LiH solvated in superfluid 4He clusters of size up to N = 100. Despite the light mass of LiH and the strongly anisotropic LiH-He potential with a large repulsion at the hydrogen end, LiH is solvated inside the cluster for sufficiently large N. Using path integral correlation function analysis, we have determined the dipole (J = 1) rotational excitations of the cluster and a corresponding effective rotational constant Beff of the solvated LiH. We predict that Beff is greatly reduced with respect to the gas-phase rotational constant B, to a value of only about 6% of B. This exceptionally large reduction of the rotational constant is due to the highly anisotropic 4He solvation structure around LiH. It does not follow the previously established trend of a relatively small B reduction for light molecules, showing the strongest reduction of all molecules in 4He to date. Comparison of the calculated rotational spectra of LiH in helium obeying Bose and Boltzmann statistics, respectively, demonstrates that the Bose statistics of helium is an essential requirement for obtaining well-defined molecule rotational spectra in helium-4.

Spectroscopic Studies of OCS-Doped 4He Clusters with 9−72 Helium Atoms: Observation of Broad Oscillations in the Rotational Moment of Inertia

High-resolution spectra of HeN-OCS clusters with N up to 39 in the microwave region and up to 72 in the infrared region were observed with apparatus-limited line widths of about 15 kHz and 0.001 cm(-1), respectively. The cold (approximately 0.2 K) clusters were produced in pulsed supersonic jet expansions of very dilute OCS + He mixtures and probed using a microwave Fourier transform spectrometer or a tunable infrared diode laser spectrometer. Consistent analyses of the microwave and infrared data yield band origins for the carbonyl stretching vibration, together with rotational parameters for the ground and excited vibrational states. The rotational constant, B, passes through a minimum at N = 9 and then rises as the He atoms uncouple from the OCS rotational motion as a result of superfluid effects. There are broad unexpected oscillations in B, with maxima at N = 24 and 47 and minima at N = 36 and 62. The change in B upon vibrational excitation, which is negative for the OCS molecule, converges to positive values for N > 15. These results help to bridge the gap between individual molecules and bulk matter with atom-by-atom resolution over a significant range of cluster sizes.

Rotational fluctuation of molecules in quantum clusters. II. Molecular rotation and superfluidity in OCS-doped helium-4 clusters

In this paper, quantum fluctuations of a carbonyl sulfide molecule in helium-4 clusters are studied as a function of cluster size N in a small-to-large size regime (2<or=N<or=64). The molecular rotation of the dopant shows nonmonotonic size dependence in the range of 10<or=N<or=20, reflecting the density distribution of the helium atoms around the molecule. The size dependence on the rotational constant shows a plateau for N>or=20, which is larger than the experimental nanodroplet value. Superfluid response of the doped cluster is found to show remarkable anisotropy especially for N<or=20. The superfluid fraction regarding the axis perpendicular to the molecular axis shows a steep increase at N=10, giving the significant enhancement of the rotational fluctuation of the molecule. On the other hand, the superfluid fraction regarding the axis parallel to the molecular axis reaches 0.9 at N=5, arising from the bosonic exchange cycles of the helium atoms around the molecular axis. The anisotropy in the superfluid response is found to be the direct consequence of the configurations of the bosonic exchange cycles.

Rotational fluctuation of molecules in quantum clusters. I. Path integral hybrid Monte Carlo algorithm

In this paper, we present a path integral hybrid Monte Carlo (PIHMC) method for rotating molecules in quantum fluids. This is an extension of our PIHMC for correlated Bose fluids [S. Miura and J. Tanaka, J. Chem. Phys. 120, 2160 (2004)] to handle the molecular rotation quantum mechanically. A novel technique referred to be an effective potential of quantum rotation is introduced to incorporate the rotational degree of freedom in the path integral molecular dynamics or hybrid Monte Carlo algorithm. For a permutation move to satisfy Bose statistics, we devise a multilevel Metropolis method combined with a configurational-bias technique for efficiently sampling the permutation and the associated atomic coordinates. Then, we have applied the PIHMC to a helium-4 cluster doped with a carbonyl sulfide molecule. The effects of the quantum rotation on the solvation structure and energetics were examined. Translational and rotational fluctuations of the dopant in the superfluid cluster were also analyzed.