Infrared spectra of seeded hydrogen clusters: (para-H2)N-N2O and (ortho-H2)N-N2O, N = 2-13

High-resolution infrared spectra of clusters containing para-H2 and/or ortho-H2 and a single nitrous oxide molecule are studied in the 2225-cm(-1) region of the upsilon1 fundamental band of N2O. The clusters are formed in pulsed supersonic jet expansions from a cooled nozzle and probed using a tunable infrared diode laser spectrometer. The simple symmetric rotor-type spectra generally show no resolved K structure, with prominent Q-branch features for ortho-H2 but not para-H2 clusters. The observed vibrational shifts and rotational constants are reported. There is no obvious indication of superfluid effects for para-H2 clusters up to N=13. Sharp transitions due to even larger clusters are observed, but no definite assignments are possible. Mixed (para-H2)N-(ortho-H2)M-N2O cluster line positions can be well predicted by linear interpolation between the corresponding transitions of the pure clusters.     

Infrared photochromism in a quantum crystal: laser pumping and probing in solid para-H2 doped with CH3F

Solid para-H(2) doped with CH(3)F at 1.8 K is studied in the ν(3) region (～1040 cm(-1)) using a quantum cascade laser source. Residual ortho-H(2) gives rise to distinct spectral features due to CH(3)F-(ortho-H(2))(N) clusters with N = 0, 1, 2, etc. The laser power (～30 mW) is sufficient to significantly affect the sample, enabling novel photochromism experiments to be performed on a solid para-H(2) matrix-isolated sample. It is found that population can be reversibly transferred between the N = 1 line and satellite features close to the N = 0 line.     

Laser induced fluorescence of Mg-phthalocyanine in He droplets: evidence for fluxionality of large H2 clusters at 0.38 K

The formation of Ar and H2 clusters, having up to 900 particles in helium droplets, has been studied via laser induced fluorescence of attached Mg-phthalocyanine (Mg-Pc) molecules. In the experiments, one Mg-Pc molecule in average was added to each He droplet either before or after the cluster species, and the shift of the spectrum of the Mg-Pc molecules was studied as a function of the cluster size. For Ar clusters, about a factor of 2 smaller matrix shift was observed for the late pickup of the Mg-Pc molecules as compared with the prior pickup, indicating that in the former case, the Mg-Pc molecules reside on the surface of the preformed Ar clusters. On the other hand, the spectra of the Mg-Pc molecules attached to H2 clusters are independent of the pickup order, which is consistent with Mg-Pc molecules residing near the center of the H2 clusters in both cases. Therefore H2 clusters remain fluxional in helium droplets at T=0.38 K. No significant differences in the spectra were observed between the para-H2 and ortho-H2 clusters.     

Rotational spectra of the van der Waals complexes of molecular hydrogen and OCS

The a- and b-type rotational transitions of the weakly bound complexes formed by molecular hydrogen and OCS, para-H2-OCS, ortho-H2-OCS, HD-OCS, para-D2-OCS, and ortho-D2-OCS, have been measured by Fourier transform microwave spectroscopy. All five species have ground rotational states with total rotational angular momentum J=0, regardless of whether the hydrogen rotational angular momentum is j=0 as in para-H2, ortho-D2, and HD or j=1 as in ortho-H2 and para-D2. This indicates quenching of the hydrogen angular momentum for the ortho-H2 and para-D2 species by the anisotropy of the intermolecular potential. The ground states of these complexes are slightly asymmetric prolate tops, with the hydrogen center of mass located on the side of the OCS, giving a planar T-shaped molecular geometry. The hydrogen spatial distribution is spherical in the three j=0 species, while it is bilobal and oriented nearly parallel to the OCS in the ground state of the two j=1 species. The j=1 species show strong Coriolis coupling with unobserved low-lying excited states. The abundance of para-H2-OCS relative to ortho-H2-OCS increases exponentially with decreasing normal H2 component in H2He gas mixtures, making the observation of para-H2-OCS in the presence of the more strongly bound ortho-H2-OCS dependent on using lower concentrations of H2. The determined rotational constants are A=22 401.889(4) MHz, B=5993.774(2) MHz, and C=4602.038(2) MHz for para-H2-OCS; A=22 942.218(6) MHz, B=5675.156(7) MHz, and C=4542.960(7) MHz for ortho-H2-OCS; A=15 970.010(3) MHz, B=5847.595(1) MHz, and C=4177.699(1) MHz for HD-OCS; A=12 829.2875(9) MHz, B=5671.3573(7) MHz, and C=3846.7041(6) MHz for ortho-D2-OCS; and A=13 046.800(3) MHz, B=5454.612(2) MHz, and C=3834.590(2) MHz for para-D2-OCS.     

High resolution quantum cascade laser studies of the ν3 band of methyl fluoride in solid para-hydrogen

Spectra of solid para-H(2) doped with CH(3)F at 1.8 K are studied in the ν(3) region (~1040 cm(-1)) using a quantum cascade laser source. As shown previously, residual ortho-H(2) in the sample (~1000 ppm) gives rise to distinct spectral features due to clusters of the form CH(3)F-(ortho-H(2))(N), with N = 0, 1, 2, 3, etc. Brief annealing at 7 K is found to give narrower spectral lines (≥0.006 cm(-1)) than conventional (5 K) annealing, and causes the N = 3 and 4 lines to fragment into two or more components. The N = 3 line is observed to be particularly stable and persistent. The N = 0 line (no ortho-H(2) neighbors) is resolved into two closely spaced (≈0.007 cm(-1)) components which are assigned to the K = 0 and 1 states of CH(3)F rotating around its C(3v) symmetry axis (ortho- and para-CH(3)F, respectively). Similar K-structure is also evident for other lines. Weak but persistent features ("N = 1/2 lines") are observed mid way between N = 0 and 1.     

Solidification of small p-H2 clusters at zero temperature

We have determined the ground-state energies of para-H(2) clusters at zero temperature using the diffusion Monte Carlo method. The liquid or solid character of each cluster is investigated by restricting the phase through the use of proper importance sampling. Our results show inhomogeneous crystallization of clusters, with alternating behavior between liquid and solid phases up to N = 55. From there on, all clusters are solid. The ground-state energies in the range N = 13-75 are established, and the stable phase of each cluster is determined. In spite of the small differences observed between the energy of liquid and solid clusters, the corresponding density profiles are significantly different, a feature that can help to solve ambiguities in the determination of the specific phase of H(2) clusters.     

Infrared spectra of N2O-(ortho-D2)N and N2O-(HD)N clusters trapped in bulk solid parahydrogen

High-resolution infrared spectra of the clusters N2O-(ortho-D2)N and N2O-(HD)N, N=1-4, isolated in bulk solid parahydrogen at liquid helium temperatures are studied in the 2225 cm-1 region of the nu3 antisymmetric stretch of N2O. The clusters form during vapor deposition of separate gas streams of a precooled hydrogen mixture (ortho-D2para-H2 or HDpara-H2) and N2O onto a BaF2 optical substrate held at approximately 2.5 K in a sample-in-vacuum liquid helium cryostat. The cluster spectra reveal the N2O nu3 vibrational frequency shifts to higher energy as a function of N, and the shifts are larger for ortho-D2 compared to HD. These vibrational shifts result from the reduced translational zero-point energy for N2O solvated by the heavier hydrogen isotopomers. These spectra allow the N=0 peak at 2221.634 cm-1, corresponding to the nu3 vibrational frequency of N2O isolated in pure solid parahydrogen, to be assigned. The intensity of the N=0 absorption feature displays a strong temperature dependence, suggesting that significant structural changes occur in the parahydrogen solvation environment of N2O in the 1.8-4.9 K temperature range studied.     

Quantum dynamics of rovibrational transitions in H2-H2 collisions: internal energy and rotational angular momentum conservation effects

We present a full dimensional quantum mechanical treatment of collisions between two H(2) molecules over a wide range of energies. Elastic and state-to-state inelastic cross sections for ortho-H(2)?+ para-H(2) and ortho-H(2)?+ ortho-H(2) collisions have been computed for different initial rovibrational levels of the molecules. For rovibrationally excited molecules, it has been found that state-to-state transitions are highly specific. Inelastic collisions that conserve the total rotational angular momentum of the diatoms and that involve small changes in the internal energy are found to be highly efficient. The effectiveness of these quasiresonant processes increases with decreasing collision energy and they become highly state-selective at ultracold temperatures. They are found to be more dominant for rotational energy exchange than for vibrational transitions. For non-reactive collisions between ortho- and para-H(2) molecules for which rotational energy exchange is forbidden, the quasiresonant mechanism involves a purely vibrational energy transfer albeit with less efficiency. When inelastic collisions are dominated by a quasiresonant transition calculations using a reduced basis set involving only the quasiresonant channels yield nearly identical results as the full basis set calculation leading to dramatic savings in computational cost.     

Hole-burning spectra of phenol-Arn (n = 1, 2) clusters: resolution of the isomer issue

The hole-burning (HB) spectra of phenol-Arn (PhOH-Arn) clusters with n = 1 and 2 have been measured in a molecular beam to clarify the possible existence of isomers. Two species were identified to give rise to signals in the S1-S0 spectrum recorded for the n = 1 cluster; however, one of the species was found to originate from dissociation of an n = 2 cluster. Similarly, three species were observed in the spectrum of the n = 2 cluster, and two of them were assigned to n = 3 and larger clusters. The spectral contamination from larger size clusters was quantitatively explained by the dissociation after photoexcitation. The analysis of the spectra demonstrates that only a single isomer exists in the molecular beam for both the n = 1 and the n = 2 clusters. In addition to two previously detected intermolecular modes, a third low-frequency mode, assigned to an intermolecular bending vibration, is observed for the first time in the HB spectrum of the n = 2 cluster. The assignments of the intermolecular vibrations were confirmed by ab initio MO calculations. The observation of the third intermolecular vibration suggests that the geometry of the n = 2 cluster has Cs or lower symmetry.     

Electron impact ionization of size selected hydrogen clusters (H2)N: ion fragment and neutral size distributions

Clusters consisting of normal H2 molecules, produced in a free jet expansion, are size selected by diffraction from a transmission nanograting prior to electron impact ionization. For each neutral cluster (H2)(N) (N=2-40), the relative intensities of the ion fragments Hn+ are measured with a mass spectrometer. H3+ is found to be the most abundant fragment up to N=17. With a further increase in N, the abundances of H3+, H5+, H7+, and H9+ first increase and, after passing through a maximum, approach each other. At N=40, they are about the same and more than a factor of 2 and 3 larger than for H11+ and H13+, respectively. For a given neutral cluster size, the intensities of the ion fragments follow a Poisson distribution. The fragmentation probabilities are used to determine the neutral cluster size distribution produced in the expansion at a source temperature of 30.1 K and a source pressure of 1.50 bar. The distribution shows no clear evidence of a magic number N=13 as predicted by theory and found in experiments with pure para-H2 clusters. The ion fragment distributions are also used to extract information on the internal energy distribution of the H3+ ions produced in the reaction H2+ + H2-->H3+ +H, which is initiated upon ionization of the cluster. The internal energy is assumed to be rapidly equilibrated and to determine the number of molecules subsequently evaporated. The internal energy distribution found in this way is in good agreement with data obtained in an earlier independent merged beam scattering experiment.     

Energetics and structures of the initial stages of nucleation of (SiO(2))(N) species: possible routes to highly symmetrical tetrahedral clusters

A detailed survey of the low energy isomer spectrum of (SiO(2))(N), N= 6-10, 13, 16 has been performed using interatomic potential based global optimisations refined via high-level density functional calculations. Within these spectra, including many isomers reported for the first time, structurally and energetically viable pathways for the initial stages of silica cluster growth through SiO(2) nucleation are identified. The role of the exceptionally stable (SiO(2))(8) ground state "magic" cluster is highlighted in the possible formation of highly symmetric fully tetrahedral clusters of size (SiO(2))(10) and (SiO(2))(16). These clusters are found to form a part of a natural (SiO(2))(N)N= 7, 10, 13, 16 sequence together with the C(3v) ground states for (SiO(2))(7) and (SiO(2))(13). The fully tetrahedral clusters are argued to be likely relatively long-lived metastable species in the process of gas phase SiO(2) nucleation due to the manner of their termination. It is speculated that larger tetrahedral (SiO(2))(40) clusters may exhibit porous structures.     

Spatial delocalization in para-H2 clusters

We applied the quantum path integral Monte Carlo method for the study of (para-H)N (N = 5-33) clusters at T = 2 K, exploring static and dynamic order, which originates from the effects of zero-point energy, kinetic energy, and thermal fluctuations in quantum clusters. Information on dynamic structure was inferred from the asymptotic tails of the cage correlation function calculated from the centroid Monte Carlo trajectory. The centroid cage correlation function decays to zero for large clusters (N = 15-33), manifesting the interchange of molecules between different solvation shells, with statistically diminishing back interchange. Further evidence for the floppiness of para-hydrogen clusters emerges from the Monte Carlo evolution of the centroid of a tagged molecule, which exhibits significant changes in the list of its first and second solvation shells due to the interchange of molecules between these shells.     

Infrared spectra of helium clusters seeded with nitrous oxide, 4HeN-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.     

Infrared spectra of CO2-doped 4He clusters, 4HeN-CO2, with N=1-60

High resolution spectra of (4)He(N)-CO(2) clusters are studied in the region of the CO(2) nu(3) fundamental band (approximately 2300 cm(-1)). The clusters are produced in a pulsed supersonic jet expansion from a cooled nozzle source and probed by direct absorption using a tunable diode laser operating in a rapid-scan mode. Four carbon dioxide isotopes ((16)O(12)C(16)O, (16)O(13)C(16)O, (18)O(13)C(18)O, and (16)O(13)C(18)O) are used to support the analysis, and because additional rotational transitions are allowed for the asymmetric one ((16)O(13)C(18)O). Resolved R(0) (J=1<--0) rotation-vibration transitions are observed for clusters up to N=60. A detailed rotational analysis is possible up to N approximately 20 and, with some assumptions, to N approximately 37 and beyond. The derived rotational constants (B values) vary smoothly with N and show evidence for broad oscillations similar to those already reported for He(N)-OCS and He(N)-N(2)O. Possible indications of a disruption are observed in the J=2 levels of larger clusters (N>22) which could be caused by interactions with a "dark" helium cluster modes.     

Impurity-stimulated heterogeneous nucleation of supercooled H2 clusters

The sizes and mass spectra of large (N=1900-13,700 molecules) cold (approximately 3.1 K) H2 clusters have been measured after scattering from CO molecules. Cluster-size measurements after between 2 and 8 collisions indicate that 7% of the H2 molecules are evaporated. This loss agrees with calculations for the number of H2 molecules evaporated by the heat released in the transition from an initial liquid state to a final solid state. Even though heterogeneous nucleation is initiated after only a few collisions with CO molecules, the mass spectra show that additional captured CO molecules coagulate to form large CO clusters with up to n=11 molecules, suggesting that the outer layer is sufficiently liquidlike to facilitate mobility of the CO molecules. Since the calculated H2 cluster temperature (approximately 3.1 K) is below the superfluid transition temperature predicted for pH2 with density between 40% and 80% of the triple-point density, a shell-like region of low density near the cluster surface can be expected to be superfluid.     

Microhydration shell structure in Cl2*-.nH2O clusters: A theoretical study

We present the results of a detailed study on structure and electronic properties of hydrated cluster Cl2*-.nH2O (n = 1-7) based on a nonlocal density functional, namely, Becke's [J. Chem. Phys. 98, 1372 (1993)] half and half hybrid exchange-correlation functional with a split valence 6-311++G(d,p) basis function. Geometry optimizations for all the clusters are carried out with various possible initial guess structures without any symmetry restriction. Several minimum energy structures (conformers) are predicted with a small difference in total energy. There is a competition between the binding of solvent H2O units with Cl2*- dimer radical anion directly through ion-molecule interaction and forming interwater hydrogen-bonding network in Cl2*-.nH2O (n > or = 2) hydrated cluster. Structure having interwater H-bonded network is more stable over the structure where H2O units are connected to the solute dimer radical anion Cl2*- rather independently either by single or double H bonding in a particular size (n) of hydrated cluster Cl2*-.nH2O. At the maximum four solvent H2O units reside in interwater H-bonding network present in these hydrated clusters. It is observed that up to six H2O units are independently linked to the anion having four double H bondings and two single H bondings suggesting the primary hydration number of Cl2*- to be 6. In all these clusters, the odd electron is found to be mostly localized over the two Cl atoms and these two atoms are bound by a three-electron hemibond. Calculated interaction (between solute and different water clusters) and vertical detachment energy profiles show saturation at n = 6 in the hydrated cluster Cl2*-.nH2O (n = 1-7). However, calculated solvation energy increases with the increase in number of solvent H2O molecules in the cluster. Interaction energy varies linearly with vertical detachment energy for the hydrated clusters Cl2*-.nH2O (n < or = 6). Calculation of the vibration frequencies show that the formation of Cl2*(-)-water clusters induces significant shifts from the normal stretching modes of isolated water. A clear difference in the pattern of IR spectra is observed in the O-H stretching region of water from hexa- to heptahydrated cluster.     

Laser ablation synthesis of new phosphorus nitride clusters from α-P3N5 via Laser Desorption Ionization and Matrix Assisted Laser Desorption Ionization Time-Of-Flight Mass Spectrometry

Phosphorus nitride clusters generated during Laser Desorption Ionization (LDI) and Matrix-Assisted Laser Desorption Ionization (MALDI) of solid P(3) N(5) were analyzed via Time-of-Flight Mass Spectrometry (TOF MS). The LDI TOF mass spectra show the formation of series of clusters: P(m)N(n)(+) {(m=1; n=8-11), (m=4; n=3-4), (m=5; n=1-5), (m=6; n=1-3, 5-8), (m=2-7; n=1), (m=5-10; n=2), (m=4-6; n=3), (m=4,5; n=4), (m=5,6; n=5)}, and P(m)N(n)(-) (m=4,5; n=1). Using 3-hydroxypicolinic acid (HPA) as a matrix the P(m)N(n)(+) species (m=1-4, 6, 8) with a high nitrogen content (n=4, 5, 8, 10-12, 20) were identified. The formation of a N(6)(-) cluster was also detected using a C(60) matrix. Under various conditions singly charged P(m)(+) (m=2-7, 9, 13), P(m)(-) (m=3-11, 13, 15, 17), N(n)(+) (n=5, 9, 10, 12, 13), and N(n)(-) (n=6, 10-15) clusters were identified in the mass spectra. Such high nitrogen content clusters (up to N(15)(-)) generated by laser desorption from a solid material are described for the first time. The stoichiometry of the P(m)N(n) clusters was determined via isotopic envelope analysis and computer modelling. The composition of the clusters with respect to the crystalline structure of α-P(3)N(5) is discussed.     

Translation-rotation energy levels of one H2 molecule inside the small, medium and large cages of the structure H clathrate hydrate

We report quantum dynamics calculations of the translation-rotation energy levels of one hydrogen molecule inside the small, medium and large cages of the structure H clathrate hydrate. The calculations are performed using the multiconfiguration time-dependent Hartree (MCTDH) method. Some low-lying states are computed for para-H(2), ortho-H(2), para-D(2), ortho-D(2) and HD, by block improved relaxation.