Van Der Waals Vibrational Modes Research Articles

Rotational State Distributions of I2(B) from Vibrational Predissociation of I2(B)-Ne

The vibrational predissociation of triatomic, i.e., atom-diatom, van der Waals complexes in transient electronic excited state has been widely investigated. The predissociation rates or lifetimes are major concerns of the previous studies. Experimentally rotational state distributions of diatomic product are hardly investigated and few theoretical stuides on rotational state distributions have appeared in literature. In this work, choosing the frequently studied <TEX>$I_2(B)-Ne$</TEX> complex as an example, we investigate the change of rotational state distributions of <TEX>$I_2(B)-Ne$</TEX> produced from predissociation of the various initial states of <TEX>$I_2(B)-Ne$</TEX>. The present study on the rotational distributions indicates that rotational state distributions depend significantly on the predissociation energy and the van der Waals vibrational modes of <TEX>$I_2(B)-Ne$</TEX>. That is, the initial state dependency of rotational state distributions is extensively discussed.

Resonant two-photon ionization spectroscopy of the van der Waals complex C6H5CH3⋯N2: Structure, binding energy, intermolecular vibrations, and internal rotation

Resonant two-photon ionization spectrum of C6H5CH3⋯N2 has been recorded, which exhibits rich information about the van der Waals vibrational modes and internal rotation of N2. An ab initio calculation shows a Cs point group for the complex, in which the mass center of N2 is ∼0.35 nm above the benzene ring and the molecular axis of N2 is in the symmetry plane but tilted slightly away from the CH3 group by ∼2°. With the help of isotopic substitution, quantum mechanics calculations, and systematic comparisons, we have tentatively assigned all the observed spectral features. The van der Waals bonding is shown to have an inhibitory effect on the internal rotation of the CH3 group. Bond dissociation energies for the complex C6H5CH3⋯N2 in the excited electronic state and in the ground electronic state are estimated to be 494 and 474 cm−1, respectively, in good agreement with the calculated value.

Vibrational predissociation of ArHF: a test of global semiempirical potential energy surfaces

Vibrational spectra and vibrational predissociation dynamics of the ArHF complex in the lowest van der Waals levels corresponding to the vibrational states of the HF fragment v = 0–7 are investigated using an accurate quantum mechanical method and recent three-dimensional potential energy surfaces (PES). The results demonstrate the accuracy of the semiempirical diatomics-in-molecule and empirically corrected ab initio PES's and allow for better understanding of the sensitivity of the vibrational predissociation lifetimes and product state distributions to the interaction potential. Main features of the fragmentation dynamics such as near-resonant vibration-to-rotation energy transfer, strong variation of the predissociation lifetimes with excitation of the van der Waals vibrational modes, and rapid increase of the decay rates with vibrational excitation of HF are analyzed.

The ArNe-N2O van der Waals trimer: a high resolution spectroscopic study of its rotational spectrum, structure and dynamics

A new ternary van der Waals complex of the type rare gas-rare gas'-linear molecule, ArNe-N2O, was investigated using a pulsed molecular beam cavity Fourier transform microwave spectrometer. The rotational spectra of six isotopomers of the trimer were studied in detail. These include Ar20Ne-14N14NO, Ar22Ne-14N14NO, Ar20Ne-15N14NO, Ar22Ne-15N14N0, Ar20Ne-14N15NO and Ar22Ne-14N15NO. Nuclear quadrupole hyperfine structures of the rotational transitions that are due to the one or two 14N nuclei were resolved and analysed. The resulting spectroscopic constants were used to provide structural and dynamical information about the trimer. Based on the quartic centrifugal distortion constants, a harmonic force field analysis was performed to estimate the frequencies of the van der Waals vibrational modes. A perturbation of the electronic charge distribution at the site of the central 14N nucleus of N20 upon complex formation was detected and discussed. Differences of structural parameters of the trimer as compared to those of the respective dimer units are indicative of the presence of significant three-body non-additive contributions to the interaction energy.

Spectroscopic investigation of the ternary Ne–Ne–OCS van der Waals cluster: additive and non-additive interactions

The ternary Ne–Ne–OCS cluster was generated in a pulsed molecular beam and was probed using a Fourier transform microwave spectrometer. In all, nine isotopomers, namely 20Ne–20Ne–OCS, 20Ne–22Ne–OCS, 22Ne–22Ne–OCS, 20Ne–20Ne–OC34S, 20Ne–22Ne–OC34S, 22Ne–22Ne–OC34S, 20Ne–20Ne–O13CS, 20Ne–22Ne–O13CS, and 22Ne–22Ne–O13CS were studied in their natural abundances, and their rotational transitions were observed and assigned. The measured transition frequencies could be fitted adequately using a semirigid rotor model. The resulting rotational constants were used to derive structural parameters. A harmonic force field analysis based on the quartic centrifugal distortion constants was performed and was used to predict the harmonic vibrational frequencies of the van der Waals vibrational modes. For the isotopomers with Cs symmetry, i.e. 20Ne–20Ne and 22Ne–22Ne containing species, both a- and c-type transitions were observed, while for the asymmetric isotopomers, i.e. species with 20Ne–22Ne, additional, weak b-type transitions were detected. These observations are qualitatively in accord with a geometry based on a pairwise additive model. More detailed structural analyses indicate that the ternary cluster is indeed dominated by additive intermolecular interactions. However, a significant lengthening of the Ne–Ne bond as compared with the Ne–Ne dimer was detected. This is indicative of the presence of three-body non-additive contributions to the interaction energy of the ternary cluster.

Nuclear Dynamics of Benzene···(Ar)n Clusters

In this paper we report on constant energy molecular dynamics (MD) simulations of nuclear dynamics of benzene−Ar and benzene−Arn (n = 2−7) clusters, utilizing the ab initio intermolecular potential of benzene−Ar. The dissociation dynamics of benzene−Ar resulting from a nonselective excitation of the intermolecular modes exhibits an excess energy dependence which can be accounted for in terms of the Rice−Ramsperger−Kassel−Marcus (RRKM) statistical theory. The time-resolved isomerization dynamics of benzeneAr2 and rigid nonrigid isomerization of benzene−Arn (n = 2−7) was investigated. The intracluster vibrational energy redistribution (ICVR) in benzene−Ar from an intramolecular vibration to the intermolecular modes is inefficient on the nanosecond time scale for all of the (one quantum or two quanta) vibrational excitation of each mode. The mode selectivity of the ICVR and its linear dependence on the excess vibrational energy were established. The vibrational predissociation (VP) times for single-mode excitations of benzene−Ar are also slow on the nanosecond time scale (>22 ns), in accord with experimental data. The retardation of ICVR and VP of benzene−Ar reflects the bottleneck effect for intracluster vibrational energy flow, due to the considerable frequency mismatch between the high-frequency vibrational modes of benzene (ω = 402−2955 cm-1) and the low-frequency van der Waals vibrational modes (ω = 26 and 40 cm-1).

Effect of polar solvents on the (2p3s) Rydberg state of diazabicyclooctane

Mass-resolved excitation spectra of the (2p3s)←(2p) 2 Rydberg transition of 1,4-diazabicyclo[2.2.2]octane (DABCO) clustered with polar and nonpolar solvents are reported. The DABCO /solvent clusters are generated in a supersonic expansion. The solvents employed in this study have diverse electronic and dipolar properties and include amines, ethers, aromatics, acetonitrile, a thioether, cyclohexanes, ethylene, N 2 , and Kr. The spectra can be analyzed in terms of cluster transition origins, van der Waals vibrational modes, and internal DABCO vibrational modes. Potential energy calculations are performed for the cluster ground state using atom-atom potentials

Pyrene-rare gas clusters: anomalous spectral shifts and van der Waals mode intensities

S 0 →S 1 electronic spectra of pyrene-R n van der Waals complexes and clusters with R=Ne, Ar, Kr, and Xe and n=1-5 were measured in pulsed supersonic jet expansions by mass-selective absorption spectroscopy employing fragmentation-free two-color resonant two-photon ionization (2C-R2PI) and time-of-flight mass separation. Electronic spectra were recorded for n=1 and 2 for Ne and Xe and n=1-5 for Ar and Kr. The spectral shifts of the n=1 and n=2 (1+1 isomer) complexes are anomalously small, being 10-15 times less than for most other aromatic substrates. Also, the intensity pattern of the van der Waals vibrational modes is very unusual

Laser threshold photoelectron spectra of the cis and trans rotational isomers of p-dimethoxybenzene–Arn (n=0,1,2): Observation of the intermolecular van der Waals stretching and bending vibrational modes in the cation

The cis and trans rotational isomers of p-dimethoxybenzene–Arn (n=0,1,2) have been studied in a supersonic free jet by two-color laser resonance enhanced multiphoton ionization threshold photoelectron spectroscopy. The two-color (1+1′) threshold photoelectron spectra recorded via the S1 state of the cis and trans isomers of the 1:1 and 1:2 argon complexes reveal well resolved vibrational structure characteristic of the low frequency bending and stretching van der Waals vibrational modes. In the case of the trans isomer of the 1:2 complex, a very low frequency progression (11 cm−1) in a nontotally symmetric van der Waals bending mode appears in single quanta in the spectrum. The equivalent spectrum recorded for the cis isomer exhibits structure characteristic of van der Waals stretching modes as well as double quanta excitation in both totally symmetric and nontotally symmetric van der Waals bending modes. The observation of single quantum excitation in formally forbidden van der Waals vibrational modes implies the possibility of a change in the overall symmetry of the complex in the ground cationic state when compared to that in the S1 state. The adiabatic ionization energies (Ia) for the cis and trans isomers of p-dimethoxybenzene–Arn (n=0,1,2) were measured as 60 774±7 (cis; n=0), 60 687±7 (cis; n=1), 60 509±7 (cis; n=2), 60 563±7 (trans; n=0), 60 479±7 (trans; n=1), and 60 295±7 cm−1 (trans; n=2).

The electronic spectroscopy and structure of complexes of argon with 3-amino-s-tetrazine in a supersonic jet

The structure and spectroscopy of van der Waals clusters between argon and 3-amino-s-tetrazine (AT) has been investigated in a supersonic jet. Low resolution vibrationally resolved fluorescence excitation spectra of AT at a variety of argon gas concentrations enable us to assign the origin transitions for the clusters AT–Arn, where n=1 through 6. The frequencies of the van der Waals vibrational modes of these clusters are determined, both in the excited states from the excitation spectra, and in the ground state from single vibronic level emission spectra. High resolution spectroscopy is used to resolve rotational structure in the origin transitions of some of the clusters, and this provides structural information about the clusters. The argon atom in AT-Ar is found to be above the tetrazine ring, and the argon atoms in AT–Ar2 are found to be above and below the tetrazine ring, in all cases having a van der Waals bond length of about 3.3 Å. This work is useful in interpreting the results of our previous study on the photodissociation dynamics of van der Waals clusters between argon and AT.

The structure and dissociation dynamics of the Ne2Cl2 van der Waals complex

The structure and dynamics of Ne2Cl2 and Ne3Cl2 are studied by laser pump–probe spectroscopy. Analysis of a rotationally resolved B←X excitation band shows that Ne2Cl2 has a distorted tetrahedral structure with a Ne–Ne bond length of 3.23 Å and Ne2 center of mass to Cl2 center of mass distance of 3.12 Å. This structure is very close to that predicted by summing the atom–atom interactions. Excitation spectral shifts suggest a Ne3Cl2 structure with the neon atoms encircling the Cl2 bond axis. The total van der Waals binding energy of Ne2Cl2 is found to be between 145.6 and 148.6 cm−1, which is 20 cm−1 greater than 2*D0(Ne–Cl2)+D0(Ne2). For Cl2 stretching levels below υ′=10, transfer of one Cl2 vibrational quantum to the van der Waals vibrational modes is sufficient to dissociate both neon atoms from the complex. This indicates that the two neon atoms need not dissociate via independent, impulsive ‘‘half-collisions’’ which would require two Cl2 vibrational quanta. Observation of a NeCl2 dissociation fragment, however, indicates that such a sequential mechanism competes with the direct dissociation. Cl2 fragment rotational state population distributions for different initial vibrational levels are characterized using a simple rotational surprisal analysis. Comparison of these surprisal plots to those of the NeCl2 dissociation shows that as the size of the complex increases, so does the degree of statistical redistribution during the reaction. Even for Ne2Cl2, however, the extent of product rotational excitation is only weakly dependent upon the amount of energy available to the products and is always less than predicted by a statistical distribution between the translational and rotational product degrees of freedom.

Coupled channel calculations on the vibrational predissociation of argon-para-difluorobenzene

Vibrational predissociation in the complex of an Ar atom with para-difluorobenzene (pDFB) in its electronically excited S 1 state is studied theoretically. A three-dimensional quantum-mechanical method is used, based on the infinite order sudden approximation, with coupled channel expansions in the vibrational states of pDFB. The method involves the calculation and characterisation of scattering resonances, within this approximation, and relating these to experimental lifetimes and product state distributions obtained by Parmenter and co-workers. Mixing of the optically active 5 1 and 6 2 states of the complex with nearresonant states having excited van der Waals vibrational modes is investigated in order to test a proposal made by Ewing to explain the experimental results.

Local modes of benzene and benzene dimer, studied by infrared–ultraviolet double resonance in a supersonic beam

We used rotational cooling of molecules to ∼5 K by supersonic expansion and state-selective, multilevel saturation spectroscopy to obtain high-resolution spectra of the fundamental and first and second overtone transitions of C–H stretching modes in ground-electronic-state benzene and its dimer. Greatly reduced linewidths (&amp;lt;3 cm−1 FWHM) in the rich spectra show that previously reported spectra have suffered from inhomogeneous congestion. Our observed spectral widths indicate that the vibrational lifetimes of the C–H stretches are at least a few ps, even at the energy of the second overtone (8800 cm−1). The ‘‘local mode’’ picture appears to apply when at least three quanta of C–H stretching motion are present. Spectra of the dimer are similar to those of the monomer but show a red shift of a few cm−1, the appearance of combination bands involving van der Waals vibrational modes, some intensity changes, and a broadening of spectral features that increases with the vibrational energy. The dimer’s predissociation lifetime at ∼3000 cm−1 vibrational energy exceeds ∼3 ps.

The rotational spectra, molecular structures, and 21Hg nuclear quadrupole coupling constants of HgHCl and HgDCl

High-resolution rotational spectra of 16 isotopic species of the gas phase van der Waals complex HgHCl have been measured using pulsed Fourier-transform microwave spectroscopy carried out in a Fabry–Perot cavity. The Hg–Cl distance is 4.097 Å in HgHCl and 4.112 Å in HgDCl, with the acid proton located between the two heavy atoms, analogous to the structures of previously studied rare gas-hydrogen halide complexes. Based on this structure, an effective van der Waals radius of 1.99 Å for mercury in HgHCl is determined. 201Hg nuclear quadrupole coupling constants were measured for two complexes: χHg (201HgH 35Cl)=+5.993 (165) MHz, and χHg(201HgD 35Cl) =+7.789 (165) MHz. From these constants, and from a similar measurement for 201HgHC 15N, we obtain the first experimental estimate of the Sternheimer shielding constant in the mercury atom γ∞ =−47(7). Force constants and normal frequencies for the van der Waals vibrational modes were also calculated and compared with those of rare gas-hydrogen chloride complexes.

The photodissociation of van der Waals molecules: Complexes of iodine, argon, and helium

We have observed the fluorescence excitation and dispersed fluorescence spectra of van der Waals molecules produced in a supersonic expansion of mixtures of iodine, helium, and argon. The spectral position of a given van der Waals molecule was found to follow a band shift rule which states that the displacement of the feature of the molecule I2AraHeb from the feature of uncomplexed I2 is given by Aa + Bb, where A and B are constants. The band shift produced by an argon atom was found to be ∼13.5 cm−1. Complexes containing more than three argon atoms were not observed, and this was attributed to rapid electronic predissociation in the larger complexes. Spectral features attributed to progressions in excited state van der Waals vibrational modes were identified. The vibrational product state distributions of the fragment I2* produced by dissociation of various complexes were measured. Larger van der Waals complexes tended to require more vibrational quanta per rare gas atom to dissociate. Excitation of a given complex to higher initial vibrational state also tended to favor channels requiring more vibrational quanta per atom. In the dissociation of the molecule I2ArHe, a single vibrational quantum appeared to be shared between the helium and argon atoms. Information on the rotational product state distribution of the fragment I2* was obtained by measurement of the fluorescence band contours.