Ar Van Der Waals Research Articles

Photodissociation of Hg–Ar van der Waals molecules: Alignment effects

We present a calculation of the polarization of the fluorescence from the Hg λ253.7 nm transition following photodissociation of the Hg–Ar van der Waals molecule. For this case, it is possible to produce appreciable photodissociation near to threshold directly from the ground vibrational state. This means one can study the nonadiabatic mixing of molecular states over a wide range of translational energies by varying the frequency of the dissociating light. The formalism of Singer et al. [J. Chem. Phys. 79, 6060 (1983)] is used to cast the observables in terms of reduced half-collision T matrix elements. These half-collision fragmentation matrix elements have been found using a time propagation method.

A b i n i t i o calculation of the OH (X 2Π, A 2Σ+)+Ar potential energy surfaces and quantum scattering studies of rotational energy transfer in the OH (A 2Σ+) state

The potential energy surfaces of OH+Ar, which correlate asymptotically with OH(X 2Π)+Ar(1S) and OH(A 2Σ+)+Ar(1S), have been calculated using the coupled electron pair approximation (CEPA) and a very large basis set. The OH–Ar van der Waals complex is found to be bound by about 100 cm−1 in the electronic ground state. In agreement with several recent experimental studies the first excited state is found to be much more stable. The A state potential energy surface has two minima at collinear geometries which correspond to isomeric OH–Ar and Ar–OH structures. The dissociation energies De are calculated to be 1100 and 1000 cm−1, respectively; both forms are separated by a barrier of about 1000 cm−1. The equilibrium distances for OH–Ar and Ar–OH are calculated to be 2.9 and 2.2 Å, respectively, relative to the center of mass of OH. In order to investigate the nature of the strong binding in the A state, we have calculated accurate dipole and quadrupole moments as well as dipole and quadrupole polarizabilities for the X and A states of the OH radical and for the Ar atom. These data are used to estimate the contributions of induction and dispersion forces to the long-range OH–Ar potential. The calculated potential energy surfaces have been fitted to an analytical function and used in quantum scattering calculations for collision induced rotational energy transfer in the A state of OH. From the integral cross sections rate constants have been evaluated as a function of the temperature. The theoretical rate constants are considerably larger than the corresponding experimental values of Lengel and Crosley [J. Chem. Phys. 67, 2085 (1977)], but in good agreement with recent measurements of Jörg, Meier, and Kohse-Höinghaus [J. Chem. Phys. (submitted)]. Our potential energy surface has also been used to calculate the bound rovibrational levels of the OH–Ar complex.

Fluorescence excitation spectrum of the Si–Ar van der Waals complex

We report here the fluorescence excitations spectrum of the Si–Ar van der Waals complex in the region of the (3p4s)3P–(3p2)3P atomic transition. Long progressions are observed, which have been assigned to a Π–Σ transition. The potential curves derived from the analysis of these progressions are discussed in terms of effects of spin–orbit coupling on van der Waals interactions.

Influence of vibrational frequency mismatch on phase-space bottlenecks to intramolecular energy redistribution and molecular fragmentation

We report studies of classical models of unimolecular fragmentation of van der Waals complexes using the methodologies of Hamiltonian mappings and flows. The effect of frequency mismatch between harmonic molecular vibrations of a host molecule and the van der Waals bond-stretching motion is shown to dominate vibrational energy redistribution and fragmentation. Our results show that a crossover exists when the frequency mismatch is increased from a stochastic regime of resonances between the nonlinear oscillators to a regular regime where the system behaves like an integrable system. Our model systems include both collinear and T-shaped oscillator displacement configurations, as well as a many-oscillator system that included both classes of motions. Oscillator parameter ranges were used that approximate vibrations in C6H6⋅He and C6H6⋅Ar van der Waals complexes.

Rotationally resolved ultraviolet spectrum of the benzene–Ar complex by mass-selected resonance-enhanced two-photon ionization

High resolution laser excitation was combined with the technique of mass-selected two-photon ionization via a resonant intermediate state to measure rotationally resolved UV spectra of benzene–Ar van der Waals clusters. When the second laser pulse in the two color experiment is delayed by 7 ns no line broadening due to the second ionizing absorption step is observed. Spectra of three vibronic bands in the S1 ←S0 transition of benzene (h6)–Ar and benzene (d6)–Ar were measured yielding a line spectrum with a linewidth of 130 MHz. Resolution is sufficient to demonstrate that no asymmetry splitting of the rotational lines occurs and the spectrum is to a high precision that of a symmetric rotor. A detailed analysis of the rotational structure yields an accurate set of rotational constants. We find that the Ar is located on the C6 rotational axis. Its distance from the benzene ring plane is 3.582 Å in the electronic ground state and decreases by 59±3 mÅ in the electronically excited state due to the increased polarizability of the benzene molecule after electronic excitation.

Interatomic potentials of the C 11 state of HgNe, HgAr, HgKr, and HgXe as studied by tunable vacuum ultraviolet laser spectroscopy

The laser induced fluorescence spectra of the C–X transitions of HgRg (Rg=Ne, Ar, Kr, and Xe) van der Waals complexes formed in a supersonic jet have been observed in the longer wavelength side of the Hg(1P1−1S0) resonance transition by use of a tunable vacuum ultraviolet laser, which is generated by the two-photon resonant four-wave sum mixing in Sr vapor. On the basis of vibrational quantum numbers assigned to the upper levels of the observed C–X transitions,potential parameters of the C state are determined. The interatomic potentials for the C state are well approximated by a Morse function and the dissociation energies for HgNe, HgAr, HgKr, and HgXe are 89, 486, 1410, and 3463 cm−1 , respectively.

Quantitative absorption spectroscopy and dissociation dynamics of I2 van der Waals complexes with He, Ar, Kr, and Xe

Noble gas (He, Ar, Kr, and Xe) van der Waals (vdW) complexes of I2 formed in axiosymmetric, pulsed jet expansions were studied by absorption using intracavity laser spectroscopy (ILS). Absorption spectra of I2He, I2Ar, I2Ar2, and I2Ar3 as well as previously undetected I2Kr, I2Kr2, I2Kr3, I2Xe, and I2Xe2 were recorded using B(3Π+0u)←X transitions (Figs. 2–4 and Table I). The formation efficiencies of nine I2–noble gas vdW complexes were studied as a function of expansion conditions (Table II). Comparisons of absorption and laser-induced fluorescence data provide an analysis of excited-state dissociation mechanisms for the overall He, Ar, Kr, and Xe chemical series and as a function of vibrational level populated in the B(3Π+0u) state for I2He and I2Ar. The competition between the electronic and vibrational predissociation of the vdW complex is discussed with respect to absorption data.

Application of a diabatic distorted wave approximation to the study of X⋅ ⋅ ⋅H2 (X=He, Ne, Ar) van der Waals molecules

A diabatic rotational decoupling scheme is applied to X⋅ ⋅ ⋅H2 van der Waals molecules using the anisotropic potentials of Tang and Toennies [J. Chem. Phys. 68, 5501 (1978); 74, 1148 (1981)]. We have developed the method proposed by Beswick and Requena [J. Chem. Phys. 72, 3018 (1980)] in an entirely numerical way. Attention is focused on the rotational predissociating levels and on the shape resonances of these systems. Among these resonances, a narrow orbiting and a narrow overbarrier resonances are found for the complexes Ne–H2 and Ar–H2, respectively. They should be amenable to experimental observation in molecular beam scattering as well as infrared spectroscopic studies.

Electron bombardment fragmentation of Ar van der Waals clusters by scattering analysis

Arn clusters generated in a supersonic nozzle beam are investigated by exploiting their different kinematical behavior in a scattering experiment. In a crossed molecular beam experiment the Arn clusters are scattered from He atoms at E=69.7 meV. By measuring the angular and velocity distributions of the scattered clusters, the different species can be separated from each other. In this way fragmentation probabilities of small Arn clusters up to n=6 by electron impact ionization are measured. The influence of collisional dissociation of the dimer is accounted for by classical trajectory calculations. The dimer fragmentation is found to be f21=0.40±0.05, while the trimer totally fragments to dimers ( f32=0.7±0.3) and monomers ( f31=0.3). All clusters with n from 4 to 6 appear with a probability of 0.95 on the dimer ion mass.

Electronic excitations of the free-base porphine–Ar van der Waals complex

In this paper we report on the excited-state energetics of the two lowest-lying intravalence electronic excitations S0 → S1x and S0 → S1yof a large van der Waals complex, consisting of Ar bound to free-base porphine (H2P). H2P•Arn complexes were synthesized and interrogated by laser-induced fluorescence in seeded supersonic expansions of Ar and He/Ar. Diagnostic methods, which were based on the dependence of the intensity of the spectral features on the stagnation pressure and on spectroscopy in He(99%)/Ar(1%) mixtures, were utilized for the identification of the vibrationless electronic excitations of H2P•Ar1. The spectroscopic data were supplemented by model calculations of the potential surface, which demonstrate that the single equilibrium configuration of H2P•R1 (R = Ne, Ar, Kr, and Xe) corresponds to the R atom being located at the twofold symmetry axis perpendicular to the porphyrin ring. While the intense S0 → S1y transition of H2P•Ar, exhibits a red, dispersive, microscopic solvent shift (δv = −24 cm−1), the S0 → S1x transition is characterized by a blue microscopic spectral shift (δv = +8 cm−1), which provides a unique example for excited-state destabilization, originating from intramolecular configurational changes induced by van der Waals binding in a large complex.

Product state and kinetic energy distributions in the ultraviolet photodissociation of the NO–Ar van der Waals molecule

The internal state and kinetic energy distributions of the X̃ NO fragments formed from the ultraviolet photodissociation of the NO–Ar van der Waals species were obtained by laser-excited fluorescence techniques. The initially excited à NO–Ar rapidly dissociates to form X̃ NO with little rotational excitation, with vibrational excitation determined by a Franck–Condon process, with a cos2 θ angular flux distribution (θ defined relative to the direction of polarization of the pump laser), and with a speed v∼4.4×105 cm/s.

The interaction potential energy surface of O 2Ar

A potential energy surface for the O 2Ar system has been obtained by a multiproperty analysis. The absolute integral cross sections at thermal energy, the high energy scattering cross sections, the second virial coefficients, and the P and R branches in the absorption infrared spectrum of the O 2Ar van der Waals molecule, have been analyzed in order to obtain the isotropic component of the interaction. This average potential curve shows a minimum of 11.5 meV at 3.72 Å. The quenching of glory structure and some features of the infrared spectrum have been used to extract information on the anisotropic component.

Scattering of thermal He beams by crossed atomic and molecular beams. II. The He–Ar van der Waals potential

Differential cross sections for He–Ar scattering at room temperature have been measured. The experimental consistency of these measurements with others performed in different laboratories is demonstrated. Despite this consistency, the present van der Waals well depth of 1.78 meV, accurate to 10%, is smaller by 20% to 50% than the experimental values obtained previously. These discrepancies are caused by differences between the assumed mathematical forms or between the assumed dispersion coefficients of the potentials used in the present paper and those of previous studies. Independent investigations have shown that the previous assumptions are inappropriate for providing accurate potentials from fits to experimental differential cross section data for He–Ar. We use two forms free of this inadequacy in the present analysis: a modified version of the Simons–Parr–Finlan–Dunham (SPFD) potential, and a double Morse–van der Waals (M2SV) type of parameterization. The resulting He–Ar potentials are shown to be equal to with experimental error, throughout the range of interatomic distances to which the scattering data are sensitive. The SPFD or M2SV potentials are combined with a repulsive potential previously determined exclusively from fits to gas phase bulk properties. The resulting potentials, valid over the extended range of interatomic distances r≳2.4 Å, are able to reproduce all these bulk properties quite well, without adversely affecting the quality of the fits to the DCS.

An accurate determination of the HeAr van der Waals potential

A new van der Waals potential for HeAr is constructed from a systematic analysis of differential cross section scattering data. The significant discrepancies between this potential and those based on similar experiments in other laboratories are shown to be due primarily to biases present in the mathematical parameterizations used previously. The new potential is valid for interatomic separations between 2.4 Å and 6.5 Å, and provides an excellent description of all available HeAr data, including transport and virial coefficients. This potential, which has a well depth of 1.78 meV, is parameterization-independent and accurate to 10%.

Determining anisotropic intermolecular potentials for van der Waals molecules

The effects of model-dependence and statistical correlation upon the determination of a potential energy surface from the spectroscopic data for isotopic H2–Ar van der Waals complexes, are critically examined. The new potential thus obtained for this system has the correct theoretical long-range behaviour, and yields predicted differential scattering cross-sections and orbiting resonance energies in good accord with experiment. It is also shown that the secular equation method previously applied to H2+ inert gas complexes may also be successfully used to calculate the properties of much more strongly anisotropic species, such as HCl–Ar.

Infrared spectrum of the HD–Ar van der Waals complex

The infrared spectrum of HD–Ar, which occurs as fine structure accompanying the collision-induced fundamental band of HD near 2.7 μm has been studied in a mixture of HD and Ar at 85°K with an absorption path of 66 m. Spectra are observed accompanying the Q(0), Q(1), S(1), and S(0) transitions of HD, but only the latter is sufficiently strong and well resolved to be accurately measured. There is a close qualitative resemblence between the HD–Ar spectrum and previously observed spectra of H2–Ar and D2–Ar van der Waals complexes. However, a quantitative comparison shows a systematic difference which is probably due to the noncoincidence of the center of rotation of HD with its geometrical center.

Infrared spectrum, structure, and properties of the O2–Ar van der Waals molecule

The infrared spectrum of a gaseous mixture of oxygen and argon at 93 °K reveals descrete features which are attributed to absorption by the O2–Ar van der Waals molecule. The spectrum occurs near the infrared inactive vibration of O2. One band, ν1, corresponds to the stretching frequency of O2 within O2–Ar. An analysis of the P and R envelope of this band provides an approximate intermolecular distance for O2–Ar of Re = 3.5 Å. Other absorption features extend as much as 50 cm−1 to the high and low frequency of the ν1 fundamental. These features are assigned as libration or hindered rotation of O2 within the van der Waals molecule in combination with the ν1 fundamental. A model is developed to account for these rotatory energy levels in which the anisotropic intermolecular interaction in O2–Ar is approximated by a simple sinosoidal potential. The internal rotatory energy levels are then obtained from a quantum mechanical calculation. Using this model the infrared data are shown to be consistent with an angular interaction which favors a ``T'' orientation with a barrier to internal rotation of 30 cm−1 (85 cal/mole). Thus a picture of O2–Ar emerges in which molecules with rotatory energy less than the barrier height undergo librational motion and will appear semirigid or ``locked'' into a ``T'' configuration. van der Waals molecules with rotatory energies greater than the barrier height will undergo hindered internal rotation. This study, in addition to other recent work, indicates the power of spectroscopy to provide a mapping of intermolecular interactions of small molecules.