J. Peter Toennies

An improved simple model for the van der Waals potential based on universal damping functions for the dispersion coefficients

Starting from our earlier model [J. Chem. Phys. 66, 1496 (1977)] a simple expression is derived for the radial dependent damping functions for the individual dispersion coefficients C2n for arbitrary even orders 2n. The damping functions are only a function of the Born–Mayer range parameter b and thus can be applied to all systems for which this is known or can be estimated. For H(1S)–H(1S) the results are in almost perfect agreement with the very accurate recent ab initio damping functions of Koide, Meath, and Allnatt. Comparisons with less accurate previous calculations for other systems also show a satisfactory agreement. By adding a Born–Mayer repulsive term [A exp(−bR)] to the damped dispersion potential, a simple universal expression is obtained for the well region of the atom–atom van der Waals potential with only five essential parameters A, b, C6, C8, and C10. The model has been tested for the following representative systems: H2 3Σ, He2, and Ar2 as well as NaK 3Σ and LiHg, which include four che...

Theoretical studies of highly expanded free jets: Influence of quantum effects and a realistic intermolecular potential

An earlier approximate theory proposed by Knuth and Fisher and Miller and Andres for solving the Boltzmann equation to calculate speed ratios and temperatures in free jet expansions has been extended to take account of quantum effects and realistic potentials in the collision integrals. The calculations show that quantum effects lead to a substantial increase in terminal speed ratios in helium but not in heavier gases. For He the speed ratios depend sensitively on the shape of the potential. Despite the approximate nature of the theory the predictions are in reasonable agreement with recent experiments. The theory predicts that the presently highest achieved speed ratio in He of 225 can be increased to 700 if condensation does not occur. At this speed ratio the quasiequilibrium gas phase temperature is roughly 10−3 K.

The rotational spectrum of single OCS molecules in liquid 4He droplets

Single OCS molecules have been embedded in large 4He droplets (N=1×103–8×103 atoms) and their infrared spectra in the vicinity of the ν3-fundamental at 2062 cm−1 have been studied using coaxial laser depletion spectroscopy. Sharp lines corresponding to the P- and R-branches with a linewidth of 160 MHz or greater are observed. From the line intensities a droplet temperature of 0.37±0.02 K is obtained and from the line positions the rotational constants B for the ground and excited states and an average centrifugal distortion constant D are determined. The former are about a factor of 2.8 smaller and the latter four orders-of-magnitude larger than for the free molecule. The decrease in B is attributed to an attached nonsuperfluid component which has the same effect as a ring of about six 4He atoms in an equatorial plane around the waist of the OCS molecule, which is carried along in the end-over-end rotation. The widths of the individual lines show an interesting asymmetry which is oppositely sloped for th...

Photoelectron spectroscopy of liquid water, some alcohols, and pure nonane in free micro jets

The recently developed technique of accessing volatile liquids in a high vacuum environment by using a very thin liquid jet is implemented to carry out the first measurements of photoelectron spectra of pure liquid water, methanol, ethanol, 1-propanol, 1-butanol, and benzyl alcohol as well as of liquid n-nonane. The apparatus, which consists of a commercial hemispherical (10 cm mean radius) electron analyzer and a hollow cathode discharge He I light source is described in detail and the problems of the sampling of the photoelectrons in such an environment are discussed. For water and most of the alcohols up to six different electronic bands could be resolved. The spectra of 1-butanol and n-nonane show two weakly discernable peaks from which the threshold ionization potential could be determined. A deconvolution of the photoelectron spectra is used to extract ionization potentials of individual molecular bands of molecules near the surface of the liquid and shifts of the order of 1 eV compared to the gas p...

Interaction potentials for alkali ion–rare gas and halogen ion–rare gas systems

The Tang–Toennies model [J. Chem. Phys. 80, 3725 (1984)] has been modified to predict the potentials for ion–atom systems. First order SCF energies are used to describe the repulsive potential. The long range second order induction and dispersion potential terms up to R−10 are either taken from ab initio calculations or estimated and each term is appropriately damped. The potentials for Li+, Na+, K+, F−, and Cl− interacting with He, Ne, and Ar are found to agree well with both theoretical and experimental data within the expected errors. For comparison with the model new ab initio calculations have been performed for Na+–Ar and the results are in excellent agreement with the model predictions (<10%).

Molecular beam scattering studies of orbiting resonances and the determination of van der Waals potentials for H–Ne, Ar, Kr, and Xe and for H2–Ar, Kr, and Xe

The velocity dependence of the integral cross sections for the scattering of H atoms and normal‐H2 molecules from the rare gases has been measured for primary beam velocities from 300 m/sec (Ec.m.=0.5 meV) to about 3000 m/sec (Ec.m.=100 meV). Three distinct resonance maxima were observed in each of the systems H–Xe, H–Kr, H2–Kr, and H2–Ar. At least one resonance maximum was found in H2–Xe and H–Ar, while H–Ne showed no maxima. The H2–Ar cross sections were also measured with a cold beam of pure para‐H2 consisting entirely of spherically symmetric j=0 molecules. No noticeable difference was found when compared to the n‐H2 cross sections indicating that the observed resonances depend only on the spherical symmetric potential. Extensive comparisons with spherical symmetric model potentials showed that all the observed resonances are due to temporary capture in the well of the effective potential. This phenomenon is usually referred to as ’’orbiting.’’ The energetic location of the observed orbiting resonance...

The nondestructive detection of the helium dimer and trimer

A transmission grating is used to nondestructively analyze a low source temperature (6–60 K) beam of helium for small clusters. He2 and He3 are clearly resolved in first order diffraction. The relative ionization and fragmentation probabilities are measured and lend support to a recent mass spectrometer experiment claiming detection of He2.

Superfluid Helium Droplets: An Ultracold Nanolaboratory

The unique environment in liquid helium droplets opens up new opportunities for molecular spectroscopy and for probing superfluid phenomena on the atomic scale.

New Combining Rules for Well Parameters and Shapes of the van der Waals Potential of Mixed Rare Gas Systems

New combining rules are presented for calculating the van der Waals well parametersε andσ as well asC6,C8 andC10 for the mixed rare gas systems from the corresponding values of the homogeneous dimers. The combining rules forε andσ are tested by comparison with the recent compilation of Aziz and found to be superior to the best previous combining rules selected by Aziz. Effective Born-Mayer repulsive potential parameters are determined from the model potential of Tang and Toennies and make possible the calculations of accurate potential curves for all combinations of rare gas atoms.

Density of superfluid helium droplets

The classical integral cross sections of large superfluid 4 He N droplets and the number of atoms in the droplets (N = 10 3 − 10 4 ) have been measured in molecular beam scattering experiments. These measurements are found to be in good agreement with the cross sections predicted from density functional calculations of the radial density distributions with a 10–90% surface thickness of 5.7u By using a simple model for the density profile of the droplets a thickness of about 6–8u is extracted directly from the data.