George B. Hess

Infrared absorption study of physisorbed carbon monoxide on graphite

Abstract We have applied infrared reflection absorption spectroscopy (IRRAS) to the study of carbon monoxide monolayers adsorbed on a single surface of graphite (HOPG). Concurrent monitoring by laser ellipsometry determines the time of appearance of the first and second layer and of bulk CO. CO is one of the few adsorbates previously studied on graphite by IRRAS. Our spectra agree with the previously published results at 35 K and extend the temperature range to 20–40 K, including all four known solid monolayer phases in this range, and unambiguously establish which spectrum corresponds to the bilayer. Integrated absorption strength provides estimates of the average tilt of the molecular axes with respect to the surface in the nominally flat phases. Frequency shifts due to dynamic dipole coupling may contain information on the short-range correlations of tilt azimuths. We find evidence for specific tilt correlations in the commensurate phases.

Tunable Fermi resonance in a C2F6 monolayer on graphite

The infrared absorption spectrum of C2F6 physisorbed on graphite in the commensurate 2×2 phase has two strong, narrow peaks associated with the ν5 molecular vibration. They are interpreted as a Fermi resonance between ν5 and the ν8+ν11 combination band, which are shifted into near-coincidence by dynamic dipole coupling between the adsorbate molecules. The splitting and relative strengths have been measured as the coupling is reduced in a lower-density, tilted, incommensurate phase and are consistent with theory. It is shown that two alternative descriptions, as Fermi resonance between exciton modes of the layer or as dynamic dipole coupling between molecules with two vibration modes, are essentially equivalent. To fit the experimental frequency shifts and absolute absorption strengths, it is necessary to use a value for the ν5 vibrational polarizability of the adsorbed molecule that is somewhat smaller than the value for the free molecule.

Study of SF6 adsorption on graphite using infrared spectroscopy

We report an experimental study of adsorbed monolayers of SF(6) on graphite using infrared reflection absorption spectroscopy supplemented by ellipsometry. The asymmetric S-F stretch mode nu(3) near 948 cm(-1) in the gas is strongly blueshifted in the film by dynamic dipole coupling. This blueshift is very sensitive to the intermolecular spacing in the SF(6) layer. We convert the measured frequency nu(3) to a lattice spacing a, using a self-consistent field calculation, calibrated by the frequency in the commensurate phase. The resolution in lattice spacing is 0.002 A, although there is a larger systematic uncertainty associated with nondynamic-dipole contributions to the frequency shift. We map the commensurate-incommensurate transition, a transition between two incommensurate phases, and the melting transition. These results are compared to previous x-ray data. We provide a new determination of the layer critical point (156 K), the layer condensation line down to 110 K, and the spreading pressure at saturation in this temperature range.

Intrinsic critical velocities in superfluid4He flow through 12-μm diameter orifices near Tλ: Experiments on the effect of geometry

We report super fluid4He flow measurements at temperatures from 1.2 K up to Tλ — 3 mK in three orifices of different mesoscopic geometry. Under conditions of our experiments, the flow usually reaches a temperature-dependent intrinsic critical velocity, where dissipation is believed to occur by thermal (or quantum) nucleation of individual quantized vortex rings or loops. The nucleation rate should be sensitive to the wall geometry of the flow channel and to any local velocity enhancement at the most favorable nucleation site. According to the Iordanskii-Langer-Fisher (ILF) theory, the radius of the “critical” vortex ring, the threshold size which can grow freely by extracting energy from the flow, increases inversely as the superfluid density on approach to the superfluid onset temperature, Tλ. Thus sufficiently near Tλ the critical ring should be large enough that the geometry relevant to the nucleation process and local velocity enhancement can be studied by scanning electron microscope (SEM). We examined our three orifices by SEM. One, a standard optical pinhole, has a relatively smooth taper on one side and a sharp lip on the other. The second is similar, but contains a 1-μm flake perpendicular to the flow, which should provide additional velocity enhancement at its edge. In the third, the sharp lip is beveled to reduce the velocity enhancement at that site. Contrary to expectation, the intrinsic critical velocities are the same, within a relative calibration error of 10%, in all three cases. Thus, local sites of enhanced velocity do not appear to be active in nucleating vortices. This raises a question whether the classical two-fluid model which underlies the ILF calculation is adequate to describe the superfluid hydro-dynamics near walls, as it affects the vortex nucleation process.

Infrared spectroscopic study of C2F6 monolayers and bilayers on graphite

We report an experimental study of adsorbed films of C(2)F(6) on graphite by using infrared reflection absorption spectroscopy supplemented by ellipsometry. The vibrational C-F stretch modes nu(5) (parallel to the molecular axis) and nu(7) (perpendicular) in the film are strongly blueshifted by dynamic dipole coupling, and these shifts are sensitive to lattice spacing and molecular tilt. The relative strength of the absorption peaks mainly depends on the tilt angle relative to the surface normal. We use the strength data to estimate the tilt angle across the known monolayer phases, information that is difficult to obtain by other techniques. Although only the surface-normal component of the induced dipole moment appreciably couples to the external infrared field, surface-parallel components contribute to the intralayer coupling and hence to the frequency shifts for tilted molecules. Comparison to model calculations for a range of herringbone tilt configurations allows us to draw conclusions regarding the pattern of tilt azimuths. On this basis, we offer a revised interpretation of the origin of the Ising-type ordering transition found by Arndt et al. [Phys. Rev. Lett. 80, 1686 (1998)] in heat capacity measurements. Our phase boundaries for monolayer phases above 80 K are in good agreement with earlier results of the Saarbrucken group. We identify three distinct bilayer phases near saturation in isothermal pressure scans from ellipsometric steps and spectroscopic signatures. In temperature scans, we find evidence for several monolayer phases more dense than the well-established 2 x 2 commensurate phase and for a stable trilayer phase below about 60 K.

Adsorption of CF4 on graphite preplated with a monolayer of CF3Cl

We report a study of the adsorption of CF(4) on graphite preplated with a monolayer of CF(3)Cl, using infrared reflection absorption spectroscopy combined with ellipsometry. The saturated vapor pressure of CF(3)Cl is nearly 3 orders of magnitude smaller than that of CF(4) at the same temperature, so the main control variables are the temperature and the pressure (or chemical potential) of CF(4), together with the initial coverage of CF(3)Cl. The temperature range covered is 60-105 K. We find that, if the initial monolayer of CF(3)Cl is liquid, CF(4) continuously displaces CF(3)Cl by substitution in the monolayer. If the initial monolayer of CF(3)Cl is solid, due to either lower temperature or compression, CF(4) condenses as a second layer on the top of the CF(3)Cl layer, with only slight mixing with the original layer. This behavior persists to multiple layers of CF(4).

Multilayer Physisorbed Films on Graphite

This paper reviews experimental work in the multilayer range on physisorption of classical rare gases and simple molecular gases on graphite. The first part outlines ideas about various phenomena which occur or may occur in these systems, such as wetting and limitations to complete wetting, surface roughening and surface melting as they are manifest in films of finite thickness, and substrate-induced effects such as interface-induced freezing and layering in liquid films. Next a number of specific systems are examined from an experimental point of view for evidence of these phenomena. For example, layer critical points in solid rare gas and methane films on graphite have been associated with surface roughening of (111) facet of the corresponding bulk crystal. Surface melting of certain facets has been identified with behavior seen in argon and other films on graphite as well as on MgO slightly below the bulk melting temperature of the adsorbate. On the other hand, freezing of several layers of thick neon and argon films above the bulk melting point is attributed to interaction with the graphite substrate. Certain liquid films, such as those of ethane, ethylene, and oxygen on graphite, exhibit a strong tendency for layering in the proximity of the substrate. Complex phase diagrams may occur in layered films due to interaction of layering with melting and possibly also orientational ordering; examples include oxygen and tetrafluoromethane on graphite. Finally, certain experimental difficulties of a general nature are discussed.

Inhomogeneous nucleation of superfluid vorticity at a sharp edge

We report an experiment in which superfluid flow through a single 10-µm diameter orifice is examined at pressure heads as low as 0.03 dyne/cm2. Accurate measurements of low pressure head are made possible by a recirculating flow cryostat, capable of generating a calibrated, continuous flow of superfluid helium. Current vs. potential data for temperatures between 1.46 and 2.14 K are analyzed according to the Iordanskii-Langer-Fisher thermal nucleation theory, modified to apply to a model in which vortex half-rings are inhomogeneously nucleated at the sharp-edged mouth of the orifice. We offer two possible interpretations of the results.

Liquid helium-II counterflow in a tube packed with angular particles and application as a superfluid pump☆

Abstract Observations are reported on the operation of a recirculating superfluid 4 He pump capable of driving a stable superfluid flow from subcritical continuously through a wide range of chemical potential head, ranging from very small to moderately large. The pump consists of a superfluid path shunting a thermal counterflow tube. The latter is packed with coarse angular particles, which generate quantized vorticity at a low and reproducible superfluid velocity in the tube, and so improve control of the superfluid flux in the shunt. We have made a limited study of the tube characteristic for conditions other than pure counterflow as a basis for analyzing the pump operation. Applied to a study of the onset of dissipation in an external sample, this pump has proved convinient and free of long relaxation times. Some precautions must be taken in matching the counterflow tube to the sample, to avoid regions of possible instability.

Solution and displacement in monolayer and multilayer binary films of SF6 and CF4 on graphite

Infrared reflection absorption spectroscopy is used to study the evolution of binary physisorbed films on graphite. A predeposited monolayer of SF6 is exposed to slowly increasing pressure of CF4 at constant temperature between 80 and 113 K. Shifts in the frequencies of the dominant vibrational mode of each species due to resonant dipole-dipole coupling serve as proxies for the areal density of each species in the monolayer. If the initial SF6 film is far below saturation (coexistence with bulk solid), the SF6 can be largely displaced by continuous solution of CF4. However, if the initial SF6 layer is at or near saturation, a layer of CF4 condenses on top at a well defined CF4 pressure after only 2%-3% dilution of the SF6 layer. Simultaneously, most of the dissolved CF4 is withdrawn from the SF6 layer. With further increase in CF4 pressure, the CF4 layer is compressed and additional layers condense, while the SF6 layer is again diluted. Still, the SF6 layer retains about 90% concentration until the CF4 pressure is very close to saturation, at which point the SF6 is rapidly displaced, apparently going into dilute solution in the rapidly growing CF4 multilayer. Monte Carlo simulations are used to quantitatively relate measured frequency shifts to concentrations in the binary monolayer.