Phase separation of carboxylic acids on graphite surface at submonolayer regime

Mixing behaviour of solid crystalline monolayers adsorbed onto graphite from different mixtures of undecanoic and dodecanoic acids at submonolayer coverage has been investigated. X-ray diffraction measurements have been collected from a variety of compositions as a function of temperature. An extensive phase separation is found for all the compositions – the scattering patterns characteristic of the pure material crystalline structures being preserved across the entire composition range. The temperature dependence of the monolayer melting points and their depression is also clearly indicative of separation of the two surface components, in clear contrast to that expected if the two carboxylic acids mixed ideally in the monolayer.     

Phase diagram and phase transitions of Krypton on graphite in the extended monolayer regime

We report high resolution x-ray diffraction studies of the structures and phase transitions of monolayer krypton, adsorbed on both powder and single crystal graphite substrates. A comprehensive series of powder diffraction profiles is used to construct the two dimensional phase diagram. The melting of the commensurate solid is shown to be strongly first order throughout the region where tricritical behavior was previously thought to occur; fluid solid coexistence extends up to the termination of the commensurate phase at 130 K. A disordered weakly incommensurate phase is shown to be a reentrant fluid, a system which may be described as a disordered network of domain walls and which evolves continuously into a more conventional 2D fluid. This evolution is marked by the disappearance of satellite peaks which are caused by the modulation of the overlayer by the substrate. The freezing of the reentrant fluid into the commensurate phase is shown to be consistent with a chiral Potts transition, its freezing into the incommensurate solid consistent with a dislocation binding transition.Single crystal experiments reveal the orientation of the weakly incommensurate phase. The reentrant fluid is found to have no visible orientational fluctuations, manifesting isotropic diffraction peaks. This is attributed to the strong epitaxy of domain walls. The incommensurate solid is shown to undergo an aligned-rotated transition which is well described by zerotemperature calculations.     

Low-temperature magnetization of submonolayer 3He adsorbed on HD preplated graphite

NMR studies of submonolayer 3He adsorbed on a bilayer of HD preplated graphite have been made down to 100 &mgr;K, which is more than 1 order of magnitude smaller than the exchange energy ( J). In the highly frustrated antiferromagnetic solid region, the magnetization obeys a Curie-Weiss law even at temperatures around J and then increases gradually down to 100 &mgr;K. Nevertheless, it does not show any anomalous behavior corresponding to a spin gap. The normalized magnetization versus the reduced temperature ( T/J) is independent of the density just after solidification. This is consistent with the result in the high-temperature region, that the main multiple-spin exchanges have a similar density dependence.     

Study of optical transitions in an individual ZnO tetrapod using two-photon photoluminescence excitation spectrum

Two-photon excited photoluminescence (PL) measurements have been carried out on an individual ZnO tetrapod at low temperature (8 K) using femtosecond laser pulses in the wavelength range of 714–850 nm. Simultaneously PL and second-harmonic generation were observed. The integrated PL intensity excitation spectrum at different two-photon excitation frequencies has eight peaks, which are in good correspondence to the exciton-phonon complexes L_1b , L_1a , and the free exciton lines B _n=3, A _n=3, B _n=2, A _n=2, B _n=1, and A _n=1 seen in ZnO film. This technique can be used to measure the optical transitions in individual nanostructures, which is very difficult to achieve using the traditional transmission/reflection method.     

Two-dimensional phase transition in xenon submonolayer films adsorbed on (0001) graphite

Ultra high vacuum molecular beam techniques coupled with LEED and Auger electron spectroscopy are particularly well suited to the study of surface chemical reactions because of the ability to assess the effect of the surface conditions on the reaction probability. Investigation of the hydrogen-deuterium exchange reaction on a series of low and high Miller index platinum single crystals has indicated that the steps present on the high index surfaces are necessary for the dissociation and subsequent recombination of hydrogen. We have undertaken a systematic study of a series of small molecule reactions on these stepped surfaces to determine the reaction probability on stepped platinum surfaces. Reactions involving dissociation of Ha₂, D₂, O₂, OH, NH, and CH bonds proceed on the stepped surfaces with much higher reaction probabilities than reactions requiring dissociation of N₂, or CO bonds. All of the reactions studied resulted in cosine product angular distributions except for the formation of CO₂, which exhibited a distribution more peaked at the normal to the surface.     

Collisional redistribution of energy in the infrared spectrum of potassium vapor under two-photon excitation

The cascade decay of the 6S(4D) levels of the potassium atom, 6S(4D)-5P-5S(3D)-4P, following two-photon excitation of potassium vapor (4S-4P-6S) is investigated experimentally. The dependences of 11 resonant IR line intensities on potassium and buffer gas densities are presented. It is shown that the presence of buffer gas leads to collisional redistribution of radiation, causing suppression of some lines and production of new ones.     

Multiphoton excitation of potassium

The Rydberg series of atomic potassium has been measured for 4S-nD transitions from n = 13 to 41 by two-photon and hybrid resonance excitation. Detection of the excitation was by means of an ionization detector. Quantum defects and term values accurate to 1 cm^-1 were computed for the measured transitions.     

Phase transitions in hadronic systems and thermodynamics of a system with an exponential spectrum

Both nuclear matter and hadronic matter at high excitations can be described by a liquid-vapor phase transition. For the hadronic systems, a system with an exponential mass spectrum (Hagedorn-like or bag-like) leads to a thermodynamics which is identical to that of a two phase coexistence at a fixed temperature.     

Phase transitions in physisorbed films on cadmium (0001): comparison with graphite (0001)

Physical adsorption of simple molecules has been studied experimentally on cadmium (0001). Adsorption isotherms of Kr and CF₄ at 77.3 K show a first-order phase transition in the first layer, characteristic of adsorption on uniform substrates.

The nature of the phase transition on cadmium (0001) in the first layer is investigated for Kr and CF₄ by a thermodynamic approach in comparison with results obtained earlier on graphite (0001).

From the comparison between the two uniform substrates, we show that (1) cadmium (0001) has a less attractive force field than graphite (0001); and (2) like graphite (0001), cadmium (0001) is a very uniform surface suitable for studying two-dimensional phases according to the Gibbsian formulation.     

Comparative study of vibrations in submonolayer structures of potassium on Pt(111)

We present results of a comparative study of the vibrational spectrum and local density of phonon states in ordered p(2 x 2) and (√3 x √3)R30° structures formed by potassium atoms on the Pt(111) surface. The calculations were performed with tight-binding interatomic interaction potentials. It was found that the mode associated with vertical displacements of K adatoms has an energy of about 20 meV in both K structures. The strength and energy of this mode slightly decreases with increasing coverage. This result is in good agreement with available experimental data. As in time-resolved second harmonic generation measurements, we observed low frequency modes for both structures considered, which are caused by the interaction of potassium with the second layer of the substrate.     

The controlled excitation of forbidden transitions in the two-photon spectrum of strontium by using collisions and electric fields

We present experimental results involving controlled configuration mixing in two-photon spectroscopy of highly-excited states by exploiting a weak external electric field and collisions. The method has allowed new extensions to high members of the two-photon forbidden J = 3 odd-parity 5snf ¹F₃ and the J = 0, even-parity 5sns ¹S₀ Rydberg series of neutral strontium to be observed. We achieve resonant two-photon transverse excitation of a high density atomic jet by using a narrow bandwidth tunable dye laser in a heat pipe setup with sensitive ionization detection. Experimental term values are extended for the 5sns ¹S₀ series up to n = 46. By suitable exploitation of the composition and pressure of the buffer gases in conjunction with the electric field strength in the excitation region and the exciting laser beam intensity we have also extended observations up to n = 44 for the 5snf ¹F₃ series and up to n = 46 for the 5snp ¹P₁ series. Our results demonstrate a novel and remarkably simple experimental method to access high Rydberg states to which transitions are forbidden from the ground state by parity and other selection rules.