Volume 36, Issue 5, May 2010
Index of content:
Low-temperature radial thermal expansion of single-walled carbon nanotube bundles saturated with nitrogen36(2010); http://dx.doi.org/10.1063/1.3432241View Description Hide Description
The effect of impurity on the radial thermal expansion coefficient of single-walled carbon nanotube bundles has been investigated in the temperature interval by the dilatometric method. Saturation of nanotube bundles with sharply increased the positive values of in the entire experimental temperature range and resulted in a very high, wide maximum in the thermal expansion coefficient at . Low-temperature impurity desorption from -saturated powder consisting of bundles of single-walled carbon nanotubes with open and closed ends has been investigated.
36(2010); http://dx.doi.org/10.1063/1.3432243View Description Hide Description
A model of an atomic impurity in an octahedral void of fullerite is suggested. The problem is solved in the spherical oscillator approximation, which is appropriate for the larger rare gas atoms. It is shown that such impurities can contribute to the negative thermal expansion at low temperatures and produce a Schottky-like maximum at higher temperatures.
36(2010); http://dx.doi.org/10.1063/1.3432244View Description Hide Description
Resonance Raman spectra of DNA-wrapped single-walled carbon nanotubesfilms were studied at 5 and in the range of radial-breathing and tangential modes. The spectra were compared with those of nanotubes in bundles. At in the spectrum of film an upshift of bands with respect to their spectrum at high temperature and the intensity redistribution among bands of two samples were observed. The magnitude of this upshift depends on the nanotube type. The influence of a temperature decrease, the environment, and the electron-phonon coupling on the Raman spectrum of nanotubes are discussed.
36(2010); http://dx.doi.org/10.1063/1.3432245View Description Hide Description
Impurity–helium condensates formed by krypton nanoclusters containing atoms and molecules of hydrogen isotopes have been studied via an electron spin resonance(ESR) technique. Analysis of the ESRspectra has shown that most of the H and D atoms reside on the surfaces of Kr nanoclusters. Very large average concentrations have been obtained for H atoms and D atoms in these experiments. Line narrowing has been observed for the highest concentration of D atoms stabilized in the Kr–He sample. Exchange tunnelingreactions have been studied in Kr–He sample containing H and D atoms.
Infrared studies of ortho-para conversion at Cl-atom and H-atom impurity centers in cryogenic solid hydrogen36(2010); http://dx.doi.org/10.1063/1.3432246View Description Hide Description
We report infrared spectroscopic studies of ortho-para (o/p) conversion in solid hydrogendoped with Cl-atoms at while the infrared-induced chemical reaction is occurring. The Cl-atom doped hydrogen crystals are synthesized using in situphotodissociation of precursor molecules. For hydrogen solids with high ortho- fractional concentrations , the o/p conversion kinetics is dominated by Cl-atom catalyzed conversion with a catalyzed conversion rate constant and the process is rate-limited by ortho- quantum diffusion. For hydrogen crystals with low ortho- concentrations , single-exponential decay of the ortho- concentration with time is observed which is attributed to H-atom catalyzed o/p conversion by the H-atoms produced during the infrared-induced reaction. The measured H-atom catalyzed o/p conversion kinetics indicates the H-atoms are mobile under these conditions in agreement with previous ESR measurements.
36(2010); http://dx.doi.org/10.1063/1.3432248View Description Hide Description
The reaction leading to HArF formation in an argon matrix is studied at temperatures down to . The effects of the precursor concentration, deuteration, IR light, and deposition temperature as well as thermal activation of this reaction are studied. It is found that HArF molecules are formed slowly but efficiently at in a photolyzed matrix, supporting the previously reported results. The formation rate of HArF (and DArF) exhibits a low-temperature limit and enhances at elevated temperatures with activation energy of about . All the data show that HArF is formed as a result of a local reaction of hydrogen atoms with the parent Ar–F centers and the tunneling mechanism is very probable here. The locality of the precursor photolysis required for this tunnelingreaction is consistent with the partial HArF formation observed during photolysis of HF in an argon matrix. The decay mechanism of cations is also studied. The present results confirm the previous conclusions that the decay of the cations is not essential to HArF formation.
Formation of centers in solid Xe via recombination: nonstationary luminescence and internal electron emission36(2010); http://dx.doi.org/10.1063/1.3432249View Description Hide Description
The formation of excimers in solid Xe doped with molecular hydrogen under irradiation by an electron beam is studied using the original two-stage technique of nonstationary (NS) cathodoluminescence(CL) in combination with current activation spectroscopy—thermally stimulated exoelectron emission(TSEE). Charged species are generated using a high-density electron beam. The species produced are probed with a low-density beam with sample gradually heated. The near-UV emission of the is used to monitor the neutralization process. It is found that the temperature behavior of the NS CL band of clearly correlates with the yield of TSEE measured after identical pre-irradiation of the sample. The fingerprints of the thermally stimulated detrapping of electrons—«internal electron emission»—-in the spectrum of NS CL point to the essential role of the neutralization reaction in the stability of protons solvated by rare-gas atoms.
Ion–molecule reactions of with nitrogen oxides , NO, and : measuring absolute pressure by shock-freezing of the collision complex36(2010); http://dx.doi.org/10.1063/1.3432259View Description Hide Description
A new method to determine the absolute pressure in an ultra-high vacuum apparatus is tested using ion molecule reactions with . In a collision with a neutral reactant the complex between and the collision partner is stabilized by evaporation of argon atoms. If reacts at the collision rate, the absolute pressure can be determined by comparing the experimental collision rate with the collision rate calculated from average dipole orientation theory. The experimental results with , NO and do indeed show that the collision complex is frozen out. Comparing the rates of primary, secondary and tertiary reaction products suggests that not all collisions of are reactive.
36(2010); http://dx.doi.org/10.1063/1.3432260View Description Hide Description
Narrow excitation features present on the transitions of manganese atoms isolated in solid Kr are analyzed within the framework of weak crystal field splitting. Use of the optical lineshape function allowed identification of multiple zero-phonon lines for individual spin-orbit states of the a transition recorded with laser-induced excitation spectroscopy. Excellent agreement exists between the predicted crystal field splitting patterns for the levels of the state isolated in the «red» tetravacancy site of solid Kr. The tetrahedral crystal field of the «red» trapping site splits levels of the and states by approximately . This report represents the first definitive evidence of crystal field splitting, induced by the weak van der Waals interactions between a neutral metal atom and the rare gas atoms surrounding it in a well-defined solid-state site.
36(2010); http://dx.doi.org/10.1063/1.3432267View Description Hide Description
The phase diagram of solid carbon monoxide was investigated in the pressure range and temperature range by infrared and Raman spectroscopy. The tentative phase diagram known from the literature was expanded and specified in detail. The -phase region is divided into two subphases— and —similar to solid nitrogen. The pressure-temperature behavior of the elementary and combined excitations was also followed up. The vibron overtone region was carefully investigated by FTIR spectroscopy as a function of temperature at different pressures; the fundamental region was investigated by Raman spectroscopy. The features of the IR-active phonon sideband to the vibron overtone were investigated in detail in the entire pressure-temperature region. The lattice-phonon spectra were studied by Raman spectroscopy as a function of pressure (at lowest temperature) and by IR spectroscopy as a function of temperature at saturated vapor pressure.
Study of cluster formation in low-temperature systems. Spectral manifestation of resonance dipole–dipole interactions between nondipole polyatomic molecules36(2010); http://dx.doi.org/10.1063/1.3432261View Description Hide Description
Resonance dipole-dipole interactions are shown to strongly manifest themselves in fundamental and overtone band shapes of low-temperature systems consisting of polar and nonpolar molecules as well with a relatively large first derivatives of the dipole moment. A rough model on the basis of the interaction between two non-degenerate, doubly degenerate, and triply degenerate oscillators was developed to study pairs of interacting molecules and clusters in low-temperature condensed systems. The concentration dependences of second- and third-order spectral band moments made it possible to estimate the number of molecules in clusters.
36(2010); http://dx.doi.org/10.1063/1.3432262View Description Hide Description
Our data on the distribution of purely electronic linewidths of single molecules of terrylene in incommensurate biphenyl crystals are compared with the data obtained by other groups for different low-temperature organic solid hosts and with results of numerical simulations. The first two moments of the distributions measured within a narrow temperature interval have been used to calculate a single dimensionless parameter characterizing each of the respective hosts—the variation coefficient. It appears that different amorphous hosts have similar values of this coefficient, but the value obtained for the incommensurate crystal of biphenyl is significantly different. One can conclude that the remarkable single-molecule line broadening in biphenyl at cannot be solely explained by the interaction with two-level systems, which is considered to cause the broadening in amorphous hosts.
36(2010); http://dx.doi.org/10.1063/1.3432263View Description Hide Description
An unexpected phenomenon was revealed in the laser induced fluorescence spectra of free-base and zinc (ZnPc) phthalocyanines trapped in rare gas and nitrogen matrices under a moderate increase in the laser intensity. In all matrices the intensity of an emission band near increased drastically when pumping the transition. This observation was assigned to stimulated emission in a four-level scheme involving a vibronic transition from the lowest electronic state to a vibrational level of the ground state. In the present work, we expose new similar results obtained with porphyrin molecules, i.e. tetra-benzoporphin (TBP). With free-base , stimulated emission was observed in Ar or matrices, but not in Xe matrices. A possible reason could be a fast inter-system crossing rate due to the heavy atom effect induced by Xe. We also report the observation of persistent burnt spectral holes, although the low efficiency of this process is not competitive with stimulated emission and no decrease of the stimulated emission with time was observed. With ZnTBP no stimulated emission could be recorded but the appearance of rather strong phosphorescence was noticed. Similar studies focusing on the stimulated emission phenomenon is that they should allow new insights into site effects and site selectivity.
36(2010); http://dx.doi.org/10.1063/1.3432264View Description Hide Description
The conformational equilibrium of the ribonucleoside uridine molecules in low-temperature Ar matrices was investigated by the FTIR matrix isolation spectroscopy and quantum-chemical calculations at the DFT and MP2 levels of theory. It was established that conformers with a carbonyl group C2O, which takes part in the intramolecular hydrogen bonds, dominate in the low temperature matrices. Populations of syn- and anti- conformers with intramolecular hydrogen bonds and , respectively, are equal practically. Unlike -deoxyuridine, the syn- conformers of Ur with -endo structure of the ribose ring (South) have been found in Ar matrices. At the same time the conformations of -endo (North) ribose ring prevail both in anti- and syn- conformers.
36(2010); http://dx.doi.org/10.1063/1.3432265View Description Hide Description
Ion implantation in diamond creates optically active defects which have emission lines in broad spectral regions and may be used in advanced photonics and optical communication applications. A brief review of the photoluminescence properties of ion implanteddiamond is presented. The Xe-related center is of particular interest as this center is one of a few centers (Ni, Si, Cr) in diamond having sharp emission lines in the infrared spectral region, specifically at 813 and . The paper discusses an approach to determine the important and difficult to measure conversion efficiency of implanted ions into emitting optical centers. The method uses micro-luminescence confocal mapping and statistical analysis based on a compound Poisson distribution, accounting for both the implanted centers and the optically excited centers statistics. Results of numerical simulations and experimental measurements are presented.