No abstract

Optical interference techniques were used to measure the real index of refraction of nitric acid/ice films representative of type I and type I1 polar stratospheric clouds (PSCs). Possible candidates for type I PSCs include amorphous HNO3/H20 mixtures as well as crystalline nitric acid trihydrate (NAT), dihydrate (NAD), and monohydrate (NAM). Amorphous and crystalline model PSC films were grown in vacuum by vapor deposition on single-crystal A1203 substrates at low temperatures. The real indices of refraction at X = 632 nm were measured for these films using the time-dependent optical interference during film deposition. The stoichiometries of the HNO3/H20 films were determined using laser-induced thermal desorption (LITD) techniques. For the amorphous films at 130 K, the refractive indices increased with increasing nitric acid content. The values ranged from n = 1.31 f 0.01 for pure ice to n = 1.47 f 0.01 for nearly pure nitric acid.A Lorentz-Lorenz analysis was in good agreement with the measured refractive indices of the amorphous HNO3/H20 films as a function of HN03 mole fraction. Growth of HNOs/H20 films at 175 K resulted in the formation of either crystalline NAM or NAD. The crystalline indices were substantially higher than their amorphous analogs. The crystalline refractive indices at 175 K were n = 1.52 f 0.01 for NAD and n = 1.54 f 0.01 for NAM. Attempts to measure the refractive index of crystalline NAT were unsuccessful because NAT films would not nucleate under allowable temperature and pressure conditions.

Investigations of energy transfer between adlayers on single-crystal surfaces provide a unique opportunity to explore electronic energy transfer in restricted geometries. In this study, laser induced fluorescence techniques and donor quantum yield measurements were used to examine the distance dependence of electronic energy transfer between donor and acceptor adlayers on Al2O3(0001). The donor adlayer was p-terphenyl, the acceptor adlayer was 9,10-diphenylanthracene, and n-butane was the variable spacer adlayer. The electronic energy transfer rates vs spacer thickness were determined at both 30 and 85 K in ultra high vacuum. The butane spacer experiments showed that the donor energy transfer rate decreased with a 1/d3 dependence, where d is the thickness of the spacer adlayer. Given a Förster quantum mechanical or a Kuhn classical energy transfer mechanism with randomly oriented dipoles, a 1/d3 distance dependence is consistent with resonance electronic energy transfer from a two-dimensional donor adlayer to a three-dimensional array of acceptors. The spacer measurements yielded a critical transfer distance of d0=44 ±4 Å at 30 K and d0=33 ±6 Å at 85 K. The differences in the critical transfer distance at 30 and 85 K could be explained by the redshift in the p-terphenyl fluorescence spectrum at 85 K that reduces the overlap between the donor fluorescence and acceptor absorption spectra. Values of d0=44 Å at 30 K and d0=35 Å at 85 K were calculated theoretically from a 1/d3 analysis and were in excellent agreement with the experimental measurements. The rate of donor–donor intralayer energy migration was also determined by measuring the electronic energy transfer rate versus donor coverage on the acceptor adlayer. The donor quantum yield measurements versus donor adlayer coverage were consistent with the spacer results and indicated that electronic energy migration does not occur within the p-terphenyl adlayer. These results vs spacer thickness and donor coverage reveal that electronic energy transfer in spatially confined geometries can be described using a modified Kuhn energy transfer mechanism.

The effect of thermal annealing and surface coverage on porous silicon photoluminescence was studied in situ in an ultrahigh vacuum chamber. These investigations correlated simultaneously temperature, surface coverage, and photoluminescence intensity. The surface coverage was monitored using transmission Fourier transform infrared spectroscopy. The results demonstrated that the photoluminescence could not be defined only according to the presence of SiH2 surface species. Likewise, the disappearance of the photoluminescence versus thermal annealing did not scale directly with H2 desorption from SiH2 species. The loss of photoluminescence versus thermal annealing was attributed to surface structural changes or the production of surface states which provide pathways for nonradiative recombination.

Surface structural dependence of cesium adsorption on the αAl2O3(0001) surface AIP Conf.