Femtosecond transfer dynamics of photogenerated electrons at a surface resonance of reconstructed InP(100).

Time-dependent two-photon photoemission spectra are used to resolve the femtosecond dynamics of hot electrons at the energetically lowest surface resonance of reconstructed InP(100). Two different cases are studied, where electrons either are lifted into the surface resonance via a direct optical transition or are captured from bulk states. These data are the first of this kind recorded with a time resolution below 70 fs. The microscopic analysis shows that electron-phonon scattering is a major mechanism for electron transfer between surface and bulk states.

Optical absorption of water: coulomb effects versus hydrogen bonding.

The optical spectrum of water is not well understood. For example, the main absorption peak shifts upwards by 1.3 eV upon condensation, which is contrary to the behavior expected from aggregation-induced broadening of molecular levels. We investigate theoretically the effects of electron-electron and electron-hole correlations, finding that condensation leads to delocalization of the exciton onto nearby hydrogen-bonded molecules. This reduces its binding energy and has a dramatic impact on the line shape. The calculated spectrum is in excellent agreement with experiment.

InP(001)-(2 x 1) surface: a hydrogen stabilized structure.

The InP(001)(2 x 1) surface has been reported to consist of a semiconducting monolayer of buckled phosphorus dimers. This apparent violation of the electron counting principle was explained by effects of strong electron correlation. Combining first-principles calculations with reflectance anisotropy spectroscopy and LEED experiments, we find that the (2 x 1) reconstruction is not at all a clean surface: it is induced by hydrogen adsorbed in an alternating sequence on the buckled P dimers. Thus, the microscopic structure of the InP growth plane relevant to standard gas phase epitaxy conditions is resolved and shown to obey the electron counting rule.

Bulk excitonic effects in surface optical spectra.

We calculate the surface optical properties of the passivated Si(110) surface using a real-space multigrid technique and ab initio pseudopotentials. Rather than from the usual eigenvalue representation, the macroscopic polarizability is obtained from the solution of an initial-value problem, which allows inclusion of excitonic and local-field effects in addition to the electronic self-energy in the surface calculations. It is shown that the electron-hole attraction is largely responsible for the peculiar line shape of the surface reflectance anisotropy.

Effect of dietary fluoride and aluminum on laying hen performance and fluoride concentration in blood, soft tissue, bone, and egg.

Changes in laying-hen performance due to the effect of graded levels of dietary fluoride (F) have been documented earlier. The present study examined the effect of dietary F and aluminum (Al) on laying hen performance, and F levels in plasma, soft tissues, muscle, bone, and eggs. Two hundred and seventy Single Comb White Leghorn (SCWL) layers were fed a basal diet containing the following F and Al levels: control; 100 ppm F; 80 ppm Al; 100 ppm F/80 ppm Al; 100 ppm F/40 ppm Al; 1300 ppm F; 1040 ppm Al; 1300 ppm F/1040 ppm Al, and 1300 ppm F/520 ppm Al. After 112 days of feeding, the blood, tissue, bone, and egg samples were collected for analysis. High F intake significantly depressed feed intake and hen-day production and elevated serum, tissue, bone, and egg F levels. Dietary Al minimized the effect of high fluoride feeding. The mean fluoride concentration for low (80 ppm Al diet) vs. high F (1300 ppm) intake was, respectively: plasma, .2 vs. 10.1 ppm; liver, 4.5 vs. 19.2 ppm; kidney, 2.9 vs. 31.8 ppm; pectoralis muscle, 3.6 vs. 6.7; tibia muscle homogenate, 3.9 vs. 21.1 ppm, and bone, 611 vs. 2600 ppm. The majority of the F transferred to the egg was incorporated into the shell (2.8 to 307.1 ppm). Low F levels were found in albumen (fresh, .4 to 1.0 ppm) and yolk (dry fat free, 3.1 to 18.4 ppm). The data suggested that dietary F is readily accumulated in bone and kidney; however, liver and muscle are less responsive.(ABSTRACT TRUNCATED AT 250 WORDS)

Fluorine toxicity and laying hen performance.

Two hundred eighty-eight Single Comb White Leghorn (SCWL) hens were treated for 252 days with sodium fluoride added to a practical wheat-soybean meal-type basal diet to supply 0, 100, 400, 700, 1,000, and 1,300 ppm fluoride (F). On Day 252, hens fed the two highest levels of F were switched to the control diet and the experiment continued for another 168 days. The two highest levels of fluoride resulted in significant (P less than .05) depression of feed intake, body weight gain, hen-day production, feed efficiency, and egg quality. Fluoride (700 ppm) tended to reduce performance but generally was not significantly different from the control and lower F diets. Long-term feeding of high levels of F (NaF) did not result in permanent production impairment. Birds fully recovered during the 168-day recovery period. In the second experiment 288 SCWL pullets were treated for 49 days with seven dietary regimens: 0 ppm F fed ad libitum (1), 1,300 ppm F fed ad libitum (2), 0 ppm F control pair-fed to 1,300 ppm F (5), an NaCl diet formulated to supply as much Cl- as F- in diet 3 (4), an NaCl diet formulated to supply as much Na as supplied by NaF in Diet 3, (5), 1,300 ppm F plus 1,040 ppm Al diet fed ad libitum (6), and the same diet pair-fed to the 1,300 ppm F diet (7). All groups were fed the control diet for a 7-day recovery period at the end of the experiment. High F intake significantly (P less than .01) decreased feed intake, hen-day production, feed efficiency, and shell quality. However, these depressions were not as severe when Al was present in the diet. The depression in performance due to F feeding was not simply due to the depressed feed intake but rather was a result of a metabolic function of F. Pair feeding the control and F/Al diets resulted in much smaller depression. Egg shell quality was more responsive to the addition of Na to the diet than the F. Although feed consumption recovered within the 7-day recovery period, this time was inadequate for complete recovery in egg production.