Index of content:
Volume 84, Issue 8, 15 October 1998
- GENERAL PHYSICS: NUCLEAR, ATOMIC, AND MOLECULAR (PACS 01-39)
84(1998); http://dx.doi.org/10.1063/1.368616View Description Hide Description
The combination of two-photon excitation 4Pi-confocal fluorescence microscopy with image restoration leads to a fundamental improvement of three-dimensional resolution in the imaging of transparent, fluorescent specimens. The improvement is exemplified by randomly dispersed fluorescent beads and with actin filaments in a mouse fibroblast cell. For an illumination wavelength of 810 nm, we obtained lateral and axial full-width at half-maxima of point-like objects of 120–140 nm, and 70–100 nm, respectively. Fluorescent beads that are 150 nm apart are imaged with an intensity dip of ∼25%. This amounts to a ∼sixfold improvement of the axial resolution over standard two-photonconfocal microscopy. In the cell, the 3D-images reveal details otherwise not resolvable with focused light.
84(1998); http://dx.doi.org/10.1063/1.368617View Description Hide Description
The tip–sample capacitance in the scanning capacitancemicroscopy (SCM) of dielectric films is described through theoretical calculations based on the method of images. The results are explained with the charge density distribution in the tip–sample system. Furthermore, capacitance signals in the tapping mode SCM of dielectric films are experimentally investigated and found to be in good agreement with the simulation results.
84(1998); http://dx.doi.org/10.1063/1.368693View Description Hide Description
A high-temperature pressurecalibration technique using Sm-doped (Sm:YAG) crystal as the pressure calibrant has been developed by studying its Y1 through Y10 fluorescence peaks (frequencies from 15 600 to 17 200 at pressures (p) from 1 bar to 19 GPa and temperatures (T) from 20 to 850 °C in externally heated diamond anvil cells. The entire spectrum was fit to a sum of ten Lorentzians plus a linear background. The positions, relative intensities and widths were represented by empirical functions of p and T. Several fitting routines for p determination were created based on these dependences, and were tested on various high-p and high-T experimental Sm:YAG fluorescence spectra. The p values obtained from the fitting routines are compared with those obtained from the ruby and the nitrogen vibron pressure scales. A fitting routine is proposed that can determine p from 20 to within an estimated uncertainty of 0.4 GPa.
A method for in situ characterization of tip shape in ac-mode atomic force microscopy using electrostatic interaction84(1998); http://dx.doi.org/10.1063/1.368618View Description Hide Description
We present a method for in situ characterization of the tip shape in atomic force microscopes that can operate in noncontact ac mode. By sweeping the voltage between tip and sample while recording the sample position as it is regulated to give a constant force gradient, we obtain curves giving information about the tip geometry. The measurements were performed in ultrahigh vacuum using electrochemically etchedtungsten tips against a surface of dopedsilicon. Our results show that the sphere model gives a good description of the interaction, and that the radii we obtain are consistent with data from scanning electron microscopy. The method can also be used to estimate the value of the Hamaker constant and the contact potential between tip and sample.
84(1998); http://dx.doi.org/10.1063/1.368619View Description Hide Description
A beam of atomic hydrogen is produced by passing hydrogen gas through a tungsten capillary heated to 2600 K by electron impact. By means of a differentially pumped quadrupole mass analyzer the angular distribution of the atomic hydrogen is measured for different hydrogen feed pressures. The angular distribution is considerably more peaked than a cosine distribution and is narrower the lower the feed pressure is. Based on the theory of beam formation by molecular flow a simple analytical model is developed which assumes a spatially limited transparent flow at the end of the capillary. The measured angular distribution can be described by the model using the length of transparent flow as the only fitting parameter.