Single-Beam Experiments

For exposure at low intensities, large-surface modifications were produced only when a field gradient and a component of electric field were simultaneously present. The bottom of the relief corresponded to the maximum in light intensity, as predicted by Kumar et This was true for a circular as well as a cylindrical Gaussian beam. For a circular Gaussian beam, a maximum deformation of 37 nm was obtained for a p-linearly polarized beam with 238 mW/cm in the center of the beam and time of exposure of 50 min. For a circularly polarized beam, with intensity 230 mW/cm in the center and time of exposure of 50 min, the maximum deformation was 

Below a certain threshold intensity, no in-phase grating could be formed even if the out-of-phase grating was completely saturated and the writing continued. This threshold intensity at 488 nm was about 24 W/cm for PD03 and 22 W/cm for HPAA-N02. These systems also showed interesting optical erasure characteristics, which are discussed in Section 14.4.10. 

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The experiments reported here were designed to demonstrate the feasibility of the measurement and to provide an initial test of the theory. As single beam experiments, the results are laser noise limited. Planned elaboration of the equipment to make double beam measurements should provide an increase in sensitivity. Other modifications which may improve detectability are cell design changes to reduce cell wall absorptions while maintaining minimal cell volume, laser output feed-back control, and signal averaging. With improved sensitivity the use of lower power tunable laser excitation will be feasible. Eventual improvement of sensitivity to the level required for use of continuum sources is at present doubtful. 

Single-beam experiments, with velocity analysis only, have been performed in the low-energy regions by Fink and King and by Henglein. Results are consistent with those described above. 

The decomposition kinetics of the N-Br-amino acids was studied spectro-photometrically by following the fall in absorbance at the wavelength of the absorbance maximum of the N-bromoamino acid, in a Milton Roy Spectronic 3000 Array or a Beckman DU65 single-beam spectrophotometer, both equipped with a cell carrier thermostated to within 0.1 °C by water flow. Kinetic experiments were initiated using a hand-driven HI-TECH SFA-12 Rapid Kinetics Accessory with a 1.00 cm flow cell. 

There are relatively few examples of C-C bond formation on solid surfaces under UHV conditions. There are virtually no examples of catalytic C-C bond formation under such conditions. Perhaps the closest precedent for the present studies on reduced Ti02 can be found in the studies of Lambert et al. on single crystal Pd surfaces. Early UHV studies demonstrated that acetylene could be trimerized to benzene on the Pd(lll) surface in both TPD and modulated molecular beam experiments [9,10]. Subsequent studies by the same group and others [11,12] demonstrated that this reaction could be catalyzed at atmospheric pressure both by palladium single crystals and supported palladium catalysts. While it is not clear that catalysis was achieved in UHV, these and subsequent studies have provided valuable insights into the mechanism of this reaction as catalyzed by metals, including spectroscopic evidence for the hypothesized metallacyclopentadiene intermediates [10,13,14]. 

The main focus of the molecular beam experiments has been to investigate the kinetic details of the catalytic reduction of NO in the presence of a reducing agent (most often CO) under isothermal steady-state conditions. This type of studies have been carried out on Rh(lll) [29], Rh(110) [30], and Pd(lll) [31] single-crystal surfaces. On Rh(lll), we have reported systematic studies as a function of surface temperature, NO + CO... 

For what kind of experiment is the use of a single-beam spectrophotometer a slow, tedious process Why is it slow and tedious ... 

Imagine an experiment in which the molecular absorption spectrum of a particular chemical species is needed. Which instrument is preferred—a single-beam or double-beam instrument Why ... 

During in situ UV-vis spectroelectrochemical work, it is easier to obtain spectra by using a single-beam instrument. At the start of the experiment, the analyst sets the absorbance to zero with the in situ cell placed in the path of the beam, so the cell then acts as a spectroscopic blank or ( reference ). Any changes in absorption will relate to the changes in the amounts of each of the redox states within the cell, rather than from the cell itself. 

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