Raman scattering from Pt(CN)42− adsorbed on Pt colloids

Raman scattering from Pt(CN)₄^2? adsorbed on Pt colloids (average diameter of 16 A) is compared with the Raman signal from the same amount of Pt(CN)₄^2? in solution. The wavelength dependence of the adsorbate Raman intensity is measured from 308 to 647 nm and compared with a Lorenz/Mie calculation of the enhancement factor for the electromagnetic field intensity on the surface of a Pt sphere as a function of sphere radius and incident wavelength.     

Application of simultaneous excitation/detection to generate real-time excitation profiles in Fourier transform ion cyclotron resonance mass spectrometry

Simultaneous excitation/detection (SED), which permits observation of ion motion during an excitation event, is used to generate real-time Fourier transform ion cyclotron resonance (FTICR) excitation profiles that track the radial extent of ion motion in a trapped-ion cell. The conventional FTICR excitation profile is collected in a series of individual experiments in which peak magnitude is monitored as excitation voltage is increased. In contrast, SED permits the single-scan detection of ion cyclotron motion within the trapped-ion cell and consequently yields the data that produces a real-time excitation profile. Data analysis techniques are presented that facilitate conversion of a time domain SED profile into an excitation profile. An order of magnitude decrease in the amount of time is required to acquire an excitation profile, while the precision of the measurement is improved. To demonstrate the utility of the technique, it is applied to the study of axial and radial ion loss mechanisms for argon, benzene, and acetophenone ions under different conditions. SED excitation profiles are also used to illustrate the facility of quadrupolar excitation for minimizing radial ion loss.     

Competitive ionization of tetraphenylporphyrin in a laser-generated metal ion plasma

The ionization of tetraphenylporphyrin (TPP) in a laser-desorbed metal ion plasma is examined by Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry. Competitive reaction pathways observed to generate abundant molecular ion species include electron detachment, cation attachment, charge exchange, metallation, and transmetallation in the positive ion mode and electron capture, metallation, and transmetallation in the negative ion mode. In general, cation attachment reactions dominate positive ion spectra below the laser irradiance threshold for plasma ignition, although the metallation product from [TPP]⁺ reaction with the metal atom, M, is observed. Negative ion products are not observed in the FT-ICR spectrum when a plasma is not formed. Under plasma ignition conditions, positive ion spectra include [TPP]⁺ formed by charge exchange with M⁺, which is also present in the spectrum. Negative ion spectra are dominated by [TPP]^?; which is formed by attachment to thermal electrons generated in the plasma. Metallation reactions involving TPP and the metal substrate are examined. Positive ion metallation products are observed both in the absence of a plasma through reaction of [TPP]⁺ with M and by a second pathway under plasma ignition conditions through reaction of TPP with M⁺. In negative ion mode, metallation is only observed under plasma ignition conditions through reaction of [TPP]^? with M. Observation of metallated products is found to be consistent with formation of stable metal oxidation states in the metallated porphyrin.     

Optimized Mueller polarimeter with liquid crystals

We demonstrate a Mueller polarimeter in which the polarization-state generator and analyzer are both composed of a linear polarizer and two liquid-crystal variable retarders. The polarimeter is designed to optimize the accuracy of the final results by minimization of the condition numbers of the modulation and analysis matrices. The polarimeter calibration, a difficult task by conventional procedures, is achieved easily by use of the eigenvalue method of Compain et al. [Appl. Opt. 38, 3490 (1999)]. The overall polarimeter performance is tested with a linear polarizer at various angles and a compensator at various retardations.     

Amplitude and phase control of ultrashort pulses by use of an acousto-optic programmable dispersive filter: pulse compression and shaping

We demonstrate experimentally that an arbitrary phase and amplitude profile can be applied to an ultrashort pulse by use of an acousto-optic programmable dispersive filter (AOPDF). Our filter has a large group-delay range that extends over 3 ps and a 30% diffraction efficiency over 150 THz. Experiments were conducted on a kilohertz chirped-pulse amplification laser chain capable of generating 30-fs pulses without additional pulse shaping. Compensating for gain narrowing and residual phase errors with an AOPDF in place of the stretcher results in 17-fs transform-limited pulses. Arbitrary shaping of these 17-fs pulses is also demonstrated in both the temporal and the spectral domains.     

Persistent photoconductivity and dangling bonds in amorphous germanium

It was shown experimentally that persistent photoconductivity occurs in an amorphous germanium film, submitted to laser irradiation. It is demonstrated that a strong electron-lattice coupling around the dangling bonds present in the material may explain this phenomenom qualitatively.     

Remeasurement at high resolving power in fourier transform ion cyclotron resonance mass spectrometry

The Fourier transform ion cyclotron resonance mass spectrometry remeasurement experiment is demonstrated and evaluated under high resolution conditions. Signal-to-noise enhancement is observed for isotopically resolved bovine insulin peaks at a resolution of ～ 31,000 (full width at half height). The experiment is sensitive to spacecharge effects and resultant changes in scan-to-scan signal-to-noise and resolution. Coulombic repulsion in the ion cloud during the high resolution remeasurement experiment can cause the cyclotron frequency to shift through the duration of the experiment, which results in broadened peak shapes when individual remeasurement spectra are coadded. By either reducing the number of ions in the cell or allowing the ion cloud to diffuse during the lifetime of the experiment, high resolution remeasurement spectra can be coadded without peak broadening or degradation of signal-to-noise ratio.