Robert M. Braun

Performance characteristics of a chemical imaging time-of-flight mass spectrometer.

A chemical imaging time-of-flight secondary ion mass spectrometer is described. It consists of a liquid metal ion gun, medium energy resolution reflectron mass analyzer, liquid nitrogen cooled sample stage, preparation chamber and dual stage entry port. Unique features include compatibility with laser postionization experiments, large field of view, cryogenic sample handling capability and high incident ion beam current. Instrument performance is illustrated by the characterization of scanning electron microscopy grids, silver and functionalized polystyrene beads and the postionization of an organic overlayer on a gold substrate.

Postionization of molecules desorbed from surfaces by keV ion bombardment with femtosecond laser pulses

We report the use of femtosecond laser photoionization of sputtered neutral molecules to enhance the sensitivity of detection and to improve the prospects for molecule-specific imaging experiments. Results are presented for patterned metal oxides, polycyclic aromatic hydrocarbons and several amino acids. In addition to increased signal levels, we find that is photoionization generally yields simpler mass spectra than the corresponding SIMS spectra, although considerable fragmentation is observed in both cases.

Coverage-dependent Bond Length of Chlorine Adsorbed on Cu{111}

The adsorption of Cl 2 on Cu{111} at 300 K has been studied using shadow-cone-enhanced secondary ion mass spectrometry (SIMS). The system has been investigated for chlorine coverages ranging from 0.08 to 0.33 monolayer (ML), including the Cu{111}-(√3 x √3)R30°-Cl surface. The secondary Cu + ion intensity has been measured as a function of the incidence angle of the primary ion beam. The enhanced intensity features in the spectra are compared with results from a two-body interaction calculation that uses the Moliere approximation to the Thomas-Fermi potential. A chlorine-copper interlayer spacing of 1.87 ± 0.04 A is measured between the coverages of 0.17 and 0.33 ML. This value corresponds to a chlorine-copper bond length of 2.38 ± 0.04 A. At 0.08 ML, the chlorine-copper bond length is expanded to 2.48 ± 0.04 A. These results suggest that the chlorine-copper bond is more ionic in the low coverage limit.

Surface and depth profile investigation of a phosphorylcholine-based contact lens using time of flight secondary ion mass spectrometry

The dehydrated surface of a commercially available contact lens containing hydroxyethyl methacrylate and phosphorylcholine is investigated by time of flight secondary ion mass spectrometry employing a 25keV Bi3+ ion beam. Results show the successful detection of hydroxylethyl methacrylate and phosphorylcholine species from an Omafilcon A lens. Utilization of a 20keV C60 ion beam allowed the bulk region of the lenses to be probed using primary ion dose densities exceeding 2×1014C60∕cm2 and indicated that the phosphorylcholine component reorganizes below the surface. X-ray photoelectron spectroscopy results are consistent with the presence of these moieties and suggest that the phosphorylcholine components may be below 100A in the dehydrated hydrogel.

Development of a New Ti:Sapphire Laser System for Femtosecond Laser Ionization at kHz Repetition Rates

We have developed a new laser system which operates at a repetition rate of one kHz and generates 3.5 mJ/pulse with 85 fs pulse widths at 800 nm. It is composed of a self‐mode locked oscillator followed by a pulse stretcher, a regenerative amplifier and post‐amplifier which are pumped by frequency doubled Nd:YAG lasers, and a compressor. The new post‐amplification stage has a four‐pass, nearly collinear geometry. This allows for improved power and shorter pulses while still providing high repetition rates. Fast repetition rates are essential for imaging experiments using a time of flight (TOF) mass spectrometer since a fast rate decreases the probability of sample drift during image acquisition and decreases the amount of time needed per image.

Design of an ultrahigh vacuum direct-drive, cryogenic sample manipulator providing two degrees of rotational freedom

An ultrahigh vacuum direct‐drive, cryogenic sample manipulator is described. It is fully bakeable in UHV, uses no lubricants, and does not require differentially pumped seals. It provides independent polar and azimuthal rotation of a sample, three degrees of translational freedom, electron‐beam heating, as well as cooling to 100 K using liquid nitrogen.