Kenneth F. Willey

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.

Ion beam induced desorption with postionization using high repetition femtosecond lasers

Abstract Static SIMS has shown itself to be a powerful tool for surface analysis with high chemical specificity. Because of the destructive nature of the sputtering process, high spatial resolution analysis (sub-200-nm regime) requires very high yields of emitted ions since there is a very limited amount of material in each image pixel. Generally the sputtered neutral yield is significantly greater than the secondary ion yield. To attain the high sensitivities required it is therefore critical that the ejected neutrals be ionized efficiently with minimal fragmentation. This paper explores the application of a high repetition rate Ti:sapphire based femtosecond laser to the ionization of sputtered and gas phase species from a variety of representative materials including silver, indium, tryptophan, benzo[ a ]pyrene, p -nitroaniline and polystyrene. The effects of photon wavelength (800, 400, and 266 nm) and power density on ionization and molecular fragmentation have been studied.

Photoionization mechanisms for Cr(CO)6 using high intensity laser pulses in the near-IR

Abstract High intensity (i.e. 10 13 W/cm 2 ) picosecond and femtosecond laser pulses are utilized to study possible photoinization mechanisms of Cr(CO) 6 . This work represents the first example of excitation of a metal carbonyl with 800 nm photons where the ionization of intact molecular species dominates photofragmentation channels. With picosecond pulse excitation, however, attenuation of the laser power appears to induce a higher degree of fragmentation. The data can best be interpreted assuming two distinct photoionization mechanisms where MPI dominates at low laser powers and a transition occurs to barrier suppression ionization as the laser power is increased.

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.

Photoionization studies of small biological molecules using femtosecond laser pulses

We report on the fs photoionization and dissociation dynamics of small biological molecules sputtered from surfaces and in the gas phase. Femtosecond postionization studies of ion-beam desorbed dopamine at 800, 400, 267, and 200 nm are presented and compared to the fragmentation patterns observed for gas phase dopamine under similar laser conditions. Differences in the amount of fragment ions originating from the molecular ion suggest that dopamine undergoes extensive fragmentation during the sputtering process. We also present fs postionization data for the amino acid alanine at 267 and 200 nm along with its positive SIMS mass spectrum.

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.

Photoionization Mechanisms of Metal Carbonyls with High Power Femtosecond Laser Pulses

High intensity pulses are utilized to explore the photoionization mechanisms involved during femtosecond excitation. A molecular ion signal is observed for both Fe(CO)5 and Cr(CO)6 at all wavelengths studied (i.e. 800, 400, and 266 nm). Two distinct ionization mechanisms are proposed. MPI dominates when the multiphoton cross section is large. If the neutral molecules, however, are exposed to sufficient laser intensity barrier suppression ionization occurs. At our highest obtainable powers for 800 nm fs excitation, intact doubly and triply charged molecular ions are observed.

Photoionization and molecular fragmentation using nanosecond and femtosecond lasers

We have incorporated a Ti:sapphire laser coupled to a regenerative amplifier to our TOF‐SIMS postionization system. The Ti:sapphire laser system operates at 1 kHz and produces pulses at 800 nm (1.1 mJ). 400 nm (470 μJ) and 266 nm (150 μJ). The kHz repetition rate reduces data acquisition times and the short pulse duration provides softer ionization than was possible with a YAG based 30 Hz ns laser system. We compare data from the ns and the fs system. We also compare molecular photoionization at 800, 400 and 266 nm.