C. L. Brummel

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.

MOLECULAR DESORPTION IN BOMBARDMENT MASS SPECTROMETRIES

Abstract Molecular dynamics simulations have been performed to gain microscopic insight into those factors which influence the ejection due to ion bombardment of molecules adsorbed on solid substrates. The specific system modeled is a pentylidyne (C 5 H 9 ) film adsorbed on Pt{111} and C{111}. Lowering the binding energy of the film to the substrate from 2.7 to 0.6 eV increases the total yield of ejected particles. The bombardment of the film bound by 2.7 eV results in considerable fragmentation of the C 5 H 9 adsorbate while the bombardment of the film bound by 0.6 eV results in the desorption of the intact molecular adsorbate.

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.