Energetic Processes (Surfaces/Solids) Instrumentation w/Lasers

EMSL offers a suite of instrumentation dedicated to understanding photoreactivity in the condensed phase, on surfaces, and at material interfaces. In addition, a variety of continuous wave and pulsed laser sources are available that are capable of providing tailored photoexcitation from the extreme ultraviolet through mid-infrared. Pulse durations of these instruments range from several nanoseconds to less than 100 femtoseconds.

Laser desorption and ablation from surfaces is a powerful experimental tool used for studying the energetics and dynamics of the interaction of molecules with surfaces. The coupling of surface science, solid-state physics, and laser technologies makes possible the measurement of total energy disposal and redistribution in material decomposition processes. Previously, these experimental methods were used to acquire detailed measurements of molecules desorbing from ionic molecular substrates. While such experiments have resulted in a fairly detailed understanding of the laser surface chemistry of ionic crystals, researchers currently do not have a similar understanding of the elementary dynamical and kinetic processes occurring on metal oxide surfaces. Such interactions are clearly important from an environmental viewpoint, since they form the molecular-level basis for the complex physiochemical processes that take place on the aqueous-mineral (geochemical) interface. The goal of EMSL researchers is to apply and extend laser desorption techniques to these systems in an effort to elucidate the relevant interactions. Toward this goal, EMSL researchers have constructed instrumentation that will allow for investigation of the dynamics and kinetics of surface laser desorption in superb detail.

All work with this equipment and at EMSL must be performed in compliance with EMSL practices and permits.

UHV Laser Desorption/Ablation Apparatus

In EMSL's energetic processes laboratory, researchers use resonance-enhanced multiphoton ionization (REMPI) techniques to obtain state distributions and total energy information from laser desorbed species. One of the instruments in this lab—the ultrahigh vacuum (UHV) surface apparatus—consists of a liquid nitrogen-trapped, diffusion-pumped chamber (base pressure 1 x 10^-10 Torr) equipped with quadrupole and time-of-flight (TOF) mass spectrometers. Samples are mounted on standardized EMSL platens and attached to a vacuum manipulator.

Several excitation, or pump, nanosecond and femtosecond lasers are available in the laboratory for sample irradiation. These include excimer lasers at 193 and 157 nm, a Nd:YAG-pumped Raman shifter operational up to the eight and ninth order, and Nd:YAG-pumped dye lasers and broadband parametric oscillators.

The probe laser system consists of a high-power Nd:YAG-pumped "Sirah" dye laser ensemble capable of 300 mJ/pulse of tunable radiation. This output can be doubled in frequency to produce output up to 201 nm. The positive ions produced in the REMPI process are detected using a Wiley-McClaren-type TOF mass spectrometer with a nominal resolution of δ m/m = 150 at m/z = 28. Ion signals are captured on a digital scope and stored on a personal computer. The target sample manipulator is mounted on a large rotary flange assembly that allows the sample to be positioned easily in x, y, z, θ, and φ positions. This instrument is equipped with a sputter ion gun for sample cleaning, and an Auger electron spectrometer (AES) and a low-energy electron diffraction (LEED) spectrometer for monitoring surface composition and order, respectively.

A separate thin film deposition chamber with dual transfer arm capability is coupled to the UHV surface apparatus. It is equipped with residual gas analyzer quadrupole, sample heater, refractory metal oven, oxygen plasma source, quartz microbalance, and reflection high-energy electron diffraction analytical instrumentation for thin film monitoring. Thin films oxides may be routinely created here, then moved in vacuo to the the UHV surface apparatus for laser desorption/ablation studies. Through the use of "vacuum luggage" (a battery-operated ion pump assembly), the samples may be transported to other analytical instrumentation within EMSL or to other laboratories on standardized EMSL platen holders.

Ultrafast Laser System

This femtosecond laser system consists of an argon ion-pumped Ti:sapphire oscillator, which acts as a seed source for the Nd:YAG-pumped regenerative amplifier. The mode-locked oscillator operates at a repetition rate of 82 MHz, producing pulse widths less than 100 fs with an average power of ~1 W. Typical experiments performed with this system require that the low-intensity oscillator pulses (10 nJ/pulse) be amplified into the mJ/pulse range. Amplification factors greater than 10⁶ are achieved by first optically stretching or chirping an input seed pulse from the oscillator to greater than 50 ps; this is followed by two stages of amplification (regenerative and linear) in Ti:sapphire rods pumped at 20 Hz with the frequency-doubled output of a Q-switched Nd:YAG laser. Finally, the chirped amplified pulse is recompressed and yields approximately 120-fs pulses with energies approximately 10 mJ/pulse.

Wavelength tuning of the amplifier output is accomplished by adjusting the wavelength of the oscillator, which can be tuned 720 nm to 850 nm with the standard optics set; this range can be extended with different optics sets. Subpicosecond ultraviolet pulses are produced by frequency doubling or tripling the amplified fundamental. Diagnostic equipment includes optical multichannel analyzers and an imaging spectrometer fitted with a CCD camera used to monitor the spectral bandwidth of the oscillator and amplifier; a single-shot autocorrelator monitors the pulsewidth of the amplifier. Frequency resolved optical gating (FROG) is being developed to simultaneously monitor the temporal pulsewidth, spectral bandwidth, and spectral phase of the amplified pulses.

Various optical layouts are possible to perform ultrafast studies related to energetic processes in materials. High-precision, computer-controlled, optical-delay stages allow pump/probe experiments to be performed with optical delays up to 1 ns with an accuracy and reproducibility better than 20 fs over the entire delay. Samples to be investigated can be mounted either on the optical table or in vacuum systems capable of heating or cooling samples from 10 K to 1000 K.

Other experimental capabilities that can be integrated into ultrafast studies include a VUV monochromator for photo-luminescence investigations; charged particle detection, including quadrupole and TOF mass spectrometers; neutral particle detection using a second multiphoton ionization or REMPI laser system; and UHV surface diagnostic equipment (AES, LEED, XPS).

All Related Publications Related Publications

1. Excitation, Ionization, and Desorption: How Sub-band gap Photons Modify the Structure of Oxide Nanoparticles.
2. Two Pathways for Water Interaction with Oxygen Adatoms on TiO2(110).
3. Laser and Electrical Current Induced Phase Transformation of In2Se3 Semiconductor thin film on Si(111) .
4. Energy and Site Selectivity in O-Atom Photodesorption from Nanostructured MgO.
5. Acetaldehyde photochemistry on TiO2(110).