Molecular beam experiments on model catalysts

Broadband spectroscopy is an invaluable tool for measuring multiple gas-phase species simultaneously. In this work we review basic techniques, implementations, and current applications for broadband spectroscopy. We discuss components of broadband spectroscopy including light sources, absorption cells, and detection methods and then discuss specific combinations of these components in commonly used techniques. We finish this review by discussing potential future advances in techniques and applications of broadband spectroscopy.

Gas-phase broadband spectroscopy using active sources: progress, status, and applications.

The development, during the last decade, of modern step-scan interferometry instrumentation has allowed FT-IR to be applied to the study of time-dependent phenomena in ways not previously possible, because of the problems of uncoupling the spectral multiplexing from the temporal domain in the continuous-scan FT-IR mode. Specifically, the time regime from tens of nanoseconds to tens of milliseconds has been accessible to time-domain measurements to only a very limited degree with continuous-scan instrumentation and not at all for modulation-demodulation (frequency-domain) experiments in this time range. The step-scan technique not only works very well in this time regime and for slower phenomena, but is only prevented from application to faster processes by the signal strength, the speed of available detectors, the intensity of sources, and the speed and sophistication of the electronics. This paper surveys the various types of experiments which are either enhanced by use of step-scan FT-IR methods or are only possible by use of these techniques. The principles of step-scan instrumentation are reviewed, particularly those of retardation control, signal generation, and data acquisition, as well as the place of step-scan FT-IR relative to other techniques of dynamic vibrational spectroscopy. The importance of path difference (phase) modulation, particularly in frequency-domain measurements, the extraction and use of the signal phase, the creation of 2D FT-IR spectra, and the strategies for acquisition of both time- and frequency-domain data in the step-scan mode are discussed and illustrated.

Investigation of Time-Dependent Phenomena by Use of Step-Scan FT-IR

We describe a newly developed ultrahigh vacuum (UHV) experiment which combines molecular beam techniques and in situ surface spectroscopy. It has been specifically designed to study the reaction kinetics and dynamics on complex model catalysts. The UHV system contains: (a) a preparation compartment providing the experimental techniques which are required to prepare and characterize single-crystal based model catalysts such as ordered oxide surfaces or oxide supported metal particles; and (b) the actual scattering chamber, where up to three molecular beams can be crossed on the sample surface. Two beams are produced by newly developed differentially pumped sources based on multichannel arrays. The latter are capable of providing high intensity and purity beams and can be modulated by means of a vacuum-motor driven and computer-controlled chopper. The third beam is generated in a continuous or pulsed supersonic expansion and is modulated via a variable duty-cycle chopper. Angular and time-resolved measureme...

A molecular beam/surface spectroscopy apparatus for the study of reactions on complex model catalysts

We present a pulsed multiplex absorption spectrometer in which the sensitivity of the cavity ring down absorption detection technique is combined with the multiplex advantage of a Fourier Transform spectrometer. A description of the Fourier transform cavity ring down (CRD) spectrometer—substantiated with first experimental results on the atmospheric band of molecular oxygen—is given. It is shown that as in the case of normal CRD spectroscopy, the measurement is independent of light intensity fluctuations provided the spectral intensity distribution of the light source is known and is constant during the measurement.

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A Fourier transform cavity ring down spectrometer

The development of a new time‐resolved Fourier transform spectrometer that is capable of 0.25 cm−1 spectral resolution and better than 10−7 s temporal resolution in the visible is reported. The time‐resolved capability of the spectrometer is achieved by coupling a step‐scan interferometer to a transient digitizer/laser system. The operation of the spectrometer is described in detail, and scattered light and laser‐induced fluorescence spectra from an I2 gas cell are presented to demonstrate the temporal and spectral resolution of the spectrometer.

Gregory V. Hartland

Papers

Time‐resolved Fourier transform spectroscopy with 0.25 cm−1 spectral and <10−7 s time resolution in the visible region