Volume 78, Issue 5, May 2007
Index of content:
Photoacoustic (PA) infrared spectroscopy enables the characterization of a wide variety of materials, affording the spectroscopist several advantages over more traditional infrared methods. While PA spectra are readily acquired using commercial instrumentation, the quality of the data can be improved substantially through the use of specialized numerical and experimental procedures. Two of these methods are the subject of this review. Specifically, this article describes (a) linearization of PA infrared spectra, a calculation that incorporates phase and amplitude information to extend the range of linearity for strongly absorbing samples, and (b) lock-in and digital signal-recovery procedures in step-scan phase-modulation PA infrared spectroscopy. Linearization yields significant improvement in band definition, especially in the low-wavenumber region. This numerical method succeeds in situations where the PA phase of the sample is less than that of the reference (carbon black). When this criterion is not met initially, the sample or reference interferograms can be manipulated prior to the calculation. The steps involved in linearization are illustrated in detail and approximations are discussed. Lock-in demodulation of the step-scan phase-modulation signal is compared to digital (software) demodulation in this study; the lock-in technique is found to be superior in several cases. The imaginary interferograms in these experiments sometimes lack a strong central feature, a situation that necessitates the application of less commonly used methods for phase correction and spectrum calculation. These methods, which are available in commercial software, include two-quadrant and stored-phase corrections. The PA phase spectrum resembles amplitude and absorption spectra when real and imaginary PA spectra are correctly calculated.
Micromechanical sensor for studying heats of surface reactions, adsorption, and cluster deposition processes78(2007); http://dx.doi.org/10.1063/1.2740165View Description Hide Description
We present a newly designed highly sensitive micromechanical sensor devoted to thermodynamic studies involving supported clusters. The thermally sensitive element of the sensor consists of a micromachined silicon cantilever array, onto which a thin metalfilm is evaporated. Due to the difference between the thermal expansion coefficients of silicon and the metal employed, thermal bending is observed when heat is exchanged with the cantilever. The sensitivity and the response time of the cantilever are studied as a function of the film material (gold or aluminum) and the thickness of the metalfilm. With our routinely prepared cantilevers, a minimum power of is measurable with a submillisecond response time, corresponding to a limit of detection in the femtojoule range. The high sensitivity of the sensor is demonstrated by measuring the heat exchange which occurs during the deposition of clusters on the cantilever. Experimentally, we illustrate the 1,3-butadiene hydrogenation reaction using a cluster model catalysts created by soft-landing palladium clusters onto the cantilever surface.