The ΦM frequency should be selected based on the characteristics of the detector and other constraints of the experiment. For example, in a polymer stretching experiment where multiple modulations are used, the ΦM frequency and the sample modulation frequency should be separated by a factor of 10 or larger. For PAS depth profiling, different ΦM frequencies are used to vary the sampling depth as described in Chapter 3 of this book.
1.4 Overview of advanced FT-IR Applications Research-grade Nicolet FT-IR research spectrometers offer a full range of step-scan operation modes, and dual-channel continuous-scan mode for polarization modulation/demodulation experiments ,as well as conventional single-channel continuous-scan operation. The spectrometers are equipped with a series of highly integrated synchronous sampling technique (SST) modules. The open architecture design of these SST modules allows the research spectrometer to be configured to perform many advanced experiments. Two electronically matched digitizers are used independently or simultaneously for step-scan time-resolved or polarization modulation/demodulation dual-channel experiments. Each digitizer has an independent, software-controlled amplifier and set of electronic high- and low-pass filters. The dual-channel capability allows simultaneous acquisition of two channel signals. For example, in a polarization modulation-based experiment such as infrared reflection absorption (PM-IRRAS), vibrational linear dichroism (VLD) or vibrational circular dichroism (VCD) measurements, both the static (reference) and dynamic differential spectra can be measured simultaneously.
Typically, the advanced experiments that research-grade Nicolet FT-IR sepctrometers perform can be classified into nine broadly defined experimental categories. These categories are based on different operation modes and demonstrated with sixteen representative applicaition examples. The nine experimental categories include: 1)Extened spectral range experiments (27000-15cm-1, i.e. from far-IR up to near ultra violet) 2)High resolution spectroscopy (better than 0.09cm-1 for gas phase measurements) 3)Single-channel rapid-scan kinetics (77 spectral/sec at 8cm-1 spetral data resolution) 4)Dual-channel polarization modulation spectroscopic experiments(IRRAS, VLD, and VCD, absorbance level on the order of 10-3 to 10-5) 5)Step-scan amplitude modulation (electroluminescence measurement) 6)Step-scan phase modulation (photoacoustic depth profiling) 7)Step-scan sample modulation (polymer streching, liquid crystal dynamics, and spectro-electrochemistry) 8)Step-scan time-resolved spectroscopy (ns chemical kenetics, polymer streching, liquid crystal dynamics and photoacoustic depth profiling) 9)Step-scan space-resolved spectroscopy (focal plane array detector-based IR imaging) The potential for other modulation and TRS experiments is unlimited. Combined techniques from different categories may also be used, enabing experiments such as TRS-based PM-IRRAS, kinetic PM-IRRAS and so forth. The relationship between these typical experiments is summarized in Figure 1.10. In the following chapters ,these experiments will be addressed in more detail.
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