2.3.2 FT-UV-Visible spectroscopy The UV-Vis spectral regions are classically defined as: far UV, 1,000,000-50,000cm-1 (10-200nm); near UV, 50,000-26,300cm-1 (200-380nm) and visible, 26,300-12,800(380-780nm). The absorption of UV-Vis radiation generally results in the excitation of bonding electrons, such as electron transitions involving π,σ,n,d and f electrons as well as charge-transfer electrons. Consequently, the wavelengths of absorption peaks can be correlated with the types of bonds that exist in the species under study. Therefore UV-Vis spectroscopy is valuable for identifying functional groups in a molecule and also quantitative determination of compounds containing absorbing groups.
Dediacted monochromator-based UV-Vis spectrometers, such as the Nicolet Evolution series, are widely used for UV-Visible spectroscopic measurements. Research -grade Nicolet FT-IR spectrometers can be optionally configured to cover the near-UV and nearly full range of visible spectroscopic regions (27000-128000cm-1). This configuration contains a quartz-halogen light source (27000-2000cm-1), a quartz beamsplitter (27,000-2,800cm-1), a silicon detector (2,700-8,600cm-1), and wide and narrow blue filters. Blue filters are used to optimize thje spectral result, to block any unwanted energy from reaching the detector (such as interference from the He-Ne laser at 15798cm-1 or energy at lower wavenumbers), and to prevent folding of adjacent spectral regions into the desired region. Single-beam spectra of the quartz-halogen source from 28,000 to 2,000cm-1 with different combinations of detector (Si or PbSe), beamsplitter (Quartz or CaF2) and optical filter (narrow blue, wide blue or red) are shown in Figure 2.6. The diffuse reflectance spectrum of yttrium chloride from 25,000 to 2,000cm-1 is shown in Figure 2.7.10 To optimize the near-IR regions, a red filter was used for spectral region 14,000-9,000cm-1, a CaF2 beamsplitter, and a PbSe detector were used for spectral region 9,000-2,000cm-1.
Chapter 3 Step-scan Phase-Resolved Spectroscopy (S2PRS) Modulation Experiments 3.1 Introduction Step-scan phase-resolved spectroscopy (S2PRS) includes three types of modulation techniques: 1)amplitude modulation (IR intensity chopping); 2) phase modulation (mirror dithering along the retardation direction or path difference modulation); 3) sample modulation (sample under an external, usually sinusoidal, physical perturbation). In S2PRS experiments, the raw detector signal can be demodulated by an external lock-in amplifer, a dedicated demodulator, or an internal DSP circuitry-based demodulator. In the early 1990's when multipe modulations were applied to the system, multiple lock-in amplifiers were required for sequential demodulations at different modulation frequencies. Today, research-grade Nicolet FT-IR spectrometers are fully equipped with internal DSP demodulators, and thus experiments requiring lock-in amplifiers have been greatly simplified. The spectrometers also keep the external input channel option when an arbitray external sinusoidal modulation and a lock-in amplifier are used, as in amplitude modulation (AM) experiments. In a typical DSP-based S2PRS data collect, two orthogonal outputs at phase angles 0 (I, in-phase) and 90 (Q, quadrature) are generated and recorded simultaneously. The following sections in this chapter will present typical applications for all three modulation techniques.
3.2 Amplitude Modulation FT-IR Electroluminescene Measurement In a step-scan amplitude modulation (AM) experiment, the intensity of the IR beam is modulated periodically by either chopping, synchronously varying the emission of a sample, or exciting the sample with synchronously varying sources. In all cases, the energy hitting the detector varies at a constant frequency. The raw signal from the detector is first fed externally to a lock-in amplifier referenced to the same modulation frequency. The maximized in-phase output signal is then digitized, recorded and Fourier transformed to obtain a step-scan AM spectrum.
A typical application of step-scan AM is the characterization of electroluminescene of light-emitting devices in the mid- and near-IR regions, with less restriction on the modulation frequency range. The Fourier frequency is eliminated in step-scan AM, and the modualtion electrical or optical excitation with a desired single frequency is applied to the entire spectrum. This allows the device modulation to be tuned in a relatively wide range of frequencies from ~10 to 100kHz. In contrast, if a continuous-scan mode is chosen, the device frequency should be at least a factor of ten times higher than the fastest Fourier frequency, fF=2vσ, in the spectral region of the measurement, where is mirror velocity (cm/sec) and σ is IR wavenumber (cm-1). This requirement restricts the range of electrical or optical modulation frequencies that can be used to produce emission output, particularly in the lower modulation frequency range (~10kHz).
Figure 3.1 shows a step-scan amplitude modulation electroluminescence spectrum of an iron-doped indium gallium aresenic phosphide (Fe:InGaAsP) LED with spectral resolution of 0.5cm-1. The phonon sidebands are associated with different acoustic, optic, and localized vibrational modes. Since the emission power is weak, on the order of 10 nW, low-duty cycles or low modulation frequency is required for this measurement. Bands at high wavenumbers around 2825cm-1 are attributed to intra-center transitions within the 5D states of the Fe2+ ions substituted for indium and isotopic shift (54Fe/56Fe). Bands between 2600 and 2500cm-1 are phonon sidebands that are associated with different acoustic, optical, and localized vibrational modes. The advantage of step-scan AM measurements on miniature and low-power output is evident in the quality of the high reolution spectrum.
3.3 Phase Modulation FT-IR - Photoacoustic (PA) Spectral Depth Profiling 3.3.1 Introduction As discussed in Chapter 1, in a research-grade Nicolet FT-IR interferometer, the step-scan phase modualtion (ФM) is produced by dithering the fixed mirror along the retardation direction at a constant frequency. One of the most significant applications of step-scan ФM is photoacoustic (PA) spectral depth profiling. Photoacoustic spectroscopy (PAS) has gaind much attention and popularity recently in chemical analysis due to a unique combination of the following features: nondestructive, non-contact measurements; simple sample preparation; depth profiling (resolving) capability; and high signal saturation limit. PAS can be used for analyzing solid samples of almost any form (polymers, rubbers, dyes, and papers, etc) in any shape (bulky, powdered, or fibriod, etc.), and it is good for spectral measurement of strong absorbers or depth profiling of heterogeneous samples. PA spectral depth profiling takes advantages of two fundamental aspects of the photothermal effect. The first is the variation of thermal probing depth with ФM frequency, and the second is the dependence of the PA signal phase on the depth (as well as on absorptivity) of the absorber(s). Both subjects will be discussed in this chapter. In addition, continuous-scan FT-IR PAS will be presented for comparison.
我的社区
签到
