Reflection mode spectrometer

In a reflection mode spectrometer of high finesse or quality factor Q, the reflected power when matched on resonance is many decibels below the incident power, which reduces the noise floor by many decibels with respect to a transmission mode resonator. When the ESR sample is resonant, the residual resonator mismatch changes, which causes the reflected power to change, and a small signal on a low background is presented to the detector. In order for the signal to be detected, however, it must be discriminated from the radiation incident on the resonator, just as in a conventional reflection mode ESR spectrometer. 

Although all laboratories that perform high-field ESR have experimented with Fabry-Perot resonators instead of fundamental mode microwave cavities, few laboratories have as yet explored quasioptical implementations of common microwave devices such as a magic T or circulator in an FIR-ESR spectrometer (see Earle and Freed, 1995 Earle et al. 1996b Smith, 1995). Part of the problem is the unfamiliar appearance of optical circuits to spectroscopists who are only familiar with 

KEITH A. EARLE, DAVID E. BUDIL AND JACK H. FREED 

Let us now trace the polarization evolution of a Gaussian beam as it traverses the optical system shown in Fig. 7. Polarizer P passes radiation linearly polarized at 45° (L450) with respect to the normal to the plane of the page. The polarization-transforming reflector (PTR) converts linearly 

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Fig. 3. Demonstration of the performance of the reflection mode spectrometer compared to the transmission mode spectrometer, (a) EPR spectrum of polyanilane at 170 GHz. The signal-to-noise ratio is 530 1. (b) EPR spectrum of polyaniline at 250 GHz. The signal to noise ratio is 180 1. (From Earle et al. (1996b), by permission of the AIP.]...

Improvements in the single side-band performance of a mixer-based receiver can be made by filtering the unwanted side band before it is down-converted in the mixer. Such a scheme, which is described in detail by Goldsmith (1982) is based on interferrometric techniques. We will not discuss single side-band filtering any further, except to note that it is a particularly apposite demonstration of the use of optical techniques to process the radiation in the spectrometer. We will discuss the use of interferometric techniques in Section IX as a means to realize a reflection mode spectrometer. These few examples indicate the flexibility of application of optical techniques to problems of instrument design in the FIR. 

We discuss in Section IX techniques for constructing a reflection mode spectrometer based on quasioptical techniques that will have a receiver input bandwidth A/ = 25 GHz. The quasioptical component that accomplishes this function is a Polarization Transforming Reflector (PTR) discussed in Howard et al. (1986) We use the PTR as a wave plate that rejects noise components outside of a narrow band. We discuss this point further in Section IX. Based on Eq. (78), this should allow a further reduction in Frt, min factor of 2 compared to the current spectrometer. We may... 

One simple method for varying the coupling is to construct the resonator from two polarizers. We can show (Tudisco, 1988) that the finesse of such a resonator is proportional to cos, where is the relative orientation of the two polarizers. This device is the quasioptical analog of the cavity coupling scheme of Lebedev (1990). There are several limitations to this scheme as pointed out by the author, namely, the radiation must be linearly polarized, which complicates transmit-receive duplexing in a reflection mode spectrometer on resonance, the power minimum occurs in transmission, which precludes using the device in a reflection mode spectrometer if we wish to work with low background levels. 

We have presented a complete analysis of the Cornell mm-wave spectrometer using quasioptical techniques. We also have developed a quasioptical formalism with sufficient flexibility to predict the performance of a novel reflection mode spectrometer with variable input coupling and transmit-receive duplexing based on polarization coding and we present a practical realization of these design concepts in Earle et al. (1996b). At every stage we have chosen parameters that correspond to practical performance values and measured response. 

A laser-induced ToF mass spectrometer (LIMA-2A) was manufactured by Cambridge Mass Spectrometry Ltd., Cambridge, UK, for micro local analysis and was used to analyze thin sections of biological samples in the transmission mode or bulk material in the reflection mode.150,151 Typical LIMA applications in microelectronics include identification of impurities in dielectrics, microlocal analysis, depth profiling, thick film analysis and investigations on hybrid circuits. 

Figure 10.21—Optical path of the 1R beam in an 1R microscope). The sample can be examined in transmission or in reflection mode. This accessory can be installed on a spectrometer that has a deflected beam.

The UV-Vis spectra were recorded on a Perkin-Elmer Lamda 16 spectrometer in the diffuse reflectance mode. ESR spectra were recorded at X-band on a Bruker ESP 300 spectrometer at 77 K The samples were loaded into 3 mm o.d by 2 mm i d. Suprasil quartz tubes, which were evacuated to a final pressure of 10 5 kPa at 695 K overnight. The activated samples were sealed, immersed in liquid nitrogen and finally exposed to y-irradiation of a 6(lCo source to a total dose of 1.1 Mrad at a dose rate of 0.18 Mrad h 1 before the ESR experiments... 

The FT-IR spectra were obtained on a Nicolet IR-300 spectrometer from Thermo-Electron Corp. The spectra were recorded either in transmittance or in reflectance mode using respectively KBr or Ge as windows. 

The experimental aspects of ex situ FUR spectroscopic studies of sensitive electrodes (e.g., Li and Ca surfaces) using a purged spectrometer (not in a glove box) have been described in detail in Refs. 36-38, 92, and 93. Briefly, it is possible to analyze thin surface films on active metal, using a reflectance mode, while the active surface is in contact with a KBr or NaCl polished window. In Refs. 37 and 38, a possible organization of the measurement chamber of an FTIR spectrometer for such measurements is described. It should be noted, however, that the performance of such measurements when the spectrometer is placed in the glove box is much more elegant and easier. 

This technique has recently been extended to the much more useful reflection mode as the LAMMA 1000 by Leybold Heraeus(18,20) and the LIMA by Cambridge Mass Spectroscopy Co.(21,22) In these Instruments, the region of Interest is located under a microscope. The laser beam is focussed and pulsed through a microscope, producing a plasma that fragments and ionizes the molecules, which are then analyzed in a time of flight mass spectrometer. One of the major parameters to be controlled and measured is the laser power, to get reproducible fragmentation ... 

In the present studies, a Fourier-transform infra-red spectrometer in attenuated total reflectance mode (FT-IR-ATR) was used to characterize the albuminated polymer membrane surface. FT-IR-ATR is a powerful surface analysis technique in which a spectrum of only a few micron thick surface layer is obtained. As albumin or enzymes were attached only on the surfaces of polypropylene, FT-IR-ATR technique was ideally suited for the their analysis. 

Figure 10.19 FTIR imaging system. Left, an optical pathway in an IR microscope associated with a spectrometer. The sample can he examined in transmission (left), or in specular reflection modes (right). This accessory is installed in a spectrometer whose beam is deflected. The Cassegrain optic has the advantage that the light is reflected at the surface of mirrors rather than having to pass through optical lenses. Right, model 400 UMA (reproduced courtesy of Varian Inc. USA).

The IR spectra were recorded with a Nicolet NEXUS FTIR spectrometer (spectral range down to 400 cm", resolution 2 cm ) using diffuse reflectance mode. The samples were diluted (ca 2 wt%) and finely ground with dried KBr. 

Nimaiyar (2004) used a Perkin-Elmer Spectrum One Fourier-Transform NIR (FT-NIR) spectrometer to scan ground soybean samples in reflectance mode. She had a calibration set of 74 samples and a validation set of 24 samples. Of 15 amino acids, she was able to best predict (% of dry weight) aspartic acid, serine, glutamic acid, and alanine. Threonine and tyrosine could not be predicted. 

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