Intensified multichannel detectors

Although the peak power of the pump laser must be high, the power of the probe laser should be kept low to avoid nonlinear effects (multiphoton absorption, stimulated Raman scattering see Section 3.9.2) and dielectric breakdown (ionization of molecules) that damage the sample. Thus, signal averaging of many pulses (high repetition rate) is made to obtain acceptable S/N ratios. Multichannel detectors such as an intensified silicon photoiode... 

Recent advances in solid state detectors have led to more efficient laser Raman spectrometers. These spectrometers are based on multichannel detectors (MCD) and are at least an order of magnitude faster than the systems based on PMT detector. However, at present these systems do not match the resolution capabilities of the scanning systems. The Spex Triplemate 1877 A is a new generation instrument which can be fitted with an intensified diode array consisting of 1024 elements. Figure 4.6.3 shows spectra obtained with these two types of detectors. 

In the past, molecular luminescence spectrometry was always conducted with single channel systems involving a photomultiplier tube (PMT) as the detector. The availability of multichannel detectors with internal gain has provided a new powerful tool for luminescence measurements, and several types of applications have been reported (1-15). This paper is concerned with the application of an intensified diode array dynamic molecular fluorescence and chemiluminescence measurements. In this paper the types of measurements and analytical systems for which multichannel detectors are used in our laboratory are introduced. Next the specific IDA system used is presented along with important hardware and software considerations. Third, the characteristics of the IDA detector are reviewed to give some perspective about its influence on the quality of measurements. Finally, some typical applications to chemical systems are presented to illustrate the advantages of multichannel detection. 

There are two practical choices for a multichannel detector, namely, PDAs and CCDs. Different detectors use different methods to transfer the photon signals into the detectable current. The CCD detector has a relatively low dark current and high quantum efficiency, especially for the intensified CCD (ICCD). 

In recent years, multichannel detectors have become more and more popular, especially intensified linear diode arrays and CCD cameras. These devices offer a sensitivity similar to that of PMTs, while a complete spectrum can be recorded simultaneously. Apart from the substantial reduction in measurement time, another advantage is the fact that in the case of signal fluctuations (e.g., due to photodegradation, hole burning) all peaks in the spectrum are equally affected. For time-resolved detection a pulsed excitation source and a fast gateable intensified camera can be used. 

Great progress has also been achieved in the field of low-level signal detection. Apart from new photomultipliers with an extended spectral sensivity range and large quantum efficiencies, new detection instruments have been developed such as image intensifiers, infrared detectors, or optical multichannel analyzers, which could move from classified military research into the open market. For many spectroscopic applications they prove to be extremely useful. 

The spectral region of interest is then detected and processed by means of an optical multichannel analyzer (EG G-PARC model 1215, OMA). The OMA detector SIT (silicon intensified target), EG G-PARC model 1254 has been used. It is operated by the detector controller, EG G-PARC, model 1216, which performs the signal digitization as well. The acquired spectra are displayed in real time on a TV display and on the OMA console. The data storage and processing are also performed by that console which has a 28K of 16 bit core memory and a floppy disk for permanent storage. 

The apparatus consists of a microscopic optical system, a grating mirror combination (Fig. 1A, IB) for spectral and topographic measurement, and an optical multichannel analyzer (OMA) equipped with a silicon intensified tube (SIT) as a photon detector (55). 

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