Low-light level spectroscopy

Sihcon charge coupled devices (CCDs), commonly used in soHd-state video cameras and in research appHcations, are being appHed to low light level spectroscopy appHcations. The main advantage of area array CCDs over linear photodiode detectors is the two-dimensional format, which provides simultaneous measurements of spatial and spectral data. 

ISPD. The ISPD provides a linear dynamic range (1-32) of at least 16,384 1 Q4 bit A/D converter). At illumination levels above 4 x 10"5 foot candle (with a corresponding output brightness of approximately 1 foot lambert), the MCP saturates and thereafter has a non-linear transfer characteristic. However, this level is way above the A/D converter saturation level and has no implications on low light level spectroscopy. 

The primary purpose of this paper has been to provide a convenient reference for researchers who are interested in applying OIDs for low-light level spectroscopy and who need a source of detailed information about these devices to enable them to select the proper detector for a particular application. 

The scope of this discussion will be limited to those spectroscopic applications where low light levels are involved. It is in these applications where most of the problems occur, and where an in-depth knowledge of detector performance is required in selecting a detector for a particular application. By definition, in this category are included all "photon-starved" areas of spectrometry. It is instructive to consider some of the causes of photon starvation in spectroscopy. 

This paper will concentrate on the unique requirements of aeronomic spectroscopy and on the application of image devices to these measurements. Spectrometer 1, Table I, was developed for rocket experiments intended to measure the NIR absorption spectra of 1 0 and 02 molecules in the middle atmosphere. A photodiode array was used as the spectrometric sensor. With this spectrometer we were able to measure the NIR solar radiation spectrum with an altitude resolution better than 2 km. Spectrometer 2, Table I, was basically of the same design as spectrometer 1, except that an image intensifier was optically coupled to the diode array to permit low light-level measurements. The resolution of this spectrometer was adequate for measurements of rotational profiles of the A-band absorption spectra of 02 molecules. We were able to measure the rotational temperature of oxygen molecules, in the stratosphere and the lower mesosphere with an accuracy of + 1.5°, and a spatial resolution better than 2 km. These experiments provided the basis for study of the dynamic processes of atmospheric molecules. Spectrometer 3,... 

The most important applications for diode array systems are in molecular spectroscopy, since in general they do not have the resolution necessary for atomic spectroscopy. In molecular spectroscopy the most useful areas of application are for (1) scanning fast reactions to determine kinetics, (2) applications involving low light levels because spectra can be stored and added to each other, increasing the intensity, and (3) detectors for HPLC and capillary electrophoresis (CE). HPLC and CE are discussed in Chapter 13. 

The present equipment, based on Fourier-transform spectroscopy, makes use primarily of the so-called "throughput advantage", i.e., the data corresponding to all wavelengths of light are collected simultaneously from an extended source. These characteristics increase the sensitivity for TL emission spectroscopy and have achieved the aim of making it possible to study TL emission phenomena at low radiation levels, comparable with those received by the mineral or phosphor samples during their actual application. 

Thermionic dark current emission occurs whenever thermally produced electrons are accelerated in the dynode train. The detection of low light energy levels is limited by the thermionic dark current. Thermionic noise cannot be balanced out of the measurement. Thermionic noise can be decreased by operating the multiplier phototube at reduced temperatures and in some applications this is done. For most analytical spectroscopy applications, however, the lower thermionic noise obtained at lower temperatures is not worth the considerable inconvenience of operating at low temperatures. 

The low MW power levels conuuonly employed in TREPR spectroscopy do not require any precautions to avoid detector overload and, therefore, the fiill time development of the transient magnetization is obtained undiminished by any MW detection deadtime. (3) Standard CW EPR equipment can be used for TREPR requiring only moderate efforts to adapt the MW detection part of the spectrometer for the observation of the transient response to a pulsed light excitation with high time resolution. (4) TREPR spectroscopy proved to be a suitable teclmique for observing a variety of spin coherence phenomena, such as transient nutations [16], quantum beats [17] and nuclear modulations [18], that have been usefi.il to interpret EPR data on light-mduced spm-correlated radical pairs. 

---