Time-Resolved Fluorescence From Dispersed Organic Material Of Permian Basin Shales By Picosecond Laser Microscopy

Time-resolved fluorescence from dispersed organic material of Permian Basin shales bypicosecond laser microscopyM.W. Pleil, C.R. Landis, W.L. Borst Texas Tech University, Department of Physics, Lubbock, Texas, 79409ABSTRACTA picosecond fluorescence microscopy technique is applied to the liptinite maceral, alginite, of the Woodford Shale. Lifetime measurements indicate that two dominant fluorophores are responsible for the observed fluorescence. The lifetimes can be grouped into two characteristic ranges from 50 to 200ps and 500 to 900ps. There are indications that the higher rank (more mature) samples have shorter component lifetimes which may explain the lower fluorescence intensity witnessed with increasing rank. Continuous wave (c.w.) spectra are also given.1. IntroductionFluorescence microscopy has been a useful analytical tool for characterizing organic material in coal and sedimentary rocks. In 1983, we began to extend fluorescence analysis from the wavelength to the time domain with our fast analog technique. 1 " 6 Some of the fluorescence decay times were at the limit of this technique (~200ps). Therefore, in 1987, a new mode locked, synchronously pumped, cavity dumped dye laser system was acquired (described below), and a single photon counting technique has been implemented. Writing the necessary data fitting routine, testing the equipment and becoming familiar with the new laser required much time and care.First results of the weak fluorescence from the microscopic organic materials (macerals) were obtained and are presented here.2. System Description and MethodThe picosecond laser microscopy system is shown schematically in Figure 1. Some operating specifications are given in Table 1. A synchronously pumped, cavity dumped dye laser (consisting of items 1-6 in Figure 1) is used as one of two different pulsed laser excitation sources available in our laboratory. (The older pulsed laser source is a nitrogen pumped tunable dye laser which is used with our fast analog technique, described elsewhere.1 ~ d )The Nd:YAG laser is mode locked and provides narrow pulses at 1.06jnm (approximately 100-200 ps FWHM) at a 82.0880MHz repetition rate. The average output power from the Nd:YAG laser is 7W. The light from this laser is frequency doubled with a high-quality KDP crystal. The resulting 530nm green light with an average power of 0.8 - 1.OW pumps the dye laser, which currently employs a Pyridine-1 dye. The laser is tuned at approximately 700 nm. The dye laser also is cavity dumped allowing a variable repetition rate from single shot to 4MHz. The current 800kHz rate gives an average power of about 50mW (lOOmW at 4MHz). The red output from the dye laser is split into two beams, one for doubling and eventual sample excitation (90%) and the other for monitoring by a photodiode. The output of the latter is used as a start pulse for the time-to-amplitude converter (TAG). The doubled red beam (345-350nm) has 0.5mW average power at a 800kHz repetition rate (<10ps FWHM). A half width of 3ps was determined at time of laser installation using an autocorrelator. The laser is expected to achieve approximately 3. OmW in the blue (200mW in the red) at 800kHz with suitable optimization.The near-ultraviolet output is linked to our Leitz MPV3 microscope system (item 10) via a liquid light guide. The beam is directed through the objective to the sample. The emitted fluorescence is redirected through the objective to the emission monochromator (l-7nm resolution). The final wavelength selected output is detected by a two-stage multichannel plate (MCP) photomultiplier tube (Hamamatsu R1564U-1). Further information on the microscope and c.w. analysis may be found elsewhere.4 ' 5 ' 6 The output of the MCP is amplified resulting in 130 - 250mV single photon pulses (~500ps FWHM).The fast photodiode and amplifier output are led into seperate discriminators (items 13 and 14). Leading edge triggering is currently employed since these disciminators are not fast enough to be used in the constant fraction mode without substantial modification. The standard TTL outputs from the discriminators are used as the start (photodiode) and stop (MCP) signals for the TAC. The output of the TAG is directed to a multichannel analyzer board (Nucleus PCA 2000) installed in the IBM/AT computer.