We report the results from a new pump/probe spectrometer for potential use in combustion diagnostics that employs asynchronous optical sampling. The instrument consists of two frequency-doubled mode-locked Nd:YAG lasers operating at slightly different repetition rates, synchronously pumping two dye lasers (rhodamine 6G) to generate the pump and probe beams. The spectral and temporal capabilities of the instrument are examined by obtaining a spectrum and an excited state decay of rhodamine B. The instrument response is shown to be proportional to pump power, probe power, and sample absorptance. Different frequency synthesizers and different modes of triggering are used to study their effect on signal stability.

We report the first results from a new pump/probe technique called asynchronous optical sampling (ASOPS). The method employs a mode-locked, frequency-doubled Nd:YAG laser operating at a repetition rate of 81.5970000 MHz as the pump laser, and a synchronously pumped dye laser (R6G) operating at a repetition rate of 81.5870000 MHz as the probe laser system. The 10-kHz beat frequency produces a repetitive relative phase walk-out of the pump and probe pulses which replaces the optical delay line used in conventional instruments. Studies of rhodamine B in methanol demonstrate that the instrument response is proportional to pump power, probe power, and sample absorptance. The fluorescence lifetime of 4 × 10−5 M rhodamine B is determined to be 2.3 ns.

Genetic similarity was estimated among a sample of 28 ecotypes of Arabidopsis thaliana. Twenty-five previously mapped genomic clones were used as probes in Southern hybridizations to detect restriction fragment length polymorphisms (RFLPs). A total of 62 polymorphic restriction fragments were classified as to their presence or absence for each genotype. The genetic similarity between each pair of ecotypes was calculated as the ratio of concordant to total bands scored. The mean genetic similarity among the 28 ecotypes was 0.69 and ranged from 0.32 to near 1.0. No relationship was observed between genetic similarity and geographical origin of the 28 ecotypes. The ecotype most distantly related to the other 27 was Niederzenz, with a mean genetic similarity of 0.55 ± 13. A bootstrap procedure was used to generate 200 random samples of bands of size n (n=8,12,16,..., 55), and the coefficient of variance (CV) was estimated for each sample. The plot of the first two principal components provided a description of the relative genetic similarity among ecotypes. The results provide information useful to investigators interested in sampling the genetic variation among Arabidopsis ecotypes.

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We report the first two-photon-excited hydrogen-atom fluorescence measurements in flames made to our knowledge. The n = 3 level of the H atom was excited by 205.1-nm radiation generated by Raman shifting a 224-nm beam produced by frequency mixing. Fluorescence was observed at 656.3 nm as a result of radiative decay from n = 3 to n = 2, the Balmer-alpha transition. A novel technique, photoionization-controlled loss spectroscopy, is proposed to eliminate the quenching dependence of the fluorescence signal.