The resonance enhanced multiphoton ionization (REMPI) spectrum of O2 a 1Δg between 305 and 350 nm is reported. The spectrum is compared to the REMPI spectrum of ground state molecular oxygen in the same electronic energy region, and a number of differences in the respective spectra are observed. Detection limits for O2 a 1Δg by REMPI is calculated to be 5×109 molecule cm−3 in the ion source of the mass spectrometer.
The mental models of both novice and advanced chemistry students were observed while the students performed a periodic table activity. The mental model framework seems to be an effective way of analyzing student behavior during learning activities. The analysis suggests that students do not recognize periodic trends through the examination of elemental data. Even simple relationships (e.g., the sequence of atomic mass values) proved important to novice understanding of the trends in the periodic table. The use of common heuristics for decision making, and well-known common errors associated with those heuristics independent of the students' chemistry background, were also observed.
Thermal lens spectrophotometry employlng a pulsed TEA-CO, laser excltatlon source Is applled to quantlflcatlon of trace amounts of dlchlorodlfluoromethane. The high radiation flux density or lntenslty of thls eTcltatlon source results In optlcal saturation of the Infrared transltlon. The resultlng slgnals show slgnlflcant devlation from the slmple theories used to describe the thermal lens process. The detectlon llmlt of dlchlorofluoromethane observed In these experiments Is 10 ppbv In argon at a total pressure of 13.3 kPa (100 torr), whlch Is extrapolated to an atmospheric pressure detection llmlt less than 1 ppbv. These detection llmlts correspond to 0.3 pg of difluorodlchloromethane in the lrradlated volume. The detectlon llmlt was found to be bound by slgnals due to gas hpurltles, wlndows, and most likely hot band transltlons of CO, in air. Suggestlons for further Improvements on the experimental setup are dlscussed.Thermal lens spectrometry (TLS) is one of several calorimetric techniques used for high-sensitivity spectrophotometry ( I ) . Since the first reported recognition of the thermal lens effect in 1964, there has been much advancement in the theory (2-6) and practice (6-8) required for analytical application of this technique. For an analyte that is weakly fluorescent, electromagnetic energy absorbed and not lost by subsequent emission will result in heating of the medium. The TLS signal is derived from a change in the refractive index resulting from this heating. Because TLS is dependent on "dark" relaxation mechanisms, it serves as a complimentary technique to the very sensitive laser excited fluorescence technique, where the signal is derived from the fluorescence of the excited analyte. The TLS signal, on the other hand, is derived from the defocusing of an optical beam as it propagates through a medium with a radially symmetric refractive index gradient. This index gradient is a consequence of the temperature rise induced by thermalizing relaxation of the excited state. TLS is thus a second-order effect where the optical signal is not necessarily derived from the radiation exciting the transition but can be derived from a second light source which "probes" the thermal lens.This fact has lead to the development of optical configurations that have evolved along two different lines ( I , 7). The single-laser optical configuration uses a continuous wave laser to both excite the analyte and probe the resulting thermal lens.The two-laser design uses one laser to excite the analyte and a second laser to probe the thermal lens. The sensitivity of the single-laser configuration is less than the two-laser one, but it has the advantage of being easier to align (7). There are advantages to the two laser configuration which make it a more convenient choice when dynamic or spectroscopic measurements are to be made ( I ) . The advantages to this technique are (1) the pump-laser wavelength can be varied so that excitation spectra can be obtained (9), (2) high-intensity pulsed lasers can be utilized for multiphot...
A new approach to teaching method assessment using student focused qualitative studies and the theoretical framework of mental models is proposed. The methodology is considered specifically for the advantages it offers when applied to the assessment of inquiry-based teaching methods. The theoretical foundation of mental models is discussed, and the framework described in detail, with examples from preliminary experimental results. It is argued that the application of this methodology provides a more learner focused approach to assessing a teaching activity, which provides more detail about the relevant cognitive processes used by the student and is a better guide for improving the activity.
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