Recent measurements of carbon isotopes in carbon dioxide using near-infrared, diode-laser-based cavity ring-down spectroscopy (CRDS) are presented. The CRDS system achieved good precision, often better than 0.2 per thousand, for 4% CO2 concentrations, and also achieved 0.15-0.25 per thousand precision in a 78 min measurement time with cryotrap-based pre-concentration of ambient CO2 concentrations (360 ppmv). These results were obtained with a CRDS system possessing a data rate of 40 ring-downs per second and a loss measurement of 4.0 x 10(-11) cm(-1) Hz(-1/2). Subsequently, the measurement time has been reduced to under 10 min. This standard of performance would enable a variety of high concentration (3-10%) isotopic measurements, such as medical human breath analysis or animal breath experiments. The extension of this ring-down to the 2 microm region would enable isotopic analysis at ambient concentrations, which, combined with the small size, robust design, and potential for frequent measurements at a remote site, make CRDS technology attractive for remote atmospheric measurement applications.

No abstract

Rheology, selfdiffusion, and microstructure of charged colloids under simple shear by massively parallel nonequilibrium Brownian dynamicsWe performed two different types of spectral diffusion experiments on persistent spectral holes. In all cases, we measured the holewidth as a function of time. The two experiments differed in their initial conditions: In the type 1 experiment ͑the ''aging experiment''͒, the sample was cooled from room temperature to the final temperatures which were 100 and 800 mK, respectively. Holes were burnt at various time intervals after the final temperature was reached. In the type 2 experiment ͑the ''cycling experiment''͒, the sample was allowed to relax for a period of about 10 days. Then, a hole was burnt and subjected to a temperature cycle. In all cases, the time dependence of the holewidths was strongly nonlogarithmic. The temperature cycled hole showed a narrowing regime which prevailed for the whole observation period of roughly one week. We will show that the deviation from the logarithmic time dependences is a nonequilibrium phenomenon. All features observed could be modelled within the standard tunneling model.

We have investigated two-level system dynamics in the time range from 10 to 10 6 s at temperatures of 500 mK and 1 K. The experiments were performed by measuring spectral diffusion in a photochemical hole-burning system. The observed power-law time dependences can be interpreted in the framework of a system of strongly interacting spins which are, at short time scales, immobilized by their mutual coupling. With this conjecture we would be able to rationalize that the density of states is seemingly low at short time scales. PACS numbers: 61.43.Fs, Many physical phenomena which deal with wide distributions of rates are characterized by algebraic time dependences. For instance, photocurrents in amorphous solids can often be quantitatively described by algebraic laws over more than 10 orders of magnitude in time [1,2]. The same basic ideas hold for the dynamics of proteins which can also be characterized by algebraic time dependences [3]. Spectral diffusion in amorphous solids has, in the past, been observed over mostly rather limited time intervals (typically 3 orders of magnitude) and on comparably short time scales. Within these time intervals a logarithmic time scale has mostly been sufficient to describe the data.Here we report experimental data over a wider time regime (5 orders of magnitude, up to 10 6 s). We observe that for long times an algebraic law describes the experimental data very well, whereas a logarithmic law would break down. Therefore we suggest a modified model for the density of states of a two-level system (TLS) which would, as a consequence, predict that in the short time regime most TLS spins are locked.The tunneling model [4,5] was originally developed to explain the anomalous temperature dependence of the specific heat of inorganic amorphous solids at low temperatures observed in the pioneering work of Zeller and Pohl [6]. Very soon after these experiments this anomaly was also found for organic materials and, specifically, for the polymer PMMA [7], which is being investigated in the present Letter. Without major modifications the standard TLS model was subsequently applied to a variety of data and techniques ranging from measurements of the thermal conductivity [6] to experiments like ultrasonic attenuation [8-10] or phonon echoes [11]. In order to explain the observed anomalies Black and Halperin [12] adopted the concept of spectral diffusion, developed for paramagnetic spin resonance [13], within the framework of the tunneling model. Later Reinecke used spectral diffusion to account for the anomalous linewidths observed in the optical spectroscopy of glasses [14]. On the basis of the standard model [4,5] theory predicts a logarithmic time dependence of the optical linewidths.There is, however, an inconsistency within the theoretical descriptions of all the above mentioned phenomena: The original model does not contain any interactions of the tunneling "pseudospins" with each other. This is quite natural since these interactions are not required for explaining most of the experimental da...