Biomedical subjects

Frank N Keutsch

Publications and source records attributed to Frank N Keutsch.

4 recordsLinked to original sources

PubMed2006

Ultrasensitive near-infrared integrated cavity output spectroscopy technique for detection of CO at 1.57 microm: new sensitivity limits for absorption measurements in passive optical cavities.

A robust absorption spectrometer using the off-axis integrated cavity output spectroscopy (ICOS) technique in a passive cavity is presented. The observed sensitivity, conceptually the detection threshold for the absorption cross section (cm2) multiplied by the concentration (cm(-3)) and normalized by the averaging time, is measured to be 1.9 x 10(-12) (1/cm square root of Hz). This high sensitivity arises from using the optical cavity to amplify the observed path length in the spectrometer while avoiding cavity resonances by careful design of the spot pattern within the cavity. The instrument is ideally suited for routine monitoring of trace gases in the near-infrared region. A spectrum showing ambient carbon monoxide at 1.57 microm is presented.

Journal Article↗

PubMed2005

Rotationally resolved absorption cross sections of formaldehyde in the 28100-28500 cm(-1) (351-356 nm) spectral region: implications for in situ LIF measurements.

The rotationally resolved ultraviolet absorption cross sections for the 2(0)(0)4(1)(0) vibrational band of the A(1)A(2)-X(1)A(1) electronic transition of formaldehyde (HCHO) at an apodized resolution of 0.027 cm(-1) (approximately 0.0003 nm at 352 nm) over the spectral range 28100-28500 cm(-1) (351-356 nm) at 298 and 220 K, using Fourier transform spectroscopy, are first reported here. Accurate rotationally resolved cross sections are important for the development of in situ HCHO laser-induced fluorescence (LIF) instruments and for atmospheric monitoring. Pressure dependence of the cross sections between 75 and 400 Torr at 298 K was explored, and an average pressure broadening coefficient in dry air of 1.8 x 10(-4) cm(-1) Torr(-1) for several isolated lines is reported. Gaseous HCHO was quantitatively introduced into a flow cell by evaporating micron-sized droplets of HCHO solution, using a novel microinjector technique. The condensed-phase concentrations of HCHO were determined by iodometric titrations to an accuracy of <1%. Accuracy of the measured absorption cross sections is estimated to be better than +/-5%. Integrated and differential cross sections over the entire band at low resolution (approximately 1 cm(-1)) obtained with our calibration technique are in excellent agreement with previous measurements. A maximum differential cross section of 5.7 x 10(-19) cm(2) molecule(-1) was observed at high resolution-almost an order of magnitude greater than any previously reported data at low resolution.

Journal Article↗

PubMed2005

Water pentamer: characterization of the torsional-puckering manifold by terahertz VRT spectroscopy.

We present the measurement and analysis of five new (D2O)5 bands via vibration-rotation-tunneling (VRT) spectroscopy as well as a preliminary description of a second (H2O)5 band. The vibrationally averaged rotational constants of all five fitted bands agree well with those from the two previously observed (D2O)5 bands and confirm that the pentamer averages to a symmetric, quasi-planar structure on the time scale of our experiment. While the spectrum of the first two bands, located at 50.7 cm(-1) (1.52 THz) and 27.3 cm(-1) (0.82 THz) are indicative of unperturbed oblate rotors, the three remaining (D2O)5 bands centered at 47.7 cm(-1) (1.43 THz), 45.4 cm(-1) (1.36 THz), and 45.0 cm(-1) (1.35 THz) are severely perturbed by first-order Coriolis coupling. This represents the first observation of this perturbation in the perdeuterated water pentamer, as well as the first observation of transitions between degenerate states of the torsional-puckering manifold. Unlike transitions from the (H2O)5 band observed by Brown et al. at 89.0 cm(-1) and the 103.8 cm(-1) band that we report here, none of the individual rovibrational transitions of any of the five (D2O)5 bands demonstrate spectral splittings due to bifurcation tunneling. We conclude, through careful analysis of these water pentamer bands, that at least three torsional-puckering manifolds have been probed and that the lowest-energy manifold is highly compacted. A plausible water pentamer torsional-puckering correlation diagram is proposed, though additional experimental data are required to unambiguously establish the energies of the torsional-puckering levels.

Models, Molecular↗

PubMed2003

The water trimer.

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Journal Article↗