Precise measurements of UV atomic lines: Hyperfine structure and isotope shifts in the 398.8 nm line of Yb

We demonstrate a technique for frequency measurements of UV transitions with sub-MHz precision. The frequency is measured using a ring-cavity resonator whose length is calibrated against a reference laser locked to the $D_2$ line of $^{87}$Rb. We have used this to measure the 398.8 nm ${^1S}_0 \leftrightarrow {^1P}_1$ line of atomic Yb. We report isotope shifts of all the seven stable isotopes, including the rarest isotope $^{168}$Yb. We have been able to resolve the overlapping $^{173}$Yb($F = 3/2$) and $^{172}$Yb transitions for the first time. We also obtain high-precision measurements of excited-state hyperfine structure in the odd isotopes, $^{171}$Yb and $^{173}$Yb. The measurements resolve several discrepancies among earlier measurements.Comment: 7 pages, 6 figure

Biases in the polarization position angles in the NVSS point source catalogue

We have examined the statistics of the polarization position angles determined for point sources in the NRAO-VLA sky survey (NVSS) and find that there is a statistically significant bias toward angles which are multiples of 45 degrees. The formal probability that the polarization angles are drawn from a uniform distribution is exponentially small. When the sample of those NVSS sources with polarizations detected with a signal to noise $\geq$3 is split either around the median polarized flux density or the median fractional polarization, the effect appears to be stronger for the more highly polarized sources. Regions containing strong sources and regions at low galactic latitudes are not responsible for the non-uniform distribution of position angles. We identify CLEAN bias as the probable cause of the dominant effect, coupled with small multiplicative and additive offsets on each of the Stokes parameters. Our findings have implications for the extraction of science, such as information concerning galactic magnetic fields, from large scale polarization surveys

What Would Happen If You Ditched Your Textbook? Gettysburg Faculty Share Strategies

How much do your course materials really cost? Do your students obtain all the books you assign? How much does access to required readings affect student success? What would happen if you ditched your textbook? If you’ve ever been tempted to toss your conventional reading list out the window and start over, this session is for you. Learn how colleagues are swapping out expensive course materials for more affordable options, including freely available materials, library-licensed items, and original creations. Our panelists are: Ian Clarke – Ian recently abandoned his $150 textbook for ENG 111 (Writing through Literature) and created an open reading list instead. Sharon Birch – In 2011, Sharon dropped the classic text for SOC 203 (Population) and replaced it with a variety of library-licensed and freely available materials. More recently she has begun developing her own open textbook for this course. Chuck Kann – Chuck has written several open educational resources and posted them in The Cupola. He uses them in CS 221 (Computer Organization and Assembly Language Programming); they have also been adopted by professors teaching at other institutions and have been downloaded around the world. You’ll leave with a clear idea of how a range of simple to complex strategies could help make your course materials more affordable, too

Polarization Dependence of Born Effective Charge and Dielectric Constant in KNbO3_3

The Born effective charge Z^{*} and dielectric tensor \epsilon_{\infty} of KNbO_3 are found to be very sensitive to the atomic geometry, changing by as much as 27% between the paraelectric cubic and ferroelectric tetragonal and rhombohedral phases. Subtracting the bare ionic contribution reveals changes of the dynamic component of Z^{*} as large as 50%, for atomic displacements that are typically only a few percent of the lattice constant. Z^{*}, \epsilon_{\infty} and all phonon frequencies at the Brillouin zone center were calculated using the {\it ab initio} linearized augmented plane-wave linear response method with respect to the reference cubic, experimental tetragonal, and theoretically determined rhombohedral ground state structures. The ground state rhombohedral structure of KNbO_3 was determined by minimizing the forces on the relaxed atoms. By contrast with the cubic structure, all zone center phonon modes of the rhombohedral structure are stable and their frequencies are in good agreement with experiment. In the tetragonal phase, one of the soft zone center modes in the cubic phase is stablized. In view of the small atomic displacements involved in the ferroelectric transitions, it is evident that not only the soft mode frequencies but also the Born effective charge and dielectric constants are very sensitive to the atomic geometry.Comment: 26 pages, revtex, no figures; to appear in Phys. Rev. B15 (Oct.), 199

Coherent Dual Comb Spectroscopy at High Signal to Noise

Two frequency combs can be used to measure the full complex response of a sample in a configuration which can be alternatively viewed as the equivalent of a dispersive Fourier transform spectrometer, infrared time domain spectrometer, or a multiheterodyne laser spectrometer. This dual comb spectrometer retains the frequency accuracy and resolution inherent to the comb sources. We discuss, in detail, the specific design of our coherent dual-comb spectrometer and demonstrate the potential of this technique by measuring the first overtone vibration of hydrogen cyanide, centered at 194 THz (1545 nm). We measure the fully normalized, complex response of the gas over a 9 THz bandwidth at 220 MHz frequency resolution yielding 41,000 resolution elements. The average spectral signal-to-noise ratio (SNR) is 2,500 for both the fractional absorption and the phase, with a peak SNR of 4,000 corresponding to a fractional absorption sensitivity of 0.025% and phase sensitivity of 250 microradians. As the spectral coverage of combs expands, this dual-comb spectroscopy could provide high frequency accuracy and resolution measurements of a complex sample response across a range of spectral regions.Comment: 18 pages, 8 figures updated references, updated to match publication versio