Study of coupled states for the (4s^{2})^{1}S + (4s4p)^{3}P asymptote of Ca_{2}

The coupled states A^{1}\Sigma_{u}^{+} (^{1}D +}1}S), c^{3}\Pi_{u} (^{3}P + ^{1}S) and a^{3}\Sigma_{u}^{+} (^{3}P +}1}S) of the calcium dimer are investigated in a laser induced fluorescence experiment combined with high-resolution Fourier-transform spectroscopy. A global deperturbation analysis of the observed levels, considering a model, which is complete within the subspace of relevant neighboring states, is performed using the Fourier Grid Hamiltonian method. We determine the potential energy curve of the A^{1}\Sigma_{u}^{+} and c^{3}\Pi_{u} states and the strengths of the couplings between them. The c^{3}\Pi_{u} and \as states are of particular importance for the description of collisional processes between calcium atoms in the ground state ^{1}S_{0} and excited state ^{3}P_{1} applied in studies for establishing an optical frequency standard with Ca.Comment: 15 pages, 12 figure

Experimental study of the Ca2 1S+1S asymptote

The filtered laser excitation technique was applied for measuring transition frequencies of the Ca$_2$ B-X system from asymptotic levels of the X$^1\Sigma_{\mathrm g}^{+}$ ground state reaching $v''=38$. That level has an outer classical turning point of about 20~\AA which is only 0.2 \rcm below the molecular $^1$S$+^1$S asymptote. Extensive analysis of the spectroscopic data, involving Monte Carlo simulation, allowed for a purely experimental determination of the long range parameters of the potential energy curve. The possible values of the s-wave scattering length could be limited to be between 250$a_0$ and 1000$a_0$.Comment: 10 pages, 7 figure

Photospheric properties and fundamental parameters of M dwarfs

M dwarfs are an important source of information when studying and probing the lower end of the Hertzsprung-Russell (HR) diagram, down to the hydrogen-burning limit. Being the most numerous and oldest stars in the galaxy, they carry fundamental information on its chemical history. The presence of molecules in their atmospheres, along with various condensed species, complicates our understanding of their physical properties and thus makes the determination of their fundamental stellar parameters more challenging and difficult. The aim of this study is to perform a detailed spectroscopic analysis of the high-resolution H-band spectra of M dwarfs in order to determine their fundamental stellar parameters and to validate atmospheric models. The present study will also help us to understand various processes, including dust formation and depletion of metals onto dust grains in M dwarf atmospheres. The high spectral resolution also provides a unique opportunity to constrain other chemical and physical processes that occur in a cool atmosphere The high-resolution APOGEE spectra of M dwarfs, covering the entire H-band, provide a unique opportunity to measure their fundamental parameters. We have performed a detailed spectral synthesis by comparing these high-resolution H-band spectra to that of the most recent BT-settl model and have obtained fundamental parameters such as effective temperature, surface gravity, and metallicity (Teff, log g and [Fe/H]) respectively.Comment: 15 pages, 10 figures, accepted for publication in A&

The potential of the ground state of NaRb

The X$^{1}\Sigma ^{+}$ state of NaRb was studied by Fourier transform spectroscopy. An accurate potential energy curve was derived from more than 8800 transitions in isotopomers $^{23}$Na$^{85}$Rb and $^{23}$Na$^{87}$Rb. This potential reproduces the experimental observations within their uncertainties of 0.003 \rcm to 0.007 \rcm. The outer classical turning point of the last observed energy level ($v''=76$, $J''=27$) lies at $\approx 12.4$ \AA, leading to a energy of 4.5 \rcm below the ground state asymptote.Comment: 8 pages, 6 figures and 2 table

Mode identification of Pulsating White Dwarfs using the HST

We have obtained time-resolved ultraviolet spectroscopy for the pulsating DAV stars G226-29 and G185-32, and for the pulsating DBV star PG1351+489 with the Hubble Space Telescope Faint Object Spectrograph, to compare the ultraviolet to the optical pulsation amplitude and determine the pulsation indices. We find that for essentially all observed pulsation modes, the amplitude rises to the ultraviolet as the theoretical models predict for l=1 non-radial g-modes. We do not find any pulsation mode visible only in the ultraviolet, nor any modes whose phase flips by 180 degrees; in the ultraviolet, as would be expected if high l pulsations were excited. We find one periodicity in the light curve of G185-32, at 141 s, which does not fit theoretical models for the change of amplitude with wavelength of g-mode pulsations.Comment: Accepted for publication in the Astrophysical Journal, Aug 200

Detailed study of dissipative quantum dynamics of K-2 attached to helium nanodroplets

We thoroughly investigate vibrational quantum dynamics of dimers attached to He droplets motivated by recent measurements with K-2 [1]. For those femtosecond pump-probe experiments, crucial observed features are not reproduced by gas phase calculations but agreement is found using a description based on dissipative quantum dynamics, as briefly shown in [2]. Here we present a detailed study of the influence of possible effects induced by the droplet. The helium droplet causes electronic decoherence, shifts of potential surfaces, and relaxation of wave packets in attached dimers. Moreover, a realistic description of (stochastic) desorption of dimers off the droplet needs to be taken into account. Step by step we include and study the importance of these effects in our full quantum calculation. This allows us to reproduce and explain all major experimental findings. We find that desorption is fast and occurs already within 2-10 ps after electronic excitation. A further finding is that slow vibrational motion in the ground state can be considered frictionless.Comment: 17 pages, 5 figure

Cost-effective flexibilisation of an 80 MWe retrofitted biomass power plants : improved combustion control dynamics using virtual air flow sensors

As they deliver dispatchable renewable energy, biomass power plants are expected to play a key role in the stability of the future electricity grids dominated by intermittent renewables. Large-scale, biomass-fired power plants are often retrofitted from coal-fired plants. Such a fuel modi-fication combined with decreasing pollutant emission limits and higher requirements in terms load flexibility can lead to a decrease of the maximum power delivered by the unit. The limiting factors are partly related to the control systems of those plants. In this paper, we present the results of the upgrading of an 80 MWe, retrofitted biomass power plant that was achieved improving the dynamic control of the combustion process. Thanks to the addition of virtual air flow sensors in the control system and the re-design of the combustion control loops, the unde-sired effects of a recent 10% power increase on NOx emissions were more than compensated. The accurate control of the local NOx production in the furnace resulted in a decrease of these emissions by 15% with an increased stability. This study will help increasing the cost-effectiveness of such conversions, and facilitate the development of dispatchable, renewable power units able to contribute to the grid stability