297 research outputs found
Research Investigation Directed Toward Extending the Useful Range of the Electromagnetic Spectrum
The report discusses completed and proposed research in atomic and molecular physics conducted at the Columbia Radiation Laboratory from July 1972 to June 1973. Central topics described include the atomic spectra and electronic structure of alkali metals and helium, molecular microwave spectroscopy, the resonance physics of photon echoes in some solid state systems (including Raman echoes, superradiance, and two photon absorption), and liquid helium superfluidity
Effects of Nitrogen Quenching Gas on Spin-Exchange Optical Pumping of He-3
We consider the degree of conservation of nuclear spin polarization in the
process of optical pumping under typical spin-exchange optical pumping
conditions. Previous analyses have assumed that negligible nuclear spin
precession occurs in the brief periods of time the alkali-metal atoms are in
the excited state after absorbing photons and before undergoing quenching
collisions with nitrogen molecules. We include excited-state hyperfine
interactions, electronic spin relaxation in collisions with He and N_2,
spontaneous emission, quenching collisions, and a simplified treatment of
radiation trapping
Instantaneous Clear Sky Radiative Forcings of Halogenated Gases
The clear sky instantaneous radiative forcings of the 14 halogenated gases
previously shown to have the largest contribution to global warming, were
found. The calculation used the absorption cross sections for the halogenated
gases which are assumed to be independent of temperature as well as over 1/3
million line strengths for the 5 naturally occurring greenhouse gases: HO,
CO, O, CH and NO, from the Hitran database. The total radiative
forcing of the halogenated gases at their 2020 concentrations is 0.52 (0.67)
W/m at the tropopause (mesopause). Over half of this forcing is due to
CFC11 and CFC12 whose concentrations are declining as a result of the Montreal
Protocol. The rate of total forcing change for all 14 halogenated gases is 1.5
(2.2) mW/m/year at the tropopause (mesopause). The calculations assumed a
constant altitude concentration for all halogenated gases except CFC11, CFC12
and SF. Using the observed altitude dependence for those 3 molecules
reduced their radiative forcings by about 10%. The global warming potential
values were comparable to those given by the Intergovernmental Panel on Climate
Change. The contribution of a gas to global warming was estimated using the
forcing power per molecule defined as the derivative of its radiative forcing
with respect to its column density. For the present atmosphere, the
per-molecule forcing powers of halogenated gases are orders of magnitude larger
than those for the 5 naturally occuring greenhouse gases because the latter
have much higher concentrations and are strongly saturated. But, the rates of
concentration increase of the 5 main greenhouse gases are orders of magnitude
greater than that of any halogenated gas. Assuming the temperature increase
caused by each gas is proportional to its radiative forcing increase, the 14
halogenated gases are responsible for only 2% of the total global warming.Comment: arXiv admin note: text overlap with arXiv:2103.16465,
arXiv:2006.0309
Optimization of cw sodium laser guide star efficiency
Context: Sodium laser guide stars (LGS) are about to enter a new range of
laser powers. Previous theoretical and numerical methods are inadequate for
accurate computations of the return flux and hence for the design of the
next-generation LGS systems.
Aims: We numerically optimize the cw (continuous wave) laser format, in
particular the light polarization and spectrum.
Methods: Using Bloch equations, we simulate the mesospheric sodium atoms,
including Doppler broadening, saturation, collisional relaxation, Larmor
precession, and recoil, taking into account all 24 sodium hyperfine states and
on the order of 100 velocity groups.
Results: LGS return flux is limited by "three evils": Larmor precession due
to the geomagnetic field, atomic recoil due to radiation pressure, and
transition saturation. We study their impacts and show that the return flux can
be boosted by repumping (simultaneous excitation of the sodium D2a and D2b
lines with 10-20% of the laser power in the latter).
Conclusions: We strongly recommend the use of circularly polarized lasers and
repumping. As a rule of thumb, the bandwidth of laser radiation in MHz (at each
line) should approximately equal the launched laser power in Watts divided by
six, assuming a diffraction-limited spot size.Comment: 15 pages, 12 figures, to be published in Astronomy & Astrophysics,
AA/2009/1310
High rate production of polarized 3He with meta-stability exchange method
Keywords: polarized 3He, meta-stability exchange, infrared laserComment: 4 figures, submitted to J. Phys. Soc. Jpn
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