https://e-reports-ext.llnl.gov/pdf/231636.pdf Global Warming and Ice Ages: Prospects for Physics-Based Modulation of Global Change
Resonant absorption and (quasi-isotropic) fluorescent re-radiation of solar (near-)optical photons is an incompletely-compelling candidate mechanism for scattering units primarily because even atoms with full dipole-transition oscillator strengths in the (near-)visible spectrum absorb with maximum radiative strength only over relatively very small wavelength intervals (∆ω/ω ~ 10-7) and secondarily because such strong absorption is typically seen only in metallic gases (and similarly low-density, effectively-collisionless circumstances, in which the natural width of the transition is a regrettably large fraction of its full width). However, the intrinsic mass efficiency of scattering units comprised of a set of resonant absorbers could be expected to be extremely high, and it likely is worth considerable applied photophysics experimental effort to spectrally broaden such resonant transitions to cover ~2% of the solar spectrum. Normal matter never works harder in sustainable electromagnetic terms than when it’s scattering radiation on a full electric-dipole-strength transition at the maximum rate given by that transition’s Einstein A coefficient (~108 sec-1, for the full-dipole strength optical transitions of present interest): i.e., an alkali-metal atom (e.g., Li) will process ~3x10-11 W of resonant radiation (≤108 photons/sec, each of 1.8 eV or 3x10-12 ergs energy) for a mass-cost of 10-23 grams (e.g., an Li6 atom, scattering on its 6708 Å resonant transition) – which corresponds to a specific scattering power of 3x1012 W/gm(!). If it were feasible to exercise matter this vigorously in scattering units, then the working-mass of an entire scattering system would be only ~1 kg. However, the ≤1018 photons/cm2-sec of (near-)optical solar flux at 1 AU only present ~1011 photons/cm2-sec within the ~30 MHz absorption-line of a typical full electric-dipole-strength optical resonant transition, which has a characteristic peak absorption cross-section σ ~ 3x10-10 cm2 (i.e., σ ~ λ2/4π, with λ ~ 6x10-5 cm). Thus, such an atom only processes ~30 photons/sec when hung-in-space in 1 AU sunlight, i.e., it works at only 3x10-7 of its maximum feasible scattering-rate. Instead of 1 kg of scattering unit-mass, 3x109 grams, or 3000 tons, would have to be employed – moreover, as a scattering-disc of free atoms positioned on the Sun-Earth line. Nonetheless, this is a small mass for the ''active'' component of a full-scale insolation modulation system; it motivates serious consideration of options based on resonant scattering physics.
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Global Warming and Ice Ages:
Prospects for Physics-Based Modulation of Global Change
Resonant absorption and (quasi-isotropic) fluorescent re-radiation of solar (near-)optical photons is an
incompletely-compelling candidate mechanism for scattering units primarily because even atoms with full
dipole-transition oscillator strengths in the (near-)visible spectrum absorb with maximum radiative strength
only over relatively very small wavelength intervals (∆ω/ω ~ 10-7) and secondarily because such strong
absorption is typically seen only in metallic gases (and similarly low-density, effectively-collisionless
circumstances, in which the natural width of the transition is a regrettably large fraction of its full width).
However, the intrinsic mass efficiency of scattering units comprised of a set of resonant absorbers could be
expected to be extremely high, and it likely is worth considerable applied photophysics experimental effort to
spectrally broaden such resonant transitions to cover ~2% of the solar spectrum. Normal matter never works
harder in sustainable electromagnetic terms than when it’s scattering radiation on a full electric-dipole-strength
transition at the maximum rate given by that transition’s Einstein A coefficient (~108 sec-1, for the full-dipole
strength optical transitions of present interest): i.e., an alkali-metal atom (e.g., Li) will process ~3x10-11 W of
resonant radiation (≤108 photons/sec, each of 1.8 eV or 3x10-12 ergs energy) for a mass-cost of 10-23 grams
(e.g., an Li6 atom, scattering on its 6708 Å resonant transition) – which corresponds to a specific scattering
power of 3x1012 W/gm(!). If it were feasible to exercise matter this vigorously in scattering units, then the
working-mass of an entire scattering system would be only ~1 kg. However, the ≤1018 photons/cm2-sec of
(near-)optical solar flux at 1 AU only present ~1011 photons/cm2-sec within the ~30 MHz absorption-line of a
typical full electric-dipole-strength optical resonant transition, which has a characteristic peak absorption
cross-section σ ~ 3x10-10 cm2 (i.e., σ ~ λ2/4π, with λ ~ 6x10-5 cm). Thus, such an atom only processes ~30
photons/sec when hung-in-space in 1 AU sunlight, i.e., it works at only 3x10-7 of its maximum feasible
scattering-rate. Instead of 1 kg of scattering unit-mass, 3x109 grams, or 3000 tons, would have to be employed
– moreover, as a scattering-disc of free atoms positioned on the Sun-Earth line. Nonetheless, this is a small
mass for the ''active'' component of a full-scale insolation modulation system; it motivates serious
consideration of options based on resonant scattering physics.