8 · Inelastic Extension (brief)

For: users who need to model the Ring effect or hyperspectral retrievals where rotational/vibrational Raman scattering matters. UV/Vis instruments, O₂ A-band, deep Fraunhofer lines.

Prev: 7 · Architecture-Agnostic Code · Next: User Guide → Compute Jacobians (loop back to task layer)

This page is intentionally brief — vSmartMOM's Raman support is a parallel solver path next to the elastic one, not central to most retrievals. For the full RT formalism see Sanghavi & Frankenberg 2023 (Part II); for the optical-property derivation see Sanghavi 2022 (Part I).

In one paragraph

vSmartMOM supports rotational Raman (RRS) and vibrational Raman (VRS, RVRS) inelastic scattering as a parallel kernel path next to the elastic one. Selection is via the RS_type configured in YAML: noRS (default; pure elastic), RRS, VS_0to1, VS_1to0, plus _plus variants for multi-band runs that share spectral structure. The bulk of retrievals (NIR shortwave for greenhouse gases, polarized aerosol over ocean) use noRS — the inelastic correction is small there. UV/Vis retrievals (Ring effect, O₂ A-band line shapes, ozone DOAS) need RRS.

When to enable it

Application	Mode
Ring effect (hyperspectral UV/Vis trace gases)	RRS
O₂ A-band line-shape correction at high precision	RRS (single-scatter approximation often suffices via rt_run_ss)
Fraunhofer-line ghosting due to N₂/O₂ vibrational Raman	VS_0to1 and/or VS_1to0
NIR-only retrievals (XCO₂, XCH₄ from OCO-2/3, GOSAT)	noRS (default)

How it's wired

The Raman path uses the same matrix-operator language as the elastic path. Sanghavi & Frankenberg 2023 §3.3 shows that the elastic adding equations (R₂₀, T₂₀, J₂₀) carry over essentially unchanged; the inelastic adding equations (Eqs. 16–21 of SF2023-II) build the inelastic contributions on top under a linear-in-inelastic-scattering approximation — only one inelastic event per photon path, multiple elastic scattering before and after. Second-order inelastic has been shown to be negligible at typical atmospheric conditions.

Architecturally:

• Mode types subtype AbstractRamanType and live in src/Inelastic/types.jl. Each carries the precomputed Cabannes/Raman greek coefficients and Dunham coefficients for N₂/O₂.

• Parallel kernel files at src/CoreRT/CoreKernel/elemental_inelastic.jl, doubling_inelastic.jl, interaction_inelastic.jl. rt_kernel! dispatches on RS_type to choose which set runs (see src/CoreRT/CoreKernel/rt_kernel.jl:48–229). The elastic kernel also runs alongside — Raman is additive on top.

• rt_run_ss (src/CoreRT/rt_run.jl:364–524) is the single-scattering approximation that exists alongside rt_run for SF2023-II Eq. (32) — the inelastic correction I₁ ≈ I₀ + (I'₁ − I'₀) that makes the linear-in-inelastic approximation usable in absorbing bands. Not a debug helper; it's the production tool for fast O₂ A-band Raman corrections.

• Linearized Raman is currently elastic-only. The _lin.jl files cover the elastic path only; derivatives through inelastic scattering are not yet implemented.

Code anchors

Concept	Source
Mode types (noRS, RRS, VS_0to1, VS_1to0, _plus variants)	src/Inelastic/types.jl
Inelastic optical properties (Cabannes, RRS, VRS, RVRS)	src/Inelastic/
Inelastic elemental	src/CoreRT/CoreKernel/elemental_inelastic.jl
Inelastic doubling	src/CoreRT/CoreKernel/doubling_inelastic.jl
Inelastic interaction	src/CoreRT/CoreKernel/interaction_inelastic.jl
_plus variants (multi-band)	src/CoreRT/CoreKernel/elemental_inelastic_plus.jl
Single-scatter inelastic correction	src/CoreRT/rt_run.jl::rt_run_ss:364–524
Cabannes vs full Rayleigh selector	src/CoreRT/LayerOpticalProperties/compEffectiveLayerProperties.jl:8–9

For developers extending the Raman path (adding a new mode, adding new molecular species), see Add a Raman Mode.

References

• Sanghavi (2022), Raman scattering in the earth's atmosphere, part I: Optical properties, JQSRT 291:108328, doi:10.1016/j.jqsrt.2022.108328. Optical-property derivation; Cabannes vs Rayleigh; RRS/VRS/RVRS phase matrices and cross-sections.

• Sanghavi & Frankenberg (2023), Raman scattering in the Earth's atmosphere, Part II: Radiative transfer modeling for remote sensing applications, JQSRT 311:108791, doi:10.1016/j.jqsrt.2023.108791. Inelastic RT formalism in the matrix-operator method; constant-N_doubl trick (Eqs. 8–9); single-scatter correction (Eq. 32).

• Crib sheet: docs/dev_notes/theory_references.md §I.