CASAC: Astrochemical Absorption Cell

• CASAC is a state-of-the-art absorption cell platform enabling high-sensitivity detection of gas-phase molecules and transient species across multiple wavelength regimes.
• The system employs advanced cell design and window engineering, including impedance matching and AR-coated materials, to minimize noise and optimize calibration.
• It supports both laboratory molecular characterization and on-sky radial velocity calibration, crucial for precision exoplanet surveys and astrochemical studies.

The Center for Astrochemical Studies Absorption Cell (CASAC) is an advanced laboratory platform for molecular spectroscopy, developed to enable high-precision measurements of gas-phase species relevant to astrochemistry, atmospheric science, and precision astronomical instrumentation. It incorporates stabilized absorption cells and frequency-modulated free-space millimeter/submillimeter spectrometers, facilitating both laboratory spectroscopic characterization and on-sky calibration for radial velocity (RV) measurements. CASAC is implemented across multiple wavelength regimes, including near-infrared (NIR), radio, and submillimeter, and supports the detection and analysis of transient, plasma-generated species as well as reference gases for exoplanet searches (Plavchan et al., 2013, Tanarro et al., 2017, Lattanzi et al., 27 Nov 2025).

1. Scientific Motivation and Functional Scope

CASAC serves dual scientific objectives in astrochemical research and astronomical instrumentation. In laboratory astrochemistry, CASAC enables broadband, high-sensitivity detection of molecules down to partial pressures below $10^{-3}$ mbar, including rotational and vibrational transitions with precise line-shape and kinetic measurements. It is particularly suited to the study of transient species, plasma-generated radicals, and complex molecular systems, as exemplified by the rotational spectroscopy of $trans$-HNSO ($200$–$530$ GHz) using a hollow-cathode discharge source (Lattanzi et al., 27 Nov 2025). In astronomical applications, CASAC cells provide stabilized references for precision RV calibration in the NIR, extending the reach of spectroscopic exoplanet surveys to M-dwarfs and low-mass stars with noise floors as low as $7$ m/s (Plavchan et al., 2013).

2. Cell Design, Construction, and Window Engineering

CASAC absorption cells are constructed for both NIR and mm-wave operation, typically using borosilicate (Pyrex) or stainless steel tubes of lengths from $125$ mm (NIR spectrographs) up to $3.0$ m (mm-wave spectroscopy), and diameters ranging from $5$ cm to $60$ cm. For laboratory systems, the cell is mounted inside a vacuum/shield enclosure with multiple ports for gas injection, pressure monitoring, and discharge or UV sources. Key to performance are the window materials and impedance-matching techniques. NIR systems use low-OH quartz windows (AR-coated on one side, wedge-polished at $1.6^\circ$ to suppress etalons); radio and mm-wave cells employ fused quartz, Upilex polyimide, or Teflon with quarter-wave anti-reflection grooved layers to minimize added receiver noise ($\leq 2$ K for Upilex, $<–30$ dB reflection loss for Teflon+AR, $\sim15$–$40$ K for uncoated quartz) (Tanarro et al., 2017). Parameters for three prototypical NIR CASAC cells are provided below.

Parameter	13CH₄ Cell	12CH₃D Cell	14NH₃ Cell
Gas pressure (20°C)	275 mbar	345 mbar	75 mbar
Cell length	125 mm	125 mm	125 mm
Window material	Quartz	Quartz	Quartz
Internal T set-point	283 K	283 K	283 K

3. Gas Selection and Source Integration

CASAC gas selection is dictated by intended spectroscopic application. For NIR RV applications, isotopic methane ($^{13}$CH₄), deuterated methane ($^{12}$CH₃D), and ammonia ($^{14}$NH₃) are chosen for line isolation, density, and coverage: $^{13}$CH₄ shifts absorption centers $\sim10$ nm from telluric $^{12}$CH₄, $^{12}$CH₃D provides $\sim3\times$ higher line density, and $^{14}$NH₃ spans H/K bands, facilitating broad astrochemical relevance (Plavchan et al., 2013).

For plasma and photochemical studies, the cell supports in-situ generation of reactive species through cold inductively coupled plasmas (RF $13.56$ MHz) or UV photolysis (main lines at $185$ nm/$254$ nm, total $20$ W) (Tanarro et al., 2017). The hollow-cathode discharge source, newly integrated for detection of $trans$-HNSO, achieves elevated electron density ($\sim10^{12}$ cm$^{-3}$) and efficient radical formation at pressures $\sim20$ mTorr (Lattanzi et al., 27 Nov 2025).

4. Spectroscopic Methodologies and Data Acquisition

Laboratory CASAC spectroscopy spans NIR Fourier Transform (FTS, $R\sim700\,000$), radio heterodyne (FFTS, $2$ GHz bandwidth, $38$ kHz channel spacing), and mm-wave modulation (FM at $50$ kHz, spectral resolution $\sim100$ kHz). For RV calibration, absorption-cell scans are divided by empty-cell references to remove ambient features (H$_2$O, CO$_2$), and Beer–Lambert and Voigt-profile models are applied:

$I(\nu) = I_0(\nu)\exp[-\sigma(\nu)NL],$

where

$$\sigma(\nu) = \sum_i S_i \phi_V(\nu-\nu_i; \gamma_D, \gamma_L).$$

Parameters such as pressure broadening ($\gamma_L$), Doppler width ($\gamma_D \propto \sqrt{T}$), and temperature exponent ($n$) are determined, with broadened widths expressed as $\gamma_L(P, T) = \gamma_0 (P/P_0)(T_0/T)^n$, $n\approx 0.5-1.0$ (Plavchan et al., 2013).

For rotational spectroscopy (e.g., $trans$-HNSO), assignments and global fits utilize Watson S-reduced Hamiltonians with rotational and centrifugal distortion constants, using SPFIT and pyLabSpec’s QtFit software (Lattanzi et al., 27 Nov 2025). The signal-to-noise ratio for single lines exceeds $10$ for integration times of $20$–$180$ s.

5. Thermal and Mechanical Control Strategies

CASAC employs precise thermal regulation—custom silicon heaters (set-point $283$ K), platinum RTD (PT100) sensors adjacent to the windows, and PID controllers (Omega CN7823) maintaining stability of $\pm0.1$ K (RV drift $<$ $1$ m/s). Mechanical mounting uses Newmark rotary stages, rubber-lined clamps, and FEA-modeled supports to ensure flexure $<$ $50$ μm and vibrational isolation ($>$ $200$ Hz natural frequency). For mm-wave cells, active liquid nitrogen cooling narrows Doppler profiles during discharge operation (Plavchan et al., 2013, Lattanzi et al., 27 Nov 2025).

6. Sensitivity, Performance, and Applications

CASAC achieves high sensitivity, routinely detecting species at partial pressures $<10^{-3}$ mbar in $<10$ s and resolving time-evolving concentrations in plasma and UV experiments. Receiver noise temperatures range from $2\,000$–$4\,000$ K (mm-wave, single-sideband); NIR RV precision reaches $7$ m/s noise floor for bright targets with the CSHELL spectrograph (RV extraction pipeline fits $25$ parameters, barycentric correction $<$ $0.1$ m/s) (Plavchan et al., 2013).

Application	Spectral Range	Precision / Limit
NIR RV (CSHELL)	$2.28$–$2.35\,\mu$m	$7$ m/s (S/N $>$ $400$)
mm-wave lab (CASAC)	$200$–$530$ GHz	SNR $10$–$30$ ($20$–$180$ s)
Radio cell (FFTS)	$41$–$49$ GHz	$<10^{-3}$ mbar/$<10$ s

Observed limitations include baseline instabilities (window reflections require AR layers/wedge profiles), small spectral grasp in some telescopes ($\sim5$ nm at NIR), and detector artifacts. Enhanced cross-dispersion and modern detectors (e.g., H2RG) on next-generation facilities (iSHELL, iGRINS, NIRSPEC) are expected to yield $\sim1$ m/s RV precision on bright M-dwarfs (Plavchan et al., 2013).

7. Analytical Frameworks and Data Products

CASAC data analysis integrates polynomial baseline correction, modulated Voigt profile fitting, and assignment to global Hamiltonians. Astrochemical by-products include high-accuracy FTIR line lists (ASCII grids) and extracted line parameters for reference databases (e.g., HITRAN, GEISA). The rotational constants for $trans$-HNSO, reproduced with an RMS of $40$ kHz (Table 1, (Lattanzi et al., 27 Nov 2025)), enable astronomical frequency predictions and facilitate tests of interstellar isomerization mechanisms.

In kinetic and fragmentation studies, time-resolved data capture chemical transformations such as photolytic CS generation and sequential CS → OCS growth from CS$_2$+O$_2$ mixtures, under plasma and UV excitation (Tanarro et al., 2017). Calibration is performed using hot/cold loads and precise sky models; repeatability in system noise and frequency coverage is $\lesssim3\%$ per run (Tanarro et al., 2017).

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The Center for Astrochemical Studies Absorption Cell system constitutes a robust, multi-faceted platform, unifying laboratory astrochemical spectroscopy and precision astronomical calibration. Its technical refinements in cell design, window engineering, source integration, and data analysis render it uniquely suited to advancing spectroscopic detection and characterization of both reference and transient molecular species in laboratory and observational contexts (Plavchan et al., 2013, Tanarro et al., 2017, Lattanzi et al., 27 Nov 2025).