Final Report Performance of a cryogenic multipath Herriott cell Vacuum coupled to the Bruker 125HR system at JPL Arlan W. Mantz1, Keeyoon Sung2, Tim J. Crawford2, Linda R. Brown2, Mary Ann H. Smith3, V. Malathy Devi4, D. Chris Benner4 1Dept. of Physics, Astronomy and Geophysics, Connecticut College, New London, CT 2Jet Propulsion Laboratory/California Institute of Technology, Pasadena, CA 3Science Directorate, NASA Langley Research Center, Hampton, VA 4Dept. of Physics, The College of William and Mary, Williamsburg, VA 23187

Advantages of FT-IR + Herriott cell FT-IR with a white light Broad-band spectroscopy securing high level of consistency (frequency, line to line, band to band intensity, etc.) Multiplex advantage leading to high S/N Radiometric stability enabling long integration Herriott cell Virtually everything is in one material Simpler and robust design (over White cell) - Two mirrors only Less susceptible to opto-mechanical disturbance – alignment preserves All materials are identical - uniform cooling Compact design saving space/sample gas - efficient cooling Shortcoming over White-type cell Absorption path length is fixed. Beam alignment is difficult for a converging beam (e.g. sharing one hole). FT-IR + Herriott cell Ideal for spectroscopy at cold temperatures Study of temperature-dependent molecular line parameters

Herriott cell in a simple design Two concave spherical mirrors, focal length = f Mirror separated by distance, d Entrance hole (ϕ in dia.) at r away from the center Focal length, f = R/2 Curvature, R C Not to scale F Mirror distance, d r ϕ Ang. Dist. each pass, θ Closure constraints cosθ = 1 – d/2f N × θ = 2π × k Total path inside the cell L = N×d + Nr4/(8df 2) Herriott cell 69th Meeting - June 16-20, 2014 - Champaign-Urbana, Illinois 3/12

Herriott Cell Installation Movie from Wikipedia Hyperboloid Herriott cell parameters # of passes N 62 Focal length 125 mm Center-to-center distance 337.031mm Entrance/Exit hole diameter 4 mm Extra path due to window 22.25 mm Total path at 293 K, L293K 20.941 m Total path at 100 K, L100K 20.882 m Twisting angle per pass, θ 110.37° closure constraint and offset ∑θ = N × θ = k × 2π,(k =19) 6842.94° ∆θ = 2.94° Beam spots simulated Entrance hole (ϕ) size (in °) 9.02° Hyperboloid of one sheet, x2/a2 + y2/b2 – z2/c2 = 1

Ray tracing and beam coupling Interferometer From the top The Herriott cell Coupler vacuum tank Bruker sample compartment From the side The Herriott cell installed [Not to scale]

Pathlength validation (using single spectrum fit, fov = 1.244 mrad) Experimental inputs in common: L = 21.005 m (assumed) 12CO (99.999%) P = 28.33 Torr T = 297.1 K Res.=0.012 cm-1 12CO (99.999%) P = 28.33 Torr T = 297.1 K Res.=0.012 cm-1 Absorption pathlength = 21.005 * (1. – 0.008) = 20.837 (± 0.147) m %discrepancy = -0.5 (± 0.7) % from the optically determined L = 20.941 m

Temperature control of the cell Body Oxygen Free High Conductivity copper# Volume 3.23 L (Cavity) 0.008 L (inlet tube) Mirrors Bare gold (4 in. dia.); R = 0.99 Cell Window CaF2 (wedged, 30′, Qty. = 1) Vacuum box Cooling system CTI Cryogenics 9600 He-compressor; 1020 CP Cryopump Refrig. agent Helium (99.9999%) Heater capacity 25 W × 2 Temp. sensor silicon diode (accuracy of 0.125K) Temp. achieved 75 – 250 K and 296 K Temp. control$ < 0.05 K for days Compressor and chiller Excellent temperature stability 0.05 K / days #Thermal conductivity, among the best in the range 70 < T < 300 (better than Al and Au) $Achieved by PID (Proportional, Integrate and Differentiate) temperature control loop adopted in a Model 331 temperature controller supplied by Lakeshore Cryotronics, Inc. Make the font bigger in the green box

Validation of the temperature assumption Assumption: Cell T = Gas T Investigated four cases between 300 and 79 K Ratio of line intensity at two different temperatures Subscript ‘R’ for Reference data set Subscript ‘X’ for Measurement data set SX/SR = QR/QX · exp{−c2 E" (1/TX −1/TR)} · [1– exp(-c2 v/TX)]/[1–exp(-c2 v/TR)] Solve for Rotational temperature, TX, iteratively log(SX/SR) – log(QR/QX) ≈ − c2E"(1/TX −1/TR) Kin.T Rad.T Rot.T Vib.T TDE.T

Validation of the gas temperatures Gas temperature offsets for CO Effective gas temperature Higher by ~0.5 K at room Temperature Lower by ~0.4 K at T ~ 80 K Other experimental factors Sensor location / cooling time / sample gas/pressure / cryodeposits etc. Temp. Readings Rot.Temp (CO) (CO2) Offsets (CO; CO2) 300.4(0.1) 301.2(1.4) 300.6(1.4) 0.8; 0.2 250.0(0.1) 250.1(0.2) 249.9(0.2) 0.1; -0.1 179.3(0.1) 179.0(0.2) 179.1(0.1) - 0.3; 0.1 78.8(0.1) 78.2(0.2) 78.6(0.2) - 0.6; -0.2 Rot.Temp – Temp.Readings NOTE: These are not the 13CH4 runs: should indicate this in words. !! Agree within ~ 0.5 K !!

Sample spectrum I Cold 12CO2/Air for the OCO-2 mission L = 20.941 m

Sample spectrum II Study of weak bands of CH4 Transition with higher Eʺ Transition with lower Eʺ

Summary and future work Herriott cell developed and integrated to FT-IR Developed and installed Characterized to be L = 20.941 m, T = 296, 250 – 75 K (or colder) Excellent radiometric stability (Througtput =~ 50 %; S/N =300 :1; vibrational perturbation mitigated) Report results (Mantz, et al., Submitted to JMS) Data acquisition at JPL Air-broadened cold CO2 and CO2 isotopes Air-broadened cold O2 Air-broadened cold CH4 and isotopes, etc. Future work Enabling cryogenic spectroscopy in the near-infrared Collisional cooling study Collision-induced absorption

The team and Acknowledgements Malathy Chris Keeyoon Linda Tim Arlan Mary Ann Acknowledgements Research described in this talk was performed at Connecticut College, the College of William and Mary, NASA Langley Research Center and the Jet Propulsion Laboratory, California Institute of Technology, under contracts and cooperative agreements with the National Aeronautics and Space Administration.