1. 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

2. 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

3. 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, Champaign-Urbana, Illinois /12

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

5. 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]

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

7. 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 ( %)
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

8. 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

9. 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 !!

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

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

12. Summary and future work
Herriott cell developed and integrated to FT-IR
Developed and installed
Characterized to be L = 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

13. 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.