THz Studies of Water Vapor

1. THz Studies of Water Vapor Vyacheslav B. Podobedov, Gerald T. Fraser and David. F. Plusquellic NIST/Optical Technology Division/Physics Lab Gaithersburg, MD 20899

2. Motivation THz studies are of importance to Climate modeling Radio Astromony Satellite-based remote sensing Acura/Aura/Far IR Space Telescope EM wave propagation over wide range of atmospheric conditions mm-wave have less sensitivity to cloud contamination vs infrared and UV Major importance for ozone chemistry and for the greenhouse effect Experimental advantages in the THz region for water vapor Discrete line shape is nearly pure Lorentzian for pressures > 1 Torr Doppler contributions are <5 MHz at room temperature Continuum aborption Insensitivity to far-wind line shape model

3. Challenges Two sources of absorption in this region Discrete line absorption Continuum absorption Self- and air-pressure broadened widths, shifts, and the temperature dependence of these parameters needed before estimates of continuum absorption

4. Far-infrared MW Pure Rotational Lines 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 The Terahertz Gap Pure Rotational Spectroscopy for H2O (18O), HDO and D2O Terahertz (THz) 1 THz  33.3 cm-1 or   300 m ν 0.06 THz to 3 THz ν2 cm-1 to 100 cm-1 λ 5 mm to 100 μm

5. Photoconductive Switches or Photomixers The photomixers are epitaxial low-temperature-grown GaAs with a gold spiral antenna structure Photomixer chip – 5 x 5 mm Two CW lasers, offset by THz, illuminate the fingers Conduction band - e- + ~850 nm Vbias 15 V Valence band + 8 x 8 m - Photoexcitation produces an acceleration of charge at the beat note of the two lasers 0.2 μm wide fingers separated by 1 μm THz radiation is emitted

6. Performance Limitations Beat Note Amplitude on Mixer Surface Conduction band e- ~850 nm NIR Driving Fields Valence band  0.25 psec time 2io2RLc()[mP1P2/Po2] [(1+22)(1+ 2RL2C2)] Prf() = 1/ 4 LT-GaAs poor conductor of heat

7. Bolometer sensitivity 1 pW/Hz1/2

8. ErAs:GaAs LT GaAs ErAs:GaAs Photomixers New Photomixers deliver more than >5-fold power

9. Why is resolution important in the THz region? S/N Limit ~1% Repeatability minimizes spectral artifactsCurrent resolution is2 parts in 10,000 ΔνLaser ~ 0.2 cm-1 (0.006 THz) ΔνLaser < 0.02 cm-1 (0.0006 THz)

10. Diode Amplifier Diode Laser + Nd:YAG (x2) Optical Isolator Ti:Sap Ring Laser BS 1/2 Wave Chopper Laser Cal. & Stabilization Single-mode Fiber Evacuated Sample Chamber 40 mWatts @ 850 nm T = 260 – 340 K Photomixer Bolometer Brass Cell Thermo-electric PID Controller ±0.2 C Fill/Pump Ports THz Photomixer Spectrometer for Line Shape Studies

11. THz Frequency Calibration System ΔνLOCK< 150 kHz ΔνLOCK< 0.5 MHz Stabilization Polarization Stabilized Lock-in HeNe Laser Electronics PID Servo PZT Heater Evacuated Reference Cavity Intensity ΔνLOCK< 0.5 MHz Stabilizer AOM Lock-in PID Servo Programmable RF Driver Ramp (12 Bits) Analog Sum 16 Bit Ramp Computer Ti:Sap Laser CW Ring Electronics Ti:Sap Laser Diode Laser Diode Laser/ Electronics Amplifier

12. Instrumental Linewidth < 3.0 MHz THz Studies of Ions and Radicals in Etching Plasmas used to Validate plasma models and improve recipes to increase etch uniformity and feature fidelity

13. 1 1 L=0.3 – 1 cm for strong lines Abs10 Abs10 0 0 L=53 cm for weak lines x175 0 THz 3.0 AM methods optimal between 10% and 90 % fractional absorption

14. Pure Lorentzian 4 MHz Doppler limited Spike small contribution to line shape Shift <1/20 of line width

15. Self-Width vs H2O Pressure Residuals Residuals

16. Self-Width vs H2O Pressure x3 different Temperatures 263, 300, 340 K

17. Self-Width vs H2O Pressure Error bars are included

18. Self-Shift vs H2O Pressure Error bars are included

19. Temperature Dependence on Width At 1.5 Torr H2O, 10-12 MHz changes 60 Γ(T) / Γ(T0)=(T0 / T)n wheren found between 0.56 – 0.81 48 36 δ(T) = (2-5) x 10-3 cm-1/atm 80-200 kHz/Torr is comparable to 100 kHz/Torr found for the 643-550 line in the mm region

20. Parameter Summary for weak lines of H2O >2-fold variation in shifts 1% on self-widths 5% on self-shifts 10-20% on temp dependence on widths V. B. Podobedov, D. F. Plusquellic, G. T. Fraser, JQSRT, 87, 377 (2004)

21. THz Studies vs HITRAN for Pure H2O at 300 K

22. THz Studies vs HITRAN for Pure H2O at 300 K Jinit= J + Ka - Kc Open – Experiment, Solid – Theorya aW. S. Benedict, L. D. Kaplan, JQSRT, 4, 453 (1964)

23. THz Instrumentation for H2O Foreign Gas Parameters FTIR Instrument 975 Torr ΔνRange = 10–250 cm-1 ΔνInst = 0.07 cm-1 Time = 35 min 0.9 cm-1/atm Ti:Sapp Instrument ΔνRange = 2-100 cm-1 / 1 cm-1 ΔνInst = 0.0005 cm-1 Time = 10 min 0.2 cm-1/atm New Ti:Sapp Instrument (single knob tunable) 15 Torr ΔνInst ~ 0.07 cm-1 (2000 MHz) ΔνRange = 2-100 cm-1 ΔνInst = <0.01 cm-1 Time = 30 min

24. Single Knob Tunable Ti:Sapp Laser stage-mounted retro-reflector M8 Ti:Sapp M1 532 nm Pump M2 10% M5 M3 OC 6:1 beam expander M = 1 M4 1800 grooves/mm M6 Stepper driven micrometer

25. High resolution Broadband THz Laser system Range >100 cm-1 at <0.02 cm-1 step resolution 2 parts in 10,000

26. Water Vapor Continuum High Sensitivity Long Path Length THz Studies Necessary for accurate retrievals of temperature and humidity profiles by EOS Water Vapor Continuum Absorption V. B. Podobedov, D. F. Plusquellic, G. T. Fraser, JQSRT, 91, 287 (2005)

27. THz White Cell Photomixer or FTFIR Spec Evacuated Sample Chamber 60 mm beam aperature M1 M0 Vol 3 ft3 M2 M3 M4 40 Pass White Cell M5 M6 Au Mirrors M0 & M6 Parabolic LHe cooled Bolometer • Path Length = 24 m • Temperature controlled to >70 C • No optical saturation issues

28. THz Water Vapor Continuum FTFIR Instrument and Sensitivity Polarizing Michelson Interferometer w/ Hg Lamp Source Range = 7-250 cm-1 Time = 35 min @ 0.07 cm-1 resolution Drift less than ±1.5 % T Abs10 = ±0.007 Minimum Values for Continuum Absorption T=297(1) K 2.5 Torr H2O 375 Torr N2 A = AR + ANR ANR = C1P2H2O + C2 PN2PH2O+ C3 P2N2

29. Pure H2O Line shape model important for local line absorption

30. Models of Local & Far-Wing Line Absorption Basic choices before application of far-wing absorption model Choice of lineshape function Lorentzian, Van Vleck Weisskopf How far to extend the lineshape Cutoff = 25 cm-1, 100 cm-1, infinite Typically 25 cm-1 useda or no cutoffb Number of water lines to consider Upper cutoff = 100 - 300 cm-1 aT. Kuhn, A. Bauer, M. Godon, S. Buhler, K. Kunzi, JQSRT 74, 545 (2002) bJ. R. Pardo, E. Serabyn, J. Cernicharo, JQSRT 68, 419 (2001)

31. Continuum Absorption of H2O Change is <10 % above 1 THz

32. Continuum Absorption of Pure H2O HITRAN 01 Γself = 4.8 Γair Expected ν2 dependence found Pair = 1.11 PN2 Windows where continuum absorbance largest relative to discrete line absorption and uncertainties in line intensities smallest

33. Continuum Absorption of H2O / N2 Mixtures

34. Continuum Absorption of H2O / N2 Mixtures ANR = ATotal - AR ANR AH2O-N2 = ANR – AH2O

35. Continuum Absorption of H2O / N2 Mixtures Potential Sources of discrepancy Near-wing line shape model Number of lines included to model resonant absorption Self-broadening and foreign parameters used From the perspective of atmospheric modeling, the total absorption is what is important! α(ν,T) = A * PH2O * PN2 * ν2 * (300/T)B Q. Ma, R. H. Tipping, J. Chem. Phys. 117, 10581 (2002) T. Kuhn, A. Bauer, M. Godon, S. Buhler, K. Hunzi, JQSRT, 74, 545 (2002)

36. Conclusions Current results on Self-width (1%), self-shift (%5) and temperature dependence of 6 weak lines from 12 cm-1 - 55 cm-1 (0.4 -1.7 THz) Continuum absorption of H2O-H2O and H2O-N2 Planned or in progress: Self-width, self-shift and temperature dependence for strong lines Foreign-width, shift and temperature dependence for strong lines Temperature dependence of the H2O-H2O and H2O-N2 continuum

38. Overlapping Scans to within +250 MHz FSR = 249.058 MHz

39. Optically pumped THz photomixer Operational range 0.1 – 4.5 THz Output power 10-6 - 10-8 W Linewidth 1 MHz Frequency drift <0.3 MHz/hour

40. Lens Lens Chamber

41. Backward Wave Oscillators B ~ 6 to 10 kG l Electron Beam ~1 to 0.03 mm d < Cathode v - e Fast R 3 to 6 kV Feedback Collector + L Slow Wave Structure Waveguide 1 to 20 mW - Strong Interaction of e and electromagnetic waves w 1 2eV = v v k = = ph e L m k e f D V w ~ v v 30% ~ ~ ph e L f

42. Continuous-Wave Backward-Wave Oscillators • Power: 1 mW to 50 mW • Linewidth: ~ 10 kHz • Frequency Range: to 1.2 THz • Bandwidth: 30 GHz to 200 GHz, dependent on frequency • Magnetic Field: 10 kG using permanent or electromagnetics. • Sensitivity approximately 0.001 % fractional absorption for 1 s integration. • BWO’s used: • 78 – 118 GHz (156 – 236 GHz with doubling). • 220-380 GHz • 450-750 GHz

43. BWO-based Spectrometer 50-850 GHz GPIB PLL Synchronizer f Frequency Synthesizer PC Reference F ref Modulation 54-118 GHz A/D & D/A Clock SRS Lock-In IF=350 MHz Mixer Low-noise BWO Control Amp BWO DF=100 MHz, t =2 ms InSb Bolometer 4.2K Beam W R=100 Splitter High Voltage Voltage Control Power Supply From D/A Card FuG

44. TT TG GG Potential of THz Methods for Detection of Chemical Agents • Agent precursor diethyl sulfide – CH2-CH3-S-CH2-CH3 • > 15% fractional absorption predicted • Detection limit using AM methodsdemonstrated near 0.2% 0.1 Torr in 100 Torr air sample Three conformers populated at room temperature Conformers intensities scaled according to MP2/ 6311++G(d,p) energies and dipole moments squared. Most vibrational sequence levels overlap within the pressure broadened linewidth ~1 GHz

47. Continuum Absorption of H2O

49. Grating-tuned Ti:Sap Laser Pump Laser Laser Cal. & Stabilization Computer Lock-in Lock-in THz Spectrometer BS BS waveplate Isolator Diode Amplifier Diode Laser BS BS Isolator 30 mW each @ 850 nm Photomixer and Si lens Bolometer Evacuated Sample Chamber Chopper Amplitude Modulation ~ 400Hz PhotoCurrent

50. Transmission Properties in the THz Region THz Scans Performed in Vacuum Plastic, Paper, Wood transparent