terahertz spectroscopy of excited water shanshan yu, john pearson, brian drouin jet propulsion...
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Terahertz spectroscopy of excited water
Shanshan Yu, John Pearson, Brian DrouinJet Propulsion Laboratory, California Institute of Technology, USA
Adam WaltersCentre d'Etude Spatialedes Rayonnements, Universite de Toulouse, France
Holger Müller and Sandra BrünkenI. Physikalisches Institut, Universität zu Köln, 50937 Köln, Germany
Copyright 2010 California Institute of Technology. Government sponsorship acknowledged.
Excited waterShanshan Yu
Herschel-HIFI (Heterodyne Instrument for the Far-IR)
• ESA and NASA joint mission
• Launch: 2009 (3 years lifetime)
• Telescope: 3.5 meter diameter, <100 K temperature
• The only space facility dedicated to the terahertz part of the spectrum
• Spectral coverage: 1910–1410 GHz; 1250–480 GHz
• Objectives: life cycle of gas and dust
Water spectroscopy
Shanshan Yu Excited water
• C2V symmetry, 11,000 cm-1 barrier to linearity
– Bond angle changes a lot (many degrees) even in pure rotation
– Bigger change upon exciting 2 mode
• Stretching slightly more rigid than bending but not much• Watson Hamiltonian does not converge!
– DKK4>AK2 at J~10
– DJ and (B+C)/2 are not much better
– Series in J(J+1) and K2 alternate in sign• Spectrum extremely difficult to extrapolate
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000
010
020
100
001
IF
IF IF
SF
IC
SC
SC
IF= Isolated Fermi ResonanceIC= Isolated Coriolis ResonanceSF=Strong Fermi ResonanceSC=Strong Coriolis Resonance
Note that this is limited to J=25.
Water spectroscopy-Strong coupling
Project Goals
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• Measure transitions to Ortho GS 1(1,0)-1(0,1) & 2(1,2)-1(0,1) – 1(1,0)-1(0,1) is done– 2(1,2)-1(0,1) remains to be done
• Measure transitions to Para GS 1(1,1)-0(0,0)
– 1(1,1)-0(0,0) done except for 22
• Measure weak and high J GS and 2 transitions
• Measure other low lying triad transitions• Check accuracy of previous measurements
– Some surprises in previous microwave measurements• Critically reviewing and fitting the lowest 5 states
– previous reduced RMS of 8.4 (some known calibration issues)
Experiments at JPL
H2O: 30 mTorr DC discharge: 400 mA, ~3 kV
Discharge
H2O
Sample cell
Pump
BeamsplitterRooftop reflector
FM
Rf Synthesizer Multiplier chain
PC
Si detector
Lock-in
×6×2
×3…
Source frequency: 300–1230, 1575–1626 GHz
Discharge cell: 1.2-meter-long
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Experiments at Cologne
• Frequency range– 290–968 GHz with BWOs;
– 1.42–1.45 THz with a VDI multiplier chain;
– 1.85–1.99 THz with a sideband spectrometer
• Numerous ways to generate hot water– RF-discharge (200 W, ~2 m);
– DC discharge (~2 kV, ~300 mA and 1.5 m);
– A pyrolysis oven (~1500 K and 50 cm absorption path);
– Heating tape (~450 K and 3.5 m)
• Pressures: 10–50 mTorr
• Detector: a composite InSb bolometer cooled with liquid He
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Excited waterShanshan Yu
Examples of hot water spectra
GS at 557 GHz; 2 at 658 GHz
3 at 524 GHz 1 at 540 GHz 2 at 793 GHz
1(1,0)-1(0,1) transitions
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High K ground state
119,3 – 1010,0 and 119,2 – 1010,1
Calculated position of 1010 is off by a few MHz
108,3 – 99,0 and 108,2 – 99,1
Laboratory spectrum!?
Absorption peak is up with our phase convention!
020 66,1-75,2
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Simple optically pumped maser
10,1
11,0
21,21669 GHz
557 GHz00,0
11,1
20,2
10,1
11,0
21,21753 GHz
658 GHz
maser
maser
1113 GHz
6 micro optical pumping
988 GHz
00,0
11,1
20,2
1205 GHz maser
899 GHz
Ground State
2 State
Ortho Para Excited waterShanshan Yu
Summary of observed H2O transitions
• 145 pure rotational transitions in its GS, 2, 22, 1 and 3 – Frequency range: 293 – 1969 GHz– 86 are new transitions– 1(1,0)-1(0,1) observed for all the five states– 2(1,2)-1(0,1) observed for GS, 1, 3, but missing for 2 (1753914
GHz), 22 (1872972 GHz)– 1(1,1)-0(0,0) observed for GS, 2, 1, 3 but missing for 22 (1332967
GHz)• Observed highest J– 18 for GS (E = 4174 cm-1)– 14 observed for 2 (E = 4174 cm-1)– 9 observed for 22 (E = 4774 cm-1)– 6 for 1 (E = 4381 cm-1)– 5 for 3 (E = 4126 cm-1)• About 10 transitions between 500–580 GHz to be measured at JPL
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Fitting water spectra with Euler series
Euler series obtained by transforming the angular momentum operators
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Pickett et al, 2005
Euler series
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D1,0 = A D0,1 = (B+C)/2
d0,0 = (B-C)/2
Pickett et al, 2005
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Euler series success
• H2O ground state and 2 to J=22 (JPL 2001)
– Subsequent TUFIR measurements agreed with in error bars – One high J line was off 3 MHz all others <1 MHz– Reduced RMS 1.9– Found a number of suspicious assignments in IR data
• D2O (Köln & JPL)
– Analyzed ground and 2 to reduced RMS of 1.6
• CH2 (Köln ~2003)
• Key is to get a clean data set and choose ‘av’ and ‘bv’
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Status of H2O data analysis
• Reviewing the lowest 5 states IR data in progress– Experimental uncertainty underestimated?– Calibration factor?– Bad measurements and blends?– Misassignments?
Thanks for your attention!
Excited waterShanshan Yu
Shanshan Yu Excited water
• Many approaches developed over the years– Padé Approximates (Burenin)– Borel Approximates (Polyansky)– Generating function in K (Tyuterev & Starikov)– Euler Series (Pickett)– Adjustable Bending potential (Coudert)– Full spectroscopic potential (Partridge & Schwenke; Tennyson, Polyansky & Zobov)
• Best for all levels is the full potential empirically adjusted by the observed spectrum– Still not to experimental accuracy (factor of a few for IR)– Not suitable for microwave transitions due to insufficient accuracy
• Euler series works and is in SPFIT– Fitted approach works can predict microwave transitions
Coping with nonconvergance
A closer look
66,1
75,2
77,0
55,0
65,2
63,3
4408.02880
4350.69931 62,4
4491.36973 64,2
72,6
74,4
4658.97471
4452.35271
54,2
52,4
4345.27202
4165.47381
4050.50370
4407.04635
4197.33874
4368.63692
4812.19276
020 State
Small moment to ground
001 State
Larger Moment to ground
Perturbation
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