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

Excited waterShanshan Yu

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

Excited waterShanshan Yu

• 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

Excited waterShanshan Yu

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

Excited waterShanshan Yu

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

Excited waterShanshan Yu

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

Excited waterShanshan Yu

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

Excited waterShanshan Yu

Fitting water spectra with Euler series

Euler series obtained by transforming the angular momentum operators

Excited waterShanshan Yu

Pickett et al, 2005

Euler series

Excited waterShanshan Yu

D1,0 = A D0,1 = (B+C)/2

d0,0 = (B-C)/2

Pickett et al, 2005

Excited waterShanshan Yu

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’

Excited waterShanshan Yu

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

Excited waterShanshan Yu