CAVIAR’s 3d Quarterly report (1 November 2007)
UCL (sent by Ross Kelly):
Science:
Since the last report, Ross has finished test calculations on a new 12D water dimer potential for
low lying energy levels using the vibration-rotation program of Gerrit Groenenboom and Ad van der
Avoird (Nijmegen). We have since submitted a tri-lateral (Nijmegen, Emory, UCL) collaborative
paper [1] about this new potential produced by the Bowman group. The results suggest that this new
potential performs very well and is the best choice for the project at this present time [1].
However, we require many more states to simulate the water dimer absorption spectrum. We have
made significant progress towards this aim.
A good improvement of the Nijmegen program has been made by the implementation of a new
diagonalisation scheme for the Hamiltonian matrix, called ARPACK.
Using 64-bit machines in the UCL department, we have been able to find the required number of
states up to the water dimer dissociation. There seems to be around 320 states for each calculation, so
that by diagonalising for 350 states should be sufficient. However, we need to perform many
calculations (nearly 100) to get a decent range of energy levels, and hence energy transitions.
One issue that is still a problem is the run times for each job: for the best accuracy computations
on a 64-bit machine with Intel Core 2 processors, one calculation for 350 states takes one month. One
possibility discussed before was the use of the 2000+ node UCL research computing facility to be in
use quite soon. Ross has since been on a parallel computing course organised by UCL computing
services (at Cambridge) and is looking to parallelise the current code.
Since Matt's arrival, work has begun on compiling a water linelist for the region of interest
including the necessary weak transitions to model the temperature effects on the absorption correctly.
Matt has been using the in-house Goblin software, together with input from Hitran and other
published sources to produce some preliminary absorption spectra of this region.
[1] X. Huang, B. J. Braams, J. M. Bowman, R. E. A. Kelly, J. Tennyson, G. C. Groenenboom, A.
van der Avoird. J. Chem. Phys. 2007, Submitted (Text of the manuscript is available on CAVIAR
internal page. /I.P./).
NPL (from Tom Gardiner)
The FTIR facility has now been moved to the new NPL laboratory and successfully
commissioned. Initial solar measurements will begin as soon as suitable weather conditions occur.
The detailed planning for the first field campaign is underway. A pre-field trial visit to the MET
Office site at Camborne was carried out in order to assess all local logistics for installing and
operating the FTIR system. This went very well, with very positive support from the local MO
personnel. A suitable location has been identified capable of housing the spectrometer and providing
the required services – a small separate building previously used for training purposes. The site
includes an ideal location for mounting and aligning of the solar tracker device during measurements.
The tracker will be mounted on a support structure bolted to an external concrete base for where the
solar beam can be easily directed into the spectrometer. From the details acquired during the visit we
have been able to design a support structure for use with the tracker and construction of this will
begin shortly.
The detailed design of blackbody, including the interfaces to the instruments, has been completed
and agreed by all the relevant groups. The blackbody is now in production, and is due to be completed
by the end of the year, together with the the liquid controller/circulator which is on order. Work on
the cooled dry purge gas system is underway, and testing of this system are in progress.
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RAL (from Robert McPheat)
We have completed manufacturing the components of the SPAC upgrade and have test assembled
most of it. The transfer optics have been designed and assembly is underway.
Following analysis of our initial mid-IR spectra at Reading we have ordered a new filter for this
region which should exhibit much reduced channel fringes. The filter is expected in December.
We have made initial LPAC measurements in the mid to near IR region 1800 to 4500 cm -1 and
2100 to 10500 cm-1 (water & water/air mixtures from 32m, 128m & 512m path length, approx. 293K,
maximum total pressure 1 bar). The initial data from this region will be supplied to the University of
Reading shortly. We believe the problems with the LPAC cooling have been resolved but have not
yet tested it.
In the next 3 months we intend completing the final assembly of the SPAC and making the first
measurements with it. These will be in the mid to near IR region, total pressures up to 0.5 MPa,
temperatures up to 450K.
There are no new staff or students.
Leicester and Cambridge (from Stephen Ball and Alex Shillings)
Leicester has now ordered cavity mirrors & high-power light emitting diodes at wavelengths
covering the 3rd, 4th and 5th overtones of the hydrogen bonded OH stretch in the dimer’s donor H2O
molecule. I’ve specified these components according to wavelengths taken from the most recent
theoretical work of Schofield, Lane & Kjaergaard (JPC A 111, 567, 2007) which, relative to their
previous calculations, predicts the dimer overtones to be shifted even further to the red of the
corresponding monomer bands: v = 3 at 980 nm, v = 4 at 755 nm and v = 5 at 620nm. If true, this will
aid detection of overtone absorptions in the Leicester (& Cambridge) lab studies.
In anticipation of starting pulsed-nozzle experiments on the 980 nm band, an undergraduate
project student has been surveying various low-power (& hence cheap!) near-IR LEDs sourced from
mainstream electronics suppliers. Our experience at visible wavelengths is that the emission spectra
of even very cheap LEDs provide good indicators of the likely bandwidths available from the much
high-powered LEDs required for cavity enhanced experiments. Thus the project student’s work
informed the choice of LEDs in our recent order. Once these arrive (ETA ≈ 1 more week), we will
identify the best light source for each set of mirrors by installing them into our BBCEAS field
instrument whose performance we know very well at visible wavelengths. The 980 nm optics & most
compatible LED will then be moved across to the pulsed-nozzle chamber to target the strongest v = 3
overtone of those currently under consideration. The chamber is available to us from next week, as
previously agreed with my Leicester colleague: there are some hardware modifications to be made to
the chamber apparatus, but these are on the timescale of a few weeks.
Laboratory work at Cambridge in this quarter has focussed on the wavelength region between 595
and 625nm, where a dimer feature is predicted to lie between major water absorption bands. The
BBCRDS system has not previously been used in this region, and as such, time consuming
investigative work on different dye solutions for the broadband laser (to generate the appropriate
colour light) has been required. We have now recorded absorption spectra of the 610 to 625 nm
region at high temperature and [H2O] (up to 20% H2O (by mass)). Preliminary analysis has shown no
obvious sign of dimer absorption features, although comprehensive analysis is ongoing. Even on a
preliminary analysis, however, we do observe significant disagreement between our measurements of
water monomer lines and the most recent HITRAN database. We are currently investigating the 595
to 610 nm region.
At the time of writing the last quarterly report, Cambridge & Leicester had been looking into
buying an incandescent light source for  > 1000 nm. However, Cambridge has since had the
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opportunity to test a super-continuum laser which, potentially, is a more easily collimated brighter
source out to 2 m.
Post-doc: combining Leicester’s CAVIAR post-doc with 6 months of field work post-doc funding
hasn’t proved possible in practice. I will therefore be advertising an 18-month CAVIAR position in
the next quarter.
Imperial College (from Paul Green)
WP3.1 TAFTS optics upgrade.
To increase the reliability of the instrument a program of renovation and upgrading has been
undertaken. The optical redesign part of this has progressed well. The linear motor is in the process of
being fitted into the instrument. Bench testing is complete, with the motor performing as hoped,
although this testing has highlighted a few issues that need to be addressed in the installation.
Electrical modifications required for the control and feedback system are now being wired into the
control box, with the new wiring schedules complete. Appropriate connectors and cabling
components are being sourced. Mechanically, the mirror mounting system is being rendered in 3D
AutoCAD for manufacture in the College workshop. An appropriate failsafe brake [needed for sudden
loss of power] is being designed at present.
The renovation of the internal laser system is progressing well. Alternate laser systems have been
researched, with a HeNe gas laser still being the most appropriate for the environmental conditions.
New potting and insulation methods are currently being investigated for the high voltage power
supply.
The installation of new beamsplitters now look impossible for the Spring 2008 calibration and
Summer 2008 flying program. (What precisely it may cause? Limitation of TAFTS accurate
measurements by 500-600 cm-1 from above…? /I.P./) The options will continue to be investigated
with possible implementation for the summer 2009 flying program. A discussion with Cardiff Uni
will take place next week [6th Nov].
The schedule for re-construction is on track for December 2007, with system testing to commence
in January 2008. FAAM has been contacted regarding a possible test flight in early 2008 to evaluate
the new components in flight.
A flight planning meeting for the Summer 2008 flight will be held on 20th November in Exeter. As
preparation, a short simulation study has been initiated to produce realistic simulated data for the
experiment. The purpose of which is to address any specific flight requirements for the continuum
study from the TAFTS and ARIES datasets.
WP3.2 TAFTS calibration at NPL
A calibration plan for the Spring 2008 calibration has been written and the outstanding issues are
being worked through.
WP5 Impact on Understanding
We are in position to be able to study the impact of changes in the continuum/dimer absorption
spectrum and its dependence on the key atmospheric parameters once new data are produced by the
project. At present, however, relatively small amounts of effort are being expended on WP5,
recognizing that all available effort is needed for the instrument upgrade.
Ralph Beeby, the IC PhD student started at the beginning of October 2007 as planned.
Met. Office (from Stuart Newman)
Stuart Newman accompanied Tom Gardiner and Marc Coleman of NPL to Camborne radiosonde
station on 19 September. This was an extremely useful visit in preparation for NPL ground-based
measurements and FAAM BAe 146 flights in 2008. A building for location of NPL's FTS was
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identified. As well as microwave and GPS water vapour instrumentation there were a number of other
radiometers set up on site – discussions continue as to which of these will be available during the
flying period. As well as the daily 1115 UTC radiosonde launches it should be possible to launch
additional sondes to coincide with FAAM overflights.
A meeting for interested parties to discuss flying plans in more detail will be held at the Met
Office on 20 November.
Reading univ. (from Keith Shine)
Igor Ptashnik finished processing of the preliminary data from the RAL (R.M. & K.S.) Long-pathabsorption-cell (LPAC) measurements in the spectral region 600 – 3000 cm-1. The channel “fringes”
in the initial interferograms (probably due to multi-layers structure of the filter) limits confidence
about continuum absorption in the band wings; however, the continuum structure within 1600 cm-1
band was retrieved rather confidently and revealed: 1) good agreement with previous short-path
measurements by David Paynter; 2) the central peak of the residual is in a good agreement with
Schofield & Kjaergaard (S&K) ab initio WD prediction for OH-stretching transitions in WD. Two
“satellite” residual peaks are (in relative units) in a good agreement with the recent ab-initio
calculations by V. Buch (Hebrew University, Israeil - unpublished) which accounts also (by contrast
with the S&K approach) for the intermolecular oscillations in WD.
/Res. 0.01 cm-1; 295K; 128.75m; 1.7mm apert., f=418.0mm/
Optical depth
1.0
CKD-2.4 continuum
MT_CKD continuum
Dimers, ab initio (S&K-2003)
RAL (Exper.-HITRAN-2004)
10 point FFT Smoothing
17.85 mb pure H2O
Threshold=3.5; Grad.=10
Keq=0.04 atm-1, hwhm=30 cm-1
Only microwindows are taken into
account during data processing, and
only those with Opt.depth < 3.5.
0.5
0.0
1200
Fig. 1. Pure water vapour RAL
(Robert McPheat & Kevin Smith)
measurements. The dots show
experimental continuum, calculated
as: <RAL – HITRAN-2004>1cm-1
1400
1600
1800
Wavenumber, cm-1
2000
More accurate numerical assessment of the best spectral regions for WD detection (using S&K
WD intensities and positions) in pure water vapour and in atmospheric conditions for near-IR spectral
region is nearly finished and will be available soon through the CAVIAR web-page.
JQSRT paper of I.P. “Evidence for the contribution of water dimers to the near-IR water vapour
self-continuum” is accepted for publication and corrected proof is available on the CAVIAR internal
web-page.
New tied student, Adam Bamford, started on 1 October 2008. To date, he has mostly been
attending MSc modules and doing some background reading. He will visit both NPL and Exeter (for
the planning meeting for the first field campaign) during November.
Any comments & suggestions are welcome.