Weak Near-Infrared Aborption Lines in Water Vapour
Global warming is a "hot topic" these days, but "traditional
spectroscopy" is seen by some as "old-fashioned" or "boring".
What is the connection? An accurate spectroscopic model of the
atmosphere is vital to measuring the earth's radiation balance (how
much heat is coming in and how much heat is going out). The model
relies on a database of spectral absorption lines (such as HITRAN),
each with a measured wavelength, absorption strength and line
shape. This is where spectroscopy comes in - to populate the
database requires detailed, painstaking and very careful experiments
followed by very sophisticated analysis to determine the quantum
transitions that give rise to each observed absorption.
Measurements are required for each atmospheric consitituent, and it was
the case until recently that weak water vapour absorption features in
the near infra-red region were relatively poorly characterised.
Water is an important greenhouse gas not only because of its absorption
spectrum (especially in the long-wave infrared) but because its
concentration in the atmosphere will rise (by evaporation from the
oceans) if the planet warms. At the same time it became clear
that there was an unexplained discrepancy between the measured
radiation flux coming through the atmosphere and that predicted on the
basis of models.
It was clearly time for some high-quality experimental work!
The practical team comprised Roland Schermaul, Dick Learner, Jim Brault
from the USA and myself.
Measurements were made at the Molecular Spectroscopy Facility at the
Rutherford Appleton Laboratory using a Bruker IFS120 spectrometer and a
long-path White cell.
The White cell is a chamber about 9 metres in length which can be
evacuated and then filled with water vapour at any desired pressure and
temperature. The cell contains three mirrors which, with very
careful alignment can pass a
beam of light many times up and down the cell so that a very long
effective path is achieved. This permits even very weak
absorptions to be detected. It also gives the operator eye-strain
and a headache because one hase to count many small light spots and the
alignment is very sensitive at high path lengths: we had the cell set
for about 900 metres at one point, and routinely used about 50 passes.
The cell is illuminated by light from a high-resolution
Fourier-transform spectrometer - a commercial Bruker IFS120
instrument. The moving mirror on this instrument has a travel of
about 3 metres, so extremely fine spectral resolution can be achieved.
The instrument is really designed for operation in the infrared, so we
were pushing it quite hard to operate in the near infrared / visible
The standard detectors and electronic filters on the IFS120 were
judged not to be adequate for the task, so I designed and built custom
replacement units and fitted them to the instrument - directly to the
signal digitisers. Special optical bandpass filters were procured
and fitted to optimise the signal quality within each waveband of
interest, and we also carried out a very careful end-to-end
re-alignment of the interferometer and all auxiliary optics to ensure
peak performance. We are grateful to the MSF staff who put up
with our "disruptive behaviour" and allowed us to "mess about" with
their very expensive instrument - they showed considerable restraint at
An extremely pure sampl of water was prepared by Roland Schermaul and
freeze-distilled to remove all traces of air. The sample, in a
tiny flask, was connecetd to the evacuated White cell and the valve
bwtween the two was opened. The water level dropped slowly as the
vapour diffused into the White cell, and measurements started...
Considerable care was taken to operate the instrument at the peak of
its capabilities: we avregaed data over many hours, being careful of
ageing effects in the light source (which was operated very gently and
burned in for each run). We did not opt for maximum resolution
but instead selected the best resolution that we could achieve given
the shape of the spectrum and the prevailing noise floor; the data we
collected were therefore minimally redundant, and we were able to use
the available observing time to best advantage to reduce noise by
averaging many scans. This strategy paid off: our final signal to
noise performance was excellent!
Analysis and Results
The final data were analysed by Roland Schermaul, Dick Learner, Jim
Brault and a group headed by Prof Tennyson at University College,
London who are experts in the quantum mechanics of water molecules. The
process comprised very careful phase correction of the raw
interferograms followed by careful calibration of the wavelength scale
(which depends on the instrument alignment and optical beam solid
angles), determination of the background (zero absorption) level and
then extraction of each line with fitting to try to determine its
strength and shape. Finally the quantum mechanics tried to assign
each line to a transition of the molecule. The results can be
seen in two papers here
The study was very successful in that many new weak absprption features
were identified and transitions were assigned to some of them. It
was also possible to make some corrections to the established database.
In a small but hopefully valuable way our models of the atmosphere have
been improved. Who says spectroscopy is boring or useless?!