MOLECULAR SPECTROSCOPY AT THE UNIVERSITY OF TORONTO
AND FUTURE APPLICATIONS TO MARS
M. O. Ishtiak, O. Colebatch, and K. Strong Department of Physics, University of Toronto, Toronto, ON,
Canada (osama.ishtiak@mail.utoronto.ca)
Introduction:
Molecular spectroscopy measurements in our lab
at the Department of Physics are being resumed with
the restoration of a small cell and the commissioning
of a multi-pass White cell. Both gas cells are coupled
to a Fourier-transform infrared spectrometer to acquire spectral data. The small cell is being used to
retrieve absorption-cross sections, whereas the multipass cell, once commissioned, will allow the measurement of weakly absorbing gases. In particular, the
multi-pass cell will enable the measurement of CO2broadened spectroscopic line parameters. These
CO2-broadened parameters have applications to Martian trace gas retrievals because the Martian atmosphere primarily consists of CO2. This presentation
will provide an overview of the laboratory spectroscopy facility, absorption cross sections obtained with
the small cell, and plans for future measurements.
The Small Cell and the Multi-pass Cell:
The small cell system consists of a high resolution (0.004 cm-1) ABB Bomem DA8 Fouriertransform spectrometer and a 10 cm sample cell inside a vacuum jacket, shown in Figures 1 and 2. The
reassembled small cell has been heat-cleaned to reduce outgassing and enable accurate pressure measurements needed to retrieve absorption crosssections. Spectra of two halogenated compounds
were recorded and analyzed to test the spectroscopic
accuracy of the system. In both cases, spectral measurements and derived absorption cross-sections
agreed with previous measurements (Godin et al.,
2017; Godin et al., 2019) within uncertainty. Therefore, the small cell can be used to retrieve absorption
cross-sections, but spectral measurements of weakly
absorbing gases are not possible because the optical
path through the cell is limited to 10 cm.
A multi-pass cell can be used to measure the
spectra of weakly absorbing gases by significantly
increasing the optical path through the sample. One
way to accomplish this is using a cell with spherical
mirrors of the same radii of curvature placed on either end, as is shown in Figure 3 (White, 1942). Indicated in red is the optical path of the multi-pass
cell currently being commissioned. This path involves a parabolic mirror focusing a parallel beam
from the spectrometer into the cell. Given the arrangement of mirrors inside the cell, any light that
illuminates mirror 1 is focused to a point on mirror 3
and reflected to a corresponding point on mirror 2,
and vice versa, increasing the optical path length.

The beam then exits the cell and is focused onto the
detector by parabolic mirrors. The mirrors inside the
cell have a theoretical reflectance of >98%, allowing
up to 25 reflections and providing a path length of 50
meters.
Applications to Mars:
Martian trace gas retrievals require CO2broadened spectroscopic line parameters. For example, efforts to retrieve atmospheric profiles of CH4
from spectra recorded by the ExoMars Atmospheric
Chemistry Suite (ACS) and the Nadir and Occultation for Mars Discovery (NOMAD) instruments
(Korablev et al., 2019) use line parameters from the
high-resolution transmission molecular absorption
database (HITRAN) (Gordon et al., 2017). Presently,
the database is missing some CO2-broadened spectroscopic line parameters for Martian trace gases
such as CH4, C2H6, and C2H4. Furthermore, some
line parameters are missing for HCl (Korablev et al.,
2021). The multi-pass system will enable the measurement of such parameters to fill the gaps in the
HITRAN database, and this effort may help with
future retrievals of trace gases in the atmosphere of
Mars.
References:
Godin, P. J., Johnson, H., Piunno, R., Le Bris, K., &
Strong, K. (2019). Conformational analysis and
global warming potentials of 1,1,1,2,3,3hexafluoropropane and 1,1,2,2,3-pentafluoropropane
from absorption spectroscopy. Journal of Quantitative Spectroscopy and Radiative Transfer, 225, 337–
350. https://doi.org/10.1016/j.jqsrt.2019.01.003
Godin, P. J., Le Bris, K., & Strong, K. (2017). Conformational analysis and global warming potentials
of 1,1,1,3,3,3-hexafluoro-2-propanol from absorption spectroscopy. Journal of Quantitative Spectroscopy and Radiative Transfer, 203, 522–529.
https://doi.org/10.1016/j.jqsrt.2017.04.031
Gordon, I., Rothman, L., Hill, C., Kochanov, R.,
Tan, Y., Bernath, P., Birk, M., Boudon, V., Campargue, A., Chance, K., Drouin, B., Flaud, J.-M.,
Gamache, R., Hodges, J., Jacquemart, D., Perevalov,
V., Perrin, A., Shine, K., Smith, M.-A., … Zak, E.
(2017). The HITRAN2016 molecular spectroscopic
database. Journal of Quantitative Spectroscopy and
Radiative Transfer, 203, 3–69.
https://doi.org/10.1016/j.jqsrt.2017.06.038

Korablev, O., Olsen, K. S., Trokhimovskiy, A.,
Lefèvre, F., Montmessin, F., Fedorova, A. A., Toplis, M. J., Alday, J., Belyaev, D. A., Patrakeev, A.,
Ignatiev, N. I., Shakun, A. V., Grigoriev, A. V.,
Baggio, L., Abdenour, I., Lacombe, G., Ivanov, Y.
S., Aoki, S., Thomas, I. R., … Vandaele, A. C.
(2021). Transient HCl in the atmosphere of Mars.
Science Advances, 7(7), eabe4386.
https://doi.org/10.1126/sciadv.abe4386
Korablev, O., Vandaele, A. C., Montmessin, F., Fedorova, A. A., Trokhimovskiy, A., Forget, F.,
Lefèvre, F., Daerden, F., Thomas, I. R., Trompet, L.,
Erwin, J. T., Aoki, S., Robert, S., Neary, L., Viscardy, S., Grigoriev, A. V., Ignatiev, N. I., Shakun,
A., Patrakeev, A., … Teams, N. S. (2019). No detection of methane on Mars from early ExoMars Trace
Gas Orbiter observations. Nature, 568(7753), 517–
520. https://doi.org/10.1038/s41586-019-1096-4

Figure 1: The 10 cm cell inside the vacuum jacket.

White, J. U. (1942). Long optical paths of large
apeture. J. Opt. Soc. Am., 32(5), 285–288.
https://doi.org/10.1364/JOSA.32.000285
Yan, P. (2008). The White cell system [Technical
Report, Department of Physics, University of Toronto].

Figure 2: A top-down view of the small cell setup
including the Bomem DA8 and the sample insertion
system. The cell is inside the large cylindrical
vacuum jacket.

Figure 3: Path of the Bomem DA8 beam through the long-path system into the multi-pass cell and to the
detector (Yan, 2008). Mirrors 1,2,3 are gold plated spherical mirrors of the same radii of curvature. Mirrors 1
and 2 are D-shaped whereas mirror 3 is T-shaped. Mirrors 4,5,7 are parabolic mirrors used to direct the beam.
Mirror 6 is a flat collimating mirror.