Professor Marissa Weichman (Princeton) "Precision Cavity-Enhanced Spectroscopy for Astrochemistry and Atmospheric Science"

"Precision Cavity-Enhanced Spectroscopy for Astrochemistry and Atmospheric Science"

Light beaming to us across space encodes features arising from the absorption and
emission of photons by a menagerie of interstellar molecules. Identification of these
astrochemical species is key to understanding the chemical underpinnings of star, planet,
and galaxy formation and the prebiotic origins of life. Interpretation and assignment of
observational data has historically been driven by laboratory spectroscopy, though resolving
individual quantum states in large molecules − even those with as few as 20 atoms −
remains a challenging endeavor on the brink of treatment with our existing toolkit. As it
becomes clear that astrophysical environments are brimming with complex,
spectroscopically-challenging species, we must develop next-generation methods with
improved resolution, sensitivity, and scope.

Cavity-enhanced frequency comb spectroscopy simultaneously permits excellent spectral
resolution, high sensitivity, and broadband readout − all essential for detailed spectroscopy
of large gas-phase molecules. This technique centers around the use of frequency combs:
light sources that emit a spectrum consisting of thousands of narrow, evenly-spaced “comb
teeth” which act as many individual stable lasers lasing synchronously. We match the
comb's spectral lines to the resonant modes of an optical cavity containing the molecular
absorber of interest. The cavity dramatically increases the pathlength of light through the
sample, yielding sensitive absorption spectra with broadband detection across the comb
spectrum.

My lab has recently commissioned a state-of-the-art cavity-enhanced comb spectrometer
operating in the long-wave mid-infrared. Building on my prior work on the C 60 fullerene − by
far the largest and highest-symmetry molecule for which a fully quantum-state-resolved
spectrum has been reported – we are now extending comb spectroscopy to achieve a
similarly clear picture of the quantum states of a library of polycyclic aromatic hydrocarbons
and fullerenes likely to survive in harsh astrochemical environments. We will directly
compare our measured spectra to observational data with the aim of identifying specific
molecular carriers of interstellar features and unlocking new astrochemical assignments.
Time-permitting, I will also touch on my group’s parallel efforts to advance laboratory
methods for atmospheric aerosol science. Aerosols – particles and droplets between 10 nm
and 10 µm in diameter – are among the largest sources of uncertainty in models of Earth’s
climate through their direct interactions with solar radiation and indirect impacts on clouds.
Studying isolated single aerosols in the laboratory allows us to directly correlate particle size
and composition with optical properties, microphysics, and chemistry. We have recently
commissioned a new single-particle electrodynamic balance in which we can trap and
levitate individual aerosols, and interrogate them with cavity-enhanced spectroscopy and
Mie scattering. We are working towards understanding how aerosols are transformed during
their atmospheric residence times via processes like chemical aging, photobleaching,
wetting, and icing.