To save content items to your account,
please confirm that you agree to abide by our usage policies.
If this is the first time you use this feature, you will be asked to authorise Cambridge Core to connect with your account.
Find out more about saving content to .
To save content items to your Kindle, first ensure email@example.com
is added to your Approved Personal Document E-mail List under your Personal Document Settings
on the Manage Your Content and Devices page of your Amazon account. Then enter the ‘name’ part
of your Kindle email address below.
Find out more about saving to your Kindle.
Note you can select to save to either the @free.kindle.com or @kindle.com variations.
‘@free.kindle.com’ emails are free but can only be saved to your device when it is connected to wi-fi.
‘@kindle.com’ emails can be delivered even when you are not connected to wi-fi, but note that service fees apply.
In dense molecular clouds, the birthplace of stars and planets, interstellar atoms and
molecules freeze onto extremely cold dust and ice particles. These ices are processed by
ultraviolet light and cosmic rays forming hundreds of far more complex species, some of
astrobiological interest. Eventually, these rain down on primordial planets where they
take part in the young chemistry on these new worlds.
Although the IR spectroscopy and energetic processing of interstellar ice analogs have
been studied for nearly 30 years, similar studies of PAH containing ices have only just
begun. This paper presents recent results from laboratory studies on the vacuum
ultraviolet (VUV) photochemistry of PAHs in water ice at low temperatures to assess the
roles they play in the photochemical evolution of interstellar ices and their relevance to
astrobiology. A number of “surprises” were found in these studies on PAH containing
water-rich ices, indicating that PAHs likely play very important, unexpected roles in
cosmic ice chemistry, physics and astrobiology.
The mid-infrared emission of Polycyclic Aromatic Hydrocarbons is found in many phases of
the interstellar medium. Towards cold dense clouds, however, the emission is heavily
quenched. In these regions molecules are found to efficiently freeze-out on interstellar
grains forming thin layers of ices. PAHs are highly non-volatile molecules and are also
expected to freeze-out. PAHs trapped in interstellar ices are likely to participate in the
overall chemistry, leading to the formation of cations and complex molecules in the
solid-state. The work presented here aims to experimentally study the chemical reactions
that PAHs undergo upon vacuum ultraviolet irradiation when trapped in interstellar
Polycyclic Aromatic Hydrocarbons (PAHs) are widespread across the Universe and influence
many stages of the Galactic lifecycle. The presence of PAHs has been well established and
the rich mid-IR PAH spectrum is now commonly used as a probe into (inter)stellar
environments. The NASA Ames PAH IR Spectroscopic Database has been key to test and refine
the “PAH hypothesis”. This database is a large coherent set (>600 spectra) of
laboratory measured and DFT computed infrared spectra of PAHs from
C10H8 to C130H28 and has been made available
on the web at (http://www.astrochem.org/pahdb). With a new spectral window opening up; the
far-IR, the study of PAH far-IR spectra and the quest for identifying a
unique member of the interstellar PAH family has begun. To guide this research, the far-IR
(>20 μm) spectra of different sets of PAHs are investigated
using the NASA Ames PAH IR Spectroscopic Database. These sets explore the influence of
size, shape, charge and composition on the far-IR PAH spectrum. The far-IR is also the
domain of the so-called “drumhead” modes and other molecular vibrations involving low
order bending vibrations of the carbon skeleton as a whole. As with drums, these are
molecule and shape specific and promise to be a key diagnostic for specific PAHs. Here,
the sensitivity of these “drumhead” modes to size and shape is assessed by comparing the
frequencies of the lowest drumhead modes of a family of circular shaped (the coronene
“family”) and rhombus shaped (the pyrene “family”) PAH molecules. From this study, some
consequences for an observing strategy are drawn.
Laboratory spectra through the mid-infrared have been used to calculate the optical constants (n and k) for a variety of pure and mixed molecular ices. The ices studied were H2O, CH3OH, CO2, OCS, CH4, CO2+CH4, CO2+OCS, CO+CH4, CO+OCS, O2+CH4, O2+OCS, N2+CH4, N2+OCS, H2O+CH4, H2O+OCS, and H2O+CH3OH+CO+NH3.
Email your librarian or administrator to recommend adding this to your organisation's collection.