xperiments were performed at Institut e de Planétologie et d’Astrophysique de Grenoble. Thin films of several carboxylic acids liquid at room… [618622]

Our e
xperiments were performed at Institut
e

de Planétologie et d’Astrophysique de
Grenoble.
Thin films of several carboxylic acids liquid at room temperature (showed at table 1)
were

produced with a cryo
–
cooler combined to a vacuum chamber maintained at 10
–
8
mbar. They
were condensed on a KBr window held at 25 K from vapor deposition. Then they were warmed
up with a resistor and a PID controller (Lake Shore 331). In a first step, the sample was heated
until sublimation and spectra were collected during warming

up. In a second step, another
experiment was run and spectra were collected on warming and cooling back to 25 K, in order to
locate amorphous
–
crystal transition and to avoid artifact due to hysteresis.

For carboxylic acids available as a powder at room t
emperature, samples were prepared
as
pellet of mixture 1 mg of carboxylic acids and 300 mg of pure KBr p
owder and
poured into the
mold with diameter 13

mm then
compressed
to 400 mbars during 2 hours to get good optical
quality.
During compressing, the pell
et underwent heating at 50°C to remove water contamination.
Thin film of carboxylic acids powder were also generated using spin coater.

The broad
3.2 µm band in the VIRTIS spectra of 67P/CG spans the 2.8
–
3.7 µm range.
Vibration modes of OH and CH bonds are potential candidat: [anonimizat]. The OH group in
carboxylic acid

is

expected
to
form a broad feature (onto which combinations or overtones of CHx.

We have presented spectra of solid carboxylic acids from single until five
number of
atom
C.
Number of atom C1
–
C4 carboxylic acids with an aliphatic side chain display a broad feature in
the 3 μm region controlled by the OH group, onto which are superimposed C
–
H stretching and
overtones

modes. This band has however components towards long waveleng
ths (3
.7
–
4.
1

μm),
which do not appear in VIRTIS spectra. Above C5, the prominent and sharp aliphatic massif at
3.4 μm dominates the 3 μm region and does not account for the broadness of the 3.2 μm feature.

For dicarboxylic (C4 fumaric acid)
the presence of

2 OH bond of acids produce the spectra
that spread from 2.9
μm

until 3.4
μm
. For

hydroxylated acids (C2 glycolic and C3 lactic acids), the
presence of
hydroxyl group

leads to a broadening of the 3 μm feature with respect to their related
monocarboxylic ac
ids, however side bands between
3.7 until
4.
5
μm still present

for glycolic acid
,
though less intense in lactic acid
.

The CO molecule has recently been identified on the phases.

icy surfaces of Triton and Pluto by Cruikshank
et al.
(1993) Precise knowledge of phase and temperature effects on

and Owen
et al.
(1993), respectively. These authors have the 0
–
2 band of CO is necessary to ensure the use of

recorded high
–
resolution near
–
infrared reflectance spectra relevant and accurate optical constants for bidirectionnal

(with UKIRT
1
CGS4 infrared detector, Mauna
Kea, models of recent Triton and Pluto spectra; however, al
–

Hawaii), in which they confidently assigned a band peaking though many studies have been devoted to the
spectrosat

p
4251 cm
2
1
(2.3524
e
m) to the first overtone of CO. copy of CO isolated in a nit
rogen matrix (Dubost 1975,

On the basis of thermodynamical considerations, they sug
–

Dubost
et al.
1982, Nelander 1976, Quirico and Schmitt

gested that this molecule would be diluted in solid N
2
. 1997), no data are available over an extended temperature

U
sing a bidirectionnal reflectance model, they estimated range and in the wavelength region of the first overtone

the relative concentration of CO with respect to N
2
(0.1
%
which appears in Triton and Pluto spectra. The aim of this

on Triton, 0.5
%
on Pluto)