Photodesorption of CO ice

Abstract

At the high densities and low temperatures found in star forming regions, all molecules other than H2 should stick on dust grains on timescales shorter than the cloud lifetimes. Yet these clouds are detected in the millimeter lines of gaseous CO. At these temperatures, thermal desorption is negligible and hence a non-thermal desorption mechanism is necessary to maintain molecules in the gas phase. Here, the first laboratory study of the photodesorption of pure CO ice under ultra high vacuum is presented, which gives a desorption rate of 3E-3 CO molecules per UV (7-10.5 eV) photon at 15 K. This rate is factors of 1E2-1E5 larger than previously estimated and is comparable to estimates of other non-thermal desorption rates. The experiments constrains the mechanism to a single photon desorption process of ice surface molecules. The measured efficiency of this process shows that the role of CO photodesorption in preventing total removal of molecules in the gas has been underestimated.

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arXiv:0705.0260v1 [astro-ph] 2 May 2007
Draft version February 1, 2008
Preprint typeset using LATEX style emulateapj v. 10/09/06
PHOTODESORPTION OF CO ICE
Karin I.¨Oberg1∗, Guido W. Fuchs1, Zainab Awad1, Helen J. Fraser2, Stephan Schlemmer3, Ewine F. van
Dishoeck4and Harold Linnartz1
Draft version February 1, 2008
ABSTRACT
At the high densities and low temperatures found in star forming regions, all molecules other than
H2 should stick on dust grains on timescales shorter than the cloud lifetimes. Yet these clouds are
detected in the millimeter lines of gaseous CO. At these temperatures, thermal desorption is negligible
and hence a non-thermal desorption mechanism is necessary to maintain molecules in the gas phase.
Here, the first laboratory study of the photodesorption of pure CO ice under ultra high vacuum is
presented, which gives a desorption rate of 3 × 10−3CO molecules per UV (7–10.5 eV) photon at 15
K. This rate is factors of 102-105larger than previously estimated and is comparable to estimates of
other non-thermal desorption rates. The experiments constrains the mechanism to a single photon
desorption process of ice surface molecules. The measured efficiency of this process shows that the role
of CO photodesorption in preventing total removal of molecules in the gas has been underestimated.
Subject headings: Molecular data — Molecular processes — ISM: abundances — Physical Data and
Processes: astrochemistry — ISM: molecules
1. INTRODUCTION
In the cold and dense interstellar regions in which
stars are formed, CO and other molecules collide
with and stick to cold (sub)micron-sized silicate par-
ticles, resulting in icy mantles (L´ eger et al. 1985;
Boogert & Ehrenfreund 2004). Chemical models of these
regions show that all molecules except for H2are removed
from the gas phase within ∼ 109/nH years, where nH
is the total hydrogen number density (Willacy & Millar
1998). For a typical density of 104cm3, this time scale
is much shorter than the estimated age of such regions
and hence molecules like CO should be completely frozen
out in these clouds. Yet, these clouds are detected in
the millimeter lines of gaseous CO (Bergin et al. 2001,
2002). Similarly cold CO gas has been detected in the
midplanes of protoplanetary disks (Dartois et al. 2003).
A recent study of several disks (Pi´ etu et al. 2007) even
finds that the bulk of the gaseous CO is at temperatures
lower than 17 K, below the condensation temperature
onto grains. Thus some desorption mechanism is needed
to keep part of the CO and other molecules in the gas
phase. Clarifying this desorption mechanism is impor-
tant in understanding the physical and chemical evolu-
tion of interstellar clouds. Because the sticking probabil-
ity of even volatile species like CO has been shown to be
unity (Bisschop et al. 2006), it is the desorption mecha-
nism and rate that controls the allocation of molecules
between gas and solid phase. This allocation of molecules
affects the gas phase and surface reactions as well as the
⋆To whom correspondence should be addressed;
oberg@strw.leidenuniv.nl.
1Sackler Laboratory for Astrophysics, Leiden Observatory, Uni-
versity of Leiden, P.O. Box 9513, NL 2300 RA Leiden, The Nether-
lands.
2Department of Physics, Scottish Universities Physics Alliance
(SUPA), University of Strathclyde, John Anderson Building, 107
Rottenrow East, Glasgow G4 ONG, Scotland.
3Physikalisches Institut, Universit¨ at zu K¨ oln, Z¨ ulpicher Str. 77,
50937 Cologne, Germany
4Leiden Observatory, University of Leiden, P.O. Box 9513, NL
2300 RA Leiden, The Netherlands.
E-mail:
dust properties.
The case of CO is of particular importance, as it
is the most common molecule after H2 and the prime
tracer of molecular gas. It is also a key constituent in
the formation of more complex and pre-biotic species
(Tielens & Charnley 1997), and its partitioning between
the grain and gas phase therefore has a large impact on
the possible chemical pathways (van Dishoeck 2006). In
dense clouds without embedded energy sources, the grain
temperature is low enough, around 10 K, that thermal
desorption is negligible and hence desorption must occur
through photon or cosmic ray induced processes. Exter-
nal UV photons from the interstellar radiation field can
penetrate into the outer regions of dense clouds and cos-
mic rays are always present, even in the most shielded
regions.
Photodesorption has been proposed as an important
desorption pathway of ices in protoplanetary disks and
other astrophysical regions with dense clumps of ma-
terial and excess UV photons (Willacy & Langer 2000;
Dominik et al. 2005). The lack of experimentally de-
termined photodesorption rates for most astrophysically
relevant molecules and conditions has prevented progress
in this area, however, and theoretical estimates range by
orders of magnitude, with desorption rates from 10−5
to 10−8CO molecules UV-photon−1(Draine & Salpeter
1979; Hartquist & Williams 1990). Due to this low esti-
mated rate, CO photodesorption has generally been re-
garded as an insignificant process in astrophysical envi-
ronments.
As the present study shows, this assumption is not
correct, and the actual desorption rate is at least two
orders of magnitude larger than the previous high esti-
mate. Here, we present the results of an experimental
study under astrophysically relevant conditions of the
photodesorption rate of CO ice and of the mechanism
involved.
2. EXPERIMENTS

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The experimental set-up has been described in detail
elsewhere (Fuchs et al. 2006). In these experiments, thin
ices of 2 to 350 monolayers (ML) are grown at 15 K
on a gold substrate under ultra-high vacuum conditions
(P < 10−10mbar).The ice films are subsequently
irradiated at normal incidence with UV light from a
broadband hydrogen microwave discharge lamp, which
peaks around 125 nm and covers 120–170 nm (7–10.5 eV)
(Mu˜ noz Caro & Schutte 2003). The lamp has a UV pho-
ton flux, measured with a NIST calibrated silicon diode,
of (6±2)×1013photons s−1cm−2at the substrate sur-
face in its standard setting. The emission resembles the
spectral distribution of the UV interstellar radiation field
that impinges externally on all clouds as well as that
of the UV radiation produced locally inside clouds by
the decay of electronic states of H2, excited by energetic
electrons resulting from cosmic-ray induced ionization of
hydrogen (Sternberg et al. 1987). The setup allows si-
multaneous detection of molecules in the gas phase by
quadrupole mass spectrometry (QMS) and in the ice by
reflection absorption infrared spectroscopy (RAIRS) us-
ing a Fourier transform infrared spectrometer.
Once an ice is grown, it remains stable until it is UV
irradiated. The layer thickness of the ice is monitored by
recording RAIR spectra (Fig. 1). The intensity of the
CO RAIRS profile is linearly correlated with the layer
thickness of the CO ice up to ∼20 monolayers (ML).
One monolayer is generally taken to consist of ∼1015
molecules cm−2and the rate of the CO photodesorp-
tion is subsequently derived from the intensity loss in
the RAIR spectra as function of time (Fig. 2). From
this loss of ice molecules and the known photon flux
hitting the surface it is possible to calculate the des-
orption rate as the number of molecules desorbed per
incident photon.Re-condensation will play a negligi-
ble role given the small surface area of the sample and
the resulting unterestimate of the actual photodesorp-
tion will be substantially lower than other sources of in-
accuracy. Above 20 ML the photodesorption rate can
no longer be calculated from the RAIRS profile as the
integrated absorbance of the peak is no longer linearly
dependent on the number of molecules. Instead a rel-
ative photodesorption rate can be determined by mass
spectrometry of the desorbed gas phase molecules. This
rate is converted to an absolute photodesorption rate by
comparison with thin layer experiments where both QMS
and RAIRS data are available. Simultaneous mass spec-
trometry of gas phase constituents shows that only CO
molecules are desorbed. Furthermore the RAIRS results
show that no other molecules are formed during the UV
irradiation (i.e. less than 0.2% of the CO ice is converted
to CO2 after 8 hours of irradiation of 8 ML CO ice).
This result is of importance as in traditional vacuum ex-
periments with substantially thicker and less pure ices,
reaction products have been identified upon UV photo-
processing (Loeffler et al. 2005) and this may affect the
photodesorption efficiency.
In these experiments the thickness of the ice, which
is needed to determine the desorption rate, was calcu-
lated from the observed difference in desorption from
multilayer coverages (constant rate) and monolayer cov-
erages (decreasing rate). From the RAIR spectra at this
turning point, the integrated absorbance of 1 ML is es-
timated to within 20%. The original thickness of the ice
can then be calculated from the integrated absorbance
of the RAIRS feature before onset of desorption. This
technique is based on the assumption that the ice is quite
flat, which is confirmed by the results of the experiments.
Fig. 1.— RAIR spectra of the C18O ν = 1 − 0 vibrational
band at 2040 cm−1(2140 cm−1for normal CO) acquired before
irradiation of a 8 ML of C18O ice and then after every hour of
irradiation during 8 hours. The drop in integrated absorbance of
the C18O ice band is linear with UV irradiation time. In most of
our experiments we used the C18O isotopologue instead of C16O to
rule out any unwanted contributions from outgassing of the vacuum
chamber. Control experiments with C16O resulted in the same
photodesorption rate within our experimental uncertainty.
Fig. 2.— The integrated absorbance of the CO RAIRS
band acquired before irradiation of a 8 ML C18O ice and then
after every 15 minutes of UV irradiation during 8 hours.
photodesorption rate is calculated from the slope of the fitted line.
The fitted line gives a photodesorption rate in loss of integrated
absorbance in integrated absorbance units (I.A.U.) per hour (here
0.015 I.A.U. hr−1). The amount of integrated absorbance per
monolayer can be derived from the integrated absorbance of 0.24
I.A.U. of 8 ML at time 0. Using the known UV flux and CO
coverage, the loss of integrated absorbance per hour is converted to
a photodesorption rate in CO molecules per UV (7-10.5 eV) pho-
ton:Rpd= (0.015 I.A.U./hr) × (hr/3600 s) × (8 ML/0.24 I.A.U.)
×`1015molecules cm−2/1 ML´×`1/6×1013photons s−1cm−2´
= 0.003 molecules photon−1
The
3. RESULTS
The evaluation of the experiments results in a constant
rate of (3±1) ×10−3CO molecules photon−1, averaged

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over the wavelength range of the lamp. This rate is fully
reproducible from repeated experiments of 8 ML cover-
age and has a standard deviation of ∼15% (Fig. 3). The
uncertainty in the absolute value is somewhat larger, up
to 30%, dominated by the uncertainty in the UV photon
flux and coverage.
The thickness of the CO ice has been varied between
2 and 350 ML. We find that the photodesorption rate of
CO is independent of the ice thickness (Fig. 3). This
suggests that only the upper layers are involved in the
photodesorption event. It is also consistent with a sur-
face that is quite smooth, since at 2 ML the entire sur-
face must still be covered to achieve the same photodes-
orption rate as for 350 ML. To confirm that the des-
orbed molecules only originate from the top layers we
performed experiments with two layers of ices compris-
ing different CO isotopologues. When 2 ML of C18O is
deposited on top of 8 ML of C16O the desorption rate
from the bottom layer drops with less than 20%. In con-
trast depositing 4 ML of C18O on top of 8 ML of C16O ice
reduces the C16O desorption rate with more than 80%.
This confirms that mainly the top few layers of the CO
ice are directly involved. Furthermore, the CO photodes-
orption rate is directly proportional to the photon flux
within the flux range covered here ((4−8)×1013photons
s−1cm−2).
In contrast to previous findings on H2O photodesorp-
tion (Westley et al. 1995), we find that the CO photodes-
orption rate is independent of the total photon dose as
well as the irradiation time, as long as 1 ML is left on the
surface. The mass signals show an onset of the photodes-
orption within the time constant of our QMS system (a
few seconds) when the UV source is turned on.
In addition to the experiments on CO ices, a thin layer
of N2 ice (8 ML) was irradiated under the same condi-
tions as the CO ices with the aim to compare the two pho-
todesorption rates. It is found that N2has no detectable
photodesorption in the present set-up, which puts an up-
per bound to the photodesorption rate of pure N2ice of
2 x 10−4molecules UV-photon−1.
Fig. 3.— The desorption rate of CO at different layer thicknesses.
From repeated experiments around 8 ML the standard deviation
in the photodesorption rate was determined, indicated by the size
of the error bars. Within the experimental uncertainty derived
from this spread, we conclude that the CO photodesorption rate is
independent of the thickness of the CO ice.
4. DISCUSSION
4.1. Photodesorption Mechanism
The above experiments can be used to constrain the
CO photodesorption mechanism.
layer thickness (demonstrated in Fig. 3) indicates that
only molecules from the top layers of the ice contribute
to the photodesorption flux.
that the CO photodesorption mechanism at these UV
wavelengths does not involve the substrate. The linear
dependence of the photodesorption rate on the UV in-
tensity is not consistent with that the desorption is due
to sublimation caused by heating of the ice as a whole.
Together these results suggest that CO photodesorption
occurs through a single photon-process, which is further
supported by the immediate on-set of the desorption once
the UV lamp is turned on. The opposite conclusion has
previously been drawn for H2O (Westley et al. 1995).
The final support for a single-photon process is given
by the different desorption rates for CO and N2ice. The
two molecules have similar inter-molecular binding en-
ergies and ice structures (Fuchs et al. 2006), which sug-
gests that any difference in photodesorption rate must
involve the internal structure of the molecules. A rele-
vant difference between the two species is that CO has
an electric dipole allowed transition in the vacuum ultra-
violet (7–10 eV) (Mason et al. 2006), exactly where the
hydrogen lamp simulates the interstellar radiation field,
while N2 does not. This transition corresponds to the
solid state equivalent of the A1Π-X1Σ+electronic exci-
tation of gaseous CO and the most plausible photodes-
orption mechanism hence involves this transition. After
UV absorption, the excited molecule relaxes via a radia-
tionless transition into vibrationally excited states of the
electronic ground state which subsequently transfer part
of this intramolecular energy to the weak intermolecu-
lar bonds with neighboring CO molecules, resulting in
a desorption event. This desorption event may consist
of more than the originally excited molecule desorbing,
but experiments with mixed CO/N2 ices are necessary
to constrain this part of the mechanism in more detail.
Its insensitivity to
In addition, it suggests
4.2. Astrophysical Implications
The single photon mechanism of CO photodesorption
means that the rate derived from these experiments can
be easily applied to astrophysical environments with-
out concerns about ice thicknesses and irradiation field
strengths. It is illustrative to compare the photodesorp-
tion rate with other possible desorption routes in dark
clouds. Since thermal desorption is negligible, species
can only be kept in the gas phase through ice desorp-
tion induced by UV photons and cosmic rays. While
cosmic rays have been proposed to directly desorb ices
(L´ eger et al. 1985), the efficiency of heating an entire
grain to the required desorption temperature rapidly
drops with grain size so that usually only spot heating at
an estimated rate of 70 molecules cm−2s−1is considered
as a viable mechanism. The cosmic rays also produce UV
photons so that the total photodesorption rate depends
on both the external interstellar radiation field and the
UV field produced inside the cloud by the cosmic rays.
For a typical galactic cosmic ray flux, the resulting UV
photon flux is of the order of 104photons cm−2s−1with
a factor of 3 uncertainty (Shen et al. 2004).

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We calculated the desorption rate due to photodesorp-
tion in a dark cloud and compared this with the desorp-
tion due to spot-heating from Shen et al. (2004). Equa-
tions 1 and 2 describe the photodesorption rates of CO
molecules from grain surfaces in molecules cm−2s−1due
to external and cosmic ray induced UV photons, respec-
tively, where IISRF−FUV= 1 × 108photons cm−2s−1is
the strength of the external irradiation field with energies
6-13.6 eV, ICR−FUVthe strength of the UV field due to
cosmic rays, γ is a measure of UV extinction relative to
visual extinction, which is ∼2 for small interstellar grains
(Roberge et al. 1991), and Ypdis the experimentally de-
termined photodesorption rate.
RUV−PD=IISRF−FUVe−γAVYpd
(1)
RCR−PD=ICR−FUVYpd
(2)
Fig. 4.— The photodesorption rate of CO for small (0.1 µm,
Rpd−sm, full line) and large (a few µm, Rpd−lg, dotted line) grains
compared to desorption due to spot heating by cosmic rays (Rspot,
dashed line). For small grains, applicable to dark clouds, the spot
heating and photodesorption rates are comparable. When grains
have grown to a few µm size the photodesorption dominates up to
AV = 15.
Applying this model shows that photodesorption dom-
inates at the edge of the cloud and becomes comparable
(within the uncertainties of a few) to spot heating in the
interior of a cloud, i.e. beyond a depth corresponding
to an extinction of 3–4 AV (Fig. 4). In the interior of
the cloud the photodesorption rate due to cosmic rays is
∼30 molecules cm−2s−1, which is equivalent to ∼ 10−8
molecules grain−1s−1for grains with a 0.1 µm radius.
A rate of this magnitude may on its own explain the
gas phase CO seen in dark clouds (Bergin et al. 2006).
In comparison with the other plausible non-thermal des-
orption mechanisms, photodesorption has the advantage
that the rate can now be determined experimentally and
unambiguously included in astrophysical models.
One particularly interesting application of our derived
photodesorption rate is to the case of CO in proto-
planetary disks, where large abundances of cold CO-gas
is observed. Aikawa & Nomura (2006) argue that the
cold CO-gas can be explained by vertical mixing and
Semenov et al. (2006) by a combination of radial and
vertical mixing of disk material.
todesorption rate of CO derived here, non-thermal des-
orption of CO may suffice to explain these observations.
An important characteristic of disks compared to dense
clouds is dust coagulation, which reduces the absorption
of the external UV field since larger grains absorb less
efficiently at short wavelengths. In Eq.
sponds to γ ≤ 0.6 (van Dishoeck et al. 2006), assuming
the grains have grown to at least µm size as indicated by
infrared observations of the silicate feature from the sur-
faces layers of the inner disk (e.g. Bouwman et al. 2001)
and by millimeter observations of the outer disk show-
ing growth up to mm size (e.g. Rodmann et al. 2006).
The photodesorption rate due to external photons then
dominates over other non-thermal desorption rates up
to AV = 15, but detailed modeling is necessary to deter-
mine whether this rate is high enough to offer an alterna-
tive to the turbulence theory in explaining the observed
CO gas. It is clear, however, that photodesorption can
no longer be ignored in astrophysical models and may ex-
plain a large part of the gas observed in cold and dense
regions.
With the high pho-
1 this corre-
We thank F.A. van Broekhuizen and W.A. Schutte for
initial work on our photodesorption instrument. Funding
was provided by NOVA, the Netherlands Research School
for Astronomy, a grant from the European Early Stage
Training Network (’EARA’ MEST-CT-2004-504604)and
a NWO Spinoza grant.
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