Desorption of Hot Molecules from Photon Irradiated
Interstellar Ices
The Surface Science Facility at the LSF
J. D. Thrower * 1, D. J. Burke 2 , M. P. Collings 1 , A. Dawes 3 , P. J. Holtom 3 , F. Jamme 4 , P. Kendall 3 , W. A. Brown 2 , I. P. Clark 5 , H. J. Fraser 6 ,
M. R. S. McCoustra 1 , N. J. Mason 3 , A. W. Parker 3
1Department
of Chemistry, School of Engineering and Physical Sciences
Heriot-Watt University, Riccarton, Edinburgh, EH14 4AS, UK
Introduction
Experimental
Despite the harsh conditions, a wide range of molecules have been detected in the interstellar medium (ISM)
using a range of spectroscopic techniques. The formation of many of these molecules has been well described
using gas-phase reactions. However, gas-phase reactions cannot account for a significant number of the
molecules that have been detected, including H2. In order to account for the observed abundance of H2, along
with many other key molecules, reactions and other physical processes that occur on the surface of interstellar
dust grains need to be included. The presence of both silicate and carbon derived dust grains coated with
mantles of molecular ices, frozen out from the gas-phase, has been revealed using infrared spectroscopy along
many lines of sight. With this in mind, our experiments have been designed to study the chemical and physical
processes occurring on the surfaces of interstellar dust grain mimics. One important class of molecule in the
interstellar medium are large networks of aromatic benzene rings known as Polycyclic Aromatic Hydrocarbons
(PAHs). There is experimental evidence for the conversion of such molecules into more complex organics
following irradiation by ultraviolet (UV) radiation. Despite this, little is known about the more fundamental
physical processes that occur. This research is focussed on studying such processes following exposure of
interstellar ice mimics on model grains to both photons and electrons. Figure 1 shows some of these processes.
The work presented here reports the study of the photodesorption of benzene and water from layered systems,
used as a model of PAH/H2O ice mixtures in the ISM.
Desorption following
resonant absorption
Photochemistry
C6H6
H2O
Sapphire
Sapphire
The work presented here was carried out at the Lasers for Science Facility (LSF) of the Central Laser Facility (CLF) at
the Rutherford Appleton Laboratory (RAL) in Oxfordshire. An ultrahigh vacuum (UHV) chamber with a base pressure
of 1×10-10 mbar was used to mimic the conditions found in the ISM. The substrate used was a 10 mm diameter sapphire
(Al2O3) disk mounted on the end of a cryostat, allowing it to be cooled to around 110 K. Sapphire was chosen as a
substrate as it is relatively easy to cool to cryogenic temperatures, and ensures that no metal mediated desorption occurs.
Such a mechanism would not be relevant in the interstellar medium. The layered benzene/water ice systems shown in
Figure 2 were deposited by backfilling the chamber to a pressure of 4×10-7 mbar for 500 s (an exposure of 200
Langmuir).
Following deposition, the ice systems were irradiated by the frequency-doubled output of a nanosecond pulsed, Nd-YAG
pumped dye laser, operating at 10 Hz. Three laser wavelengths were used in this study. 250.0 nm is on resonance with a
vibronic peak in the 1B2u←1A1g band of benzene, whilst 248.8 sits in a minimum within the same band (near-resonance).
275.0 nm was used as an off-resonance reference wavelength, being located well away from the absorption band. Two
pulse energies were used, 1.1 mJ (low) and 1.8 mJ (high).
Desorbed H2O and C6H6 were detected using a pulse counting quadrupole mass spectrometer (QMS) situated at the end
of a liquid nitrogen cooled line-of-sight tube. Time-of-flight (ToF) profiles were recorded with a multichannel scaler
(MCS) that was triggered before each laser pulse. Data was accumulated over 200 pulses for each of, typically, 30 spots
on the sample surface. Figure 3 shows a schematic of the experiment
Photon Induced Desorption Curves
Figure 1 – Some of the possible processes following
irradiation of benzene/water ice mixtures with UV radiation
H2O
C6H6
Sapphire
Sapphire
Liquid N2
QMS
1000
500
0
30
40
Time (s)
Photon Induced
Desorption
trigger
Figure 2 – The pure and layered systems
deposited on the sapphire substrate for
this work
D
La ye
se
r
6
MCS
Nd
:Y
AG
mcs counts
Substrate Mediated
Desorption
H2O
Mass 78 SEM counts/s
1500
C6H6
4
2
0
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
time-of-flight (ms)
Time of Flight
(TOF)
Figure 3 – Schematic of the experiment showing the detection technique
Layer Configuration and Pulse Energy Dependence
See Figures 4 & 5.
•A H2O overlayer inhibits desorption of C6H6 by introducing a physical barrier.
•More H2O is desorbed when C6H6 is present indicating energy transfer from C6H6 to H2O.
λ=250.0 nm
1300 K
•Some H2O is desorbed without C6H6 being present suggesting that substrate mediated
desorption also plays a role.
540 K
•It is possible that some two-photon absorption contributes to the desorption of H2O, though
this is only a small effect. Further power dependence experiments are required to understand
more fully the origin of H2O desorption
•More H2O is desorbed when in contact with the substrate – suggests stronger coupling to the
substrate than for C6H6 increasing the substrate mediated component.
λ=248.8 nm
•A very slow, broad component is observed only in the desorption of H2O from
Al2O3/C6H6/H2O.
540 K
1100 K
•The amount of C6H6 desorption is approximately proportional to the laser pulse energy. This
suggests that the desorption is due to a single photon process. This is important because multiphoton processes in the ISM are negligible due to the low photon flux.
λ=275.0 nm
540 K
Wavelength Dependence and
Maxwell-Boltzmann Fitting
Figure 6 shows 2 component Maxwell-Boltmann
fits to the C6H6 ToF profiles. The slow component
observed off-resonance (c) is attributed to
substrate mediated desorption. This component
has been fixed at 540 K for fitting at the other two
wavelengths. The two component fitting for C6H6
at (a) 250.0 nm shows a much more intense, faster
component with a temperature of around 1300 K.
This can be attributed to desorption following
resonant absorption by the C6H6 molecules. At the
near-resonance wavelength (b) 248.8 nm
desorption appears to be dominated by the
substrate mediated component, with a small, fast
resonant desorption component. For both
components the molecular temperatures are
greatly elevated with respect to the substrate
temperature – i.e. desorption of hot molecules.
The desorption of H2O can also be fitted with
distributions having T~400 K, also indicating the
desorption of hot molecules. The origin of H2O
desorption when C6H6 is absent still needs to be
understood fully through future experiments
Figure 6 – ToF profiles and Maxwell-Boltzmann
fits for C6H6 desorption from Al2O3/C6H6
Conclusions and Future Work
These experiments have focussed on the photodesorption of C6H6 and H2O from layered systems deposited on a Al2O3 substrate
under conditions that mimic an interstellar dust grain. The observed desorption of hot molecules with molecular temperatures of
over 1000 K is important in an astrophysical context. Typically, interstellar dust grains in the ISM are extremely cold with
temperatures ranging from 10 K to 100 K. UV Irradiation of the icy mantles surrounding these grains provides a mechanism for
the return of molecules to the gas-phase with greatly elevated molecular temperatures, with respect to the grain temperature. It is
likely that the presence of energetic molecules has an impact on the gas phase chemistry that occurs within the ISM. It has also
been observed that the desorption of H2O is enhanced by the presence of C6H6, indicating energy transfer between the two
molecules.
Figure 4 – ToF profiles for (a) C6H6 and (b)
H2O (λ=250.0 nm, 1.8 mJ/pulse). Grey lines
are averaged data, other lines are smoothed.
red=(a) Al2O3/C6H6 (b) Al2O3/H2O,
blue=Al2O3/H2O/C6H6,
black=Al2O3/C6H6/H2O
*Contact email: [email protected]
Figure 5 – ToF profiles for (a) C6H6 and
(b)H2O (λ=250.0 nm). Desorption is from the
Al2O3/H2O/C6H6 system. Red (blue) lines
correspond to a laser pulse energy of 1.1 mJ
(1.8 mJ)
Two desorption channels have been identified, desorption following resonant absorption by the C6H6 and substrate mediated
desorption. The latter is also relevant in the ISM where the grains themselves are likely to absorb incident UV radiation. The
molecules desorbed have a lower temperature than those desorbed by the resonant process, but are still hot with respect to the
substrate temperature.
The desorption of C6H6 is assigned to a single photon absorption process which would be relevant in the ISM. The possible small
two-photon component of the H2O desorption would not be relevant in such an environment with a low photon flux. It is therefore
important to conduct further power dependence experiments to determine the origin of H2O desorption .
The desorption characteristics are likely related to the morphology of the ice, and future experiments will aim to study more
realistic ice mixtures. It will also be important to extend this work to the study of PAHs themselves and it is anticipated that a
study of electron stimulated processes in these systems will be conducted.
2
Department of Chemistry, University College London, 20 Gordon Street, London, WC1H 0AJ, UK
3
Department of Physics and Astronomy, The Open University, Walton Hall, Milton Keynes, MK7 6AA, UK
4
SOLEIL Synchrotron, BP 48, L’Orme des Merisiers, F-91192 Gif sur Yvette Cédex, France
5 Central
6
Laser Facility, CCLRC Rutherford Appleton Laboratory, Chilton, Didcot, Oxon, OX11 0QX, UK
Department of Physics, University of Strathclyde, John Anderson Building, 107 Rotten Row, Glasgow, G4 0NG, UK
Funding and Facilities