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Phosphorus is the 17th most abundant element in the universe. CP has been detected in carbon rich circumstellar envelopes 1, and PN has been
observed in hot molecular cores 2. Although phosphine (PH3) has been detected in the atmospheres of Jupiter and Saturn 3, it is believed to have a
short life time in the gas phase, keeping its abundance low and making its detection difficult 4.
• Infrared spectroscopy was performed in the reflection absorption
(RAIRS) configuration, at an angle of 75 o to the surface normal, with a
resolution of 1 cm-1.
Phosphorus is an elemental component of many of the biological molecules that are essential for life on earth, principally in the form of phosphates.
However, since phosphates {PO(OR)3} have low solubility in water, they are an unlikely source of phosphorus in pre-biotic chemistry. Phosphonates
{PO(OR)2R} on the other hand are more water soluble. Alkylphosphonates have been isolated from the Murchison meteorite 5, indicating a
potentially extraterrestrial origin for pre-biotic phosphorus chemistry.
The formation of phosphonates by reaction of PH3 in the icy mantles accreted on interstellar dust particles has been suggested. In this project we
have studied the behaviour of PH3 adsorbed with water ice during thermal processing under conditions of relevance to the interstellar medium.
Given the explosive reactivity of PH3 in air at room temperature, we have searched for evidence of reaction between PH3 and H2O in our interstellar
ice analogues.
• The symmetric stretch feature at 2305 cm-1 appears as a shoulder on
the asymmetric stretch band at 2315 cm-1 in the amorphous PH3 film
deposited at 8K. These features sharpen slightly when the film is
annealed to 35 K, indicating crystallisation (Figure 4).
Figure 3. RAIR spectrum of a 10 L exposure of PH3 deposited at 8 K onto a bare
gold substrate. Assignments are made from references 9 and 10.
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• When the crystalline film is cooled, the broad stretch and the symmetric
bend features were found to sharpen into an array of partially resolved
fine structure (Figures 4 and 5). This behaviour has been previously
attributed to freezing of rotational motion of the PH3 molecules 10.
• The desorption of phosphine was detected via its principal ion PH+,
at mass 32 amu. TPD experiments were performed at a mean
temperature ramp of 0.1 K s-1.
• Although PH3 desorbs at higher temperatures than CO 6,7, there are
strong similarities in the desorption characteristics, both showing
diffusion into the water ice and trapping.
• The freezing of rotational motion was also observed to occur in PH3
films deposited on water films (figure 5), with the onset of rotational
freezing occurring between 37 and 30 K (Figure 6).
• Solid PH3 shows zero order desorption kinetics with a single
desorption peak in the 60 K region (Figure 1).
• In contrast, the behaviour of PH3 is dissimilar to that of the
electronically analogous, but lighter molecule, ammonia. NH3 desorbs
at higher temperatures, and shows a very limited ability to diffuse into
porous water 7,8.
• The P-H stretch feature of a PH3 film adsorbed on top of a water film
shows a distinct broadening when annealed commencing at about 35 K.
This change marks the temperature at which the PH3 molecules become
sufficiently mobile to diffuse into the porous bulk of the water film.
• PH3 desorbs from the surface of water ice in a broad peak in the
60~100 K range (Figure 2). For exposures of 20 L, multilayer PH3
desorption is not evident, indicating that PH3 is readily able to diffuse
into the porous structure of the water. The strong peak at 145 K, and
the minor peak at 165 K can be respectively attributed to the escape
of trapped PH3 in molecular volcano desorption as the amorphous
water crystallises, and a co-desorption with crystalline water
desorption.
cooled 10 K
heated 35 K
heated 20 K
• We therefore class PH3 as a weakly bound CO-like species 7.
• Hydrogen sulphide (H2S) was similarly found to interact less
strongly than its lighter electronic analogue water 7. These results
demonstrate that hydrogen bonding is weaker in molecules with third
period elements.
• No evidence for the desorption of any products of reaction between
PH3 and H2O was observed.
deposited 8 K
Figure 4. RAIR spectra of a 10 L
exposure of PH3 deposited at 8 K
onto a bare gold substrate, annealed
for 30 minutes at the temperatures
indicated and cooled back to 10 K.
Figure 5. RAIR spectra of a 20 L
exposure of PH3 adsorbed on a predeposited 100 L H2O film at 54 K
during gradual cooling.
• The bands of anticipated reaction products occur at 1230~1260 cm-1
(P=O, phosphonates), 1100~1200 cm-1 (P=O, phosphates), ~2630,
~2220, ~1665 and ~1900 cm-1 (P-O-H) and ~1900 cm-1 (P-O-C) 11. No
unattributed bands were detected in the above experiments.
• In an effort to induce reaction between PH3 and H2O, the following
experiments were performed:
- co-dose a 33% PH3 in H2O mixture at 54 K and anneal at 130 K for 3 hours.
- dose 1×10–7 mbar PH3 onto H2O at 130 K for 3 hours.
- co-dose partial pressures of 1×10–7 mbar of both PH3 and H2O at 180 K and 200 K
for 3 hours.
No evidence of reaction was detected in any of these experiments.
Figure 6. RAIR spectra of a 10 L exposure of PH3 adsorbed at 8 K onto a predeposited 100 L H2O film, during slow annealing.
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Figure 1. TPD traces for varying exposures of PH3 deposited at 8 K onto a bare
gold substrate; inset: the four lowest exposures shown on an expanded scale.
TPD traces for a 5 L exposure of NH3 and a 10 L exposure of CO are shown for
comparison. Note that due to the directed method of dosing, equal exposures of
different molecules are not necessarily equivalent, and that the NH3 and CO
traces have been arbitrarily scaled for clarity.
*Contact email: [email protected]
Project Funded by:
Figure 2. TPD traces for varying exposures of PH3 adsorbed at 8 K onto a 100 L
film of H2O pre-deposited at 8 K; TPD traces of the H2O, and of a 5 L exposure of
NH3 and a 10 L exposure of CO on similar H2O films are shown for comparison.
Note that the H2O, NH3 and CO traces have been arbitrarily scaled for clarity. The
NH3 trace is truncated due to interference from the H2O desorption at mass 17
amu.
Phosphine interacts weakly with water ice and can be classed as a weakly bound CO-like species. Although it reacts explosively in air, it will not
react with water under conditions of pressure and temperature at which it is retained astrophysical ices. Although not studied here, it seems unlikely
that it will react with other ‘stable’ species present in astrophysical ices. Reactions with atomic, radical and ionic species accreted in icy mantles
remain possible, and reaction during processing by UV irradiation or cosmic ray bombardment may also occur.
Detection of the infrared absorptions of phosphine in interstellar ices will be difficult, even if phosphine is present at relatively high concentrations.
The strongest absorption feature due to P-H stretches at ~2310 cm-1 overlies the C-O stretch of solid carbon dioxide. The weaker absorption modes
due to the symmetric and asymmetric H-P-H bending modes at 985 and 1100 cm-1 overlie bands due to water ice and silicates.
1
M. Guelin, J. Cernichavo, G. Paubert, and B. E. Turner, Astron. Astrophys. 1990, 230, L9
2
T. J. Millar, Astron. Astrophys. 1991, 242, 241
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Astron. Soc. 2004, 94, 1133
8
T. W. Weston, M. P. Collings, and M. R. S. McCoustra, unpublished results.
9
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10
M. D. Francia, and E. R. Nixon, J. Chem. Phys. 1973, 58, 1061
11
http://www.cem.msu.edu/~reusch/VirtualText/Spectrpy/InfraRed/infrared.htm
S. B. Charnley, and T. J. Millar, Mon. Not. R. Astron. Soc. 1994, 270, 570
G. W. Cooper, W. M. Onzo, and J. R. Cronin, Geochim. Cosmochim. Acta 1992, 56, 410
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