Formation of hydroxylamine on dust grains via ammonia oxidation

0.35 MB·English

by  Jiao He

Draftversion January21,2015
PreprinttypesetusingLATEXstyleemulateapjv.12/16/11
FORMATION OF HYDROXYLAMINE ON DUST GRAINS VIA AMMONIA OXIDATION
Jiao He, Gianfranco Vidali
PhysicsDepartment, SyracuseUniversity,Syracuse,NY13244,USA
Jean-Louis Lemaire
ParisObservatory,Paris,France
5
1
Robin T. Garrod
0
CenterforRadiophysicsandSpaceResearch,CornellUniversity,Ithaca, NY14853,USA
2
Draft version January 21, 2015

n
a ABSTRACT
J
The quest to detect prebiotic molecules in space, notably amino acids, requires an understanding of

0 the chemistry involving nitrogen atoms. Hydroxylamine (NH OH) is considered a precursor to the
2
2 amino acid glycine. Although not yet detected, NH OH is considered a likely target of detection
2
with ALMA. We report on an experimental investigation of the formation of hydroxylamine on an
] amorphous silicate surface via the oxidation of ammonia. The experimental data are then fed into a
M
simulationof the formationof NH OH in dense cloud conditions. On ices at 14 K and with a modest
2
I activation energy barrier, NH OH is found to be formed with an abundance that never falls below a
2
.
h factor 10 with respect to NH3. Suggestions of conditions for future observations are provided.
p Keywords: ISM: molecules — ISM: atoms — ISM: abundances — ISM:dust, extinction — Physical
- Data and Processes: astrochemistry
o
r
t
s 1. INTRODUCTION ylamine have been done by Pulliam et al. (2012) ob-
a
serving seven bright sources toward IRC+10216, Orion
[ With molecules detected in space increasing in num-
ber and growing in complexity, one of the main goals in KL, Orion S, Sgr B2(N), Sgr B2(OH), W3IRS5, and
1 W51M using the NRAO 12m telescope on Kitt Peak,
astronomy today is to find a link between astrochem-
v and by Charness (2013) toward L1157-B1 using the
istry and astrobiology (Wincel et al. 2000). Prebiotic
1 CARMA Radio Telescope Array. Glycine of cometary
molecules have been already observed that can be con-
0 origin has been detected in STARDUST-returned sam-
sideredas precursorsofcomplexorganiccompounds like
7 ples (Glavin et al. 2008; Elsila et al. 2009), but it is un-
aminoacidsandprebioticmaterial. N-bearingmolecules
4 known whether it represents pristine interstellar mate-
are necessary precursors of such prebiotic material and
0 rial. Achemicallyrelatedspeciesthatisapossibledirect
amongthemhydroxylamine(NH OH)appearstobeone
1. of the best candidates. Both e2xperimental and theo- precursor of glycine, amino acetonitrile (NH2CH2CN),
0 retical studies (Blagojevic et al. 2003; Snow et al. 2007) has been detected (Belloche et al. 2008) in Sgr B2(N).
5 Glycine detection in the ISM or CSM is claimed to
have shown that in its protonated form it can react in
1 be highly plausible using ALMA towards bright nearby
the gas phase with acetic and propanoic acids to yield
: sources with narrow emission lines (Garrod 2013). Such
v protonated glycine (the simplest non-chiral amino-acid)
sources are luminous high-mass young stellar objects
i and protonated alanine (the second simplest and chi-
X and low-mass protostars, also called hot cores and cori-
ral amino-acid). However, the question is still under
nos. Theyareregionssurroundedbydensematerial(108
r debate whether such a synthesis happens in the gas
a phase or by surface reactions. Actually, in a recent cm−3 for hot cores and 106 cm−3 for hot corinos) and
workBarrientos et al.(2012)dismissthe possibilitythat warm(∼250Kand100K,respectively)closetothecore.
precursors of glycine could be efficiently produced from This material is much colder farther away. Deep within
the reactions of hydroxylamine-derived ions with acetic such dense regions, cosmic-ray (CR) ionized helium,
acid in the gas phase. This is confirmed by recent work He+, breaks up the very stable N2 molecule, liberating
(Garrod 2013) using a new three-phase chemical model N atoms that interact with abundant H atoms to yield
that fully couples gas-phase, grain-surface and bulk-ice NH3, which then sticks to cold grains (Tielens 2005).
chemistry,whichshowsthatglycinecanbeformedmuch It is worth noting that the composition of hot-cores re-
moreefficientlyongrainsurfacesthanthroughgas-phase sembles that of interstellar ices with high abundances of
reactions. hydrogenated species as H2O and NH3 (Tielens 2005).
Currently, the main problem is the lack of astronom- Hot cores and corinos are in general places where many
ical detection (either in the inter- or circum-stellar me- terrestrial-like organic molecules are observed. More re-
dia, ISM and CSM) of both hydroxylamine and glycine cently, radiativetransfer calculations of glycine emission
(NH CH COOH), and this is obviously correlated to show that low-mass star-forming regions, in their earli-
2 2
their low predicted abundances. Searches for hydrox- eststagesofcoldpre-stellarcores,maybe betterregions
for the detection of glycine (Jim´enez-Serra et al. 2014).
[email protected] Water detection in the low-mass pre-stellar core L1544
2
(Caselli et al.2012)indicatesthatafractionofthegrain age. Such a process occurs without external energetic
mantleshasdesorbedintothegasphase. Thismakesthe input but implies several successive steps of hydrogena-
observation of glycine plausible, assuming it co-desorbs tion. It is worth mentioning a crossed molecular beam
withwater. Afterthefirstdetectionofnitrogenhydrides study of the O(1D)+NH reaction at 295 K (Shu et al.
3
inSgr B2(Goicoechea et al. 2004), a complexregionen- 2001) forming vibrationally excited NH OH with a sub-
2
compassing a variety of physical conditions and in par- sequentdissociationinto twodifferentreactionchannels:
ticular shocks, the new detection with Herschel/HIFI of OH+NH andNHOH/NH O+Hwith90%and10%yield
2 2
NH(andND),NH ,andNH towardstheexternalenve- respectively. This reaction has been also studied theo-
2 3
lopeofthe protostarIRAS16293-2422(Hily-Blant et al. retically (Wang et al. 2004). Finally, very recently, hy-
2010;Bacmann et al.2010)hasnotimprovedourknowl- droxylamine have been formed in a two-step mechanism
edgeoftheinterstellarchemistryofnitrogenhydrides. In through the reaction of ammonia with hydroxylradicals
a recent publication, Le Gal et al. (2014) develop a new in a Neon matrix at 4 K (Zins & Krim 2014).
modelthatcouldexplaintheformationofthesehydrides However, most of the objects mentioned above, if not
in the gas phase while some of the modelers mentioned all, either in high-mass or low-mass star-formingregions
above claim surface formation. Concerning oxygen, one as well as in proto-stellarcores,share the commonchar-
of the conclusions of deep observations of O towards acteristic of having a large abundance of NH (and also
2 3
NGC1333IRAS4Aprotostar(Yildiz et al.2013)isthat H O), in any case far higher than the NO abundance.
2
the observed low molecular oxygen abundance is due to Thislattermoleculeappearstobepresentinarestricted
the freeze-out of atomic O onto grains. All these obser- number of objects relating to high-massstar-forming re-
vationsleadtotheconclusionthatonthegrainsindense gions. The abundanceofNH isassertedinobservations
3
clouds surrounding protostellar cores, there can be NH of low-mass young stellar objects (Bottinelli et al. 2010)
3
and O present. and even dark clouds (as the Bok globule Barnard 68
On the experimental side, two important facts are of (Di Francesco et al. 2002)) and comets (as very recently
interest. First, concerning NH , it has been experimen- detectedon67P/C-GbytheROSINAinstrumentaboard
3
tally demonstrated (Hidaka et al. 2011) that the succes- ROSETTA (Altwegg 2014)). Then the formation pro-
sive hydrogenation of N atoms trapped in a solid N cessinstar-formingregionsofhydroxylaminewe present
2
matrix at 10 K leads to NH formation. Second, the in this paper is a very likely more direct (single step)
3
formation of NH OH has been observed after irradiat- and realistic (considering the species involved) mecha-
2
ing anammonia-waterice at 90-130K with UV photons nism,comparedtothe one alreadyknown(Congiu et al.
(Nishi et al. 1984), and at 10 and 50 K with 5 keV elec- 2012b).
trons (Zheng & Kaiser 2010). The chemical processing Actually this process is well suited with respect to to
of H O-NH ices induced by heavy ions (Bordalo et al. hot-coresandprotostellarcores. In additionto N result-
2 3
2013) leads also, by radiolysis, to the formation of ing from N dissociation under cosmic-rays irradiation,
2
N O, NO, NO , and NH OH. As mentioned above, atomicOisalsopresentforthesamereason. BothNand
2 2 2
(Blagojevic et al. 2003) showed that the reaction of hy- Ostickonthecolderbareoricescoveredgrains,theones
droxylamine ions, NH2OH+, with acetic or propanoic deepintothe cloudsurroundingthe core(Av ∼ 15-20).
acid makes ionized glycine and alanine. Much earlier, Averysimplisticschemecouldthenbe the following: (i)
from about 1960to mid 1980s,the NH +O reaction has Verycoldgrainsarecoveredbyatoms(H, O,andN),ei-
3
been the subject of numerous experiments reviewed by theronthe surfaceorinbulk,(ii) whenthe temperature
(Cohen 1987). They have been performed at moderate begins to increase (>5-10 K), H atoms become mobile
temperature (450 to 850 K) in flowing and static sys- and form NH (and H O) by hydrogenation surface re-
3 2
tems(Perry1984;Baulch et al.1984)wheregroundstate actions , (iii) as the temperature continues to increase
O(3P) is obtained by laser photolysis, or at higher tem- (>14 K in interstellar space conditions - corresponding
peratures (850 to 2200 K) in flames (Fenimore & Jones in laboratory experiments to >20-25 K)), O atoms be-
1961) or in shock tube experiments (Fujii et al. 1986). comemobiletofinallyreactwithNH3(andH2O)already
Evenifsuchareactionhasneverbeenstudiedatverylow formed to to produce in particular NH2OH through an
temperature,ithasbeenproposedthatexcitedNH Oin- insertion mechanism.
3
termediate couldrearrangeto givestable hydroxylamine In this paper we present experimental evidence of the
NH2OH (Baulch et al. 1984). The formation of complex formationprocessofhydroxylamineviaoxidationofNH3
organicmoleculesinvolvingatomicadditionreactionson on dust grain analogs (Section 3). The experimental
the cold surface of ices-covered interstellar grains in di- findings are then used in a 3-phase gas-grain model of
verse environments, has been extensively reviewed re- a dark cloud (Section 4). A comparison of NH2OH for-
cently (Herbst & van Dishoeck 2009). As NO has been mationmechanismsandadiscussionaboutsuitableenvi-
detected in the gas phase towards a few high-mass star- ronmentswhereNH2OHcouldbeobservedarepresented
forming regions, it has been suggested that hydroxy- in Sections 5 and 6.
lamine could be formed in the gas phase through non-
energetichydrogenationofNOiceunderdarkcloudcon- 2. EXPERIMENTALSETUP
ditions (Charnley et al. 2001). But recently, an NO ice
2.1. Apparatus
hydrogenation surface reaction at low temperature has
shown that an efficient route to NH OH formation was Theapparatuswasdescribedelsewhere(He et al.2011;
2
alsopossiblewithinterstellarrelevantices(Congiu et al. Jing et al. 2013); here we summarize the main features
2012a,b; Fedoseev et al. 2012). It takes place at 10 to that are important for this study. The experiments
15 K at both NO submonolayer and multilayer cover- were carried out in an ultra-high vacuum main cham-
ber connected to an atomic/molecular beam line. The
Formation of hydroxylamine 3
main chamber is pumped by a combination of a cryop-
ump,turbomolecularpump,andionpumpandcanreach
1.2×10−10 torr after a bake-out. At the center of the eight 0.1
w
cefihllemacmtsrbaomenrpbtleheaegmrreowpishnyaosni1caaµlm1vacmtpho2ircgkdoeladpmoposliratptiohendo.ucsoTpshipleiecraddteeitsaktihlbeindy ounts/K) 105 Relative 0.05
preparation and characterization of the sample can be e (c 0
at 2800 3000 3200 3400
found in Jing et al. (2013). The sample can be cooled n r Desorption energy (K)
o
to 8 K by a liquid helium-cooled sample holder and be pti
or
heatedupto450Kbyacartridgeheaterbehindthesam- es
D
ple. At the beginning of each day, the sample is heated
upto400Ktocleanitbeforecoolingitdown. Thecham- 4
10
ber pressure after cooling down is about 5×10−11 torr.
During exposure of the sample to the atomic/molecular
80 100 120 140 160 180
beam, the pressure increases to about 1 × 10−10 torr. Temperature (K)
At this pressure, the amount of water that stick on the Figure 1. TPDs with different exposure of NH3 at 70 K. The
sample from the chamber background gas is negligible. exposure times are, from bottom to top, 0.5, 1, 2, 4, and 8 min-
A triple-pass quadrupole mass spectrometer (QMS) is utes, respectively. Desorption energy distribution of NH3 calcu-
lated from the 2 minutes exposure curve (1 ML deposition) is in
mountedonarotaryplatformtorecorddesorbedspecies
theinset.
from the sample surface or to measure the beam com-
position. The QMS is placed in a differentially pumped Bolina et al. (2005) for NH desorption from a graphite
3
enclosure and is fitted with a cone aimed at a sample. surface. With the current experimental settings, 1 ML
Thedistance betweenthe tipofthe coneandthe sample NH coverage is achieved by 2 minutes exposure with
3
can be changed by repositioning the sample via a XYZ 0.3sccmgasflow. Thusthe NH flowis 0.5ML/minute,
3
sample holder manipulator. This arrangement prevents or equivalently, 5×1014 cm−2minute−1, assuming 1 ML
products loosely adsorbed on sample holder parts from ∼ 1015 cm−2. Following the procedures as described in
reaching the QMS, thus reducing the background level He & Vidali (2014), the desorption energy distribution
and improving the signal/noise. A radio-frequency dis- of NH from amorphous silicate surface can be obtained
3
sociationsourceismountedinthefirststageofthetriple by direct inversion of the 1 ML trace in Figure 1. The
differentially pumped beam line. A mass flow controller resulted distribution is shown in the inset of Figure 1.
(Alicat MCS-5SCCM) is used to ensure stable gas flow. Additional TPD runs are performed at a surface tem-
perature of 10 K, 30 K, and 50 K. They show almost
2.2. Beam flux calibration identical TPD curves as the one at 70 K. This suggests
thatthestickingofammoniaonthesilicatesurfaceat70
The beam fluxes are calibrated by TPD experiments.
K is unity, and the desorption rate at 70 K is negligible.
With the the amorphous silicate sample kept at 70 K,
In the atomic oxygen exposure, the O gas flow pass-
ammoniagasisintroducedfromthemolecularbeamline. 2
ing through the flow controller is set to 0.1 sccm. At
The gas flow is set to 0.3 sccm on the mass flow con-
this flow the dissociation rate of O is measured to be
troller. After ammonia exposure, the sample is cooled 2
42%. The calibration of oxygen flux needs to be done
down to below 40 K and then heated up at 1 K/s to
differently from ammonia because O is volatile and
about 200 K to do a TPD. In both ammonia exposure 2
the sticking is not necessarily unity. The direct beam
stage and TPD stage, the gas phase molecules are mea-
intensities of O gas at both 0.3 sccm and 0.1 sccm
sured using the QMS. Since ammonia fragments in the 2
QMS ionizer, both mass 17 amu (NH+) and 16 amu are measured. Since it is known that 0.3 sccm corre-
3 sponds to 0.5 ML/minute, the flow at 0.1 sccm can be
(NH+) are measured. The fragmentation of ammonia
into2NH+/N+ (mass 15/14 amu) is found to be negli- cisalacbuloautted0.c3o4rrMesLp/omndininugtley.(3.T4h×e 1O0214flcomw−a2tmi0n.1utes−cc1m).
gible by direct beam measurements. A series of TPD
When the RF is on, the beam intensities of O and O
2
experimentalrunswithdifferentammoniadosesareper- are 0.29 ML/minute (2.9 × 1014 cm−2minute−1) and
formed. The TPD spectra are shown in Figure 1. We
0.20ML/minute(2.0×1014cm−2minute−1),respectively.
calibrate the NH coverage by analyzing the shape of
3 Withtheradiofrequency(RF)poweronandtheoxygen
the TPD profile. In the submonolayer region the TPD
sent into the dissociation source, beam contamination is
peak temperature should shift to lower values, because
checked. The main contaminant is NO (mass 30 amu)
in the low coverage region molecules occupy the deep
due the small leak of air into the dissociation source.
sites (higher desorption energies). Here a layer is actu-
The NO (mass 30) signal is less than 3% of the O sig-
ally an equivalent layer, since it is unclear whether am-
nal, which is trivial in the context of the experiments
moniawouldformclustersorislandsonthesurface;little
performed here.
experimental evidence support the existence of clusters
orislands. As the deepsites getfilledupwith increasing
2.3. Experimental procedures
exposure, the TPD profile and peak temperature shift
to lower temperatures (He & Vidali 2014). In the mul- The ammonia oxidation reactions were studied in am-
tilayer region, the TPD spectra should show a common monia and atomic oxygen sequential exposure TPD ex-
leadingedge (Kolasinki2008), whichisa typicalfirstor- periments. The surface was covered with ammonia be-
derdesorptionbehavior. ThetrendofTPDtracesshown foreintroducingatomicoxygen. Twoammoniacoverages
in Figure 1 is in agreement with the one obtained by wereused,2MLand1/4ML,representativeofmultilayer
4
x 104 x 105
3
16 1187 aammuu 300 0 min
0.5 min
MS signal (count/s) 11102468 1333te6203m aaaapmmmmeuuuura xxxtu 333re000 + 20000 122505000 Temperature (K) MS signal (counts/K) 12..255 124681 6mmmmm miiiiinnnnnin
Q Q
4 100 17 1
s
s
2 50 ma
0.5
0 5 10 15 20
Time (min)

Figure 2. A typical TPD of a NH3 + O sequential exposure 80 90 100 110 120 130 140 150 160
experimentat70KinwhichtheQMSrecordssimultaneouslymul- Temperature (K)
tiplesignals. The temperature ramp(black line)shows that after
Figure 3. Mass 17 amu QMS signal of the TPD from an ice
theexposureisterminated,thesampleiscooledto40Kbeforethe
heatingisstarted. consistingofNH3(4minor2ML)followedbyexposuretovarious
dosesofOat70K.
coverages and submonolayer coverages. After prepara- neath. After about 6 minutes of O exposure, the first
tion of the ammonia sample, the residual ammonia in peak almost disappears, indicating the top layer of NH
3
the beam line and dissociation source was cleaned by is almost all converted to NH OH or other products.
2
flushing the beam line several times using oxygen. This InFigure 4 two desorptionpeaks, peak A andpeak B,
ensures that almost no ammonia was mixed with oxy- are visible. Peak A shows up at O exposure as low as
gen. The sample temperature in the exposure stage was 0.5 minutes. This indicates that the ammonia oxidation
chosen to be 70 K so that O/O2/O3 does not stick onto reaction is efficient. As the O exposure increases from 0
the surface while the sticking of NH3 is unity. After the to 4 minutes, the area of peak A increases at first, but
exposurestage,the surfacewascooleddowntobelow 40 then it decreases from 4 minutes to 16 minutes of O ex-
K and then heated up linearly at 1 K/s to above 320 K posure. Peak A is followed, at larger O exposures, by
to desorb species from the surface. The QMS recorded peak B at ∼ 260 K. Notice that peak B starts to ap-
simultaneouslythe signalsofvariousmassesduringboth pear at 4 minutes of O exposure time at the expense of
exposureandTPDstages. Figure2showstheTPDspec- peakA.Asmentionedabove,peakAfallsinthetemper-
tra of a typical experimental run. ature range of the TPD mass 33 amu peak obtained by
Congiu et al. (2012a). Therefore we attribute peak A to
3. RESULTSANDANALYSIS the desorptionof NH OH. Concerning peak B, however,
2
3.1. Multilayer NH + O in a subsequent paper on an NO hydrogenationexperi-
3 2
ment(Ioppolo et al.2014),amass33peak,confirmedby
IntheNH +OsequentialexposureTPDexperiments,
3
infraredmeasurementspectra,appearsatatemperature
mass 16, 17, 18, 30, 32, and 33 amu were measured by
T∼ 245 K, close to the temperature of our second peak
the QMS, see Figure 2. Mass 16 amu is due to the frag-
(peakBinourFigure4). Nevertheless,oddlythereisno
mentation of NH in the QMS ionizer; mass 17 amu is
3
mention in their paper of a peak A at 160 K.
due to NH and fragmentation of H O; mass 32 amu is
3 2
TofindoutwhatpeakBrepresents,weshowinFigure
due to O and fragments of O . Mass 30 amu could be
2 3
5 a comparison of TPD traces for various masses for 2
duetoNOfromthebeamline(buttheamountissmall),
MLofNH +8or16minutesofOexposure,seeleftand
or fragments of NH OH; mass 33 amu could be due to 3
2
right panels of Figure 5, respectively. Peak A and peak
NH OH. HO has the same mass but it is unlikely to
2 2
B are marked by vertical lines. In both panels of Figure
be there because of the lack of detection of the prod-
5, peak A (see trace of mass 33 amu) is accompanied by
ucts it fragments into. The mass 33 amu peak centered
desorption of mass 30 amu, 17 amu, and 16 amu, while
at around 180 K is accompanied by mass 30 amu and
this is not true for peak B. This suggests that peak B is
very small signals with masses 16, 17 and 32 amu at the
not due to NH OH. It could be the product of a frag-
same temperature; this suggests that the mass 33 amu 2
mentationofa dimer (Del Bene 1972) orofanoxidation
peak at 180 K is due to NH OH. This agrees with the
2
product of NH OH. At about 280 K, there is a peak C
TPD peak attribution in Congiu et al. (2012a) (in the 2
range of 160-200 K, peaking at T∼ 190 K) obtained in showing up for mass 30 amu, 17 amu, and 16 amu, but
only when the O dose is high (16 minutes of O expo-
an NO hydrogenation experiment and confirmed by in-
sure), see right panel of Figure 5. It could be due to a
frared measurements (RAIRS). Mass 18 amu is due to
yetunidentifiedproductformedinafurther oxidationof
water.
NH OH.PeakBandpeakC differsinbothpositionand
Figure 3 shows the mass 17 amu (NH ) signal after 2
3
shape, therefore they should be attributed to different
depositing various doses of O on top of 2 ML (4 min-
species.
utes exposure) NH . As the O exposure time increases,
3
theNH peakdecreasesandthe peakpositionalsoshifts
3 3.2. Submonolayer NH + O
to higher temperatures. This is because the top layer of 3
NH isgraduallyconvertedtoNH OHorotherproducts, Experiments were also carried out at submonolayer
3 2
thus hindering the desorption of NH molecules under- NH coverage. We exposed the silicate sample to NH
3 3 3
Formation of hydroxylamine 5
4000 NH3isgraduallyconvertedtoNH2OHorotherproducts,
0 min O thuspreventingNH3 moleculesunderneathfromdesorb-
K) 3500 peak A 0.5 min O ing. After about 6 minutes of O exposure,the first peak
unts/ peak B 12 mmiinn OO almostdisappears,indicatingthatthetoplayerofNH3is
gnal (co 3000 468 mmmiiinnn OOO aitlmtaoksetsaalltcmonovster6temditnooNfHO2O(1H.74orMotLh)ertoprcoodnuvcetrst.1TMhuLs
S si 2500 16 min O of NH3, suggestion an efficiency for the reaction of each
M
Q oxygen atom of at least 0.575 (=1/1.74).
mu 2000 Using this efficiency, an upper limit for the activation
a
ass 33 1500 eensteirmgaytebda.rrier, EA, for the O + NH3 reaction may be
m
We firstly assume that the incoming oxygen atom is
1000
thermalized on the surface, before overcoming the acti-
140 160 180 200 220 240 260 280 300 320 vation energy barrier to reaction. This thermalization
Temperature (K)
is the most likely immediate outcome of the impact, ex-
Figure 4. AsinFigure3butforQMSmass33amusignal. cept in a small minority of ”direct-hit” trajectories, as
theoxygenatomdissipatesenergyasitisdrawnintothe
for 0.5 minutes, or about 1/4 of a layer, with the same
multi-dimensional potential of the surface. The atom is
proceduresastheonesfollowedtoobtainthedatainFig-
nevertheless available for immediate reaction if the ac-
ure1. This wasfollowedby exposuretoO.Mass16amu
tivation energy barrier can then be overcome; in such a
was chosen to represent the NH amount because the
3
case, the reaction may be classed as Eley-Rideal, as it is
mass 17 amu signal has a non-trivial contribution from
not mediated by a diffusion process.
water fragmentation when the NH signal is weak. In
3
Assuming that the main loss mechanism for an oxy-
Figure6weshowtheintegratedmass16amuTPDtrace
gen atom on a pure NH surface is either reaction with
for different exposures to O. With O exposure from 0 to 3
NH or thermal desorption, the reaction efficiency may
2 minutes, the amount of NH follows more or less an 3
3
be formulated thus:
exponential decay. This is because when the O amount
is small, oxidation dominates and the NH destruction
3
rate is proportional to the NH3 amount. With a further 1/1.74=exp[−E /T]/(exp[−E /T]+exp[−E (O)/T])
A A des
increase in O exposure, the NH decay does not follow
3 (2)
a simple exponential decay anymore because of possible
where E (O) is the desorption energy for an oxy-
des
secondary reactions. In Figure 6, a straight line is fit-
gen atom and T is the surface temperature. Adopting
ted to the loge plot from 0 to 2 minutes. The slope is E (O) = 1500 K and T = 70 K, this expression pro-
a−m0.o4u6n±t o0f.0N6Hm3 oinnutteh−e1s.urIfanceansheoxupldonfoelnltoiwaledxepc(a−yσ,φtth)e, dudTcesehsisavvaalulueecEonAs(tOit+utNesHa3n) =up1p4e7r9bKou.ndary of the reac-
where σ and φ are the reaction cross-section area and tion energy barrier because we assume a sticking coeffi-
the beam flux,respectively. Fromit, weobtainthe cross cient of unity, and thence that each oxygen atom that
section σ =−slope/φ=(1.6±0.2)×10−15 cm2. reaches the surface participates in a reaction. If this is
notthecase,thenthereactionenergybarrierforthefirst
3.3. Reaction energy barrier of NH3 + O oxidation should be even smaller.
IntypicalISMconditions,itis unlikelythatsituations The above treatment does not consider the possibil-
arisesimilartotheonesthatledtotheappearanceofthe ity that surface oxygen atoms could meet each other
second TPD peak of mass 33 amu at high temperature and react, reducing the efficiency of the reaction. Un-
attributed to a subsequent oxidation process. Therefore der conditions in which such reactions could be impor-
we focus on the first oxidation: tant, then in order for the O + NH3 reaction to be ef-
ficient and the O + O reaction to be minimized, the
NH3 + O → NH2OH (1) reaction barrier for O + NH3 would need to be lower
than the oxygendiffusion barrier. This quantity has not
When the surface is fully covered with NH and the
3 been measured in the laboratory, but there are rules-
surface temperature is 70 K, O could react with NH
3 of-thumb based on experimental data from other sys-
also via the Eley-Rideal or hot-atom mechanisms, i.e.
tems. The ratio of the diffusion energy barrier to the
reaction without complete thermal accommodation of
desorption energy is typically found to be ∼0.3 for crys-
the oxygen atom. Based on prior experiments (He et al.
talline surfaces (Bruch et al. 2007). For rough surfaces,
2014), we assume that at 70 K the residence time of O
values between 0.5 and 0.8 have been used (Katz et al.
on NH ice is negligible. We also assume that all the
3 1999;Garrod & Herbst2006),althoughGarrod & Pauly
reactionstakeplaceduringthe exposurestageinsteadof
(2011)foundthatvalueslowerthan∼0.4weremostcon-
the TPD stage, because at 70 K the mobility of oxygen
sistent with observed interstellar extinction thresholds
should be relatively high. The degree of conversion of
for H O, CO and CO ices. Garrod (2013) adopted a
NH can be gauged by looking at what remains of the 2 2
3 value of0.35,whichwouldyieldanupper limit onthe O
NH layersaftervariousdosesofoxygen. Figure3shows
3 + NH activation energy barrier as low as 525 K.
the TPD mass 17 signal (NH ) for various doses of O. 3
3
The NH dose is fixed at 4 minutes, which is equivalent
3 3.4. Control experiments
to about 2 ML. As the O exposure time increases, the
NH peak decreases, and the peak position also shifts Controlexperimentswerecarriedouttoverifywhether
3
to higher temperatures. This is because the top layer of ammonia react with molecular oxygen or ozone. We ex-
6
5 5
10 10
mass 18 mass 18
peak A peak A peak C
s/K) mass 17 s/K) mass 17
nt nt
al (cou 104 mass 16 al (cou 104 mass 16
n n
g g
S si peak B mass 30 S si peak B mass 30
M M
Q Q
mass 33 mass 33
103 103
100 150 200 250 300 100 150 200 250 300
Temperature (K) Temperature (K)
Figure 5. QMSsignaloftheTPDfromaniceofNH3 (4min)+8(left)and16(right)minutesofOexposure.
12.5
Experimental 4000 2 ML NH3 + 1.1 ML O @70 K
Fitting 2 ML NH + 1.4 ML O @70 K
12 3 2
mass 16 TPD peak area) 11011..155 slope=-0.46±0.06/min 33 QMS signal (counts/K) 12233505050000000000 1.3 ML O3 + 2 ML NH3 @50 K
log(e mass 1000
10
500
9.5 0
0 2 4 6 8 10 12 14 16 100 150 200 250 300
O exposure time (min) Temperature (K)
Figure 6. Integrated TPD signs mass 16 amu from adeposit of Figure 7. Mass33QMSsignalintheTPDfromNH3 +O2 and
NH3 (0.5minutesor1/4ofaML)+variousexposuresofOat70 O3 + NH3 deposited at 70K compared with that of NH3 + O at
K. 50K.Curvesareoffsetforclarity.
posed the ammonia layers to O and found no mass 33
2
or 30 amu peaks in the TPD, signifying that there has all species, E = 0.35 E , following (Garrod 2013).
diff des
been no conversionof ammonia to hydroxylamine in the Due to the low temperatures at which grain-surface ice
presence of molecular oxygen. We also checked the re- mantlesareformed,onlysurfacechemistryisswitchedon
activity of ozone with ammonia. Ozone was prepared in this model, although the three-phase treatment con-
on a clean silicate following the procedure described in siders separate grain/ice surface and bulk mantle popu-
He et al.(2014); then, 2 ML of ammonia were deposited lations for all species (as well as gas-phase abundances).
on it. Again, there was no mass 33 or 30 amu peaks in The three-phase model allows the composition of each
the following TPD, see Figure 7. layer within the ice mantle to be traced as it is de-
posited during the chemical evolution of the cloud (see
4. MODELING Garrod & Pauly (2011)). Only Langmuir-Hinshelwood
In order to test the astrophysical importance of the (i.e. diffusional) processes are considered in this model,
O + NH → NH OH reaction investigated here, we in- as the Eley-Rideal mechanism tends to require surface
3 2
corporate this surface mechanism into the recent three- coverages of close to unity (in models of astrophysical
phase gas-grain chemical kinetics model MAGICKAL grains) to compete with L-H mediated reactions.
(Garrod 2013). The model parameters used here are in- Reactions are also included in the network allowing
tended to approximate those present under dark cloud NO molecules to be hydrogenated to NH OH by the se-
2
conditions, during which significant ice mantles are quential addition of atomic H. Following (Garrod2013),
formed on the surfaces of dust grains. The model uti- this includes a barrier-mediated reaction, H + HNO →
lizes the chemical network and initial elemental abun- HNOH, as well as an alternative H-abstraction branch,
dancesusedbyGarrod(2013),assumingageneric,static H+HNO →H +NO,forwhichthe barrieris assumed
2
dark-cloud gas density of n = 2 × 104 cm−3 and vi- to be 1500 K in both cases. Following (Hasegawa et al.
H
sual extinction A = 10. A gas temperature of 10 K 1992), tunneling through activation energy barriers is
V
is used in all model runs. We assume a binding energy treatedusingarectangularbarriertreatment,withade-
for atomic oxygen of 1500 K, based on an experiment fault width of 1 ˚A. The faster of tunneling and thermal
that measuredthe desorptionenergy ofO from compact penetration is used, according to the grain temperature.
amorphous water ice (He & Vidali 2014). Diffusion bar- The model described above is used to produce a grid
riers are set to a uniform fractionof binding energies for ofmodelsforarangeofdust-graintemperaturesfrom10
Formation of hydroxylamine 7
–20 K,andusing activationenergybarriers,E ,for the production.
A
O + NH → NH OH reaction ranging from 0 – 2500 K. Models were also run to compare the case where the
3 2
Asmallersetofmodelsisalsoruntocomparetheeffects hydrogenationofNObyHatomsisassumedtooccurei-
of NH OH produced by this mechanism, with cases in ther (i) without barriers or (ii) not to occur at all. Case
2
whichthe barrierto the H + HNO →HNOH reactionis (i),removalofthe barriertoH+HNO →HNOH(while
altered. the alternative hydrogen-abstraction branch retains its
barrier of 1500 K), in fact results in no appreciable dif-
4.1. Model results
ferencefromtherunsthatassumeabarrierof1500Kfor
Figure8,panel(a),showsthecompositionofeachlayer H + HNO (the zero-barrier results are thus not plotted
in the grain mantle ice for the model run corresponding explicitlyinthefigure). NH OHproductionthroughthis
2
to E =0 and T = 10 K. Typically observed com- mechanism is therefore already at its highest efficiency.
A grain
ponents are shown, along with NH OH. In this case, The alternative regime, case (ii), in which the H +
2
NH OH production is relatively small, and all such pro- HNO → HNOH barrier is increased to arbitrary height,
2
duction is a result of the hydrogenation of NO. At 10 such that the reaction does not occur, results in negligi-
K, oxygen atoms are insufficiently mobile to be able to ble NH OH production at 12 K and below, because all
2
reach and react with NH before they are hydrogenated NH OH formation is dependent on the O + NH reac-
3 2 3
to produce water, even under the assumption of a zero tion,whichisinefficientatthesetemperatures. Above12
barrier for the O + NH reaction. Here, the majority K, NH OH is formed efficiently via O + NH , and there
3 2 3
of NH OH is produced via NO hydrogenation, which is isonlyasmalldifferencebetweentheseresultsandthose
2
strongly peaked at late time/outer layers. in which the H + HNO reaction is allowed to occur at
Panels (b) and (c) show the results at 14 K, for acti- maximum efficiency.
vation energy barriers of E =1000 and 2000 K, respec-
tively. Atthistemperature,Aoxygenatomsaresufficiently 5. DISCUSSION
mobile to meet ammonia molecules on the surface be- Experimentsofoxygenexposureonsubmonolayerand
fore being hydrogenated. In the E =1000 K case, the multilayer ammonia ices show an efficient formation of
A
reactionbarrier is low enough (in comparisonto the dif- NH OHwithareactionenergybarrierthatcanbeaslow
2
fusion barrier for the oxygen atom to move away from as ∼525K and no higher than 1479K. The experiments
the ammonia molecule) to allow reaction to occur with weredone on samples at 70 K to avoidcontaminationof
high efficiency. In this case, the majority of NH OH is molecular oxygen and ozone. At this temperature NH
2 3
formed through this reaction, and NH OH abundance is stuck on the surface while O has a short residence
2
closely follows that of NH , never falling below a factor time. Experiments have been performed at both bilayer
3
of ten lower than NH at any layer in the ice. NH OH and submonolayer coverage. Simulations were done to
3 2
maintains a fairly steady abundance throughout the ice, find what rates of hydroxylamine formation one would
up to around 1 Myr (170 ML), but the NH OH produc- obtain in simulated dense cloud conditions. According
2
tionis enhancedat alltimes/depths. Inthe E =2000K to the chemical models, at dust temperatures around 14
A
case,thereactionbarrieristoohightoallowreactionbe- K or higher, the O + NH reaction is found to be an
3
fore hydrogenationof oxygenoccurs,making the NH + efficientmechanismfortheproductionofNH OH.While
3 2
O reaction inefficient; here NH OH production is again temperatures this high are not the typically considered
2
the result of NO hydrogenation. dark-clouddust temperatures,it is certainthat the dust
Figure 9 shows the final (maximum) grain-mantle in dark clouds must pass through such temperatures on
abundances of NH OH as a function of total hydrogen theirwaytothe8–10Kthatismoretypicallyassumed.
2
in the cloud, for model runs assuming a range of grain Furthermore,interstellardustwillagainpassthroughthe
temperaturesfrom10–20K,andactivationenergiesfor ∼14Kthresholdatwhichthisreactionbecomesefficient
the O + NH reaction ranging from 0 – 1500 K. Results as part of the evolution of hot cores.
3
fortheE =500Kcasearenotshown,astheyareidenti-
caltotheAE =0case;bothvaluesaresufficientlybelow 6. ASTROPHYSICALIMPLICATIONS
A
theatomicoxygendiffusionbarrierassumedinthemodel NH and H O have been detected in the gas phase
3 2
(525K),suchthatthereactionwithNH occursatanef- aroundtransientprotostellarcoresorinferredforOfrom
3
ficiency close to unity, if the temperature is high enough the strong O depletion. These cores are also classi-
2
to allow the oxygen atom to diffuse and reach an NH fied as hot cores or hot corinos in the case of mas-
3
before it is hydrogenated and becomes immobile. This sive and medium/low mass nascent stars, respectively.
is the case for models of temperature 14 K and above; These cores whose temperature can be as high as 100
allmodelsat12K orlessproducethe sameresults,with K close to the protostellar source (and even 250 K for
NH OH produced by NO hydrogenation. the densest ones) are much colder far from the core due
2
Assuming a barrier of 1000 K, the reaction is still rel- to the strong extinction provided by the dusty molec-
atively efficient, falling by a factor ∼2 at grain tempera- ular clumps in which they are embedded. A possible
tures of ∼16 K and above, compared to the E =0/500 scenario for the coldest regions where NH and O are
A 3
K results. However, with a 1500 K barrier, reaction is stuck on the grains and approaching the core is that in
inefficient, and yet higher barriers produce the same re- a first stage (at temperature in the 14-50 K range) O is
sults;insuchcases,NH OHisproducedoverwhelmingly releasedin the gasphase andis then able to collide with
2
by NO hydrogenation. A run with the O + NH bar- NH coveredgrains,inducingchemicalreactionsthatcan
3 3
riersetto 1250K allowsthis reactionto become slightly synthesize new species. These would later desorb from
dominant over the NO hydrogenation mechanism, pro- thegrainswhenthecoretemperatureincreases. Alterna-
ducing a small, though significant, increase in NH OH tively, NH OH may be produced at lower temperatures,
2 2
8
Figure 9. Final model abundances of ice-mantle NH2OH with
respect to total hydrogen. The grain temperature is varied for a
selection of activation-energy values for the NH3 + O reaction.
The dashed line shows the results assuming no barrier for either
theNH3 +→NH2OHorH+HNOH→HNOHreactions.
during the formation of the ice mantles, to be released
at later times. (Models of such scenarios are beyond the
scope of this primarily experimental study, but will be
considered in future work).
Consequently, the present laboratory astrophysics ex-
periment on O interaction with NH should be useful to
3
interpretthe observationaldata. Then the questionis of
thenon-observationofhydroxylaminewhile,atthesame
time, it has been demonstrated that it can be made in
the laboratory in conditions almost similar to those in
protostellar cores.The upper limit relative abundance of
NH OH with respect to H , as deduced from observa-
2 2
tions(Pulliam et al.2012) is foundto be in the 3×10−9
to 8×10−12 range, depending on the object. Interest-
ingly, combining this result with the relative abundance
of NH with respect to H in a high-mass star-forming
3 2
region, 4.6 × 10−8 (Battersby et al. 2014) (a value of 2
× 10−8 is already reported in the case of a cold dense
cloud like TMC-1 (Turner 2000)), we deduce an aver-
agefractionalabundanceofNH OHwithrespecttoNH
2 3
in the 0.7 × 10−1 to 2 × 10−4 range, close to the val-
ues deduced from the proposed model. Several reasons
may explain the non-observation: the yield of such re-
actions could be quite low due to the narrow tempera-
ture range in which they have to take place as well as
a possible spatial confinement of dusty clouds where the
required temperature conditions for formation are ful-
filled. Inaddition,recentobservations(Sakai et al.2013,
2014a,b; Sanna et al. 2014) have shown that the orien-
tation of the protostellar core in the plane of the sky
could be also important, particularly if it is associated
to infall-disk-outflow systems and rotation. It could be
expected that the amount of chemical species resulting
from the NH +O interaction will be almost equal both
3
in high-mass and medium- to low-mass stars because of
a higher species density in the first case and a longer in-
teraction time in the other. All these conditions could
explain the difficulty to observe such species as hydrox-
Figure 8. Fractional composition of ices within each layer as a ylamine that have, however, been made in the labora-
function of depth into the ice, lower horizontal scale, or, equiva- tory. ALMA could provide the required sensitivity and
lently,asafunctionofageofthecloud,tophorizontalscale. Panel
(a) The activation energy barrier for the O+NH3 reaction is 0 K
andthetemperatureoftheiceis10K.Panel(b)EA=1000Kand
T=14K.Panel(c)EA=2000KandT=14K.
Formation of hydroxylamine 9
the sub-arcsecondangular resolution to observe them. Fedoseev, G.,Ioppolo,S.,Lamberts,T.,etal.2012,J.Chem.
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