The rate of the F + H2 reaction at very low temperatures

Article abstract of DOI:10.1038/nchem.1835

The prototypical F + H₂ → HF + H reaction possesses a substantial energetic barrier (～800 K) and might therefore be expected to slow to a negligible rate at low temperatures. It is, however, the only source of interstellar HF, which has been detected in a wide range of cold (10-100 K) environments. In fact, the reaction does take place efficiently at low temperatures due to quantum-mechanical tunnelling. Rate constant measurements at such temperatures have essentially been limited to fast barrierless reactions, such as those between two radicals. Using uniform supersonic hydrogen flows we can now report direct experimental measurements of the rate of this reaction down to a temperature of 11 K, in remarkable agreement with state-of-the-art quantum reactive scattering calculations. The results will allow a stronger link to be made between observations of interstellar HF and the abundance of the most common interstellar molecule, H 2, and hence a more accurate estimation of the total mass of astronomical objects.

Full text of DOI:10.1038/nchem.1835

ARTICLES
PUBLISHED ONLINE: 12 JANUARY 2014 | DOI: 10.1038/NCHEM.1835
The rate of the F1H2 reaction at very
low temperatures
1
1
2
1
1
´
´
Meryem Tizniti , Sebastien D. Le Picard , Franc¸ois Lique , Coralie Berteloite , Andre Canosa ,
Millard H. Alexander3 and Ian R. Sims1
*
*
The prototypical F 1H2 ꢀ HF1H reaction possesses a substantial energetic barrier (∼800 K) and might therefore be
expected to slow to a negligible rate at low temperatures. It is, however, the only source of interstellar HF, which has been
detected in a wide range of cold (10–100 K) environments. In fact, the reaction does take place efficiently at low
temperatures due to quantum-mechanical tunnelling. Rate constant measurements at such temperatures have essentially
been limited to fast barrierless reactions, such as those between two radicals. Using uniform supersonic hydrogen flows
we can now report direct experimental measurements of the rate of this reaction down to a temperature of 11 K, in
remarkable agreement with state-of-the-art quantum reactive scattering calculations. The results will allow a stronger link
to be made between observations of interstellar HF and the abundance of the most common interstellar molecule, H2, and
hence a more accurate estimation of the total mass of astronomical objects.
he F þ H2 ꢀ HF þ H reaction, and its isotopic variants, is temperature24,25 and none below 190 K, far above the temperatures
perhaps one of the most intensively investigated chemical in the ISM where HF is observed (10–100 K). Aquilanti et al.10
T
reactions. Because of its simplicity and experimental accessi- have used theoretical simulations to estimate the rate coefficients
bility, it has become the paradigm for exothermic triatomic down to 10 K, but with an uncertainty too great for use in
chemical reactions1. Sophisticated crossed molecular beam astrophysical modelling.
methods2–5 provide details of the dynamics of this reaction
We report here the first experimental determination of the rate
and reveal the existence and influence of reactive resonances— coefficient of the title reaction at temperatures between 11 and
short-lived metastable quantum states3,4,6. This experimental 295 K. The results are compared to state-of-the-art quantum reac-
work, as well as the availability of high-quality ab initio FH2 tive scattering calculations involving multiple PESs, which, in
potential energy surfaces (PESs)7,8, has stimulated many theoreti- addition, shed light on the importance of non-adiabatic effects at
cal studies of this reaction5,9–12. Because it involves transfer of a low temperatures. Our results will enable a more accurate prediction
light hydrogen atom and because of the high barrier to reaction of the abundance of HF in the different interstellar regions where it
(ꢁ800 K)8, only quantum-mechanical simulations can provide has been observed, resulting in a better calculation of the abundance
an accurate description of the dynamics, particularly at low of the commonest interstellar molecule, H2, and therefore of the
temperatures, where the reaction occurs exclusively through total mass distribution.
quantum-mechanical tunnelling5,9,12
.
The study of the title reaction at low temperatures is challen-
The discovery of interstellar HF in 199713, and the realization of ging. Because it possesses an energetic barrier, its rate coefficient
its ubiquity in the Universe thanks to very recent observations by the is expected to be small (k , 1 × 10212 cm3 molecule21 s21), in
Herschel Space Observatory14–20, has engendered renewed interest contrast to the majority of barrierless chemical reactions,
in the F þ H2 reaction, which is the sole source of HF in these gen- which have rate coefficients at temperatures below 100 K often
erally very cold environments. Molecular hydrogen is essentially exceeding 1 × 10210 cm3 molecule21 s21
.
In addition, the
unobservable at low temperatures21 and a common proxy is the Born–Oppenheimer (BO) forbidden reaction of the spin–orbit
CO molecule, which is easily observable, abundant and chemically excited F atom, F(2P1/2), can have a significant influence on
stable, but suffers from photodissociation at lower H2 column den- the reaction rates at low temperature5,26. For the experimental
sities21. The HF molecule can serve as an alternative tracer for H2 in measurements reported here, we used the CRESU (cinetique
´
´
´
these more diffuse regions owing to its high stability. Recent obser- de reaction en ecoulement supersonique uniforme, or reaction
vations reveal16,20 that HF is the main reservoir of fluorine in the kinetics in uniform supersonic flow) technique to provide a
interstellar medium (ISM) due to the large exothermicity of the low-temperature gas-phase environment, employing for the
F þ H2 reaction22 and the inefficiency of processes that destroy first time pure hydrogen flows to compensate for the relatively
the HF molecule. Reactions of HF with most of the interstellar slow F þ H2 reaction rate and pulsed laser photolysis–pulsed
elements are substantially endothermic and negligibly slow at the laser-induced fluorescence (PLP–LIF) to measure rate constants.
low temperatures that characterize molecular clouds. Only the reac- The reaction was monitored by following the increase in
tion of HF with Cþ, leading to the formation of CFþ, is expected to product hydrogen atoms. These experiments, as well as
take place in the ISM23.
complementary quantum reactive scattering calculations on
There have been, to our knowledge, only two direct determi- multiple PESs, are fully described in the Methods and in
nations of the rate of the F þ H2 reaction rate below room Supplementary Section 1.
1
2
´
´ ´
Institut de Physique de Rennes, UMR CNRS-UR1 6251, Universite de Rennes 1, 263 Avenue du General Leclerc, 35042 Rennes Cedex, France, LOMC,
3
´
UMR 6294, CNRS-Universite du Havre, 25 rue Philippe Lebon, BP 540, 76058, Le Havre, France, Department of Chemistry and Biochemistry and Institute
for Physical Science and Technology, University of Maryland, College Park, Maryland 20742-2021, USA. e-mail: mha@umd.edu; ian.sims@univ-rennes1.fr
*
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NATURE CHEMISTRY DOI: 10.1038/NCHEM.1835
ARTICLES
overcome the energy barrier in a preferential fashion to react immedi-
ately with the nearby H2 molecule in the complex.
a
The nature of the initial prompt signal was verified as being due to
the rapid production of hydrogen atoms (within the pulse duration of
the excimer laser). A spectroscopic line profile recorded at a very short
delay time (50 ns) yielded a width (full-width at half-maximum,
FWHM) of 3.8 pm, significantly wider than the laser-limited
2.3 pm line width observed at longer delay times, indicating that
these ‘prompt’ hydrogen atoms are produced with significant excess
energy (a pure Doppler line width of 3.8 pm for the Lyman a tran-
sition at 121.57 nm would correspond to a temperature of 1,940 K).
A potential source for these prompt, hot hydrogen atoms could be
photolysis of low levels of hydrogen-containing impurities. However,
the fact that this prompt, hot hydrogen-atom signal is not observed at
77 K and 295 K would be difficult to understand in this case. Instead,
we propose that this prompt signal is due to a reaction initiated within
a small concentration of F2–H2 van der Waals complexes, which could
be formed during the expansion and subsequent cold flow, in a similar
way to a number of reactions studied within the Wittig group, among
others27. Although the formation of such weakly bound complexes
would be slow under the relatively low pressure conditions of
CRESU flow, photolysis of F2 within the complex would yield two
fluorine atoms with high translational energy, one of which would
almost immediately encounter an H2 molecule in a potentially favour-
able orientation for reaction and with a favourable impact parameter
and relatively high energy. This would result in a prompt signal, the
magnitude of which would increase in comparison with the normal
0
100
200
300
Delay time (µs)
10
b
8
6
4
2
T = 77 K
k = (2.3 0.5) × 10–12 cm3 s–1
0
0
a
1
2
3
[n-H2] (1016 molecule cm–3
)
Figure 1 | Results for the determination of the F1H2 reaction rate
coefficient at 77 K. a, Hydrogen atom product LIF intensity ILIF as a function
of photolysis–probe laser delay time at 77 K for an H2 concentration of
6.91 × 1015 cm23, fit to a function of the form ILIF ¼ A(exp(2kdiff,Ht) 2
exp(2k1stt)) (red line). b, Pseudo-first-order rate coefficients k1st obtained
from a series of such experiments plotted against the concentration of normal
hydrogen, [n-H2]. Error bars correspond to twice the standard error in k1st
.
T = 11.3 K
k = (2.5 1.9) × 10–13 cm3 s–1
Results
Figure 1a presents the LIF trace of hydrogen atoms formed during the
reaction at 77 K for a given density of H2 introduced into the flow. This
trace was fitted using a sum of exponential functions, which included
the first-order rate coefficient for hydrogen atom formation (k1st), as
well as the rate coefficient for loss of hydrogen atoms by diffusion in
the flow (kdiff,H). The values of k1st were plotted as a function of n-H2
(normal hydrogen, see later) density to yield a straight line with slope
corresponding to the second-order rate coefficient at 77 K (Fig. 1b).
Results at 295 K are provided in Supplementary Fig. 3. Figure 2
shows the fitted LIF traces of hydrogen atoms formed during reaction
at 11.3 K and 20.4 K. Because n-H2 was used as the buffer gas at
these temperatures, it was not possible to vary its density. However,
second-order rate coefficients could be obtained as the ratio of the
first-order rate coefficients extracted from the fits of the LIF traces
and corrected for diffusive loss both of product hydrogen atoms and
reagent fluorine atoms (Supplementary Section 1.4), divided by the
respective n-H2 densities. The measured rate coefficients at the four
temperatures are listed in Table 1 and plotted as a function of tempera-
ture in Fig. 3, together with the results of the theoretical calculations.
As can be seen in Fig. 2, a prompt formation of hydrogen atoms
was observed within the first microsecond at the lowest temperatures
of 11.3 and 20.4 K. We attribute this to the formation of a small con-
centration of F2–H2 complexes in the very low-temperature superso-
nic flow. These complexes are then photodissociated by the photolysis
laser, yielding translationally ‘hot’ fluorine atoms that may be able to
0
50
100
150
200
Delay time (µs)
b
T = 20.4 K
k = (2.2 0.7) × 10–13 cm3 s–1
0
50
100
150
200
250
Delay time (µs)
Figure 2 | Hydrogen atom product LIF intensity as a function of
photolysis–probe laser delay time. a,b, LIF signal at 11.3 K for a density
of [n-H2] ¼ 3.92 × 1016 cm23 (a) and at 20.4 K for a density of
[n-H2] ¼ 5.10 × 1016 cm23 (b). The traces are fit (red line) by an
exponential growth (reaction) and decay (loss by diffusion), including an
initial prompt component, as described in Supplementary Section 1.5.
142
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NATURE CHEMISTRY DOI: 10.1038/NCHEM.1835
ARTICLES
Table1 | Rate coefficients for the reaction between F(2PJ) atoms and n-H2 between 11 and 295 K.
Temperature (K)
Buffer gas
Total density (1016 molecule cm23
)
[H2] (1016 molecule cm23
)
k (10213 cm3 molecule21 s21
)
11.3
20.4
77
H2
H2
He
He
4.02
5.15
101
3.92
5.10
0.364–3.41
0.100–0.390
2.48+1.90*
2.18+0.72
23.2+4.8
249+37
295
28.1
*Errors correspond to 95% statistical confidence limits combined with an estimated 10% systematic error (20% at 77 K).
reactive signal as the temperature decreased and the formation of by Stevens and colleagues25. As can be seen by comparison with
F2–H2 van der Waals complexes was thereby enhanced. Such a depen- the extrapolated recommendation from the review of Persky and
dence was indeed observed, with negligible prompt signal at 77 and Kornweitz28, also shown in Fig. 3, our results at the lowest tempera-
295 K and a prompt signal at 20 K of about 40% of the fitted magni- tures display a strongly non-Arrhenius temperature dependence.
tude of the reactive signal, rising to be as large as the reactive signal at This confirms the importance of tunnelling in this reaction.
11 K. Measurements were also made using a hydrogen flow at 5 K, but
The results of the simulation shown in Fig. 3 demonstrate excel-
the prompt signal had grown so large in comparison with the lent agreement with the experimental measurements, confirming
reactive signal that it was impossible to measure the rate constant the importance of using an appropriate theoretical description. In
accurately for the F þ H2 reaction at this temperature.
particular, it is essential to take into account the open-shell charac-
To test this explanation, we performed ab initio calculations to ter of the fluorine atom. In our experiment the F(2P1/2) excited spin–
determine the dissociation energy of the F2–H2 van der Waals orbit state is collisionally relaxed on a timescale much shorter than
complex, as detailed in Supplementary Section 2.1. The complex the reaction, especially at low temperature. This is fully discussed in
was indeed found to have a global minimum in a T-shaped configur- Supplementary Section 1.2. Thus, reaction occurs only on the
ation, with the F2 normal to the H–H bond, and a value for the dis- ground adiabatic PES (Supplementary Fig. 4). Nevertheless, a
sociation energy (De) on the order of 100 cm21. The F2–H2 van der correct multi-state scattering treatment includes coupling between
Waals complex will therefore form at low concentrations via three- the three FH2 PESs (Supplementary Fig. 4) and the presence of
body collisions at the very low temperatures of the CRESU flow. non-zero electronic and spin angular momenta, all neglected in
Photodissociation of F2 within this complex will result in F þ H2 col- the usual (BO) single-PES treatment. Consequently (Fig. 3), even
lisions that are favourably oriented for reaction, providing a plausible at low temperature26, the multi-state cross-sections and rate coeffi-
explanation for the prompt hydrogen atom signal.
cients are lower than those obtained within a single-PES treatment.
Aquilanti et al.10,29 have discussed in detail the inadequacies in
the original Stark–Werner PES, adding modifications to include
Discussion
Excellent agreement was obtained between our experimental rate the spin–orbit Hamiltonian and to describe better the long-range
coefficients and previous measurements down to 221 K obtained F–H2 interaction. The rate constants predicted by calculations
with this PES (which they designate PES III) are also shown in
Fig. 3. They appear to be a significant underestimate at low
10–10
4× 10–11
3× 10–11
10–10
10–11
10–12
10–13
2× 10–11
1× 10–11
10–11
10–12
10–13
200
300
F + H2 (LTE, Zhu et al.)
F + H2 ( j = 0)
F + H2 (LTE)
1
10
100
1,000
F + H2 ( j = 1)
Temperature (K)
Figure 3 | Comparison of rate coefficients k for the F(2PJ)1n-H2 ꢀ HF1H
reaction as a function of temperature. Symbols present experimental
results: from this work (filled red circles, with error bars corresponding to
95% confidence limits) and from Stevens and colleagues25 (open triangles,
with their quoted uncertainties). The solid and dashed blue lines represent
the predictions of our scattering calculations with, respectively, inclusion and
exclusion of non-BO coupling. In these calculations we assume a ratio of
ortho (j ¼ 1) to para (j ¼ 0) H2 of 3:1. Also shown is the Arrhenius
temperature dependence proposed by Persky and Kornweitz28 (alternating
short–long dashed dark green line), the predictions of the single-PES
quantum scattering calculations of Zhu and colleagues22 (long-dashed
magenta line) and the quantum reactive scattering calculations of Aquilanti
and colleagues10,29 (light green dashed–dotted line) performed on their PES
III, which is a modification of the original Stark–Werner PES.
1
10
100
Temperature (K)
Figure 4 | Low-temperature F1H2 (j 5 0, 1) rate coefficients. The blue
solid line shows the full calculation described in the text, including non-BO
effects, for the reaction of fluorine in its lower spin–orbit 2P3/2 state with H2
(j ¼ 0), while the red dashed line shows the same calculation for H2 (j ¼ 1).
The dotted black line represents the calculated rate coefficient determined
under the assumption of a Boltzmann population at temperature Tof the
j ¼ 0 and j ¼ 1 rotational levels, and the thin black line which is coincident
with this plot shows the fit to this LTE value (described in the text and
recommended for use in astrochemical models). The magenta dashed–dotted
line shows the rate coefficient currently used in astrochemical calculations of
HF densities23,30,31 based on calculations by Zhu and colleagues22,32
.
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NATURE CHEMISTRY DOI: 10.1038/NCHEM.1835
ARTICLES
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
temperature, probably a consequence of an overestimate of the
height of the barrier to reaction8. We explore this disagreement
further in Supplementary Section 2.2, where we compare our
results with the full set of calculations of Aquilanti and colleagues.
At the four temperatures investigated, the new rate coefficients are
between 1.5 and 5 times lower than those used in the interstellar flu-
orine chemistry model of Neufeld and Wolfire30. As an example of the
likely impact of this work upon astrochemical models, we have taken
the simplified interstellar fluorine chemistry model based on the work
of Neufeld and Wolfire30 and used by Godard et al.31 to calculate the
ratio of abundances of n(HF) to n(H2) in diffuse clouds. This can then
be used to relate observations of HF by the Herschel Space
Observatory to the difficult-to-observe hydrogen abundance and
hence the total mass of the object under study. Under such conditions,
the abundance of HF is controlled by four chemical reactions:
10
100
F + H2 ꢀ HF + H
HF + hn ꢀ F + H
HF + C+ ꢀ CF+ + H (k2)
(k1)
(zHF
Temperature (K)
Figure 5 | Values of the predicted abundance ratio n(HF) to n(H2) for
typical diffuse interstellar cloud conditions. Equation C.3 from Godard et al.31
is used with atomic hydrogen density nH ¼ 50 cm23, visual extinction
)
Av ¼ 0.3 and molecular fraction fH = 1. The dashed black line shows the
2
CF+ + e− ꢀ F + C
(k3)
calculation using the expression for the F þ H2 rate coefficient supplied by
Zhu et al.22,32 and used by Godard et al.31, and the solid blue line shows the
same calculation using our new value for the rate coefficient. The black
dotted line shows the actual value of the abundance ratio (3.6 × 1028) used
by Godard et al. for their comparisons.
Godard et al.31 used the value for the rate coefficient k1 calculated by
Zhu et al.22,32 and communicated to Neufeld and Wolfire30, namely
k1Zhu et al. = 1.00 × 10−10[exp(−450 K/T) + 0.078 exp(−80 K/T)
+ 0.0155 exp(−10 K/T)] cm3 s−1
In passing, we note that Godard et al., in their calculations,
have in fact neglected the left-hand side of the denominator of
their equation C.3, and used a constant value for the abundance
ratio (3.6 × 1028), which overestimates the new value for the abun-
This is claimed to be valid down to 10 K, and is plotted in Fig. 4
together with the results of our calculations for H2 in j ¼ 0 and dance ratio by more than an order of magnitude at
low temperatures.
j ¼ 1 rotational states and a thermally averaged value (LTE, local
Fluorine chemistry is less well understood in the densest
(105–106 cm23) and coldest environments (T ¼ 10–50 K) where
a significant fraction of HF is expected to condense on dust
grains15,33. A better knowledge of this chemistry, in which the
reaction F þ H2 plays a central role, is crucial to interpret new
observations of HF in such cold, dense regions of the ISM.
In summary, we have reported the experimental determination
of rate coefficients for the F þ H2 reaction down to 11 K, where
thermodynamic equilibrium). We have fitted this thermally
averaged value of k1 to give the following equation, which we recom-
mend for use in astrochemical models:
k1This work = 4.71 × 10−13[exp(−0.364 K/T) + 3.38 exp(−38.9 K/T)
+ 212 exp(−881 K/T) + 124 exp(−414 K/T)] cm3 s−1
This fit is also plotted in Fig. 4, and differs from the value of Zhu et al. the reaction takes place exclusively by quantum-mechanical tunnel-
by a factor of ꢁ3 in the range 10–100 K. The ratio of abundances of ling. The results agree very well with the predictions of non-adia-
n(HF) to n(H2) under typical diffuse interstellar cloud conditions is batic, multiple-PES quantum scattering calculations, confirming
the importance of non-BO effects. The new rate coefficients
should significantly improve the estimation of the density of inter-
stellar molecular hydrogen by means of observations of HF.
given by equation C.3 in ref. 31:
n(HF)
[F]
ꢀ
ꢁ
ꢂ
ꢃ
=
zHF
n H2
ꢂ
ꢃ
[C]k2 +
Methods
f
k2
k3
nH
H2
We used the CRESU technique34 to measure rate coefficients for the title reaction
over temperatures ranging from 11 to 295 K. This method uses isentropic supersonic
expansions of a buffer gas through convergent–divergent Laval nozzles to generate a
cylinder of cool, thermalized gas that can be used as a reactor to perform kinetics
experiments. Fluorine atoms were generated by excimer laser photolysis of
molecular fluorine at 248 nm, followed by rapid removal of any non-thermal
distribution of fluorine spin–orbit states by collision with the buffer gas, as discussed
in Supplementary Section 1.2. The rate coefficients were measured by monitoring
the hydrogen atoms formed using resonant vacuum ultraviolet LIF at a wavelength
of 121.6 nm. A schematic diagram of the experimental apparatus is shown in
Supplementary Fig. 1.
+
1 +
k1
2
Using their values for zHF, k2, k3 and the fluorine and carbon
elemental gas-phase abundances relative to hydrogen nuclei, [F]
and [C] (see ref. 31 for the values), and typical diffuse cloud
conditions with atomic hydrogen density nH ¼ 50 cm23, visual
extinction Av ¼ 0.3 and molecular fraction fH (defined as
2
fH = 2n(H2)/nH) set to 1, we calculated the n(HF) to n(H2) abun-
2
dance ratio using both the Zhu et al. expression for k1 and the
new value from this work. The results are presented in Fig. 5 and
show that our new values result in a significant lowering of
the predicted n(HF) to n(H2) abundance ratio over the temperature
range 10–100 K, by up to a factor of two. Thus, for a given
observation of HF, the predicted value of the total H2 abundance
would be significantly increased (by up to a factor of two) using
the new data.
Experiments were performed at 295, 77, 20 and 11 K. At 11 and 20 K, where
we anticipated small rate coefficients, we used pure hydrogen as both the buffer gas
and reagent. This required the use of Laval nozzles, which, together with the
hydrogen gas, were precooled to 77 K using liquid nitrogen to avoid problems
associated with incomplete rotational relaxation. The high quality of the uniform
supersonic flows can be seen in the temperature profiles shown in Supplementary
Fig. 2. The measurements at 77 and 295 K were performed by introducing various
densities of H2 into a subsonic flow of helium obtained by reducing the pumping
speed. In all cases, a room-temperature population ratio of 3:1 was maintained for
144
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NATURE CHEMISTRY DOI: 10.1038/NCHEM.1835
ARTICLES
the ortho:para spin isomers of H2 (so-called normal hydrogen, n-H2). The use of
pure hydrogen gas in the CRESU technique is considered in more detail in
Supplementary Section 1.1.
20. Sonnentrucker, P. et al. Detection of hydrogen fluoride absorption in diffuse
molecular clouds with Herschel/HIFI: an ubiquitous tracer of molecular gas.
Astron. Astrophys. 521, L12 (2010).
A full theoretical description of the F þ H2 reaction requires three BO PESs as
well as an off-diagonal coupling potential and the dependence on geometry of the
spin–orbit coupling between atomic states9 (Supplementary Fig. 4).
Time-independent, fully quantum, reactive scattering calculations were carried
out to predict rate coefficients, based on a formalism presented earlier9 and
incorporating the very recent Li–Werner–Alexander–Lique (LWAL) PESs26, which
are a fit to highly correlated ab initio calculations. In particular, and of crucial
importance at low temperature, the external correlation energy was scaled so that the
calculated F(2P3/2) þ H2(v ¼ 0, j ¼ 0) ꢀ HF(v ¼ 3, j ¼ 0) þ H endoergicity
(22.73 meV) agrees with experiment (22.33+0.61 meV) to within the experimental
uncertainty8,35. Furthermore, the reaction barrier of 71.18 meV is very close to the
current best estimate of 70.86 meV (ref. 36). This is an indication of the high
accuracy and quantitative nature of the PES.
21. van der Tak, F. The first results from the Herschel-HIFI mission. Adv. Space Res.
49, 1395–1407 (2012).
22. Zhu, C., Krems, R., Dalgarno, A. & Balakrishnan, N. Chemistry of hydrogen
fluoride in the interstellar medium. Astrophys. J. 577, 795–797 (2002).
23. Neufeld, D. A., Wolfire, M. G. & Schilke, P. The chemistry of fluorine-bearing
molecules in diffuse and dense interstellar gas clouds. Astrophys. J. 628,
260–274 (2005).
24. Wurzberg, E. & Houston, P. L. The temperature-dependence of absolute
rate constants for the F þ H2 and F þ D2 reactions. J. Chem. Phys. 72,
4811–4814 (1980).
25. Stevens, P. S., Brune, W. H. & Anderson, J. G. Kinetic and mechanistic
investigations of F þ H2O/D2O and F þ H2/D2 over the temperature-range
240–373 K. J. Phys. Chem. 93, 4068–4079 (1989).
All scattering calculations were performed with the ABC code of Manolopoulos
and co-workers37, extensively modified to include the three FH2 PESs as well as the
spin–orbit and non-adiabatic coupling between them9. The resulting state-to-state
reaction cross-sections were transformed into thermal rate coefficients by
appropriate averaging over reactant and product rovibrational states and collision
energies, as described in Supplementary Section 1.7.
26. Lique, F., Li, G. L., Werner, H. J. & Alexander, M. H. Communication: non-
adiabatic coupling and resonances in the F þ H2 reaction at low energies.
J. Chem. Phys. 134, 231101 (2011).
27. Jaques, C. et al. Photoinitiated processes in complexes—subpicosecond studies
of CO2-HI and stereospecificity in Ar-HX. J. Chem. Soc. Faraday Trans. 89,
1419–1425 (1993).
Further details of the experimental and theoretical methods can be found in the
Supplementary Information.
28. Persky, A. & Kornweitz, H. The kinetics of the reaction F þ H2 ꢀ HF þ H.
A critical review of literature data. Int. J. Chem. Kinet. 29, 67–71 (1997).
29. Aquilanti, V. et al. Exact activation energies and phenomenological description
of quantum tunneling for model potential energy surfaces. The F þ H2 reaction
at low temperature. Chem. Phys. 398, 186–191 (2012).
30. Neufeld, D. A. & Wolfire, M. G. The chemistry of interstellar molecules
containing the halogen elements. Astrophys. J. 706, 1594–1604 (2009).
31. Godard, B. et al. Comparative study of CHþ and SHþ absorption lines observed
towards distant star-forming regions. Astron. Astrophys. 540, A87 (2012).
32. Zhu, C., Krems, R., Dalgarno, A. & Balakrishnan, N. Erratum: ‘Chemistry of
hydrogen fluoride in the interstellar medium’ (vol. 577, p. 795, 2002).
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Acknowledgements
The authors acknowledge support from the French Agence Nationale de Recherche (ANR
Blanc Programme, Project CRNS) and the Centre National de la Recherche Scientifique
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Additional information
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Supplementary information is available in the online version of the paper. Reprints and
permissions information is available online at www.nature.com/reprints. Correspondence and
requests for materials should be addressed to M.H.A. and I.R.S.
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Competing financial interests
The authors declare no competing financial interests.
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