Infrared spectra of (CH 3) 2 O and (CH 3) 2 O+ H 2 O at low temperature

Infrared spectra of (CH3)2O and(CH3)2O þ H2O at low temperature

L. Schriver-Mazzuolia,b,*, J.M. Coangaa, A. Schrivera, P. Ehrenfreundc

aLaboratoire de Physique Moleculaire et Applications, Unite Propre du CNRS, Universite Pierre et Marie Curie,

Tour 13, Case 76, 4 Place Jussieu, 75252 Paris Cedex 05, FrancebLaboratoire d’Etude des Nuisances Atmospheriques et de Leurs Effets, Universite Paris Nord,

IUP Ville et Sante, Rue de la Convention, Bobigny 93017, FrancecLeiden Observatory, P.O. Box 9513, 2300 Leiden, RA, The Netherlands

Received 20 February 2002; received in revised form 16 April 2002; accepted 16 April 2002

Abstract

Fourier transform infrared reflection spectroscopy (incidence angle of 58) was used to characterize thin films of dimethyl ether

(DME) and of mixtures containing water and DME between 10 and 160 K under a pressure of 10�7 mbar. Solid DME has two

solid phases: an amorphous phase which is obtained below 65 K and a crystalline phase >65 K. From 90 K, DME begins to

sublimate with surface binding energy of 20 � 2 kJ mol�1. Vibrational spectrum of DME trapped in water ice remains nearly

unchanged from 30 to 120 K. Between 120 and 130 K, a large part of DME is released and strong changes in the frequencies and

the profile of the absorptions of DME are observed. This behavior suggests the formation of clathrate hydrate. Below 120 K, the

trapped DME is hydrogen-bonded to water molecules.

# 2002 Elsevier Science B.V. All rights reserved.

Keywords: Infrared spectroscopy; Interstellar molecules; Ices; Laboratory studies

1. Introduction

The search for the solid dimethyl ether (DME)

features in the IR spectra of interstellar clouds requires

laboratory data. Observations at radio, millimeter, sub-

millimeter, and infrared wavelengths have led to the

discovery of well over a hundred different molecules in

interstellar clouds and circumstellar shells. Many of

these are organic species of considerable complexity,

with HC11N [1] and diethyl ether [2] being the largest

detected so far. Despite their relatively low abundances,

the variety and complexity of organic compounds

currently detected in space indicates an active chem-

istry and ubiquitous distribution [3].

A large part of the molecular complexity found in

these regions is due to gas–grain interactions. Radio-

astronomical molecular line surveys of well-known

sources, such as the dense star-forming cores in the

Orion and Sagittarius molecular clouds, show the

signature of DME [4–6]. The sources mentioned

above are examples of so-called ‘‘hot cores’’, dense

(106 cm3), hot (�200 K) regions in the immediate

vicinity of massive protostars. The high abundances

of many molecular species found in hot cores cannot be

explained by gas-phase chemistry and one must invoke

molecule formation on catalytic icy grain surfaces [3]

Vibrational Spectroscopy 30 (2002) 245–257

* Corresponding author. Tel.: þ33-1-4427-4475;

fax: þ33-1-4427-7033.

E-mail address: [email protected] (L. Schriver-Mazzuoli).

0924-2031/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved.

PII: S 0 9 2 4 - 2 0 3 1 ( 0 2 ) 0 0 0 4 5 - 0

during the cold dark cloud phase of these cores. Heating

by the newly formed protostar and/or energetic pro-

cesses such as outflows producing shock waves lead

to evaporation of the grain mantles into the gas, which

is followed by gas-phase reactions. Pathways to the

formation of larger molecules include alkyl-cation

transfer [7]. For example, infrared observations of ices

towards embedded protostars have been extensively

revisited with the infrared space observatory (ISO) [8].

Currently, the largest molecule detected in interstellar

ices is methanol. In order to identify larger molecules

residing on interstellar grains, laboratory data are

of crucial importance. DME is a molecule with C2v

symmetry and its rotational transitions have been

extensively studied in the laboratory [9]. Also the IR

spectrum of DME has been investigated 30 years ago

([10,11] and references therein). However, no study has

examined ice mixtures of DME, and hence, spectral

properties of the DME fundamentals in ice are not

known. In the present work, we report an infrared

spectroscopic study of mixed films containing water

and DME with a particular attention to temperature

effects between 10 and 170 K. Prior to this study

spectra of pure DME at the same temperature have

also been recorded and the surface binding energy

of DME has been determined. Vibrational spectra of

interstellar ice analogs have been the object of numer-

ous investigations [12–15]. Temperature effects in band

shape and positions of the (OH) absorption related to

structural differences with our experimental device

have been recently described [16].

2. Experimental

The experiments were performed using a rotatable

closed cycle helium refrigerator (Air Product, Displex

202 A), which was pumped continuously (10�7 mbar

background pressure range). Thin films of ice or DME

or H2O–DME mixtures were supported inside upon a

thermostated gold side of a substrate cube. The tem-

perature of the metal substrate (10–170 K) was control-

led by a silicon diode (Scientific Instruments 9600-1).

Water was de-ionized, triply distilled, and then

degassed by freeze–thaw cycles under vacuum before

use. Water layers were grown from water vapor via a

capillary connected to pulsed dosing system, which

provides pulses of gas during 2 ms at a frequency of one

pulse per 2 s leading to a deposition rate of about

2 � 10�3 mmol h�1. DME (from UCAR) was con-

densed at 30 K with helium as carrier gas. The initial

mixture was prepared in a 1 l glass bulb on a vacuum

line. The ratio DME/He was typically 1/25 with a total

pressure of Helium of 150 Torr. Gas was deposited via

a capillary with a rate of 3 mmol h�1. The deposition

nozzle parameters were 1 mm inner diameter and a

distance of 20 mm from the cold substrate. Mixed films

of H2O/DME were prepared with the same method.

Typical ice film thickness were less than 500 nm. The

infrared measurements (from 5000 to 400 cm�1) were

made in reflection absorption geometry with an inci-

dence angle of 58 to the surface normal using a Bruker

113v spectrometer. Spectra were recorded at 0.5 cm�1

nominal resolution from co-addition of 100 interfero-

grams. Temperature was slowly increased (2 K min�1)

then stabilized at the desired value.

3. Results and discussion

3.1. Pure DME ice

Vibrational spectra at 90 K of films of crystalline

DME including deuterated species have been described

in detail 30 years ago [17]. It was suggested that the two

methyl groups were not equivalent in a site of symmetry

lower than C2v with at least four molecules in the unit

shell. Fig. 1 shows in the fundamental regions typical

spectra of a thin film of DME deposited at 11 K then

annealed at 60, 70 and 90 K. From 93 K, DME begins

to sublimate. A strong difference is observed between

spectra recorded below 60 K and spectra recorded

above 70 K which are comparable to those described

in [17]. This behavior provides evidence for the exis-

tence of two phases of DME: an amorphous phase at

low temperature and a more ordering phase which

appears from 65 K. Condensation at 90 K was also

carried out in order to investigate the effect of deposi-

tion temperature on the DME fundamentals. Resulting

spectra were similar to those obtained after annealing

of amorphous DME condensed at 30 K. Positions, full

width at half maximum (FWHM) and relative inten-

sities of the fundamental bands of DME at 11 and 90 K

are summarized in Table 1.

In the amorphous phase, the n(CH3) region is

characterized by two distinct single bands at 2986.8

246 L. Schriver-Mazzuoli et al. / Vibrational Spectroscopy 30 (2002) 245–257

and 2814.0 cm�1 separated by three overlapped

absorptions at 2920.5, 2883.8 and 2866.6 cm�1, the

two later corresponding to overtone or combination

bands of CH3 deformation modes in Fermi resonance

with fundamentals at higher frequency. When going

from amorphous phase to crystalline phase, the

absorption pattern narrows and the two single bands

split into two doublets measured at 2990.0–2984.8 and

2822.4–2814.0 cm�1. This splitting could be due to

the existence of two types of molecule in the unit cell

or to the non-equivalence of the two methyl group in

the same molecule as suggested in [17].

In the deformation region, the spectrum of crystalline

DME is very complex as compared to the spectrum of

the amorphous phase. The broad band observed at

1458.4 cm�1 with a weak shoulder at 1480 cm�1 at

11 K splits into 10 narrow bands above 65 K, the two

more intense features being located at 1481.9 and

1467.7 cm�1. In fact six bands are expected in this

region for the two CH3 groups (4das(CH3), 2ds(CH3)).

Additional features can be due to molecules in non-

equivalent positions and orientations or to 18O and 13C

satellites lines. As a matter of fact, in solid SO2, for

several vibrational modes, the relative integrated inten-

sity absorption coefficients of the 34SO2 isotopic bands

in regard to the bands of natural species is high and not

in good agreement with the natural abundance of 34S

[18]. The minor isotope species are diluted in the solid

main isotope and can be less coupled with crystal

vibrations due to their different mass [19].

Fig. 1. Vibrational spectrum of solid DME recorded at: (a) 11 K; (b) 60 K; (c) 70 K; (d) 90 K.

L. Schriver-Mazzuoli et al. / Vibrational Spectroscopy 30 (2002) 245–257 247

Smaller changes between the two phases are seen in

the rocking mode regions. The three broad bands

located at 1247.1, 1159.3, 1088.5 cm�1 in the amor-

phous phase, and assigned to the r(CH3)(A1), and to

the two r(CH3) coupled with the n(COC), respectively,

narrow by a factor of 4 and blue shift weakly (about

2 cm�1) in the crystalline phase. The two weak satel-

lites at 1158.5. and 1083.3 cm�1 are due to isotopic

species (13C). The r(CH3)(B2) appears as a very weak

feature at 1179 cm�1 and the r(CH3)(A2) expected

about 1148 cm�1 is not observed.

At last, the n(COC) mode is characterized by a band

at 916 cm�1 with a FWHM of 10.5 cm�1 in the

amorphous phase. It narrows by a factor of 6.5 and

splits into three overlapped components at 917.5,

916.4 and 913.1 cm�1 with a weak feature at

907.5 cm�1 due to isotopic DME, (13C) in the crystal-

line phase at 90 K.

Dependence on the band positions with tempera-

ture was accurately examined. No change was

observed between 10 and 60 K for the amorphous

DME. Very weak shifts were measured for crystal-

Table 1

Frequencies in cm�1, relative optical density, FWHM in cm�1, of fundamental absorptions of DME at 11 and 90 K

Amorphous DME (11 K) Crystalline DME (90 K) Assignment

n OD FWHM n OD FWHM

2986.8 0.3 23 2990.0 0.3 5 n0sðCH3Þ2984.8 0.3 5

2920.5 04 –a 2924.5 0.2 25 naðCH3Þ2903.0 sh 2dðCH3Þ

2883.8 0.3 –a 2887.1 0.4 9

2866.6 sh –a 2871.9 0.2 10

2814.0 1 12 2822.4 0.8 3 nsðCH3Þ2814.0 0.4 3

1480.1 sh 1481.9 0.1 2.5 dðCH3Þ1458.4 0.1 24 1474.2 0.02

1467.7 0.2 2

1460.1 0.07

1457.1 0.05

1452.9 0.04

1447.2 0.03

1443.3 0.02

1429.1 0.006

1247.1 0.04 11.5 1249.3 0.05 rðCH3Þ1179.0 0.02

1159.3 0.4 8 1161.8 0.6 3 nðCOCÞ þ rðCH3Þðb1Þ1158.5 sh 13C

1088.5 0.4 8 1090.2 1 1

1083.3 0.06 13C

1072.7 18O

916.4 0.15 10 917.5 sh n(COC)

916.4 0.25 1.5

913.1 0.1

907.6 0.02 13C

sh: shoulder.a These bands are overlapped with a total FWHM of 85 cm�1.


line DME between 70 and 100 K. Bands located at

1482, 1468 cm�1 at 70 K were red-shifted with a

Dn/DT of �0.04 and �0.025 cm�1 K�1, respectively.

Bands located at 1160, 1090 and 916 cm�1 shifted

towards the blue with a Dn/DT of þ0.018, þ0.007

and þ0.01 cm�1 K�1, respectively. In the n(CH3)

region, the two components of each doublet shifted

with an inverse sign, the higher component with a

Dn/DT of �0.025 cm�1 K�1 and the lower com-

ponents with a Dn/DT about of þ0.04 and

þ0.02 cm�1 K�1.

In addition to the fundamental bands of DME, two

weak absorptions (optical density ðODÞ ¼ 0:012) were

observed at 2075.3 and 2002 cm�1 in the previous

films deposited at 30 K. They narrow by a factor of

6 above 65 K and the former one splits into three

overlapped components. These bands can be assigned

to the combinations of the rocking modes (1159.3 and

1088.5 cm�1) with the n(COC) mode at 916.4 cm�1,

respectively. In order to observe other combination

or overtones bands in the 4500–3600 cm�1 region, we

have recorded the spectrum of a thicker sample at

different temperatures. As shown in the Fig. 2A,

numerous weak bands appear. Their intensity are very

weak in regard to those located between 2100 and

2000 cm�1 which are displayed Fig. 2B for the same

sample. The Table 2 summarizes the observed fre-

quencies. On the basis of changes in the spectrum

Fig. 2. (A) Absorptions of a film of DME in the 4500–3650 cm�1 and (B) 2100–1980 cm�1 regions: (a) 30 K; (b) 70 K.

Table 2

Frequencies in cm�1 of additional bands observed between 4450–3700 and 2100–1990 cm�1

30 K 70 K Tentative assignment

4427.0 4440.5–4415.2 n0sðCH3 þ dðCH3ÞÞ4353.1 4365.7 naðCH3Þ þ dðCH3Þ4244.1 4254–4242.7–4230.0 nsðCH3Þ þ dðCH3Þ4152.7 4150.2–4143.6 naðCH3Þ þ rðCH3Þ4091 4105 n0sðCH3Þ þ ðnðCOCÞ; rðCH3ÞÞ4056 4059.4 n0sðCH3Þ þ ðnðCOCÞ; rðCH3ÞÞ4023 4028.4

3976.6 3983.3–3975.0 naðCH3Þ þ ðnðCOCÞ; rðCH3ÞÞ3906.0 3910.9–3901.2 nsðCH3Þ þ ðnðCOCÞ; rðCH3ÞÞ3734.0 3738–3730 nsðCH3Þ þ ðnðCOCÞ; rðCH3ÞÞ2075.3 2090.4–2075.0–2071 ðnðCOCÞ; rðCH3ÞÞ þ nðCOCÞ2002 2000 ðnðCOCÞ; rðCH3ÞÞ þ nðCOCÞ


between the amorphous and the crystalline phases, a

tentative assignment has been given.

In order to derive the surface binding energy of

DME in our experimental conditions, kinetic studies

of the sublimation of crystalline DME were performed

at 94, 96, 98, 100 and 102 K. Several experiments

were done. In each experiment, pure DME was deposi-

ted near the temperature to be studied then the spectra

were recorded at the chosen temperature at different

times. The concentration of DME which remains at

each time was monitored by the measurement of the

integrated intensity of different absorptions of DME.

Fig. 3A shows plots of the integrated intensity of the

1090.2 cm�1 band versus time at some different tem-

peratures. As it can be seen, a zero order is found as

expected for a three-dimensional phase. For presenta-

tion, curves have been translated at the same origin.

Slope of the curves does not give the absolute rate

constant k which is dependent on the absorption

coefficient of the band of DME monitoring the kinetic

loss of DME. However, variation of ln k versus reci-

procal temperature leads to activation energy from the

Arrhenius law. As shown in Fig. 3B, the logarithm of

the sublimation rates varies linearly with reciprocal

temperature and the activation energy for sublimation

of neat crystalline DME was deduced from the slope

of such a plot. Ea,sub related to the enthalpy of sub-

limation was found to be 20 � 2 kJ mol�1.

3.2. Proton affinity of DME

The proton affinity of DME has been found of about

803 kJ mol�1 indicating that hydrogen bonds stronger

than in the water dimer can be formed between DME

and water for which proton affinity is of about

685 kJ mol�1. In nitrogen matrices, three complexes

(H2O)m(DME)n (m ¼ n ¼ 1; m ¼ 1, n ¼ 2; m ¼ 2,

n ¼ 1) have been spectroscopically identified from

shifts in the infrared absorptions of water sub-mole-

cule [20]. The 1:1 complex, HOH DME is charac-

terized by strongly decoupled oscillators, an

unperturbed one absorbing at 3700 cm�1 and an other

one perturbed with a frequency close to 3510 cm�1. In

the 1:2 complex, B H–O–H B in which the water

molecule keeps it C2v symmetry, n3 is measured at

3592 cm�1 and n1 at 3520 cm�1.

The 2:1 complex is characterized by two bands at

3520 and 3425 cm�1 corresponding to two perturbed

OH of water molecules. Vibrational spectrum of DME

sub-molecule in the complex was not reported. In

order to examine spectrum of DME molecule com-

plexed by water molecules, we have recorded some

nitrogen matrix spectra of DME co-deposited with an

Fig. 3. (A) Kinetic curves for pure DME desorption at 93, 97 and 100 K. The integrated intensity of the DME absorption at 1088 cm�1 is

plotted vs. time for each temperature. (B) Arrhenius plots of ln k vs. 1/T.


excess of water at 17 K in a single jet deposition

(DME/H2O/N2 ¼ 1/20/1000). Fig. 4 compares the

spectrum obtained with that of isolated DME in

nitrogen without water. As can be seen, addition of

an excess of water gives rise to new DME absorptions

in almost all the fundamental regions. Positions of

bands characterizing DME isolated in nitrogen and

those of DME perturbed by water molecules are

summarized in Table 3. In the n(CH3) region new

bands are blue-shifted by about 8–9 cm�1 from fun-

damental bands of DME, while the rocking modes

coupled with the n(COC) mode are red-shifted. The

stronger red shift was found for the n(COC) band

which is shifted by 7 cm�1 and appears as a doublet.

3.3. Interaction of DME with ice surfaces

Before studying the spectral properties of mixed

(DME þ H2O) films we have recorded the spectrum of

a water ice film first deposited at 11 K then exposed to

DME vapor. Difference spectrum between the water

film spectrum and that of water film recovered by

Fig. 4. FTIR spectra in the 3020–2800, 1275–1050 and 945–900 cm�1 spectral DME regions of: (a) DME/N2, 1/5000; (b) DME/H2O/N2,

1/20/5000. All spectra are recorded at 11 K after deposition at 17 K.


some layers of DME did not reveal new bands in the

n(OH) region of ice and in the DME fundamental

regions. DME fundamentals were qualitatively similar

to the spectrum of pure solid obtained at this tem-

perature. This observation in particular in the n(OH)

region of water ice is puzzling because as previously

reported earlier, DME has a great proton affinity and

hydrogen bond between the first layer of DME with

the free OH of ice surface are expected. Two explana-

tions can be thought of. First, the perturbed dangling

bands (free water OH oscillator bands) are perhaps too

weak to be observed. Generally, such bands are

observed when the impurity is trapped in the micro-

pores of amorphous ice which has a high coverage of

free OH surface groups compared to crystalline ice

surface. [21,22]. Thus, coverage of DME in the pores

could be negligible, an assumption supported by the

study of a sandwich H2O/DME/H2O which was kept at

90 K during 12 h. No migration of DME in the water

lattice was observed. However, an alternative expla-

nation is that the absence of perturbed free water OH

in our spectrum may be due to an orientation effect. In

our device, because of the surface dipole selection

rule, only the transversal mode (TO) can be excited. If

the dangling perturbed OH band is nearly perpendi-

cular to the metal surface, the intensity of the OH

stretch would be strongly reduced.

3.4. Mixed DME/H2O films

Mixed films were obtained mainly by co-depositing

through two inlets of DME and H2O diluted in He as

carrier gas. Some experiments were also made by

depositing through one inlet a gaseous mixture of

DME and water in helium. They led to the same

Table 3

Absorption band frequencies in cm�1 of DME and DME in

interaction with water molecules in nitrogen matrices

DME/N2 DME/N2/H2O Assignment

2997.8–2994.8–2991.3 3001.4 n0sðCH3Þ2937.1–2934.4 2942.6 2dðCH3Þ2922.0 2929.6 naðCH3Þ2908.6 2913.0 2dðCH3Þ2890.8–2887.1 2896.4

2872–2869 2875.7

2822.0–2818.1 2830.0 nsðCH3Þ1476.1 1478.4 dðCH3Þ1459.5–1456.1 1460.7–1457.6

1245.2 1249.5 rðCH3Þ1168.7 1167.2 sh nðCOCÞ þ rðCH3Þðb1Þ1096.5 1092.9

925.2 919.1–916.9 n(COC)

Fig. 5. Evolution with the temperature of vibrational spectra of films of DME containing water: (A) initial gas ratio DME/H2O, 10/1—(a)

30 K; (b) 50 K; (c) 61 K; (d) 63 K. (B) Initial gas ratio DME/H2O, 4/1: (a) 30 K; (b) 52 K; (c) 70 K; (d) 74 K; (e) 78 K.


results. Several initial ratios of H2O and DME were

studied as H2O/DME ¼ 1/100, 1/10, 1/4, 5/1 and 10/1.

3.4.1. Mixtures rich in DME

Fig. 5A (trace a) shows the spectrum of a film rich in

DME by co-depositing at 30 K of DME and H2O with

a H2O/DME ratio of 1/10. Spectrum of DME with

traces of water (<1/100) was similar. In the n(OH)

region two relative narrow bands appear at 3537 and

3473 cm�1 with in the d(OH) region a weak feature at

1664 cm�1. With temperature increase no change in

intensity and position is observed till 65 K. At 65 K,

when transformation of amorphous DME to crystal-

line DME occurs, the three bands disappear. These

bands can be assigned to an aggregate (H2O)(DME)2.

When the water concentration in DME is increased

(DME/H2O ¼ 4/1) four absorptions are observed at

3534, 3474, 3417 and 3296 cm�1 in the n(OH) region

(Fig. 5B) as well as two bands at 1664 and 1640 cm�1

in the d(OH) region. Some small changes are observed

in the DME spectrum. The n(CH3) absorptions are

blue-shifted by 1.6 cm�1 and a shoulder at 2939 cm�1

appears on the high side of the 2921 cm�1 absorption.

The rocking mode at 1090.2 cm�1 broadens and is

red-shifted by 1.5 cm�1, while a shoulder at 910 cm�1

appears on the low side of the n(COC) band. When the

film is annealed at 78 K, crystalline DME is formed

and its spectrum is similar to that observed for cry-

stalline DME without water. In the water region, the

3534 cm�1 band disappears as well as the 1640 cm�1

absorption. We observe a broad band with three sub-

maxima at 3469, 3380 and 3264 cm�1 and an absorp-

tion at 1670 cm�1 characteristic of water ice. These

observations suggest that upon heating some water

molecules can diffuse through the DME matrix to

form larger aggregates besides the DME–H2O clus-

ters. Monomers, and dimers of isolated water mole-

cules are observed at higher frequencies compared to

multimers and polymeric water ice [23]. The shift

towards lower frequencies of the water OH stretching

features shown in Fig. 5B, indicates the restructuring

of H2O ice.

3.5. Mixture with excess of water

Fig. 6 (trace a) shows in two DME spectral regions a

typical spectrum of a mixed film (H2O/DME ¼ 10)

deposited at 30 K with He as carrier gas. Vibrational

spectrum is qualitatively comparable to that of pure

amorphous DME. Bands are broad non-structured, but

shifted from those of amorphous DME in particular in

the n(CH3) and in the n(COC) regions as illustrated in

the Fig. 7. The n(CH3) absorptions are blue-shifted by

about 7–20 cm�1 and the n(COC) band is red-shifted

by 6 cm�1. The rocking band at 1086 cm�1 is broa-

dened and presents a shoulder at 1071 cm�1. Such a

behavior is comparable to that observed in nitrogen

matrix when excess of water was deposited with

Fig. 6. Spectral changes with temperature in DME spectral regions after condensation at 30 K of a H2O/DME mixture rich in water

(H2O/DME ¼ 10/1): after deposition at 30 K (a); after annealing at different temperatures, T (K): 110 (b); 120 (c); 130 (d); 140 (e); 150 (f).


DME. Furthermore, the relative intensities of bands in

the n(CH3) region and <1200 cm�1 are different of

those observed in pure amorphous DME. In water ice,

the most intense absorptions are the two rocking bands

at 1161 and 1086 cm�1.

After deposition the mixed film was annealed at

different temperatures at a rate of 1 K min�1. Evolu-

tion of the spectrum with temperature is also shown in

Fig. 6 (traces b–f). Between 30 and 110 K, shapes and

positions of the bands remain the same. No partial

crystallization of DME is observed indicating that

DME is well diluted in the water amorphous phase.

Between 120 and 130 K, a strong change occurs.

Bands narrow and strongly shift as summarized in

Table 4 which compares the frequencies of DME

trapped in water ice <120 K and frequencies of

DME after annealing at 130 K. In parallel, a loss of

DME is observed. At 130 K, 60% of initial DME is

lost and at 150 K, 18% of initial DME remains.

However, no change in the frequency is observed

between 130 and 150 K. Above 160 K, DME disap-

pears as a result of the complete evaporation during

ice crystallization. The Fig. 8 shows the temperature

dependence of frequencies of the 2826 and 1086 cm�1

bands as well that of the FWHM and the integrated

intensity of the 2826 cm�1 absorption. The marked

band shapes and positions observed between 120

and 130 K is the result of the progressive transforma-

tion of low density amorphous ice Ial in a third

amorphous ice form Iar and in cubic ice Ic above

130 K as probed by the evolution of water n(OH)

band. A complete evaporation of DME is observed

at 160 K when water crystallization is achieved. The

release of volatile species trapped in ice which probes

the gradual transformation of water ice structure, has

been shown by various techniques [24–28]. The

trapped species escape from ice mixture mainly in

two temperature ranges: the first one lies between 30

and 80 K, that corresponds to the transformation of Iah

into Ial and the progressive collapse of the pores, the

second one is roughly between 120 and 140 K, that

corresponds to the transformation of Ial into Iar þ Ic.

The rearrangement of water molecules opens some

blocked channels and results in loss of trapped species.

In the case of DME, the non-evolution of DME

spectrum between 30 and 110 K is the indication

that DME, as expected, is not trapped in the pores

of amorphous ices. Furthermore, the significant fre-

quency shift and the strong change in the shape of the

bands observed for DME in the 120–130 K tempera-

ture range, which is not typical for other species, is

Fig. 7. Comparison of spectra of amorphous pure DME (a) and of DME trapped in ice (b).

Table 4

Frequencies in cm�1 of fundamental absorptions of DME trapped

in excess of water ice

30 K 110 K 130 K Assignment

2995.6 (0.2) 2996.7 3006.0 n0sðCH3Þ2942.5 (0.3) 2940.9 2932.6–2919.4 naðCH3Þ2892.9 (0.2) 2891.7 2884.8 2dðCH3Þ2870.4 (0.1) 2870.4 2868.0

2826.0 (0.4) 2826.0 2816.7–2811.4 nsðCH3Þ1478.1 (0.15) 1476.3 1474.1 dðCH3Þ1458.2 (0.3) 1456.5 1456.5

1250.6 (0.2) 1250.6 1246.7 rðCH3Þ1176.5 (0.15) 1174.5 –

1161.4 (1) 1161.4 1168.4 nðCOCÞ; rðCH3Þðb1; a1Þ1086.2 (1) 1086.6 1096.3

910.4 (0.6) 910.4 – n(COC)


difficult to explain. It could be suggested the formation

of a clathrate hydrate of DME. Clathrate hydrates of

some ethers were shown to form in the course of co-

deposition of water vapor mixtures with guest ether

species [29]. The latter, being incorporated in the

clathrate structure, are characterized by a very specific

spectrum with respect to their solution in other phases

of an ether. The DME–H2O bonding is comparable in

strength to water–water bonding and DME can break

the H-bonds of ice and reorder ice to form an hydrate

Fig. 8. Temperature dependence on frequencies of the 2826 and 1086 cm�1 bands and on integrated intensities and FWHM of the 2826 cm�1

absorption.


clathrate. Thus, the spectrum of DME in water ice at

140 K may correspond to an intermediate absorbate,

and hence, the observed DME frequencies in ice at

this temperature are not representative of DME in the

evenly averaged water molecules environment.

An interesting observation is made in the difference

spectra obtained by differences between spectrum at

110 K and spectra at 120 and 130 K as shown in Fig. 9.

The loss of DME is accompanied by the progressive

disappearance of bands at 3473 and 1680 cm�1 in

the water ice regions. This observation indicates that

<120 K, DME is probably in a substitution site of the

ice lattice and forms an hydrogen bond with nearest

water molecules.

An upper limit of DME in interstellar icy grains is

currently determined by comparing a H2O:DME ¼100:10 mixture at 10 K to recent ISO data of the high-

mass protostar W33A.

Acknowledgements

This work was performed with support from the

Programme de Chimie du Milieu Interstellaire (PCMI).

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Documents

Infrared spectra of (CH 3) 2 O and (CH 3) 2 O+ H 2 O at low temperature