Transcript Herrero CO2H2O.ppt

Cometary ice analogs: TPD and FTIR
study of CO2/H2O ices
V.J. Herreroa, R. Escribanoa, B. Matéa, O. Gálveza, I.K. Ortegaa
B. Martín-Llorentea and P. J. Gutiérrezb
a) Instituto de Estructura de la Materia, CSIC, Madrid, Spain
b) Instituto de Astrofísica de Andalucía, CSIC, Granada, Spain
Outline



Introduction
Ices of CO2/H2O
• Experimental method
• TPD experiments
• FTIR measurements
• Desorption energies
Summary
Cometary ices
Comets are the most primitive objects in the Solar System and
contain a most valuable information about its origin
Cometary nuclei are made of frozen volatile compounds (“ices”) rocky
material and complex organic matter. They are often described as
“dirty snowballs” or “icy dirtballs”
The main components of
commetary ices are H2O, CO, CO2 ,
CH3OH and CH4
H2O is by far the most abundant
Ices with the same components are
also found in other Solar System
bodies and interstellar grain mantles
Space missions to comets
1986. Nucleus of comet Halley
photographed from the Giotto
spacecraft. Image: ESA MPAe
Lindau
2005. The Deep Impact probe
collides with comet Temple 1.
Image: NASA/JPL-Caltech/UMD
Studies of cometary ices
Due to the large temperature
differences along the orbit of a
comet, various ice phases, in
particular, amorphous and
crystalline water ice may be
relevant for the physics of the
cometary nucleus
Laboratory data are needed for the interpretation of the observations
and for the modelling of cometary behavior
Studies by different groups show that the structural and dynamic
properties of these ices may depend on their formation conditions
In the present work we have studied H2O/C2O ice samples,
generated under different conditions, in the 80-190 K
temperature range
Experimental setup
Experimental details
Ices produced by vapor deposition of suitable precursors
Chamber:
Bckground pressure 10-8 mbar
Deposition pressure 10-5-10-4 mbar
Cryostat (liquid nitrogen)
Temperature range 80-325 K
heating ramp for TPD 0.3 K s-1
FTIR Spectrometer (Bruker, Vertex 70)
Detector MCT
Resolution 2 cm-1
Deposition substrate
Si, Ge (transmisson); Al, Au (RAIRS)
Quadrupole mass spectrometer (Inficon, Transpector2)
Mass range 0-100 amu
Experimental setup
Ice samples studied
Composition of the mixtures  H2O/CO2 : ~ 15:1
Deposition temperature: 80 K
Preparation of the samples
CO2 with amorphous solid water (ASW)
a) Sequential deposition (first H2O, then CO2)
b) Co-deposition
c) Inverse sequential deposition (first CO2, then H2O)
CO2 with crystalline water ice
d) Sequential deposition over crystalline ice (first
amorphous H2O, then annealed, then cooled
to 80 K, then CO2 on top)
TPD of H2O/CO2 mixtures
2
Sequential
(a)
CO2 desorbs completely from
crystalline water ice and
partly from amorphous ice
below 130 K (not shown),
but…
CO2 Sequential
CO2
1
0
-8
signal
QMS
Pp (mbar)
x 10
2
(b)
Co-deposition
CO2 Co-deposition
some CO2 is retained within the
amorphous ice and gives rise
to further desorption peaks
CO2
1
0
2
Inverse sequential
CO2 Inverse Sequential
(c)
CO2
1
15
10
5
0,4
(d)
H2O
H
2O
Thickness
Thickness
0,2
0
0,0
130 135 140 145 150 155 160 165 170 175 180 185 190
T (K)
Thickness
Thickness ((μm)
m)
0
150-165 K: “Volcano”
desorption associated with the
crystallization of ice
180-190 K: Co-desorption
of H2O and remaining CO2
Remarks on TPD measurements



CO2 does not enter the crystalline ice, but is incorporated to some
extent into the amorphous ice samples
There is a great similitude between the CO2 desorption peaks of
samples obtained from co-deposition and inverse sequential
deposition (CO2 first)
Comparison with previous works:
Present results are consistent with those of Bar Nun et
al.(Icarus 63, 317, 1985) and of Hudson and Donn (Icarus
94,326, 1991) for co-deposited samples
The peak structure is in agreement with that of Collings et
al.(Mon Not. R. Astron. Soc.354, 1133, 2004) for sequential and
co-deposited samples, but our peaks are shifted by 15-20 K to
higher temperatures.
IR spectra of CO2 on amorphous and crystalline ice
0.6
Sequential
on ASW
(a) Sequential
CO2 80 K
Absorbance
0.4
CO2 105 K
0.2
Appreciable differences between
the two spectra are readily
observable in the OH stretching
band of H2O (beyond 3000 cm-1)
and in the ν3 asymmetric stretch
band of CO2
0.0
1.2
(b) Crystalline Sequential
1.0
0.8
0.6
CO2 80 K
0.4
CO2 105 K
0.2
0.0
-0.2
4000
3500
3000
2500
2000
1500
1000
A temperature rise from 80 to
105 K produces large variations
in the ν3 stretch band of CO2 but
leaves the H2O spectra
unchanged
The asymmetric stretch (ν3) band of CO2
0.4
-1
(a) Sequential
0.3
IR spectra evolve with T between 80 K
and 105 K, then they do not change
until desorption.
2344,3 cm
CO2 80 K
0.2
CO2 105 K
0.1
-1
2340,9 cm
Absorbance
0.0
-1
2343,1 cm
0.5
0.4
0.3
0.2
0.1
0.0
(b) Co-deposition
0.4
(c) Inverse Sequential
CO2 80 K
2340,2 cm
CO2 105 K
0.3
CO2 80 K
0.2
CO2 105 K
For ASW/CO2 samples the spectra lose
the higher frequency part of the band
upon heating. The new peak is shifted by
3-4 cm-1 .
-1
-1
2343,3 cm
-1
2339,7 cm
0.1
0.0
-1
1.0
0.5
The CO2 band on crystalline ice is
narrower and similar to that of pure
CO2. It disappears completly upon
heating
2343,8 cm
(d) Crystalline Sequential
CO2 80 K
CO2 105 K
CO2 pure 80 K
0.0
2420
2400
2380
2360
2340
-1
Wavenumber (cm )
2320
2300
The ν3 band of
-0.03
13CO
(a) Sequential
CO2 80 K
CO2 105 K
-0.04
Observation of the ν3 band of 13CO2
suggessts that the different CO2/H2O
interactions can be broadly classified
into two types: CO2 ext. and CO2 int.
2282.5 cm -1
-0.05
Absorbance
2
-0.06
-0.02
(b) Co-deposition
2275.2 cm -1
CO2 80 K
CO2 105 K
-0.03
Upon heating to 105 K , the band
corresponding to CO2 int. remains
whereas that of CO2 ext. virtually
disappears
-0.04
CO2 ext
2290
CO2 int
2280
Wavenumber (cm-1)
2270
Remarks on IR results
The ν3 band of CO2 is similar in all the CO2/ASW samples at 80 K.
This band is composed by two absorption features and only one
remains for T=105 K.
The spectra of samples from codeposition and from inverse
sequential deposition have a similar evolution with temperature
Comparison with previous results:
Rough agreement with the results of Sandford and
Allamandola ( ApJ 355, 357, 1990 ) in the narrowing of the ν3
band of CO2 and in the redshift of the peak when T is increased to
150 K (in 20:1 H2O:CO2 samples codeposited at 10 K)
Good agreement with the measurements of Kumi et al. (J.
Phys.Chem. A 110, 2097, 2006) in the efficient diffusion of CO2
into ASW and in the identification of the high T (150 K) peak as CO2
embedded in the ASW.
Desorption energies
The activation energy, Ed , for the desorption of CO2 is determined
under the assumption that the growth of CO2 ceases when its
residence time on the ice surfaces is equal to the time required for
the formation of a monolayer (cf. Sandford and Allamandola ApJ 355,
357,1990).
For the residence time, ts, we assume the model of Langmuir and the
time for the formation of a monolayer, tm , is estimated from gaskinetic theory :
t s   01 exp( Ed / RT )
Ed  RT ln( 0tm )
with ν0 = 2.9 x 1012 s-1 (Sandford & Allamandola 1990) derived from
librational modes of CO2.
CO2 sticking on crystalline ice vs T
0,3
0,2
86
88
0,1
90
25
00
0,0
92
24
00
94
PCO2= 4.8 x10-5 mbar
22
00
23
00
96
98
CO2 Desorption Energies
Ed /kJ mol-1 Ed /kJ mol-1
Amorphous crystalline
Experiment
Technique
19.9 ± 2
Sticking vs T
(Sequential
deposition)
FTIR
Gas-surface
scattering
Molecular
beam TOF
FTIR
Present results
20.7± 2
Anderson et al.
2004
------
21.2± 1.9
Sandford &
Allamandola
1990
23.7± 1.7
--------
Sticking vs T
(Codeposition)
Bryson and
Levenson
1974
25.1
-------
TPD
Quartz
microbalance
(sequential
deposition with
molecular beam)
Summary
There are basically two types of interaction between CO2
and amorphous water ice. The stronger of these interactions
leads to a slight weakening of the C-O bond (IR spectra).
Only weak surface interactions take place between CO2 and
crystalline ice. CO2 does not enter the bulk
CO2 diffuses efficently into the amorphous ice between 80
and 105 K (similtude between codeposition and inverse
sequential deposition data), due probably to the porous structure
and dynamic nature of this solid.
The activation energy for desoprtion of CO2 from
crystalline ice determined in this work is in good
agreement with recent molecular beam measurements
The available values of the desorption energy of CO2 from
amorphous ice show a higher dispersion reflecting a more
complex dynamics.
Funding agencies:



CAM, FSE for studentship
CSIC: studentship for UA with University of Jaén,
Juan de la Cierva Program, PIF 200550F0051
“Hielocris”
Spanish Ministry of Education, Project FIS200400456, Sabbatical grant
Future work
Theoretical investigation of the observed
modifications in the CO2 IR bands
Extension of the experiments to other formation
conditions, especially to lower temperatures
Extension of the studies to other ice mixtures
(binaary and ternary)
(a)
TPD peak positions
CO2 Sequential
2
1
The location of the TPD peaks depends on the
desorption kinetics (ν0 , Ed)
and also on the sample density (ρ) thickness (x) and heating rate (vh). It
0
may differ for different experimental conditions.
2
(b)
CO2 Co-deposition
xf
xi
Pp (mbar) x 10

dx
 E 
  0 exp   d 
dt

 RT 
-8
The evolution of the layer thickness with temperature can be estimated
approximately:
1
Tf
dx   
Ti
0
 E
exp   d
vh(c)

 RT
0
2

dT
 CO Inverse Sequential
2
0
15
10
5
0,4
(d)
H2 O
0,2
Thickness
0
0,0
130 135 140 145 150 155 160 165 170 175 180 185 190
T (K)
Thickness (m)
Example of calculations for
H2O desorption under our
experimental conditions
with literature values forEd,
ν0 and ρ (see Maté et al.
JPC B 110, 7396, 2003).
H2O QMS signal
1