adsorption and desorption process in lab for h20, co2, co and mixture of h2o-co2 on au sample

8/17/2019 Adsorption and desorption process in lab for H20, CO2, CO and mixture of H2O-CO2 on AU sample

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as-grain modeling for interstellar chemistry

Project 09

Aix-Marseille University

Master space 201-201!

o"ri #$A%MA

#"pervise &y- Marco MINISSALE – P. THEULE

April, 22, 2016


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Contents

1 |Introduction 3

2 |Theoretical approach 7

3 |Experimental approach 103.1 Experimental setup . . . . . . . . . . . . . . . . . . . . . . . . 103.2 Temperature-programmed desorption (TPD) . . . . . . . . . 123.3 Deposition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

4 |Experiments 154.1 H 2O (

me

= 18) experiment . . . . . . . . . . . . . . . . . . . . 15

4.1.1 Experimental data and simulation fitting . . . . . . . 174.1.2 Discussion on H 2O . . . . . . . . . . . . . . . . . . . . 194.1.3 Conclusion: . . . . . . . . . . . . . . . . . . . . . . . . 23

4.2 CO (me

= 28) Experiment . . . . . . . . . . . . . . . . . . . . 234.2.1 Experimental data and simulation fitting . . . . . . . 244.2.2 Discussion on CO . . . . . . . . . . . . . . . . . . . . 264.2.3 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . 28

4.3 CO2 (me

= 44) Experiment . . . . . . . . . . . . . . . . . . . 284.3.1 Experimental data and simulation fitting . . . . . . . 294.3.2 Discussion on CO2 . . . . . . . . . . . . . . . . . . . . 304.3.3 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . 32

4.4 Experiment: Mixture of CO2 with H2O . . . . . . . . . . . . 324.4.1 Deposition of mixture and discussion . . . . . . . . . . 324.4.2 Result . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

5 |Conclusion 36

6 |References 37

7 |Acknowledgment 38

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Abstract

Objective of this project is:

• Discussion on importance of gas grain modeling in astrophysics.

• Simulate adsorption and desorption model in multi-layer regime.

• Perform temperature programmed desorption (TPD) experimentin lab.

• Extract desorption rate of different species.

• Compare the simulation and experimental result and make aanalysis on desorption energy and discuss the physical signifi-cance.

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1 |Introduction

The life-cycle of stars, recycles the matter of universe in galaxies (shownin fig 1). During intermediate life of star, heavy elements creates in theinterior of star, while during death of star these elements scattered in Inter-stellar medium (ISM). So we can say, ISM is actually matter that existsin between stars and galaxies. This matter includes gas in ionic, atomic,molecular phase and dust as well as cosmic rays. All these phases of gasesblends smoothly in the surrounding intergalactic space. This means wehave all phases of gases everywhere. But spectroscopic observations verifies,this smoothness is not uniformly distributed as we thinks, It depends onsurrounding temperature where matter resides. Alternatively, in all phases,

ISM is tenuous by terrestrial standards. In hot, diffuse regions (near todeveloped star) matter is in ionized form. In cool dense regions (wherestar is in forming state) matter is in molecular form and forms a molecularclouds.

Figure 1: Life-cycle of star, credit: Minissale M. presention(lecture1)

Over the last decades it is clear and proven experimentally and obser-vationally, purely gas phase study cannot explain the variety and richnessof chemistry finds in ISM. Specially in star forming regions, in such a envi-

ronment gas-grains plays an important role. For example formation of mostabundant molecules hydrogen and water is not clear in gas-phase modelbut formation of these species is clear in gas-grain model, In which grain(dust: graphite, silicate) surface work like a catalyst to perform reactionmore stably.

Star forming region is best environment to form dust and then in sucha cool and dense environment molecules get stuck and froze on the surfaceof dust-grains and form ice mantles. This ice is effected by cosmic rays,ultraviolet photosynthesis and shock waves, these process transfers energy tomantles consequently different molecules (species, atoms) form on icy grains.

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During cloud collapse or due to enough cosmic ray hits and else, when grain

surface get enough temperature then molecules on grains transform theirphase (solid to gas) and these gas phase species we observed in observationalspectroscopy. To evaluating these environment it is necessary to understandsublimation behavior of every particular species and ice mantles such thatunderstanding of sublimation behavior is aim of this project.

we studied in our course till today 180 chemical compound found in ISMand all these evidences comes from observational spectroscopy. Spectroscopyalso points, in ISM dust is only 1 percent and gas 99 percent and experimentssays this 1 percent dust plays a very important role in molecule formation.we studied H 2O, CO, CO2 and mixture of H 2O − CO2 in order to explainthe importance of dust and sublimation behavior of molecules in ISM en-

vironment. But before I start about project, I would like to give some keyprocess and definitions, those are important to understand physics of gas-grain modeling. There are many more processes those takes place to formmolecules but for project purpose solid-gas interaction and surface physicsprocesses are important, These processes are following:

• Adsorption

• Desorption

• Sticking

•

DiffusionBefore I discuss these processes in brief, I would like to mention temperatureand density places an important role to explain these processes. Temper-ature control the residence time of species on surface. If surface is hot theybounce and if surface is cool they stick for long time. Density controls theprobability of interaction, thus more dense cloud means more probability of sticking molecules.

1. Adsorption: In this process gas phase particle stuck on grain sur-face and transform their phase gas to solid. There are two kind of adsorption, physical adsorption and chemical adsorption. For physics

purpose, we will totally focus on physisorption (physical adsorption).In physisorption we talk about inter-molecular forces, these inter-molecular forces comes due to potential difference that arises due tostructure of molecules. There are two kinds of potential one is dueto Van der waals interaction force other is due to Pauli repulsive po-tential. Combined forces is defined by Lennard-Jones potential.Fig(2) explains the adsorption process and Fig(3) explains the limit of physisorption.

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Figure 2: Adsorption and desorption on grain surface, Credit:Philip Hof-

mann

Figure 3: physisorption, credit: Minissale M. presentation(lecture 1)

2. Desorption: When the temperature of grain surface exceeds the sub-limation temperature of stuck species then they transform solid to gas.Alternatively, I could say, due to thermal changes, when molecules getssufficient energy to cross the activation barrier (create due to potentialdifference) of desorption they leads to gases phase. Fig(2) shows a hintof desorption.

3. Sticking: It is the most important factor that helps in forming molecules

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on grain surface. It is calculated by sticking coefficient.

[sticking coefficient] = [molecules adsorb][molecules spread on surface]

sticking depend on mass of specie, kinetic energy of specie, structureof specie, surface temperature and surface coverage. Molecules that Iused in my experiment has sticking coefficient nearly equal to 1. [Ref:Bishop et al. 2006]

4. Diffusion: In this process atoms and molecules comes to the surfaceand diffuse (sit or stick) on surface vacancies more than the usualresidence time, then wait until perfect pair of specie comes. once theperfect pair comes they both get react and form a new molecule. I

will not discuss this process further but just mentioning because mostabundant hydrogen molecule form by this way.

There are two kinds of reaction that can take place after species adsorbon the surface to form another molecule:

• Exothermic reaction: When system release (transfer) energy to sur-rounding in form of heat after reaction.

• Endothermic reaction: When system absorbs energy from surroundingafter reaction to make molecule stable.Endothermic reactions cannotpossible in ISM because of low temperature.

When any reaction take place that has to follow specific pattern shownin fig(4).

Figure 4: A potential energy profile of exothermic reaction. The height of the barrier between reactant and product is the activation energy of reaction.image credit: P.Theule (lecture notes)

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Above figure shows, whichever is the reaction, to form a product it has

to cross the activation barrier. To cross this activation barrier either weincrease the temperature of surface or we introduce the catalyst. Increasingtemperature is not possible all the time but in presence of catalyst by givingthe small amount of energy, reactant can cross the the activation barrierand product forms. This exact process that I talked in second paragraphof introduction i.e. grain behave like catalyst by little heat from cosmic rayor else, they form new molecules. All this discussion shows the importanceof gas-grain modeling in ISM physics. All above points are very basic andimportant point to understand the gas-grain modeling.

2 |Theoretical approachIn second section of the project our goal was to develop the standard sim-ulation for absorption and desorption rate for specific desorption energies(taken from literature). For this purpose we used Polanyi-Wigner Equation:

Rdes = −dN

dt =

A

β N n exp(

−E desT

) (1)

Where,β = Heating ramp rate (Kelvin/min)T = Temperature of surface (Kelvin)

E des = Desorption energy (Kelvin)n = Order of desorption (for multilayer its zero). There are three orderof desorption: mono-layer (n = 1), sub-mono-layer (n = 2) and multi-layer(n = 0)N = Number of molecules per square cmA = Frequency factor or normalization constant. It’s unit depend on orderof desorption. But we can write base unit as molecules1−ncm−2+2nsec−1.dN dt

= Rate of desorption (molecules cm−2K −1), In experiment it is desorp-tion signal.

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Figure 5: simulation sample of adsorption and desorption for different ramprate

Figure clearly explains the behavior of adsorption and desorption, ini-tially when molecules adsorb on surface and surface temperature is not highenough to desorb them, we see full coverage in graph, as temperature in-creases desorption takes place and coverage start falling at the same pointdesorption peaks start rising. Figure 5 is simple model created by using

python code from equation (1), code is described below

1 im p o rt numpy a s np2 i m po rt m a t p l o tl i b . p y pl o t a s p l t3 im po rt s c i py a s s p4 f ro m s c i p y . i n t e g r a t e i mp or t o d e i nt5 ’ ’ ’6 we a r e m aking s i m u l a t i on f o r : r a t e = (A/B) ∗ (N^n) ∗ exp(E/kT)

∗ d t f o r H2O7 TPD s p e c tr a f rom a s e r i e s o f ( a ) z er ot h−o rd e r ( A= 1 0 ^ 3 0

m o l e c u l e s8 cm−2 s−1 )9 A i s i n s ec ^ −1

10 B = b e t a = r amp r a t e = K/ mi n11 N = no . o f m o l ec u l e s / s q ur e cm12 n = o r de r o f d e s or p ti o n13 Eads = a d s o rp t i o n e ne gy i n K14 K = b o l tz m an n c o n t a n t15 ’ ’ ’16 A = 1 0 .0∗∗1 317 B = 1 018 C = [ 2 , 5 , 1 0 , 2 0 , 5 0 , 1 0 0 ] # no . o f m on ol ay er s i n m u l t i l a ye r s19 n = 0 . 0 # f o r z e ro t h o r de r ( m u l t il a y e r )20 E = 5 9 0 0. #d e s o r p t i o n e n e rg y21 k = 8 . 6 1 6 ∗ 10∗∗ (−5)22 T 1 = 2 0 . 0

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23 T 2 = 3 0 0 . 0

2425 ’ ’ ’26 f u n ct i o n s t o c a l u c u l at e P ol an yi−Wi gner E qu at io n f o r d i f f e r e n t

ramps27 ’ ’ ’28

29 d e f e q u a t i o n ( t h i t a , T ) : # f un c ti o n f o r b et a 10

30 n0 = t h i t a [ 0 ]

31 r = −((A∗6 0 . ) / 1 0 . ) ∗ ( n 0 ) ∗ np . ex p(−E/ ( T) )

32 r e t u r n r

33 T = n p . l i n s p a c e ( T1 , T2 , 1 0 0 0 )

34 e c = 1 . 035 s 1 = o d e i n t ( e q u at i o n , e c , T ) #c a l c u l a t i n g c o ve r a ge36 T3 = (T2 − T1) /10 00.37 s 2 = n p . z e r o s ( l e n ( s 1 ) )38 f o r i i n r a ng e ( l e n ( s 1 )−1) :39 s 2 [ i ] = −(s 1 [ i +1] − s1 [ i ] ) / T3 # c a l c u l a t i on f o r d es o rp ti o n

r a t e40

41

42

43 d e f e 2 ( t h i t a , T ) : # fu n ct i on f o r b et a 644 n0 = t h i t a [ 0 ]45 r 2 = −((A∗6 0 . ) / 6 . ) ∗ n0 ∗ np . exp (−E/ ( T) )46 r e t u r n r 247 T = n p . l i n s p a c e ( T1 , T2 , 1 0 0 0 )48 e c = 1 . 049

s 1 2 = o d e i n t ( e 2 , e c , T)50 T3 = (T2 − T1) /10 00.51 s 2 2 = n p . z e r o s ( l e n ( s 1 2 ) )52 f o r i i n r a ng e ( l e n ( s 1 2 ) −1) :53 s 2 2 [ i ] = −(s1 2 [ i +1] − s12 [ i ] ) / T354

55

56

57 d e f e 3 ( t h i t a , T ) : # fu n ct i on f o r b et a 458 n0 = t h i t a [ 0 ]59 r 3 = −((A∗6 0 . ) / 4 . ) ∗ n0 ∗ np . ex p(−E/ ( T) )60 r e t u r n r 361 T = n p . l i n s p a c e ( T1 , T2 , 1 0 0 0 )62 e c = 1 . 0

63 s 1 3 = o d e i n t ( e 3 , e c , T)64 T3 = (T2 − T1) /10 00.

65 s 2 3 = n p . z e r o s ( l e n ( s 1 3 ) )

66 f o r i i n r a ng e ( l e n ( s 1 3 ) −1) :

67 s 2 3 [ i ] = −(s1 3 [ i +1] − s13 [ i ] ) / T3

68

69

70 p l t . p l o t ( T, s 1 , ’ . ’ )71 p l t . x l im ( T1 , T2 )72 p l t . p l o t ( T, s 2 , c o l o r =’ bla ck ’ )73 p l t . p l o t ( T, s 22 , c o l o r = ’ r e d ’ )74 p l t . p l o t ( T, s 23 , c o l o r = ’ gre en ’ )

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75 p l t . x l a b e l ( ’ Temperature(K) −−−−> ’ )

76 p l t . y l a b e l ( ’ H2O d e s o r p t i o n r a t e ( m o l e c u l e s cm^ −

2 K^ −

1) −−−−−

> ’ )77 p l t . l e g e n d ( ( ’ Co v e rag e ’ , ’ b e t a 1 0 K/m ’ , ’ b e t a 6 K/m ’ , ’ b e t a 4 K/

m ’ ) , l o c =0)78 p l t . t i t l e ( ’ s i m u l a t i o n s a mp l e ’ )79 p l t . g r i d ( Tru e )80 p l t . s a v e f i g ( ’ s im u la t io n _s a m p l e .p n g ’ , fo rm a t= ’ png ’ )81 p l t . s ho w ( )

3 |Experimental approach

3.1 Experimental setup

Instrument consist of stainless steal ultra-high-vacuum (UHV) chamber thatoperates on 9.3× 10−9mbar pressure and cooled by helium till 4K, this pro-vides very similar conditions those found in ISM [Ref: Helen J. Fraser et al.2001]. The chamber equipped with two instrument, effusive gas dispositionsystem, a quadrupole mass spectrometer (QMS) and a mid infrared spec-trometer (MIR) (cooled by liquid nitrogen), Fig(6) shows the experimentalsetup used in experiment.

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Figure 6: Experimental septup

QMS is used for temperature-programmed desorption (TPD) experiment(I will discuss about this in next subsection). Mid infra-red (MIR) spectrom-eter for monitoring the film of icy species on sample. We used copper samplecoated by gold film, capable to reach 10K (approx) and free to rotate on 360degrees. TPD over this sample (when covered by specie’s ice) is controlledby positive feedback loop from the Lakeshore instrument (see in fig(7)),this instrument control the temperature of sample during TPD and capableto work under 10K to 340K.

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Figure 7: Lakeshore instrument, Image credit: Lake Shore Cryotronics, Inc.

3.2 Temperature-programmed desorption (TPD)

Adsorbed molecules bound to surface in potential well of depth E des, Prob-ability that molecule will desorb at any one temperature, to produce anequilibrium vapor pressure, is governed by Boltzmann statistics. Once thedesorption starts, QMS handles the TPD. QMS principle: It consistsof four parallel metal rods. Each opposing rod pair is connected togetherelectrically, and a radio frequency (RF) voltage with a DC offset voltageis applied between one pair of rods and the other. Ions travel down thequadrupole between the rods. Only ions of a certain mass-to-charge ratiowill reach the detector for a given ratio of voltages: other ions have un-stable trajectories and will collide with the rods. This permits selection of

an ion with a particular m/e that allows the operator to scan for a rangeof m/e-values by continuously varying the applied voltage (see in fig 8b).During TPD formation, a linear temperature ramp is applied on sample bylakeshore. when sample get heated, species start transforming there phasefrom solid to gas and Mass spectrometer (QMS) measured the amount of adsorbate that desorbs as a function of temperature.

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(a) schematic diagram of TPD mechanismimage credit:Trustees of Wellesley College

(b) schematic diagram of QMS mecha-nism

Figure 8: QMS working principle and TPD extraction internal view

TPD can give us certain information:

• Heat of adsorption (if adsorption and desorption are reversible non-dissociative processes). Alternatively, temperature range of adsorptionin which molecules could remain on surface.

• coverage information.

• Energy information.

• Kinetic information about desorption process.

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3.3 Deposition

In this section I will try to explain the whole procedure that we followedin order to obtain TPD of species : Before taking any step towards experi-ment, we need to note down pressure in sample chamber, if it is above 10−7mbar, we can not proceed for experiment. For doing experiment we followfollowing steps:STEP-1: In specimen chamber, first we take fixed amount of particularmolecules. The amount taken in experiment is measured by pressure differ-ence in specimen chamber. In all experiments we have taken only 10mbarmolecules.STEP-2:Spray these molecules on copper sample.STEP-3:Turn our sample toward MIR spectrometer for taking spectrum inorder to verify species on sample.STEP-4:Turn sample towards QMS to start TPD.STEP-5:Start QMS software by giving mass no. of species (taken for exper-iment).STEP-6:Set temperature in lakeshore (according to particular sample), thenwe wait to obtain TPD by QMS.

In experiments we fixed the amount of molecules in each case but we obtainTPD for two different cases:

• For different exposure at fixed ramp rate(β ). for example we spray

molecules on sample for 20 sec at 10 K/min ramp, then in next exper-iment we sprayed molecules for 25 sec at same ramp and so on.

• For different ramp rate but at fix exposure of molecules. For examplewe sprayed molecules for 20 sec for 10 K/min ramp. Then in nextexperiment we sprayed molecules again for 20 sec for 6 K/min rampand so on.

These are the two conditions that we followed for pure molecules H 2O, C O2and CO, to analyze the changes in TPD and how these changes effect thekinetics of desorption and adsorption.

For Mixture of H 2O − CO2 we repeat the experiment for different ratioof mixture (1:10 and 1:1) on fix ramp rate but In this case spraying timedepend on pressure in specimen chamber (In order to not to break QMS).

Mixture preparation: We followed few steps: We have two tube inspecimen chamber both are connected and separated by valves. First wewill pump-out all tubes (evacuation of all gasses, those are already availablein tubes). Second: we open valve of tube one and fill gas1 for certain pressurethen close the valve. Next step is to open valve of another tube and fill gas2according to the ratio which we want for our experiment. After filling gases

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in both tube, we will close all other valves and open the valve of both tubes

(to connect tubes) in order to mix gases. We left gases to mix for someseconds or for a minute then mixture is ready to use in experiment.

4 |Experiments

4.1 H 2O (me

= 18) experiment

We repeated H 2O experiment four time, three time for different exposuretime by fixing ramp rate. Two time for different ramp-rate for fix exposuretime. We have taken 10mbar amount of molecules in each case. Depositionstart at 80K and desorption was started at 140K and finished before 200K.

For analysis, I used-

• Simple matplotlib library, numpy and scipy modules in python pro-gramming

• For energy calculation direct conversion of equation(1)

• For area calculation, I use simple trapezoidal rule.

• For uncertainty in measurement, I used statistical error analysis func-tion for large dataset (normal distribution):The average of all data values

Ravg =

ni=0 RiN

where R is desorption signal and N is the total no. of observed points.

Uncertainty in a single measurement of R is given by equation (2):

Error = ∆x = σ =

N i=0(Ri − Ravg)

2

N (2)

• For calculating average time in which process occur and residence time

of surface (τ ), I followed following equation:

t = 1

K (3)

where

K = A exp(−E des

T )

equation 2 gives Average time in which process occur.

τ = τ o exp(E des

T ) (4)

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where τ o is 1

A. Equation 3 gives the residence time of surface. Every

coefficient has there usual meaning.[Ref:http : //www.chem.qmul.ac.uk/surfaces/scc/scat26.htm]

• Number of mono-layers I have calculated by langmuir (L). I use fol-lowing equation [Ref:wikkipedia]:

layers = Pressure × time

1L (5)

Where pressure is in torr. (1bar = 750.062 torr)1L = 10−6torr − sec

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Results are shown below:

Figure 9: Result of H2O experiment: Energy is in Kelvin, pre-exponentialfactor (A) is in sec−1,Area is in cm2, unit of x-peak and y-peak is same astemperature and desorption signal respectively.

4.1.1 Experimental data and simulation fitting

Figure:10,11,12,13 shows the best fitted experimental data by simulationfor energies 5355±61.53K, 5400±55.7K, 5455±49.34K and 5420±59.00K re-spectively with pre-exponential factor (frequency-factor) shown in table-1(fig9).

Figure 10: Fitting plot of H2O for 20sec exposure and beta 10K/min; unitof Y-axis is molecules cm−2K −1

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Figure 11: Fitting plot of H2O for 25sec exposure and beta 10K/min; unitof Y-axis is molecules cm−2K −1

Figure 12: Fitting plot of H2O for 30sec exposure and beta 10K/min; unit

of Y-axis is molecules cm−2K −1

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Figure 13: Fitting plot of H2O for 30sec exposure and beta 6K/min ; unitof Y-axis is molecules cm−2K −1

Conclusion: On the basis of this I concluded H 2O has desorption en-ergy (E) 5407.5 ± 56.7 kelvin with pre-exponential factor (A) 1012±1 sec−1.Desorption peaks are in between 177K to 182K, these result are nearly sameas available in literature [Ref: J.A. Noble et al.].

4.1.2 Discussion on H 2O

We deposited H 2O molecules on sample at 80K for each TPD extraction.Fig:14 shows the change in the intensity of desorption signal as the sampleget heated. Plot clearly shows the rising edge of TPD coincide and peaktemperature increases with the exposure time. This signifies intensity (areaunder curve) increases as more and more material condensed on sample butin every case desorption take place on same temperature. The shift in peaktemperatures occur due to desorption rate(eq:1) increases exponentially withtemperature, rate increases until all the layers get sublimate then it dropdown rapidly. A simple comment I would like to make on multi-layer and

mono-layer regime is: monolayer is much more tightly bound from the sur-face than the multilayer, consequently we may find desorption peak of mono-layer TPD at higher temperature than the multilayer regime ***********.I would also point, I tried to de-convolve monolayer from multilayer TPD(by integrating area under peak and comparing area covered at each smalltemperature deference) but I couldn’t successful. so I will say multilayerdesorption process is independent of number of molecules that covered thesample. However, If we look on above subsection figures, multilayer simula-tion provides a good estimated fit to data (this simulation doesn’t containcontain the no. of molecules) so its verified we worked in multilayer regime

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and its independent of coverage.

Peak desorption signal for any curve is in between 177K to 182K, whichco-relate our experiment with published result and verifies or experiment.Below fig shows the amount of area covered in each exp. and there peakvalues.

Figure 14: Experiment of H2O for 20, 25, 30sec exposure and beta 10K/min;

unit of Y-axis is molecules cm−2K −1

Figure(15) shows, the there is slight shoulder observed at leading edgesof each TPD around 154K. This is due to phase transition of H 2O fromamorphous to crystalline. With this we can conclude, TPD peak of amor-phous ice will occur at low temperature, this is due to vapor pressure inamorphous ice is 3-time grater than the crystalline ice. Although, crys-talline ice peaks dominates the TPD because amorphous phase transitionis for very little time always in such conditions. With this we can concludeamorphous water is hard to find in such conditions, its so quickly transform

in crystalline phase.

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Figure 15: Amorphous water desorption peak at 154K; unit of Y-axis ismolecules cm−2K −1

Fig16 shows the condition of different ramp rate (β ) for fix exposure time.In this we seen desorption leading edges totally overlap therefore desorptiontake place at same time for different ramp, but Area covered by beta 6 ismuch lesser than the area covered by beta 10, peak intensity also shifted bythe difference of 4. Difference in area under curve is due to some mistake

in experiment (for beta 10 we have taken farady integration and for beta 6we have taken sel configuration) otherwise as we seen in simulation samplewhatever is the beta area covered by curves will be same always. Intensitypeak shifted towards the left is natural when we slow the ramp rate. Thissignifies that when we have high ramp (means high temperature) thereforetime spent on each kelvin will be more thus high signal and vice-versa .

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Figure 16: H2O experiment for 30sec exposure and beta 10K/min and6K/min

In case of H2O (mass 18), we can see one more interesting feature duringTPD formation, Some time we find isotope of H2O (mass 17). This is dueto diffuse discharge electrons from electronics some time ionize the H2O(mass 18) molecule and form H2O

+ (mass 17) but probability of happeningthis phenomenon is very less. Below figure(17) shows the condition of this

stage. In figure we could clearly see mass-17 has very less signal (due toless probability) but desorption take place at same temperature, desorptionpeaks also coincide. simulation fits the mass-17 for energy 5520±100K withfrequency factor 1012±1 sec−1.

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Figure 17: mass17 and mass18 H2O comparison; unit of Y-axis ismolecules cm−2K −1

4.1.3 Conclusion:

we studied desorption energy and behavior of crystalline and amorphousH 2O under the conditions same as observed in denser region of interstellarclouds. The result implies: In dense ISM, H 2O desorption occur at high tem-

perature (182K) with desorption energy 5407.5±57.7 K and frequency factor1012±1 sec−1. These types of results observed in hot-cores, which are verydense clumps of gas and remnants of a collapsing cloud that form the mas-sive star. We know from observation in such conditions trapped radiationand pressure heats the core at 100K to 300K, such that water ice evaporateand we trace that in observed spectra with other tracers (molecules) [Ref:Millar 1993]. Our experimental data shows the same conditions as observa-tion. But point to be noted here is, TPD temperature peak may not similarto the observation because of the chemistry (mixture of molecules) find inISM (we worked for pure H2O ice) but initial and final range of desorptiontemperature will be same or in between.

we followed same procedure, same equations for calculations ineach set of experiment as in H 2O.

4.2 CO (me

= 28) Experiment

We performed CO experiment four times, three time for different exposuretime (10s, 15, 20s) at same ramp rate (10K/min) then twice for differentdifferent ramp (10 and 6K/min) by keeping exposure time fix at 20sec. Westarted deposition at 20K and desorption started at 25K and finished at

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45K. (we set set-point between 20 to 80K for CO).

We obtained following results:

Figure 18: Result of CO experiment: Energy is in Kelvin, pre-exponentialfactor (A) is in sec−1,Area is in cm2. Unit of x-peak and y-peak is sameas temperature and desorption signal respectively. * shows there was some

connection problem in pressure measurement equipment, so we could notget appropriate readings, that is why I estimated there is error in layercalculation.


Fig: 19,20,21,22 shows best fitted experimental data by simulation for theenergies 930±39K, 1060±52.61K, 945±39.11K and 1060±30K respectivelywith pre-exponential factor (A) shown in fig 18. I tried but, in some cases,I was unable to fit onset position.

Figure 19: Fitting plot of CO for 10sec exposure and beta 10K/min; y-axis:CO desorption signal molecules cm−2K −1, x-axis: temperature

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Figure 20: Fitting plot of CO for 15sec exposure and beta 10K/min; y-axis:CO desorption signal, x-axis: temperature

Figure 21: Fitting plot of CO for 20sec exposure and beta 10K/min; y-axis:

CO desorption signal molecules cm−2K −1, x-axis: temperature

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Figure 22: Fitting plot of CO for 15sec exposure and beta 6K/min;y-axis:CO desorption signal molecules cm−2K −1, x-axis: temperature

NOTE: I missed my y-axis title due to zoom in matplot.

4.2.2 Discussion on CO

Figure:23 shows the TPD formation of CO for different exposure on fix ramp.

we could clearly see as we increase exposure time, all leading edges concideand peak temperature of TPD increases, this increment in peak signifies aswe deposit (condensed) more material on sample, intensity increases justbecause more material more desorption signal.The shift in peak temperatures occur due to desorption rate(eq:1) increasesexponentially with temperature, rate increases until all the layers get subli-mate then it does not drop down rapidly, since in CO since energy is not ashigh enough as in H 2O desorption, that is why we have smooth curves bothside.Area covered by TPD increases as be condensed more molecules (increasedexposure).

There is second peak (right side) in each TPD but more clearly visible in20sec TPD. This peak has no significance, Its just the disambiguation of QMS, as I mentioned (sec-3.2) QMS work on mass no. so when it foundmolecules of similar mass like CO, which has mass no. equal to nitrogen N2,it disambiguate and showed us two peaks since CO simulation fits the firstpeak we could say first peak is due to CO. But there can be another reason,this N2 signal comes from any outside layer or leakage in chamber.We found peak desorption signal in each TPD signal is in between 32Kto 37K, which co-relate the results with literature and verify our experi-ment.[Ref: J.A. Noble et al.]

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Figure 23: CO experiment for 10, 15, 20sec exposure and beta 10K/min;y-axis: CO desorption signal molecules cm−2K −1, x-axis: temperature

Below fig shows the condition of different ramp rate (10 and 6K/min)with fix exposure (15sec). We could see desorption of leading edges overlapstherefore desorption take place at same time, Area covered by both peaksis nearly equal of difference 0.04 × 10−6. This result verifies the simulation(Fig:5). Again I would mention this little difference is due to non precise

measurement in exposure time while spraying molecules otherwise TPD hasto cover same area. We can verify our mistake by looking on no. of layersin these two experiments (see in figure 18). Shift in the peak occur dueto slowdown in ramp, signifies the same reason as I mentioned in H2O (itsnormal).

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Figure 24: CO experiment for 15sec exposure and beta 10K/min and6K/min

4.2.3 Conclusion

On the basis of this I conclude CO has desorption energy (E) 987±36K withfrequency factor 1012±1 sec−1. Desorption arises in between 32K to 37K,this environment typically observed around young stars.

4.3 CO2 (me

= 44) Experiment

As usual we performed C O2 experiment four time, thrice for different expo-sure (15s, 20s, 25s) by keeping fix ramp rate at 10K/min and then two timesfor different ramp rate (10 and 6K/min) on fixed exposure time. We starteddeposition of molecules at 50K and desorption start at 85K and finishedat 102K. The peak of TPD is in between 95 to 100K which is very similarthat we finds in available literature. In this experiment we fixed set pointsin between 50 to 130K. We obtained certain results:

Figure 25: Result of CO2 experiment: Energy is in Kelvin, pre-exponentialfactor (A) is in sec−1,Area is in cm2. Unit of x-peak and y-peak is same astemperature and desorption signal respectively.

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Figure: 26,27,28,29 shows the best fitted data by modeled simulation fordesorption energy 2550±250K, 2450±250K, 2480±30K and 2500±165K re-spectively. Pre-exponent factor is shown in figure 25.

Figure 26: Fitting plot of CO2 for 15sec exposure and beta 10K/min; unitof Y-axis is molecules cm−2K −1


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Figure 29: Fitting plot of CO2 for 20sec exposure and beta 6K/min; unit of

Y-axis is molecules cm−2K −1

4.3.2 Discussion on CO2

Fig: 30 shows the TPD formation of CO2 observed in same conditions asin ISM. Experiment has done for three different exposure (15s,20s,25s)onfix ramp of 10K/min. I will first point-out there is some mistake in 25secTPD experiment, we may have done huge mistake in calculating the timeduring spraying molecules. We were running out of time so we couldn’t

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repeat this experiment. But still I can try to concluded, as happened in

other cases here also leading edges coincide and peak intensity increases aswe condensed more material on sample, significance of same natural reasonas discussed in above two experiment.we can see in figure area covered by TPD increases as we increase the expo-sure.Peak desorption signal occurs in between 96K to 99K which co-relate orresult with standard published result [Ref: J.A. Noble et al.].

Figure 30: CO2 experiment for 15, 20, 25sec exposure and beta 10K/min;unit of Y-axis is molecules cm−2K −1; x-axis is in Kelvin

Below fig:31 express the condition of different ramp rate 10K/min and6K/min at fix exposure time of 20sec. I don’t have idea why every timearea differ with different ramp, according to model it should not differ onlypeaks has to shift. figure clearly points, desorption takes place at same timeat same temperature for different ramp.

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Figure 31: CO2 experiment for 20sec exposure and beta 10K/min and6K/min

4.3.3 Conclusion

Since CO2 is the second most abundant molecule mixed in interstellar icesaccording to spitzer observations(Oberg et al. 2011). Therefore it is im-portant to understand the sublimation behavior of pure CO2. I concluded,

the desorption energy that governed the CO experiment is 2495±50K withpre exponential factor of order 1012±1 sec−1. For experiment taken by usdesorption peaks arises in between 95K to 99K which verifies the accuracyof experiment from available literature.

4.4 Experiment: Mixture of CO2 with H2O

As I discussed earlier in introduction section, In pre-stellar cores, cold outerproto-stellar envelop and proto-planetary disks, most molecules frozen outon dust grains, forming ice mantles. The main component of young stellarobjects is H2O with the mixture of CO and CO2 according to spitzer ob-

servations(Oberg et al. 2011). Near Infra-red (NIR) observation shows theevidence of it [Ref: Oberg et al. (2001), Knez et al. (2005)]. Therefore it isimportant to understand mixture desorption.

4.4.1 Deposition of mixture and discussion

Figure:32 shows the experiment of mixture in two different conditions.

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1. When we mixed CO2 with H2O for 1:1 respectively. The amount of

molecules taken in experiment is 10mb. Exposure time is 25sec withramp rate 10K/min.

2. When we mixed CO2 with H2O for 1:10 respectively. The amountthat we have taken is approx 25mb. Exposure time is 10sec with ramp10K/min. we reduced exposure, In order to not to destroy QMS,because after mixing for ratio 1:10 pressure in specimen chamber wasmax.

Preparation of mixture is discussed in section 3.3 . In both of the cases wedeposited mixture on sample at 80K. we run our TPD for 80K to 200K.

Experiment shows mixture of CO2 with H2O dose not behave as pure

CO2 and H2O. This is due to interaction behavior of CO2 with H2O, this be-havior differ the binding energies of mixture molecules consequently we seedifferent results then pure molecule TPD. CO2 known as volatile molecule,due to its volatile nature its traps in H2O and desorbs twice in single experi-ment. Trapping fraction depend on the ratio of mixture. If volatile moleculefraction increases with H2O tapped fraction decreases. Theoretical modelthat defines such behavior is three phase model. In this model, gas-graininteractions are addressed by considering three phases: the gas phase, thesurface of the ice and the bulk/mantle of the ice. (this model is bit hardto explain systematically but this will my future work), still I would like topoint diffusion plays a key role in this condition. This diffusion focused on

two important conditions: the energy barrier that volatile molecule has toovercome to swap the neighboring molecule and fraction of mantle moleculesclose enough to participate in swapping surface molecules. [Ref: Edith C.Fayolle at al.(2011)]

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(a) CO2 - H2O mixture of (1:1)

(b) CO2 - H2O mixture of (1:10)

Figure 32: Mixture desorption experiment; x-axis is in kelvin; Y-axis is inmolecules cm−2K −1

In the cases of mixture we observed CO2 desorbs twice or more thantwice. As we could see in the ratio of 1:1 mixture, CO2 has more than twopeaks, the first peak that arise nearly 90K is due to pure CO 2 nature andother rest peaks are due to trapping behavior. In mixture 1:10 (CO2 andH2O resp.) we seen CO2 desorption twice, both peaks shows the behaviorof CO2 and H2O chemistry.

We usually discuss on two peaks because 1:1 ratio is not that muchrecommended in ISM (nearly we can not find such condition). First des-orption peak of CO2 is called molecular volcano: Molecules those are nottrapped deeply in surface easily diffused in ice mental and desorbs so we de-

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tect them. There is another strong reason, when H2O transform amorphous

to crystalline phase there is sudden leakage CO2 from upper layers and weobserved this peak. That is why, it is named as molecular volcano. Secondpeak of CO2 is due to co-desorption: This is due to trapping of CO2 in thedeep layers of H2O. This signifies mixing can change sublimation tempera-ture of molecules or this is due to, diffusion quickly slowdown as the surfacelayers saturate with H2O molecules or alternatively all accessible pores havebeen emptied with which CO2 were diffusing and sublimating.

4.4.2 Result

• When we have ratio of mixture 1:1, we seen first desorption peak of

CO2 is at 93.11K. this is nearly the case of pure CO2 therefore itsshows the independence from coverage (thickness of layers) such thatwe can conclude from the top layer of mixture CO2 desorbs at samedesorption temperature of pure CO2.

H2O peak is at 172 K which is in the range of pure H2O desorptionrange. CO2 second peak is at 149.83K. Therefore this second peakshows the huge difference (by 50K) in the sublimation temperature of CO2 in desorption process.

At the end I can point, for the mixture of equal ratio fist peak of CO2and water peak behaves same as pure molecules. But CO2 second peakshows the behavior of trapping and points on change in sublimation

temperature due to mixing.

• When we have ratio of mixture 1:10 (CO2 and H2O resp.). This exper-iment clearly shows the behavior of trapping and sublimation tempera-ture of CO2 in mixture increased with huge factor, first peak observedat 154.5K. This shows the clear dependency of desorption tempera-ture on ratio of mixture. This implies, as the water start desorbing,surface layers start saturating with H2O molecules, consequently dif-fusion quickly slowdown or alternatively all accessible pores have beenemptied with which CO2 were diffusing and sublimating.

Its is also amazing to see second peak of CO2 coincide with H2O,

both peak observed at 177.6K. This is due CO2 is less mobile in H2OTherefore, CO2 that has to sublimate before water spent more timeon each kelvin such that defines the science of trapping.

• In the ratio of 1:1, area covered by CO2: curve1 is 1.077×10−9 curve2

is 6.1 × 10−10. Area covered in H2O curve is 1.2 × 10−8.

• In the ratio of 1:10, area covered by CO2: curve1 is 1.547×10−10 curve2

is 2.26×10−10. Area covered in H2O curve is 1.2×10−8. we seen with

this result area covered by both CO2 curves is same. This may signifies

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that CO2 molecules that traps in deep layer of H2O is same as in above

layers (less abundence but equal distribution of CO2 with H2O).

• For ration 1:1 mixture experiment sample was covered by ∼ 25 mono-layers and in 1:10 mixture experiment sample was covered by ∼ 38mono-layers.

5 |Conclusion

These experiment was aimed to extract the two important parameter des-orption energy (Edes) and pre-exponential factor A (frequency factor) fromeach set of experiment. For pure molecule experiment, results are given

below:

Figure 33: Energy is in Kelvin, pre-exponential factor (A) is in moleculescm−2sec−1, Temperature is in Kelvin, residence time is in years.

While fitting simulation I found A and E both are co-related, so just forcuriosity I tried to find the co-relation between them, I found co-relationcoefficient 0.422.

These A and E parameters are so important for modeling interstellar en-vironment specially in calculating the residence time of particular specie ongrains. I have calculated the residence time of H2O, CO and CO2 at therepeak desorption temperature for the typical lifetime of inter-stellar cloud107years [Ref: Wikipedia inter-stellar clouds], results are shown in figure 33.Results implies, at the peak desorption temperature of molecules, there res-idence time is huge. This means they take very long time to desorb but we

can see them because of there large number density. Due to huge residencetime, species have sufficient time to grown-up and perform reaction to formnew molecules in ice mantles on grain surface.

Future work: I would like to work more on Three-phase model and resi-dence time co-relation in cloud collapse conditions.

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6 |References

[1] Thermal desorption of water ice in interstellar medium, (Helen J. Fraseret al. 2001)[2] Desorption of CO and O2 interstellar ice analogs (K. Acharya et al. 2014)[3] J. A. Noble et al. 2011, Thermal desorption characteristic of CO, O2 andCO2 on non porous water, crystalline water and silicate surface at sub-monolayer and multi coverage, Pg. no. 5 (reference for energies )[4] Edith C. Fayolle et al. 2001, Laboratory H2O:CO2 ice desorption: entrap-ment and its parameterization with an extended three-phase model "H2OCO2 ice desorption data, and three phase model"[5] Astro-chemistry lecture notes by P. Theule.

[6] Marco Minissale master thesis[7] https : //en.wikipedia.org/wiki/Langmuir[8] http : //www.chem.qmul.ac.uk/surfaces/scc/scat26.htm[9] "error-analysis" lab manual of university of pennsylvania, Pg. no. 6[10] Oberg et al. 2011

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7 |Acknowledgment

I am very thankful to my supervisor Marco Minissale and Co-supervisor P.Theule for there consistent availability and fruitful discussion on my everymode of difficulty. I am thankful to my colleague Ny kieu for helping indoing experiment. Thanks for my mind, it was functioning properly (I hopeit functioned properly). If some thing is missing in report, I really apologiesto my supervisors please forgive me: "God tells me every day, see gauri If you use your brain in more amount, It will finish fast, strictly speaking Iwont give you more again and again. so please control on yourself, you haveto survive till 100 years (with tongue smiley)" That’s the reason!!!

adsorption and desorption process in lab for h20, co2, co and mixture of h2o-co2 on au sample

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