Refractive indices vs deposition temperature of thin films of ethanol, methane and nitrous oxide in the vicinity of their phase transition temperatures
This paper presents results of an experimental study of the dependence of deposition temperature on the refractive in dices of ethanol, methane and nitrous oxide, in the form of cryovacuum deposited thin films, in the vicinity of their structural transformation temperatures. The measurements were ma...
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Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України
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irk-123456789-1752292021-02-01T01:28:22Z Refractive indices vs deposition temperature of thin films of ethanol, methane and nitrous oxide in the vicinity of their phase transition temperatures Drobyshev, A. Aldiyarov, A. Sokolov, D. Shinbayeva, A. Tokmoldin, N. Физические свойства криокристаллов This paper presents results of an experimental study of the dependence of deposition temperature on the refractive in dices of ethanol, methane and nitrous oxide, in the form of cryovacuum deposited thin films, in the vicinity of their structural transformation temperatures. The measurements were made using a two-beam laser interferometer in the temperature range 14–130 K. The refractive indices of ethanol, methane and nitrous oxide demonstrate clear non-monotonic dependence on deposition temperature in the vicinity of their structural phase transformation temperatures. It is speculated that the observed abrupt changes in the refractive indices are due to a difference in the number of translational and rotational degrees of freedom of the molecules existing in different phase states of these cryocrystals. 2017 Article Refractive indices vs deposition temperature of thin films of ethanol, methane and nitrous oxide in the vicinity of their phase transition temperatures / A. Drobyshev, A. Aldiyarov, D. Sokolov, A. Shinbayeva, N. Tokmoldin// Физика низких температур. — 2017. — Т. 43, № 10. — С. 1521-1524. — Бібліогр.: 21 назв. — англ. 0132-6414 PACS: 78.20.Ci, 65.20.Jk http://dspace.nbuv.gov.ua/handle/123456789/175229 en Физика низких температур Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України |
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Физические свойства криокристаллов Физические свойства криокристаллов |
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Физические свойства криокристаллов Физические свойства криокристаллов Drobyshev, A. Aldiyarov, A. Sokolov, D. Shinbayeva, A. Tokmoldin, N. Refractive indices vs deposition temperature of thin films of ethanol, methane and nitrous oxide in the vicinity of their phase transition temperatures Физика низких температур |
| description |
This paper presents results of an experimental study of the dependence of deposition temperature on the refractive in dices of ethanol, methane and nitrous oxide, in the form of cryovacuum deposited thin films, in the vicinity of their structural transformation temperatures. The measurements were made using a two-beam laser interferometer in the temperature range 14–130 K. The refractive indices of ethanol, methane and nitrous oxide demonstrate clear non-monotonic dependence on deposition temperature in the vicinity of their structural phase transformation temperatures. It is speculated that the observed abrupt changes in the refractive indices are due to a difference in the number of translational and rotational degrees of freedom of the molecules existing in different phase states of these cryocrystals. |
| format |
Article |
| author |
Drobyshev, A. Aldiyarov, A. Sokolov, D. Shinbayeva, A. Tokmoldin, N. |
| author_facet |
Drobyshev, A. Aldiyarov, A. Sokolov, D. Shinbayeva, A. Tokmoldin, N. |
| author_sort |
Drobyshev, A. |
| title |
Refractive indices vs deposition temperature of thin films of ethanol, methane and nitrous oxide in the vicinity of their phase transition temperatures |
| title_short |
Refractive indices vs deposition temperature of thin films of ethanol, methane and nitrous oxide in the vicinity of their phase transition temperatures |
| title_full |
Refractive indices vs deposition temperature of thin films of ethanol, methane and nitrous oxide in the vicinity of their phase transition temperatures |
| title_fullStr |
Refractive indices vs deposition temperature of thin films of ethanol, methane and nitrous oxide in the vicinity of their phase transition temperatures |
| title_full_unstemmed |
Refractive indices vs deposition temperature of thin films of ethanol, methane and nitrous oxide in the vicinity of their phase transition temperatures |
| title_sort |
refractive indices vs deposition temperature of thin films of ethanol, methane and nitrous oxide in the vicinity of their phase transition temperatures |
| publisher |
Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України |
| publishDate |
2017 |
| topic_facet |
Физические свойства криокристаллов |
| url |
http://dspace.nbuv.gov.ua/handle/123456789/175229 |
| citation_txt |
Refractive indices vs deposition temperature of thin films of ethanol, methane and nitrous oxide in the vicinity of their phase transition temperatures / A. Drobyshev, A. Aldiyarov, D. Sokolov, A. Shinbayeva, N. Tokmoldin// Физика низких температур. — 2017. — Т. 43, № 10. — С. 1521-1524. — Бібліогр.: 21 назв. — англ. |
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Физика низких температур |
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Low Temperature Physics/Fizika Nizkikh Temperatur, 2017, v. 43, No. 10, pp. 1521–1524
Refractive indices vs deposition temperature of thin films
of ethanol, methane and nitrous oxide in the vicinity
of their phase transition temperatures
A. Drobyshev, A. Aldiyarov, D. Sokolov, A. Shinbayeva, and N. Tokmoldin
Institute of Experimental and Theoretical Physics, Almaty, Kazakhstan
E-mail: Andrei.Drobyshev@kaznu.kz
Received January 17, 2017, revised February 17, 2017, published online August 27, 2017
This paper presents results of an experimental study of the dependence of deposition temperature on the re-
fractive in dices of ethanol, methane and nitrous oxide, in the form of cryovacuum deposited thin films, in the
vicinity of their structural transformation temperatures. The measurements were made using a two-beam laser in-
terferometer in the temperature range 14–130 K. The refractive indices of ethanol, methane and nitrous oxide
demonstrate clear non-monotonic dependence on deposition temperature in the vicinity of their structural phase
transformation temperatures. It is speculated that the observed abrupt changes in the refractive indices are due to
a difference in the number of translational and rotational degrees of freedom of the molecules existing in differ-
ent phase states of these cryocrystals.
PACS: 78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflec-
tion and transmission coefficients, emissivity);
65.20.Jk Studies of thermodynamic properties of specific liquids.
Keywords: refractive index, laser interferometer, ethanol, methane, nitrous oxide, thin films.
1. Introduction
In recent years, renewed interest has been noticed in the
studies of thin films of ethanol [1,2], nitrogen oxide [3,4]
and methane [5–7] deposited from the gas phase at cryo-
genic temperatures, specifically as part of astrophysical
investigations. This paper aims at a more detailed study on
deposition temperature dependence of the refractive indi-
ces in the vicinity of phase transition temperatures of the
mentioned materials. The latter are often associated with
abrupt changes in the number of translational and rotation-
al degrees of freedom of the molecules in cryofilms. For
example, phase transitions in solid N2, CO, N2O, C2H5OH
up on decrease in temperature may result in partial or
complete freeze-out of orientational ordering of rotational
subsystem in these cryocrystals. The influence of these
processes on mechanical and optical properties of thin
films, such as refractive index, density and polarizability,
is not yet fully clarified. This relationship becomes even
more intricate due to the role of deposition temperature on
the structural and phase state of these materials. This report
focuses on the study of thin films of solid ethanol, methane
and nitrous oxide in the temperature range between 12 and
130 K in which the mentioned structural phase transitions
are observed.
2. Results and discussion
The measurements of influence of deposition tempera-
ture on the refractive indices of ethanol, methane and ni-
trous oxide were performed using the experimental setup
described in detail elsewhere [9]. The experiments were
conducted in the temperature range 14–130 K, comprising
temperatures of structural transformations of the studied
materials. All experiments were performed by same depo-
sition pressure of P = 1.2⋅10–4 Torr and sample thickness
of d = 1.5 μm. The refractive index was measured using a
two-beam laser interferometer. The main source of error
was associated with measurement of the period of interfer-
ence during the film growth, whereas the random error was
0.3%. As such, the total error of the refractive index meas-
urements was estimated to be no more than 1%. It must be
noted that each point representing a refractive index value
in the diagrams below corresponds to an individual exper-
iment conducted at a given deposition temperature. After
completion of each experiment the setup was prepared
again to a new experimental cycle.
It is well known that cryofilms of ethanol, methane
and nitrous oxide exist in various states depending on
both their deposition temperature and consequent thermal
history [10–12]. For example, following rapid quenching
© A. Drobyshev, A. Aldiyarov, D. Sokolov, A. Shinbayeva, and N. Tokmoldin, 2017
A. Drobyshev, A. Aldiyarov, D. Sokolov, A. Shinbayeva, and N. Tokmoldin
to 80–90 K liquid ethanol exhibits transition into the glassy
state [13], which upon annealing transforms sequentially in
to the super-cooled liquid (SCL) at temperatures in the
vicinity of T = 97 K, plastic crystal (PC) at temperatures
above 110 K and monoclinic crystal at temperatures above
140 K. Cooling of ethanol in the plastic crystal state below
95–97 K results in its reversible transition into the
orientational glass (OG) state. A similar behavior is ob-
served for thin films deposited at cryogenic temperatures
from the gaseous phase. However, in contrast to monolithic
samples, a transition from the amorphous state (ASE) to the
glassy state is observed in the region of 70–80 K [14]. Thus,
the following characteristic temperature intervals can be
identified for stable molecular states of ethanol deposited
as thin films at lower temperatures.
— Deposition temperature above T = 145 K — mono-
clinic crystal.
— Deposition temperature between 120 and 90 K —
plastic crystal, cubic face-centered lattice with rotating
molecules in lattice points. Cooling below T = 90 K results
in “freezing-out” of the rotational component and for-
mation of structural glass.
— Deposition temperature below T = 90 K — amor-
phous phase comprising orientationally disordered crystal-
lites with various degrees of ordering [15].
Figure 1 demonstrates the influence of deposition tem-
perature on the refractive index of ethanol. Gradual rise in
the refractive index value with deposition temperature is
observed in the range between 12 and 60 K, which agrees
with results of other authors for similar materials [5,6,8].
The influence of sample porosity on the refractive index
is considered to be the main cause of this behavior. In
our measurements the refractive index values for etha-
nol varied between n = 1.250 at T = 16 K and n = 1.352
at T = 60 K. In the range of 60–90 K the refractive index
remains practically constant.
Further increase in deposition temperature above 90 K
leads to an abrupt rise of the refraction index value from
around n = 1.352 at 90 K to n = 1.430 at 98 K. Be-
tween100 K and 130 K the refractive index of ethanol is
again practically independent of deposition temperature
having an average value of n = 1.430. This is consistent
with the given interval corresponding to the range of exist-
ence of the plastic crystal phase only [13].
The influence of deposition temperature on the refrac-
tive index of nitrous oxide is shown in Fig. 2. As reported
by us earlier [16,17], at T = 40 K this material undergoes
transition from the crystalline cubic face-centered state
(T > 40 K) to the partially orientationally disordered Pa3
state (T < 40 K). It is therefore of interest how the refractive
index of nitrous oxide is influenced by this transition. As
seen in the diagram, the refractive index of nitrous oxide
gradually rises with temperature from n = 1.254 at 16 K to
n = 1.310 at 40 K. In the vicinity of 45 K, it undergoes an
abrupt increase in value from n = 1.315 at T = 45 K to n =
= 1.410 at T = 52 K. Further increase in the deposition
temperature from 50 K to 70 K results in continuation of
the gradual rise pattern.
For comparison, we include into Fig. 2 the refractive
index values of carbon dioxide measured experimentally
using a technique similar to ours [5]. In this case, a monot-
onous rise is observed between 10 K and 55 K before pla-
teauing at higher temperatures.
The influence of deposition temperature on the refrac-
tive index of methane deposited under high vacuum at cry-
ogenic temperatures is demonstrated in Fig. 3. In this case,
in the interval between 14 K and 20 K the refractive index
rises monotonously with deposition temperature going
from n = 1.280 at T = 14 K to n = 1.291 at T = 19 K. In the
vicinity of T = 20 K the refractive index undergoes an abrupt
increase jumping from 1.291 to 1.310. Further increase in
deposition temperature results in recovery of the monotonous
rise with n reaching the value of 1.342 at T = 30 K.
The quantum nature of CH4 molecules, intramolecular
spin-nuclear conversion processes in methane and peculi-
arities of the intermolecular interactions related to these
Fig. 1. Influence of deposition temperature on the refractive in-
dex of ethanol.
1.48
1.44
1.40
1.36
1.32
1.28
1.27
1.20
0 20 40 60 80 100 120 140
Deposition temperature, K
R
ef
ra
ct
iv
e
in
de
x
n
Fig. 2. Influence of deposition temperature on the refractive indi-
ces of nitrous oxide. For comparison, data for carbon dioxide are
shown [5].
Nitrogen oxide
Carbon dioxide [5]
1.50
1,45
1.40
1.35
1.30
1.25
1.200 10 20 30 40 50 60 70 80 90
Deposition temperature, K
R
ef
ra
ct
iv
e
in
de
x
n
1522 Low Temperature Physics/Fizika Nizkikh Temperatur, 2017, v. 43, No. 10
Refractive indices vs deposition temperature of thin films of ethanol, methane and nitrous oxide …
processes may act as some of the factors affecting the re-
fractive index behavior at low temperatures [18–20]. For
example, it is known that the solid methane undergoes
transition from the orientationally disordered α-phase
phase (at higher temperature) to the partially ordered β-
phase phase (at lower temperature) at T = 20.4 K [21],
which coincides with the temperature of the abrupt change
in the refractive index observed in Fig. 3.
For comparison we include into Fig. 3 data points ob-
tained dynamically using a two-beam laser interferometer
and reported by other authors [6,7]. Our results are in satis-
factory agreement with these data, particularly taking into
account relatively high scattering in Ref. 6.
3. Conclusions
This study is dedicated to elucidation of the influence of
deposition temperature on the refractive indices of
cryovacuum deposited films of molecular materials exist-
ing in a variety of solid phases at low temperature. All ma-
terials under study: ethanol, methane and nitrous oxide,
exhibit transitions resulting in changes in the number of
molecular degrees of freedom. It must be noted that this
study does not consider thermally stimulated transfor-
mations, but rather focuses on measurements at fixed dep-
osition temperatures in the intervals of existence of stable
and quasi-stable states of the studied films.
Based on the correlation between our data and the phase
diagram of ethanol [13], the following characteristic tem-
perature regions corresponding to its various states can be
distinguished (Fig. 1):
Between 14 K and 65 K. This interval corresponds to
amorphous solid ethanol (ASE). Rise in deposition tem-
perature results in increasing of the refractive index, poten-
tially due to reduction in sample porosity.
Between 65 K and 90 K. This interval corresponds to
amorphous glassy state (GS) with the refractive index remain-
ing constant. ASE and GS appear to represent to different
phases which is confirmed by IR-spectrometric studies [9].
Between 90 K and 100 K. This interval corresponds to
three different phases of ethanol obtained in various ways:
1) plastic crystal (PC) with cubic face-centered lattice hav-
ing freely rotating ethanol molecules in lattice points (li-
brations); 2) orientational glass (OG) obtained by rapid
quenching of PC below T = 97 K, suggesting that this state
cannot be realized in our experiments; this results in freez-
ing out of the rotational subsystem and formation of an
orientationally disordered cubic lattice; 3) super-cooled
liquid (SCL) obtained by annealing of GS, suggesting that
this state also cannot be realized in our experiments. Ab-
rupt increase in the refractive index between 90 K and
100 K correlates with transition from ASE to PC with the
freely moving rotational subsystem.
Between 100 K and 130 K. The refractive index shows
weak temperature dependence. This range is associated
with existence of the PC phase.
A similar jump-like dependence of the refractive index
on deposition temperature is observed for nitrous oxide
(Fig. 2). As we reported earlier [16,17] cryovacuum depos-
ited films of nitrous oxide undergo a structural transition at
temperatures below 40–45 K interpreted as amorphization
of the samples. Thus, upon annealing to 37–42 K partially
or fully amorphous films deposited at cryogenic tempera-
tures become unstable and transform into the Pa3 struc-
ture. The interval between 14 K and 40 K corresponds to a
cubic phase of nitrous oxide with an orientationally disor-
dered rotational subsystem. Gradual rise in the refractive
index in this interval is most likely related to an increase in
porosity of the samples.
Transition from the amorphous state of nitrous oxide to
the Pa3 structure occurs in the range of 40–50 K which is
accompanied by an abrupt increase in the refractive index
from n = 1.310 at T = 40 K to n = 1.405 at T = 50 K.
Above 50 K the refractive index exhibits a gradual rise
with deposition temperature reaching n = 1.425 at T = 70 K.
In the vicinity of 70 K significant scattering of data points
is observed which is explained by the proximity of the
deposition pressure to the equilibrium vapor pressure at
this temperature, which in turn makes it challenging to
maintain given thin film deposition conditions.
The observed dependence of the refractive index of me-
thane on deposition temperature (Fig. 3) correlates with the
α–β-transition at T = 20.4 K reported elsewhere [18–20].
As such, a distinct gap in the n–T data is observed in the
vicinity of the phase transition temperature. It may there-
fore be speculated that the data points lying in the range of
16–19 K relate to the partially ordered β-phase of methane,
whereas the temperature interval 20.4–30 K corresponds to
the α-state with freely rotating methane molecules located
in the crystal lattice sites. In this case, one can argue that
the α–β-transition resulting in orientational ordering of as
many as three quarters of molecules [19,20] in the system
is also reflected in a substantial and abrupt change in the
refractive index of methane.
Fig. 3. Influence of deposition temperature on the refractive in-
dex of methane.
1.36
1.32
1.28
1.248 12 16 20 24 28 32
Temperature, K
R
ef
ra
ct
iv
e
in
de
x
n
This paper
Sattore [6]
Hudson [7]
Linear approximation 1
Linear approximation 2
1
2
Low Temperature Physics/Fizika Nizkikh Temperatur, 2017, v. 43, No. 10 1523
A. Drobyshev, A. Aldiyarov, D. Sokolov, A. Shinbayeva, and N. Tokmoldin
Results presented herein are in good agreement with the
above studies and may find use in further astrophysical
research
Acknowlegments
This research was supported by the Ministry of Educa-
tion and Science of the Republic of Kazakhstan, Grant
N 3118/GF4-15.
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1. Introduction
2. Results and discussion
3. Conclusions
Acknowlegments
|