IR Spectrometric Studies of CCL4 and Ar Gas Mixtures

IR spectrometric studies were performed to examine the cryo-deposition processes and properties of thin cryo-vacuum condensate films containing a mixture of carbon tetrachloride and argon, which were obtained by physical cryo-vacuum deposition at various concentrations. The measurements were carried out in a temperature range of 11–100 K, and the pressure of the gas phase was 10–8 Torr-10–4 Torr. The film thickness in all experiments performed was d = 2 μm. The vibrational spectra of CCl4 in an argon matrix were measured in a frequency range of 400–4200 cm−1 at mixture concentrations (CCl4 + Ar) of 10–90%, 5–95%, and 1–99% and after the argon matrix underwent sublimation. Thermal desorption curves for the yield of the matrix gas (Ar) were obtained in the temperature range of 40–90 K for the (CCl4 and Ar) system. It was found that the CCl4 molecules in the Ar matrix substance underwent cryocapture. The results obtained suggest that carbon tetrachloride can be used as a matrix in low-temperature studies on self-organization processes in thin films.


Introduction
The physics and chemistry of ice surfaces has attracted considerable research interest in diverse areas of science, such as environmental, interstellar, and biological chemistry [1]. The molecular structure of ice surfaces is also a fundamental interest. Ice is often regarded as a model system for studying the surface properties of molecular solids experimentally due to the availability of extensive and theoretical data on intermolecular interactions in liquid water and ice [2][3][4][5].
The main task when studying condensed matter physics is to determine the patterns of formation of disordered, glassy states and subsequent structural transformations in the samples. Thus, along with theoretical studies, experimental results play a large role. Research pure, single-element samples, it is possible to determine with high accuracy the temperature ranges of structural transformations and the behavior of films formed after being subjected to thermally stimulated transformations [6][7][8].
Notably, it is important to apply the cryomatrix isolation method with a mixture of objects under study as a guest matrix. The goal of this kind of experimental task is to establish a relationship between the structure of condensed molecules and the conditions of their deposition, as well as the properties of cryocondensates formed at low temperatures, such as the degree of kinetic stability [9]. In view of the above, IR spectrometric studies of the processes of formation and structural transformation of cryovacuum condensates of carbon tetrachloride were carried out [10]. Thus, IR spectrometric studies can provide information about the individual vibrations of tetrachloromethane molecules and their response to thermally stimulated transformations in the samples under study. The aim of this research was to study the features of carbon tetrachloride cryocondensation and to determine the glass transition temperature of cryofilms formed at low temperatures.
The studies of cryo-deposition processes and the properties of thin films of cryovacuum condensates confirm the thesis that physical cryo-vacuum deposition is an effective method to obtain the simplest gases in glass-like amorphous states. Therefore, the method has been intensively used in the last few years to study a wide range of glass-forming substances [11][12][13][14][15]. The mobility of small molecules, which is an obstacle in the formation of amorphous states, is sufficiently compensated by the low temperature of the substrate, limiting their lifetime in the adsorbed layer. In this case, the small thickness of the formed films ensures that heat is efficiently removed from the interface to the substrate surface, which prevents the condensation surface from heating due to the phase transition heat.
It is also worth mentioning our earlier studies, as we used the cryomatrix isolation method in an inert gas environment to determine and study nanocluster structures [16]. Thus, when studying ethanol in a nitrogen matrix, we determined the monomeric and dimeric structures, their contribution to structural transformation during thermal cycling transformation in film samples. In this sense, we note the importance of using the cryomatrix isolation method with a mixture of gas condensates to confirm and supplement the experimental data for the objects under study. This paper presents a continuation of our study using the cryomatrix isolation method with a mixture of carbon tetrachloride and argon gases in a wide range of temperatures for the development of cryocondensates. The purpose of this research is to confirm the temperatures of structural transformation of the research object under study. In addition, by answering this question, this research is aimed at improving our understanding of the interaction between the gas phase and the chemistry of the solid phase in a cryocondensable medium.
In view of the foregoing, we carried out IR spectrometric studies to examine the processes of formation and structural transformation with cryovacuum condensates containing carbon tetrachloride under conditions of joint condensation with an inert gas argon. IR spectrometric studies can provide information about individual vibrations of the tetrachloromethane molecule and their response to thermally stimulated transformations in the samples under study.

Materials and Methods
The experimental setup and measurement technique have been described in more detail by us previously [17]. The cylindrical vacuum chamber with a diameter and height of 450 mm is composed of stainless steel (Fig. 1). The vacuum in the chamber was formed with a Turbo-V-301 turbomolecular pump up to the ultimate vacuum of 10 -8 Torr. The chamber pressure readings were measured by an FRG-700 transducer with an AGC-100 controller. An Extorr XT100 mass spectrometer was used to determine the qualitative analysis of desorption substances. On the lower carrier plate of the vacuum chamber, a mirror substrate (copper coated with a layer of gold) is fixed on the upper flange of the Gifford-McMahon cryogenic system. The limiting temperature of the cryostat cooling is T = 11 K. The temperature was measured by a platinum thermistor using an M335/20 s temperature controller. The absorption spectra were recorded using an FSM 2203 Fourier spectrometer in the frequency range of 400-4200 cm -1 . Measurements of the condensation rate and the growth of the film thickness were controlled using a two-beam laser interferometer. The laser wavelength was 420 nm. The purity of carbon tetrachloride was 99.8%. Immediately before measurements, the vessel with CCl 4 was evacuated to a pressure of 0.1 Torr.
Thus, the order of the experiments was as follows. The chamber was evacuated to a pressure of 10 -8 Torr, after which the substrate was cooled to the condensation temperature (T c = 11 K). Furthermore, by utilizing a leak valve connected to the gas preparation system for a mixture of CCl 4 and Ar gases, the gas was admitted into the chamber until the working pressure of cryocondensation (10 -5 Torr) was reached. With the help of laser interferometers, growth interferograms were recorded for two different angles of incidence. The film thickness of the film was measured using two interference patterns generated by a diode laser and two photomultipliers. The measurements were carried out at a frequency of 100 Hz, which makes it possible to determine the oscillation period with an accuracy of ± 0.05; these results were added to the work with reference to our publication [18]. Furthermore, the gas puffing was stopped, and at a fixed film thickness and substrate temperature, the absorption spectrum of the condensed film was measured. Then, the samples were heated up with the temperature increments of 5 K and the IR spectrum was measured after each heating step. The entire process of thermal variation of the samples was recorded by measuring the pressure in the chamber, and the desorption of film elements was determined by mass spectrometry.

Experimental Results
The cryomatrix isolation method showed promise in our previous work on the study of the cluster composition of ethanol recondensates and nitrogen matrix [16].
The aim of our research was to study the features of carbon tetrachloride cryocondensation in an argon matrix. The effect of thermal cycling processes with cryocondensate films samples on the structural transformations and the behavior of vibrational modes of molecules in the IR range were determined. Previously, we published a work devoted directly to the glass transition processes in cryofilms containing pure carbon tetrachloride [10]. Temperature intervals were determined, which were interpreted as the transition between the amorphous glassy state of the cryocondensate film to the crystalline state. This paper presents a demonstrated technique for the cryomatrix isolation of a research object in an inert gas matrix that is optically transparent in the IR range of research. Figure 2 shows the absorption spectra of carbon tetrachloride measured by us in an argon matrix with various concentrations of the gas mixture (1%; 5%; 10%).
The data are presented near the vicinity of the vibrational frequencies of the carbon tetrachloride molecule. In the presented figure for the percentage (1%) CCl4, the peaks at frequencies 761 cm −1 , 764 cm −1 , and 767 cm −1 are split. For the percentage composition (5%) in the frequency range of 790 cm −1 , the absorption band peak is clearly split into two maxima at 785 cm −1 and 795 cm −1 .  Figure 3 shows the thermal desorption curves obtained while heating cryocondensates at various percentages of gas mixtures.
The authors applied the inert gas argon as the matrix, the quantitative ratio of which exceeds the test substance (guest) CCl 4 . In the temperature range of 40 K, desorption of the main composition of the matrix, argon molecules, occurs, and CCl 4 recondenses onto the substrate. Recondensation is a term introduced by researchers studying ethanol molecules in a nitrogen (N 2 ) matrix [6]. Furthermore, when the temperature reaches 100 K, the trapped argon is released by the structural formations of CCl 4 . In this temperature range, anomalous behaviors for molecular vibrations were found for a pure CCl 4 film, and these behaviors are interpreted as a transition to a supercooled liquid (see [10]).
The presented data show that for mixtures of cryocondensates, argon desorption begins at a surface temperature of 38-40 K, which is in good agreement with the reference data at a given thermodynamic value. Furthermore, some specifics were found; for example, the desorption peaks of gas release from the substrate surface are also found in the temperature range of 90-100 K. According to the mass spectrometer XT 100, these desorption peaks correspond to the elements of the film of cryocaptured argon. The temperature range of 89 K corresponds to the gas yield for a mixture obtained at (5%) CCl 4 concentration, for a mixture obtained at (10%) CCl 4 concentration, the argon desorption temperature corresponds to 98 K, and for a (1%) CCl 4 mixture, the desorption temperature is 105 K. This result reveals that structural transitions occur in films of carbon tetrachloride cryocondensates in the given temperature range, which is consistent with previous data [10]. This subject is not the main goal of this work and requires a more detailed experimental study.
It is known that the mixtures of cryocondensate studied by us exhibit different sublimation temperatures, which, under certain temperature conditions, can lead to sharp changes. Thus, for a two-component sample, when the temperature of the medium reaches a value above the sublimation temperature of the lower-temperature component, it will evaporate, resulting in the separation of the second component and its subsequent recondensation. As an example, a schematic diagram of the recondensation process is shown in Fig. 4. Thus, the authors believe that films recondensing after argon sublimation may consist of a porous structure. It would be incorrect to speak more precisely about the structure by the method of IR spectroscopy. However, the change in the absorption bands of the characteristic vibration modes of the substance under study makes it possible to determine the temperature ranges of transformations in films. Figure 5 shows the absorption spectra of carbon tetrachloride in argon measured by us at a certain concentration of a mixture of gases (10%-90%).
The left figure shows the spectra of a mixture of carbon tetrachloride and argon gases obtained at a condensation temperature of 11 K and then after heating to 55 K and 110 K. After annealing the cryocondensate film with a temperature of 110 K, the substrate was cooled to a temperature of 11 K, and the spectrum was again measured, as shown in the left figure. These spectra are marked (T = 11 K rec) and (T = 100 K rec). To determine more accurate temperature limits of structural changes, as previously performed [10], we used an observational method with the spectrometer at a fixed frequency; the data are presented in the right figure. In our case, the sample was heated from the condensation temperature T = 11 K to T = 100 K to measure the spectrometer signal. The absorption Fig. 5 Comparison of the vibrational spectra of carbon tetrachloride in an argon matrix at a certain concentration of a mixture of gases (10-90%) Fig. 6 Comparison of the vibrational spectra of carbon tetrachloride in an argon matrix at a certain concentration of a mixture of gases (5-95%) band intensity for two observation frequencies corresponding to ν obs = 800 cm −1 and ν obs = 780 cm −1 was measured while the cryocondensate samples underwent thermal cycling. The heating rate was V heat = (0.1 ± 0.05) K/sec in the temperature range 16-40 K and V heat = (0.01 ± 0.005) K/sec in the temperature range 40-110 K. Figures 6 and 7 show the absorption spectra measured analogously to the (10%) concentration of carbon tetrachloride in an argon matrix, but the concentrations of the mixture of gases were (5-95%) and (1-99%), respectively.
Thus, from the experimental data presented above, homogeneous behavior is clearly observed upon heating for the concentrations (10% and 5%) of carbon tetrachloride in the argon matrix. Moreover, for the (1%) composition of carbon tetrachloride in argon, heating the film causes a more pronounced structural transformation, as shown by a change in the vibrational modes of the molecules.
The cryopreservation of argon molecules by CCl 4 recondensation during is considered by the authors as an anomaly for the behavior of films upon heating. The authors assume that the structural state and possible porosity of the film CCl 4 could serve as an argon molecule capture factor. Thus, the temperature range above 90 K and the desorption of argon serve as experimental evidence for the existence of structural transformations in the CCl 4 film. The authors previously found anomalies in this range for pure samples [10]. Thus, this observation is an indirect confirmation for the appearance of mobility in CCl 4 molecules in the temperature range for the existence of a supercooled liquid, which is analogous with previous studies [10].
More detailed results are possible with structural studies and X-ray analysis, which can provide sufficient information and confirm or refute the proposed authors assumptions.

Discussion and Conclusions
The studies conducted on the cryomatrix isolation of a mixture of gases and the properties of thin cryo-vacuum condensate films confirm the thesis that, together with physical cryo-vacuum deposition, these methods can effectively obtain Fig. 7 Comparison of the vibrational spectra of carbon tetrachloride in an argon matrix at a certain concentration of a mixture of gases (1-99%) information about the structural states in cryo-condensates of gases. The mobility of small molecules, which is an obstacle in the formation of amorphous states, is sufficiently compensated by the low temperature of the substrate, limiting their lifetime in the adsorbed layer. In this case, the small thickness of the formed films ensures that heat is efficiently removed from the interface to the substrate surface, which prevents the condensation surface from heating due to the phase transition heat. Thus, the role of the condensation temperature was determined, which was also confirmed by the results presented in our article.
Increasing the temperature of the cryocondensate of the mixture caused a shift in the absorption bands, which we attribute to structural transformations in the samples. Thus, based on the obtained thermograms, we assume that with an increase in temperature, the structure of the carbon tetrachloride film is rearranged. The values of the thermal desorption curves for the argon yield with 40 K and 90 K temperature ranges are presented, and we interpret the results so that the temperature value is T = 40 K; this interval corresponds to the tabular values for the substance. However, the temperature interval T = 90 K can be defined as an increase in mobility in carbon tetrachloride molecules at a structural transition to the state of supercooled liquid. The data are in good agreement with previous work. [10] Thermally stimulated transformations at observation frequencies of 780 cm −1 and 800 cm −1 were studied. Based on the data obtained, CCl 4 cryocondensates in this temperature range are capable of cryocapturing molecules of other gases; thus, it is concluded that CCl 4 cryocondensates can be used as a matrix element when studying the formation of molecular compounds at low temperatures and high vacuum. The results obtained show that when conducting low-temperature studies of self-organization processes in thin films, carbon tetrachloride can be used as a matrix. Publisher's Note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Springer Nature or its licensor (e.g. a society or other partner) holds exclusive rights to this article under a publishing agreement with the author(s) or other rightsholder(s); author self-archiving of the accepted manuscript version of this article is solely governed by the terms of such publishing agreement and applicable law.