# FTIR and Raman Spectroscopy Study of Soot Deposits Produced in the Infrared Multiphoton Dissociation of Vinyl Bromide

Authors: B. Samoudi; O. Bendaou; I. Hanafi; A. Asselman; K. Haboubi
Journal: Journal of Spectroscopy
 (2022)

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## Abstract

Even at low concentrations, poly-aromatic hydrocarbons found in soot have substantial health implications. Soot deposits have been reported and studied using FTIR and Raman spectroscopy. Using a CO2 photolysis laser, the samples were obtained via infrared multiphoton dissociation (IRMPD) of vinyl bromide (VBr, C2H3Br) molecules. The solid deposit formed in the IRMPD of VBr when a relatively high fluence of the order of 204 J.cm−2 was analyzed by FTIR, and it was discovered that the majority of its composition is aromatic and aliphatic hydrocarbons. Significant fullerene-type carbonaceous soot particles are also found, which could correspond to C60 and C70 or other carbonaceous agglomerates of a higher order; however, the disappearance of this fullerene on Raman spectra cast doubt on this explanation. Our samples’ Raman spectroscopy has been compared to Tamor and Vassell’s research, which may indicate that they have a lesser degree of hardness and density than these authors’ results, indicating a larger hydrogen content in our samples. The optical gap has been calculated, yielding a very limited range of values ranging only between 1.0 and 1.2 eV, resulting in a crystalline size of 0.58 to 1.12 nm.


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## Body

## 1. Introduction

Soot, also known as black carbon, is a significant substance produced by the incomplete combustion of hydrocarbons, with a large body of research devoted to its generation, characterization, characteristics, and effects on the environment and human health [1]. Soot can be found in the atmosphere, oceans, soil, sediments, and ice sheets, among other places [2]. In terms of climate change, current estimates imply that black carbon, after CO2, is the second most significant contributor to global warming [1]. From a health aspect, black carbon particles have been shown to have negative impacts on human health and have a considerable impact on human life expectancy [3–6]. Vehicle motors [7], biomass burners [8], and hydrocarbon fuels [9] are only a few of the many sources of soot.Understanding the morphological and chemical properties of soot is critical for improving particle soot emission reduction techniques. Spectroscopy (i.e., Raman spectroscopy [10–12, Fourier-transform infrared spectroscopy [12–14], UV-vis spectroscopy [15, 16], X-ray photoelectron spectroscopy [12, 17, 18], and electron energy loss spectroscopy [19, 20]) is often used to characterize the chemical features of soot, while the microscopy (i.e., scanning probe microscopy [21–23], transmission electron microscopy with high resolution [20, 24], transmission electron microscopy with high resolution [25–27], SEM [28, 29], and helium ion microscopy [30, 31]) is used for morphological properties studies. Microscopy creates pictures of the spatial distribution of density, composition, or morphology, whereas spectroscopy determines the chemical composition and physical structures of soot. The most appropriate approach is determined by the parameter of interest and the stage of soot production in question.Soot samples collected from flames and combustion have been characterized in previous studies. They looked through a guideline for choosing between several microscopy and spectroscopic techniques for analyzing the characteristics of soot at various phases of its development (incipient, young, and mature soot). However, depending on the combination of investigated attributes, amount of soot maturity, and characterization approach is chosen, accuracy, quantification, and time and cost may differ [32].Recent investigations were conducted to resolve the chemical and physical aspects of soot categorized by aerodynamic diameter using Raman spectroscopy and TEM analysis [11]. From the tiny to the big aerodynamic diameter, the Raman spectra revealed a constant change. The total (unclassified) soot samples produced by an inverted burner employing ethylene burning in bottled air have spectra that are comparable to the middle stages, which makes sense given that the middle stages capture the majority of the aerosol mass. The Raman spectra are fitted to a five-band model in decreasing ratios as the size increases. This indicates that the bigger particles have more organized graphitic domains and are larger.Popovicheva et al. presented an intriguing study [8] to examine the environmental implications of generated soot at various fuel mixtures and operation circumstances. The scientists used microscopic methods such as scanning electron microscopy and high-resolution TEM analysis, Raman, and FTIR spectroscopic characterization to examine the soot of those particles [8]. Primary particles, such as fullerenic nuclei, are agglomerated and surrounded by a graphitic layer in this study. In addition, when the heating value of the fuel combination grew, the ratio of disordered to ordered carbon dropped.Kazemimanesh et al. [9] published another study that used the same characterization methodologies to describe the morphological, physical, and nanostructure soot particles generated by a buoyant turbulent diffusion flame. The scientists found that when the fuel mix varied from light to medium to heavy, the particle median diameter and concentration rose. The morphological characterization was also unaffected by fuel type, flow rate, or flare size. The D1/G ratio of soot particles dropped as the fuel mixture composition varied from light to heavy, showing that heavier fuels had higher graphitic material.Other research employing Raman spectroscopy and TEM analysis looks at the size and shape of soot formed from a big, buoyant turbulent diffusion flame situated at the size of soot from gas flares as a function of fuel and water addition. Fuels with a higher HHVv form bigger aggregates in greater amounts on average than fuels with a lower HHVv, suggesting a graphitic structure [33].Several studies showing the positive impact on the environment and potential benefits of soot aerosols as a reinforcing agent in the polymer and automotive industries. A method for making feasible multifunctional nanocomposites with carbon-based, metallic, or ceramic nanoparticles or nanofibers that considers both wettability and conductivity in the air has been reported [34]. Similarly, a recent review paper [35] investigated for the first time-optimal approaches for postsynthesis reuse of carbonaceous aerosols to develop superhydrophobic coatings for various industrial uses. Furthermore, the focus of this research has shifted to recyclability procedures and the practical implications of any sort of soot that is extremely water-repellent.To our knowledge, no deposits of soot have been documented as a result of the infrared multiphoton dissociation (IRMPD) procedure of vinyl bromide. The goal of this investigation is to see if FTIR and Raman spectroscopy can be used to investigate the black soot created during the VBr IRMPD process. First, we will go through the results of IR spectroscopy on soot deposition. To determine the physical parameters of our samples, we compared them to data gathered from Tamor and Vassell’s earlier work [36].


## 2. Materials and Methods

### 2.1. IRMPD Process Using a CO2 Photolysis Laser

IRMPD is an important process in physics and chemistry, in addition to being an elementary process of radiation-matter interaction. The energy of the electromagnetic field absorbed by the molecule in the ground state through an active mode is transformed into its internal energy, and if this energy exceeds the vibrational potential barrier, overcoming the anharmonicity of molecules, dissociation or photofragmentation can occur [37]. After irradiating a vibrational absorption band with a suitably powerful pulsed laser beam, this procedure is simple; however, several (10 to 40) infrared photons are required in most circumstances [38]. Although large levels of vibrational excitation can be seen in both the generated species and the parent molecule, this energy is usually insufficient to fill higher electronic states of photofragments [39]. However, electronic excitation of the products can occur in some situations, resulting in UV or visible emission. Unlike the IRMPD process, ultraviolet multiphoton dissociation occurs primarily in the excited electronic state, with the absorption of 1 to 3 photons causing electronic transitions that lead to molecular photodissociation [40].IRMPD in polyatomic molecules is one of the more widespread approaches for initiating unimolecular reactions from the ground state and acquiring free radicals and fragments of interest in kinetic investigations [38]. In the high-temperature chemistry of hydrocarbons, elementary processes involving tiny carbon species such as C2 and C3 play an essential role, as shown in the production and decay of the newly found C60 fullerene [41, 42]. C2 is created by IRMPD of a range of polyatomic compounds [43, 44], and vinyl halides are one of them [45, 46], using easy laser-induced chemistry. IRMPD of VBr using the 10P(24) line of a TEA CO2 laser has previously produced C2H2, HBr, and noncondensable gas, thought to be hydrogen, as final stable products [39, 47]. When a sharply concentrated beam was utilized, generally with a 10 cm focal length lens, more C4H2 and black carbon soot was produced. In each experiment, the 24 cm lens produced no solid products. Another paper [40] provided a time-resolved analysis of the IR fluorescence emissions caused by IRMPD of VBr under various experimental settings. Three species have been investigated: the parent VBr, the generated HBr†, and C2H2†, where † is the vibrational energy [39].Figure1 shows the experimental setup used for the IRMPD procedure of the VBr samples, which has been largely detailed in earlier publications [39, 47]. Only a basic explanation of the experimental setup will be offered in this section. The CO2 photolysis laser is a Lumonics model K-101 transverse excitation atmospheric (TEA) laser with frontal Ge multimode optics (35% reflection) and a 135 lines/mm rear diffraction grating blazed at 10.6 μm. The irradiation is done with the 10P(24) line at 940.56 cm−1, which is roughly resonant with the VBr molecule’s ν5 torsion mode. A 16-A spectrum analyzer is used to examine the wavelength (Optical Eng. Co.). In all of studies, the ratio of CO2:N2:He was set to 8:8:84. The temporal form of the TEA-CO2 laser pulse in this setting (see Figure 1) was a pronounced spike of full width at half maximum (FWHM) 64 ns followed by a long-lasting tail of lower energy, which was tracked shot by shot with a Rofin Sinar 7415 photon drag detector. A Lumonics 20 D with an area of 18 cm2 and an energy detection threshold of 5 mJ was used to monitor the CO2 laser’s energy in front of the lens. The pyrometer’s calibration error is 5%, and the fluctuation of its response on the surface is a maximum of 5% when it comes to the one in the center. A pyroelectric array detector Delta Development Mark IV, which consists of a linear array of 60 pyroelectric detectors with a total length of 26.2 mm and a resolution of 0.4375 mm, was used to measure the focused radius of the laser beam at the target point. (7.5 0.6) × 10−3 cm2 was the measured focused-spot area. The greatest laser energy observed was (1,530 ± 76) mJ, resulting in fluence of (204 ± 6) J.cm−2.Figure 1
Experimental arrangements employed for the IRMPD of the VBr samples for inducing black soot.


### 2.2. FTIR Spectroscopy

The absorption of IR photons by molecules is the basis of infrared (IR) spectroscopy. The process of absorption can occur when the energy of the IR photon to be absorbed corresponds with the difference in energy between two levels of molecule rovibrational energy. The frequency-domain spectrum of the resultant interferogram is then Fourier transformed, giving birth to the term Fourier-transform infrared (FTIR) spectroscopy [48].An infrared spectrophotometer, Perkin-Elmer model 1725X, was used to record FTIR transmission spectra in all experiments. This device uses a ceramic incandescent source that is heated to 1400°C. A DTGS (deuterated triglycine sulfate with quick recovery) detector with a cesium iodide window was used. The beam splitter is made of KBr and is multicoated. It spans the spectrum range of 400 cm−1 to 10,000 cm−1, yet it is only practical up to 600 cm−1 in these instances owing to the NaCl optics’ transmittance. The spectral resolution was 2 cm−1, and each sample yielded 100 scans, resulting in an excellent signal-to-noise ratio. There was no substantial absorption over 4,000 cm−1 in any of the experiments.The traditional KBr pellet approach is used in this FTIR investigation. Both the samples and the KBr were dried in a vacuum before analysis to reduce the influence of moisture on the spectra. The transmission signal associated with the clean KBr pellet was removed from each spectrum to get the spectrum of the simple.


### 2.3. Raman Spectroscopy

Raman scattering has long been acknowledged as a potent spectroscopic tool for determining the degree of structural organization of carbonaceous matter, whether it is a natural substance or an organic material that has been intentionally generated (graphitized) in experimental settings [36, 49–56]. Carbon-carbon bonding may occur in a variety of ways to produce molecular and crystalline structures, which is exceptional when compared to other elements in the periodic table.Carbon films, both amorphous and nanocrystalline, may exist in a variety of matrices and are frequently doped with a considerable quantity of hydrogen, making the materials even more complex. Hybridizations of sp3 and sp2 are the most prevalent chemical linkages in amorphous and nanocrystalline carbon. A carbon atom in the sp3 configuration produces four sp3 orbitals, forming a strong connection with the next atom [57]. Some Raman spectral features of several forms of amorphous carbon have been linked to deposition circumstances and macroscopic qualities [58–60].Tamor and Vassell investigated a variety of amorphous carbonaceous materials from various sources and manufactured them using various procedures [36]. They correlated the adjustment parameters with the physical attributes of the samples, such as hydrogen concentration, optical gap, hardness, and density, by fitting the Raman spectra to two types of Gaussian lines. The analysis of this database indicates several relationships that may be utilized to predict the qualities of other films. Furthermore, despite their phenomenological character, these correlations appear to apply to all amorphous carbon films and hence may be beneficial to all carbonaceous material researchers.A Renishaw inVia Raman equipment with an electrically cooled CCD camera and a Leica DM 2500 microscope was used to detect the Raman spectra. The equipment was fitted with laser sources, allowing it to be used. As an excitation source, a diode laser (HPNIR 785 model, Renishaw) with a wavelength of 785 nm was utilized, producing power of 2 mW. Raman scattering was filtered with a holographic notch filter system to minimize Rayleigh's elastic dispersion of photons with appropriate diffraction grids and then captured with a 400 × 575 pixel electrically air-cooled CCD detector.—Two wavenumber was chosen as the resolution. Without any additional preparation, samples placed on glass and put into VBr photolysis cells were examined immediately. Each time, a 10 s scan was taken.All of the Raman spectra were fitted with two Gaussians to determine the locations and intensities of the G and D peaks. The criteria we employ here are the same as those applied elsewhere [36]: the breadth of a Gaussian curve, σ. The FWHM (full width at half maximum) is around 2.30 σ. Because it gives good fits for all the soot deposits, these two Gaussian fits properly duplicate the data and more complicated line shape functions. This might be backed up by scientific evidence.The acquired spectra should be deconvoluted to identify the precise placements, bandwidths, and relative intensities. The Raman spectra were processed using the built-in peak fitting analysis capabilities in OriginPro 7.5 software (OriginLab Corporation, USA).


## 2.1. IRMPD Process Using a CO2 Photolysis Laser

IRMPD is an important process in physics and chemistry, in addition to being an elementary process of radiation-matter interaction. The energy of the electromagnetic field absorbed by the molecule in the ground state through an active mode is transformed into its internal energy, and if this energy exceeds the vibrational potential barrier, overcoming the anharmonicity of molecules, dissociation or photofragmentation can occur [37]. After irradiating a vibrational absorption band with a suitably powerful pulsed laser beam, this procedure is simple; however, several (10 to 40) infrared photons are required in most circumstances [38]. Although large levels of vibrational excitation can be seen in both the generated species and the parent molecule, this energy is usually insufficient to fill higher electronic states of photofragments [39]. However, electronic excitation of the products can occur in some situations, resulting in UV or visible emission. Unlike the IRMPD process, ultraviolet multiphoton dissociation occurs primarily in the excited electronic state, with the absorption of 1 to 3 photons causing electronic transitions that lead to molecular photodissociation [40].IRMPD in polyatomic molecules is one of the more widespread approaches for initiating unimolecular reactions from the ground state and acquiring free radicals and fragments of interest in kinetic investigations [38]. In the high-temperature chemistry of hydrocarbons, elementary processes involving tiny carbon species such as C2 and C3 play an essential role, as shown in the production and decay of the newly found C60 fullerene [41, 42]. C2 is created by IRMPD of a range of polyatomic compounds [43, 44], and vinyl halides are one of them [45, 46], using easy laser-induced chemistry. IRMPD of VBr using the 10P(24) line of a TEA CO2 laser has previously produced C2H2, HBr, and noncondensable gas, thought to be hydrogen, as final stable products [39, 47]. When a sharply concentrated beam was utilized, generally with a 10 cm focal length lens, more C4H2 and black carbon soot was produced. In each experiment, the 24 cm lens produced no solid products. Another paper [40] provided a time-resolved analysis of the IR fluorescence emissions caused by IRMPD of VBr under various experimental settings. Three species have been investigated: the parent VBr, the generated HBr†, and C2H2†, where † is the vibrational energy [39].Figure1 shows the experimental setup used for the IRMPD procedure of the VBr samples, which has been largely detailed in earlier publications [39, 47]. Only a basic explanation of the experimental setup will be offered in this section. The CO2 photolysis laser is a Lumonics model K-101 transverse excitation atmospheric (TEA) laser with frontal Ge multimode optics (35% reflection) and a 135 lines/mm rear diffraction grating blazed at 10.6 μm. The irradiation is done with the 10P(24) line at 940.56 cm−1, which is roughly resonant with the VBr molecule’s ν5 torsion mode. A 16-A spectrum analyzer is used to examine the wavelength (Optical Eng. Co.). In all of studies, the ratio of CO2:N2:He was set to 8:8:84. The temporal form of the TEA-CO2 laser pulse in this setting (see Figure 1) was a pronounced spike of full width at half maximum (FWHM) 64 ns followed by a long-lasting tail of lower energy, which was tracked shot by shot with a Rofin Sinar 7415 photon drag detector. A Lumonics 20 D with an area of 18 cm2 and an energy detection threshold of 5 mJ was used to monitor the CO2 laser’s energy in front of the lens. The pyrometer’s calibration error is 5%, and the fluctuation of its response on the surface is a maximum of 5% when it comes to the one in the center. A pyroelectric array detector Delta Development Mark IV, which consists of a linear array of 60 pyroelectric detectors with a total length of 26.2 mm and a resolution of 0.4375 mm, was used to measure the focused radius of the laser beam at the target point. (7.5 0.6) × 10−3 cm2 was the measured focused-spot area. The greatest laser energy observed was (1,530 ± 76) mJ, resulting in fluence of (204 ± 6) J.cm−2.Figure 1
Experimental arrangements employed for the IRMPD of the VBr samples for inducing black soot.


## 2.2. FTIR Spectroscopy

The absorption of IR photons by molecules is the basis of infrared (IR) spectroscopy. The process of absorption can occur when the energy of the IR photon to be absorbed corresponds with the difference in energy between two levels of molecule rovibrational energy. The frequency-domain spectrum of the resultant interferogram is then Fourier transformed, giving birth to the term Fourier-transform infrared (FTIR) spectroscopy [48].An infrared spectrophotometer, Perkin-Elmer model 1725X, was used to record FTIR transmission spectra in all experiments. This device uses a ceramic incandescent source that is heated to 1400°C. A DTGS (deuterated triglycine sulfate with quick recovery) detector with a cesium iodide window was used. The beam splitter is made of KBr and is multicoated. It spans the spectrum range of 400 cm−1 to 10,000 cm−1, yet it is only practical up to 600 cm−1 in these instances owing to the NaCl optics’ transmittance. The spectral resolution was 2 cm−1, and each sample yielded 100 scans, resulting in an excellent signal-to-noise ratio. There was no substantial absorption over 4,000 cm−1 in any of the experiments.The traditional KBr pellet approach is used in this FTIR investigation. Both the samples and the KBr were dried in a vacuum before analysis to reduce the influence of moisture on the spectra. The transmission signal associated with the clean KBr pellet was removed from each spectrum to get the spectrum of the simple.


## 2.3. Raman Spectroscopy

Raman scattering has long been acknowledged as a potent spectroscopic tool for determining the degree of structural organization of carbonaceous matter, whether it is a natural substance or an organic material that has been intentionally generated (graphitized) in experimental settings [36, 49–56]. Carbon-carbon bonding may occur in a variety of ways to produce molecular and crystalline structures, which is exceptional when compared to other elements in the periodic table.Carbon films, both amorphous and nanocrystalline, may exist in a variety of matrices and are frequently doped with a considerable quantity of hydrogen, making the materials even more complex. Hybridizations of sp3 and sp2 are the most prevalent chemical linkages in amorphous and nanocrystalline carbon. A carbon atom in the sp3 configuration produces four sp3 orbitals, forming a strong connection with the next atom [57]. Some Raman spectral features of several forms of amorphous carbon have been linked to deposition circumstances and macroscopic qualities [58–60].Tamor and Vassell investigated a variety of amorphous carbonaceous materials from various sources and manufactured them using various procedures [36]. They correlated the adjustment parameters with the physical attributes of the samples, such as hydrogen concentration, optical gap, hardness, and density, by fitting the Raman spectra to two types of Gaussian lines. The analysis of this database indicates several relationships that may be utilized to predict the qualities of other films. Furthermore, despite their phenomenological character, these correlations appear to apply to all amorphous carbon films and hence may be beneficial to all carbonaceous material researchers.A Renishaw inVia Raman equipment with an electrically cooled CCD camera and a Leica DM 2500 microscope was used to detect the Raman spectra. The equipment was fitted with laser sources, allowing it to be used. As an excitation source, a diode laser (HPNIR 785 model, Renishaw) with a wavelength of 785 nm was utilized, producing power of 2 mW. Raman scattering was filtered with a holographic notch filter system to minimize Rayleigh's elastic dispersion of photons with appropriate diffraction grids and then captured with a 400 × 575 pixel electrically air-cooled CCD detector.—Two wavenumber was chosen as the resolution. Without any additional preparation, samples placed on glass and put into VBr photolysis cells were examined immediately. Each time, a 10 s scan was taken.All of the Raman spectra were fitted with two Gaussians to determine the locations and intensities of the G and D peaks. The criteria we employ here are the same as those applied elsewhere [36]: the breadth of a Gaussian curve, σ. The FWHM (full width at half maximum) is around 2.30 σ. Because it gives good fits for all the soot deposits, these two Gaussian fits properly duplicate the data and more complicated line shape functions. This might be backed up by scientific evidence.The acquired spectra should be deconvoluted to identify the precise placements, bandwidths, and relative intensities. The Raman spectra were processed using the built-in peak fitting analysis capabilities in OriginPro 7.5 software (OriginLab Corporation, USA).


## 3. Results and Discussion

### 3.1. FTIR Spectroscopy Study of the Soot Deposits

Figure2 shows the 2.5 hPa FTIR spectrum of a BrV pure simple in a gaseous state, recorded between 600 cm−1 and 4,000 cm−1 before and after being bombarded with more than 600 infrared pulses (Figures 2(a) and 2(b)), respectively. Along with the decrease in the bands belonging to the BrV, additional bands appear with the original spectrum, showing that the BrV has decoupled, as can be seen in the spectrum obtained after the irradiation. The bands at 730 cm−1 and 3,270 cm−1, attributed to the torsional vibrations of the CH bond of the five modes and the three asymmetric CH stretch modes, respectively, have been recognized as belonging to acetylene (C2H2) [61–63].Figure 2
(a) FTIR spectrum of a 2.5 hPa of VBr pure sample in a gaseous state, recorded between 600 cm−1 and 4,000 cm−1. (b) FTIR spectrum after being irradiated with more than 600 infrared pulses.
(a)(b)The transitions of vibrationally excited hydrogen bromide HBr (1–3) have been attributed to the bands occurring between 2,400 cm–1 and 2,700 cm−1 [61, 62, 64]. Furthermore, a solid deposit has been seen clinging to the cell walls and windows. Bands at 1,065 cm–1 and 1,165 cm−1 may be found in Figure 2(b) of this deposit. There are no solid deposits when a small fluence is utilized. Table 1 summarizes the detection of gaseous end products and the characterization of soot deposits.Table 1
Identification of products obtained from IRMPD of VBr.
SampleTechnique usedProductsBands (cm−1)VBrFTIRC2H2730, 3,270FTIRHBr2,400–2,700FTIRC2H43,329FTIRH2—Soot depositsFTIRSolid deposit1,065, 1,165FTIRC60527FTIRC70795, 1,134, 1,460Lussier and Steinfeld [65] achieved similar results in the IRMPD of vinyl chloride under identical circumstances to ours. They identify stable end products C2H2, HCl, C4H2, H2, and a solid deposit when the laser energy is highly concentrated in this situation. When the sample is irradiated with a reduced fluence, the only products generated are C2H2 and HCl, according to Lussier’s team. Figure 3 shows the FTIR spectrum of a deposit solid that was observed clinging to cell walls and windows. The vibrational transitions of numerous bonds present in the structure create peaks in the IR spectra. Table 2 shows some typical band locations in poly-aromatic hydrocarbon molecules and soot [8, 13, 14, 32, 66] due to certain chemical functionalities. At various frequencies, the spectrum reveals multiple unique absorption peaks. Because of the amount of material combined into the pellet, the bands in the figure have a substantial intensity. The absorption peaks in this spectrum (Figure 3) correspond to the C–H aliphatic functional groups and have wavenumbers of 2,850 cm−1, 2,925 cm−1, and 2,960 cm−1. The peak corresponding to the wavenumber 2,925 cm−1 is the most intense of these peaks. Similar behavior has been documented in previous soot experiments [13, 14, 66–68]. The spectrum also presents many absorption peaks at 1,390 cm−1 and 1,460 cm−1, showing the presence of a C–H aliphatic bond, which is caused by scissoring and bending deformation in CH3 and CH2 [13, 14]. The peak origin at 1,600 cm−1 could be due to aromatic C=C functionalities. Literature [13, 32] attributes the 1,720 cm−1 wavenumber range to the carbonyl C=O stretch. A large vibration band in the range of 3,200–3,600 cm−1 is also visible in the FTIR spectra [68]. The stretching vibration of the OH– functional group present in the sample is responsible for the peak (3,435 cm−1) at this position. The existence of this peak, on the other hand, may be attributable to a water molecule absorbed by the KBr salt during sample preparation or spectral acquisition. The existence of tiny (sp2 or sp3) aromatic clusters of amorphous carbon soot is indicated by certain weak bands found at low wavenumbers 1,048 cm−1 and 669 cm−1 [69].Figure 3
FTIR spectrum of soot deposits obtained in the IRMPD of 2.5 hPa of VBr.Table 2
IR absorption wavenumber of soot and polyaromatic hydrocarbons summarized from the literature [8, 13, 14, 66].
Main peak wavenumber (cm−1)Chemical functional group to which assigned (cm−1)1,000–700Aromatic C=C–H out of plane deformation1,300–1,000Aromatic C–C and C–H plane deformation and ether C–O–C stretching1,460Aromatic C=C stretchingAliphatic C–H bending and scissoring of CH3 and CH21,600Aromatic C=C stretching1,720Carbonyl C=O stretching2,850, 2,925, and 2,960Asymmetric and symmetric C–H stretching of CH3 and CH2 aliphatic functional groups3,039Aromatic compounds C=C–H stretching3,435Water O−H stretch3,000–3,600Hydroxyl O=H groupWe can also see some weak bands and shoulders, which we believe are caused by C60 and C70 fullerenes [70] or higher-order carbonaceous agglomerates. This identification was made feasible by comparing the spectra of the soot deposits to that of a fullerene combination synthesized at the Rocasolano Institute of Physical Chemistry (Madrid, Spain) and illustrated in Figure 4. Previously, fullerene-type carbonaceous structures were discovered in pyrolysis experiments using IR lasers pulsed in acetylene, ethene, and benzene [71–73]. These findings show that the high temperatures obtained in pulsed IR pyrolysis (>2,000 K) promote the production of curved carbonaceous structures that can evolve into fullerenes, rather than the more prevalent flat graphene sheets generated at lower temperatures (1,500 K–1,800 K).Figure 4
(a) Spectrum of the soot deposits formed after irradiating a 10 hPa of VBr pure sample with the CO2 laser. φ = 204 J·cm−2. (b) Spectrum of a synthetic fullerene sample.
(a)(b)We conducted multiple tests with Raman spectroscopy analysis to gain a better understanding of the generation of fullerene in the IRMPD process. The absence of C60 and C70 fullerenes in our samples’ Raman spectra cast doubt on this explanation.


### 3.2. Raman Spectroscopy Study of the Soot Deposits

The Raman spectra of all solid deposits obtained from the IRMPD of VBr include the characteristic two primary features: the G band at 1,550 cm−1 and the D band at around 1,350 cm−1 (Figure 5). The physical origins of these two Raman modes are described in detail in [9, 74]: the sp2 sites are responsible for the G and D peaks. The bond stretching of all pairings of sp2 atoms in both rings and chains causes the G peak. The D peak is caused by defects along graphitic edge planes and is caused by the breathing modes of sp2 atoms in rings. There have been no bands observed in the second-order region of the spectrum, between 2,000 cm−1 and 3,600 cm−1, which is dominated by a modulated bump formed of overtone and combination bands [75], nor around 600 cm−1, where broadband has been detected in hydrogen-free carbon films [68, 76]. We get values for the G band and its associated breadth that are comparable to those reported by various studies [10, 36, 74, 75, 77], even though they are concentrated in the higher frequency area. These findings, together with the lack of a band at 600 cm−1, indicate that our deposits are hydrogenated. Our results for the D band, on the other hand, show a lower frequency location than those reported by these writers. It is worth noting that their Raman spectra were obtained using an argon laser at 514.5 nm, whereas ours were obtained using a diode laser at 785 nm. We validated that the identical spectra are produced at 785 nm using an argon laser at 514.5 nm; however, in the first case, a significantly stronger fluorescence background emerges, for which we offer the data obtained at 785 nm. The parameters obtained for our samples, including the results of these authors for the hydrogenated samples, are shown in Figures 6(a) and 6(b).Figure 5
Typical Raman spectra of the soot deposits obtained in the IRMPD process of VBr.Figure 6
(a) Position of D band versus position of G band. (b) G bandwidth versus G band position;, this work; •, work from Tamor and Vassell [36].
(a)(b)Tamor and Vassell [36] discovered relationships between the width of the G band and macroscopic deposit characteristics including hardness and density. We will be able to compare our results with theirs and estimate the hardness, density, and optical gap of our samples since the placements of the G band in our samples, as well as their associated widths, are comparable to those acquired by these authors. Figures 7(a)–7(c) show the findings obtained.Figure 7
(a) Hardness of soot deposits as a function of G bandwidth. (b) Density of soot deposits as a function of G bandwidth. (c) Optical gap of soot deposits as a function of the Raman G band position;, this work; •, work from Tamor and Vassell [36].
(a)(b)(c)Hardness is the most important attribute for protective coatings in tribological applications. The resistance to the penetration of a hard counterbody is defined by the ratio of applied force to the indentation area. The peak position shift of G band is associated with an increase in sp3 concentration for an a-C:H and an increase in hardness, according to Ferrari and Robertson [74]. In addition, a rise in the full width at half maximum (FWHM) of G band suggests a decrease in the in-plane correlation length (La, commented later) and an increase in carbon disorder, both of which are signs of the nucleation of tiny sp3 sites formed from sp2 bonds [74].As can be observed, our samples had a lesser degree of hardness (i.e., up to 7 GPa) and density (i.e., ranging between 1.4 g.cm−3 and 2.0 g.cm−3) than the ones tested by Tamor and Vassell [36], which might indicate a larger hydrogen concentration in our samples based on their findings.The optical gap has been estimated (Figure7(c)), obtaining a very narrow range of values varying only between 1.0 eV and 1.2 eV. So the lower bandgap can be interpreted; as the overall hydrogen content decreases, the π-bonded clusters increase in size and modify their shape. The dependence of the optical gap on the hydrogen content is explained by invoking a more pronounced sp2 character in the less-hydrogenated films. Hence, higher hydrogen contents are associated with larger gaps. In this trend, many studies of hydrogenated diamond-like carbon comparable with our gap values (i.e., 1.0–2.0 eV) using a Raman spectroscopy estimated an H content (20%–40%) and confirm this purpose [78, 79].The maximum intensities ratio (ID/IG) has long been used as a useful indicator of crystal structure disorder in soot samples, and it is related to lattice width (La). Tuinstra and Koenig [56] were the first to show a linear connection between the relationship of intensities given by the areas of bands D and G, ID/IG, of the Raman spectra and the effective crystallite size, La, in the graphite plane direction. The proportionality constant is dependent on the wavelength laser, La, employed to acquire the Raman spectra [80–82]. This dependency is linear in the range 400nm<λL<700nm. Various phrases have been given in the literature to estimate La as follows:(1)La2nm=Bλ.IDIG,with Bλ=1.82 for laser excitation energy, El=2.41eV, used in Raman experiment has been proposed by Ferrari and Robertson [83]. Cançado et al. [84] utilized a generic equation to demonstrate ID/IG.EL4 is proportional to1/La when using Raman spectroscopy to determine the crystallite size La of nanographite. Other research [20, 74, 75, 84] has lately provided a clear explanation for this pattern. The ratio between the integrated intensities of the disorder-induced D and G Raman bands, ID/IG, in nanographite samples with different crystallite sizes La and using different excitation laser energies has been studied in detail, resulting in a general equation for determining La using any visible laser energy. These results reveal that ID/IG is inversely proportional to the fourth power of the laser energy utilized in the experiment, indicating that the linearity in the visible region is confirmed. Our samples’ ID/IG values (Figure 8) are in the range of 1.0 to 3.7. If we use the known proportionality constant for excitation laser wavelengths of 785 nm as an example, the crystalline size, La, for them varies between 0.58 nm and 1.12 nm, depending on the following formula [75, 85]:(2)La2nm2=5.4.10−2.El4eV4.IDIG.Figure 8
The ratio of the integrated areas of the Raman D and G bands (ID/IG) as a function of the G band position; , this work; , work from Tamor and Vassell [36].


## 3.1. FTIR Spectroscopy Study of the Soot Deposits

Figure2 shows the 2.5 hPa FTIR spectrum of a BrV pure simple in a gaseous state, recorded between 600 cm−1 and 4,000 cm−1 before and after being bombarded with more than 600 infrared pulses (Figures 2(a) and 2(b)), respectively. Along with the decrease in the bands belonging to the BrV, additional bands appear with the original spectrum, showing that the BrV has decoupled, as can be seen in the spectrum obtained after the irradiation. The bands at 730 cm−1 and 3,270 cm−1, attributed to the torsional vibrations of the CH bond of the five modes and the three asymmetric CH stretch modes, respectively, have been recognized as belonging to acetylene (C2H2) [61–63].Figure 2
(a) FTIR spectrum of a 2.5 hPa of VBr pure sample in a gaseous state, recorded between 600 cm−1 and 4,000 cm−1. (b) FTIR spectrum after being irradiated with more than 600 infrared pulses.
(a)(b)The transitions of vibrationally excited hydrogen bromide HBr (1–3) have been attributed to the bands occurring between 2,400 cm–1 and 2,700 cm−1 [61, 62, 64]. Furthermore, a solid deposit has been seen clinging to the cell walls and windows. Bands at 1,065 cm–1 and 1,165 cm−1 may be found in Figure 2(b) of this deposit. There are no solid deposits when a small fluence is utilized. Table 1 summarizes the detection of gaseous end products and the characterization of soot deposits.Table 1
Identification of products obtained from IRMPD of VBr.
SampleTechnique usedProductsBands (cm−1)VBrFTIRC2H2730, 3,270FTIRHBr2,400–2,700FTIRC2H43,329FTIRH2—Soot depositsFTIRSolid deposit1,065, 1,165FTIRC60527FTIRC70795, 1,134, 1,460Lussier and Steinfeld [65] achieved similar results in the IRMPD of vinyl chloride under identical circumstances to ours. They identify stable end products C2H2, HCl, C4H2, H2, and a solid deposit when the laser energy is highly concentrated in this situation. When the sample is irradiated with a reduced fluence, the only products generated are C2H2 and HCl, according to Lussier’s team. Figure 3 shows the FTIR spectrum of a deposit solid that was observed clinging to cell walls and windows. The vibrational transitions of numerous bonds present in the structure create peaks in the IR spectra. Table 2 shows some typical band locations in poly-aromatic hydrocarbon molecules and soot [8, 13, 14, 32, 66] due to certain chemical functionalities. At various frequencies, the spectrum reveals multiple unique absorption peaks. Because of the amount of material combined into the pellet, the bands in the figure have a substantial intensity. The absorption peaks in this spectrum (Figure 3) correspond to the C–H aliphatic functional groups and have wavenumbers of 2,850 cm−1, 2,925 cm−1, and 2,960 cm−1. The peak corresponding to the wavenumber 2,925 cm−1 is the most intense of these peaks. Similar behavior has been documented in previous soot experiments [13, 14, 66–68]. The spectrum also presents many absorption peaks at 1,390 cm−1 and 1,460 cm−1, showing the presence of a C–H aliphatic bond, which is caused by scissoring and bending deformation in CH3 and CH2 [13, 14]. The peak origin at 1,600 cm−1 could be due to aromatic C=C functionalities. Literature [13, 32] attributes the 1,720 cm−1 wavenumber range to the carbonyl C=O stretch. A large vibration band in the range of 3,200–3,600 cm−1 is also visible in the FTIR spectra [68]. The stretching vibration of the OH– functional group present in the sample is responsible for the peak (3,435 cm−1) at this position. The existence of this peak, on the other hand, may be attributable to a water molecule absorbed by the KBr salt during sample preparation or spectral acquisition. The existence of tiny (sp2 or sp3) aromatic clusters of amorphous carbon soot is indicated by certain weak bands found at low wavenumbers 1,048 cm−1 and 669 cm−1 [69].Figure 3
FTIR spectrum of soot deposits obtained in the IRMPD of 2.5 hPa of VBr.Table 2
IR absorption wavenumber of soot and polyaromatic hydrocarbons summarized from the literature [8, 13, 14, 66].
Main peak wavenumber (cm−1)Chemical functional group to which assigned (cm−1)1,000–700Aromatic C=C–H out of plane deformation1,300–1,000Aromatic C–C and C–H plane deformation and ether C–O–C stretching1,460Aromatic C=C stretchingAliphatic C–H bending and scissoring of CH3 and CH21,600Aromatic C=C stretching1,720Carbonyl C=O stretching2,850, 2,925, and 2,960Asymmetric and symmetric C–H stretching of CH3 and CH2 aliphatic functional groups3,039Aromatic compounds C=C–H stretching3,435Water O−H stretch3,000–3,600Hydroxyl O=H groupWe can also see some weak bands and shoulders, which we believe are caused by C60 and C70 fullerenes [70] or higher-order carbonaceous agglomerates. This identification was made feasible by comparing the spectra of the soot deposits to that of a fullerene combination synthesized at the Rocasolano Institute of Physical Chemistry (Madrid, Spain) and illustrated in Figure 4. Previously, fullerene-type carbonaceous structures were discovered in pyrolysis experiments using IR lasers pulsed in acetylene, ethene, and benzene [71–73]. These findings show that the high temperatures obtained in pulsed IR pyrolysis (>2,000 K) promote the production of curved carbonaceous structures that can evolve into fullerenes, rather than the more prevalent flat graphene sheets generated at lower temperatures (1,500 K–1,800 K).Figure 4
(a) Spectrum of the soot deposits formed after irradiating a 10 hPa of VBr pure sample with the CO2 laser. φ = 204 J·cm−2. (b) Spectrum of a synthetic fullerene sample.
(a)(b)We conducted multiple tests with Raman spectroscopy analysis to gain a better understanding of the generation of fullerene in the IRMPD process. The absence of C60 and C70 fullerenes in our samples’ Raman spectra cast doubt on this explanation.


## 3.2. Raman Spectroscopy Study of the Soot Deposits

The Raman spectra of all solid deposits obtained from the IRMPD of VBr include the characteristic two primary features: the G band at 1,550 cm−1 and the D band at around 1,350 cm−1 (Figure 5). The physical origins of these two Raman modes are described in detail in [9, 74]: the sp2 sites are responsible for the G and D peaks. The bond stretching of all pairings of sp2 atoms in both rings and chains causes the G peak. The D peak is caused by defects along graphitic edge planes and is caused by the breathing modes of sp2 atoms in rings. There have been no bands observed in the second-order region of the spectrum, between 2,000 cm−1 and 3,600 cm−1, which is dominated by a modulated bump formed of overtone and combination bands [75], nor around 600 cm−1, where broadband has been detected in hydrogen-free carbon films [68, 76]. We get values for the G band and its associated breadth that are comparable to those reported by various studies [10, 36, 74, 75, 77], even though they are concentrated in the higher frequency area. These findings, together with the lack of a band at 600 cm−1, indicate that our deposits are hydrogenated. Our results for the D band, on the other hand, show a lower frequency location than those reported by these writers. It is worth noting that their Raman spectra were obtained using an argon laser at 514.5 nm, whereas ours were obtained using a diode laser at 785 nm. We validated that the identical spectra are produced at 785 nm using an argon laser at 514.5 nm; however, in the first case, a significantly stronger fluorescence background emerges, for which we offer the data obtained at 785 nm. The parameters obtained for our samples, including the results of these authors for the hydrogenated samples, are shown in Figures 6(a) and 6(b).Figure 5
Typical Raman spectra of the soot deposits obtained in the IRMPD process of VBr.Figure 6
(a) Position of D band versus position of G band. (b) G bandwidth versus G band position;, this work; •, work from Tamor and Vassell [36].
(a)(b)Tamor and Vassell [36] discovered relationships between the width of the G band and macroscopic deposit characteristics including hardness and density. We will be able to compare our results with theirs and estimate the hardness, density, and optical gap of our samples since the placements of the G band in our samples, as well as their associated widths, are comparable to those acquired by these authors. Figures 7(a)–7(c) show the findings obtained.Figure 7
(a) Hardness of soot deposits as a function of G bandwidth. (b) Density of soot deposits as a function of G bandwidth. (c) Optical gap of soot deposits as a function of the Raman G band position;, this work; •, work from Tamor and Vassell [36].
(a)(b)(c)Hardness is the most important attribute for protective coatings in tribological applications. The resistance to the penetration of a hard counterbody is defined by the ratio of applied force to the indentation area. The peak position shift of G band is associated with an increase in sp3 concentration for an a-C:H and an increase in hardness, according to Ferrari and Robertson [74]. In addition, a rise in the full width at half maximum (FWHM) of G band suggests a decrease in the in-plane correlation length (La, commented later) and an increase in carbon disorder, both of which are signs of the nucleation of tiny sp3 sites formed from sp2 bonds [74].As can be observed, our samples had a lesser degree of hardness (i.e., up to 7 GPa) and density (i.e., ranging between 1.4 g.cm−3 and 2.0 g.cm−3) than the ones tested by Tamor and Vassell [36], which might indicate a larger hydrogen concentration in our samples based on their findings.The optical gap has been estimated (Figure7(c)), obtaining a very narrow range of values varying only between 1.0 eV and 1.2 eV. So the lower bandgap can be interpreted; as the overall hydrogen content decreases, the π-bonded clusters increase in size and modify their shape. The dependence of the optical gap on the hydrogen content is explained by invoking a more pronounced sp2 character in the less-hydrogenated films. Hence, higher hydrogen contents are associated with larger gaps. In this trend, many studies of hydrogenated diamond-like carbon comparable with our gap values (i.e., 1.0–2.0 eV) using a Raman spectroscopy estimated an H content (20%–40%) and confirm this purpose [78, 79].The maximum intensities ratio (ID/IG) has long been used as a useful indicator of crystal structure disorder in soot samples, and it is related to lattice width (La). Tuinstra and Koenig [56] were the first to show a linear connection between the relationship of intensities given by the areas of bands D and G, ID/IG, of the Raman spectra and the effective crystallite size, La, in the graphite plane direction. The proportionality constant is dependent on the wavelength laser, La, employed to acquire the Raman spectra [80–82]. This dependency is linear in the range 400nm<λL<700nm. Various phrases have been given in the literature to estimate La as follows:(1)La2nm=Bλ.IDIG,with Bλ=1.82 for laser excitation energy, El=2.41eV, used in Raman experiment has been proposed by Ferrari and Robertson [83]. Cançado et al. [84] utilized a generic equation to demonstrate ID/IG.EL4 is proportional to1/La when using Raman spectroscopy to determine the crystallite size La of nanographite. Other research [20, 74, 75, 84] has lately provided a clear explanation for this pattern. The ratio between the integrated intensities of the disorder-induced D and G Raman bands, ID/IG, in nanographite samples with different crystallite sizes La and using different excitation laser energies has been studied in detail, resulting in a general equation for determining La using any visible laser energy. These results reveal that ID/IG is inversely proportional to the fourth power of the laser energy utilized in the experiment, indicating that the linearity in the visible region is confirmed. Our samples’ ID/IG values (Figure 8) are in the range of 1.0 to 3.7. If we use the known proportionality constant for excitation laser wavelengths of 785 nm as an example, the crystalline size, La, for them varies between 0.58 nm and 1.12 nm, depending on the following formula [75, 85]:(2)La2nm2=5.4.10−2.El4eV4.IDIG.Figure 8
The ratio of the integrated areas of the Raman D and G bands (ID/IG) as a function of the G band position; , this work; , work from Tamor and Vassell [36].


## 4. Conclusions

A new approach for soot formation by IRMPD of VBr was applied in this study while applying a rather high fluence of the order of 204 J.cm−2. FTIR and Raman’s spectroscopy have been used to characterize such goods. FTIR spectroscopy was used to examine the soot deposits formed in the IRMPD of VBr when a relatively high fluence of the order of 204 J.cm−2 was used. It was discovered that the majority of the soot particles were the aliphatic and aromatic groups. On the other hand, we had also found some indication that the deposited soot is fullerene-type carbonaceous soot, which could correspond to C60 and C70 or other carbonaceous agglomerates of a higher order; however, the tests realization by Raman spectroscopy make in doubt this purpose. When employing Raman spectroscopy, we discovered that they had a lesser degree of hardness and density than the samples analyzed by Tamor and Vassell, which might indicate a larger hydrogen content in our samples, based on their findings. The optical gap has been calculated, yielding a very restricted range of values ranging only between 1.0 eV and 1.2 eV, resulting in a crystalline size of 0.58 nm to 1.12 nm for them.



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