The formation of chemical compounds or individual molecules is an indispensable element of nature. It is important to know what conditions are conducive to the formation of specific chemical compounds. In the case of plasma, this requires exceeding of the ionization, excitation, or dissociation energies. They are different for each element/molecule, and these energies usually start at a few electron volts. Various species are created in this manner. It can lead to the formation of new molecular species, not existing in the initial gaseous mixture. The molecular species can be stable or transient. Their formation requires suitable plasma conditions.
Such processes can be induced by extreme ultraviolet (EUV) or soft X-ray (SXR) radiation. In normal terrestrial conditions, we do not encounter this radiation. It stops in the upper parts of the atmosphere, ionizing its components. It can lead to the escape of particles from the planet’s atmosphere or contribute to the molecular processes mentioned above. Similar processes may occur on other planets or moons with their own atmospheres [1, 2], especially taking into account the ubiquitous presence of this type of radiation in space, e.g., of stellar origin, including the Sun. The formation of molecules under such conditions can lead to more complex structures, including the foundations of life [3]. Another phenomenon that can cause ionization of both interstellar dust and the atmosphere of the celestial body is the entry of a meteor into it [4].
Many laboratory solutions can be used to obtain atomic, ionic, or molecular processes. EUV or SXR radiation can be produced in various ways. One way to get a broad spectrum from X-rays to infrared (IR) is through synchrotron systems, but it is an expensive and complex setup; another is free electron lasers (FELs), and yet another is plasma source. Plasma sources include those based on high-power discharge-produced plasma (DPP) or laser-produced plasmas (LPPs).
In this work, the LPP source was used to generate radiation in the EUV/SXR range. Such solutions have been used many times to generate this type of radiation [5]. Owing to the use of a gas target by such a source, various gas mixtures, suitably prepared in advance, can be used. They can include simple molecular gases, e.g., nitrogen, and more complex mixtures of various gases, e.g., a mixture of krypton, sulfur, fluorine, and helium. This solution allows the observation of the formation of new particles as a result of the interaction of the used gases with radiation from the LPP source.
The experimental system (Fig. 1) consists of a small vacuum chamber that can be filled with the investigated gas mixture under low pressure (1–50 mbar). The system shown in Fig. 1 can be divided into two parts: the first one designed for production of the LPPs, and the second one dedicated for the investigation of low-temperature plasmas formed around the LPP. The LPPs were formed using a neodymium-doped yttrium aluminum garnet (Nd:YAG) laser, which was focused onto a gas puff target. The target consists of an identical gas mixture that fills the chamber, but its density is much higher, close to the atmospheric density (~1019 m−3). The second system allows for measurements of the ultraviolet/visible (UV/VIS) radiation emitted from plasmas produced by the LPP source. Measurements are carried out using an optical spectrometer and streak camera.
The LPP source was based on the Nd:YAG NL laser system 129-EXPLA and the gas puff target system. The laser system with a wavelength of
The optical emission of excited molecules and ions that arise in the vicinity of the laser plasma was investigated in a wide spectral range (200–780 nm) using the ESA-4000 echelle spectrometer (LLA Instruments GmbH & Co KG, Berlin, Germany). The spectrometer is equipped with an intensified charge-coupled device (ICCD) camera with a Kodak KAF 1001 detector (spectral resolution of the system was about
To prepare the gas mixtures, a special system was constructed (Fig. 2), with the aim of creating a mixture to fill the chamber and form the gas puff target. In this work, noble and molecular gases such as Kr, Xe, Ar, He, Ne, H2, N2, CO2, and SF6 were used to create the mixtures.
Spatial–temporal measurements were made using the optical streak camera for various conditions of low-temperature plasma formation. At first, comparative studies of argon plasmas induced in the vicinity of the LPP, under different conditions, were performed. LPPs were created in the gas puff target, formed in a chamber remaining under a low vacuum (5 × 10−1 mbar) or a low pressure of argon (~1.3 mbar). In our experiments, the term low vacuum means a pressure maintained in the vacuum chamber by a roughing pump. The low pressure was obtained by the controlled injection of a gas into the chamber. The laser was operating in a long-pulse regime with a pulse energy of ~3.9 J. The investigated area was located 6.6 mm above the LPP source. Importantly, the radiation recorded in the streak camera images refers to the EUV/SXR radiation from the laser plasma. The dark area on the
The influence of pressure on the optical emission of plasmas induced in the ambient gas was also investigated. Measurements were conducted for nitrogen plasmas created in a chamber filled with nitrogen at various pressures: 2.5 mbar, 5 mbar, and 10 mbar (Fig. 4). The observed region, as in the case of argon, was 6.6 mm above the LPP source, and the streak camera entrance slit was set at 60 μm. The laser was operating in a short-pulse regime with a pulse energy of ~7.5 J. As can be seen in Fig. 4a, at the ambient pressure of 2.5 mbar, the intensity is strong, but only for the first 250 ns after start of the laser pulse. The same is true under higher pressures, in which case, after this time point, the intensity is still high enough to be seen in the pictures. The FWHM for the regions located 5 mm from the nozzle axis in a horizontal direction (1 mm and 12 mm in the
It is also worth looking at how the change of the observation region over the LPP source affects the time–space distribution. Figure 5 shows the streak images recorded for three different distances over the LPP: 2.8 mm, 6.6 mm, and 8.5 mm, respectively. The wider scale makes it possible to observe the formation of high-intensity regions, which widen with distance. In addition, an interesting phenomenon is the occurrence of increased intensity after some time. This is clearly seen in Fig. 5b, at 0.6 μs from the start of the registration process. In Fig. 5c, where the distance from the LPP is the greatest, this intensity area becomes more blurry.
The aim of the measurements using the echelle spectrometer was to determine whether it is possible to find molecular spectra in the range of 200–780 nm for the system with the LPP source. The most representative and intense in the studied range were diatomic molecules. Molecular bands expected in this range were carbon compounds and excimers; hence, the prepared mixtures contained selected noble gases CO2 and N2 [7–10]. The measurement results presented in Fig. 6 come from the study of the region located 6 mm above the LPP source, and the spectra were observed in different time moments of the low-temperature plasma existence, up to 200 ns after creation of the LPPs. These measurements were performed for a mixture consisting of [2% SF6, 18% Xe, and 80% He], under the ambient pressure of about 10 mbar and acquisition window of 100 ns. The visible molecular spectra belong to the XeF excimer, and they correspond to the transitions
Similar measurements were performed for mixtures containing the molecular gases N2 and CO2, at the ambient pressure of about 10 mbar and an acquisition window of 1000 ns. Using such mixtures, apart from the N2 and CO2 molecular bands, spectra corresponding to the CN species were also recorded. These spectra, however, were very weak; hence, a Xe admixture was added. In Fig. 8(right), a CN band spectrum recorded for a gas mixture composed of 10% Xe, 10% N2, and 80% CO2 is shown.
This work is based on the study of low-temperature plasmas induced in gas mixtures. Using various gas mixture parameters, both as an injected mixture and as a mixture filling the chamber, the formation of new molecules, i.e., not constituting the components of the premixture, was observed. This experiment showed that the LPP source is able to provide conditions for the formation of molecules and their registration using spectral methods.
Based on the pictures presented in Fig. 3, it can be concluded that the propagation of radiation from the LPP source depends on the conditions in the chamber. Low vacuum causes fewer instances of photoionization and ionization than does a chamber filled with gas under low pressure. It is worth noting that in the images from the streak camera in the area close to and above the nozzle, we observe a visibly lower intensity than in the diagonal direction. This is due to the fact that the SXR radiation is strongly absorbed near the LPP source; therefore, it can hardly propagate in the gas injection path. On the other hand, at the border of the injected gas with the ambient gas, this radiation can propagate and ionize the gas. The resulting low-temperature plasma emits light, which is visible as intense maxima on the right and left sides of the streak images [11]. Optical radiation recorded for the central area above the LPP, occurring at a time point of approximately 600 ns (Figs. 4 and 5), cannot be induced by the EUV/SXR radiation. At this time point, the hot and dense plasma emitting the energetic photons does not exist. The emission maximum recorded after such a long time could be a result of the propagation of the shockwave or the formation of low-temperature plasma by the fast ions or electrons from the LPP. A detailed analysis concerning occurrence of this emission maximum supported by the corresponding numerical simulations was reported by Bartnik
Apart from spatial–temporal measurements, spectral measurements in the optical range were also performed. Contrary to the atomic spectra, molecular ones are composed of bands, corresponding to molecular electronic transitions. Such spectra can be simulated using one of the available numerical codes, e.g., PGOPHER [12]. To perform the simulations for a specific molecule, the necessary vibration and rotational constants have to be applied. Due to the fact that real molecules have anharmonic potentials, the energies of the vibration levels have to be calculated based on Eq. (1).
where
where
The necessary molecular constants can be found in the database of the National Institute of Standards and Technology (NIST) [13].
The simulation results for CN molecules were compared with the spectral observations using a mixture of [10% Xe, 10% N2, and 80% CO2] and are shown in Fig. 8. Transitions for the selected spectra come from the states
In this work, emission of LPP formed in low-pressure ambient gas from the gas puff target was studied. As part of this work, the research was divided into methods that used streak cameras for spatial–temporal measurements and methods using spectroscopic measurements of molecular spectra. The analysis of images from the streak camera allowed us to examine the nature of radiation emission and the regions where the expected emission is the highest. This, in turn, contributed to the registration of molecular spectra, along with their subsequent analysis and simulations in numerical programs. The excimers found showed interesting spectra; the time in which they were created was a few/several hundred nanoseconds. These studies provide the basis for further molecular analyses in such a system, as well as premises for analyses in wider spectral ranges, e.g., in the infrared region.