Investigation of low-temperature plasmas formed in low-density gases surrounding laser-produced plasmas

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.

Fig. 1.

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 λ = 1064 nm can provide energy in the range of 1–10 J (3–9 J in this experiment), pulse time duration t = 1 ns or 10 ns (1–2 ns in the short-pulse and 5–10 ns in the long-pulse regime), repetition rate f = 10 Hz, and laser beam diameter = 25 mm (top-hat intensity distribution). The laser beam is focused onto the gas puff target using a plano-convex lens. The resulting intensity reached 10¹⁴ W/cm⁻² or 2 × 10¹³ W/cm⁻² for short- and long-pulse regimes, respectively. In such conditions, hot plasma is generated directly above the nozzle of the gas valve. The ambient gas filling the chamber is converted to a low-temperature plasma due to exposure to various factors accompanying the LPP formation.

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 λ/Δλ ~20 000) [6]. The diameter of the acquisition region is limited by the collecting optical system, approximately 1 mm full width at half maximum (FWHM) in the focal point of the collecting system. The light was acquired at a distance of 6 mm from the LPP. In order to investigate the behavior of the low-temperature plasma, spatial–temporal measurements were carried out. They were made using an optical streak camera (Hamamatsu C10910). The optical axis of the streak camera was perpendicular to the axis of the laser beam, as shown in Fig. 1. Between the entrance slit of the camera and the plasma position, a focusing lens was mounted. It allowed the formation of a plasma image in the plane of the entrance slit. By changing the position of the lens, it was possible to select a narrow observation region at various distances above the LPP.

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.

Fig. 2.

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 et al. [11].

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).

1 E(v)=Te+ωe(v+12)−ωexe(v+12)2+ωeye(v+12)3+…, $$\matrix{ {E(v) = } & {{T_e} + {\omega _e}(v + {\raise0.5ex\hbox{$\scriptstyle 1$} \kern-0.1em/\kern-0.15em \lower0.25ex\hbox{$\scriptstyle 2$}}) - {\omega _e}{x_e}{{(v + {\raise0.5ex\hbox{$\scriptstyle 1$} \kern-0.1em/\kern-0.15em \lower0.25ex\hbox{$\scriptstyle 2$}})}^2}} \cr {} & { + {\omega _e}{{\rm{y}}_e}{{(v + {\raise0.5ex\hbox{$\scriptstyle 1$} \kern-0.1em/\kern-0.15em \lower0.25ex\hbox{$\scriptstyle 2$}})}^3} + \ldots ,} \cr } $$

where v is the vibrational quantum number, T_e is the minimum electronic energy (in inverse centimeters [cm⁻¹]), ω_e is the first vibrational term (cm⁻¹), ω_ex_e is the second vibrational term (cm⁻¹), and ω_ey_e is the third vibrational term (cm⁻¹). If we consider a molecule with an anharmonic potential, the rotational constant changes slightly with the vibrational state. The rotational constant B_v for a given vibrational state can be described by the expression in Eq. (2).

2 BvBe−αe(v+12)+γe(v+12)2+… $${B_{\rm{v}}}{B_e} - {\alpha _e}(v + {\raise0.5ex\hbox{$\scriptstyle 1$} \kern-0.1em/\kern-0.15em \lower0.25ex\hbox{$\scriptstyle 2$}}) + {\gamma _e}{(v + {\raise0.5ex\hbox{$\scriptstyle 1$} \kern-0.1em/\kern-0.15em \lower0.25ex\hbox{$\scriptstyle 2$}})^2} + \ldots $$

where B_e is the rotational constant in equilibrium position (cm⁻¹), α_e is the first rotational term (cm⁻¹), and γ_e is the rotation–vibration interaction constant (cm⁻¹).

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 B²∑⁺ – X²∑⁺. As you can see, the simulations reflect the nature of the observations quite well. The process of formation of CN molecules is much longer than that of the excimer molecules – compare Figs. 8 and 9. Their formation lasts for tens of microseconds.

Fig. 9.

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.