**Plasma Switching by Laser Ablation**

Ryosuke Hasegawa, Kazunori Fukaya and Akiyoshi Matsuzaki1 *Mie University Japan* 

### **1. Introduction**

286 Materials Science and Technology

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Plasma offers great potential for materials science [Lieberman & Lichtenberg, 2005]. Many interesting materials have been synthesized through plasma such as chemical vapour deposition (CVD). Besides, plasma etching is also known as one of the significant techniques utilizing plasma. In general, plasma is generated by direct electrical discharges of gases. On the other hand, we recently found a new method to form *pulse* plasma in an electric field *lower* than that necessary for the plasma formation by the direct discharges [Nakano et al., 2010; Tanaka et al, 2003a, 2003b, 2004]; the laser ablation of metals switches plasma formation and extinction alternately. We call this phenomenon PLASLA (plasma switching by laser ablation) and the plasma formed by this method PLASLA (plasma switched by laser ablation). The facts that PLASLA is the pulse plasma completely synchronized with laser ablation and can be formed in a lower electric field suggest PLASLA as a promising metalcatalysis reaction for materials science. For instance, the former fact suggests that we will be able to set the most suitable conditions for materials synthesis by appropriately integrating the ablation laser and the laser for the photochemical reactions and controlling their timing. As for the latter fact, the lower discharge potential of PLASLA can exclude the discharge of impurity gases. Furthermore, our previous experiments indicate that the products of the PLASLA reactions are different in the presence and absence of a magnetic field [Nakano et al., 2010]. In the present manuscript, as well as the overview of our previous study on PLASLA, we discuss PLASLA, focusing on its spatial discharge patterns and temporal waves. These ordered structures are of great interest, since plasma is a nonlinear process.

## **2. Generation and properties of PLASLA**

Since the details of the systems and procedures of PLASLA experiments are described in the previous papers [Nakano et al., 2010; Tanaka et al, 2003a, 2003b, 2004], here we describe how to generate PLASLA with its main properties.

#### **2.1 Generation of PLASLA**

As schematically shown in Fig. 1(a), in a reaction chamber filled with CF4 at a pressure of 0.2 Torr, with the laser ablation of Cu by the irradiation of a spinning Cu target with the fundamental beam of a Nd3+:YAG laser (1.064 m in wavelength, 45 mJ/pulse in power, 10

<sup>1</sup> Corresponding Author

Plasma Switching by Laser Ablation 289

PLASLA has been confirmed to form in a CF4 gas through the laser ablation of various metals, i.e., Cu, Al, Ag, Zn, Co, Ni, Ti, Mo, and W [Nakano et al., 2010]. In view of the electronic configurations, these metals can be classified into three groups: Al, Cu, Ag, and Zn in Group-1, Co and Ni in Group-2, and Ti, Mo, and W in Group-3. The configurations in Group-1 are s2p1 in Al, d10s1 in Cu, d10s2 in Zn, and d10s1 in Ag, and have the closed highest inner sub-shells. The configurations in Group-2 are d7s2 in Co and d8s2 in Ni, and have the greater-than-half-filled highest inner sub-shells. The configurations in Group-3 are d2s2 in Ti, d5s1 in Mo, and d4s2 in W, and have the less-than-or-equal-to-half-filled highest inner subshells. On the other hand, we have experimentally found that PLASLA is most stable in Group-1, middle in Group-2, and least in Group-3 [Nakano et al., 2010], suggesting that neutral chemical reactions compete with the PLASLA plasma reactions in Groups-2 and 3 metals, since they have open highest inner sub-shells. For instance, we have confirmed that the PLASLA formation rate is fastest in Group-1, middle in Group-2, and slowest in Group-3; that the PLASLA extinction time is longest in Group-1, middle in Group-2, and shortest in

We have found the various interesting facts on the interaction of PLASLA with a magnetic field [Nakano et al., 2010]. The magnetic field effects on PLASLA are due to the magnetohydro-dynamics (MHD) processes and the magnetic effect on the chemical reactions. The main MHD processes in PLASLA are the *E*x*B* drift and the cyclotron circulation of ions and electrons. These processes increase the PLASLA potential, at which PLASLA is formed, and the activation energy of PLASLA plasma reactions, since they interrupt the movements of ions and electrons for the plasma chemical processes. As a magnetic field increases the PLASLA potential, the shape of PLASLA pulse varies considerably. By the analysis of the magnetic field effect on the activation energy of PLASLA, it is found that a magnetic field increases the activation energy of the charge-transfer processes and decreases that of the ionic chemical reactions accompanying chemical-bond cleavages. The TOF-mass spectrometry shows that the PLASLA product materials formed in a magnetic field are different from those of zero-field PLASLA. That is, for PLASLA in a magnetic field, the detected carbon clusters are rather small in size and are free from

Thus, in view of materials science, PLASLA is a promising metal-catalysis reaction.

In this section, we study the mechanism of PLASLA by quantitative analysis of PLASLA

The upper panel in Fig. 2 shows the present imaging system. The lower panel in this figure shows the typical images of DC-plasma, laser ablation (CuF chemiluminescent reaction), PLASLA with a Cu target. In DC-plasma, bright plasma sheaths are formed around the electrodes. In the laser-ablation image, the bright sphere of CuF chemiluminescence is observed near the Cu target. In PLASLA, the bright plasma sheaths around the electrodes

and the bright sphere of CuF chemiluminescence near the Cu target are observed.

Group-3.

impurities such as metals and fluorine.

discharge patterns with various metal targets.

**3. PLASLA discharge pattern** 

**3.1 Discharge pattern imaging** 

Hz in repetition rate), after increasing a direct-current electric field applied between the parallel Cu plates of electrodes, stable PLASLA is formed at an electric field of 400 V, which is inadequate to form DC-plasma (direct current plasma), which is formed by the direct discharge at a potential of 500 V without laser ablation. The oscilloscope trace of PLASLA luminescence signal in Fig. 1(b) indicates that plasma is formed by the first laser ablation, quenched by the next ablation, formed again by the third ablation, quenched again by the fourth ablation, and so on, indicating that the PLASLA formation and quenching completely synchronize with laser ablation.

Fig. 1. (a) The schematic illustration of the PLASLA experimental system. (b) The oscilloscope trace of PLASLA luminescence signal.

#### **2.2 Properties of PLASLA**

In PLASLA, the plasma can be obtained under milder conditions, since the laser ablation of metals induces the plasma in an electric field lower than that for the direct discharge in DC-plasma, suggesting the availability of the new products different from the ones of the DC-plasma. This also makes us possible to avoid the discharge of impurity gases with their low ionization potentials. For instance, in case that a minor constituent N2 gas as the impurity discharges instead of the main constituent CF4 gas in DC-plasma, it can be avoided in PLASLA, in which a CF4 gas discharges as a predominant process in the same experimental system [Nakano et al., 2010]. Since the PLASLA plasma is the pulse completely synchronizing with laser ablation, the collaboration between the ablation laser and the laser for photochemical reactions will offer great potential for new materials syntheses.

The products of PLASLA in the gaseous Cu-CF4 system are found to be polymeric carbon materials by time–resolved luminescence spectroscopy [Tanaka et al., 2003a, 2003b, 2004] and TOF (time-of-flight) mass spectrometry [Takenaka et al. unpublished data, Nakano et al., 2010]. Since various polymeric carbon materials have been extensively used for manufactures, PLASLA will be very promising in industry, as well as materials science.

Hz in repetition rate), after increasing a direct-current electric field applied between the parallel Cu plates of electrodes, stable PLASLA is formed at an electric field of 400 V, which is inadequate to form DC-plasma (direct current plasma), which is formed by the direct discharge at a potential of 500 V without laser ablation. The oscilloscope trace of PLASLA luminescence signal in Fig. 1(b) indicates that plasma is formed by the first laser ablation, quenched by the next ablation, formed again by the third ablation, quenched again by the fourth ablation, and so on, indicating that the PLASLA formation and quenching completely

**Luminescence intensity / a.u.**

**Time / s**

synchronize with laser ablation.

**400V**

+

CF4

Laser


syntheses.

**Cu target**

(a) (b)

oscilloscope trace of PLASLA luminescence signal.

Fig. 1. (a) The schematic illustration of the PLASLA experimental system. (b) The

In PLASLA, the plasma can be obtained under milder conditions, since the laser ablation of metals induces the plasma in an electric field lower than that for the direct discharge in DC-plasma, suggesting the availability of the new products different from the ones of the DC-plasma. This also makes us possible to avoid the discharge of impurity gases with their low ionization potentials. For instance, in case that a minor constituent N2 gas as the impurity discharges instead of the main constituent CF4 gas in DC-plasma, it can be avoided in PLASLA, in which a CF4 gas discharges as a predominant process in the same experimental system [Nakano et al., 2010]. Since the PLASLA plasma is the pulse completely synchronizing with laser ablation, the collaboration between the ablation laser and the laser for photochemical reactions will offer great potential for new materials

The products of PLASLA in the gaseous Cu-CF4 system are found to be polymeric carbon materials by time–resolved luminescence spectroscopy [Tanaka et al., 2003a, 2003b, 2004] and TOF (time-of-flight) mass spectrometry [Takenaka et al. unpublished data, Nakano et al., 2010]. Since various polymeric carbon materials have been extensively used for manufactures, PLASLA will be very promising in industry, as well as materials science.

Chemiluminescence

**PLASLA**

**2.2 Properties of PLASLA** 

plasma

PLASLA has been confirmed to form in a CF4 gas through the laser ablation of various metals, i.e., Cu, Al, Ag, Zn, Co, Ni, Ti, Mo, and W [Nakano et al., 2010]. In view of the electronic configurations, these metals can be classified into three groups: Al, Cu, Ag, and Zn in Group-1, Co and Ni in Group-2, and Ti, Mo, and W in Group-3. The configurations in Group-1 are s2p1 in Al, d10s1 in Cu, d10s2 in Zn, and d10s1 in Ag, and have the closed highest inner sub-shells. The configurations in Group-2 are d7s2 in Co and d8s2 in Ni, and have the greater-than-half-filled highest inner sub-shells. The configurations in Group-3 are d2s2 in Ti, d5s1 in Mo, and d4s2 in W, and have the less-than-or-equal-to-half-filled highest inner subshells. On the other hand, we have experimentally found that PLASLA is most stable in Group-1, middle in Group-2, and least in Group-3 [Nakano et al., 2010], suggesting that neutral chemical reactions compete with the PLASLA plasma reactions in Groups-2 and 3 metals, since they have open highest inner sub-shells. For instance, we have confirmed that the PLASLA formation rate is fastest in Group-1, middle in Group-2, and slowest in Group-3; that the PLASLA extinction time is longest in Group-1, middle in Group-2, and shortest in Group-3.

We have found the various interesting facts on the interaction of PLASLA with a magnetic field [Nakano et al., 2010]. The magnetic field effects on PLASLA are due to the magnetohydro-dynamics (MHD) processes and the magnetic effect on the chemical reactions. The main MHD processes in PLASLA are the *E*x*B* drift and the cyclotron circulation of ions and electrons. These processes increase the PLASLA potential, at which PLASLA is formed, and the activation energy of PLASLA plasma reactions, since they interrupt the movements of ions and electrons for the plasma chemical processes. As a magnetic field increases the PLASLA potential, the shape of PLASLA pulse varies considerably. By the analysis of the magnetic field effect on the activation energy of PLASLA, it is found that a magnetic field increases the activation energy of the charge-transfer processes and decreases that of the ionic chemical reactions accompanying chemical-bond cleavages. The TOF-mass spectrometry shows that the PLASLA product materials formed in a magnetic field are different from those of zero-field PLASLA. That is, for PLASLA in a magnetic field, the detected carbon clusters are rather small in size and are free from impurities such as metals and fluorine.

Thus, in view of materials science, PLASLA is a promising metal-catalysis reaction.
