Absorption spectroscopy of reactive species generated by non-thermal process plasmas

Atomic radicals in process plasmas are very important species to determine the process reactions. Absorption spectroscopy is one of optical methods for measuring the behaviors of atomic radicals. The atomic radicals have resonance lines in the vacuum ultraviolet (VUV) region. The absorption spectroscopy in the VUV region basically needs specialized VUV light source. Wavelength conversion methods in the VUV region are very powerful tools for obtaining VUV laser radiations although they need large tunable laser systems and special technique for frequency mixing in non-linear gas medium. In our group, micro-discharge hollow cathode lamp (MHCL) has been developed as a VUV light source [1], and the VUV absorption spectroscopy with the MHCL has been applied to measure the absolute densities of atomic radicals. From the results, the reaction mechanisms in low pressure plasma processes have been clarified in our group. Moreover, the behaviors of atomic species generated by atmospheric pressure plasma jets have been also investigated with VUV absorption spectroscopy [2-5]. Recently, new VUV light source with a self-absorbing effect has been successfully developed for the measurement of background absorption near the absorption line of target atomic radical caused by molecular species or particles in the process plasma. These can contribute to pushing the study of reaction mechanism due to atomic radicals in the plasma processes. [1] S. Takashima, M. Hori, T. Goto, A. Kono, M. Ito, K. Yoneda, Appl. Phys. Lett. 75 (1999) 3929. [2] M. Iwasaki, K. Takeda, M. Ito, T. Yara, T. Uehara, M. Hori, Jpn. J. Appl. Phys. 46 (2007) L540. [3] K. Takeda, M. Kato, F. Jia, K. Ishikawa, H. Kano, M. Sekine, M. Hori, J. Phys. D: Appl. Phys. 46 (2013) 464006. [4] K. Takeda, K. Ishikawa, H. Tanaka, M. Sekine, M. Hori, J. Phys. D: Appl. Phys. 50 (2017) 195202. [5] K. Takeda, T. Kumakura, K. Ishikawa, H. Tanaka, M. Sekine, M. Hori, Appl. Phys. Express 10 (2017) 036201.