Volume 53, Number 6, 1999 APPLIED SPECTROSCOPY 719 0003-7028 / 99 / 5306-0719$2.00 / 0 q 1999 Society for Applied Spectroscopy Shock Excitation and Cooling Stage in the Laser Plasma Induced by a Q-Switched Nd:YAG Laser at Low Pressures WAHYU SETIA BUDI, HERY SUYANTO, HENDRIK KURNIAWAN, MAY ON TJIA, and KIICHIRO KAGAWA * Applied Spectroscopy Laboratory, Graduate Program in Optoelectrotechniques and Laser Applications, The University of Indonesia, 4 Salemba Raya, Jakarta 10430, Indonesia, (W.S.B., H.S., H.K.); Department of Physics, Faculty of Science and Mathematics, Bandung Institute of Technology, 10 Ganesha, Bandung, Indonesia (M.O.T.); and Department of Physics, Faculty of Education, Fukui University, 9-1 bunkyo, 3 chome, Fukui 910, Japan (K.K.) An experimental study has been carried out on the dynamical pro- cess taking place in the secondary plasma generated by a Q- switched Nd:YAG laser (80 mJ, 8 ns) on a copper target at reduced pressure. Accurate dynamical characterization of the cross-sectional view of the plasma has been made possible by the unique combi- nation of a plasma con®nement con®guration and the time-resolved measurement technique. In addition to reaf®rming the role of the blast-wave mechanism in the generation of secondary plasma, an analysis of the time-resolved spatial distributions of emission inten- sities and the time-resolved spatial distributions of temperature was made. As a result, the occurrence of two-stage emission processes, the ``shock excitation stage’’ and ``cooling stage,’’ has been proved. For instance, at 2 Torr it is shown that the emission process is initiated by a brief shock excitation process (; 1 ms) and followed by a longer cooling process ( ; 3 ms). The experimental results con- cerning the characteristics of the plasma can be well understood by considering the two-stage processes. Index Headings: Laser-induced plasmas; Shock-wave plasma; Shock excitation; Cooling stage; Spectrochemical application. INTRODUCTION The plasma generated by a laser pulse focused on a solid target in air under reduced pressures is a very in- teresting phenomenon, for both scienti®c and practical reasons. For instance, some years ago, Basov et al. 1,2 re- ported that a laser-induced shock wave is produced along with the plasma when a high-power Q-switched solid state laser, such as a ruby and Nd-glass laser, is sharply focused on a solid target. The physical mechanism of this shock wave has since become a subject of continued re- search interest from the viewpoint of high-temperature hydrodynamics. 3±6 Nowadays, from a different point of view, laser-induced shock wave research has attracted considerable attention since Geoheagen 7 and Kumuduni et al. 8 showed that a laser-induced shock wave plays an important role in the production of the thin ®lm of high- temperature superconductors by mean of the excimer la- ser ablation method. On the other hand, the present au- thors have also pointed out the great potential promised by laser-induced shock wave plasma for practical appli- cations to spectrochemical analysis. The technique of laser ablation emission spectrometric analysis (LAESA) is well known as one of the typical applications of lasers. 9±11 In this technique, a Q-switched laser is focused on the sample in air at 1 atm. As a con- sequence, high-temperature and high-density plasma is Received 20 August 1998; accepted 18 January 1999. * Author to whom correspondence should be sent. generated, giving rise to a high-intensity continuous emission spectrum, accompanied by undesirable self-ab- sorption processes. This limitation is the major obstacle in yielding the linearity and sensitivity required for an accurate spectroscopic calibration. More recently, how- ever, some of the disadvantages faced by the conventional LAESA techniques have been overcome, to some extent, by the use of new spectral detection systems. Presently, two general strategies are being pursued in LAESA de- velopment. One, employing atmospheric pressure, usu- ally called laser-induced breakdown spectroscopy (LIBS), has been developed by Cremers and colleagues. 12±14 In this method a pulsed laser with high peak power and short duration, such as the Nd:YAG laser, is focused onto the sample at atmospheric pressure. In order to remove the interfering background from the high-intensity con- tinuous emission due to the high-density plasma, a gated optical multichannel analyser (OMA) is incorporated into the detection system. Another LAESA method involves the use of low surrounding gas pressures. Namely, the laser plasma is produced under reduced pressure with the objective of suppressing the background emission inten- sity of the spectrum. During the study of the plasma generation under re- duced pressure, Kagawa and co-workers 15±21 showed that a plasma having characteristics favorable to spectrochem- ical analysis can actually be generated by using a pulsed gas laser with short duration, such as the nitrogen laser, 15 carbon dioxide laser, 17,18 and excimer laser, 21 when the pressure of the surrounding gas is reduced to around 1 Torr. In these cases, the plasma invariably consists of two distinct parts. The ®rst part, which is called the primary plasma, occupies a small area and gives off intense con- tinuous emission spectra for a short time just above the surface of the target. The other part, called the secondary plasma, expands with time around the primary plasma with near-hemispherical shape, emitting sharp atomic spectral lines with negligibly low background. The sec- ondary plasma displays favorable characteristics for ele- mental and analytical analysis because of the linear re- lationships between the emission line intensities and the contents of associated elements in the target. By means of time-resolved experiments using a car- bon-dioxide laser 17,18 and excimer laser, 21 our groups demonstrated that this secondary plasma was excited by the shock wave, while the primary plasma acted as an initial explosion energy source. We have offered a theory with respect to the excitation mechanism of the second-