- Email: [email protected]
Translational motion of SiO+ produced by laser ablation
Applied Surface Science 197–198 (2002) 202–206
Translational motion of SiOþ produced by laser ablation Takashi Mogia,b, Yoshimitsu Fukuyamaa, Tohru Kobayashia, Isao Tanihataa, Kiyoji Ueharaa,b, Yukari Matsuoa,* b
a RI Beam Science Laboratory, RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan Faculty of Science and Technology, Department of Physics, Keio University, 3-14-1 Hiyoshi, Kouhoku-ku, Yokohama 223-8522, Japan
Abstract We have measured the Doppler width of laser-induced fluorescence of SiOþ ions produced by laser ablation of a Si target with a pulsed Nd:YAG laser in oxygen gas. The technique of pulsed amplification of a narrow-band cw laser was used to obtain a probe pulse whose bandwidth was much narrower than the Doppler width. The measured Doppler width was converted into an effective temperature representing the distribution of the velocity component of the SiOþ ions along the path of the probe laser pulse. By changing the delay time between the ablation and probe laser pulses, we have found that the effective temperature does not decrease monotonically to room temperature but forms humps. This implies that the ablation cloud may pulsate while diffusing. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Laser-induced fluorescence; Doppler width; Laser produced ablation plasma; Molecular ion
1. Introduction Laser ablation can provide atomic and molecular beam sources that are otherwise difficult to be produced. For instance, it can prepare metal-containing molecules, including those which can be produced only from refractory materials. Such capability of laser ablation has attracted much attention not only from the field of materials science but also from the field of laser spectroscopy. For the proper use of such molecular beams, precise knowledge on dynamical characteristics of the ablation cloud is indispensable. Recently, various methods have been used to diagnose ablation clouds, e.g., optical detection of the plasma emission [1], laser-induced fluorescence *
Corresponding author. Tel.: þ81-48-462-1111; fax: þ81-48-462-4689. E-mail address: [email protected] (Y. Matsuo).
(LIF) measurements [2,3], time-of-flight spectrometry [4], and so on. Especially optical detection has provided a powerful means for studying the dynamics of the products because the ablation cloud can be imaged directly. However, in most cases the behavior of an ablation cloud has been observed from the intensity variation of emission, but little effort has been made to investigate the dynamics from the spectral lineshape of the ablation product [5]. In the previous work we observed the rotational structure of the LIF spectrum of the SiOþ ions produced by laser ablation of a solid Si sample in oxygen ambient gas [6]. The rotational temperature (e.g. [7]) derived from the rotational state distribution reached the thermal equilibrium with room temperature (300 K) within 20 ms. In the present experiment we have measured the Doppler width of a single rotational component of the SiOþ emission spectrum, and investigate the translational motion of the ablation cloud.
0169-4332/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 2 ) 0 0 3 6 0 - 4
T. Mogi et al. / Applied Surface Science 197–198 (2002) 202–206
203
Fig. 1. The experimental setup for the LIF detection of SiOþ ions produced by laser ablation. The pulse-amplified cw ring Ti:Sapphire laser is used as the probe laser for LIF detection.
2. Experimental setup A schematic diagram of the experimental setup is shown in Fig. 1. The laser ablation was performed in a vacuum chamber, in which a solid silicon sample was rotated in order to maintain the ablation condition unchanged. The chamber was filled with oxygen gas of 10 Pa. The fundamental output of a Q-switched Nd:YAG laser (pulse duration: 10 ns; repetition rate: 10 Hz) irradiated the sample at normal incidence, and was focused to a spot size of 0.5 mm2 on the sample surface. The fluence was approximately 2 J/cm2 per pulse. SiOþ ions were produced by the reaction of the ejected Siþ ions with the oxygen gas. After each ablation pulse, the SiOþ ions were irradiated with a probe laser pulse to observe the LIF signal. The probe laser was prepared as follows. A cw ring Ti:Sapphire laser output was pulse-amplified twice with an XeCl excimer laser in order to gain enough power density for second harmonic generation in a BBO crystal. The frequency-doubled output was pulse-amplified further to obtain the strong LIF signal. Thus 385 nm pulses with a bandwidth as narrow as 200 MHz (FWHM) and a duration of 15 ns were generated. Then the Ti:Sapphire laser wavelength was scanned to measure the Doppler width of the SiOþ spectrum (FWHM: 3 GHz) precisely. The LIF signal was averaged
using a boxcar integrator with a 100 ns gate width and stored in a computer. The time sequence of laser pulses, and typical signals of the laser-induced plasma emission and the LIF are depicted in Fig. 2.
3. Results and discussion In a previous study we observed the clearly resolved rotational structure of the B2Sþ–X2Sþ (0,0) band of the SiOþ LIF spectrum at 385 nm (see the upper part of Fig. 3) [6]. In the present work we precisely measured the lineshape of a selected rotational line, the R(19) line of this band. A typical spectrum is shown in the lower part of Fig. 3. The R(19) line is split into two components due to the spin-rotation interaction. Their separation is 5.735 GHz [8] and can be used as a frequency scale. The separation is wide enough not to suffer from overlap of two components. The spectrum was fitted to a combination of two Gaussian profiles to calculate the Doppler width, which is typically around 3 GHz (FWHM). Then the width was converted into TDv using a relation [7] TDv ¼
Mc2 Dn2D 8ðln 2ÞkB n20
(1)
204
T. Mogi et al. / Applied Surface Science 197–198 (2002) 202–206
Fig. 2. The time sequence diagram and typical signals of the plasma emission and the LIF.
Fig. 3. The observed LIF spectra. The upper part shows the rotational structure of the SiOþ B2Sþ–X2Sþ (0,0) band. The lower part shows the two split components of the R(19) rotational line.
where M is the mass of SiOþ, c the velocity of light in vacuum, DnD the Doppler width (FWHM), kB the Boltzmann’s constant, and n0 the center frequency. TDv is the effective temperature representing the translational velocity distribution along the path of the probe laser pulse (see the inset of Fig. 1). We measured the temporal variation of TDv by changing the delay time after ablation. Similar measurements were carried out for various distances d between the Si sample and the path of the probe laser pulse. Fig. 4 shows TDv versus delay time for d ¼ 10, 15 and 20 mm, respectively. The LIF signal before 4 ms cannot be observed because it is submerged in the plasma emission. An advantage of this method is that we can measure the velocity distribution directly from the width of the spectrum, not indirectly from the intensity variation of the emission. It should be noted that TDv does not decrease monotonically to room temperature as a function of the delay time but increases again after a decrease. In the case of d ¼ 15 mm, e.g., TDv decreases until 9 ms and turns to an increase forming a peak at 13 ms. Similarly, in the cases of d ¼ 10 and 20 mm, the peak appears at 8 and 50 ms, respectively. The peak appears later as the d becomes larger. It means that the part of the cloud, where the distribution of the velocity component of the ablation cloud along the probe laser path is the broadest, moves outward. The average speed of the peak position is estimated to be approximately 1.0 and 0.1 km/s in the regions d ¼ 10–15 mm and 15– 20 mm, respectively. It is considered that the effective temperature TDv representing the velocity distribution of the ablation cloud is dominated not by the thermal motion of the ejected particles but by the translational motion of the whole ablation cloud. The reasons are as follows. First, because the rotational temperature reaches thermal equilibrium with room temperature (300 K) within 20 ms [6], the translational motion of each particle is also expected to equilibrate by 20 ms. Secondly, the average speed of 1.0 km/s for d ¼ 1015 mm is comparable to the initial velocity of ejected particles [2]. Besides, if the Doppler width is converted into the velocity distribution Dv using the relation Dv ¼ cDnD =n0 (see the right axes of Fig. 4), it is also comparable to the initial velocity of the ejected particles. Therefore the hump of TDv implies that the speed of the ablation cloud oscillates while the cloud moves
T. Mogi et al. / Applied Surface Science 197–198 (2002) 202–206
205
microsecond. In their 1D LIF measurement, the layer was found moving backward after it stopped. As another example, Bulgakov and Bulgakova [9] numerically simulated the dynamics of cloud expansion into ambient gas, and reached conclusion that the ablation cloud does not stop upon reaching the maximum propagation distance but moves repeatedly back and forth up to 200 ms after ablation. In the present experiment we probe all the particles present along the path of the probe laser pulse, making a clear interpretation of our result difficult. For the clearcut diagnostics of the ablation cloud, spatially resolved measurements should be carried out. We are now in preparation for constructing a 2D LIF imaging system using a CCD camera for this purpose.
4. Conclusion
Fig. 4. Effective temperature derived from the measured Doppler width as a function of delay time after ablation. The distance between the sample and the probe laser is: (a) 10 mm; (b) 15 mm; (c) 20 mm. The vertical axis of the right side represents velocity distribution Dv of the SiOþ ions corresponding to the effective temperature.
outward. In the case of d ¼ 15 mm we can see that the peak appears not only at 13 ms but also at 100 ms. It is interpreted that these peaks are caused by the pulsation of the ablation cloud. TDv finally reaches the thermal equilibrium with room temperature (300 K). Our result that the temperature TDv increases after a decrease indicates the ablation cloud pulsates, i.e., moves back and forth. There have been some reports that the ablation cloud moves back and forth. Nakata and coworkers [2,3] measured one- and two-dimensional timeresolved density distributions of ground state oxide molecules by the LIF detection using a probe laser sheet beam. They found that an oxide molecule layer expanded as it was decelerated by background oxygen gas within 20 ms and then stopped at a few tens of
We have observed the lineshape of the single rotational component of the SiOþ spectrum precisely by the LIF measurement. We defined the effective temperature TDv derived from the measured Doppler width as the temperature representing the velocity distribution of the ablation cloud along the probe laser pulse direction and measured the temporal variation of TDv . It has been found that TDv does not decrease to room temperature monotonically but reaches thermal equilibrium with room temperature after forming a few humps. This implies that the ablation cloud may pulsate while moving outward. It is the first time that the information on the velocity pulsation of the ablation cloud is obtained directly from the spectral lineshape.
Acknowledgements This work was partly supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science, Sports and Culture. References [1] D.B. Geohegan, Thin Solid Films 220 (1992) 138. [2] W.K.A. Kumuduni, Y. Nakayama, Y. Nakata, T. Okada, M. Maeda, J. Appl. Phys. 74 (1993) 7510. [3] Y. Nakata, H. Kaibara, T. Okada, M. Maeda, J. Appl. Phys. 80 (1996) 2458.
206
T. Mogi et al. / Applied Surface Science 197–198 (2002) 202–206
[4] K.J. Koivusaari, J. Levoska, S. Leppa¨ vuori, J. Appl. Phys. 85 (1999) 2915. [5] H.C. Le, R.W. Dreyfus, W. Marine, M. Sentis, I.A. Movtchan, Appl. Surf. Sci. 96–98 (1996) 164. [6] Y. Matsuo, T. Nakajima, T. Kobayashi, M. Takami, Appl. Phys. Lett. 71 (1997) 996.
[7] W. Demtro¨ der, Laser Spectroscopy, 2nd Edition, Springer, Berlin, 1996, p. 67 for TDv, p. 529 for rotational temperature. [8] R. Cameron, T.J. Scholl, L. Zhang, R.A. Holt, S.D. Rosner, J. Mol. Spectrosc. 169 (1995) 352. [9] A.V. Bulgakov, N.M. Bulgakova, J. Phys. D 28 (1995) 1710.
Translational motion of SiOþ produced by laser ablation Takashi Mogia,b, Yoshimitsu Fukuyamaa, Tohru Kobayashia, Isao Tanihataa, Kiyoji Ueharaa,b, Yukari Matsuoa,* b
a RI Beam Science Laboratory, RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan Faculty of Science and Technology, Department of Physics, Keio University, 3-14-1 Hiyoshi, Kouhoku-ku, Yokohama 223-8522, Japan
Abstract We have measured the Doppler width of laser-induced fluorescence of SiOþ ions produced by laser ablation of a Si target with a pulsed Nd:YAG laser in oxygen gas. The technique of pulsed amplification of a narrow-band cw laser was used to obtain a probe pulse whose bandwidth was much narrower than the Doppler width. The measured Doppler width was converted into an effective temperature representing the distribution of the velocity component of the SiOþ ions along the path of the probe laser pulse. By changing the delay time between the ablation and probe laser pulses, we have found that the effective temperature does not decrease monotonically to room temperature but forms humps. This implies that the ablation cloud may pulsate while diffusing. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Laser-induced fluorescence; Doppler width; Laser produced ablation plasma; Molecular ion
1. Introduction Laser ablation can provide atomic and molecular beam sources that are otherwise difficult to be produced. For instance, it can prepare metal-containing molecules, including those which can be produced only from refractory materials. Such capability of laser ablation has attracted much attention not only from the field of materials science but also from the field of laser spectroscopy. For the proper use of such molecular beams, precise knowledge on dynamical characteristics of the ablation cloud is indispensable. Recently, various methods have been used to diagnose ablation clouds, e.g., optical detection of the plasma emission [1], laser-induced fluorescence *
Corresponding author. Tel.: þ81-48-462-1111; fax: þ81-48-462-4689. E-mail address: [email protected] (Y. Matsuo).
(LIF) measurements [2,3], time-of-flight spectrometry [4], and so on. Especially optical detection has provided a powerful means for studying the dynamics of the products because the ablation cloud can be imaged directly. However, in most cases the behavior of an ablation cloud has been observed from the intensity variation of emission, but little effort has been made to investigate the dynamics from the spectral lineshape of the ablation product [5]. In the previous work we observed the rotational structure of the LIF spectrum of the SiOþ ions produced by laser ablation of a solid Si sample in oxygen ambient gas [6]. The rotational temperature (e.g. [7]) derived from the rotational state distribution reached the thermal equilibrium with room temperature (300 K) within 20 ms. In the present experiment we have measured the Doppler width of a single rotational component of the SiOþ emission spectrum, and investigate the translational motion of the ablation cloud.
0169-4332/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 2 ) 0 0 3 6 0 - 4
T. Mogi et al. / Applied Surface Science 197–198 (2002) 202–206
203
Fig. 1. The experimental setup for the LIF detection of SiOþ ions produced by laser ablation. The pulse-amplified cw ring Ti:Sapphire laser is used as the probe laser for LIF detection.
2. Experimental setup A schematic diagram of the experimental setup is shown in Fig. 1. The laser ablation was performed in a vacuum chamber, in which a solid silicon sample was rotated in order to maintain the ablation condition unchanged. The chamber was filled with oxygen gas of 10 Pa. The fundamental output of a Q-switched Nd:YAG laser (pulse duration: 10 ns; repetition rate: 10 Hz) irradiated the sample at normal incidence, and was focused to a spot size of 0.5 mm2 on the sample surface. The fluence was approximately 2 J/cm2 per pulse. SiOþ ions were produced by the reaction of the ejected Siþ ions with the oxygen gas. After each ablation pulse, the SiOþ ions were irradiated with a probe laser pulse to observe the LIF signal. The probe laser was prepared as follows. A cw ring Ti:Sapphire laser output was pulse-amplified twice with an XeCl excimer laser in order to gain enough power density for second harmonic generation in a BBO crystal. The frequency-doubled output was pulse-amplified further to obtain the strong LIF signal. Thus 385 nm pulses with a bandwidth as narrow as 200 MHz (FWHM) and a duration of 15 ns were generated. Then the Ti:Sapphire laser wavelength was scanned to measure the Doppler width of the SiOþ spectrum (FWHM: 3 GHz) precisely. The LIF signal was averaged
using a boxcar integrator with a 100 ns gate width and stored in a computer. The time sequence of laser pulses, and typical signals of the laser-induced plasma emission and the LIF are depicted in Fig. 2.
3. Results and discussion In a previous study we observed the clearly resolved rotational structure of the B2Sþ–X2Sþ (0,0) band of the SiOþ LIF spectrum at 385 nm (see the upper part of Fig. 3) [6]. In the present work we precisely measured the lineshape of a selected rotational line, the R(19) line of this band. A typical spectrum is shown in the lower part of Fig. 3. The R(19) line is split into two components due to the spin-rotation interaction. Their separation is 5.735 GHz [8] and can be used as a frequency scale. The separation is wide enough not to suffer from overlap of two components. The spectrum was fitted to a combination of two Gaussian profiles to calculate the Doppler width, which is typically around 3 GHz (FWHM). Then the width was converted into TDv using a relation [7] TDv ¼
Mc2 Dn2D 8ðln 2ÞkB n20
(1)
204
T. Mogi et al. / Applied Surface Science 197–198 (2002) 202–206
Fig. 2. The time sequence diagram and typical signals of the plasma emission and the LIF.
Fig. 3. The observed LIF spectra. The upper part shows the rotational structure of the SiOþ B2Sþ–X2Sþ (0,0) band. The lower part shows the two split components of the R(19) rotational line.
where M is the mass of SiOþ, c the velocity of light in vacuum, DnD the Doppler width (FWHM), kB the Boltzmann’s constant, and n0 the center frequency. TDv is the effective temperature representing the translational velocity distribution along the path of the probe laser pulse (see the inset of Fig. 1). We measured the temporal variation of TDv by changing the delay time after ablation. Similar measurements were carried out for various distances d between the Si sample and the path of the probe laser pulse. Fig. 4 shows TDv versus delay time for d ¼ 10, 15 and 20 mm, respectively. The LIF signal before 4 ms cannot be observed because it is submerged in the plasma emission. An advantage of this method is that we can measure the velocity distribution directly from the width of the spectrum, not indirectly from the intensity variation of the emission. It should be noted that TDv does not decrease monotonically to room temperature as a function of the delay time but increases again after a decrease. In the case of d ¼ 15 mm, e.g., TDv decreases until 9 ms and turns to an increase forming a peak at 13 ms. Similarly, in the cases of d ¼ 10 and 20 mm, the peak appears at 8 and 50 ms, respectively. The peak appears later as the d becomes larger. It means that the part of the cloud, where the distribution of the velocity component of the ablation cloud along the probe laser path is the broadest, moves outward. The average speed of the peak position is estimated to be approximately 1.0 and 0.1 km/s in the regions d ¼ 10–15 mm and 15– 20 mm, respectively. It is considered that the effective temperature TDv representing the velocity distribution of the ablation cloud is dominated not by the thermal motion of the ejected particles but by the translational motion of the whole ablation cloud. The reasons are as follows. First, because the rotational temperature reaches thermal equilibrium with room temperature (300 K) within 20 ms [6], the translational motion of each particle is also expected to equilibrate by 20 ms. Secondly, the average speed of 1.0 km/s for d ¼ 1015 mm is comparable to the initial velocity of ejected particles [2]. Besides, if the Doppler width is converted into the velocity distribution Dv using the relation Dv ¼ cDnD =n0 (see the right axes of Fig. 4), it is also comparable to the initial velocity of the ejected particles. Therefore the hump of TDv implies that the speed of the ablation cloud oscillates while the cloud moves
T. Mogi et al. / Applied Surface Science 197–198 (2002) 202–206
205
microsecond. In their 1D LIF measurement, the layer was found moving backward after it stopped. As another example, Bulgakov and Bulgakova [9] numerically simulated the dynamics of cloud expansion into ambient gas, and reached conclusion that the ablation cloud does not stop upon reaching the maximum propagation distance but moves repeatedly back and forth up to 200 ms after ablation. In the present experiment we probe all the particles present along the path of the probe laser pulse, making a clear interpretation of our result difficult. For the clearcut diagnostics of the ablation cloud, spatially resolved measurements should be carried out. We are now in preparation for constructing a 2D LIF imaging system using a CCD camera for this purpose.
4. Conclusion
Fig. 4. Effective temperature derived from the measured Doppler width as a function of delay time after ablation. The distance between the sample and the probe laser is: (a) 10 mm; (b) 15 mm; (c) 20 mm. The vertical axis of the right side represents velocity distribution Dv of the SiOþ ions corresponding to the effective temperature.
outward. In the case of d ¼ 15 mm we can see that the peak appears not only at 13 ms but also at 100 ms. It is interpreted that these peaks are caused by the pulsation of the ablation cloud. TDv finally reaches the thermal equilibrium with room temperature (300 K). Our result that the temperature TDv increases after a decrease indicates the ablation cloud pulsates, i.e., moves back and forth. There have been some reports that the ablation cloud moves back and forth. Nakata and coworkers [2,3] measured one- and two-dimensional timeresolved density distributions of ground state oxide molecules by the LIF detection using a probe laser sheet beam. They found that an oxide molecule layer expanded as it was decelerated by background oxygen gas within 20 ms and then stopped at a few tens of
We have observed the lineshape of the single rotational component of the SiOþ spectrum precisely by the LIF measurement. We defined the effective temperature TDv derived from the measured Doppler width as the temperature representing the velocity distribution of the ablation cloud along the probe laser pulse direction and measured the temporal variation of TDv . It has been found that TDv does not decrease to room temperature monotonically but reaches thermal equilibrium with room temperature after forming a few humps. This implies that the ablation cloud may pulsate while moving outward. It is the first time that the information on the velocity pulsation of the ablation cloud is obtained directly from the spectral lineshape.
Acknowledgements This work was partly supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science, Sports and Culture. References [1] D.B. Geohegan, Thin Solid Films 220 (1992) 138. [2] W.K.A. Kumuduni, Y. Nakayama, Y. Nakata, T. Okada, M. Maeda, J. Appl. Phys. 74 (1993) 7510. [3] Y. Nakata, H. Kaibara, T. Okada, M. Maeda, J. Appl. Phys. 80 (1996) 2458.
206
T. Mogi et al. / Applied Surface Science 197–198 (2002) 202–206
[4] K.J. Koivusaari, J. Levoska, S. Leppa¨ vuori, J. Appl. Phys. 85 (1999) 2915. [5] H.C. Le, R.W. Dreyfus, W. Marine, M. Sentis, I.A. Movtchan, Appl. Surf. Sci. 96–98 (1996) 164. [6] Y. Matsuo, T. Nakajima, T. Kobayashi, M. Takami, Appl. Phys. Lett. 71 (1997) 996.
[7] W. Demtro¨ der, Laser Spectroscopy, 2nd Edition, Springer, Berlin, 1996, p. 67 for TDv, p. 529 for rotational temperature. [8] R. Cameron, T.J. Scholl, L. Zhang, R.A. Holt, S.D. Rosner, J. Mol. Spectrosc. 169 (1995) 352. [9] A.V. Bulgakov, N.M. Bulgakova, J. Phys. D 28 (1995) 1710.