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Trace element analysis by laser ablation atomic fluorescence spectroscopy
15 August 1994 OPTICS COMMUNICATIONS Optics Communications 110 (1994) 298-302
ELSEVIER
Trace element analysis by laser ablation atomic fluorescence spectroscopy Yuji Oki, Tomohiro Tani, Nobutaka Kidera, Mitsuo Maeda Department of Electrical Engineering, Kyushu University, Higashi-ku, Fukuoka 812, Japan Received 28 October 1993; revised manuscript received 30 May 1994
Abstract A very sensitive method to make a trace element analysis in pure water, the laser ablation atomic fluorescence (LAAF) spectroscopy, is proposed and demonstrated, where the atomic detection by laser induced fluorescence (LiF) is combined with excimer laser ablation atomizing. A theoretical calculation predicts that the detection limits of 0.01-0.1 ppt can be expected in an ideal case. A detection limit of 1.7 ppt or 20 fg was experimentally obtained for Na.
1. Introduction
The quantitative analysis of trace impurity elements in water is an important technique in various fields. The flame atomic absorption spectroscopy is the standard method for this purpose, but its sensitivity is not enough to detect impurities less than ppb (ng/ml). At present, the atomic absorption method with a graphite furnace atomizer or the ICP mass spectrometry have an excdlent detection limit less than 1 ppb for many elements. However, in atomic power plants or highly integrated semiconductor factories, a higher detection sensitivity in the order of I ppt (pg/ml) is required. Laser induced fluorescence (LIF) spectroscopy has a potentiality in the extremely high sensitivity of the level of one atom detection [ 1,2 ], and there are many reports on the application of impurity analysis in water [ 3 ]. We have also reported LIF detection with a flameless atomizer using a microwave discharge [4]. However, the detection limits in these reports are not good enough in comparison with the expected potentiality of LIF spectroscopy. The most important problem lies in the atomizers. In the usual atomic LIF de-
tection, a pulsed dye laser is used to cover a wide tunable range. Because the pulse duration is typically only 10 ns, a pulsed atomizer synchronized with the laser is efficient to generate high density atoms. This paper proposes and demonstrates a new type of atomizer for LIF detection using the excimer laser ablation technique. The excimer laser ablation is a well known technique in the field of optical processing, and it is also used as an ionizer in the laser mass spectroscopy. The laser ablation atomic fluorescence (LAAF) spectroscopy has the following merits. The atomization by laser ablation has a high efficiency and produces high density atoms in a small volume during a very short time (~10 ns). Secondly, a small background noise can be expected because the radiation from the plume of the ablation products is not so strong and the discrimination by time-gating is effective. Lewis and Piepmeier have tried atomic LIF detection in a microprobe plume generated by an IR laser [5 ]. However, the detection sensitivity was not so high because of the strong background emission. The analytical simulation of the ablation process shows that LAAF spectroscopy had a detection sensi-
0030-4018/94/$07.00 (~) 1994 Elsevier Science B.V. All fights reserved
SSD10030-4018 ( 94 ) 00324-N
Y. Oki et al. / Optics Communications 110 (1994) 298-302
tivity less than 0.1-0.01 ppt in the ideal case. In the demonstration for Na atoms, a detection limit of 1.7 ppt or 20 fg was obtained.
2. Estimation of sensitivity of LAAF spectroscopy According to our experience, when the LIF method is applied to detect atoms in an environment with a small background or stray light, such as in an atomic vapor cell, detection limits of 104-106 and 106-107 atoms.cm -3 are easily attained for CW and pulsed dye laser excitation, respectively. However, it is not so easy to realize these detection limits in conventional atomizers, because of severe background radiation from flame or plasma and quenching by atmospheric gas. Fig. I shows the configuration of the setup of LAAF spectroscopy. The procedure of the analysis is as follows: (i) a small amount of sample water (~10/al) is put on a target stage, which is terminated by a polished tungsten rod, and dried up by heated He gas flow, (ii) the dried sample is ablated by an excimer laser beam focused on the target stage, (iii) after a delay time from the ablation, a probe dye laser pulse is injected to observe LIF. The LAAF spectroscopy in an open air environment is more convenient than in vacuum for a practical application to the element analysis. We tried the LAAF experiments both in a vacuum chamber and in He gas at atmospheric pressure. In the excimer laser ablation at a high fluence, we can expect an efficient production of atoms. Although the atomic plume is rapidly dissipated by diffusion, a high density atomic cloud will be kept around the tar-
l
Tr, 1 pulUer
2C~annel
Excimer Laser Lens
UV cut-off FiRer Interference Filter
C os
1
~'gJ Integrator 1<
~ Stage
ITrg • I
Laser
Sample
(II#L)
Heated He gas
Trg.
Fig. 1. Setup of LAAFspectroscopy.
299
get for a short time. This is a suitable characteristic of the atomizer for the pulsed LIF spectroscopy. LIF can be measured without background radiation because the strongest stray light from the excimer laser can be efficiently reduced through a time-gated bandpass filter. Radiation from the ablation plume is much weaker than those of traditional atomizers, and the gating is also effective. To confLrm the high sensitivity of the LAAF spectroscopy, the density of atoms generated by ablation was estimated. The atomic density was calculated as functions of space and time under following assumption: (i) all atoms on the target are ablated from a single point by a single shot of an excimer laser, (ii) all ablated atoms are dissociated at an efficiency of 100%, (iii) ablated atoms have a shifted Maxwellian distribution, and the angular distribution is reported to follow a cosj 0 (8 < p < 14) law [6,7], where 0 is the angle against ablated target. Since the efficiency in (ii) can not reach 100% in practice, the atomic density calculated under these assumptions gives the maximum value that can be produced. The assumption (iii) has already been confirmed in the case of the fabrication ofYBa2Cu3OT_6 (YBCO) high-Tc superconducting thin films by the excimer laser ablation in our laboratory [8,9]. In this simulation, according to these assumptions, the velocity distribution f is given by the following formulae [7],
f (v~,vy, Vz) dv~dvydvz = B × exp ( -
m [v2 + v 2 + (Vz_U)2])dvxdvydvz, (1)
where B is the normalization coefficient, m is the atomic mass, k is the Boltzman constant, T is the effective temperature, vx, vy, Vz are the particle velocity components of the atoms, and u is the flow velocity. In a non-shifted Maxwellian distribution (u = 0), the atomic density n follows the cos4 0 law. In laser ablation, however, the shifted Maxwellian distribution can be approximated by a cosp (9 < p _< 13) law [6]. It resulted from various calculations, that the value of p, changed between 9 and 13, didn't influence the results of the simulation much. Therefore, the value of p is chosen as 9, and the atomic density n in polarcoordinate is given by
300
n(r,O,t)
Y. Oki et al. / Optics Communications 110 (1994) 298-302
(2)
-2-k-T
c°s90exp
= B'
where B' is the normalization coefficient. In Fig. 2, the calculated velocity distributions of the Na density profile obtained by Eq. ( 1 ) at heights of 5 mm and 13 mm from the stage surface, are shown by a solid line. We considered sample water of 12 pl with a concentration of l ppt of Na is put in the ablation stage in vacuum. The temperature T is determined to be 5000 K by the fitting with the experimental data. The dotted lines denote the obtained profiles through the time of flight (TOF) LIF measurement. The TOF data were obtained for different delay times between the excimer laser pulse and the dye laser pulse. The velocity distribution of the Na density is obtained by the LIF intensity changing the delay time. Though there exists a low velocity component in the experimental
(a) 7mm height 1.2
.... :Calculation for T = .5000K
0 . 8 ....
t ................
:. . . . . . . . . . . . . . . . . . . . .
~: 0.6 ----. ._J 0.4 0.2 f
0
1.2 1,0
(b)
.
10
Time of Flight
20
[/~s]
18 mm Height
-O-:Experimental A ----:Calculation ...............J i l l - , ',i for T = 5000K
0.8 0.6 u. 0.4 -q 0.2 0
i0
Time of Flight
[#s]
Density[cm :l]
(a) Height5mm
1x 10!~ ~ ~ ~ m m m m m m l m ~
8
5xlO~
10 -10 Horizontal Pos. [m5,,,)
Density[cm :i] lx10 '~ ~ 1
10
Time of Flight
[#s]
Na
3
m
m
5x10' 0 / ~ -10 ~- ~ ' - - ~ - ~ ~--'~J- 1 ~ Horizontal Pos. [mm] 10
~8 6
10
Time of Flight
[#s]
Fig. 3. Calculated space-time distribution of Na atom density by ablation at heights of 5 ram(a) and 13 ram(b).
--g-:Experimental
1.0------~,'~.................. ~ =
Na
20
Fig. 2. Calculated and measured TOF velocity distributions of ablated Na atoms at heights of 7 mm (a) and 18 mm(b).
profile, the assumption of a Maxwellian distribution is almost valid. It shows that the peak of the experiment TOF profile is at about 2500 m/s. Figs. 3a and 3b show space-time distribution of Na atom density calculated by Eq. ( 1 ) at the heights of 5 mm and 13 mm, respectively. The atoms are rapidly dissipated by their initial velocity and angular distribution. It can be seen from Fig. 3 that the Na density of 109 cm -3 is obtained over an horizontal region of 3 mm at 1.2 as after the ablation for 5 mm height, and 108 cm -3 over 6 mm at 3 as for 13 mm height, when the sample water is 12 pl and 1 ppt. According to our experience, a detection limit of 1 0 6 - 1 0 7 c m -3 is easily attained in LIF detection with a pulsed dye laser, if the stray light and background light are weak enough such as in the Na vapor cell. A detection limit of 107 cm -3 corresponds to the Na concentration of 0.01-0.1 ppt in the LAAF detection, if the assumptions (i)-(iii) are satisfied. This calculation probes the high sensitivity of the LAAF spectroscopy. The calculation above is made in high vacuum condition where there is no collisional process. In atmospheric He gas, this model is not valid because of severe collisions. We shall discuss that case in Sec. 4.
Y. Oki et aL / Optics Communications I10 (1994) 298-302
301
300
3. LAAF experiment in vacuum chamber
Na I0 ppt Sample
At first, the LAAF experiment was performed in a vacuum chamber, after a Na sample water of 12 #1 was put on the target stage and dried by heated He gas. The chamber was evacuated to 30 mTorr where the collisional process could be ignored. A nitrogen laser pumped dye laser (pulse duration 4 ns, tuned at the Na D2 line) was used for the probe laser, and LIF was observed at the same wavelength. LIF was measured by a time-gated photomultiplier tube (PMT, Hamamatu R928 + C1392) through a UV cut filter, an interference filter (IF, bandwidth 1 nm, maximum transmittance 70%), and a focusing lens ( f = 50 mm). The signal from the PMT was integrated by a boxcar averager (SRS, model-SR250) and recorded by a microcomputer. The beam ofArF ( 193 nm) or KrF (249 nm) laser was focused by a lens in a cross section of 2 x 4 mm 2 on the target stage. Since the diameter of the stage was 2 mm, the ablation laser beam covered the whole stage. Fig. 4 shows the LIF intensities obtained by each shot of KrF laser with the pulse energy of 180 mJ. It seems that most of the sample can be ablated by only two laser shots. The ArF laser provided better ablation efficiency in the small laser energy, but the KrF laser is also usable by increasing the energy. The height of the probe laser beam was 13 mm from the surface of the target stage. The beam diameter and the energy were approximately 2 mm and 5 #J/pulse, respectively. The delay time between two lasers was set at the optimum value 6 #s. Fig. 5 shows an example of the LIF signal from Na atoms at a concentration of 10 ppt. The pure water, 1.2
0.12
1.0
0.I0 -~
~ loo
~
0 -I00
/
Pure Water for Solvent
i
0
10
i
20
,
i
,
,
30 40 TIME (sec)
i
i
50
i
60
Fig. 5. An example of LIF signal through boxcar integrator at a Na concentration of 10ppt and for pure water used as solvent. which was sampled directly from a circulating system through two ion-exchange-resin filters, also provided a LIF signal that corresponded to the Na concentration of about 10 ppt. Since no signal was obtained when no sample water was put on the stage, this signal comes from the residual Na atoms in the pure water for the solvent. After subtracting this residual component, a detection limit of 1.7 ppt is obtained from the noise fluctuation in Fig. 5. It corresponds to the absolute detection limit of 20 fg. The background radiation from KrF laser scattering and ablation plume can be completely cut off by the UV filter and the time-gated PMT. Therefore, the main noise component is the dye laser scattering from the target stage and window materials. If the probe beam height is set lower to increase the LIF intensity, the main stray light comes from the stage. The detection limit can be improved further by decreasing the stray light.
4. LAAF experiment in atmospheric He gas
m
e o.8
0.08~
~'o.6
D.06~
~,~ 0.4 0.02
~) 0.2
c
U-
o.oo -=.-._=
-.~ 0.0 -0.2
~. 200
0
i
0mbero,S,,ot
-0.02 6
Fig. 4. LIF intensity for each KrF laser shot.
A similar LAAF experiment was made placing the target stage in a slow He flow at 760 Torr as shown in Fig. 1. In this case, however, the LIF intensity became weak, and no LIF was observed when the probe laser beam was set higher than 3 ram. Until the He pressure was decreased to 0.5 Torr, the LIF intensity increased monotonically, then it saturated. The TOF velocity distribution became slower as the He pressure was increased. The ratio of LIF intensity at 760 Torr and 3 Torr is 1:40, as shown in Fig. 4, although
302
Y. Oki et al. / Optics Communications 110 (1994.) 298-302
an accurate comparison of the LIF intensity between different pressures is difficult, because the optimum probe laser heights and delay times are different depending on the pressure. However, the sensitivity in the atmospheric pressure seems to be less than 1 percent in the vacuum. Nevertheless, it should be noted in Fig. 4 that the ablation is finished within two shots also in the atmospheric pressure. This sensitivity reduction cannot be explained by the LIF quenching in atmospheric He, because Na LIF quenching was only 50% in atmospheric He, as we reported before [ 10 ]. The reduction was mainly caused by the dissipation of the produced atoms through very frequent collisions with He atoms. Recently in our laboratory, a detailed observation of the dynamic behavior of YO molecules scattered from a target of YBCO ceramics by laser ablation in 02 buffer gas, by the TOF-LIF spectroscopy [ 11,12 ]. This investigation provides the following conclusion. At an 02 buffer gas pressure of 1 Torr, the ablated molecules make a flight decreasing their initial speed by the collision with 02 molecules. At a flight distance of 35 mm, most of the YO molecules lose their initial momentum, then they are rapidly dissipated by the thermal diffusion. This observation suggests that the flight distance of the ablated Na atoms is less than 0.1 mm in the atmospheric pressure. Therefore, it is difficult to transport the atoms to the position of the probe laser beam. To avoid this severe dissipation of atoms, we are now trying to ablate the sample in a supersonic He gas flow. Such gas flow can be easily obtained by using a high-pressure pulsed jet nozzle that is synchronized with the lasers. Even if the He jet is supersonic, a collision of the ablated atoms with He may occur, because the initial velocity of the ablated atoms is larger than the flow speed of He gas as shown in Fig. 2. However, a preliminary experiment shows that LIF detection at a height more than 10 mm is possible by this scheme. The LAAF experiment in open air will be reported elsewhere in detail.
5. Conclusion The LAAF spectroscopy, in which the trace element detection by LIF is combined with the atomizer using the excimer laser ablation technique, was proposed and demonstrated. The feature of this atomizing technique lies in the potentiality to produce a high atomic density and in the low background noise component. A simple estimation predicts that atomic densities of 108-109 atoms.cm -3 can be produced from a 1 ppt, 12/zl sample water when the atomizing efficiency is assumed to be 100%. Even for 10% efficiency, we can expect a detection limit of 0.1-1 ppt in this scheme. A detection limit of 1.7 ppt or 20 fg was obtained for Na. However, the LAAF experiment in the atmospheric pressure He gas showed a poor sensitivity, because of the severe dissipation of the atoms by collisions. To avoid this problem, we proposed a new scheme using a supersonic gas jet nozzle.
References [1] G.W. Greenlees, D.L. Clark, S.L. Kaufman, D.A. Lewis, J.F. Tonn and J.H. Broadhurst, Optics Comm. 23 (1977) 236. [2] V.I. Balykin, V.S. Letokhov, V.I. Mishin and V.A. Semchishen, JETP Lett. 26 (1977) 358. [3] D.J. Butcher, J.P. Dougherty, F.R. Preli, A.P. Walton, G. Wei, R.L. Irwin and R.G. Michel, J. Anal. At. Spectrom. 3 (1988) 1059. [4] Y. Oki, T. Izuha, M. Maeda, C. Honda, Y. Hasegawa, H. Futami, J. Izumi and K. Matsuda, Anal. Chem. 65 (1993) 2096. [5] A.L.I. Lewis and E.H. Piepmeier, Appl. Spectrosc. 37 (1983) 523. [6] T. Vankatesan, X.D. Wu, A. Inam and J.B. Wachtman, Appl. Phys. Lett. (1988) 1193. [7] K.L. Saenger, J. Appl. Phys. 70 (1991) 5629. [8 ] T. Okada, N. Shibamaru, Y. Nakayama and M. Maeda, Appl. Phys. Lett. 60 (1992) 941. [9 ] T. Okada, N. Shibamaru, Y. Nakayama and M. Maeda, Jpn. J. Appl. Phys. 31 (1992) 367. [10] Y. Oki, H. Uda, C. Honda, M. Maeda, J. Izumi and T. Moromoto, Anal. Chem. 62 (1990) 680. [ 11] T. Okada, Y. Nakayama, K.A.K. Wanniarachchi and M. Mitsuo, Appl. Phys. Lett. 61 (1992) 2368. [12] K.A.K. Wanniarachchi, Y. Nakayama, Y. Nakata, O. Tatsuo and M. Mitsuo, Appl. Phys. 32 (1993) 271.
ELSEVIER
Trace element analysis by laser ablation atomic fluorescence spectroscopy Yuji Oki, Tomohiro Tani, Nobutaka Kidera, Mitsuo Maeda Department of Electrical Engineering, Kyushu University, Higashi-ku, Fukuoka 812, Japan Received 28 October 1993; revised manuscript received 30 May 1994
Abstract A very sensitive method to make a trace element analysis in pure water, the laser ablation atomic fluorescence (LAAF) spectroscopy, is proposed and demonstrated, where the atomic detection by laser induced fluorescence (LiF) is combined with excimer laser ablation atomizing. A theoretical calculation predicts that the detection limits of 0.01-0.1 ppt can be expected in an ideal case. A detection limit of 1.7 ppt or 20 fg was experimentally obtained for Na.
1. Introduction
The quantitative analysis of trace impurity elements in water is an important technique in various fields. The flame atomic absorption spectroscopy is the standard method for this purpose, but its sensitivity is not enough to detect impurities less than ppb (ng/ml). At present, the atomic absorption method with a graphite furnace atomizer or the ICP mass spectrometry have an excdlent detection limit less than 1 ppb for many elements. However, in atomic power plants or highly integrated semiconductor factories, a higher detection sensitivity in the order of I ppt (pg/ml) is required. Laser induced fluorescence (LIF) spectroscopy has a potentiality in the extremely high sensitivity of the level of one atom detection [ 1,2 ], and there are many reports on the application of impurity analysis in water [ 3 ]. We have also reported LIF detection with a flameless atomizer using a microwave discharge [4]. However, the detection limits in these reports are not good enough in comparison with the expected potentiality of LIF spectroscopy. The most important problem lies in the atomizers. In the usual atomic LIF de-
tection, a pulsed dye laser is used to cover a wide tunable range. Because the pulse duration is typically only 10 ns, a pulsed atomizer synchronized with the laser is efficient to generate high density atoms. This paper proposes and demonstrates a new type of atomizer for LIF detection using the excimer laser ablation technique. The excimer laser ablation is a well known technique in the field of optical processing, and it is also used as an ionizer in the laser mass spectroscopy. The laser ablation atomic fluorescence (LAAF) spectroscopy has the following merits. The atomization by laser ablation has a high efficiency and produces high density atoms in a small volume during a very short time (~10 ns). Secondly, a small background noise can be expected because the radiation from the plume of the ablation products is not so strong and the discrimination by time-gating is effective. Lewis and Piepmeier have tried atomic LIF detection in a microprobe plume generated by an IR laser [5 ]. However, the detection sensitivity was not so high because of the strong background emission. The analytical simulation of the ablation process shows that LAAF spectroscopy had a detection sensi-
0030-4018/94/$07.00 (~) 1994 Elsevier Science B.V. All fights reserved
SSD10030-4018 ( 94 ) 00324-N
Y. Oki et al. / Optics Communications 110 (1994) 298-302
tivity less than 0.1-0.01 ppt in the ideal case. In the demonstration for Na atoms, a detection limit of 1.7 ppt or 20 fg was obtained.
2. Estimation of sensitivity of LAAF spectroscopy According to our experience, when the LIF method is applied to detect atoms in an environment with a small background or stray light, such as in an atomic vapor cell, detection limits of 104-106 and 106-107 atoms.cm -3 are easily attained for CW and pulsed dye laser excitation, respectively. However, it is not so easy to realize these detection limits in conventional atomizers, because of severe background radiation from flame or plasma and quenching by atmospheric gas. Fig. I shows the configuration of the setup of LAAF spectroscopy. The procedure of the analysis is as follows: (i) a small amount of sample water (~10/al) is put on a target stage, which is terminated by a polished tungsten rod, and dried up by heated He gas flow, (ii) the dried sample is ablated by an excimer laser beam focused on the target stage, (iii) after a delay time from the ablation, a probe dye laser pulse is injected to observe LIF. The LAAF spectroscopy in an open air environment is more convenient than in vacuum for a practical application to the element analysis. We tried the LAAF experiments both in a vacuum chamber and in He gas at atmospheric pressure. In the excimer laser ablation at a high fluence, we can expect an efficient production of atoms. Although the atomic plume is rapidly dissipated by diffusion, a high density atomic cloud will be kept around the tar-
l
Tr, 1 pulUer
2C~annel
Excimer Laser Lens
UV cut-off FiRer Interference Filter
C os
1
~'gJ Integrator 1<
~ Stage
ITrg • I
Laser
Sample
(II#L)
Heated He gas
Trg.
Fig. 1. Setup of LAAFspectroscopy.
299
get for a short time. This is a suitable characteristic of the atomizer for the pulsed LIF spectroscopy. LIF can be measured without background radiation because the strongest stray light from the excimer laser can be efficiently reduced through a time-gated bandpass filter. Radiation from the ablation plume is much weaker than those of traditional atomizers, and the gating is also effective. To confLrm the high sensitivity of the LAAF spectroscopy, the density of atoms generated by ablation was estimated. The atomic density was calculated as functions of space and time under following assumption: (i) all atoms on the target are ablated from a single point by a single shot of an excimer laser, (ii) all ablated atoms are dissociated at an efficiency of 100%, (iii) ablated atoms have a shifted Maxwellian distribution, and the angular distribution is reported to follow a cosj 0 (8 < p < 14) law [6,7], where 0 is the angle against ablated target. Since the efficiency in (ii) can not reach 100% in practice, the atomic density calculated under these assumptions gives the maximum value that can be produced. The assumption (iii) has already been confirmed in the case of the fabrication ofYBa2Cu3OT_6 (YBCO) high-Tc superconducting thin films by the excimer laser ablation in our laboratory [8,9]. In this simulation, according to these assumptions, the velocity distribution f is given by the following formulae [7],
f (v~,vy, Vz) dv~dvydvz = B × exp ( -
m [v2 + v 2 + (Vz_U)2])dvxdvydvz, (1)
where B is the normalization coefficient, m is the atomic mass, k is the Boltzman constant, T is the effective temperature, vx, vy, Vz are the particle velocity components of the atoms, and u is the flow velocity. In a non-shifted Maxwellian distribution (u = 0), the atomic density n follows the cos4 0 law. In laser ablation, however, the shifted Maxwellian distribution can be approximated by a cosp (9 < p _< 13) law [6]. It resulted from various calculations, that the value of p, changed between 9 and 13, didn't influence the results of the simulation much. Therefore, the value of p is chosen as 9, and the atomic density n in polarcoordinate is given by
300
n(r,O,t)
Y. Oki et al. / Optics Communications 110 (1994) 298-302
(2)
-2-k-T
c°s90exp
= B'
where B' is the normalization coefficient. In Fig. 2, the calculated velocity distributions of the Na density profile obtained by Eq. ( 1 ) at heights of 5 mm and 13 mm from the stage surface, are shown by a solid line. We considered sample water of 12 pl with a concentration of l ppt of Na is put in the ablation stage in vacuum. The temperature T is determined to be 5000 K by the fitting with the experimental data. The dotted lines denote the obtained profiles through the time of flight (TOF) LIF measurement. The TOF data were obtained for different delay times between the excimer laser pulse and the dye laser pulse. The velocity distribution of the Na density is obtained by the LIF intensity changing the delay time. Though there exists a low velocity component in the experimental
(a) 7mm height 1.2
.... :Calculation for T = .5000K
0 . 8 ....
t ................
:. . . . . . . . . . . . . . . . . . . . .
~: 0.6 ----. ._J 0.4 0.2 f
0
1.2 1,0
(b)
.
10
Time of Flight
20
[/~s]
18 mm Height
-O-:Experimental A ----:Calculation ...............J i l l - , ',i for T = 5000K
0.8 0.6 u. 0.4 -q 0.2 0
i0
Time of Flight
[#s]
Density[cm :l]
(a) Height5mm
1x 10!~ ~ ~ ~ m m m m m m l m ~
8
5xlO~
10 -10 Horizontal Pos. [m5,,,)
Density[cm :i] lx10 '~ ~ 1
10
Time of Flight
[#s]
Na
3
m
m
5x10' 0 / ~ -10 ~- ~ ' - - ~ - ~ ~--'~J- 1 ~ Horizontal Pos. [mm] 10
~8 6
10
Time of Flight
[#s]
Fig. 3. Calculated space-time distribution of Na atom density by ablation at heights of 5 ram(a) and 13 ram(b).
--g-:Experimental
1.0------~,'~.................. ~ =
Na
20
Fig. 2. Calculated and measured TOF velocity distributions of ablated Na atoms at heights of 7 mm (a) and 18 mm(b).
profile, the assumption of a Maxwellian distribution is almost valid. It shows that the peak of the experiment TOF profile is at about 2500 m/s. Figs. 3a and 3b show space-time distribution of Na atom density calculated by Eq. ( 1 ) at the heights of 5 mm and 13 mm, respectively. The atoms are rapidly dissipated by their initial velocity and angular distribution. It can be seen from Fig. 3 that the Na density of 109 cm -3 is obtained over an horizontal region of 3 mm at 1.2 as after the ablation for 5 mm height, and 108 cm -3 over 6 mm at 3 as for 13 mm height, when the sample water is 12 pl and 1 ppt. According to our experience, a detection limit of 1 0 6 - 1 0 7 c m -3 is easily attained in LIF detection with a pulsed dye laser, if the stray light and background light are weak enough such as in the Na vapor cell. A detection limit of 107 cm -3 corresponds to the Na concentration of 0.01-0.1 ppt in the LAAF detection, if the assumptions (i)-(iii) are satisfied. This calculation probes the high sensitivity of the LAAF spectroscopy. The calculation above is made in high vacuum condition where there is no collisional process. In atmospheric He gas, this model is not valid because of severe collisions. We shall discuss that case in Sec. 4.
Y. Oki et aL / Optics Communications I10 (1994) 298-302
301
300
3. LAAF experiment in vacuum chamber
Na I0 ppt Sample
At first, the LAAF experiment was performed in a vacuum chamber, after a Na sample water of 12 #1 was put on the target stage and dried by heated He gas. The chamber was evacuated to 30 mTorr where the collisional process could be ignored. A nitrogen laser pumped dye laser (pulse duration 4 ns, tuned at the Na D2 line) was used for the probe laser, and LIF was observed at the same wavelength. LIF was measured by a time-gated photomultiplier tube (PMT, Hamamatu R928 + C1392) through a UV cut filter, an interference filter (IF, bandwidth 1 nm, maximum transmittance 70%), and a focusing lens ( f = 50 mm). The signal from the PMT was integrated by a boxcar averager (SRS, model-SR250) and recorded by a microcomputer. The beam ofArF ( 193 nm) or KrF (249 nm) laser was focused by a lens in a cross section of 2 x 4 mm 2 on the target stage. Since the diameter of the stage was 2 mm, the ablation laser beam covered the whole stage. Fig. 4 shows the LIF intensities obtained by each shot of KrF laser with the pulse energy of 180 mJ. It seems that most of the sample can be ablated by only two laser shots. The ArF laser provided better ablation efficiency in the small laser energy, but the KrF laser is also usable by increasing the energy. The height of the probe laser beam was 13 mm from the surface of the target stage. The beam diameter and the energy were approximately 2 mm and 5 #J/pulse, respectively. The delay time between two lasers was set at the optimum value 6 #s. Fig. 5 shows an example of the LIF signal from Na atoms at a concentration of 10 ppt. The pure water, 1.2
0.12
1.0
0.I0 -~
~ loo
~
0 -I00
/
Pure Water for Solvent
i
0
10
i
20
,
i
,
,
30 40 TIME (sec)
i
i
50
i
60
Fig. 5. An example of LIF signal through boxcar integrator at a Na concentration of 10ppt and for pure water used as solvent. which was sampled directly from a circulating system through two ion-exchange-resin filters, also provided a LIF signal that corresponded to the Na concentration of about 10 ppt. Since no signal was obtained when no sample water was put on the stage, this signal comes from the residual Na atoms in the pure water for the solvent. After subtracting this residual component, a detection limit of 1.7 ppt is obtained from the noise fluctuation in Fig. 5. It corresponds to the absolute detection limit of 20 fg. The background radiation from KrF laser scattering and ablation plume can be completely cut off by the UV filter and the time-gated PMT. Therefore, the main noise component is the dye laser scattering from the target stage and window materials. If the probe beam height is set lower to increase the LIF intensity, the main stray light comes from the stage. The detection limit can be improved further by decreasing the stray light.
4. LAAF experiment in atmospheric He gas
m
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0.08~
~'o.6
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~,~ 0.4 0.02
~) 0.2
c
U-
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-.~ 0.0 -0.2
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0
i
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Fig. 4. LIF intensity for each KrF laser shot.
A similar LAAF experiment was made placing the target stage in a slow He flow at 760 Torr as shown in Fig. 1. In this case, however, the LIF intensity became weak, and no LIF was observed when the probe laser beam was set higher than 3 ram. Until the He pressure was decreased to 0.5 Torr, the LIF intensity increased monotonically, then it saturated. The TOF velocity distribution became slower as the He pressure was increased. The ratio of LIF intensity at 760 Torr and 3 Torr is 1:40, as shown in Fig. 4, although
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an accurate comparison of the LIF intensity between different pressures is difficult, because the optimum probe laser heights and delay times are different depending on the pressure. However, the sensitivity in the atmospheric pressure seems to be less than 1 percent in the vacuum. Nevertheless, it should be noted in Fig. 4 that the ablation is finished within two shots also in the atmospheric pressure. This sensitivity reduction cannot be explained by the LIF quenching in atmospheric He, because Na LIF quenching was only 50% in atmospheric He, as we reported before [ 10 ]. The reduction was mainly caused by the dissipation of the produced atoms through very frequent collisions with He atoms. Recently in our laboratory, a detailed observation of the dynamic behavior of YO molecules scattered from a target of YBCO ceramics by laser ablation in 02 buffer gas, by the TOF-LIF spectroscopy [ 11,12 ]. This investigation provides the following conclusion. At an 02 buffer gas pressure of 1 Torr, the ablated molecules make a flight decreasing their initial speed by the collision with 02 molecules. At a flight distance of 35 mm, most of the YO molecules lose their initial momentum, then they are rapidly dissipated by the thermal diffusion. This observation suggests that the flight distance of the ablated Na atoms is less than 0.1 mm in the atmospheric pressure. Therefore, it is difficult to transport the atoms to the position of the probe laser beam. To avoid this severe dissipation of atoms, we are now trying to ablate the sample in a supersonic He gas flow. Such gas flow can be easily obtained by using a high-pressure pulsed jet nozzle that is synchronized with the lasers. Even if the He jet is supersonic, a collision of the ablated atoms with He may occur, because the initial velocity of the ablated atoms is larger than the flow speed of He gas as shown in Fig. 2. However, a preliminary experiment shows that LIF detection at a height more than 10 mm is possible by this scheme. The LAAF experiment in open air will be reported elsewhere in detail.
5. Conclusion The LAAF spectroscopy, in which the trace element detection by LIF is combined with the atomizer using the excimer laser ablation technique, was proposed and demonstrated. The feature of this atomizing technique lies in the potentiality to produce a high atomic density and in the low background noise component. A simple estimation predicts that atomic densities of 108-109 atoms.cm -3 can be produced from a 1 ppt, 12/zl sample water when the atomizing efficiency is assumed to be 100%. Even for 10% efficiency, we can expect a detection limit of 0.1-1 ppt in this scheme. A detection limit of 1.7 ppt or 20 fg was obtained for Na. However, the LAAF experiment in the atmospheric pressure He gas showed a poor sensitivity, because of the severe dissipation of the atoms by collisions. To avoid this problem, we proposed a new scheme using a supersonic gas jet nozzle.
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