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ANALYTICAL CHEMISTRY, VOL. 51, NO. 6, MAY 1979
region by using dye lasers in conjunction with argon ion and krypton ion lasers. In the future and with the advent of lasers encompassing all wavelengths including the UV region, LIPAS should become a powerful tool for ultratrace analysis.
ACKNOWLEDGMENT One of the authors (M. N.)is indebted to Keiichi Furuya of the Science University of Tokyo for his encouragement.
(3) E. G.Burkhardt. C. A. Lambert. and C. K . N. Patel, Science, 168, 1 1 1 1 (1975). (4) P. C. Claspy, C. Ha, and Y. H. Pao, Appl. Opt., 16, 2972 (1977). (5) L. G.Rosengren, E. Max, and S. T. Eng, J. Phys. E.. 7, 125 (1974). (6)L. B. Kreuzer. N. D. Kenyon, and C. K. N. Patel, Scisnce, 177, 347 (1972). (7) L. B. Kreuzer, Anal. Chem., 46, 239A (1974). (8)W. Lahmann, H. J. Luckwig, and H. Welling, Anal. C b m . , 49,549 (1977). (9) S.Oda, T. Sawada, and H. Karnada, BunsekiKagaku, 27, 269 (1978). (10) S. Oda, T. Sawada. and H. Karnada. Anal. Chem., 50, 865 (1978). ( 1 1 ) M. Karnikura, F. Endo, and H. Sasaki, J . FocdHyg. SOC.Jpn., 13, 555 (1972).
LITERATURE CITED (1) L. B. Kreuzer and C. K. N. Patel, Science, 173, 45 (1971). (2) C. K. N. Patel, E. G. Burkhardt, and C. A. Lambert, Science, 184, 1173 (1974).
for review November
273
Ig78. Accepted January
22, 1979.
Laser-Induced Photoacoustic Spectroscopy of Some Rare Earth Ions in Aqueous Solutions Tsuguo Sawada, * Shohei Oda, Hiromichi Shimizu, and Hitoshi Kamada Department of Industrial Chemistry, Faculty of Engineering, The University of Tokyo, 7-3- I , Hongo, Bunkyo-ku, Tokyo, Japan
The photoacoustic spectra of some trivalent rare earth ions in aqueous solutions were obtained by laser-induced photoacoustic spectroscopy (LIPAS). This method was suitable for the spectral measurements of extremely weak absorption such as a forbidden transition of rare earth ions in liquid. Compared with the absorption and emission spectra in the visible region, the photoacoustic spectrum offered complementary spectroscopic information, particularly with respect to the radiationless decay process.
Photoacoustic spectroscopy is currently being used to provide spectral information on a wide variety of solid and liquid samples ( I , 2), which was previously difficult or impossible to measure by ordinary transmission and reflection methods. The availability of dye lasers as an excitation source capable of a high intensity flux within narrow, tunable bandwidths makes photoacoustic detection a promising method for the investigation of the very weak molecular absorption spectra. Dixon et al. ( 3 ) have measured photoacoustic spectra of forbidden transitions in some unstable gaseous sulfur compounds with a tunable CW dye laser. Recently, Lahmann et al. ( 4 ) and the present authors (5)have successfully applied this method to the ultra trace analysis of liquid samples with a n argon ion laser. Laser-induced photoacoustic spectroscopy (LIPAS) has brought a considerable improvement in the detection limit in comparison with conventional absorption spectrometry. I n the present paper, high resolution photoacoustic spectra of some trivalent rare earth ions (Pr3+,Nd”, Eu3+,Ho”, and Er3+) in aqueous solution were measured by the LIPAS, though the lasing wavelength of dyes used limited the measurable region. It is well known that all the lines of rare earth ions at visible and near-infrared regions are forbidden in nature. The intensities of the transitions are, therefore, extremely weak. The LIPAS is expected to be most suitable for the measurements of such forbidden transition in liquid.
EXPERIMENTAL Reagents and Procedure. Each rare earth nitrate was prepared by dissolving the oxide (99.9%) in nitric acid. These
solutions were diluted with distilled water and adjusted to a given concentration of rare earth ion with 0.5 N nitric acid. Apparatus. A block diagram of the LIPAS is shown in Figure 1. A tunable dye laser (Spectra Physics Model 375), pumped by an argon ion laser (Spectra Physics Model 164-03), was modulated at about 200 Hz by a light chopper and divided into two beams with a beam splitter. One laser beam was directed into the sample cell and the other, into the reference cell through the collecting lenses cf = 50 cm). The dyes used as pumped media were Rhodamine 110, Rhodamine 6G, and Rhodamine B with lasing ranges of about 540-600, 570-630, and 600-670 nm, respectively. The dye laser operated in the multimode with a bandwidth of 1 cm-’. The wavelength was read with a monochromator. The average dye laser power was about 300 mW. The laser power variation with wavelength was monitored by a calibrated photocell. The pressure fluctuation induced in the solution by absorbed radiation was detected by a piezoelectric ceramic (NPM, N-21 supplied by Tohoku Kinzoku Co. Ltd.). A lock-in amplifier was used to amplify the modulated output signal, which was obtained by subtracting the photoacoustic signal of the reference cell from that of the sample cell with a differential amplifier. The photoacoustic background signal is mainly due to the absorption of the solvent. The absorption coefficient of water in the 500-nm region is very small, that is, approximately cm-’, and might change with wavelength according to purity. However, the photoacoustic signal caused by absorption of the solvent is large. The double-beam operation enabled this background signal to be greatly reduced. Other experimental descriptions have appeared in the previous literature ( 5 ) . The absorption spectrum was obtained with a 90-mm pathlength cell by using a Shimazu MPS-5000 spectrophotometer. All spectral measurements were carried out at room temperature. RESULTS AND DISCUSSION I t is well known that the sharp lines of the rare earth ions are associated with transitions between configurations of the electrons within the inner 4f shell. All of the sharp lines are forbidden to the first approximation as ordinary electric dipole transitions by Laporte’s rule. However, the inversion of center of the electric field of the free ion is removed by the perturbing surrounding field so the lines appear as electric dipole radiation. These transitions, which are so-called forced electric dipole transitions, are all weak, being of about the intensity of natural magnetic dipole transitions. Praseodymium Ion. The photoacoustic spectrum of Pr3+ ion in aqueous solution in the yellow region (-590 nm) is
0003-2700/79/0351-0688$01.00/00 1979 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 51, NO. 6, MAY 1979
Figure 1.
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Block diagram of the double-beam LIPAS apparatus
0.6
I'
A
Wovelength i n m l
Figure 2. Photoacoustic spectrum of Pr3+ ion (-) in aqueous solution and photoacoustic background signal of the solvent (- - -)
shown in Figure 2. The spectrum a t 590 nm is considered to correspond to a transition from the ground state 3H4to the excited state 'Dz. As is suggested in the photoacoustic spectrum, fine structure is present, though a similar spectrum could not be obtained by absorption spectrometry. Since a molar absorptivity of Pr3+ ion in aqueous solution a t 590 nm is about 1.6 L mol-' cm-', slight variation in absorptivity a t large transmission would be extremely difficult to detect by absorption spectrometry, in which the absorptivity of a t most cm-' is measured. On the other hand, the LIPAS could detect the change of an absorptivity of cm-' (6). The apparent splitting is due to the internal Stark effect caused by the surrounding local electric field. A detailed spectroscopic analysis of the split lines of the Pr3+ ion in solution has not been reported yet. However, the five features shown in Figure 2 may be explained as follows. The upper state 'Dz is split into the three levels, characterized by p = 0, fl, and f2, by the internal Stark effect. The spectrum results from the transition from the two lowest ground state 3H4 (F = * 2 and 3) to the above split levels. The dashed line shown in Figure 2 is the variation of photoacoustic background signal caused by the absorption of water with wavelength by a single beam apparatus. Slight wavelength dependencies were observed. Neodymium Ions. Figure 3 shows the photoacoustic spectrum of the Nd3+ ion in the range a t around 575 nm (b) and 625 nm (c) which are assigned to the electronic transitions from the ground state (419,z) to respectively the 2G,*G mixed states and the 2H11/2 state. The absorption spectrum over the wide wavelength region is shown in Figure 3a. In the photoacoustic spectrum, fine structures were clearly observed,
-
560
5 80
600 610 Wavelength ( n m )
630
650
Absorption (a) and photoacoustic spectra [(b)and (c)] of Nd3+ ion in aqueous solution. Concentration: (a) and (c),20 mM; (b), 2 mM Flgure 3.
though they could not be detected in the absorption spectrum as in the case of the Pr3+ ion. Generally speaking, the photoacoustic spectrum obtained by measuring pressure fluctuation caused by heat generated through the radiationless relaxation does not necessarily coincide with the absorption spectrum. In the crystal, the Nd3+ions in the 2G,4Glevels decay nonradiatively to the *F3p metastable state, and then undergo radiative transitions to the 419,2, 4111,2,and 4113izmultiplets (7). In aqueous solution, however, the ions undergo rapid radiationless relaxation because of the transfer of electronic excitation energy to high-frequency vibrations such as OH in the solvent (8). Accordingly, the photoacoustic spectrum of the Nd3+ ion in aqueous solution might be expected to coincide with the absorption spectrum. Europium Ion. The intensity of the lines of t,he Eu3+ion in the yellow region is extremely weak. The photoacoustic spectrum, shown in Figure 4b, could be observed in the concentration of 0.2 M. The line a t 579 nm is assigned to the transition from the ground state 4Foto the upper 5Dostate. I t is well known that the Eu3+ion fluoresces at 613 nm as the result of absorbing radiation a t 579 nm. The quantum yield of 5D0emission for Eu3+ion in methanol for direct excitation of the 5Dolevel is 0.17 (9). This is one of the reasons that the photoacoustic spectrum cannot clearly be observed for concentrations under 0.1 M. The line a t 579 nm appears surprisingly sharp in the solution a t room temperature. This
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ANALYTICAL CHEMISTRY, VOL. 51, NO. 6, MAY 1979 la)
0.6
+
1
I
1
540
Wavelength
( nm j
aqueous solution. Concentration: 0.2 M 10.06
&4!!/'---560 630 Wavelength ( nm j
540
-
650
Figure 5. Absorption (- -) and photoacoustic spectra (-) in aqueous solution. Concentration: 2 mM
660
Figure 6. Photoacoustic spectrum of Er3+ ion in aqueous solution. Concentration: 2 mM
Figure 4. Absorption (a) and photoacoustic spectra (b) of Eu3+ ion in
4
560 '' 640 Wavelength I n m j
of Ho3+ ion
probably shows that the molecules of the solvent are under compression close to the Eu3+ion and held relatively stable configuration in the region surrounding the ion, whereas the line a t 591 nm shows fine structure. It is tentatively assigned as a magnetic dipole transition from thermally excited level, though it has not been clarified yet (IO). Holmium Ion. Figure 5 shows the photoacoustic spectrum in comparison with the absorption spectrum. All the lines did not evidence definite fine structure. For the case of the transitions to the upper 5F4and 5Szlevel from the ground level (51& around 540 nm, the absorption spectrum could not clearly be measured because of weak transitions, and the photoacoustic spectrum could not be observed over the whole wavelength region because of the limiting wavelength of the lasing dyes used. However, there were some differences between the photoacoustic and the absorption spectra, though precise comparison was somewhat difficult. Slight differences might arise from different cascade processes in the two excited states, radiatively and partly nonradiatively, to the ground state.
For the case of the 518-5F5transition a t 641 nm, three peaks were observed. The intensity ratios of the peak a t 641 nm to that a t 656 nm, were 2.3:l for the photoacoustic spectrum and 3.2:l for the absorption spectrum. This result may be explained as follows. The peaks a t 641 and 656 nm are considered to correspond to the transitions from the ground state (51s)to two different excited states. The calculations of Crozier e t al. (11) suggest that the terminal state of the transition a t 641 nm was the 5F5,and that a t 656 nm might be one component of the level resulting from the spin-orbit coupling of the 3G. The energy difference between the peaks a t 641 and 651 nm was about 240 cm-', which is in good agreement with the magnitude of the ground state splitting (7). Therefore, the peak a t 651 nm was interpreted as being due to the terminal population in the upper split ground state. Erbium Ion. Figure 6 shows the photoacoustic spectrum of Er3+ ion. The spectrum a t 542 nm is assigned to the 4115,2-4s, and the spectrum a t 652 nm, the 4115/2-4F9/2, respectively. Both spectra could not be measured over the whole wavelength region because of the limiting of lasing wavelength. Molar absorptivities of the above two electronic transitions are extremely small. The photoacoustic spectrum of E$+ ion, which was clearly detected by the LIPAS, has revealed simple splitting features.
LITERATURE CITED (1) Rosencwaig, A. Anal. Chem. 1975, 4 7 , 592A-604A. (2) Adams, M. J.; King, A. A,; Kirkbright, G. F. Analyst(London) 1976, 707, 73-85. (3) Dixon, R. N.; Haner, D. A,; Webster, C. R . Chem. Phys. 1977, 22, 199-206. (4) Lahmann, W.; Ludewig, H. J.; Welling, H. Anal. Chem. 1977, 49, 549-551. (5) Qda, S.;Sawada, T.; Kamada, H. Anal. Chem. 1978, 5 0 , 865-867. (6) Sawada, T.; Oda, S.;Kamada, H. Proc. Jpn. Acad. Ser. 6 ,1978, 5 4 , 189- 193. (7) Snitzer, E. Phys. Rev. Lett. 1961, 7 , 444-446. ( 8 ) Heller, A. Appl. Phys. Lett. 1966, 9 , 106-108. (9) Dawson, W. R.; Kropp, J. L. J . Opt. SOC. A m . 1965, 55, 822-828. (10) Miller, D. G.; Sayer,E. V.; Freed, S. J . Chem. phys. 1958, 29. 454-455. (11) Crozier, M. H.; Dunciman, W. A. J. Chem. Phys. 1961, 35, 1392-1409.
RECEIVED for review December 22, 1978. Accepted February 5, 1979.
