Anal. Chem. 1992, 64, 2615-2617
2615
Laser Two-Photon Ionization Detection of Aromatic Molecules on a Metal Surface in Ambient Air Teiichiro Ogawa' and To-oru Yasudat Department of Molecular Science and Technology, Kyushu University, Kasuga-shi, Fukuoka 816, Japan
Hirofumi Kawazumi Division of Science, Kitakyushu University, Kitakata, Kokura-Minami, Kitakyushu 802, Japan
A laser two-photon process can ionize a photoabsorblng molecule selectlvely though a one-photon resonant process wlthout lonlzlng the bulk surface. By meesurlng the lonlzed speclef~through photoconductlvlty, thls technlque has been found to be successful for hlghly sensltlvedetectionof aramatlc mokcuks on a metalsurface In ambient air. The photocurrent was due to electron drM and oxygen anlon drM In amblent alr. The photocurrent slgnal of varlous aromatlc molecules was approxlmately proportlonal to the absorbance at the laser wavelength. The detectlon Ilmlts were on the order of 10-ls mol/cm2 In favorable cases, whlch correspond to less than 1% coverage of the surface.
INTRODUCTION There are many practical needs for a highly sensitive analysis of organic molecules on surfaces in ambient air. However, most of the current techniques for highly sensitive detection on surfaces require the sample to be in a high vacuum; the laser desorption of surface molecules and succeasive mawspectrometric detection is a typical example.1-3 Surface molecules emit photoelectrons in vacuum upon a multiphoton process; ionization of several organic compounds adsorbed on thin metal films has been measured with a conductivity technique, and the wavelength dependence of ionization has been analyzed.4 However, multiphoton ionization of surface molecules in ambient air and its analytical applications have not been investigated. Laser two-photon ionization is a sensitive analytical technique applicable to photoabsorbing molecules in solution.5 By using a stepwise two-photon process, only those molecules which can absorb an incident laser photon are selectively ionized, while ionization of bulk molecules such as solvents can usually be ignored owing to their transparency at the laser wavelength. The ions thus produced can be sensitively detected with a conductivity measurement. Ppb to ppt detection limits have been reported for organic photoabsorbing molecules in solution.5-14 An application of the laser two-photon ionization technique to a surface analysis would
* Author to whom correspondence should be addressed.
Present address: Mitsubishi Electric Co.,Ltd., Itami 664,Japan. (1)Tembreull, R.; Lubman, D. M. Anal. Chem. 1987,59,1003-1006. (2)Zare, R. N.; Hahn, J. H.; Zenobi, R. Bull. Chem. SOC.Jpn. 1988, 61,87-92. (3)Stahl, B.;Steup, M.; Karas, M.; Hillenkamp, F. Anal. Chem. 1991, 63,1463-1466. (4)Naaman, R.;Petrank, A.; Lubman, D. M. J.Chem. Phys. 1983,79, 4608-4612. (5)Yamada, S.;Ogawa, T. h o g . Anal. Spectrosc. 1986,9,429-463. (6)Voigtman, E.; Jurgensen, A.; Winefordner, J. D. Anal. Chem. 1981, 53,1921-1923. (7) Yamada, S.;Kano, K.; Ogawa,T. Bumeki Kagaku 1982,31,E247+
be a natural extension of studies in solution and would make highly sensitive detection possible for photoabsorbing molecules on surfaces.15 The molecule a t the surface emits a photoelectron upon optical irradiation below 200 nm even in ambient air, and the work function of various surfaces has been determined with this technique.16 However, because this technique is based on one-photon ionization, an ionization signal originating from a trace amount of molecular adsorbates will be obscured by a large signal from the bulk surface. Thus, trace determination on the surface cannot be carried out with short-wavelength photons. The solution and the surface are similar in a sense that trace solutes or adsorbate molecules, which can absorb an incident laser photon, can be selectively ionized by a onephoton resonant process without ionizing the solvent or bulk surface. In the present paper we are reporting an application of the two-photon ionization technique to aromatic molecules on metal surfaces in ambient air.
EXPERIMENTAL SECTION Apparatus. The schematic diagram of the experimental apparatus is shown in Figure 1. A pulsed nitrogen laser (MolectronUV 24 337 nm, 9 mJ, 10ns) was focused by a quartz lens softly onto the sample of about 1cm2. A mesh electrode was located 10 mm above the sample and was connected to a highvoltage power supply unit (Ikegami HD2.5K-M); the typical applied voltage was 2.5 kV. The sample and the electrode were kept in an aluminum box to shield them from any external noise. The effect of atmosphere in the sample compartment was measured in an evacuable chamber. The photoionizationsignal (charge)was taken from the sample with a Keithley 427 current amplifier and a Canopus Analog Pro I1 analog-to-digital converter. The data were accumulated, analyzed, and stored with an NEC PC9801RX microcomputer. The time profile of the current signal was obtained with the current amplifier (100 kHz) and an Iwatsu DS6411 digital storagescope(40 MHz). The time resolution was determined by the former. The absorption spectra of samples were recorded on a Shimadzu UV2200 spectrophotometer with a Shimadzu ISR240 spherical integration attachment. Sample Preparation. A hexane solution (10 ML)of the aromatic molecule was dispersed on a metal plate (15 X 15 mm), and the plate was dried in air. The sample molecules were adsorbed as a thin layer of about 1 cm2,which was somewhat irregular. Most of the sample surfacewas irradiated by the laser (10)Fujiwara, K.;Voigtman, E.; Winefordenr, J. D. Spectrosc. Lett. 1984. 17. 9-20. (11)Yamada, S.;Ogawa, T.; Zhang, P. H. Anal. Chim. Acta 1986,183, 251-256. (12)Sato, N.; Yamada, S.; Ogawa, T. Anal. Sci. 1987,3, 109-111. Anal. Chem. 1987, (13)Yamada,S.;Sato,N.;Kawazumi,H.;Ogawa,T.
.---(14)Ogawa,T.;Kise, M.; Yasuda, T.; Kawazumi,H.;Yamada, S. Anal.
E250. .
59.2719-2721. _.
(8)Voigtman, E.;Winefordner, J. D. Anal. Chem. 1982,54, 18341839. (9)Yamada, S.;Hino, A.; Kano, K.; Ogawa, T. Anal. Chem. 1983,55, 1914-1917.
Chem. 1992,64,1217-1220. (15)Ogawa, T.; Yasuda, T.; Kawazumi, H. Anal. Sci. 1992,8,81-82. (16)Kirihata, H.; Uda, M. Reu. Sci. Imtrum. 1981,52,68-70.
0003-2700/92/0364-2615$03.00/0
0 1992 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 64, NO. 21, NOVEMBER 1, 1992 e
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radiation so that the effect of nonuniformity of the sample could be reduced as much as possible. The metal surface was polished with Maruto RDX K-1 cerium oxide powder. The aromatic molecules used were research grade and were used without further purification.
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RESULTS AND DISCUSSION Conductivity is the simplest and very sensitive technique for detecting charged species and has been applied for measuring electrons and ions produced by the laser multiphoton ionization in solution.k14 The adsorbed molecules on a metal surface also can give an intense signal upon laser irradiation. The photoionization signal was clearly observed when the mesh electrode was positively biased, and almost no signal was observed when it was negatively biased. Thus, the charge carrier should be species with a negative charge. The signal from the adsorbed molecule was quadratic to the incident laser pulse energy as shown in Figure 2, indicating a twophoton process. At a higher laser pulse energy, the signal was larger in the beginning, but it gradually decreased due probably to sample desorption. Thus, the laser pulse energy should be intense enough to observe a two-photon process but should not be very intense in order to avoid desorption of the adsorbed molecule. Typical time profiles of current induced in laser two-photon ionization of pyrene on nickel and their dependence on the applied voltage are shown in Figure 3. The profile consists of two components. The first one is fast and sharp, and it seems to be faster than the time resolution of the system (10 ps). The second one is slow and broad and extends to 1 ms
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after the laser irradiation at 500 V/cm. The fast component increased as the applied voltage increased, and thus it would be due to electrons ejected from the molecular adsorbate. They would be captured before they arrived at the mesh electrode; when the applied voltage was small, their velocity was small and they would be captured quickly. The slow component decreased in height and became broader at smaller applied voltages. Thus, the slow component shows time-offlight behavior, and we can assign it as negative ions. The dependence of the time profile on the gas, and its pressure in the sample compartment was measured by using
ANALYTICAL CHEMISTRY, VOL. 64, NO. 21, NOVEMBER 1, 1992
2617
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an evacuable chamber, as shown in Figure 4. The gases used were air, nitrogen, oxygen, and SF6. When nitrogen is used, the slow component disappeared. Meanwhile, the fast component decreased for oxygen and disappeared for SF6. SF6 is a famous electron scavenger, and the above results indicate that the fast component is indeed electrons. Timeof-flight behavior of the slow component varies with the gas, and it is slower for a heavier gas. Thus, we can conclude that the slow component is anions of the gas in the sample compartment, and it was anions of oxygen in ambient air, because the time profiles of the slow component in air and in oxygen were identical. The anion should be produced by capture of an emitted electron by a gas molecule. The mobility ( p ) of the anion in ambient air can be determined by analysis of the applied voltage (E, V/cm) dependence and the gas pressure @, Torr) dependence of the time-of-flight profile, as shown in Figure 5. These results showed that the velocity of the anion (u, cm/s) is expressed bY u = D(76O/p) The mobility of the oxygen anion was determined to be 2.3 cmZ/(V.s), which agreed with the reported value." The photoionization signals of 16 molecules which have an absorption at 337 nm have been observed, and their absorbance dependence is summarizedin Figure 6. The absorbance of the molecular absorbate was measured by using a spherical integral optics; the aromatic molecule was deposited on a quartz plate for absorbance measurements because an opaque sample was difficult to measure on a metal plate. The surface molecular density was l@gmol/cm2,and the laser pulse energy was 3 mJ/pulse. As is shown there, the photoionization current increases as the absorbance at 337 nm increases. A similar conclusion has been reached for aromatic molecules in solution.9J4 The efficiencyof excitation to the initial excited state seems to be more important for producing charged species than that of the ionization by the second photon. The photoionization signals of pyrene on a few metal plates were measured and were almost identical for all metals used, but the blank signals were different. The metal with a larger work function produced a smaller blank signal and is more convenient for highly sensitive detection. Thus, among the metals examined platinum offered the lowest blank signal and hence the highest S/Nratio for all sample molecules. However, the observed blank signal seems to indicate that (17)Handbook ofPhysica1 Constants;Asakura: Tokoyo 1978,p 115.
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1 Flgure 6. Photolonizatlon current and molar absorptivity. (1) 9-methylanthracene, (2) I-chioroanthracene, (3) 9-bromoanthracene, (4) anthracene, (5) naphthacene, (6) 2-methylanthracene, (7) coumarin 440,(8)2aminoanthracene.(9)9,1Wlmethylanthracene,(lO)perylene, (1 1)DPH. (12)tetraphene,(13)BBOT, (14)PBD, (15)pyrene,(16)BBQ.
Table I. Detection Limits of Aromatic Molecules Excited at 337 nm on a Platinum Surface compound pyrene tetraphene anthracene BBQ" (1
detection limit, mol/cm2 8.2 x 10-13 3.1 X 10-l2 4.5 x 10-12 2.5 x 1043
4,4'-Bis[(2-butylocty1)oxyl-p-quaterphenyl(ref 14).
even at 4.6 eV (337 nm) some photoelectric effect of the metal surface would occur. The detection limits defined as S/N= 3 are summarized in Table I. The highest sensitivity was obtained for BBQ on platinum. Its value, 2.5 X 10-13mol/cm2,correspondsto about l / 2 ~of a monolayer coverage, which was estimated by assuming flat adsorption with a surfacearea of 3 nm2/molecule. These results shows that the trace surface molecules can be selectively ionized by the laser two-photon ionization technique, and this method is a very sensitive technique for surface analysis of photoabsorbing molecules in ambient air.
RECEIVED for review May 27, 1992. Accepted July 29, 1992.
