Anal. Chem. 1997, 69, 4524-4529
Enhancement of the Molecular Ion Peak from Halogenated Benzenes and Phenols Using Femtosecond Laser Pulses in Conjunction with Supersonic Beam/Multiphoton Ionization Mass Spectrometry Junichi Matsumoto, Cheng-Huang Lin, and Totaro Imasaka*
Department of Chemical Science and Technology, Faculty of Engineering, Kyushu University, Hakozaki, Fukuoka 812, Japan
Halogenated benzenes and phenols are measured by multiphoton ionization mass spectrometry using femtosecond (150, 500 fs) and nanosecond (15 ns) laser pulses. The molecular ion is strongly enhanced for monohalobenzenes when the pulse width of the ionization laser is shorter than the lifetimes of their excited states. This is attributed to the reduction of intersystem crossing by a spin-orbit interaction, the so-called internal heavyatom effect, and to rapid dissociation from the triplet state. A femtosecond laser pulse was deemed to be useful for the efficient ionization of dichlorobenzene and trichlorobenzene although their lifetimes are unknown, since polychlorinated benzenes are thought to have shorter lifetimes as the result of a stronger spin-orbit interaction. The ionization efficiencies of o-chlorophenol and p-chlorophenol are also obtained using femtosecond and nanosecond pulses. In the case of o-chlorophenol, intersystem crossing occurs more efficiently by stabilization of the triplet state by intramolecular hydrogen bonding, and as a result, the femtosecond pulse is more effective in ionizing o-chlorophenol, which has a shorter lifetime. These results indicate that an ultrashort laser pulse is very useful in improving the ionization efficiency for a molecule with a short lifetime, such as polychlorinated dioxins and their precursors. Polychlorinated aromatic hydrocarbons, such as dioxins, which are released into the atmosphere, are known to be potent carcinogens. Attempts have been made to analyze the exhaust gas from an incinerator by gas chromatography, interfaced with a mass spectrometer (GC/MS). Though a fast GC has been developed, a long time is required for separation of the sample consisting of a mixture of many constituents including isomers and isotopes. Thus, conventional GC/MS cannot be used for real-time monitoring of exhaust gases such as these, since it takes more than 30 min for the elution of the components, and more than 1 day is required to pretreat the sample prior to making the measurement. As a result, there is an urgent need for a new analytical method that can be used in the real-time monitoring of polychlorinated aromatic hydrocarbons and their precursors, which are generated during incineration. Needless to say, a selective analytical method is essential, since many types of molecular species with closely 4524 Analytical Chemistry, Vol. 69, No. 22, November 15, 1997
related structures but with different carcinogenicities would be expected to be present in the exhaust gas. Supersonic jet spectrometry has been developed and used as a sensitive and selective means for analysis of aromatic hydrocarbons, since supersonic jet expansion produces cold and isolated molecules, therefore providing well-resolved ultraviolet and visible spectra.1,2 A combination of supersonic jet spectrometry with timeof-flight mass (TOF-MS) spectrometry has a great deal of potential for the identification of aromatic hydrocarbons.3-5 This approach has high spectroscopic selectivity and also is capable of real-time monitoring. It would be expected, therefore, that this technique could be used for the determination of dioxins and their precursors. However, the ionization efficiencies for chlorinated aromatic hydrocarbons are rather poor, especially when a nanosecond laser is employed for excitation and succeeding multiphoton ionization.5 In addition, a molecular ion sometimes undergoes dissociation to produce fragment ions, which is attributed to fast energy relaxation and the succeeding dissociation from an intermediate state. It is well-known that intersystem crossing to the triplet manifolds occurs by a spin-orbit interaction, the so-called internal heavy-atom effect. In particular, polychlorinated dioxins, e.g., 2,3,7,8-tetrachlorodibenzo-p-dioxin, would be expected to have short lifetimes, relative to that of monochlorobenzene, because of the larger number of chlorine atoms on the benzene ring. Two mechanisms, i.e., a ladder mechanism and a ladderswitching mechanism, are used to explain the relaxation process in multiphoton ionization and the succeeding dissociation of aromatic hydrocarbons.6-8 In the ladder mechanism, only a parent molecule is assumed to absorb photons, and fragment ions are produced from the molecular ion. In the ladder-switching mechanism, photons are also absorbed by neutral and ionic fragments, in addition to the parent molecule. The ladder mechanism would (1) Hayes, J. M.; Small, G. J. Anal. Chem. 1983, 55, 565A-574A. (2) Levy, D. H. Annu. Rev. Phys. Chem. 1980, 31, 197-225. (3) Tembreull, R.; Lubman, D. M. Anal. Chem. 1987, 59, 1082-1088. (4) Williams, M. W.; Beekman, D. W.; Swan, J. B.; Arakawa, T. Anal. Chem. 1984, 56, 1348-1350. (5) Weickhardt, C.; Zimmermann, R.; Schramm, K. W.; Boesl, U.; Schlag, E. W. Rapid Commun. Mass Spectrom. 1994, 8, 381-384. (6) Gedanken, A.; Robin, M. B.; Kuebler, N. A. J. Phys. Chem. 1982, 86, 40964107. (7) Dietz, W.; Neusser, H. J.; Boesl, U.; Schlag, E. W.; Lin. S. H. Chem. Phys. 1982, 66, 105-127. (8) Yang, J. J.; Gobeli, D. A.; El-Sayed, M. A. J. Phys. Chem. 1985, 89, 34263429. S0003-2700(97)00462-9 CCC: $14.00
© 1997 American Chemical Society
Figure 1. Experimental apparatus for supersonic beam/multiphoton ionization/time-of-flight mass spectrometry.
be predicted to be more important under ultrashort laser pulses, since the interaction time between a molecule and photons is very short. As a result, the molecular ion is strongly enhanced under ultrashort laser pulses. A high-intensity laser, however, accelerates photodissociation, and the pulse energy should then be adjusted to an optimum value, in order to maximize the signal for the molecular ion. Several investigations of nonjet spectroscopy using a picosecond laser for multiphoton ionization have been reported.8-14 The ionization efficiencies of halogenated benzenes with short lifetimes are substantially improved by using a picosecond laser instead of a nanosecond laser.14 Recently, a femtosecond laser has become commercially available and could be used for the ionization of a molecule within a short time period before relaxation, a process that substantially improves the ionization efficiency. This multiphoton ionization technique using a femtosecond laser has already been applied to supersonic beam spectroscopy and spectrometry as well.15-17 However, a femtosecond laser has not yet been used to examine the effect of the short laser pulse relative to the excited-state lifetime on the ionization efficiency in detail. In this study, fluorobenzene, chlorobenzene, bromobenzene, and iodobenzene are employed as analytical samples, since the lifetimes of their first excited states have already been reported and are known to range from the nanosecond to the femtosecond scales. The present investigation was performed using 150-fs, 500fs, and 15-ns excimer laser pulses emitting at 248 nm. We also applied this technique to m-dichlorobenzene and 1,2,4-trichlorobenzene, in order to examine the influence of the number of (9) Szaflarski, D. M.; El-Sayed, M. A. J. Phys. Chem. 1988, 92, 2234-2239. (10) Hering, P.; Maaswinkel, A. G. M.; Kompa, K. L. Chem. Phys. Lett. 1981, 83, 222-225. (11) Larciprete, R.; Stuke, M. J. Phys. Chem. 1986, 90, 4568-4573. (12) Larciprete, R.; Stuke, M. Appl. Phys. B 1987, 42, 181-184. (13) Wilkerson, C. W., Jr.; Colby, S. M.; Reilly, J. P. Anal. Chem. 1989, 61, 22692673. (14) Wilkerson, C. W., Jr.; Reilly, J. P. Anal. Chem. 1990, 62, 1804-1808. (15) Lin, C. H.; Matsumoto, J.; Ohtake, S.; Imasaka, T. Talanta 1996, 43, 19251929. (16) Matsumoto, J.; Lin, C. H.; Imasaka, T. Anal. Chim. Acta 1997, 343, 129133. (17) Weinkauf, R.; Aicher, P.; Wesley, G.; Grotomeyer, J.; Schlag, E. W. J. Phys. Chem. 1994, 98, 8381-8391.
Figure 2. Femtosecond/nanosecond laser system: (A) 150 fs; (B) 500 fs; (C) 15 ns. QCDL, quenched cavity dye laser; SCDL, short cavity dye laser; DFDL, distributed-feedback dye laser; BBO, β-barium borate crystal.
chlorine atoms substituted on a benzene ring. Finally, chlorophenols, which are known to be precursors of dioxins, were examined in terms of whether a femtosecond laser is useful for the efficient ionization of the precursor of dioxins, as well as dioxins themselves. EXPERIMENTAL SECTION A supersonic beam/time-of-flight mass spectrometer used in this study is shown in Figure 1. Three types of laser systems, shown in Figure 2, are used for excitation and the succeeding multiphoton ionization. The supersonic beam spectrometer and the laser system used in this study have been reported in detail elsewhere15,18,19 and are described only briefly here. The sample Analytical Chemistry, Vol. 69, No. 22, November 15, 1997
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is placed in a reservoir and is heated at 80-150 °C by a tape heater to control and optimize the vapor pressure. The vaporized sample is introduced from a pulsed nozzle into a vacuum chamber through an orifice (0.8 mm i.d.). The pressure of the argon gas in the nozzle is 150 Torr. The femtosecond laser system consists of a femtosecond dye laser (Lambda Physik, LPD 500fs) and a KrF excimer amplifier (Lambda Physik, LPX 305i). This system produces 500-fs, 10-mJ laser pulses at 248 nm. The pulse width can be reduced to 150 fs (3 mJ) by means of a pulse compressor consisting of a prism pair. In contrast, nanosecond laser pulses are generated by a double-pass feedback of the amplified spontaneous emission (ASE) by placement of a reflective mirror, as shown in Figure 2C. This laser produces 15-ns, 10-mJ pulses at 248 nm. The laser beam is focused by a quartz lens (focal length, 1 m) into a supersonic beam, and the laser pulse energy is altered by adjusting the diameter of pinhole 1. The beam waist size was maintained constant as much as possible in the nanosecond and femtosecond experiments by adjusting the diameter of pinhole 2. The laser pulse energy was monitored by a Joule meter (Molectron, J3-05DW). The mass spectrum is recorded, and 200 signals are averaged by a digital oscilloscope (LeCroy 9360). The data are processed by a personal computer. The samples were purchased from Wako Pure Chemical Industries (Tokyo, Japan) and were used without further purification. RESULTS AND DISCUSSION Monohalogenated Benzenes. Figure 3 shows the mass spectra for fluorobenzene, chlorobenzene, bromobenzene, and iodobenzene, which are obtained by using 150-fs, 500-fs, and 15ns laser pulses, respectively. The pulse energies were adjusted to 0.3 mJ for fluorobenzene and chlorobenzene and to 0.2 mJ for bromobenzene and iodobenzene, respectively, in order to obtain the largest signals for molecular ions in the mass spectra. Wilkerson and co-workers reported that the first excited-state (S1) lifetimes of fluorobenzene, chlorobenzene, and bromobenzene are 5.5 ns, 600 ps, and 30 ps, respectively.14 Intersystem crossing to triplet levels is strongly enhanced by a spin-orbit interaction, which substantially decreases the lifetime with increasing atomic weight of the halogen atom. Thus, the lifetime of bromobenzene is shortest, among these molecules. Although they used the fourth harmonic emission of a Nd:YAG laser (266 nm) to excite the molecules, we used an excimer laser (248 nm) instead. Therefore, fluorobenzene, chlorobenzene, and bromobenzene are considered to be excited to higher vibrational levels in the S1 states by absorbing the first photon. Table 1 shows the wavelength of the 0-0 transition and the ionization potential for monohalogenated benzenes. It is likely that the molecule excited to higher vibrational levels relaxes to the lowest vibrational level in the S1 state very rapidly by an internal conversion. Thus, the lifetime of the first excited state, which is measured by using a 248-nm laser, may not differ significantly from the values reported by Wilkerson and co-workers using a 266-nm laser. Cheng and coworkers reported two dynamic channels in the dissociation process for iodobenzene: a direct (coherent)-mode dissociation with a decay time of 400 fs and a predissociation with a decay time of 600 fs.20 The excitation (S0 f S1) of a halogenated benzene (18) Imasaka, T.; Hozumi, M.; Ishibashi, N. Anal. Chem. 1992, 64, 2206-2209. (19) Lin, C. H.; Fukii, H.; Imasaka, T.; Ishibashi, N. Anal. Chem. 1991, 63, 14331440. (20) Cheng, P. Y.; Zhong, D.; Zewail, A. H. Chem. Phys. Lett. 1995, 237, 399405.
4526 Analytical Chemistry, Vol. 69, No. 22, November 15, 1997
Figure 3. Mass spectra obtained by using 150-fs, 500-fs, and 15ns laser pulses. Samples: (a) fluorobenzene; (b) chlorobenzene; (c) bromobenzene; (d) iodobenzene. The pulse energy was adjusted to (a, b) 0.3 or (c, d) 0.2 mJ to maximize the molecular ion peak. Table 1. Wavelength of 0-0 Transition and Ionization Potential for Monohalogenated Benzenes
a
compound
0-0 transition (nm)
IPa (eV)
flourobenzene chlorobenzene bromobenzene iodobenzene
264.44 269.81 270.33 272.1
9.20 9.06 8.98 8.69
IP, ionization potential.
is based on π f π* transition, and the ionization from the S1 state corresponds to a removal of a π electron from a neutral molecule.21 However, iodobenzene is known to be excited by an ultraviolet photon to a repulsive potential through an n f σ* transition, a so-called A-band continuum, which extends from approximately 240 to 320 nm. This is related to the transition to a repulsive potential for a chemical bond between carbon and chlorine atoms, in addition to the π f π* transition.22,23 Mixing of these π* and σ* natures in the upper state results in both predissociation and direct dissociation for the case of iodobenzene. As shown in Figure 3a, the ionization efficiency for fluorobenzene is independent of the laser pulse width. Since the pulse (21) Tembreull, R.; Sin, C. H.; Li, P.; Pang, H. M.; Lubman, D. M. Anal. Chem. 1985, 57, 1186-1192. (22) Hwang, H. J.; El-Sayed, M. A. J. Chem. Phys. 1992, 96, 856-858. (23) Freitas, J. E.; Hwang, H. J.; El-Sayed, M. A. J. Phys. Chem. 1993, 97, 1248112484.
Figure 4. Relationships between the intensities of molecular ions and laser pulse energy for monohalogenated benzenes. Samples: (a) fluorobenzene; (b) chlorobenzene; (c) bromobenzene; (d) iodobenzene.
widths used in this study are comparable to, or less than the S1 lifetime of fluorobenzene, the molecule is efficiently ionized by absorbing the second photon prior to relaxation to the triplet or ground state. As shown in Figure 3b, the ionization efficiency for chlorobenzene, obtained under 150-fs laser pulses, is identical to that obtained under 500-fs pulses. As in the case of fluorobenzene, chlorobenzene is readily ionized by femtosecond laser pulses prior to relaxation, though the S1 lifetime (600 ps) is much shorter than that of fluorobenzene. However, the intensity of the molecular ion peak obtained using a nanosecond laser is one-tenth the intensity obtained using the femtosecond laser, due to a shorter relaxation time relative to the laser pulse width. It is also evident from Figure 3b and c that the results obtained for bromobenzene are apparently different from those for chlorobenzene, since no molecular ion is observed under nanosecond laser pulses, and the molecular ion obtained, even under 500-fs pulses, is slightly smaller than that obtained under 150-fs pulses, due to a short S1 lifetime (30 ps). As shown in Figure 3d, the molecular ion of iodobenzene ionized under 150-fs pulses is 3-fold larger than that obtained under 500-fs pulses. This suggests a shorter S1 lifetime for iodobenzene. As described previously, this may be ascribed to fast dissociation processes (direct dissociation and predissociation) for iodobenzenes. Figure 4 shows the relationships between the intensities of the molecular ion signals for monohalogenated benzenes and laser pulse energy. The ion signal intensity in the mass spectrum generally increases with increasing pulse energy. However, the ionization efficiency is smaller for a molecule with a shorter lifetime and deviates from a linear relationship at higher pulse energies. This tendency is more serious when a longer laser pulse is used. In the extreme case, no signal is observed for bromobenzene and iodobenzene under nanosecond laser pulses, even when the pulse energy is increased to 1.0 or 1.6 mJ, respectively; a strong background signal appeared, and as a result, the experiment was not performed above these pulse energies. Thus, the advantage of the femtosecond laser in application to multiphoton ionization is evident.
Figure 5 shows the mass spectra for chlorobenzene, bromobenzene, and iodobenzene obtained with 150- and 500-fs laser pulses, in which the pulse energy is increased to enhance the fragment ions. No fragment ion was observed in the mass spectrum for fluorobenzene, and these data were omitted from Figure 5. In the mass spectra, large fragment ion peaks such as C6H5+ are observed, in addition to a molecular ion peak. No appreciable change is observed in the mass spectra measured for chlorobenzene under 150-fs and 500-fs laser pulses. On the other hand, bromobenzene appears to be slightly more dissociative. The lifetimes of chlorobenzene and bromobenzene are estimated to be in the picosecond ranges and are much longer than the laser pulse widths. These results are reasonable and expected. In contrast, iodobenzene is dissociated from the S1 state and has a very short lifetime, due to direct photodissociation from a repulsive state. Therefore, the intensity of the molecular ion peak relative to the fragment peak decreases with increasing pulse width of the laser. Thus the fragmentation can substantially be suppressed when the sample molecule is ionized by using a laser pulse shorter than the lifetime of the excited state. This behavior indicates that the ladder mechanism is more important than the ladder-switching mechanism in the multiphoton ionization of iodobenzene. Dichlorobenzene. Figure 6 shows the mass spectra for m-dichlorobenzene obtained using 150-fs, 500-fs and 15-ns pulses at different pulse energies. The ion signals are observed in the mass spectra only when the femtosecond laser pulses are used. As shown in Figure 6a-c, the molecular ion signals are dominant but rather weak. The fragment ions, which were formed by releasing one or two chlorine atoms, are detected, in addition to the molecular ion, with increasing laser pulse energy, as shown in Figure 6b and c. The mass spectral patterns for m-dichlorobenzene are similar to those for bromobenzene; molecular ion peaks are observed as major components under both 150- and 500-fs pulses, but no signals are observed when 15-ns laser pulses are used. This suggests that the lifetime of m-dichlorobenzene is several tens of picoseconds, since the lifetime of bromobenzene is reported to be 30 ps.14 The lifetime of monochlorobenzene is reported to be 600 ps. These results indicate that the lifetime decreases by 1 order of magnitude for each chlorine atom as the result of an internal heavy-atom effect. Shimoda and co-workers have discussed the relationship between the fluorescence quantum yield and the excited-state lifetime for o-, m-, and p-dichlorobenzene. The rate of nonradiative transition for p-dichlorobenzene excited at the 0-0 absorption band is generally thought to be slower than those for o- and m-dichlorobenzene.24 This can be attributed to inefficient intersystem crossing for p-dichlorobenzene. It is known that intersystem crossing is inefficient for a molecule with high symmetry due to a sparse triplet manifold to be coupled with the S1 state. As a result, the lifetime of p-dichlorobenzene becomes longer than those for o- and m-dichlorobenzene. However, excess vibrational energies for these isomers are larger than 2000 cm-1 at 266 nm, and as a result, the rate of radiationless transition through a singlet-triplet coupling would not be expected to significantly differ among these molecules at such high energies.24 In this study, we employed 248-nm laser pulses, which have excess energies of 4000 cm-1, and therefore, the mass spectra for o- and (24) Shimoda, A.; Hikida, T.; Mori, Y. J. Phys. Chem. 1979, 83, 1309-1312.
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Figure 5. Mass spectra obtained using 150-fs, 500-fs, and 15-ns laser pulses. Samples: (a) chlorobenzene; (b) bromobenzene; (c) iodobenzene. The pulse energy was adjusted to (a) 1.0, (b) 0.75, and (c) 0.50 mJ to enhance fragment ion peaks.
Figure 6. Mass spectra for m-dichlorobenzenes ionized using 150-fs, 500-fs, and 15-ns pulses. Pulse energy: (a) 40, (b) 100, and (c) 150 µJ.
p-dichlorobenzenes are expected to be similar to that for mdichlorobenzene. Trichlorobenzene. Figure 7 shows the mass spectra for 1,2,4trichlorobenzenes obtained using 150-fs, 500-fs, and 15-ns laser pulses. No ion signals are observed when 15-ns pulses are used. In Figure 7a, a molecular ion is observed as a major component, but a fragment ion is also observed under 500-fs pulses. Fragmentation is strongly enhanced by increasing the laser pulse energy, as shown in Figure 7b. These results indicate that the lifetime of 1,2,4-trichlorobenzene is in the subpicosecond range. The lifetime is strongly affected by intersystem crossing to the triplet manifolds, as described above. There are two possibilities for the fragmentation of 1,2,4-trichlorobenzene under 248-nm pulses. (1) Intersystem crossing, which is very fast but remains the rate-determining process, is followed by a predissociation from the triplet state, due to potential curve crossing. (2) Direct dissociation occurs from the excited state, as for the case of iodobenzene, by substitution of three chlorine atoms. In either case, the molecular ion signal is observed with difficulty when many chlorine atoms are substituted. It should be emphasized that the molecular ion can be detected as a major component under femtosecond pulses, even when three chlorine atoms are substituted on a benzene ring. 4528 Analytical Chemistry, Vol. 69, No. 22, November 15, 1997
Figure 7. Mass spectra for 1,2,4-trichlorobenzenes ionized using 150-fs, 500-fs, and 15-ns pulses. Pulse energy: (a) 100 and (b) 300 µJ.
Chlorophenols. Figure 8a shows the mass spectra for p-chlorophenol obtained using 150-fs, 500-fs and 15-ns laser pulses. The ionization efficiency for p-chlorophenol obtained under 150fs pulses is nearly identical to that obtained under 500-fs pulses. In contrast, the intensity of the molecular ion peak decreases to
Figure 8. Mass spectra obtained using 150-fs, 500-fs, and 15-ns laser pulses. Samples: (a) p-chlorophenol; (b) o-chlorophenol. The pulse energy was adjusted to (a) 100 and (b) 40 µJ to maximize the molecular ion peak.
one-thirtieth of the intensity, when the pulse width is changed from nanosecond to femtosecond. There is no essential change in the spectral pattern between the mass spectra measured for p-chlorophenol and chlorobenzene. This implies that the lifetime is not strongly affected by para substitution of an OH group. Figure 8b shows the mass spectra for o-chlorophenol. No ion signal is observed when 15-ns pulses are used. Yamamoto and co-workers have reported that the triplet potential curve crosses near the bottom of the S1 state because of stabilization of the triplet state by intramolecular hydrogen bonding for ochlorophenol.25 The intersystem crossing then occurs more efficiently and rapidly, and as a result, the lifetime of o-chlorophenol may be substantially shortened, in comparison with those for p- and m-chlorophenols. Thus, the substitution of the OH group at an ortho position reduces the ionization efficiency and makes the analysis of the o-chlorophenol derivatives more difficult using a nanosecond laser. It should be again pointed out, however, that the femtosecond laser is useful for efficient ionization of such derivatives. It should also be noted that dioxins have no hydrogen bond capabilities, and as a result, such an additional lifetime shortening effect may not occur for these types of molecules. (25) Yamamoto, S.; Ebata, T.; Ito, M. J. Phys. Chem. 1989, 93, 6340-6345.
CONCLUSIONS This work clearly demonstrates that ultrashort laser pulses can be very useful for excitation and the succeeding multiphoton ionization of the benzene derivatives that contain halogen atoms in a molecule and have short lifetimes as a result of spin-orbit coupling. In addition, such an ultrashort pulse usually provides a molecular ion as the major component. For polychlorobenzenes, the lifetime decreases with an increase in the number of chlorine atoms. Thus, a femtosecond laser is essential for efficient ionization and for observing the molecular ion for polychlorobenzenes that contain multiple chlorine atoms. The lifetime of o-chlorophenol, which is known to be an important precursor of dioxins, decreases as a result of intramolecular hydrogen bonding, compared to other precursors such as m- and p-chlorophenols. As demonstrated in this study, the ionization efficiency can generally be improved by using shorter, e.g., 500- and 150-fs laser pulses. The present approach, using short laser pulses, might be useful in the sensitive detection of even dioxins themselves, since their lifetimes are generally considered to be rather short, due to the presence of many chlorine atoms in the molecule. However, the femtosecond laser is not always advantageous and sometimes is disadvantageous. The line width of the femtosecond laser is in the order of nanometers, and as a result, the high spectral selectivity in supersonic beam spectrometry is, to some extent, lost. In addition, efficiency enhanced by resonance ionization may decrease. It should, however, be noted that the spectral line width (∆λ ) 0.6 nm for 150-fs pulses at 248 nm) is much smaller than the spectral bandwidth of the room temperature spectrum and that differentiation of isomeric species is possible in most cases. The ionization efficiency is not reduced when the spectral line width is equal to or is broader than the line width of the laser. Thus, a femtosecond laser is useful for efficient ionization of the molecule with a short lifetime, i.e., a broad spectral bandwidth. It would be expected that chlorinated aromatic hydrocarbons, which are known to be precursors of dioxins, or, dioxins themselves, can be sensitively and selectively measured, when the molecules are ionized by a laser whose pulse width is identical to the lifetime of the molecule and the spectral width is infinitely minimized (Fourier transform-limited pulse). Received for review May 7, 1997. Accepted August 21, 1997.X AC9704624 X
Abstract published in Advance ACS Abstracts, October 1, 1997.
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