J. Phys. Chem. 1995,99, 7867-7870
7867
Condensation of Supersaturated Vapors on Benzene Ions Generated by Resonant Two-Photon Ionization: A New Technique for Ion Nucleation David Kane, George M. Daly, and M. Samy El-Shall* Department of Chemistry, Virginia Commonwealth University, Richmond, Virginia 23284-2006 Received: January 25, 1995; In Final Form: March 17, 1995'
A new technique is described here that allows the study of ion-induced nucleation by well defined ions. The technique is based on resonant two-photon ionization (R2PI) of a chromophore molecule present in small concentrations in a supersaturated host vapor. With this method it is now possible, for the first time, to selectively and unambiguously generate specific ions of interest and study their nucleating behaviors during the process of condensation of different supersaturated vapors. The new method is demonstrated by studying the ion-induced nucleation of supersaturated methanol and acetonitrile vapors by benzene molecular ions. B2" 6; resonance transition. The The benzene ions are generated by R2PI using the benzene's AI, measurements are carried out in a thermal diffusion cloud chamber. The nucleation count vs wavelength exhibits a characteristic peak at 258.9 nm, which matches the mass spectrum obtained by R2PI of benzene in a high-pressure mass spectrometric source. The observed nucleation count in supersaturated methanol vapor increases by about 20% for benzene cations as compared to negative ions, which suggests an apparent enhancement for the condensation of this supersaturated vapor on positive ions.
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Detailed knowledge about ion nucleation in the vapor phase is crucial for the understanding of many important problems such as the reactions in the ionosphere, the condensation of interstellar dust, the formation of acid rain, and environmental pollutants in the presence of SO3+ and NO*+ ions and for other applications in radiation chemistry and combustion processes. Complete understanding of the role of various molecular properties in the ion nucleation process is necessary for the development of proper molecular theories for nucleation, with the implicit assumption that these theories will provide a better picture of how ion-molecule and intermolecular interactions lead to the rich dynamic behavior of condensed phase systems. Because of the strong ion-dipole and ion-induced dipole forces, ion nucleation occurs preferentially with respect to homogeneous nucleation. This means that ion nucleation can be induced at lower vapor supersaturations than those required for homogeneous nucleation. In fact, it is a common practice in nucleation measurements to apply an electric field to remove the stationary ions created by cosmic rays from the nucleation zone before homogeneous nucleation takes place. This phenomenon was first discovered by Wilson almost 100 years ago when he reported observations of dense clouds induced by ions in supersaturated water vapor.' It is, however, surprising that in spite of its considerable scientific and applied interest, ioninduced nucleation has received less attention than homogeneous or binary nucleation. This is evident from the few experimental studies that have been devoted to the subject since the early experiments of Wilson in an expansion cloud These studies were limited by their inability to control the generation of ions and by the unknown identity of the nucleating ions since in virtually all studies the ions were generated by either X-rays, radioactive sources, or uncharacterized photoionization. Under these conditions, molecular fragment, doubly charged, carrier gas and impurity ions are generated among some other undesirable species such as free radicals which can induce nucleation through radical addition reactions.8-'0 Since different ionic species are characterized by different mobilities, it is almost impossible to obtain accurate quantitative data on ion @
Abstract published in Advance ACS Abstracts, April 15, 1995.
0022-365419512099-7867$09.00/0
nucleation without unambiguously identifying the nucleating ions. These problems were recognized more than 20 years ago, and in a key paper by Castleman and Tang," the authors concluded that experiments in cloud chambers containing unidentified ionic species cannot yield significant results for ion nucleation. Unfortunately, to date, still no experiment on ion nucleation has unambiguously identified the nature of the nucleating ions. This knowledge is particularly important if energetic and structural information on small cluster ions is to be used in determining the free energy of the embryonic clusters and consequently the barrier height to n u ~ l e a t i o n . ' ~ , ' ~ Here, we report on a new approach for ion nucleation based on resonant two-photon ionization (R2PI), in which well-defined molecular ions act as nucleation centers for the condensation of supersaturated vapors. Our approach differs from previous studies of ion nucleation in that the identity of the nucleating ions is known and the ionization process is controlled. For this purpose we use a chromophore molecule present in small concentrations within a supersaturated nucleating vapor, produced in a thermal diffusion cloud chamber (DCC). Molecular ions of the chromophore produced by R2PI induce the condensation of the supersaturated host vapor. To verify the generation of the molecular ions of interest, high-pressure mass spectrometry (HPMS) using R2PI in the unsaturated host vapor has been utilized. The comparison between the nucleation rate as a function of laser wavelength in the nucleation experiment with the mass spectrum from the HPMS experiment provides the first unambiguous evidence for the condensation of a supersaturated vapor on well-defined molecular ions. The systems of interest in the present study consist of molecular benzene cations (CsHs') upon which supersaturated methanol or acetonitrile vapors condense to form liquid droplets. Selective ionization of benzene is achieved through the resonance of the AI, B2" 6; transition as the intermediate state, which occurs at 38 609 cm-' in isolated C6H6 and exhibits an isolated-molecule lifetime of 100 ns.I4 In the ion nucleation experiment, we used an upward thermal DCC to produce a supersaturated methanol vapor under welldefined conditions of temperature, pressure, and supersaturation.
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0 1995 American Chemical Society
1868 J. Phys. Chem., Vol. 99, No. 20, 1995
This chamber is designed so that to a high degree of approximation, one dimensional diffusion takes place through a carrier gas from a lower heated pool of liquid to an upper metal plate at which the vapor molecules are condensed.I5 As vapor pressure is approximately an exponential function of temperature while temperature and partial pressure profiles in the chamber are almost linear, the vapor is supersaturated throughout the chamber. The supersaturation can be made as large as desired by increasing the temperature gradient. The critical chamber state for homogeneous nucleation is defined as the point at which the temperature difference across the chamber is sufficient to yield a regular rain of drops (-1 drop/(cm3 s)). The onset of nucleation is determined by observing the forward scattering of light from drops falling through a horizontal He-Ne laser beam positioned at an elevation lower than 0.50 (reduced height) of the cloud chamber. These drops originate near the elevation at which the maximum (peak) supersaturation occurs (-0.75 reduced height). A solution of 0.1% benzene in methanol (2000 ppm) was used as the working fluid on the lower plate of the DCC chamber. This small concentration of benzene has very little effect on the homogeneous nucleation of methanol, and to a good approximation, the system can still be treated as a homomolecular one component condensable vapor. The temperature gradient between the top and bottom plates was adjusted to obtain a maximum supersaturation of methanol vapor of 1.80 at 274 K. Under these conditions no nucleation could be observed in the absence of the UV laser light having the characteristic resonant frequency of the 6; transition of benzene. The tunable radiation is provided by an XeCl excimerpumped dye laser (Lambda Physik LPX-101 and FL3002, respectively). Coumarin 500 dye laser output passes through a P-BaB204 crystal (CSK Co.) cut at 52" to generate continuously s pulses. The spatially tunable frequency-doubledoutput of filtered ultraviolet radiation passes through the DCC at an elevation of about 0.55 (reduced height) just above and parallel to the He-Ne laser beam. Figure l a exhibits a sketch of the ion nucleation experiment, and Figure l b displays temperature, pressure, and supersaturation profiles within the chamber under typical experimental conditions. A single laser pulse (lo-* s) of a given frequency is introduced into the chamber, and if drops are formed they fall through the He-Ne beam, thus scattering a stray signal of light. The UV laser is slightly focused near the center of the chamber, and nucleation is normally observed as a thin dense cloud of small droplets distributed equally across the chamber, thus indicating that the ions are generated throughout the W laser beam. The forward light scattering is collected using a photomultiplier and a computer. At higher supersaturation the nucleating droplets formed with a single laser pulse fall together and thus become indistinguishable as individual droplets by the photomultiplier. Therefore, the average integrated intensity is used as an indication of the relative number of nucleation events. Average integrated intensity counts due to at least five individual laser pulses are measured at each wavelength. The wavelength is scanned in 0.1 nm steps, and sufficient time is allowed between steps to ensure that the chamber achieves its equilibrium state. Figure 2, a and b, shows the dependences of the number of nucleation events (integrated intensity count) on the wavelength of the dye laser in the absence and presence of 0.1% benzene in the methanol pool; respectively. In the absence of benzene, no nucleation occurs at any wavelength, as shown in Figure 2a. Similar results were obtained for acetonitrile as a working fluid and host vapor. It is clear from Figure 2, b and c, that in
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Figure 1. (a) Schematic of the ion nucleation experiment. (b) Profiles of temperature (T, K), partial pressure (P,Torr), equilibrium vapor pressure (Pe, Torr), vapor density (d, g/cm3), and supersaturation. (S) in a diffusion cloud chamber for supersaturated methanol vapor.
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Figure 2. Ion nucleation spectra of (a) supersaturated methanol vapor containing no benzene [S,,,(T) = 1.93, T = 271.1 K; S and T in the nucleation zone are estimated as 1.92 and 272.6 K, respectively]; (b) = 1.80, T supersaturated methanol vapor containing benzene [Smm(T) = 274 K; S and T in the nucleation zone are estimated as 1.80 and 275.3 K, respectively]; and (c) supersaturated acetonitrile vapor = 2.49, T = 271 K; S and T in the containing benzene [S,,,(i') nucleation zone are estimated as 2.45 and 274.2 K, respectively].
both cases (methanol and acetonitrile) the nucleation takes place only at the frequency corresponding to the benzene's 6; transition.
J. Phys. Chem., Vol. 99, No. 20, 1995 7869
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Figure 4. Dependence of the observed nucleation count on the strength and sign of the electric field between the top and bottom plates of the cloud chamber for a supersaturated methanol vapor containing benzene. S and T in the nucleation zone are estimated as 1.83 and 277.1 K,
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Figure 3. (a) Ion intensity vs wavelength from HPMS of 0.5% benzene (B) in acetonitrile (A) vapor mixture with a total pressure of 9 x Torr. NZ is used as a carrier gas. Wavelength scan obtained by monitoring the ion signal of C&+. (b) Same as in (a) except the scan was obtained by monitoring the ion signal of C,&+*C&CN. (c) Mass spectrum of the benzene/acetonitrile vapor mixture obtained by R2PI I= 258.85 nm in the HPMS experiment. with ,
To verify the generation of the benzene ions, we used R2PI in a high pressure mass spectrometric source. The details of the experiment are reported elsewhere.I6 Briefly, the spatially filtered ultraviolet radiation (0.4 mT per pulse) is shaped with a 60 cm lens to provide a 0.09 m2beam across the high pressure source (0.05-1 Torr) which is placed inside a vacuum chamber. Ions generated by R2PI exit the source through a 0.02 cm diameter hole. A quadrupole mass filter (Extrel C-50) is mounted coaxially to the ion exit hole. The operating pressure in the mass spectrometer region is typically (1-5) x Torr. Figure 3a exhibits the R2PI spectrum of benzene (0.5%) in a vapor mixture containing 7.3% acetonitrile in Nz (total pressure 9 x lo-* Torr). This spectrum was obtained by scanning the frequency-doubled output of the dye laser and setting the quadrupole mass filter to only collect the ions corresponding to C & , + (i.e., m/z 78). A similar spectrum was obtained by monitoring the ion signal due to the C&+(CH3CN) species as shown in Figure 3b. It is clear that the maximum ion signals shown in Figure 3, a and b, match well the intense vibronic transition of benzene at 258.9 nm." The broad congestion red shifted from the sharp peak corresponds to the room temperature rotational population of the ground state benzene molecule^.^^.^^ By using the R2PI at a fixed wavelength (i.e., 1 = 258.9 nm), one can obtain the mass spectrum of the secondary ions produced following the generation of the benzene ion in the HP source. This mass spectrum is displayed in Figure 3c for a mixture of 7% acetonitrile and 0.5% benzene in a total pressure of 1.9 x lo-' Torr of Nz and a temperature of 299 K. Under these conditions, the major ions produced in the source are
C&+, C&+(CH3CN), and (CsH&+. Under the conditions of lower source temperature, higher concentration of acetonitrile, smaller concentration of benzene, and higher total pressure, the series C#6+(CH3CN), with n > 1 could also be observed. This indicates that, at the extreme conditions of low temperatures and high pressures which resemble a typical nucleation system, condensation will take place via the growth of the sequence C6H6+(CH3CN), in the ion source. It is clear that the ion signal vs wavelength scan obtained from the HPMS experiment is remarkably similar to the nucleation intensity count vs wavelength curve obtained from the DCC experiment. This similarity is strong evidence that the supersaturated vapor is condensing on benzene ions. It is also important to note that the nucleation spectrum obtained at 0.01 nm steps near the 6; resonance (Figure 2c) reveals the rotational envelope of the benzene molecules as displayed in Figure 3a. This demonstrates the high sensitivity of ion nucleation and suggests new analytical and separation capabilities for this technique. For example, it should be possible, in principle, to condense a supersaturated vapor by selective ionization through R2PI of a known impurity present in the vapor. This method may also be adapted for isotope separation from supersaturated vapors by selectively ionizing specific isotopes which exhibit distinct resonance features. To investigate the dependence of the nucleation events on the sign and magnitude of the electric field, we measured the integrated nucleation counts (at 258.9 nm) as a function of the applied field across the chamber plates. The results are shown in Figure 4. By applying a negative field at the top plate, the average nucleation count increases slightly and then starts to decrease slowly at higher fields. We attribute this behavior to the increase in the number of benzene ions reaching the nucleation zone by drift velocities which depend on their mobilities and the magnitude of the applied field. In the absence of the field the motion of the ions is described in terms of ion diffusion, and the observed nucleation events reflect the number of ions reaching the nucleation zone by diffusion. In the limit of high field the ions may be rapidly pulled through the nucleation zone before they have had time to nucleate, and this causes the observed decrease in nucleation events at higher fields. When a positive field is applied on a top plate, negative ions are pulled to the nucleation zone. We cannot presently unambiguously identify the nature of the negative ions, but these are probably solvated electrons of the type e-*(CH3OH),. These
7870 J. Phys. Chem., Vol. 99, No. 20, 1995 species have been observed in clusters following the injection of electrons into supersonic jet expansions.'* It appears that the nucleation count is smaller for these species compared to the benzene ions (by about 20%),but understanding the reasons for this behavior must await a direct identification of these ions. It is interesting, however, to note that recent density functional theory for ion nucleation predicts a sign preference for the condensation of dipolar molecule^.'^ In conclusion, the use of R2PI in a DCC and in HPMS for ion nucleation studies makes it possible, for the first time, to identify the nature of the nucleating ions and study their nucleation behaviors in supersaturated vapors. The implications of this technique are numerous because one can now selectively ionize specific guest or impurity species and condense their supersaturated host vapors. Current research is concerned with the nucleation behavior of different molecular ions and also in different host vapors characterized by different properties such as dipole moment, polarizability, and symmetry factors. It is hoped that these experiments will provide the first comprehensive test of ion nucleation theories.
Acknowledgment. This research is supported by the National Science Foundation Grant CHE 93 11643. Acknowledgment is also made to the donors of the Petroleum Research Fund (2764-AC6), administered by the American Chemical Society, to the Thomas F. and Kate Miller Jeffress Memorial Trust (5-302), and to the Exxon Education Foundation for the partial support of this research.
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References and Notes (1) Wilson, C. R. SOC.London. Philos. Trans. A 1989, 193, 289. (2) Loeb, L. B.; Kipp, A. F.; Einarson, A. W. J. Chem. Phys. 1938, 6, 264. (3) Scharrer, L. Ann. Phys. 1939, 35, 619. (4) Rabeony, H.; Mirabel, P. J. Phys. Chem. 1987, 91, 1815. (5) Adachi, M.; Okuyama, K.; Seinfeld, J. H. J. Aerosol Sei. 1992, 23, 327. (6) He, F.; Hopke, P. J. Chem. Phys. 1993, 99, 9972. (7) Katz, J.; Fisk, J.; Charkarov, V. J. Chem. Phys. 1994, 101, 2309. (8) Reiss, H.; Marvin, D. C.; Heist, R. H. J. Colloid Interface Sei. 1977, 58, 125.
(9) McGraw, R.; Reiss, H. J. Colloid Interface Sei. 1979, 72, 172. (10) Getler, A. W.; Berg, 0.;El-Sayed, M. A. Chem. Phys. Lett. 1978, 57, 343. Gertler, A. W.; Almeida, B.; El Sayed, M. A,; Reiss, H. Chem. Phys. 1979, 42, 429. (11) Castleman, A. W., Jr.; Tang, I. N. J. Chem. Phys. 1972, 57, 3629 (12) Lee. N.: Keesee. R. G.: Castleman. A. W.. Jr. J. Colloid Interface Sei: 1986, 75, 355. (131 Castleman. A. W.. Jr.: Keesee. R. G. Science 1988. 241. 36. (14) Boesl, U. J. Phys. Chem. 1991, 95, 2949. (15) Wright, D.; Caldwell, R.; Moxely, C.; El-Shall, M. S. J. Chem. Phys. 1993, 98, 3356 and references therein. (16) Daly, G. M.; Meot-Ner, M.; El-Shall, M. S. To be submitted to J. Phys. Chem. For details of the HPMS source see: Daly, G. M.; El-Shall, M. S. J. Phys. Chem. 1994, 98, 696. (17) Atkinson, G., Parmenter, C. J. Mol. Spectrosc. 1978, 73, 52. (18 ) Lee, G. H.; h o l d , S. T., Eaton, J. G.; Sarkas, H. W.; Bowen, K. H.; Ludewizt, C.; Haberland, H. Z. Phys. D: Ar., Mol. Clusters 1991, 20, 9. (19) Kusaka, I., Wang, Z. G.; Seinfeld, J. H. J. Chem. Phys. 1995, 102, 913. Jp9502565
