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J. Phys. Chem. B 1999, 103, 838-843
Formation of NaI Aggregates on Ethanol Solution Surface Hisashi Matsumura,† Fumitaka Mafune´ , and Tamotsu Kondow* Cluster Research Laboratory, Toyota Technological Institute, East Tokyo Laboratory, Genesis Research Institute, 717-86 Futamata, Ichikawa, Chiba 272-0001, Japan ReceiVed: July 28, 1998; In Final Form: NoVember 25, 1998
A sodium iodide (NaI) solution in ethanol was introduced into a vacuum as a continuous liquid flow (liquid beam) and was irradiated with an ultraviolet laser beam. Ions ejected into the vacuum were mass-analyzed by a time-of-flight mass spectrometer. The mass spectra show that Na+(EtOH)m (m ) 0-6) and Na+(NaI)n(EtOH)m (n ) 1-7) are ejected from the liquid beam. The results are interpreted as that these cluster ions are produced by unimolecular dissociation of nascent cluster ions composed of Na+ solvated with ethanol and/or (NaI)n, which are born on the liquid surface. The dependence of the ion intensities on the NaI concentration indicates formation of NaI aggregates on the liquid beam surface in the entire NaI concentration range studied.
Introduction A solvation structure on a solution surface is different from that inside the solution, as a surface molecule is not completely surrounded with the other solute and solvent molecules on the surface. This incomplete solvation manifests itself in a surface concentration,1-3 an ionization potential,4-11 and orientation12 of the solute molecule on the solution surface. For example, the ionization potential is lower than that in the gas phase,13,14 mainly because the solute ion produced by photoexcitation is more greatly stabilized by the solvent than the neutral solute molecule is. Evidently, the ionization potential of the solute molecule on the solution surface is higher than that inside the solution because of the incomplete solvation of the solute ion on the solution surface.15 As described above, this specificity of a solution surface is reflected on the surface solute concentration. A phenol molecule which possesses both hydrophilic and hydrophobic groups tends to orient on an aqueous solution surface with its hydrophilic OH end pointed inward.16,17 The surface concentration of phenol estimated by a surface tension measurement18,19 and a second harmonic generation (SHG) spectroscopy16,17 is found to increase almost linearly with the bulk concentration, until the surface is covered with one monolayer of phenol at the bulk concentration of 0.2 M. Above one monolayer, the surface concentration continues to increase more gradually. In addition, our previous study by using the liquid beam of an aqueous phenol solution has revealed a phase change of a twodimensional structure of phenol on the solution surface at the phenol concentration of 0.55 M in the solution, although the SHG spectroscopy has not shown any change.20 As exemplified in the aqueous solution of phenol, isolation and detection of species on a solution surface should be performed so as to obtain microscopic information. The method employed in the study of the aqueous phenol solution consists of introduction of a continuous liquid flow into vacuum (liquid beam),21-26 multiphoton ionization of the solute molecule by a laser, and mass spectroscopic detection of ionic species ejected * Corresponding author. † Present address: Department of Chemistry, School of Science, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113, Japan.
from the solution surface.27,28 Typically, ions produced by multiphoton absorption of the solute molecule are ejected from the surface by Coulomb repulsion of charges built by electron depletion after the multiphoton ionization. The composition of the ions reflects the composition of the solute and solvent molecules on the solution surface,29 so that one can derive the surface composition by analyzing the composition of the ions ejected from the surface.30 In the present work, we investigated the surface of an alcohol solution of NaI by using the liquid beam technique together with a multiphoton ionization-mass spectroscopy. In the alcohol solution, NaI is partly dissociated into Na+ and I- as31-34
NaI a Na+ + I-
(1)
The dissociation constant for eq 1 gives the approximate relative concentrations of NaI (CNaI) and Na+ (CNa+) to be 0.87 and 0.13, respectively. Incomplete solvation on the solution surface gives rise to less dissociation of NaI, since the ion pair of Na+ and I- is less stabilized by the solvent. In addition, NaI is inclined to aggregate on the solution surface, as NaI is more concentrated on the surface. 35-42 Experimental Section Figure 1 shows a schematic diagram of a liquid beam source and a reflectron TOF mass spectrometer used in the present study. A continuous laminar liquid flow of an ethanol solution of NaI was introduced into a vacuum chamber from a nozzle having an aperture with a diameter of 20 µm. Ethanol and NaI (> 99.5% Wako Pure Chemical Industries, Ltd.) were used without any further purification. A constant liquid flow was supplied by a Shimadzu LC-6A pump designed for a liquid chromatograph. The flow rate was maintained at 0.2 mL/min with a pressure of typically 30 atm inside the nozzle. The liquid beam was trapped at 5 cm downstream from the nozzle by a cylindrical cryopump cooled by liquid N2. The source chamber was evacuated by a 1200 L s-1 diffusion pump. The ambient pressure was typically 10-5 - 10-6 Torr during injection of the liquid beam. Traveling a distance of 5 mm from the nozzle, the liquid beam was crossed with a UV laser beam in the first acceleration region
10.1021/jp9832037 CCC: $18.00 © 1999 American Chemical Society Published on Web 01/20/1999
NaI Aggregates on Ethanol Solution Surface
Figure 1. Schematic diagram of the liquid beam source and the reflectron TOF mass spectrometer used in the present study.
of the TOF mass spectrometer. The UV laser beam was obtained by frequency-doubling of the output of a Quanta-ray PDL-3 dye laser (440 nm) pumped by the third harmonics of a Quantaray GCR-3 Nd:YAG laser. The laser power (40 µJ/pulse) was monitored by a photodiode calibrated against a Scientech Astral AA30 power meter. The laser was focused into the liquid beam by a lens with a focal length of 450 mm. The mass-to-charge ratio, m/z, of ions produced by laser photoionization was measured by the reflectron TOF mass spectrometer. The ions ejected from the liquid beam were accelerated by a pulsed electric field in the first acceleration region in the direction perpendicular to both the liquid and the laser beams. A delay time from ionization to ion extraction was varied in the range of 0-3 µs in order to obtain a higher mass resolution. The ions were then steered and focused by a set of vertical and horizontal deflectors and an einzel lens. The first and the third plates of the einzel lens were grounded to make a field-free region of 1.5 m beyond it. The reflectron was provided with a reversing field tilted by 2° off the beam axis. After traveling a 0.5-m field-free region, a train of spatially separated ions was detected by a Murata EMS-6081B Ceratron electron multiplier. Signals from the multiplier were amplified and processed by a Yokogawa DL 1200E transient digitizer based on an NEC 9801 microcomputer. The mass resolution, defined as m/∆m, was 300 in the present experimental condition. Results Figure 2 shows a typical mass spectrum of ions produced by irradiation of a 220 nm laser on a liquid beam of a 0.5 M NaI solution in ethanol. A sodium ion, Na+, solvated with ethanol molecules (Na+(EtOH)m (m ) 1-5)), and with sodium iodide cluster and ethanol molecules (Na+(NaI)n(EtOH)m (n ) 1-5)) are observed in the mass spectrum. The peaks assignable to Na+(NaOH) and Na+(NaOEt) are also observed, as reported elsewhere.43 Figure 3 shows mass spectra of ions produced from a 0.5 M NaI solution in ethanol by irradiation of a 220 nm laser having 20 µJ/pulse (a) and 30 µJ/pulse (b). The intensities of Na+(EtOH)m and Na+(NaI)n(EtOH)m increase with an increase in the irradiation laser power, while the size giving the maximum intensity in the distribution tends to be lowered for both the cluster ions, Na+ (EtOH)m and Na+(NaI)n(EtOH)m. Figure 4 shows the intensities of Na+(EtOH) and Na+(EtOH)2 and the total intensity of all Na+(EtOH)m ions, Σ[Na+(EtOH)m
J. Phys. Chem. B, Vol. 103, No. 5, 1999 839
Figure 2. Mass spectrum of ions produced by irradiation of a 220 nm laser on a liquid beam of a 0.5 M NaI solution in ethanol. A sodium ion Na+ solvated with ethanol molecules, Na+(EtOH)m (m ) 1-5), and sodium iodide cluster ions solvated with ethanol molecules Na+(NaI)n(EtOH)m (n ) 1-5) are observed in the mass spectrum. The peaks at m/z ) 63 and 91 are assignable to Na+(NaOH) and Na+(NaOEt), respectively.
Figure 3. Mass spectra of ions produced from a 0.5 M NaI solution in ethanol by irradiation of a 220 nm laser at 20 µJ/pulse (a) and at 30 µJ/pulse (b). The ion intensities of Na+(EtOH)m and Na+(NaI)n(EtOH)m increase with an increase in the laser power. The intensities of small ion clusters tend to dominate with increasing laser power.
], (denoted as Itotal hereafter) as a function of the laser power. As the laser power increases, Itotal and the intensity of Na+(EtOH) increase monotonically with the laser power, while the intensities of Na+(EtOH)2 reaches maximum at 25 µJ/pulse and then decreases. The intensity of Na+(EtOH)m (m g 3) is found to exhibit a dependence similar to that of Na+(EtOH)2. Figure 5 shows the intensities of Na+(NaI)n(EtOH)m (n ) 1-2; m ) 0-2) as a function of the laser power. As the laser power increases, the intensities of Na+(NaI) and Na+(NaI)2 increase, while those of Na+(NaI)n(EtOH)m (m ) 1, 2) increase and then gradually decrease. The laser powers which give the maximum ion intensity of Na+(NaI)n(EtOH) with n ) 1 and 2 are 28 and 20 µJ/pulse, respectively.
840 J. Phys. Chem. B, Vol. 103, No. 5, 1999
Matsumura et al. SCHEME 1
Figure 4. The intensities of Na+(EtOH) (open circles) and Na+(EtOH)2 (solid circles) and the total ion intensity of all Na+(EtOH)m ions, Σ[Na+(EtOH)m], (solid squares) as a function of the laser power. The total ion intensity and the intensity of Na+(EtOH) monotonically increase with the laser power, while that of Na+(EtOH)2 levels off at 25 µJ/pulse and then decreases.
laser used in the present experiment can excite I- into I-* in the ethanol solution.45,46 The excited ion, I-*, thus produced releases an electron into ethanol as a solvated electron, es-. As the average time (23 ps) necessary for I-* to release the electron is much shorter than the laser pulse width,47 the solvated electron can be ejected into a vacuum by absorbing another photon within the duration of the same laser pulse. The sequential photoionization scheme is expressed as
I-(s) + hν f I-* (s)
(2)
I-*(s) f I(s) + es-
(3)
es-(s) + hν f ef-
(4)
where ef- is a free electron in a vacuum, and (s) represents the solution phase. On the other hand, a resonant two-photon ionization process without formation of a solvated electron, Figure 5. The intensities of Na+(NaI)n(EtOH)m plotted as a function of the laser power. The open circles, the solid circles, and open squares represent the data for Na+(NaI)n(EtOH)m (m ) 0, 1, and 2), respectively. The intensities of Na+(NaI) and Na+(NaI)2 increase, while Na+(NaI)n(EtOH)m (m ) 1, 2) increase and then level off as the laser power increases.
Discussion Photoionization of I-. In the ethanol solution, NaI undergoes electrolytic dissociation into Na+ and I-, and the UV absorption band associated with electron transfer from I- to the solvents is known as a CTTS (charge transfer to solvent) band.44 The vacuum level of I- is considered to be located by ∼2 eV above the CTTS state; the threshold ionization energy of I- (2P3/2) in an aqueous solution is reported to be 7.19 eV,4 while the CTTS state is located 1.7 eV lower than the threshold ionization energy. The energy diagram associated with the photoionization process is shown in Scheme 1. The absorption maximum of the CTTS band in the ethanol solution is located at 218.5 nm (5.68 eV), so that the 220 nm
I-(s) + 2hν f I(s) + ef-
(5)
is ruled out (see Scheme 1), because heating of a nascent cluster ion by the laser excitation supports involvement of the solvated electron in the photoionization process as discussed in the following section. Cluster Ion Formation. The ion ejection and cluster ion formation are explained in terms of a Coulomb ejection scheme. The electron generated by the two-photon ionization of I- has a fate of being (1) captured by a neutral molecule as a negative ion, (2) delocalized in the solution as a solvated electron, (3) neutralized at a certain site via geminate recombination, or (4) liberated as a free electron in the gas phase. As a mean free path of the electron liberated from I- is in the order of several nanometers, only electrons generated as deep as several nanometers from the liquid surface can escape into a vacuum. Therefore, only a surface region (at most, several nanometers deep) turns out to be positively charged due to the electron depletion, which causes the expulsion of surface cations out into the vacuum by a Coulomb repulsion force built by it.
NaI Aggregates on Ethanol Solution Surface
J. Phys. Chem. B, Vol. 103, No. 5, 1999 841
Let us consider that a sodium ion, Na+, leaves the solution surface with accompanying m′ EtOH molecules as a form of Na+(EtOH)m′. The nascent cluster ion, Na+(EtOH)m′ undergoes unimolecular dissociation in the gas phase before it is detected as Na+ (EtOH)m,
Na+(s) + (EtOH)m′(s) f Na+(EtOH)m′(g) Na+(EtOH)m′(g) f Na+(EtOH)m(g) + (m′-m) EtOH
(6) (7)
As Na+ is reported to be solvated with (5 ( 1) methanol molecules in a methanol solution,48 Na+ is also likely to accompany (5 ( 1) ethanol molecules when it is released from the surface, that is, the nascent cluster ion has a form of Na+ (EtOH)5(1. Practically, Na+(EtOH)m (m > 7) are not descernible in the spectrum. By taking these into consideration, one can conclude that the nascent cluster ion is Na+(EtOH)6. Similarly, the detection of Na+(NaI)n(EtOH)m indicates that NaI aggregates are formed in the vicinity of Na+ and are ejected together with Na+. The ejection and unimolecular dissociation of a nascent cluster ion, Na+(NaI)n′(EtOH)m′ are expressed as
Na+(NaI)n′(s) f Na+(NaI)n′(EtOH)m′(g)
(8)
Na+(NaI)n′(EtOH)m′(g) f Na+(NaI)n(EtOH)m(g) + (n′-n)NaI + (m′-m)EtOH (9) Evaporative cooling of the nascent cluster ion proceeds mainly via loss of EtOH rather than NaI, because of a tighter binding between Na+ and NaI than that between Na+ and EtOH. Coulomb Ejection Scheme. Figure 4 shows that the total ion intensity, Itotal, increases with increase in the irradiation laser power. The dependence of Itotal on the laser power is explained in terms of the Coulomb ejection scheme in which two photons are involved as reported previously.29 As the laser power increases, the surface is much more charged and hence many more ions are ejected from the surface. On the contrary, the intensities of Na+(EtOH)m (m g 2) increase, level off, and then decrease with increase in the laser power. This contradictionary result is explained by evaporation of the nascent cluster ion, Na+(EtOH)6 as described below. Unimolecular Dissociation of a Nascent Cluster Ion. Figure 6 shows the cluster size distributions of Na+(EtOH)m at 4 different laser powers. The maximum of the size distribution appears to shift to a smaller size with the laser power. This behavior is explained in such a manner that the nascent cluster ion, Na+(EtOH)6, gains internal energy from the laser through ionization-recombination cycles mediated by solvated electrons as described below: In the ethanol solution, NaI is partly dissociated into Na+ + I-. Under irradiation by a 220 nm laser, I- releases an electron into ethanol (solvated electron, es-). The solvated electron is likely to recombine with I within a picosecond time scale to regenerate I- with releasing the recombination energy to the ethanol solution.49 The regenerated I- is excited again by absorption of a photon and then releases an electron. The excitation-recombination cycle causes the photon energy of 5.65 eV to transmit into the solution as heat within the duration of single laser pulse, resulting in an increase of the internal energy of the nascent cluster ion.50 The scheme of the energy transmission via the excitationrecombination cycles is supported further by the result of inefficient transmission of the photon energy to solutions in which no solvated electron is present, such as ethanol solutions
Figure 6. The size distributions of Na+(EtOH)m at different irradiation laser powers. The size distribution shifts toward a smaller size with increase in the irradiation laser power.
of phenol, aniline, and anisole. In a solution of this kind, the cluster size distribution remains unchanged with the laser power,29 namely, the internal energy of the nascent cluster ion does not increase with the laser power. The rate equation for the unimolecular dissociation of a cluster ion, (Na+(EtOH)p), is given by51,52
d[Na+(EtOH)p] ) kp+1[Na+(EtOH)p+1] dt kp[Na+(EtOH)p] (10) and the rate constant for each step can be estimated by RRK theory. As Na+(EtOH)p has 6p-3 intermolecular vibrational degrees of freedom, the rate constant for the unimolecular dissociation of Na+(EtOH)p into Na+(EtOH)p-1 is given by
( )
kp ) A 1 -
Vp Ep
6p-4
(11)
where A is the frequency of the internal mode related to the rupture of a Na+(EtOH)p-1-EtOH bond, Ep is the internal energy of Na+(EtOH)p, and Vp is the energy for removing one EtOH molecule from Na+(EtOH)p. The pre-exponential factor, A, is chosen to be 4 × 1012 s-1.52 As no data for Vp is available, Vp (1 e p e 4) is assumed to be the energy of removing one MeOH from Na+(MeOH)p,53 and Vp (p ) 5,6) are approximated by the heat of evaporation of pure liquid ethanol. The internal energy, Ep, is given by the energy conservation as
Ep ) Ep+1 - Vp+1 - Ep,trans
(12)
where Ep,trans is the kinetic energy release by an EtOH molecule leaving Na+(EtOH)p, and is set to be twice as large as the
842 J. Phys. Chem. B, Vol. 103, No. 5, 1999
Matsumura et al.
Figure 7. The size distributions of Na+(EtOH)m (a) and Na+(NaI)n (b) at different concentrations of NaI in the ethanol solution under the irradiation of the 220 nm laser with 30 µJ/pulse.
available energy per one vibrational degree of freedom at the transition state:54
(13)
Figure 8. The initial internal energy, E, of Na+ (EtOH)6 estimated from eqs 10-13 is plotted as a function of the NaI concentration at the laser power of 30 µJ/pulse.
The probability, Pp(E,t), that Na+(EtOH)6 having the initial internal energy, E, dissociates into Na+ (EtOH)p after a time t is calculated from eqs 10-13. The internal energy of the nascent cluster ion is estimated by using Pp(E,t) and the cluster-size distribution, and is given to be 2.0 ( 0.5 eV for the NaI solution of 0.2 M under the irradiation of the 220 nm laser with 30 µJ/ pulse. Under the present experimental condition, the ions are extracted by a pulsed electric field 2 µs after the photoionization pulse. As no fragment ion is observed in the mass spectrum, the dissociation should not take place between the acceleration region and the reflectron of the TOF mass spectrometer. In other word, the evaporation must have been completed before the cluster ion starts to be accelerated, and hence must take place in a time less than the detection time (10-6 s) determined by the configuration of the TOF mass spectrometer. Concentration Dependence. As shown in Figure 7a, the size distribution of Na+(EtOH)m shifts toward a smaller size with an increase in the NaI concentration. Similarly to the laser power dependence, the shift is ascribable to the increase of the initial internal energy of the nascent cluster ion with the NaI concentration. This internal energy increase with the NaI concentration arises from an increase in the concentration of the chromophor, I-, which is approximately proportional to the NaI concentration in the ethanol solution of NaI. In other words, the number of photons absorbed in the ethanol solution increases with the chromophor, I-, concentration, and hence, the energy transmitted to the solution increases with the NaI concentration. As shown in Figure 8, the internal energy, E, of the nascent cluster ion, Na+(EtOH)6, is approximately proportional to the NaI concentration (220 nm laser with 30 µJ/pulse) as expected. Formation of Na+(NaI)n. In addition to Na+(EtOH)m, Na+(NaI)n(EtOH)m are observed in the mass spectrum. The observation of Na+(NaI)n(EtOH)m implies that NaI aggregates are present in the vicinity of the liquid surface, and are ejected along with Na+ and EtOH into the vacuum as a nascent cluster ion,
Na+(NaI)n′(EtOH)m′. The nascent cluster ion undergoes unimolecular dissociation with EtOH being removed preferentially, because the ethanol molecules in the Na+(NaI)n′(EtOH)m′ are more weakly bound than NaI; the binding energies of Na+NaI and Na+(NaI)-NaI are 1.71 and 1.45 eV, respectively,55 while the binding energy of Na+ (EtOH)m-1-EtOH is considered to be comparable to the binding energy of Na+ (MeOH)m-1MeOH (1.15 eV for m ) 1, 0.88 eV for m ) 2, 0.75 eV for m ) 3, and 0.68 eV for m ) 4).56 According to the RRK theory, the rate of the unimolecular dissociation for the EtOH loss is estimated to be by 2-3 orders of magnitude larger than that for the NaI loss, on the assumption that the binding energies of Na+(NaI)n-1(EtOH)m-NaI and Na+(NaI)n(EtOH)m-1-EtOH are same as those of Na+(NaI)n-1-NaI and Na+(EtOH)m-1-EtOH, respectively. The preferential release of EtOH is actually observed (see Figure 5); when more internal energy is introduced into the system by increasing the laser power, the number of the ethanol molecules included in Na+(NaI)n(EtOH)m decreases. Presence of NaI Aggregates. Evaporation of NaI from the cluster ion is negligible as discussed above. The size distribution of the NaI aggregates on the liquid beam surface can be estimated from the n distributions of the product cluster ions, Na+(NaI)n(EtOH)m. The abundance of the NaI aggregate thus estimated is found to decrease smoothly with the size, where no magic number seems to be present in the size distribution. In comparison with the presence of magic numbers (n ) 4, 13, 22, ...) in the mass spectrum of Na+(NaI)n from a solid NaI by laser ablation,57 no detection of the magic numbers in the size distribution is in accord with the conjecture that significant evaporation of NaI from Na+(NaI)n′ (EtOH)m′ does not take place. Figure 9 shows the average size of the NaI aggregate on the solution surface plotted as a function of the NaI concentration in the solution. The result shows that the average size of the NaI aggregate does not charge, regardless of the bulk NaI concentration.
Ep,trans )
2(Ep+1 - Vp+1) 6(p+1)-3
NaI Aggregates on Ethanol Solution Surface
Figure 9. The average sizes of the surface NaI aggregate at different NaI concentrations.
Acknowledgment. The authors are grateful to Messrs. Yoshihiro Takeda and Jun-ya Kohno for their assistance in the early stage of the liquid beam studies. This work was supported by the International Joint Research of NEDO and the Special Cluster Research Project of the Genesis Research Institute, Inc. References and Notes (1) Ballard, R. E.; Jones, J.; Sutherland, E. Chem. Phys. Lett. 1984, 112, 452. (2) Ballard, R. E.; Jones, J.; Read, D. Chem. Phys. Lett. 1985, 121, 45. (3) Ballard, R. E.; Jones, J.; Sutherland, E.; Read, D.; Inchley, A. Chem. Phys. Lett. 1984, 112, 452. (4) Delahay, P. Acc. Chem. Res. 1982, 15, 40. (5) Delahay, P.; Dziedzic, A. Chem. Phys. Lett. 1984, 108, 169. (6) Ballard, R. E.; Jones, J.; Inchley, A.; Cranmer, M. Chem. Phys. Lett. 1988, 149, 29. (7) Bo¨kman, F.; Bohman, O.; Siegbahn, H. O. G. Acta Chem. Scand. 1992, 46, 403. (8) Bokman, F.; Bohman, O.; Siegbahn, H. O. G. Chem. Phys. Lett. 1992, 189, 414. (9) Keller, W.; Morgner, H.; Mu¨ller, W. A. Mol. Phys. 1986, 56, 1039. (10) Lacmann, K.; Koizumi, H.; Schmidt, W. F. In Linking the Gaseous and Condensed Phases of Matter, Christophorou, L. G., Illenberger, E., Schmidt, W. F., Eds.; Plenum Press: New York, 1994. (11) Faubel, M.; Steiner, B.; Toennies, J. P. Mol. Phys. 1996, 90, 327. (12) Eisenthal, K. B. Acc. Chem. Res. 1993, 26, 636. (13) Delahay, P.; Dziedzic, A. J. Chem. Phys. 1986, 84, 936. (14) Morgner, H. In Linking the Gaseous and Condensed Phases of Matter, Christophorou, L. G., Illenberger, E., Schmidt, W. F., Eds.; Plenum Press: New York, 1994. (15) Faubel M.; Steiner B.; Toennies, J. P. J. Chem. Phys. 1997, 106, 9013.
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