10210
J. Phys. Chem. 1996, 100, 10210-10214
Photoionization Efficiency Spectrum and Ionization Energy of HSSH Produced from Gaseous Self-Reaction of HS Radicals Bing-Ming Cheng* Synchrotron Radiation Research Center, No. 1, R&D Road VI, Hsinchu Science-Based Industrial Park, Hsinchu 30077, Taiwan, ROC
Wen-Ching Hung Department of Chemistry, National Tsing Hua UniVersity, No. 101, Sec. 2, Kuang Fu Road, Hsinchu 30043, Taiwan, ROC ReceiVed: February 19, 1996; In Final Form: April 11, 1996X
The spectral dependence of the photoionization efficiency of HSSH was obtained over the wavelength range 110-140 nm by means of a discharge flow and a photoionization mass spectrometer coupled to a synchrotron as the radiation source. The HSSH was generated in the flow tube at room temperature via the sequential reactions: O + H2S f OH + HS; HS + HS (+ M) f HSSH (+ M), M being any third body. The ionization energy of HSSH was determined to be 9.40 ( 0.02 eV. This result agrees satisfactorily with the reported value measured from photoelectron spectra. The heat of formation of the ion HSSH+, ∆fH°298(HSSH+), was calculated to be 220.6 ( 2 kcal mol-1. The generation of HSSH from the self-reaction of HS radicals might indicate a previously neglected role in the atmospheric sulfur cycle.
Introduction The importance of the HS radical in the atmosphere is recognized for its crucial role as an intermediate in the oxidation of H2S, which may contribute to the production of atmospheric precipitation of sulfuric acid.1 In the atmosphere H2S is removed primarily by reaction with the hydroxyl radical to generate the HS radical:2-4
OH + H2S f HS + H2O
(1)
To study oxidation processes of atmospheric sulfur compounds, substantial effort has been directed to atmospheric chemical reactions of HS with species NOx, Ox, HxOy, and halides.5-6 Considering the kinetics of the HS radical in the atmosphere, we take a suggestion from the ClO radical, which was observed in large concentrations in the Antarctic stratosphere during the spring season. The self-reaction of ClO yields chlorine peroxide (ClOOCl):
ClO + ClO + M f ClOOCl + M
(2)
Both reaction 2 and ClOOCl are of considerable interest because of their roles in the mechanism of polar depletion of ozone.7-9 Analogous to the self-reaction of ClO radicals, HS might also produce disulfane (HSSH) in the gaseous phase, with or without a third body:
HS + HS (+ M) f HSSH (+ M)
(3f)
However, the self-reaction of HS has not been explored by any means. The rate of formation of HSSH from reaction 3f is likely to be rapid. If this is true, then HSSH cannot be neglected in the atmospheric sulfur cycle. Disulfane is an accidental nearly prolate symmetric rotor, which displays many subtle effects. Its rotational and vibrational-rotational spectra have been studied in some detail in the millimeter-wave and infrared regions by many spectrosco* To whom correspondence should be addressed. X Abstract published in AdVance ACS Abstracts, May 15, 1996.
S0022-3654(96)00496-0 CCC: $12.00
Figure 1. Experimental setup of discharge flow and a photoionization mass spectrometer: MW, microwave discharge cavity; P, pressure meter; SR, synchrotron radiation source; STP-1000, turbomolecular pump (1000 L s-1); TH520, turbomolecular pump (520 L s-1).
pists for both experimental and theoretical objectives,10-14 but few spectra of HSSH are reported in the UV and visible regions. The photoionization efficiency (PIE) spectrum of HSSH has not been reported. The only published value for the ionization energy of HSSH is derived from photoelectron spectra.15-17 The kinetics and thermochemistry of HSSH are scantily investigated. In this work we made the first determination of the spectral dependence of the photoionization efficiency of HSSH by using discharge flow and a photoionization mass spectrometer coupled to a synchrotron as the radiation source. HSSH was generated directly from the self-reaction of HS radicals in the gaseous phase. The ionization energy of HSSH was also measured in this experiment. Experimental Section The apparatus used for this work is a discharge flowphotoionization mass spectrometer coupled to a synchrotron as the radiation source, which is shown in Figure 1. Since descriptions of the setup and general measurement technique are presented elsewhere,18 only matters particular to this experiment are explained here. The beam of ionizing photons was conducted through a 1 m Seya-Namioka monochromator attached to a beam line at the 1.3 GeV storage ring of the Synchrotron Radiation Research Center in Taiwan.19 A grating © 1996 American Chemical Society
HSSH from HS Radicals
J. Phys. Chem., Vol. 100, No. 24, 1996 10211
with 600 grooves/mm was used to obtain an optimal photon flux in the spectral range 110-150 nm, which delivers ∼3 × 1011 photons s-1 at an energy of about 10 eV with a slit width of 0.2 mm, corresponding to a spectral resolution of 0.5 nm. The intensity of the vacuum ultraviolet radiation was monitored via a window coated with sodium salicylate before a photomultiplier tube. We adjusted the intensity of the zeroth-order light to calibrate the monochromator at the beginning and verified it at the end of each injection of the storage ring. The calibration with zeroth-order light is typically accurate within 0.04 nm. A LiF window (2 mm thick) was placed between the ionizing chamber and the exit port of the beam line to eliminate radiation of second and higher orders from the grating. The principles of the discharge flow technique have been described previously.20,21 In this work, a Pyrex flow tube of length 300 mm and 25 mm i.d. was fitted with internal Teflon tubing (22 mm i.d.) to reduce possible surface reactions. A movable injector of 8 mm o.d. permitted adjustment for the optimal reaction distance. The reaction pressure was measured with an MKS Baratron capacitance manometer at approximately the midpoint of the reaction zone. The rates of gaseous flow were monitored with mass flow transducers. HS radicals were generated in the reaction of atomic oxygen with H2S at room temperature:
O + H2S f OH + HS
(4a)
The rate coefficient of this reaction is 2.2 × 10-14 cm3 molecule-1 s-1 at 298 K.6 Oxygen atoms were produced on passing O2 through a microwave discharge and conducted into the flow system through a sidearm. The sidearm (10 mm i.d.) has two separate regions, shown in Figure 1. The discharge region (length 70 mm) is connected to the radical-generating region (length 100 mm) via a tube of 3 mm i.d. and length 12 mm, whereas the latter is connected to the flow tube via a tube of 2 mm i.d. and length 15 mm. H2S was carried in He and added to the flow system either from the inlet port at the radicalgenerating region of the sidearm or from the movable injector. The pressure of the flow tube was regulated with a rotary pump and maintained at 0.5 Torr. The effluents in the flow tube were sampled into a second chamber through a Teflon skimmer with a hole diameter of 2 mm. A turbomolecular pump (520 L s-1) was employed to maintain a pressure below 1 × 10-4 Torr in the second chamber. The effluents were subsequently sampled through a Teflon diaphragm with an orifice diameter of 3 mm into the ionization region of the mass spectrometer. Another turbomolecular pump (1000 L s-1) served to maintain a pressure below 8 × 10-6 Torr in this ionizing chamber. The ions were selected with a quadruple mass filter aligned in the axial direction and detected with a channeltron operated in the pulse-counting mode. Typical experimental flow conditions were as follows: total flow rate FT ) 4.8 STP cm3 s-1; O2 flow rate FO2 ) 6 × 10-3 STP cm3 s-1; H2S flow rate FH2S ) 0.23 STP cm3 s-1; He flow rate to carry O2 FO2/He ) 2.0 STP cm3 s-1; He flow rate to carry H2S FH2S/He ) 1.0 STP cm3 s-1. Laboratory gases (Matheson) were He (99.9995%), O2 (99.997%), and H2S (99.5%) and used without further purification. Results and Discussion To calibrate the wavelength of this photoionization mass spectrometer system, we first measured the ionization threshold of reactant H2S. The photoionization efficiency near the threshold of H2S over the wavelength range λ ) 117.0-121.0
Figure 2. Photoion threshold region of H2S (m/z ) 34) over the wavelength range λ ) 117-121 nm at a nominal resolution of 0.13 nm and with 0.1 nm steps.
nm is plotted in Figure 2. This spectrum of H2S in the threshold region was obtained at a slit width of 0.05 mm of the beam line, corresponding to a nominal resolution of about 0.13 nm. The spectrum displays an abrupt onset, characteristic of direct ionization dominated by the 0 f 0 transition and of similar geometries of the ground states of the ion (X ˜ 2B1 for H2S+) and 1 the neutral molecule (X ˜ A1 for H2S) and some features originating from the indirect process of autoionization resonance with superimposed vibrational progression of the cation. The midrise point of the onset in Figure 2 was taken as the threshold, which thus occurs at 118.53 nm and which corresponds to an adiabatic ionization energy of 10.460 ( 0.011 eV. The uncertainty is derived from the beam-line resolution. This value agrees satisfactorily with recently reported values of 84 432 ( 2 cm-1 (10.4682 eV)22 measured with a nonresonant two-photon zero kinetic energy photoelectron technique and 84 434 ( 2 cm-1 (10.4685 eV)23 determined with a similar one-photon technique. This agreement demonstrates that the wavelength calibration, established by finding of the zeroth-order position, is reliable. In this case the vibrational frequencies of H2S+ could not be analyzed because of interference from autoionizing features immediately above the threshold.24,25 The weak tail of the signal that extends toward greater wavelengths is probably due to a slit function of the beam line or to the thermal effects of internal energy. Such tails are common in photoionization spectra. Since this tail is so small, extensive rotational cooling might have been achieved during expansion in the pseudomolecular beam. Thus, the threshold is not affected within the experimental uncertainty by thermal effects.26-28 HS radicals were generated in the flow system via reaction 4a, which subsequently reacted with themselves to form HSSH. The reaction products were examined over the mass range m/z ) 30-90 at a photoionization wavelength of 110.0 nm (11.27 eV) after the flow system had been conditioned for at least 1 h. Reactant H2S was added into the flow system either from the injector or from the inlet port in the radical-generating region of the sidearm. In both cases we detected a signal at m/z ) 66 (HSSH+). However, the ratio of signals at m/z ) 66 and 34 (H2S+) increased to 0.11% when H2S was added through the inlet port of the sidearm compared with 0.01% through the injector. This result indicates that the self-reaction of HS radicals to form HSSH involved a termolecular process. The pressure in the radical-generating region of the sidearm was 2025 Torr, whereas that in the main flow tube was 0.5 Torr. When HS was generated under the greater pressure, the termolecular reaction 3f promoted production of HSSH better than that under conditions of low pressure.
10212 J. Phys. Chem., Vol. 100, No. 24, 1996
Cheng and Hung
Figure 3. Mass spectrum of reactants and products from the reaction O + H2S in the flow system over the range m/z ) 30-90 at excitation wavelength of 110 nm (11.27 eV) and with m/z ) 0.2 steps.
The characteristic mass spectrum of reactants and reaction products when H2S was added through the inlet port in the sidearm at a photon energy of 11.27 eV is shown in Figure 3. Observable ion peaks were recorded at m/z ) 32 (0.04%), 33 (questionable, masked by the feature at next greater mass), 34 (100%), 36 (4.5%), 38 (0.04%), 48 (0.09%), 66 (0.11%), and 80 (0.22%), corresponding to ions S+, HS+, H232S+, H234S+, H236S+, SO+, HSSH+, and S2O+, respectively. Reaction 4a produced HS and OH. The latter then reacted rapidly with reactant H2S via reaction 1 (the rate coefficient at 298 K is 4.7 × 10-11 cm3 molecule-1 s-1)6 to further form HS. Another possible channel for the self-reaction of HS is the generation of S atoms:
HS + HS f H2S + S
(5)
S atoms reacted also with O2 to form SO in the flow system via reaction 6; the rate coefficient at 298 K is 2.3 × 10-12 cm3 molecule-1 s-1.6 Another possible route to generate SO in this system is the reaction O + HS or S + OH:
S + O2 f SO + O
(6)
O + HS f SO + H
(7)
S + OH f SO + H
(8)
The rate coefficients of reactions 7 and 8 at 298 K are 1.6 × 10-10 and 6.6 × 10-11 cm3 molecule-1 s-1, respectively.6 Then S atoms subsequently reacted with SO to produce S2O, or SO reacted with S atoms on a surface to form S2O:
S + SO f S2O
(9)
SO + Swallf S2O
(10)
This summary of possible reactions in the flow system accounts for the products observed. There may be two channels of reaction O + H2S: an abstraction channel producing OH + HS and an addition channel forming HSO + H:
O + H2S f OH + HS
(4a)
f H + HSO
(4b)
The abstraction channel is proposed by most authors. However, the nonlinearity in the Arrhenius plot of the rate coefficient of this reaction might indicate a change in the reaction mechanism from abstraction to addition. Direct observations of the product HSO were made in reactive scattering experiments by Clemo
Figure 4. Photoionization efficiency spectrum of HSSH (m/z ) 66) over the wavelength range λ ) 110-140 nm at a nominal resolution of 0.5 nm and with 0.4 nm steps.
TABLE 1: Comparison of Measured Ionization Energies/eV with Prior Results compound
this work
prior work
reference
H2S HS HSSH SO S 2O
10.460 ( 0.011 10.45 ( 0.04 9.40 ( 0.02 10.34 ( 0.04 10.60 ( 0.04
10.4682 ( 0.0002 10.4219 ( 0.0004 9.41 10.294 ( 0.004 10.584 ( 0.005
22 31 17 32 32
et al.29 and by Davidson et al.30 The ionization energy of HSO is unknown but expected to be less than 11.27 eV. As shown in Figure 3, we detected no signal at m/z ) 49 at the photoionization energy 11.27 eV. The result indicates that either the addition channel is minor or HSO is labile under the experimental conditions. We varied flow conditions to test for any HSO generated but failed to see any HSO+ ion. Hence, a minor importance of the addition channel is likely. The signal of the parent ion of H2S was intense; it masked the unresolved HS+ signal in the mass spectrum (Figure 3). We increased the resolution of the mass filter to resolve the HS+ signal and then measured the photoionization efficiency spectrum of the HS radical. However, when this mode was operating, the signal was weak. Although the quality of the observed spectrum of HS was much worse than that of H2S, the onset region of ionization for HS yielded an ionization energy of 10.45 ( 0.04 eV (118.7 nm). This result conforms satisfactorily with the recently reported value of 10.4219 ( 0.0004 eV determined with a nonresonant two-photon pulsed field ionization technique.31 We also examined the photoionization efficiency spectra of reaction products SO and S2O. The thresholds for ionization of SO and S2O, obtained from the abrupt onsets, were determined to be 10.34 ( 0.04 and 10.60 ( 0.04 eV, respectively. These values are similar to those recently reported by Norwood and Ng: 10.294 ( 0.004 and 10.584 ( 0.005 eV.32 These results and other data appear in Table 1. The photoionization efficiency spectrum of HSSH over the wavelength region λ ) 110-140 nm is depicted in Figure 4. This spectrum was scanned at a slit width of 0.2 mm of the beam line (corresponding to a resolution of 0.5 nm) and with 0.4 nm steps. The spectrum is characterized by a sharp autoionization feature at 118.8 nm (10.44 eV) and a long weak tail extending to wavelengths greater than 130 nm. Since the signal decreases rapidly toward greater wavelengths, the FranckCondon factors for ionization are poor near the threshold. This phenomenon is similar to that for CH3SSCH3. This neutral molecule has C2 symmetry and a CSSC dihedral angle of 85.1°, whereas the CH3SSCH3+ ion has C2h symmetry and a dihedral
HSSH from HS Radicals
J. Phys. Chem., Vol. 100, No. 24, 1996 10213 H + C2H4S in discharge flow experiments and on pulsed photolysis of H2S. However, all chemical systems for the investigation of atmospheric reactions of HS have been susceptible to errors associated with secondary processes that can regenerate HS. Control of secondary chemistry is thus a general problem in the kinetics of HS. As a secondary reaction in these chemical systems, HS can self-react to form HSSH. Disulfane is unstable and may decompose to regenerate HS:
HSSH (+ M) f 2 HS (+ M)
Figure 5. Photoion threshold region of HSSH over the wavelength range λ ) 122-135 nm at a nominal resolution 0.25 nm and with 0.1 nm steps. The upper curve shows a 5-times enlargement of the signal. The threshold of ionization (analyzed with a linear least-squares method applied to the linearly ascending portion of the curve and indicated by an arrow) at λ ) 131.9 nm yields IE(HSSH) ) 9.40 eV.
angle of 180°. Because the structure alters upon ionization, such a transition near the threshold is expected to have poor Franck-Condon overlap. For this reason the photoionization efficiency spectrum of CH3SSCH3 possesses a long and weak signal toward greater wavelengths.33 The dihedral angle of neutral HSSH is 88.68°,34 but that of the ion HSSH+ is expected to be 180°. This structural alteration accounts for the unfavorable ionization transition near the threshold for HSSH, shown in Figure 4. Other than the stable structure of HSSH+ with C2h symmetry (trans-HSSH+, dihedral angle of 180°), another structure with C2V symmetry (cis-HSSH+, dihedral angle of 0°) is also expected to exist. The rapid rise in the spectrum at 124.8 nm (9.93 eV) might be attributed to formation of cis-HSSH+. For improved measurement of the threshold for ionization of HSSH, a detailed examination near the onset region was performed. Figure 5 shows the onset region of ionization for HSSH over the wavelength range λ ) 122-135 nm. The spectrum was recorded at a nominal resolution of 0.25 nm (with a slit width of 0.1 mm) and with 0.1 nm steps. Each datum point required in total 400 s (80 scans of counting period of 5 s) of ion count. The upper curve in Figure 5 shows a 5-times enlargement of the signal. Because the onset was gradual, the threshold was analyzed with a linear least-squares method applied to the linear portion of the curve, and the fitted line was extrapolated to the background level to derive the ionization energy. To derive the threshold, we used 38 points (from 128.0 to 131.8 nm) to define the linearly ascending portion of the curve. The adiabatic ionization energy of HSSH was accordingly determined to be 9.40 ( 0.02 eV (131.9 nm) from the photoion threshold. This position is indicated with an arrow in Figure 5. This value agrees with the first adiabatic ionization energy of 9.41 eV obtained by Frost et al. with a photoelectron spectrometer.17 According to the review of the thermochemistry of sulfurcontaining molecules and radicals by Benson, the heat of formation of HSSH at 298 K, ∆fH°298(HSSH), is 3.8 kcal mol-1 (Table 1 in ref 35). The heat of formation of HS at 298 K, ∆fH°298(HS), is 34.2 ( 1 kcal mol-1.6 Thus, the enthalpy change of reaction 3f, ∆H°298(3f), is calculated to be -64.6 ( 2 kcal mol-1 at 298 K. The heat of formation of the ion HSSH+, ∆fH°298(HSSH+), is also derived from the adiabatic ionization energy of HSSH, that is, 220.6 ( 2 kcal mol-1. For the study of the kinetics of HS the radicals are generally produced in the reactions F + H2S, H + H2S, OH + H2S, and
(3r)
Thus, for accurate determinations of the rate coefficients of these chemical reactions, the self-reaction of HS should be considered in the experimental measurements. Disulfane may be formed from reaction 3f in the atmosphere. Other than to decompose to regenerate HS, HSSH may be photolyzed to produce the HSS radical:36-38
HSSH + hν f H + HSS
(11)
HSS is isovalent with HSO, which may have similar atmospheric chemistry:
HSS + O2 f OH + S2O
(12a)
f HS + SO2
(12b)
HSS + O3 f HS + SO3
(13a)
f HSO + SO2
(13b)
f HS + SO + O2
(13c)
HSS + NO f SO + HNS HSS + NO2 f HS + NO + SO
(14) (15)
If HSS is indeed formed in the atmosphere, it becomes introduced thus into the atmospheric sulfur cycle. To discover its role in this cycle, further laboratory work on HSS reactions should be performed. Conclusions The spectral dependence of the photoionization efficiency of HSSH was measured with a discharge flow and a photoionization mass spectrometer coupled to a synchrotron as the radiation source. HSSH was generated from the self-reaction of HS radicals produced in a gaseous flow via reaction O + H2S f OH + HS. The photoionization efficiency spectrum of HSSH over the wavelength range λ ) 110-140 nm is characterized by a sharp autoionization at 118.8 nm (10.44 eV) and a long weak tail extending to wavelengths greater than 130 nm. The structural alteration upon ionization, from the ground state of neutral HSSH (C2 symmetry, dihedral angle of 88.68°) to the ground state of the ion HSSH+ (in trans form and C2h symmetry, dihedral angle of 180°), accounts for the unfavorable transition near the threshold for ionization of HSSH. The adiabatic ionization energy of HSSH is determined to be 9.40 ( 0.02 eV, in agreement with the previous value derived from photoelectron spectra. A rapidly rising second onset in the spectrum beginning from 9.93 eV might be due to formation of cisHSSH+. The heat of formation of the ion HSSH+, ∆fH°298(HSSH+), was calculated to be 220.6 ( 2 kcal mol-1. The generation of HSSH from self-reaction of HS radicals may have a role in atmospheric chemistry.
10214 J. Phys. Chem., Vol. 100, No. 24, 1996 Acknowledgment. Support from Synchrotron Radiation Research Center in Taiwan and National Science Council, Republic of China, is gratefully acknowledged. References and Notes (1) Tyndall, G. S.; Ravishankara, A. R. Int. J. Chem. Kinet. 1991, 23, 483. (2) Leu, M. T.; Smith, R. H. J. Phys. Chem. 1982, 86, 73. (3) Sze, N. D.; Ko, M. K. W. Atmos. EnViron. 1980, 4, 1223. (4) Cox, R. A.; Sandalls, F. J. Atmos. EnViron. 1974, 8. 1269. (5) Atkinson, R.; Baulch, D. L.; Cox, R. A.; Hampson, R. F.; Kerr, J. A.; Troe, J. J. Phys. Chem. Ref. Data 1992, 21 (6), and references therein. (6) DeMore, W. B.; Sander, S. P.; Golden, D. M.; Hampson, R. F.; Kurylo, M. J.; Howard, C. J.; Ravishankara, A. R.; Kolb, C. E.; Molina, M. J. Chemical Kinetics and Photochemical Data for Use in Stratospheric Modeling. Evaluation No. 11. JPL Publication 94-26; Jet Propulsion Laboratory: Pasadena, CA, 1994, and references therein. (7) Molina, L. T.; Molina, M. J. J. Phys. Chem. 1987, 91, 433. (8) Cheng, B.-M.; Lee, Y.-P. J. Chem. Phys. 1989, 90, 5930. (9) Cox, R. A.; Hayman, G. D. Nature 1988, 332, 796. (10) Wilson, M. K.; Badger, R. M. J. Chem. Phys. 1948, 17, 1232. (11) Winnewisser, G.; Yamada, K. M. T. Vib. Spectrosc. 1991, 1, 263. (12) Marsden, C. J.; Smith, B. J. J. Phys. Chem. 1988, 92, 347. (13) Mittler, P.; Yamada, K. M. T.; Winnewisser, G.; Birk, M. J. Mol. Spectrosc. 1994, 164, 390, and references therein. (14) Samdal, S.; Mastryukov, V. S.; Boggs, J. E. J. Mol. Struct. 1995, 346, 35. (15) Solouki, B.; Bock, H. Inorg. Chem. 1977, 16, 665. (16) Wagner, G.; Bock, H. Chem. Ber. 1974, 107, 68. (17) Frost, D. C.; Lee, S. T.; McDowell, C. A.; Westwood, N. P. C. J. Electron. Spectrosc. Relat. Phenom. 1977, 12, 95. (18) Hung, W.-C.; Shen, M.-y.; Lee, Y.-P.; Wang, N.-S.; Cheng, B.-M. In preparation. (19) Tseng, P.-C.; Hsieh, T.-F.; Song, Y.-F.; Lee, K.-D.; Chung, S.-C.; Chen, C.-I.; Lin, H.-F.; Dann, T.-E.; Huang, L.-R.; Chen, C.-C.; Chuang,
Cheng and Hung J.-M.; Tsang, K.-L.; Chang, C.-N. ReV. Sci. Instrum. 1995, 66, 1815. (20) Howard, C. J. J. Phys. Chem. 1979, 83, 3. (21) Cheng, B.-M.; Lee, Y.-P. Int. J. Chem. Kinet. 1986, 18, 1303. (22) Fischer, I.; Lochschmidt, A.; Strobel, A.; Niedner-Schatteburg, G.; Mu¨ller-Dethlefs, K.; Bondybey, V. E. J. Chem. Phys. 1993, 98, 3592. (23) Wiedmann, R. T.; White, M. G. Proc. SPIEsInt. Soc. Opt. Eng. 1992, 1638, 273. (24) Walters, E. A.; Blais, N. C. J. Chem. Phys. 1981, 75, 4208. (25) Dibeler, V. H.; Liston, S. K. J. Chem. Phys. 1968, 49, 482. (26) Monks, P. S.; Stief, L. J.; Krauss, M.; Kuo, S.-C.; Klemm, R. B. J. Chem. Phys. 1994, 100, 1902. (27) Monks, P. S.; Stief, L. J.; Tardy, D. C.; Liebman, J. F.; Zhang, Z.; Kuo, S.-C.; Klemm, R. B. J. Phys. Chem. 1995, 99, 16566. (28) Zhang, Z.; Monks, P. S.; Stief, L. J.; Liebman, J. F.; Huie, R. E.; Kuo, S.-C.; Klemm, R. B. J. Phys. Chem. 1996, 100, 63. (29) Clemo, A. R.; Davidson, F. E.; Duncan, G. L.; Grice, R. Chem. Phys. Lett. 1981, 84, 509. (30) Davidson, F. E.; Clemo, A. R.; Duncan, G. L.; Browett, R. J.; Hobson, J. H.; Grice, R. Mol. Phys. 1982, 46, 33. (31) Hsu, C.-W.; Baldwin, D. P.; Liao, C.-L.; Ng, C. Y. J. Chem. Phys. 1994, 100, 8047. (32) Norwood, K.; Ng, C. Y. Chem. Phys. Lett. 1989, 156, 145. (33) Li, W.-K.; Chiu, S.-W.; Ma, Z.-X.; Liao, C.-L.; Ng, C. Y. J. Chem. Phys. 1993, 99, 8440. (34) Peil, G.; Yamada, K. M. T.; Winnewisser, G. J. Mol. Spectrosc. 1993, 159, 507. (35) Benson, S. W. Chem. ReV. 1978, 78, 23. (36) Porter, G. Discuss. Faraday Soc. 1950, 9, 60. (37) Gosavi, R. K.; Desorgo, M.; Gunning, H. E.; Strausz, O. P. Chem. Phys. Lett. 1973, 21, 318. (38) Holstein, K. J.; Fink, E. H.; Wildt, J.; Zabel, F. Chem. Phys. Lett. 1985, 113, 1.
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