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Spectroscopic Study of the I2 Formation from the Photolysis of Iodomethanes (CHI3, CH2I2, CH3I, and CH2ICl) at Different Wavelengths Cian-Ping Tu, Hsin-I Cheng, and Bor-Chen Chang* Department of Chemistry, National Central University, 300 Jungda Road, Jhongli 32001, Taiwan ABSTRACT: Emission spectra following the photolysis of iodomethanes (CHI3, CH2I2, CH3I, and CH2ICl) at 266 nm were recorded in a slow flow cell. In addition to emission from the electronically excited species including CH (A2Δ, B2Σ−, and C2Σ+), C2 (d3Πg), and atomic iodine (4Po), a series of emission bands was observed in the 12 000−19 000 cm−1 region. The dominant structure of these emission bands was verified as the I2 B3Π+0,u−X1Σ+g emission at the 532 nm excitation, and the observed I2 was formed from collisions between iodine atoms generated from the C−I bond dissociation in these iodomethanes. The I2 emission spectra following the photolysis of CH2I2 at different wavelengths were acquired, and the threshold energy for the first C−I bond cleavage was determined to be 208 ± 1 kJ mol−1. We also obtained the emission spectra of pure I2 at several visible excitation wavelengths for comparison with those from the photolysis of iodomethanes, and a least-squares global fit of the observed I2 emission bands yields more accurate anharmonicity parameters for the vibrational structure in the I2 B−X transition.

I. INTRODUCTION Halomethanes have been well-known for their important role in the catalytic depletion of the Antarctic ozone layer during the past few decades,1−8 and their photochemistry has thus attracted much interest. Although the atmospheric abundance of bromomethanes or iodomethanes is much lower than that of the famous chlorofluorocarbons (CFCs), the bromine/iodine atoms released by the photolysis of these compounds still significantly contribute to the ozone depletion. In the 1980s, Class et al.5 and Barrie et al.6 reported the contribution of bromine atoms to the ozone destruction, and Chameides and Davis9 proposed that the iodine contribution could be even more important. Thereafter, there have been several studies on the photochemical reactions of iodomethanes and their impacts on the ozone layer. 10−15 Saiz-Lopez et al. 16 recently summarized the atmospheric chemistry of iodine and concluded that the atmospheric iodine atoms are mostly generated from the photolysis of iodomethanes such CH2I2, CH3I, CH2ICl, and CH2IBr. The photolysis of bromoform (CHBr3)17−31 and of other bromomethanes (CH 3 Br, CH 2 Br 2 , CHBr 2 Cl, and CHBrCl2)32−37 has been well studied during the past years, but the photolysis reactions of iodomethanes are relatively less investigated. There were several reports on the photolysis of CH2I2 and that of CH3I.38−48 Tweeten et al.49 reported the molecular iodine formation from the multiphoton photodissociation of CHI3, and Senapati et al.50 investigated the photodissociation dynamics of CH2ICl at various wavelengths. In 2011, Chen et al.51 used cavity ring-down spectroscopy (CRDS) to probe I2 generated from the molecular elimination channel of CH2I2 and found the yield to be only 0.0040 ± 0.0025. We previously52 reported the first direct measurements © 2013 American Chemical Society

of the intermediates or products in the multiphoton photolysis of CHBr3, CHBr2Cl, CHBrCl2, and CH2Br2 using nascent emission and dispersed fluorescence spectroscopy. In the present study, we use similar techniques to study the photolysis of iodomethanes (CHI3 , CH 2 I 2 , CH 3 I, and CH 2 ICl). Interestingly, strong I2 emission bands were found in the photolysis of these iodomethanes, and the observed I2 is the collisional product following the C−I bond breakage. We also previously observed very weak Br2 emission bands in the photolysis of CHBr3 and CHBr2Cl, but the emission signals are too weak for any further characterization. Therefore, the threshold energy for the first C−I bond dissociation could be determined on the basis of the I2 formation at different photolysis wavelengths. In addition, as we tried to assign the vibrational structure in the I2 emission spectra following the photolysis of iodomethanes, the anharmonicity parameters for the I2 B3Π+0,u−X1Σ+g transition reported in the previous studies53−55 were found to be inaccurate for determining the high vibrational energy levels observed in the present work. Hence, the emission spectra of pure I2 at several excitation wavelengths were recorded and assigned. A least-squares global fit of the observed I2 emission data results in better anharmonicity parameters for the vibrational structure in the I2 B3Π+0,u−X1Σ+g transition. Special Issue: Terry A. Miller Festschrift Received: July 30, 2013 Revised: August 15, 2013 Published: August 16, 2013 13572

dx.doi.org/10.1021/jp407599x | J. Phys. Chem. A 2013, 117, 13572−13577

The Journal of Physical Chemistry A

Article

II. EXPERIMENTAL SECTION The schematic diagram of our experimental setup was described in detail previously.52 Iodomethanes (CHI3, CH2I2, CH3I, and CH2ICl) were commercially acquired (purity >98%) and were introduced to a glass cell by passing a flow of high purity helium gas (60 sccm) over a sample reservoir. For recording the emission spectrum of pure I2, solid iodine was heated to 30−40 °C and the I2 vapor from sublimation was carried by a helium flow into the sample cell. Another flow of helium gas (60 sccm) was also introduced to the cell at the two ends to avoid the deposition on the windows. The whole system was pumped by a mechanical pump (Alcatel 2015SD) through a liquid nitrogen trap and the pumping speed was adjusted to maintain a total pressure of 0.05−0.5 Torr in most experiments. In the photolysis experiments, a softly focused (using a f/40 focal lens) photolysis laser beam was generated from either the fourth harmonic generation output (266 nm) of a Nd:YAG laser (Spectra-Physics INDI-40-10) or a Nd:YAG (Spectra-Physics GCR-170) laser pumped dye laser (SpectraPhysics PRSC-G-18). Nascent emissions of the electronically excited species from the photolysis reaction were collected and imaged onto a monochromator (Acton Research Corp. SP308) adapted with an intensified charge-coupled device (ICCD, Andor DH720-18H-13) detector for recording emission spectra. For a photolysis-excitation scheme experiment, the photolysis laser beam was the pure 266 nm output of a Nd:YAG laser using a set of Pellin-Broca prisms for separating the residual 532 nm laser light, and the excitation laser beam came from the dye laser. The time sequence of our apparatus and the amplification gates of ICCD were controlled and optimized by two delay generators (Stanford Research System DG535).

Figure 1. Emission spectra between 12 000 and 19 000 cm−1 following the 266 nm photolysis of different iodomethanes: (a) CH2ICl, (b) CH3I, (c) CH2I2, and (d) CHI3. The sharp peak at 12 430 cm−1 corresponds to the atomic iodine emission.

laser beams results in an emission spectrum like those illustrated in Figure 1. This suggests that the formation of these emission bands possibly requires two photons with the first photon for dissociating the iodomethanes and the second photon for exciting some species to the excited emissive states. A single 532 nm laser beam is also employed for the photolysis as well as excitation, no emission signal is found when the precursor is CH2ICl or CH3I, but an emission spectrum similar to those in Figure 1, without the atomic iodine emission, is observed when the precursor is CH2I2 or CHI3. This can be explained by the fact that CH2I2 and CHI3 have pale yellow color but CH2ICl and CH3I are colorless. Nevertheless, the yellowish color of CH2I2 may result from the natural decomposition. As this issue will be discussed later (vide infra), the observed emission bands arise from the photolysis of CH2I2 rather than from the natural decomposition. In other words, CH2I2 and CHI3 have weak absorption in the visible wavelengths, whereas CH2ICl and CH3I have no visible absorption at all. The power dependence experiment of the 532 nm photolysis of CH2I2 also indicates this is a two-photon (1 + 1) process of one photon for photolysis and another photon for excitation. The emission bands in Figure 1 apparently arise from the 532 nm laser excitation. Isotopomers (CD2I2 and 13CH2I2) are used for verifying the carrier of these bands, and the results show that the carrier does not contain any hydrogen or carbon atoms; i.e., it is molecular iodine. The emission spectrum following the 532 nm excitation of pure I2 is therefore recorded for comparison and is very similar to those spectra following the photolysis of iodomethanes as depicted in Figure 2, except there are two differences. First, each peak in the emission spectrum of pure I2 seems to split into two unresolved peaks, whereas the peaks are like congested structures in the molecular dissociation experiment. Second, the line widths (∼24 cm−1) of the pure I2 emission spectrum are significantly smaller than those (∼40 cm−1) of the emission bands in Figure 1. Nevertheless, this still clearly indicates the dominant structure of the emission bands in Figure 1 to be the I2 B3Π+0,u → X1Σ+g transition, and the broad line widths may attribute to a mixing of emissions from several upper levels and/or hotter rotational distributions in these emission bands.

III. RESULTS AND DISCUSSION A. Emission Spectra Following the Photolysis of Iodomethanes. The emission spectra following the 266 nm photolysis of iodomethanes (CHI3, CH2I2, CH3I, and CH2ICl) are similar to those following the corresponding photolysis of bromomethanes (CHBr3, CHBr2Cl, CHBrCl2, and CH2Br2).52 The strong CH emission (A2Δ → X2Π, B2Σ− → X2Π, and C2Σ+ → X2Π) and the C2 emission (d3Πg → a3Πu, i.e., the Swan Band) are observed. As shown in Figure 1, in the 12 000−19 000 cm−1 region of the emission spectra following the 266 nm photolysis of iodomethanes, one can see a series of emission bands, which is not found in the emission spectra following the photolysis of bromomethanes, and a sharp emission peak at 12 430 cm−1 (804.5 nm) of atomic iodine emission (6p → 6s, 4Po3/2 → 4P5/2). Except for a small difference in the intensity distribution due to the experimental difficulty of maintaining constant vapor pressures of the precursors, the emission spectra following the 266 nm photolysis of the interested iodomethanes (CHI3, CH2I2, CH3I, and CH2ICl) are almost identical. Because the 266 nm photolysis laser beam comes from the fourth harmonic generation of a Nd:YAG laser, the photolysis laser beam contains some residual 532 nm output. To clarify the formation mechanism of these bands, a set of Pellin-Broca prisms is used to separate the 266 and 532 nm laser beams. Very interestingly, a pure 266 nm laser beam does not generate the emission bands observed in Figure 1, except the excited state atomic iodine signal at 12 430 cm−1 in the photolysis of these iodomethanes, whereas a combination the 266 and 532 nm 13573



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the photon energy at 575 nm. Because the observed I2 emission spectrum results from photolysis, the formation of I2 can serve as a probe for determining the first C−I bond dissociation energy. To study this, a dye laser at wavelengths from 509 nm (235 kJ mol−1) to 610 nm (196 kJ mol−1) is adopted for the photolysis and excitation laser. Figure 4 shows the I2 emission

Figure 2. Comparison between the emission spectrum (the upper trace) following the 266 nm photolysis of CH2I2 and the emission spectrum (the lower trace) of pure I2 at the 532 nm excitation.

The formation of I2 in the photolysis of iodomethanes may come from several sources such the direct photolysis product, the natural decomposition of iodomethanes, or the collisional product following the photolysis.48 Because the I2 emission bands in Figure 1 are observed in the photolysis of CH2I2/CHI3 as well as in that of CH3I/CH2ICl, which contains only one iodine atom, the direct molecular formation following the photolysis can be excluded. It should be noted that Chen et al.51 found the direct I2 yield to be only 0.004 in the molecular elimination channel of the 248 nm photolysis of CH2I2. On the other hand, the iodomethanes may naturally decompose iodine atoms for the I2 formation. In Figure 3, when a photolysis laser

Figure 4. Emission spectra following the photolysis and excitation at different wavelengths. The labeled vibrational assignments correspond to the I2 B → X emission.

spectra following the laser photolysis and excitation at 568, 575, and 578 nm, respectively. It is clear to see that there is no I2 detectable emission signal at 578 nm, but there are strong I2 emission signals at 568 nm. On the basis of the results, we can therefore determine the threshold dissociation energy for the first C−I bond in CH2I2 to be 208 ± 1 kJ mol−1. B. Vibrational Structure in the I2 B−X Transition. Figure 2 shows a portion of the I2 emission spectrum at 532 nm excitation. Because in the low frequency region each I2 emission band splits into two unresolved peaks, this implies that there may be more than one upper level. When we tried to fit the observed I2 vibrational structure with one upper level, the vibrational parameters significantly deviate from those reported by the previous studies.53−55 The emission spectra of pure I2 following excitation at different wavelengths from 509 to 641 nm are acquired for better characterization of the vibrational structure in the I2 B3Π+0,u−X1Σ+g transition, and the I2 emission spectrum at the 532 nm excitation is found to be dominated by the emissions from v′ = 33 and v′ = 36, which correspond to the two unresolved peaks in the pure I2 emission spectrum shown in Figure 2. In total, the emission spectra at nine different excitation wavelengths are recorded to provide data for a least-squares global fit of the I2 B−X vibrational structure. For example, the emission spectrum following the 516.8 nm excitation is shown in Figure 5. In Figure 5, one can clearly see the vibrational progression of the I2 B−X transition, and therefore, the vibrational assignments for the lower levels can be assigned. We start with fitting each emission spectrum separately for determining the vibrational parameters of the X1Σ+g state. Interestingly, for the emission spectra at longer excitation wavelengths such as 641 nm, the determined vibrational parameters are consistent with the previous values,53−55 but the fit shows deviations as the excitation wavelength decreases, because emissions to higher vibrational levels are observed at the shorter pump wavelengths. An example of these deviations is illustrated in Figure 6, which

Figure 3. Emission spectrum following the 582 nm excitation when the 266 nm photolysis laser is on (the upper trace) or off (the lower trace).

at 266 nm and an excitation laser at 582 nm are introduced into the slow flow cell containing CH2I2, the I2 emission spectrum can be clearly observed, but the I2 spectrum disappears when the 266 nm photolysis laser is turned off. This confirms that the observed I2 is formed from collisions of iodine atoms generated from the photolysis instead of the natural decomposition. Gilchrist et al.48 reported the formation of molecular iodine from the 193 nm photolysis of CH3I via atomic iodine recombination and then the dissociation of I2 to produce I(2P1/2) and I(2P3/2). Chen et al.51 also reported an ab initio calculation that predicts the threshold energy for the first C−I bond dissociation in CH2I2 is 208 kJ mol−1, which corresponds to 13574



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vibrational energy based on Williamson’s parameters55 shows a linear deviation as the vibrational quantum number increases. Interestingly, if we adopted alternative vibrational assignments for v″, the calculated errors based on Williamson’s parameters may be significantly reduced. Nevertheless, the alternative assignments will dramatically increase the errors based on the other two sets of previous parameters53,54 as well as those of this work. For diatomic molecules, the vibrational term value can be described as ⎛ ⎛ ⎛ 1⎞ 1 ⎞2 1 ⎞3 G(v) = ωe⎜v + ⎟ − ωexe⎜v + ⎟ + ωeye ⎜v + ⎟ ⎝ ⎝ ⎝ 2⎠ 2⎠ 2⎠ 4 ⎛ 1⎞ + ωeze⎜v + ⎟ + ... ⎝ (1) 2⎠

The X state vibrational parameters from the individual fit of each emission spectrum are quite consistent. Therefore, a global fit of combining these nine emission spectra of I2 has been conducted. Unfortunately, the upper vibrational levels also exhibit significant errors based on the B state vibrational parameters previously reported.53,54 This is possibly due to the fact that the highest B state vibrational level in our data is approximately only 400 cm−1 below the dissociation limit,56,57 where the simple anharmonicity parameters cannot accurately describe the vibrational energy. We find that the third anharmonicity parameter, ωeze, is needed to improve the global fit in addition to some revisions of the upper level assignments. For instance, the emission bands observed by the 516.8 nm excitation (Figure 5) are originally assumed to originate from v′ = 42,58 but after careful analysis, most of the strong emission is found to originate from v′ = 43 because the P branch of v′ = 43 is overlapped with the R branch of v′ = 42. We have thus determined more accurate anharmonicity parameters for the X state and those for the B state, and the fit results are listed in Table 1. This global fit includes 385 emission bands with v′ up to 54 and v″ up to 45, and the standard deviation of the fit is approximately 4.4 cm−1. In Table 1, the harmonic frequency, ωe, is fixed at the values determined from the high resolution data for improving the accuracy of anharmonicity parameters. The X state vibrational parameters determined in this work are more accurate and consistent with those reported by Williamson55 in 2011. However, the improved accuracy of anharmonicity parameters is crucial for the present data because a small deviation in anharmonicity parameters such ωexe and ωeye could result in large errors at high vibrational levels; e.g., the difference in ωeye between this work and the previous value55 will make a frequency difference of 33 cm−1 for v″ = 45. For the B state, the

Figure 5. Emission spectrum of pure I2 at the 516.8 nm excitation.

Figure 6. Calculated errors vs the X state vibrational quantum numbers based on different vibrational parameters. The experimental data used here came from the emission spectrum of pure I2 at the 509.5 nm excitation with v′ = 54.

shows the errors (experimental − calculated) of calculating the X state vibrational energy using different vibrational parameters.53−55 The calculated values based on the previous parameters53−55 have larger and systematic errors when the X state vibrational quantum number is larger than 30 because, in the previous studies, the highest X state vibrational level was v = 33, whereas in the present study we can assign the X state vibrational level up to v = 45. In Figure 6, the calculated X state

Table 1. Vibrational Parameters (cm−1) for the I2 B−X Transition Herzberga BΠ 3

+

0,u

X1Σ+g

ωe ωexe ωeye ωeze ωe ωexe ωeye Te

128 0.834

214.57 0.6127 −0.000895 15641.6

Gerstenkorn and Lucb 125.67 0.7503 −0.004144 0.0002249 214.53 0.6129 −0.0001027

Williamsonc

213.0(3) 0.580(22) −0.0024(4)

this workd,e 125.67 (fixed) 0.5247(53) −0.01536(23) 0.0001682(25) 214.53 (fixed) 0.5919(26) −0.002090(64) 15643.0(6)

Reference 53. bReference 54. cReference 55. dOne standard deviation in parentheses. eThe least-squares fit includes 385 emission bands and the standard deviation of the fit is 4.4 cm−1. a

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(3) Solomon, S.; Garcia, R. R.; Rowland, F. S.; Wuebbles, D. J. On The Depletion of Antarctic Ozone. Nature 1986, 321, 755−758. (4) McElroy, M. B.; Salawitch, R. J.; Wofsy, S. C.; Logan, J. A. Reductions of Antarctic Ozone Due to Synergistic Interactions of Chlorine and Bromine. Nature 1986, 321, 759−762. (5) Class, T.; Kohnle, R.; Ballschmiter, K. Chemistry of Organic Traces in Air VII: Bromo- and Bromochloromethanes in Air over the Atlantic Ocean. Chemosphere 1986, 15, 429−436. (6) Barrie, L. A.; Bottenheim, J. W.; Schnell, R. C.; Crutzen, P. J.; Rasmussen, R. A. Ozone Destruction and Photochemical Reactions at Polar Sunrise in The Lower Arctic Atmosphere. Nature 1988, 334, 138−141. (7) Garcia, R. R.; Solomon, S. A New Numerical Model of The Middle Atmosphere: 2. Ozone and Related Species. J. Geophys. Res.: Atmos. 1994, 99, 12937−12951. (8) Saiz-Lopez, A.; Mahajan, A. S.; Salmon, R. A.; Bauguitte, S. J. B.; Jones, A. E.; Roscoe, H. K.; Plane, J. M. C. Boundary Layer Halogens in Coastal Antarctica. Science 2007, 317, 348−351. (9) Chameides, W. L.; Davis, D. D. Iodine: Its Possible Role in Tropospheric Photochemistry. J. Geophys. Res.: Oceans 1980, 85, 7383−7398. (10) Chatfield, R. B.; Crutzen, P. J. Are There Interactions of Iodine and Sulfur Species in Marine Air Photochemistry? J. Geophys. Res.: Atmos. 1990, 95, 22319−22341. (11) Carpenter, L. J.; Sturges, W. T.; Penkett, S. A.; Liss, P. S.; Alicke, B.; Hebestreit, K.; Platt, U. Short-Lived Alkyl Iodides and Bromides at Mace Head, Ireland: Links to Biogenic Sources and Halogen Oxide Production. J. Geophys. Res.: Atmos. 1999, 104, 1679−1689. (12) Hölscher, D.; Zellner, R. LIF Study of The Reactions of The IO Radical with NO and NO2 Over An Extended Range of Temperature and Pressure. Phys. Chem. Chem. Phys. 2002, 4, 1839−1845. (13) Carpenter, L. J. Iodine in the Marine Boundary Layer. Chem. Rev. 2003, 103, 4953−4962. (14) Saiz-Lopez, A.; Plane, J. M. C. Novel Iodine Chemistry in The Marine Boundary Layer. Geophys. Res. Lett. 2004, 31, L04112. (15) McFiggans, G.; Coe, H.; Burgess, R.; Allan, J.; Cubison, M.; Alfarra, M. R.; Saunders, R.; Saiz-Lopez, A.; Plane, J. M. C.; Wevill, D. J.; et al. Direct Evidence for Coastal Iodine Particles from Laminaria Macroalgae − Linkage to Emissions of Molecular Iodine. Atom. Chem. Phys. 2004, 4, 701−713. (16) Saiz-Lopez, A.; Plane, J. M. C.; Baker, A. R.; Carpenter, L. J.; von Glasow, R.; Gómez Martin, J. C.; McFiggans, G.; Saunders, R. W. Atmospheric Chemistry of Iodine. Chem. Rev. 2012, 112, 1773−1804. (17) Simons, J. P.; Yarwood, A. J. Decomposition of Hot Radicals. Part 1. The Production of CCl and CBr from Halogen-Substituted Methyl Radicals. Trans. Faraday Soc. 1961, 57, 2167−2175. (18) Lichtin, D. A.; Berman, M. R.; Lin, M. C. NH(A3Π→X3Σ−) Chemiluminescence from the CH(X2Π) + NO Reaction. Chem. Phys. Lett. 1984, 108, 18−24. (19) Chen, C.; Ran, Q.; Yu, S.; Ma, X. Quenching of CH(A2Δ and B2Σ−) by NO, CHBr3 and Amine Molecules. Chem. Phys. Lett. 1993, 203, 307−313. (20) McGivern, W. S.; Sorkhabi, O.; Suits, A. G.; Derecskei-Kovacs, A.; North, S. W. Primary and Secondary Processes in the Photodissociation of CHBr3. J. Phys. Chem. A 2000, 104, 10085− 10091. (21) Lindner, J.; Ermisch, K.; Wilhelm, R. Multi-Photon Dissociation of CHBr3 at 248 and 193 nm: Observation of The Electronically Excited CH(A2Δ) Product. Chem. Phys. 1998, 238, 329−341. (22) Liu, W.-L.; Chang, B.-C. Transient Frequency Modulation Spectroscopy and 266 nm Photodissociation of Bromoform. J. Chin. Chem. Soc. 2001, 48, 613−617. (23) Xu, D.; Francisco, J. S.; Hung, J.; Jackson, W. M. Ultraviolet Photodissociation of Bromoform at 234 and 267 nm by Means of Ion Velocity Imaging. J. Chem. Phys. 2002, 117, 2578−2585. (24) Peterson, K. A.; Francisco, J. S. Should Bromoform Absorb at Wavelengths Longer Than 300 nm? J. Chem. Phys. 2002, 117, 6103− 6107.

anharmonicity parameters are significantly different from the reported values.53,54 It should be noted that the parameters reported by Herzberg53 resulted from data that did not include the high vibrational levels in the B state, and the anharmonicity parameters reported by Gerstenkorn and Luc54 were obtained from a global fit using Dunham’s formula with 16 vibrational parameters (Yi0, i = 1−16) for the B state. We tried to use their parameters to fit our data, but the results show a much larger deviations (>15 cm−1). Hence, on the basis of our data, we have determined more accurate anharmonicity parameters for the vibrational structure in the I2 B3Π+0,u−X1Σ+g transition, and this also paves the foundation for determining a better potential curve for both states.57

IV. SUMMARY We recorded the emission spectra following the 266 nm photolysis of several iodomethanes (CHI3, CH2I2, CH3I, and CH2ICl) in a slow flow cell at ambient temperature. The electronically excited species including CH (A2Δ, B2Σ−, and C2Σ+), C2 (d3Πg) were observed like those in the photolysis of bromomethanes,52 whereas the atomic iodine emission (6p → 6s, 4Po3/2 → 4P5/2) and a series of emission bands were observed in the region 12 000−19 000 cm−1. The dominant structure of these emission bands was confirmed to correspond to the I2 B3Π+0,u−X1Σ+g transition by comparison with the emission spectrum of pure I2 at the 532 nm excitation, and the observed I2 was verified as the collisional product between iodine atoms generated from the C−I bond dissociation in these iodomethanes. We obtained the I2 emission spectra following the photolysis of CH2I2 at different wavelengths, and the threshold energy for the first C−I bond dissociation was thus determined to be 208 ± 1 kJ mol−1, consistent with the previous ab initio calculation.51 In addition, the emission spectra of pure I2 at various excitation wavelengths were acquired for comparison with those from the photolysis of iodomethanes. Large errors were found in describing the high vibrational energy levels using the previously reported vibrational parameters.53−55 A least-squares fit of totally 385 emission bands was conducted and produced more accurate anharmonicity parameters for the vibrational structure in the I2 B−X transition.

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AUTHOR INFORMATION

Corresponding Author

*B.-C. Chang: e-mail, [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the support of this work from the National Science Council, Taiwan and that from National Central University. We also want to thank Miss PeiChun Hsu, Miss Sin-Hua Yang, Miss Chia-Hsin Chen, and Miss Jia-Jen Du for their helps in collecting data.

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REFERENCES

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