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short communications\(\def\hfill{\hskip 5em}\def\hfil{\hskip 3em}\def\eqno#1{\hfil {#1}}\)

Journal logoJOURNAL OF
SYNCHROTRON
RADIATION
ISSN: 1600-5775
Volume 5| Part 3| May 1998| Pages 1041-1043
https://doi.org/10.1107/S0909049597016075

Photoionization studies of sulfur radicals and products of their reactions

Bing-Ming Cheng,a* Eh Piew Chew,a Wen-Ching Hung,b Jürg Eberhardb and Yuan-Pern Leeb‡

aSynchrotron Radiation Research Center, No. 1, R&D Road VI, Hsinchu Science-Based Industrial Park, Hsinchu 30077, Taiwan, and bDepartment of Chemistry, National Tsing Hua University, No. 101, Sec. 2, Kuang Fu Road, Hsinchu 30043, Taiwan
*Correspondence e-mail: bmcheng@alpha1.srrc.gov.tw

(Received 4 August 1997; accepted 10 November 1997)

A discharge flow–photoionization mass spectrometric system coupled to a synchrotron is employed to study intermediates and products of sulfur radical reactions related to atmospheric chemistry. Sulfur radicals are generated from reactions of oxygen or chlorine atoms with sulfur compounds in a flow tube. The gaseous reaction products are sampled into the ionization region via a three-stage differential pumping scheme. Photoionization spectra and ionization energies are measured by dispersing synchrotron radiation to ionize the samples. Using this technique, photoionization spectra and ionization energies of HSO, CH3SO, C2H5SO, HSCl, and some secondary reaction products, SSCl, HSSCl, HSSSH, CH3SOH and CH3SS(O)CH3, were measured for the first time.

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1. Introduction

Anthropogenic and natural sulfur are emitted into the atmos­phere in various chemical forms: SO2, H2S, CS2, OCS, CH3SH, CH3SCH3 and CH3SSCH3, to name a few. Roughly, on a global basis, sulfur from natural origins and that from anthropogenic origins contribute equally to the atmosphere. Most of this sulfur returns to the oceans (90%) and the land (10%) as a sulfate in precipitation. The oxidation of anthropogenic SO2 to H2SO4 is the major cause of acid rain. Also, the oxidation of naturally occurring sulfur compounds leads to sulfuric acid particles, which act as condensation nuclei for water to generate clouds and thus change the albedo of the Earth. Therefore, atmospheric sulfur chemistry has immense significance to the biosphere in general and man in particular. Substantial efforts have been directed towards the study of the atmospheric chemistry of these sulfur species (Tyndall & Ravishankara, 1991[Tyndall, G. S. & Ravishankara, A. R. (1991). Int. J. Chem. Kin. 23, 483-527.]).

Photoionization mass spectrometry is currently used to study the kinetics and mechanisms of atmospheric reactions of sulfur species (Dominé et al., 1990[Dominé, F., Murrells, T. P. & Howard, C. J. (1990). J. Phys. Chem. 94, 5839-5847.], 1992[Dominé, F., Ravishankara, A. R. & Howard, C. J. (1992). J. Phys. Chem. 96, 2171-2178.]). However, photoionization spectra are lacking for most sulfur radicals and their reaction intermediates. Thus, it is important to measure photoionization spectra of sulfur radicals and their reaction products. From such measurements, an optimal wavelength for the ionization of sulfur species in kinetic experiments can be established. Ionization energies (IEs) of these sulfur compounds can also be derived from the thresholds of their photoionization spectra. We can expect to learn about their thermochemistry from the IE data obtained.

2. Experimental and results

We use a discharge flow–photoionization mass spectrometer (DF–PIMS) to measure the photoionization spectra of sulfur radicals and their reaction products (Cheng & Hung, 1996[Cheng, B.-M. & Hung, W.-C. (1996). J. Phys. Chem. 100, 10210-10214.]). Sulfur radicals are generated from reactions of oxygen or chlorine atoms with sulfur compounds (see Table 1[link]), in a flow tube by using a discharge flow technique. The flow reactor is a Pyrex tube (with inner diameter 24 mm and length 600 mm) fitted with a movable injector (outer diameter 8 mm). The gas effluents of the reaction system are sampled into an ionization chamber via a three-stage differential pumping scheme. Then, photoionization spectra and ionization energies of mass-selected ions are measured by using a synchrotron as the photoionization source. The detection system is a standard PIMS system with a quadrupole mass filter operated in the pulse-counting mode.

Table 1
Sulfur radicals produced from various reactions in this work

Radicals Reactions
HS O + H2S → HS + OH
  Cl + H2S → HS + HCl
SO O + CS2 → SO + CS
  O + OCS → SO + CO
HSO O + C2H5SH → HSO + C2H5
  O + 2-C3H7SH → HSO + 2-C3H7
  O + HSC2H4SH → HSO + HSC2H4
CH3SO O + CH3SSCH3 → CH3SO + CH3S
  O + C2H5SCH3 → CH3SO + C2H5
C2H5SO O + C2H5SC2H5 → C2H5SO + C2H5
  O + C2H5SSC2H5 → C2H5SO + C2H5S

We measured the photoionization spectra of various sulfur compounds. Ionization energies of these compounds were determined from the thresholds of their photoionization spectra near the onset. Three methods were employed to determine the threshold of ionization; the choice of method depends on the characteristics of each spectrum. When the photoion yield curve shows an abrupt rise near the onset, the maximum of its first derivative is well defined and is therefore taken as the threshold. When the spectrum appears as a distinct step but rises less rapidly than in the former case, the midrise point of the onset is taken as the threshold. The thresholds obtained by these two methods typically agree with each other. In some cases, the photoion yield curve has a gradually rising edge, presumably due to poor Franck–Condon overlap between the neutral and the ionic species. In these cases, the threshold is derived from the intersection of the background baseline with the rising edge fitted to a line by a least-squares method. The IEs obtained for various sulfur compounds are listed in Table 2[link]; the results are in agreement with literature values. Thus, it demonstrates that our DF–PIMS system operates properly.

Table 2
Ionization energies (eV) of various sulfur compounds determined in this work

Species This work Previous work
CS2 10.078 (7) 10.0782 (6)a
H2S 10.460 (11) 10.4682 (2)b
OCS 11.171 (7) 11.1736 (15)c
CH3SH 9.46 (2) 9.455 (5)d
C2H5SH 9.280 (7) 9.285 (5)e
2-C3H7SH 9.143 (7) 9.14f
HSC2H4SH 9.280 (14) –
CH3SCH3 8.69 (2) 8.710 (5)g
C2H5SCH3 8.55 (2) 8.54 (1)h
C2H5SC2H5 8.40 (1) 8.41 (1)i
CH3SSCH3 8.34 (3) 8.18 (3)j
C2H5SSC2H5 8.05 (1) <8.27 (3)h
References: (a) Fischer et al. (1993a[Fischer, I., Lochschmidt, A., Strobel, A., Niedner-Schatteburg, G., Müller-Dethlefs, K. & Bondybey, V. E. (1993a). Chem. Phys. Lett. 202, 542-548.]); (b) Fischer et al. (1993b[Fischer, I., Lochschmidt, A., Strobel, A., Niedner-Schatteburg, G., Müller-Dethlefs, K. & Bondybey, V. E. (1993b). J. Chem. Phys. 98, 3592-3599.]); (c) Ono et al. (1981[Ono, Y., Osuch, E. A. & Ng, C. Y. (1981). J. Chem. Phys. 74, 1645-1651.]); (d) Morgan, Puyuelo et al. (1995[Morgan, R. A., Puyuelo, P., Howe, J. D., Ashfold, M. N. R., Buma, W. J., Wales, N. P. L. & deLange, C. A. (1995). J. Chem. Soc. Faraday Trans. 91, 2715-2721.]); (e) Watanabe et al. (1962[Watanabe, K., Nakayama, T. & Mottl, J. (1962). J. Quant. Spectrosc. Radiat. Transfer, 2, 369-382.]); (f) Ogata et al. (1973[Ogata, H., Onizuka, H., Nihei, Y. & Kamada, H. (1973). Bull. Chem. Soc. Jpn, 46, 3036-3040.]); (g) Morgan, Orr-Ewing et al. (1995[Morgan, R. A., Orr-Ewing, A. J., Ashfold, M. N. R., Buma, W. J., Wales, N. P. L. & deLange, C. A. (1995). J. Chem. Soc. Faraday Trans. 91, 3339-3346.]); (h) Lias et al. (1988[Lias, S. G., Bartmess, J. E., Liebman, J. F., Holmes, J. L., Levin, R. D. & Mallard, W. G. (1988). J. Phys. Chem. Ref. Data, 17, Suppl. 1.]); (i) Ma et al. (1993[Ma, Z.-X., Liao, C.-L., Yin, H.-M., Ng, C. Y., Chiu, S.-W., Ma, N. L. & Li, W.-K. (1993). Chem. Phys. Lett. 213, 250-256.]); (j) Li et al. (1993[Li, W.-K., Chiu, S.-W., Ma, Z.-X., Liao, C.-L. & Ng, C. Y. (1993). J. Chem. Phys. 99, 8440-8444.]).

We have successfully studied photoionization spectra of several sulfur radicals using this DF–PIMS technique. Up to now, we have focused on some interesting radicals: HS, HSO, CH3S, CH3SO and C2H5SO. The IE of the HS radical was determined to be 10.45 (4) eV, consistent with the value of 10.4219 (4) eV measured with a nonresonant two-photon pulsed field ionization technique (Hsu et al., 1994[Hsu, C.-W., Baldwin, D. P., Liao, C.-L. & Ng, C. Y. (1994). J. Chem. Phys. 100, 8047-8054.]). The IE of CH3S was determined to be 9.27 (3) eV, in good agreement with the value of 9.2647 (10) eV measured with a nonresonant two-photon pulsed field ionization technique (Hsu & Ng, 1994[Hsu, C.-W. & Ng, C. Y. (1994). J. Chem. Phys. 101, 5596-5603.]). For HSO, CH3SO and C2H5SO radicals, photoionization spectra were measured for the first time by our group and their IEs were obtained as 9.918 (16) (Cheng, Eberhard et al., 1997a[Cheng, B.-M., Eberhard, J., Chen, W.-C. & Yu, C.-H. (1997a). J. Chem. Phys. 106, 9727-9733.]), 8.99 (2) (Hung et al., 1996[Hung, W.-C., Shen, M.-Y., Lee, Y.-P., Wang, N.-S. & Cheng, B.-M. (1996). J. Chem. Phys. 105, 7402-7411.]) and 8.71 (2) eV (Cheng, Hung et al., 1997[Cheng, B.-M., Hung, W.-C., Chen, W.-C., Yu, C.-H. & Lee, Y.-P. (1997). J. Chem. Phys. 107, 8794-8799.]), respectively. Fig. 1[link] shows the photoionization spectrum near the threshold region of HSO; the IE was derived from the distinct step shown at the onset, either from the midrise point or the first derivative of the onset, as indicated by the arrow.

[Figure 1]
Figure 1
The photoionization threshold region of HSO at a nominal resolution of 0.2 nm and with 0.1 nm steps. The arrow indicates the ionization energy of HSO.

In our studies of sulfur radicals, sometimes secondary reactions could not be avoided. For example, in the case of the CH3SO radical, we also obtained photoionization spectra of CH3SOH, CH3SS and CH3SS(O)CH3; the IEs determined were 8.67 (3), 8.62 (5) and 8.82 (5) eV, respectively. In the reaction system Cl/Cl2/H2S, photoionization spectra of products HSCl, HSSH, HSSSH, SSCl and HSSCl were measured for the first time; the IEs were 9.887 (16), 9.06 (2) (Cheng, Eberhard et al., 1997b[Cheng, B.-M., Eberhard, J., Chen, W.-C. & Yu, C.-H. (1997b). J. Chem. Phys. 107, 5273-5274.]), ≤9.01, 9.01 (3) and 9.266 (14) eV, respectively (Eberhard et al., 1997[Eberhard, J., Chen, W.-C., Yu, C.-H., Lee, Y.-P. & Cheng, B.-M. (1998). J. Chem. Phys. 108. In the press.]). Other products observed in this reaction system include S2, S3 and SCl2; their photoionization spectra and IEs were also measured (see Table 3[link]).

Table 3
Ionization energies (eV) of sulfur species produced from reactions in this work

Species This work Previous work
HS 10.45 (4) 10.4219 (4)a
SO 10.34 (4) 10.294 (4)b
S2 9.350 (14) 9.356 (2)c
HSO 9.918 (16) –
HSCl 9.887 (16) –
SCl2 9.57 (3) 9.45 (3)d
S2O 10.60 (4) 10.584 (5)b
SSCl 9.04 (3) –
S3 9.63 (3) 9.68 (3)e
HSSH 9.06 (2) 9.41f
HSSCl 9.266 (14) –
HSSSH ≤9.09 –
CH3S 9.27 (3) 9.2647 (10)g
CH3SS 8.62 (5) 8.67 (2)h
CH3SO 8.99 (2) –
CH3SOH 8.67 (3) –
C2H5SO 8.71 (2) –
CH3SS(O)CH3 8.82 (5) –
References: (a) Hsu et al. (1994[Hsu, C.-W., Baldwin, D. P., Liao, C.-L. & Ng, C. Y. (1994). J. Chem. Phys. 100, 8047-8054.]); (b) Norwood & Ng (1989[Norwood, K. & Ng, C. Y. (1989). Chem. Phys. Lett. 156, 145-150.]); (c) Liao & Ng (1986[Liao, C. L. & Ng, C. Y. (1986). J. Chem. Phys. 84, 778-782.]); (d) Kaufel et al. (1981[Kaufel, R., Vahl, R., Minkwitz, R. & Baumgartel, H. (1981). Z. Anorg. Allg. Chem. 481, 207-217.]); (e) Berkowitz & Lifshitz (1968[Berkowitz, J. & Lifshitz, C. (1968). J. Chem. Phys. 48, 4346-4350.]); (f) Frost et al. (1977[Frost, D. C., Lee, S. T., McDowell, C. A. & Westwood, N. P. C. (1977). J. Electron Spectrosc. Relat. Phenom. 12, 95-109.]); (g) Hsu & Ng (1994[Hsu, C.-W. & Ng, C. Y. (1994). J. Chem. Phys. 101, 5596-5603.]); (h) Ma et al. (1994[Ma, Z.-X., Liao, C.-L., Ng, C. Y., Cheung, Y.-S., Li, W.-K. & Baer, T. (1994). J. Chem. Phys. 100, 4870-4875.]).

Footnotes

‡Also affiliated with the Institute of Atomic and Molecular Sciences, Academia Sinica, Taiwan.

Acknowledgements

We thank the Synchrotron Radiation Research Center of Taiwan and the National Science Council of the Republic of China for supporting this work.

References

First citationBerkowitz, J. & Lifshitz, C. (1968). J. Chem. Phys. 48, 4346–4350.  CrossRef CAS Web of Science
 First citationCheng, B.-M. & Hung, W.-C. (1996). J. Phys. Chem. 100, 10210–10214.  CrossRef CAS Web of Science
 First citationCheng, B.-M., Eberhard, J., Chen, W.-C. & Yu, C.-H. (1997a). J. Chem. Phys. 106, 9727–9733.  CrossRef CAS Web of Science
 First citationCheng, B.-M., Eberhard, J., Chen, W.-C. & Yu, C.-H. (1997b). J. Chem. Phys. 107, 5273–5274.  CrossRef CAS Web of Science
 First citationCheng, B.-M., Hung, W.-C., Chen, W.-C., Yu, C.-H. & Lee, Y.-P. (1997). J. Chem. Phys. 107, 8794–8799.  CrossRef CAS Web of Science
 First citationDominé, F., Murrells, T. P. & Howard, C. J. (1990). J. Phys. Chem. 94, 5839–5847.
 First citationDominé, F., Ravishankara, A. R. & Howard, C. J. (1992). J. Phys. Chem. 96, 2171–2178.
 First citationEberhard, J., Chen, W.-C., Yu, C.-H., Lee, Y.-P. & Cheng, B.-M. (1998). J. Chem. Phys. 108. In the press.
 First citationFischer, I., Lochschmidt, A., Strobel, A., Niedner-Schatteburg, G., Müller-Dethlefs, K. & Bondybey, V. E. (1993a). Chem. Phys. Lett. 202, 542–548.  CrossRef CAS Web of Science
 First citationFischer, I., Lochschmidt, A., Strobel, A., Niedner-Schatteburg, G., Müller-Dethlefs, K. & Bondybey, V. E. (1993b). J. Chem. Phys. 98, 3592–3599.  CrossRef CAS Web of Science
 First citationFrost, D. C., Lee, S. T., McDowell, C. A. & Westwood, N. P. C. (1977). J. Electron Spectrosc. Relat. Phenom. 12, 95–109.  CrossRef CAS Web of Science
 First citationHsu, C.-W., Baldwin, D. P., Liao, C.-L. & Ng, C. Y. (1994). J. Chem. Phys. 100, 8047–8054.  CrossRef CAS Web of Science
 First citationHsu, C.-W. & Ng, C. Y. (1994). J. Chem. Phys. 101, 5596–5603.  CrossRef CAS Web of Science
 First citationHung, W.-C., Shen, M.-Y., Lee, Y.-P., Wang, N.-S. & Cheng, B.-M. (1996). J. Chem. Phys. 105, 7402–7411.  CrossRef CAS Web of Science
 First citationKaufel, R., Vahl, R., Minkwitz, R. & Baumgartel, H. (1981). Z. Anorg. Allg. Chem. 481, 207–217.  CrossRef CAS Web of Science
 First citationLi, W.-K., Chiu, S.-W., Ma, Z.-X., Liao, C.-L. & Ng, C. Y. (1993). J. Chem. Phys. 99, 8440–8444.  CrossRef CAS Web of Science
 First citationLiao, C. L. & Ng, C. Y. (1986). J. Chem. Phys. 84, 778–782.  CrossRef CAS Web of Science
 First citationLias, S. G., Bartmess, J. E., Liebman, J. F., Holmes, J. L., Levin, R. D. & Mallard, W. G. (1988). J. Phys. Chem. Ref. Data, 17, Suppl. 1.
 First citationMa, Z.-X., Liao, C.-L., Ng, C. Y., Cheung, Y.-S., Li, W.-K. & Baer, T. (1994). J. Chem. Phys. 100, 4870–4875.  CrossRef CAS Web of Science
 First citationMa, Z.-X., Liao, C.-L., Yin, H.-M., Ng, C. Y., Chiu, S.-W., Ma, N. L. & Li, W.-K. (1993). Chem. Phys. Lett. 213, 250–256.  CrossRef CAS Web of Science
 First citationMorgan, R. A., Orr-Ewing, A. J., Ashfold, M. N. R., Buma, W. J., Wales, N. P. L. & deLange, C. A. (1995). J. Chem. Soc. Faraday Trans. 91, 3339–3346.  CrossRef CAS Web of Science
 First citationMorgan, R. A., Puyuelo, P., Howe, J. D., Ashfold, M. N. R., Buma, W. J., Wales, N. P. L. & deLange, C. A. (1995). J. Chem. Soc. Faraday Trans. 91, 2715–2721.  CrossRef CAS Web of Science
 First citationNorwood, K. & Ng, C. Y. (1989). Chem. Phys. Lett. 156, 145–150.  CrossRef CAS Web of Science
 First citationOgata, H., Onizuka, H., Nihei, Y. & Kamada, H. (1973). Bull. Chem. Soc. Jpn, 46, 3036–3040.  CrossRef CAS Web of Science
 First citationOno, Y., Osuch, E. A. & Ng, C. Y. (1981). J. Chem. Phys. 74, 1645–1651.  CrossRef CAS Web of Science
 First citationTyndall, G. S. & Ravishankara, A. R. (1991). Int. J. Chem. Kin. 23, 483–527.  CrossRef CAS Web of Science
 First citationWatanabe, K., Nakayama, T. & Mottl, J. (1962). J. Quant. Spectrosc. Radiat. Transfer, 2, 369–382.  CrossRef Web of Science

© International Union of Crystallography. Prior permission is not required to reproduce short quotations, tables and figures from this article, provided the original authors and source are cited. For more information, click here.

Journal logoJOURNAL OF
SYNCHROTRON
RADIATION
ISSN: 1600-5775
Volume 5| Part 3| May 1998| Pages 1041-1043
https://doi.org/10.1107/S0909049597016075
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