- Email: [email protected]
Dissociative excitation of carbon disulfide by electron impact
ARTICLE IN PRESS
Radiation Physics and Chemistry 68 (2003) 323–328
Dissociative excitation of carbon disulfide by electron impact Mariusz Zubeka,*, Jowita Gackowskaa, Alexander Snegurskyb a
! University of Technology, 80-952 Gdansk, ! Department of Physics of Electronic Phenomena, Gdansk Poland b Institute of Electron Physics, 88000 Uzhgorod, Ukraine
Abstract Dissociative excitation of carbon disulfide (CS2) producing CS fragments in the A1P excited state has been studied by optical excitation method in the electron energy range from threshold to 30 eV. The threshold energy for the dissociative excitation has been determined to be 9.3370.06 eV. It has been also verified that the vibrational population of the CS radicals is inverted for v ¼ 0 and 1 levels. r 2003 Elsevier Science Ltd. All rights reserved. Keywords: Carbon disulfide; Dissociative excitation; Fluorescence emission; Electron impact
1. Introduction Detailed experiments on the dissociation of molecules provide data on the dissociation dynamics and also help in constructing potential energy diagrams of the excited states of parent molecules. These data obtained on the excited atomic and molecular fragments e.g. OH, SO, CS resulted from electron- and photon-impact dissociative excitation of various molecular species are also used to explain processes occurring in the planetary atmospheres and laboratory plasma. Dissociation processes in carbon disulfide (CS2) have been widely investigated using different experimental methods, which include discharge and afterglow studies (Wu, 1985; Xu et al., 1993) and photon impact technique (Okabe, 1972; Lee and Judge, 1975; Ashfold et al., 1980; Yang et al., 1980; Day et al., 1982; Kitsopoulos et al., 2001). The possible channels of dissociative fragmentation which had to be taken into account in these investigations involve the ground and excited states of CS and S fragments. Recent photodissociation studies of carbon disulfide performed by Kitsopoulos et al. (2001) at a wavelength of 193 nm provided new data on the excited states which dissociate into the CS in its ground X1S+ state. Measured velocity distributions for state *Corresponding author. Tel.: +48-58-3472284; fax: +48-583472821. E-mail address: [email protected] (M. Zubek).
selected S(3P2,1,0) and S(1D2) fragments allowed to determine their branching ratio and indicated substantial vibrational excitation of the CS(X1S+) radical. It has also been concluded that carbon disulfide molecule is highly bent before dissociating and that the dissociation time is of the order of the rotational period. In this work we have studied electron impact dissociative excitation of carbon disulfide in the electron energy range below 30 eV which results in the formation of excited CS radicals in the A1P state and sulfur atoms in the ground 3P state, * 1 Sþ Þ þ e-CSðA1 PÞ þ Sð3 PÞ þ e: CS2 ðX g
ð1Þ
Excitation of CS radicals has been detected by observing optical emission of the A1P-X1S+ transition in the wavelength region 240–280 nm. The first measurements of the dissociative excitation of (1) in carbon disulfide have been carried out using an optical method by Ajello and Srivastava (1981). They have obtained emission spectra within the 240–290 nm wavelength region at 20 and 100 eV incident electron energies. They have also measured emission crosssection in the energy range up to 125 eV which revealed a broad peak in the vicinity of 12 eV and a maximum close to 25 eV which was assigned to higher-lying process of dissociation or dissociative ionization. The absolute value of the emission cross-section at 100 eV was measured with 25% uncertainty to be
0969-806X/03/$ - see front matter r 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0969-806X(03)00309-8
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324
1.2 1021 m2. Recently Tokue et al. (1994) discussed dynamics of the dissociation of (1) within the framework of the impulsive half-collision model of Simons and Tasker (1973) which was previously applied in the photodissociation studies by Lee and Judge (1975). They also found the threshold energy for the dissociative excitation to be at 9.770.3 eV.
were obtained with an optical resolution (Dl/l) of 0.008. These spectra were corrected for the wavelength variation of the sensitivity of the photon detection channel which has been determined using the molecular branching ratio method together with monitoring well-known emission systems in nitrogen and carbon monoxide. The energy dependence of the emission cross-section has been measured at a fixed wavelength corresponding to selected optical lines. The electron beam intensity in the collision region as monitored by a beam collector was constant to better than 10% and the acquired excitation functions have not been corrected for the energy variation in the electron beam intensity. The incident electron energy has been calibrated to within 730 meV against position of the 2S+ g resonance at 11.50 eV in nitrogen observed in mixture with carbon disulfide. The wavelength in the emission spectra has been calibrated to within 70.2 nm against the position of the strong (0 0 0)-(0 0 0)3/2 vibrational line at 282.0 nm of the *2 B2S+ u -X Pg ionic transition in carbon disulfide (Lee et al., 1975). Carbon disulfide has been degassed several times in the vacuum container to remove gaseous impurities before introducing through a regulated leak valve into the collision cell.
2. Experimental The present measurements have been performed using trochoidal electron spectrometer. The electron beam leaving trochoidal selector with an intensity of about 150 nA and an energy spread of 200 meV was accelerated before entering the collision cell to an energy in the range from 8 to 30 eV. Molecular optical emission generated in the collision region filled with vapor of carbon disulfide was directed through a quartz light guide to the entrance of the 0.25 m Ebert optical monochromator. The monochromator was equipped with a grating blazed at 300 nm which has 1181 lines/ mm. The molecular emission was detected by a 9813QB photomultiplier cooled to 30 C. More details on the construction and performance of the spectrometer are given in previous articles (Zubek, 1994; Olszewski and Zubek, 2001). During the measurements to remain in the regime of linear pressure dependence of the detected photon intensity the gas pressure in the collision cell as measured by a baratron was maintained below 5 103 Pa. The emission spectra of carbon disulfide × 10
3. Results and discussion 3.1. Emission spectra Figs. 1 and 2 show emission spectra of carbon disulfide measured in the 240–280 nm wavelength range
3
CS
5
AΠ-XΣ 1
(4,v'') v''=
Intensity (counts)
4
2
(3,v'') v''= (2,v'') v''=
1
3
1 0
3
1
(0,v'') v''=
5
4
2
4
2
(1,v'') v''= 0
3
+
3
1 0
2 1
CS 2 E i = 15 eV
2
1
0 240
250
260
270
280
W avelength (nm) Fig. 1. Emission spectrum of carbon disulfide obtained at an incident electron energy of 15 eV. A smoothly varying background has been subtracted from the original spectrum. The spectrum is corrected for the wavelength dependence of the sensitivity of the optical detection channel. The spectrum has been resolved into indicated vibrational lines of the CS (A1P -X1S+) transition. The sum of all individual optical lines is shown by a full line.
ARTICLE IN PRESS M. Zubek et al. / Radiation Physics and Chemistry 68 (2003) 323–328 × 10
3
CS
AΠ -XΣ 1
8 (4,v'') v''=
7 6
Intensity (counts)
325
2
(3,v'') v''= (2,v'') v''=
1
+
0
3
1
4 3
2
0
(0,v'') v''=
5
4
2
(1,v'') v''=
5
1
3
2
1 0
1
CS 2 E i = 29 eV
4 3 2 1 0 240
250
260
270
280
W avelength (nm)
Fig. 2. Emission spectrum of carbon disulfide obtained at an incident electron energy of 29 eV. A smoothly varying background has been subtracted from the original spectrum. The spectrum is corrected for the wavelength dependence of the sensitivity of the optical detection channel. The spectrum has been resolved into indicated vibrational lines of the CS (A1P -X1S+) transition. The sum of all individual optical lines is shown by a full line.
at incident electron energies of 15 and 29 eV, respectively. The detected fluorescence is emitted by the CS fragments in the excited A1P state during their decay to the ground X1S+ state. The spectra of Figs. 1 and 2 have been analyzed using a least-square fitting procedure to resolve vibrational lines of the A1P-X1S+ emission and to determine their relative intensities in the spectra. In this procedure the experimental width (FWHM) of the individual lines has been taken to be equal to the width (2.15 nm) of the observed wellseparated lines of the (0 0 0)-(0 0 0)3/2 and (0 0 0)*2 (0 0 0)1/2 vibrational transitions of the B2S+ u -X Pg ionic bands at 282.0 and 285.5 nm, respectively. The wavelength positions of the vibrational lines have been fixed at the corresponding values given by Pearse and Gaydon (1976). Table 1 presents relative line intensities measured for both energies and normalized to that of the (1,1) bands. It also compares the present measurements with the electron impact data of Ajello and Srivastava (1981) obtained at 20 eV and with the photodissociation results of Lee and Judge (1975) which were taken at a wavelength of 1239 A. Our both distributions of line intensities are in general agreement with the results of Ajello and Srivastava (1981) except for the (0,0) line which in our spectra has lower intensity than the (1,1) line and are also consistent with the data of Lee and Judge (1975). As it is seen from Table 1 the relative intensities of the emission lines for 15 and 29 eV which corresponds to a maximum and an increase in the excitation cross-section respectively (see Fig. 3) show similar overall behavior.
This may indicate that the dissociation at both energies involve the same excited dissociative state. However, increase in the cross-section above 22 eV suggests an opening of a new channel for dissociation. This higherlying state would then necessitate predissociation into the observed excited state. Table 2 displays relative vibrational emission crosssections obtained at 15 eV which have been determined by summing the (v0 ; v00 ) line intensities over v00 for a given v0 level of the CS(A1P) state. Obtained distribution of the cross-sections reveals substantial vibrational excitation of the CS dissociation fragments up to the v0 ¼ 4 level and also a distinct population inversion between v0 ¼ 0 and v0 ¼ 1 levels. This observations agree well with the results of Lee and Judge (1975), Ajello and Srivastava (1981) and those of Xu et al. (1993) who investigated dissociative excitation of carbon disulfide by metastable argon atoms. Tokue et al. (1994) however in their electron impact studies did not report inverted population of n0 ¼ 0 and n0 ¼ 1 levels of the A1P state. It is to note that in recent studies Xu et al. (1993) found that higher pressure of carbon disulfide in the reaction cell produces collisional quenching of the v0 > 0 vibrational levels of the CS radicals which does not permit determination of the initial population of the excited fragments. Dissociation dynamics leading to vibrational excitation of CS has been considered by several authors (Xu et al., 1993; Tokue et al., 1994) within the frame work of the impulsive half-collision model which has been generalized by Simons and Tasker (1973). This model predicts population inversion of the vibrational v0 ¼ 0 and 1 levels in agreement with the present results.
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Table 1 Relative emission intensities of the (v0 ; v00 ) vibrational transitions of CS(A1P-X1S+) observed in the fluorescence spectra of carbon disulfide Vibrational transition (v0 ; v00 )
(0,0) (0,1) (1,0) (1,1) (1,2) (2,0) (2,1) (2,2) (2,3) (3,1) (3,2) (3,3) (3,4) (4,2) (4,3) (4,4) (4,5)
Wavelength (nm)a
257.56 266.26 250.73 258.96 267.70 244.48 252.32 260.59 269.32 246.02 253.87 262.16 270.89 247.70 255.58 263.89 272.67
Present work
Other data
Relative line intensity 15 eV
Relative line intensity 29 eV
Electron impactb 20 eV
Photon impactc
0.86 0.04 0.35 1.00 0.16 0.07 0.35 0.46 0.14 0.05 0.44 0.13 0.08 0.16 0.38 0.04 0.05
0.86 0.03 0.47 1.00 0.14 0.08 0.42 0.43 0.13 0.08 0.48 0.15 0.06 0.23 0.46 0.05 0.06
1.12 0.12 0.33 1.00 0.21 0.08 0.37 0.36 0.19 0.08 0.38 0.18 0.12 0.09 0.34 0.05 0.08
0.80 0.14 0.24 1.00 0.31 0.04 0.32 0.44 0.31 0.07 0.27 0.12 0.21 0.08 0.38 0.04 0.15
a
Pearse and Gaydon (1976). Ajello and Srivastava (1981). c Lee and Judge (1975). b
Table 2 Relative emission cross-sections for v0 vibrational levels of CS(A1P) state in dissociative excitation of carbon disulfide at an energy of 15 eV Vibrational level v0
Emission cross-section
0 1 2 3 4
0.59 1.00 0.67 0.46 0.42
3.2. Emission cross-section The emission cross-section of the dissociation determined in the electron energy range of 20 eV above excitation threshold is shown in Fig. 3. The energy dependence of the cross-section has been obtained in the measurements of fluorescence excitation function at a wavelength of 258 nm within a spectral window 5 nm wide (FWHM) which includes emission lines of the main peak in the spectra of Figs. 1 and 2. As it is seen in Fig. 3 the cross-section rises sharply above threshold to a broad maximum at 15 eV followed by a slow increase of the cross-section at about 22 eV which indicates opening of a new dissociative channel.
The threshold energy of the observed dissociative excitation has been derived from an analysis of the cross-section to be 9.3370.06 eV. Here the excitation cross-section in the 0.5 eV energy range above threshold energy Eth (see inset in Fig. 3) has been described by an exponential function f ðEÞ ¼ bðE Eth ÞP
ð2Þ
and for EoEth f ðEÞ ¼ 0: This function has been convoluted with a Gaussian profile representing energy spread of the experiment and then fitted to the measured emission intensity. In the fitting procedure apart from the threshold energy Eth ; the scaling factor b and exponent p ¼ 1:33 have been determined. Inset of Fig. 3 shows the fitted curve and the obtained energy dependence of the cross-section in the threshold region. Our value of Eth is in good agreement with the threshold dissociation energy of Okabe (1972), 9.27370.014 eV but does not support the result of Tokue et al. (1994). The threshold energy can be compared with the dissociation energy D0 for the CS(A1P)+S(3P) limit. This energy is calculated from D0 ¼ D00 þ Eð0; 0Þ; where D00 is the CS–S dissociation energy from the lowest * 1 Sþ state of carbon vibrational level of the ground X g disulfide and Eð0; 0Þ is the CS excitation energy from 1 + v00 ¼ 0 of the ground X S state into the v0 ¼ 0 level of A1P state. The estimated D0 is equal to 9.2970.02 eV
ARTICLE IN PRESS
Cross section
Cross section
M. Zubek et al. / Radiation Physics and Chemistry 68 (2003) 323–328
327
9.33 ± 0.06 eV
0 8.8
9.0
9.2
9.4
9.6
9.8
Electron energy (eV)
0 10
15
20
25
30
Electron energy (eV) Fig. 3. Cross-section for dissociative excitation of carbon disulfide. In the inset, fit to the experimental data obtained in the threshold region is shown by the dashed line and the analytical excitation cross-section is shown by a full line.
CS 2 10
S(3P) + CS (A1Π)
E (eV)
E(0,0)
5
S(3P) + CS (X1Σ+)
D 00 +
0
X1 ∑ g v=0
r (S - CS) Fig. 4. Potential energy curves (qualitative) of the ground * 1 Sþ and dissociative states of carbon disulfide with respect to X g the S–CS internuclear distance.
taking D00 ¼ 4:4870:02 eV (Coppens et al., 1979) and Eð0; 0Þ ¼ 4:810 eV (Huber and Herzberg, 1979). Our threshold energy of the dissociative excitation coincides within the experimental uncertainty with the above estimation. The concurrence of the measured threshold and estimated dissociation energies imply that the potential energy curve of the excited dissociative state of carbon disulfide crosses the Franck-Condon region of the S–CS
internuclear distance at about 9.3 eV. In high-resolution electron energy loss spectra (Hubin-Franskin et al., 1983; Wilden and Comer, 1980) and also in the threshold excitation spectrum of Zubek and King (1995) a peak with an origin at 9.3 and 0.5 eV wide was detected which most likely corresponds to the dissociative state detected in the present work. Combining these results with the present measurements a qualitative potential energy curve of the excited state has been constructed and it is shown in Fig. 4. In conclusion, we have studied dissociative excitation of carbon disulfide which produces CS(A1P) excited fragments in the near-threshold electron incident energy region. The excitation threshold energy has been measured, with an accuracy higher than previous, to be 9.3370.06 eV. Obtained emission spectra indicate that the vibrational population of the CS radical is inverted for v ¼ 0 and 1 vibrational levels.
Acknowledgements This work has been supported by the Polish State Committee for Scientific Research under DZ KBN 3570/ R01/R03.
References Ajello, J.M., Srivastava, S.K., 1981. UV studies of electron impact excitation of CS2. J. Chem. Phys. 75, 4454–4463.
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Ashfold, M.N.R., Quinton, A.M., Simmons, J.P., 1980. Energy disposal in the vacuum ultraviolet photodissociation of CS2 and CSe2. J. Chem. Soc. Faraday II 76, 905–915. Coppens, P., Reynaert, J.C., Drowart, J., 1979. Mass spectrometric study of the photoionization of carbon disulphide in the wavelength interval 125–60 nm. J. Chem. Soc. Faraday Trans. II 75, 29–301. Day, R.L., Sato, M., Lee, L.C., 1982. Photodissociation yields of CS2 at 1060–1520 A. J. Phys. B 15, 4403–4409. Huber, K.P., Herzberg, G., 1979. Molecular Spectra and Molecular Structure IV. Constants of Diatomic Molecules. Van Nostrand-Reinhold, New York. Hubin-Franskin, M.-J., Delwiche, J., Poulin, A., Leclerc, B., Roy, P., Roy, D., 1983. Study of CS2 in the 3–10 eV energy range by electron energy loss spectroscopy. J. Chem. Phys. 78, 1200–1212. Kitsopoulos, T.N., Gebhardt, C.R., Rakitzis, T.P., 2001. Photodissociation study of CS2 at 193 nm using slice imaging. J. Chem. Phys. 115, 9727–9732. Lee, L.C., Judge, D.L., 1975. CS(A1P- X1S+) fluorescence from photodissociation of CS2 and OCS. J. Chem. Phys. 63, 2782–2786. 2 + 2 Lee, L.C., Judge, D.L., Ogawa, M., 1975. CS+ 2 (B Su , A Pu2 X Pg) Fluorescence from photoionization excitation in CS2 vapor. Can. J. Phys. 53, 1861–1868. Okabe, H., 1972. Photodissociation of CS2 in the vacuum ultraviolet; determination of D00 (SC-S). J. Chem. Phys. 56, 4381–4384. Olszewski, R., Zubek, M., 2001. A study of electron impact excitation of the A2S+ state of nitric oxide in the nearthreshold energy range. Chem. Phys. Lett. 340, 249–255.
Pearse, R.W.B., Gaydon, A.G., 1976. The Identification of Molecular Spectra. Wiley, New York, p. 120. Simons, J.P., Tasker, P.W., 1973. Energy partitioning in photodissociation and photosensitization. Part 2. Quenching of Hg(63P1) by CO and NO and the near ultra-violet photodissociation of ICN. Mol. Phys. 26, 1267–1280. Tokue, I., Kusakabe, M., Ogawa, H., Ito, Y., 1994. Vibrational population of CS(A1P) produced by electron-impact dissociation of CS2 and OCS. J. Chem. Phys. 98, 3588–3591. Wilden, D.G., Comer, J., 1980. A study of spin and symmetry forbidden transitions in CS2 by high resolution energy-loss spectroscopy. Chem. Phys. 53, 77–84. Wu, K.T., 1985. Dynamics of CS(A1P) formation in the dissociation of CS2. J. Phys. Chem. 89, 4617–4621. Xu, D., Li, X., Shen, G., Wang, L., Chen, H., Lou, N., 1993. Vibrational population inversion in CS(A) through the dissociative excitation of CS2 by Ar(3P2). Chem. Phys. Lett. 210, 315–321. Yang, S.C., Freedman, A., Kawasaki, M., Bersohn, R., 1980. Energy distribution of the fragments produced by photodissociation of CS2 at 193 nm. J. Chem. Phys. 72, 4058–4062. Zubek, M., 1994. Excitation of the C3Pu state of N2 by electron impact in the near-threshold region. J. Phys. B 27, 573–581. Zubek, M., King, G.C., 1995. Threshold electron impact excitation of CS2. Abstracts of Contributed Papers, J.B.A. Mitchell, J.W. McConkey, C.E. Brion (Eds.), XIXth International Conference on the Physics of Electronic and Atomic Collisions, Whistler, Canada, 26 July–1 August 1995, pp. 419, to be published.
Radiation Physics and Chemistry 68 (2003) 323–328
Dissociative excitation of carbon disulfide by electron impact Mariusz Zubeka,*, Jowita Gackowskaa, Alexander Snegurskyb a
! University of Technology, 80-952 Gdansk, ! Department of Physics of Electronic Phenomena, Gdansk Poland b Institute of Electron Physics, 88000 Uzhgorod, Ukraine
Abstract Dissociative excitation of carbon disulfide (CS2) producing CS fragments in the A1P excited state has been studied by optical excitation method in the electron energy range from threshold to 30 eV. The threshold energy for the dissociative excitation has been determined to be 9.3370.06 eV. It has been also verified that the vibrational population of the CS radicals is inverted for v ¼ 0 and 1 levels. r 2003 Elsevier Science Ltd. All rights reserved. Keywords: Carbon disulfide; Dissociative excitation; Fluorescence emission; Electron impact
1. Introduction Detailed experiments on the dissociation of molecules provide data on the dissociation dynamics and also help in constructing potential energy diagrams of the excited states of parent molecules. These data obtained on the excited atomic and molecular fragments e.g. OH, SO, CS resulted from electron- and photon-impact dissociative excitation of various molecular species are also used to explain processes occurring in the planetary atmospheres and laboratory plasma. Dissociation processes in carbon disulfide (CS2) have been widely investigated using different experimental methods, which include discharge and afterglow studies (Wu, 1985; Xu et al., 1993) and photon impact technique (Okabe, 1972; Lee and Judge, 1975; Ashfold et al., 1980; Yang et al., 1980; Day et al., 1982; Kitsopoulos et al., 2001). The possible channels of dissociative fragmentation which had to be taken into account in these investigations involve the ground and excited states of CS and S fragments. Recent photodissociation studies of carbon disulfide performed by Kitsopoulos et al. (2001) at a wavelength of 193 nm provided new data on the excited states which dissociate into the CS in its ground X1S+ state. Measured velocity distributions for state *Corresponding author. Tel.: +48-58-3472284; fax: +48-583472821. E-mail address: [email protected] (M. Zubek).
selected S(3P2,1,0) and S(1D2) fragments allowed to determine their branching ratio and indicated substantial vibrational excitation of the CS(X1S+) radical. It has also been concluded that carbon disulfide molecule is highly bent before dissociating and that the dissociation time is of the order of the rotational period. In this work we have studied electron impact dissociative excitation of carbon disulfide in the electron energy range below 30 eV which results in the formation of excited CS radicals in the A1P state and sulfur atoms in the ground 3P state, * 1 Sþ Þ þ e-CSðA1 PÞ þ Sð3 PÞ þ e: CS2 ðX g
ð1Þ
Excitation of CS radicals has been detected by observing optical emission of the A1P-X1S+ transition in the wavelength region 240–280 nm. The first measurements of the dissociative excitation of (1) in carbon disulfide have been carried out using an optical method by Ajello and Srivastava (1981). They have obtained emission spectra within the 240–290 nm wavelength region at 20 and 100 eV incident electron energies. They have also measured emission crosssection in the energy range up to 125 eV which revealed a broad peak in the vicinity of 12 eV and a maximum close to 25 eV which was assigned to higher-lying process of dissociation or dissociative ionization. The absolute value of the emission cross-section at 100 eV was measured with 25% uncertainty to be
0969-806X/03/$ - see front matter r 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0969-806X(03)00309-8
ARTICLE IN PRESS M. Zubek et al. / Radiation Physics and Chemistry 68 (2003) 323–328
324
1.2 1021 m2. Recently Tokue et al. (1994) discussed dynamics of the dissociation of (1) within the framework of the impulsive half-collision model of Simons and Tasker (1973) which was previously applied in the photodissociation studies by Lee and Judge (1975). They also found the threshold energy for the dissociative excitation to be at 9.770.3 eV.
were obtained with an optical resolution (Dl/l) of 0.008. These spectra were corrected for the wavelength variation of the sensitivity of the photon detection channel which has been determined using the molecular branching ratio method together with monitoring well-known emission systems in nitrogen and carbon monoxide. The energy dependence of the emission cross-section has been measured at a fixed wavelength corresponding to selected optical lines. The electron beam intensity in the collision region as monitored by a beam collector was constant to better than 10% and the acquired excitation functions have not been corrected for the energy variation in the electron beam intensity. The incident electron energy has been calibrated to within 730 meV against position of the 2S+ g resonance at 11.50 eV in nitrogen observed in mixture with carbon disulfide. The wavelength in the emission spectra has been calibrated to within 70.2 nm against the position of the strong (0 0 0)-(0 0 0)3/2 vibrational line at 282.0 nm of the *2 B2S+ u -X Pg ionic transition in carbon disulfide (Lee et al., 1975). Carbon disulfide has been degassed several times in the vacuum container to remove gaseous impurities before introducing through a regulated leak valve into the collision cell.
2. Experimental The present measurements have been performed using trochoidal electron spectrometer. The electron beam leaving trochoidal selector with an intensity of about 150 nA and an energy spread of 200 meV was accelerated before entering the collision cell to an energy in the range from 8 to 30 eV. Molecular optical emission generated in the collision region filled with vapor of carbon disulfide was directed through a quartz light guide to the entrance of the 0.25 m Ebert optical monochromator. The monochromator was equipped with a grating blazed at 300 nm which has 1181 lines/ mm. The molecular emission was detected by a 9813QB photomultiplier cooled to 30 C. More details on the construction and performance of the spectrometer are given in previous articles (Zubek, 1994; Olszewski and Zubek, 2001). During the measurements to remain in the regime of linear pressure dependence of the detected photon intensity the gas pressure in the collision cell as measured by a baratron was maintained below 5 103 Pa. The emission spectra of carbon disulfide × 10
3. Results and discussion 3.1. Emission spectra Figs. 1 and 2 show emission spectra of carbon disulfide measured in the 240–280 nm wavelength range
3
CS
5
AΠ-XΣ 1
(4,v'') v''=
Intensity (counts)
4
2
(3,v'') v''= (2,v'') v''=
1
3
1 0
3
1
(0,v'') v''=
5
4
2
4
2
(1,v'') v''= 0
3
+
3
1 0
2 1
CS 2 E i = 15 eV
2
1
0 240
250
260
270
280
W avelength (nm) Fig. 1. Emission spectrum of carbon disulfide obtained at an incident electron energy of 15 eV. A smoothly varying background has been subtracted from the original spectrum. The spectrum is corrected for the wavelength dependence of the sensitivity of the optical detection channel. The spectrum has been resolved into indicated vibrational lines of the CS (A1P -X1S+) transition. The sum of all individual optical lines is shown by a full line.
ARTICLE IN PRESS M. Zubek et al. / Radiation Physics and Chemistry 68 (2003) 323–328 × 10
3
CS
AΠ -XΣ 1
8 (4,v'') v''=
7 6
Intensity (counts)
325
2
(3,v'') v''= (2,v'') v''=
1
+
0
3
1
4 3
2
0
(0,v'') v''=
5
4
2
(1,v'') v''=
5
1
3
2
1 0
1
CS 2 E i = 29 eV
4 3 2 1 0 240
250
260
270
280
W avelength (nm)
Fig. 2. Emission spectrum of carbon disulfide obtained at an incident electron energy of 29 eV. A smoothly varying background has been subtracted from the original spectrum. The spectrum is corrected for the wavelength dependence of the sensitivity of the optical detection channel. The spectrum has been resolved into indicated vibrational lines of the CS (A1P -X1S+) transition. The sum of all individual optical lines is shown by a full line.
at incident electron energies of 15 and 29 eV, respectively. The detected fluorescence is emitted by the CS fragments in the excited A1P state during their decay to the ground X1S+ state. The spectra of Figs. 1 and 2 have been analyzed using a least-square fitting procedure to resolve vibrational lines of the A1P-X1S+ emission and to determine their relative intensities in the spectra. In this procedure the experimental width (FWHM) of the individual lines has been taken to be equal to the width (2.15 nm) of the observed wellseparated lines of the (0 0 0)-(0 0 0)3/2 and (0 0 0)*2 (0 0 0)1/2 vibrational transitions of the B2S+ u -X Pg ionic bands at 282.0 and 285.5 nm, respectively. The wavelength positions of the vibrational lines have been fixed at the corresponding values given by Pearse and Gaydon (1976). Table 1 presents relative line intensities measured for both energies and normalized to that of the (1,1) bands. It also compares the present measurements with the electron impact data of Ajello and Srivastava (1981) obtained at 20 eV and with the photodissociation results of Lee and Judge (1975) which were taken at a wavelength of 1239 A. Our both distributions of line intensities are in general agreement with the results of Ajello and Srivastava (1981) except for the (0,0) line which in our spectra has lower intensity than the (1,1) line and are also consistent with the data of Lee and Judge (1975). As it is seen from Table 1 the relative intensities of the emission lines for 15 and 29 eV which corresponds to a maximum and an increase in the excitation cross-section respectively (see Fig. 3) show similar overall behavior.
This may indicate that the dissociation at both energies involve the same excited dissociative state. However, increase in the cross-section above 22 eV suggests an opening of a new channel for dissociation. This higherlying state would then necessitate predissociation into the observed excited state. Table 2 displays relative vibrational emission crosssections obtained at 15 eV which have been determined by summing the (v0 ; v00 ) line intensities over v00 for a given v0 level of the CS(A1P) state. Obtained distribution of the cross-sections reveals substantial vibrational excitation of the CS dissociation fragments up to the v0 ¼ 4 level and also a distinct population inversion between v0 ¼ 0 and v0 ¼ 1 levels. This observations agree well with the results of Lee and Judge (1975), Ajello and Srivastava (1981) and those of Xu et al. (1993) who investigated dissociative excitation of carbon disulfide by metastable argon atoms. Tokue et al. (1994) however in their electron impact studies did not report inverted population of n0 ¼ 0 and n0 ¼ 1 levels of the A1P state. It is to note that in recent studies Xu et al. (1993) found that higher pressure of carbon disulfide in the reaction cell produces collisional quenching of the v0 > 0 vibrational levels of the CS radicals which does not permit determination of the initial population of the excited fragments. Dissociation dynamics leading to vibrational excitation of CS has been considered by several authors (Xu et al., 1993; Tokue et al., 1994) within the frame work of the impulsive half-collision model which has been generalized by Simons and Tasker (1973). This model predicts population inversion of the vibrational v0 ¼ 0 and 1 levels in agreement with the present results.
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Table 1 Relative emission intensities of the (v0 ; v00 ) vibrational transitions of CS(A1P-X1S+) observed in the fluorescence spectra of carbon disulfide Vibrational transition (v0 ; v00 )
(0,0) (0,1) (1,0) (1,1) (1,2) (2,0) (2,1) (2,2) (2,3) (3,1) (3,2) (3,3) (3,4) (4,2) (4,3) (4,4) (4,5)
Wavelength (nm)a
257.56 266.26 250.73 258.96 267.70 244.48 252.32 260.59 269.32 246.02 253.87 262.16 270.89 247.70 255.58 263.89 272.67
Present work
Other data
Relative line intensity 15 eV
Relative line intensity 29 eV
Electron impactb 20 eV
Photon impactc
0.86 0.04 0.35 1.00 0.16 0.07 0.35 0.46 0.14 0.05 0.44 0.13 0.08 0.16 0.38 0.04 0.05
0.86 0.03 0.47 1.00 0.14 0.08 0.42 0.43 0.13 0.08 0.48 0.15 0.06 0.23 0.46 0.05 0.06
1.12 0.12 0.33 1.00 0.21 0.08 0.37 0.36 0.19 0.08 0.38 0.18 0.12 0.09 0.34 0.05 0.08
0.80 0.14 0.24 1.00 0.31 0.04 0.32 0.44 0.31 0.07 0.27 0.12 0.21 0.08 0.38 0.04 0.15
a
Pearse and Gaydon (1976). Ajello and Srivastava (1981). c Lee and Judge (1975). b
Table 2 Relative emission cross-sections for v0 vibrational levels of CS(A1P) state in dissociative excitation of carbon disulfide at an energy of 15 eV Vibrational level v0
Emission cross-section
0 1 2 3 4
0.59 1.00 0.67 0.46 0.42
3.2. Emission cross-section The emission cross-section of the dissociation determined in the electron energy range of 20 eV above excitation threshold is shown in Fig. 3. The energy dependence of the cross-section has been obtained in the measurements of fluorescence excitation function at a wavelength of 258 nm within a spectral window 5 nm wide (FWHM) which includes emission lines of the main peak in the spectra of Figs. 1 and 2. As it is seen in Fig. 3 the cross-section rises sharply above threshold to a broad maximum at 15 eV followed by a slow increase of the cross-section at about 22 eV which indicates opening of a new dissociative channel.
The threshold energy of the observed dissociative excitation has been derived from an analysis of the cross-section to be 9.3370.06 eV. Here the excitation cross-section in the 0.5 eV energy range above threshold energy Eth (see inset in Fig. 3) has been described by an exponential function f ðEÞ ¼ bðE Eth ÞP
ð2Þ
and for EoEth f ðEÞ ¼ 0: This function has been convoluted with a Gaussian profile representing energy spread of the experiment and then fitted to the measured emission intensity. In the fitting procedure apart from the threshold energy Eth ; the scaling factor b and exponent p ¼ 1:33 have been determined. Inset of Fig. 3 shows the fitted curve and the obtained energy dependence of the cross-section in the threshold region. Our value of Eth is in good agreement with the threshold dissociation energy of Okabe (1972), 9.27370.014 eV but does not support the result of Tokue et al. (1994). The threshold energy can be compared with the dissociation energy D0 for the CS(A1P)+S(3P) limit. This energy is calculated from D0 ¼ D00 þ Eð0; 0Þ; where D00 is the CS–S dissociation energy from the lowest * 1 Sþ state of carbon vibrational level of the ground X g disulfide and Eð0; 0Þ is the CS excitation energy from 1 + v00 ¼ 0 of the ground X S state into the v0 ¼ 0 level of A1P state. The estimated D0 is equal to 9.2970.02 eV
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Cross section
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9.33 ± 0.06 eV
0 8.8
9.0
9.2
9.4
9.6
9.8
Electron energy (eV)
0 10
15
20
25
30
Electron energy (eV) Fig. 3. Cross-section for dissociative excitation of carbon disulfide. In the inset, fit to the experimental data obtained in the threshold region is shown by the dashed line and the analytical excitation cross-section is shown by a full line.
CS 2 10
S(3P) + CS (A1Π)
E (eV)
E(0,0)
5
S(3P) + CS (X1Σ+)
D 00 +
0
X1 ∑ g v=0
r (S - CS) Fig. 4. Potential energy curves (qualitative) of the ground * 1 Sþ and dissociative states of carbon disulfide with respect to X g the S–CS internuclear distance.
taking D00 ¼ 4:4870:02 eV (Coppens et al., 1979) and Eð0; 0Þ ¼ 4:810 eV (Huber and Herzberg, 1979). Our threshold energy of the dissociative excitation coincides within the experimental uncertainty with the above estimation. The concurrence of the measured threshold and estimated dissociation energies imply that the potential energy curve of the excited dissociative state of carbon disulfide crosses the Franck-Condon region of the S–CS
internuclear distance at about 9.3 eV. In high-resolution electron energy loss spectra (Hubin-Franskin et al., 1983; Wilden and Comer, 1980) and also in the threshold excitation spectrum of Zubek and King (1995) a peak with an origin at 9.3 and 0.5 eV wide was detected which most likely corresponds to the dissociative state detected in the present work. Combining these results with the present measurements a qualitative potential energy curve of the excited state has been constructed and it is shown in Fig. 4. In conclusion, we have studied dissociative excitation of carbon disulfide which produces CS(A1P) excited fragments in the near-threshold electron incident energy region. The excitation threshold energy has been measured, with an accuracy higher than previous, to be 9.3370.06 eV. Obtained emission spectra indicate that the vibrational population of the CS radical is inverted for v ¼ 0 and 1 vibrational levels.
Acknowledgements This work has been supported by the Polish State Committee for Scientific Research under DZ KBN 3570/ R01/R03.
References Ajello, J.M., Srivastava, S.K., 1981. UV studies of electron impact excitation of CS2. J. Chem. Phys. 75, 4454–4463.
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