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Dissociative electron attachment in chloroalkanes and the correlation with vertical attachment energies
23 January 1998
Chemical Physics Letters 282 Ž1998. 398–402
Dissociative electron attachment in chloroalkanes and the correlation with vertical attachment energies K. Aflatooni, G.A. Gallup, P.D. Burrow Department of Physics and Astronomy, UniÕersity of Nebraska at Lincoln, Lincoln, NE 68588-0111, USA Received 16 June 1997; in final form 1 October 1997
Abstract Measurements of dissociative attachment ŽDA. cross sections and temporary negative ion energies in a series of dichloroalkanes show an exponential decrease of the peak DA cross section with increasing vertical attachment energy. Including similar data for monochloroalkanes, the exponential behavior spans five orders of magnitude in the peak cross section. q 1998 Elsevier Science B.V.
The peak cross section for a dissociative attachment ŽDA. reaction e q AB ™ ABy) ™ A q By is well-known w1x to be a strongly decreasing function of the energy of the temporary negative ion intermediary ABy) . This trend is illustrated more fully by Christophorou et al. w2x in a collection of data from a large number of compounds and peak energies from 0 to 15 eV. Considering the variety of temporary negative ion configurations, e.g. both ‘shape’ and core-excited resonances are represented, and the wide range of lifetimes as well as the sensitivity of DA to other molecular properties, it would indeed be surprising if such a broadly inclusive plot was quantitatiÕely predictive. In the present Letter, we explore these relationships in a much more restricted range of compounds, namely, a series of monochloro- and dichloroalkanes and two trichloroalkanes. In these compounds, the low-lying empty orbitals have s ) symmetry, in a local sense, and are associated with the C–Cl bonds.
In previous work, Pearl and Burrow w3x observed that to good approximation the peak DA cross sections in monochloroalkanes decline exponentially with increasing vertical attachment energy ŽVAE., that is, with the energy to form the temporary negative ion state. This dependence was not evident when the peak cross section was plotted against the energy at which the peak in the DA cross section occurs. In molecules with repulsive temporary anion potential surfaces and relatively short lifetimes, the DA peak may be shifted well below the actual VAE at which the resonance is formed because of the small survival factor w4,5x. In the monochloroalkanes, the VAEs that were encountered vary over a relatively narrow energy range. Extending our measurements to the dichloroalkanes in the present work, we find that the same exponential dependence continues to still lower VAEs, with only one significant exception observed thus far. The measured DA peak cross sections now span a range of 10 3, and with somewhat weaker evidence at higher VAEs, a range of about 10 5.
0009-2614r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 0 0 9 - 2 6 1 4 Ž 9 7 . 0 1 1 4 1 - X
K. Aflatooni et al.r Chemical Physics Letters 282 (1998) 398–402
Absolute total DA cross sections were measured using the apparatus and methods described in Ref. w3x. Vertical attachment energies were determined from resonance structure in the total scattering cross sections using electron transmission spectroscopy w6x ŽETS. in a separate dedicated apparatus. Energy dependences of the DA cross sections and a detailed discussion of the individual compounds along with model calculations will be presented elsewhere w7x. In Fig. 1 we show a semilog plot of the peak DA cross sections as a function of the lowest VAE in each compound. The solid circles indicate the previous results w3x in a series of primary, secondary and tertiary monochloroalkanes. The open circles show the present results in a series of dichloroalkanes, including linear alkanes with the chlorine atoms on opposite ends as well as compounds in which the chlorines are geminally and vicinally positioned. The open triangles show two trichloro compounds: 1,2,3-trichloropropane and 1,1,2-trichloroethane Ž1.. The straight line shows the best fit to the data lying between VAEs of 1.3 and 2.4 eV and yields the expression
s DA s 1.86 = 10yŽ1 3q2.55 VAE . cm2 for the peak DA cross section, where VAE is in eV.
399
For VAEs higher than 2.4 eV, we cannot as confidently verify the continued exponential decrease because of the paucity of related compounds we have studied with such high attachment energies. Compound Ž2., syn-7-chloro-2-norbornene, shown in the inset of Fig. 1, is an alkene rather than an alkane. However, the chlorine atom is located in the nodal plane bisecting the carbon atoms in the C s C double bond, and the C–Cl s ) orbital is thus forbidden by symmetry to couple with the p ) orbital w8x. ETS measurements reported by Pearl et al. w8x locate the s ) resonance at 2.75 eV. Only a small amount of this compound was available and the DA cross section could not be determined with confidence. Nevertheless an estimated peak cross section of 2 = 10y2 0 cm2 was reported by these authors which falls, fortuitously, on the line. Points Ž3. and Ž4. in Fig. 1 represent CH 3 Cl, which has such a small DA cross section that it is uncertain whether impurities play a role in the apparent signal w9x. An experimental estimate w9x of 2 = 10y2 1 cm2 , likely to be an upper bound, is plotted Ž3. at the ETS value of 3.45 eV w10x for the VAE. A theoretical estimate is available from the calculations of Fabrikant w11x that are in part parametrized with resonance properties taken from vibrational excita-
Fig. 1. Peak cross sections for DA as a function of the VAE for a series of mono-, di- and trichloroalkanes. The straight line is the best fit to the data lying between 1.3 and 2.4 eV. The line marked pl2 indicates the theoretical upper bound.
400
K. Aflatooni et al.r Chemical Physics Letters 282 (1998) 398–402
tion measurements w12x. The DA cross section computed for a temperature of 300 K is shown as the solid circle marked Ž4.. These two estimates bracket the best-fit line. The strong correlation observed in Fig. 1 is not evident if the peak cross sections are plotted against the energies at which the peaks in the DA cross sections occur rather than the VAEs. This latter comparison is illustrated in Fig. 2. The results emphasize that DA peak energies should not in general be identified with VAEs as done in most earlier DA work. Table 1 summarizes, for the di- and trichloroalkanes, the peak DA cross sections, the energies at which the peaks occur, the VAEs and the widths of the resonance features measured in ETS. Corresponding data for the monochloro compounds are presented in Ref. w3x. In Fig. 1, the peak DA cross sections are evaluated at their peak energies, Ep , of course, but are plotted at the corresponding VAEs which lie higher in energy. This gives rise to a subtle distortion of the data that should be noted. Theoretical expressions for the DA cross section contain in the capture cross section a factor of 1rE arising from the square of the electron wavelength. To put our data on a more equal footing, we can remove this weak energy
dependence and scale each DA cross section by multiplying it by Ep , the energy of the associated maximum in the cross section. The results are shown in Fig. 3. The good correlation is not noticeably altered. As noted previously for the monochloroalkanes w3x, the exponential decline in peak cross sections with VAE can be rationalized from the simplest possible interpretation of the theoretical expression w4,5x for s DA , namely, s DA s scap exp wytsep Gr" x, where scap is the electron capture cross section, tsep is the separation time required for the temporary anion to reach the crossing between the anion and neutral potential curves, and "rG is the average lifetime of the temporary anion. We observe experimentally that the resonance widths, G , increase in an approximately linear fashion with VAE. By assuming that the capture cross sections and the separation times along the anion surface are the same for all the compounds, an exponential decrease of s DA with VAE is obtained. We conclude that for VAEs above 1.3 eV in this set of compounds, variations in the survival factor dominate the DA cross sections. Returning to Fig. 1, extension of the best-fit line toward lower VAEs encounters pl2 , the theoretical maximum cross section for an inelastic process pro-
Fig. 2. Peak cross sections for DA as a function of the energy at which the peak in the cross section occurs.
K. Aflatooni et al.r Chemical Physics Letters 282 (1998) 398–402
401
Table 1 Peak DA cross sections and peak energies, and VAEs and resonance widths Compound
Peak sDA Ž10y18 cm2 .
DA peak energy ŽeV.
VAE ŽETS. ŽeV.
ETS FWHM a ŽeV.
dichloromethane 1,3-dichloropropane 1,4-dichlorobutane 1,5-dichloropentane 1,6-dichlorohexane 1,8-dichlorooctane 1,1-dichloroethane 1,1-dichloropropane 1,2-dichloropropane 2,2-dichloropropane 1,2-dichloro-2-methylpropane 1,3-dichlorobutane 2,3-dichlorobutane trans-1,2-dichlorocyclohexane 1,2,3-trichloropropane 1,1,2-trichloroethane
5.17 1.79 1.49 0.75 1.14 0.57 39.4 21.1 15.2 66.8 59.8 12.0 33.4 35.8 89.0 190
0.43 1.14 1.09 1.17 1.23 1.25 0.96 0.90 0.76 1.16 0.87 1.07 0.89 0.94 0.30 0.36
1.01 1.91 2.07 2.04 2.01 2.18 1.36 1.39 1.64 1.41 1.40 1.79 1.56 1.45 1.40 0.94
0.92 1.73 1.73 1.85 1.83 1.67 0.68 0.70 1.23 0.74 1.01 1.37 1.26 1.01 1.51 0.88
a
Energy separation between the minimum and maximum in the derivative of the transmitted electron current.
ceding through a single spatially non-degenerate resonance, just above a VAE of 0.8 eV. A clear change in relative importance of the factors comprising the DA cross section must occur in this vicinity. For
VAEs below about 1.0 eV, the survival factor is likely to be much closer to unity, and characteristics of the capture cross section related to the slope and intercept of the temporary negative ion potential
Fig. 3. The product of peak energy and peak DA cross section as a function of the VAE.
402
K. Aflatooni et al.r Chemical Physics Letters 282 (1998) 398–402
curve with the neutral curve are of key importance. Unfortunately, the more heavily chlorinated alkanes with VAEs in this region, such as compound Ž1., 1,1,2-trichloroethane, produce overlapping DA peaks arising from more than one resonance, making an interpretation of the data less clear-cut. The DA cross section of compound Ž5., CH 2 Cl 2 , lies two orders of magnitude below the line, and at present we have no explanation for this anomalous behavior. This compound is distinguished by having the closest proximity of the two C–Cl s ) orbitals. However, we note that in other geminally substituted compounds such as 1,1-dichloroethane, the cross sections fall close to the line. At the most practical level, the correlation observed here permits the peak DA cross sections of relevant compounds to be estimated solely from their VAEs, which are straightforward to determine experimentally by ETS. Such resonance energies, in principle, can also be computed at various levels of approximation with a variety of theoretical techniques. It must be said, however, that such efforts for higher-lying s ) orbitals have not been notably successful. For example, no one, to our knowledge, has yet accounted for the substantial difference in VAEs of two of the most fundamental monochloro compounds, methyl- and ethylchloride. The results of the present work suggest that, by focussing on restricted sets of compounds and prop-
erly determining negative ion energies, other groups of molecules will be found within which a more quantitative relationship between structure and DA cross section may be determined. This work was supported by the National Science Foundation.
References w1x L.G. Christophorou, J.A.D. Stockdale, J. Chem. Phys. 48 Ž1968. 1956. w2x L.G. Christophorou, D.L. McCorkle, A.A. Christodoulides, in: Electron-Molecule Interactions and Their Applications. Vol. 1, Ed. L.G. Christophorou ŽAcademic Press, New York, 1984.. w3x D.M. Pearl, P.D. Burrow, J. Chem. Phys. 101 Ž1994. 2940. w4x J.N. Bardsley, A. Herzenberg, F. Mandl, Proc. Phys. Soc. 89 Ž1966. 321. w5x T.F. O’Malley, Phys. Rev. 150 Ž1966. 14. w6x L. Sanche, G.J. Schulz, Phys. Rev. A 5 Ž1972. 1672. w7x K. Aflatooni, G.A. Gallup, P.D. Burrow, manuscript in preparation. w8x D.M. Pearl, P.D. Burrow, J.J. Nash, H. Morrison, K.D. Jordan, J. Am. Chem. Soc. 115 Ž1993. 9876. w9x D.M. Pearl, P.D. Burrow, Chem. Phys. Lett. 206 Ž1993. 483. w10x P.D. Burrow, A. Modelli, N.S. Chiu, K.D. Jordan, J. Chem. Phys. 77 Ž1982. 2699. w11x D.M. Pearl, P.D. Burrow, I.I. Fabrikant, G.A. Gallup, J. Chem. Phys. 102 Ž1995. 2737. w12x X. Shi, V.K. Chan, G.A. Gallup, P.D. Burrow, J. Chem. Phys. 104 Ž1996. 1855.
Chemical Physics Letters 282 Ž1998. 398–402
Dissociative electron attachment in chloroalkanes and the correlation with vertical attachment energies K. Aflatooni, G.A. Gallup, P.D. Burrow Department of Physics and Astronomy, UniÕersity of Nebraska at Lincoln, Lincoln, NE 68588-0111, USA Received 16 June 1997; in final form 1 October 1997
Abstract Measurements of dissociative attachment ŽDA. cross sections and temporary negative ion energies in a series of dichloroalkanes show an exponential decrease of the peak DA cross section with increasing vertical attachment energy. Including similar data for monochloroalkanes, the exponential behavior spans five orders of magnitude in the peak cross section. q 1998 Elsevier Science B.V.
The peak cross section for a dissociative attachment ŽDA. reaction e q AB ™ ABy) ™ A q By is well-known w1x to be a strongly decreasing function of the energy of the temporary negative ion intermediary ABy) . This trend is illustrated more fully by Christophorou et al. w2x in a collection of data from a large number of compounds and peak energies from 0 to 15 eV. Considering the variety of temporary negative ion configurations, e.g. both ‘shape’ and core-excited resonances are represented, and the wide range of lifetimes as well as the sensitivity of DA to other molecular properties, it would indeed be surprising if such a broadly inclusive plot was quantitatiÕely predictive. In the present Letter, we explore these relationships in a much more restricted range of compounds, namely, a series of monochloro- and dichloroalkanes and two trichloroalkanes. In these compounds, the low-lying empty orbitals have s ) symmetry, in a local sense, and are associated with the C–Cl bonds.
In previous work, Pearl and Burrow w3x observed that to good approximation the peak DA cross sections in monochloroalkanes decline exponentially with increasing vertical attachment energy ŽVAE., that is, with the energy to form the temporary negative ion state. This dependence was not evident when the peak cross section was plotted against the energy at which the peak in the DA cross section occurs. In molecules with repulsive temporary anion potential surfaces and relatively short lifetimes, the DA peak may be shifted well below the actual VAE at which the resonance is formed because of the small survival factor w4,5x. In the monochloroalkanes, the VAEs that were encountered vary over a relatively narrow energy range. Extending our measurements to the dichloroalkanes in the present work, we find that the same exponential dependence continues to still lower VAEs, with only one significant exception observed thus far. The measured DA peak cross sections now span a range of 10 3, and with somewhat weaker evidence at higher VAEs, a range of about 10 5.
0009-2614r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 0 0 9 - 2 6 1 4 Ž 9 7 . 0 1 1 4 1 - X
K. Aflatooni et al.r Chemical Physics Letters 282 (1998) 398–402
Absolute total DA cross sections were measured using the apparatus and methods described in Ref. w3x. Vertical attachment energies were determined from resonance structure in the total scattering cross sections using electron transmission spectroscopy w6x ŽETS. in a separate dedicated apparatus. Energy dependences of the DA cross sections and a detailed discussion of the individual compounds along with model calculations will be presented elsewhere w7x. In Fig. 1 we show a semilog plot of the peak DA cross sections as a function of the lowest VAE in each compound. The solid circles indicate the previous results w3x in a series of primary, secondary and tertiary monochloroalkanes. The open circles show the present results in a series of dichloroalkanes, including linear alkanes with the chlorine atoms on opposite ends as well as compounds in which the chlorines are geminally and vicinally positioned. The open triangles show two trichloro compounds: 1,2,3-trichloropropane and 1,1,2-trichloroethane Ž1.. The straight line shows the best fit to the data lying between VAEs of 1.3 and 2.4 eV and yields the expression
s DA s 1.86 = 10yŽ1 3q2.55 VAE . cm2 for the peak DA cross section, where VAE is in eV.
399
For VAEs higher than 2.4 eV, we cannot as confidently verify the continued exponential decrease because of the paucity of related compounds we have studied with such high attachment energies. Compound Ž2., syn-7-chloro-2-norbornene, shown in the inset of Fig. 1, is an alkene rather than an alkane. However, the chlorine atom is located in the nodal plane bisecting the carbon atoms in the C s C double bond, and the C–Cl s ) orbital is thus forbidden by symmetry to couple with the p ) orbital w8x. ETS measurements reported by Pearl et al. w8x locate the s ) resonance at 2.75 eV. Only a small amount of this compound was available and the DA cross section could not be determined with confidence. Nevertheless an estimated peak cross section of 2 = 10y2 0 cm2 was reported by these authors which falls, fortuitously, on the line. Points Ž3. and Ž4. in Fig. 1 represent CH 3 Cl, which has such a small DA cross section that it is uncertain whether impurities play a role in the apparent signal w9x. An experimental estimate w9x of 2 = 10y2 1 cm2 , likely to be an upper bound, is plotted Ž3. at the ETS value of 3.45 eV w10x for the VAE. A theoretical estimate is available from the calculations of Fabrikant w11x that are in part parametrized with resonance properties taken from vibrational excita-
Fig. 1. Peak cross sections for DA as a function of the VAE for a series of mono-, di- and trichloroalkanes. The straight line is the best fit to the data lying between 1.3 and 2.4 eV. The line marked pl2 indicates the theoretical upper bound.
400
K. Aflatooni et al.r Chemical Physics Letters 282 (1998) 398–402
tion measurements w12x. The DA cross section computed for a temperature of 300 K is shown as the solid circle marked Ž4.. These two estimates bracket the best-fit line. The strong correlation observed in Fig. 1 is not evident if the peak cross sections are plotted against the energies at which the peaks in the DA cross sections occur rather than the VAEs. This latter comparison is illustrated in Fig. 2. The results emphasize that DA peak energies should not in general be identified with VAEs as done in most earlier DA work. Table 1 summarizes, for the di- and trichloroalkanes, the peak DA cross sections, the energies at which the peaks occur, the VAEs and the widths of the resonance features measured in ETS. Corresponding data for the monochloro compounds are presented in Ref. w3x. In Fig. 1, the peak DA cross sections are evaluated at their peak energies, Ep , of course, but are plotted at the corresponding VAEs which lie higher in energy. This gives rise to a subtle distortion of the data that should be noted. Theoretical expressions for the DA cross section contain in the capture cross section a factor of 1rE arising from the square of the electron wavelength. To put our data on a more equal footing, we can remove this weak energy
dependence and scale each DA cross section by multiplying it by Ep , the energy of the associated maximum in the cross section. The results are shown in Fig. 3. The good correlation is not noticeably altered. As noted previously for the monochloroalkanes w3x, the exponential decline in peak cross sections with VAE can be rationalized from the simplest possible interpretation of the theoretical expression w4,5x for s DA , namely, s DA s scap exp wytsep Gr" x, where scap is the electron capture cross section, tsep is the separation time required for the temporary anion to reach the crossing between the anion and neutral potential curves, and "rG is the average lifetime of the temporary anion. We observe experimentally that the resonance widths, G , increase in an approximately linear fashion with VAE. By assuming that the capture cross sections and the separation times along the anion surface are the same for all the compounds, an exponential decrease of s DA with VAE is obtained. We conclude that for VAEs above 1.3 eV in this set of compounds, variations in the survival factor dominate the DA cross sections. Returning to Fig. 1, extension of the best-fit line toward lower VAEs encounters pl2 , the theoretical maximum cross section for an inelastic process pro-
Fig. 2. Peak cross sections for DA as a function of the energy at which the peak in the cross section occurs.
K. Aflatooni et al.r Chemical Physics Letters 282 (1998) 398–402
401
Table 1 Peak DA cross sections and peak energies, and VAEs and resonance widths Compound
Peak sDA Ž10y18 cm2 .
DA peak energy ŽeV.
VAE ŽETS. ŽeV.
ETS FWHM a ŽeV.
dichloromethane 1,3-dichloropropane 1,4-dichlorobutane 1,5-dichloropentane 1,6-dichlorohexane 1,8-dichlorooctane 1,1-dichloroethane 1,1-dichloropropane 1,2-dichloropropane 2,2-dichloropropane 1,2-dichloro-2-methylpropane 1,3-dichlorobutane 2,3-dichlorobutane trans-1,2-dichlorocyclohexane 1,2,3-trichloropropane 1,1,2-trichloroethane
5.17 1.79 1.49 0.75 1.14 0.57 39.4 21.1 15.2 66.8 59.8 12.0 33.4 35.8 89.0 190
0.43 1.14 1.09 1.17 1.23 1.25 0.96 0.90 0.76 1.16 0.87 1.07 0.89 0.94 0.30 0.36
1.01 1.91 2.07 2.04 2.01 2.18 1.36 1.39 1.64 1.41 1.40 1.79 1.56 1.45 1.40 0.94
0.92 1.73 1.73 1.85 1.83 1.67 0.68 0.70 1.23 0.74 1.01 1.37 1.26 1.01 1.51 0.88
a
Energy separation between the minimum and maximum in the derivative of the transmitted electron current.
ceding through a single spatially non-degenerate resonance, just above a VAE of 0.8 eV. A clear change in relative importance of the factors comprising the DA cross section must occur in this vicinity. For
VAEs below about 1.0 eV, the survival factor is likely to be much closer to unity, and characteristics of the capture cross section related to the slope and intercept of the temporary negative ion potential
Fig. 3. The product of peak energy and peak DA cross section as a function of the VAE.
402
K. Aflatooni et al.r Chemical Physics Letters 282 (1998) 398–402
curve with the neutral curve are of key importance. Unfortunately, the more heavily chlorinated alkanes with VAEs in this region, such as compound Ž1., 1,1,2-trichloroethane, produce overlapping DA peaks arising from more than one resonance, making an interpretation of the data less clear-cut. The DA cross section of compound Ž5., CH 2 Cl 2 , lies two orders of magnitude below the line, and at present we have no explanation for this anomalous behavior. This compound is distinguished by having the closest proximity of the two C–Cl s ) orbitals. However, we note that in other geminally substituted compounds such as 1,1-dichloroethane, the cross sections fall close to the line. At the most practical level, the correlation observed here permits the peak DA cross sections of relevant compounds to be estimated solely from their VAEs, which are straightforward to determine experimentally by ETS. Such resonance energies, in principle, can also be computed at various levels of approximation with a variety of theoretical techniques. It must be said, however, that such efforts for higher-lying s ) orbitals have not been notably successful. For example, no one, to our knowledge, has yet accounted for the substantial difference in VAEs of two of the most fundamental monochloro compounds, methyl- and ethylchloride. The results of the present work suggest that, by focussing on restricted sets of compounds and prop-
erly determining negative ion energies, other groups of molecules will be found within which a more quantitative relationship between structure and DA cross section may be determined. This work was supported by the National Science Foundation.
References w1x L.G. Christophorou, J.A.D. Stockdale, J. Chem. Phys. 48 Ž1968. 1956. w2x L.G. Christophorou, D.L. McCorkle, A.A. Christodoulides, in: Electron-Molecule Interactions and Their Applications. Vol. 1, Ed. L.G. Christophorou ŽAcademic Press, New York, 1984.. w3x D.M. Pearl, P.D. Burrow, J. Chem. Phys. 101 Ž1994. 2940. w4x J.N. Bardsley, A. Herzenberg, F. Mandl, Proc. Phys. Soc. 89 Ž1966. 321. w5x T.F. O’Malley, Phys. Rev. 150 Ž1966. 14. w6x L. Sanche, G.J. Schulz, Phys. Rev. A 5 Ž1972. 1672. w7x K. Aflatooni, G.A. Gallup, P.D. Burrow, manuscript in preparation. w8x D.M. Pearl, P.D. Burrow, J.J. Nash, H. Morrison, K.D. Jordan, J. Am. Chem. Soc. 115 Ž1993. 9876. w9x D.M. Pearl, P.D. Burrow, Chem. Phys. Lett. 206 Ž1993. 483. w10x P.D. Burrow, A. Modelli, N.S. Chiu, K.D. Jordan, J. Chem. Phys. 77 Ž1982. 2699. w11x D.M. Pearl, P.D. Burrow, I.I. Fabrikant, G.A. Gallup, J. Chem. Phys. 102 Ž1995. 2737. w12x X. Shi, V.K. Chan, G.A. Gallup, P.D. Burrow, J. Chem. Phys. 104 Ž1996. 1855.