Electroionization study of ethane: structures in the ionization and appearance curves1

Transcription

1 Electroionization study of ethane: structures in the ionization and appearance curves1 PIERRE PLESSIS AND PAUL MARMET Herzberg Institute of Astrophysics, National Research Courzcil of Canada, Ottalva, Ont., Canada KIA OR6 Received January 23, 1987 PIERRE PLESSIS and PAUL MARMET. Can. J. Chem. 65, 2004 (1987). Ionization efficiency curves of very high signal-to-noise ratio are presented for ethane and most fragment ions containing at least one carbon atom. These curves, which are produced by monoenergetic electron impact, extend up to 10 ev above the initial onsets. Novel structures are observed in most curves. Certain features appear at the same energy in several of the curves indicating that they belong to a common progenitor, either C2H6 * or C2&*, and not directly to the detected ions. These sets of structure are attributed to negative ion states (Feshbach resonances) and/or to Rydberg autoionizing states. PIERRE PLESS~S et PAUL MARMET. Can. J. Chem. 65, 2004 (1987). Des courbes de section efficace d'ionisation de trks grand rapport signal sur bruit sont prcsentces pour 1'Cthane et ses fragments ioniques composcs d'au moins un atome de carbone. Ces courbes sont produites par impact d'clectrons monocnergctiques et s'ktendent jusqu'i 10 ev au-dessus des seuils initiaux. De nouvelles structures sont observces dans la plupart des courbes. Certaines structures apparaissent aux mcmes Cnergies dans plusieurs courbes, ce qui indique qu'elles n'appartiennent pas directement aux ions dctectcs mais A une particule qui est a l'origine de tous ces ions, soit le C2H; * ou le C2Hg. Ces groupes de structures sont attribucs i des Ctats d'ions ncgatifs (rksonances Feshbach) et/ou i des Ctats de Rydberg autoionisants. 1. Introduction where S is a parameter controlling the amount of low frequency component that is eliminated and N is the number of channels used. For Ethane and its fragment ions have been studied the special case of S = 1 and using the transmission factor -TrttSS, in the and appearance the result is identical to the second derivative. It has been shown threshold regions. Our data for these regions have been qualitatively that for small values of S (S S 100) the result is similar to presented in a recent paper (1). Much less work has been done the second derivative but contains a larger amount of low frequency at energies above the immediate onset regions. Ionization components (10). The vertically inverted filtered curves (-T,,,,s) are efficiency curves at higher energies are reported by Suzuki presented in this paper in order to have direct resemblance with the and Maeda (2) using a monoenergetic electron beam, Selim second derivative. (3) using the (pseudo-monoenergetic) electron impact energy distribution difference technique, and ~erkowitz (4) using photoionization mass spectrometry. In this paper very high signal-to-noise ratio electroionization efficiency (EIE) data extending up to 10 ev above onset are presented for ethane and all fragment ions containing at least one carbon atom, except CH;. Numerous low intensity features are observed for the first time in most curves. Assignments are proposed for many of these structures. 2. Apparatus and method More complete descriptions of the apparatus have been given elsewhere (5, 6). It consists of two 127 electrostatic selectors in series producing a monoenergetic electron beam. In this work the full width at half maximum (FWHM) resolution of the beam is approximately 60 mev for a current of 3 X A. The molecular beam is bombarded by the electrons; the ions produced are mass analysed by a large quadrupole mass filter and the transmitted ions are detected by a dynode type electron multiplier. Data acquisition is computer controlled. An individual EIE curve takes about 10 s to obtain and the final curves consist of thousands of individual curves summed over periods of about 130 h. The energy scale of the curves is calibrated typically every 24 h using the clear threshold structure of argon ( ev). The energy drift per 24 h period was usually no greater than 20 mev and this was taken into account in adding the data. Most structures in EIE curves appear as narrow (high frequency) and relatively weak perturbations on an intense, broad (low frequency), and smoothly varying ionization continuum. Filtering out the intense low frequency components allows the high frequency features to be seen much more readily. This is accomplished by a specialized Fourier filtering technique (7-9) whose transmission factor for the mth frequency component is given by ' NRCC No Results and discussion The raw data EIE curves of ethane and of the fragments studied in this work are presented in Figs. 1 and 2. The data points, originally taken at an interval of 20 mev per channel, are added 5 by 5 to give 100 mev per channel in order to improve the signal-to-noise ratio and allow weaker features to be detected. Very little structure of detectable intensity narrower than 300meV could be seen in the curves, so virtually no information was lost by increasing the energy interval. The intensity of the curves of Figs. 1 and 2 increases uniformly with energy and, apart from the initial onset regions that have been analysed previously (I), they show no apparent narrow distinct features. At the vertical scale shown, the general shape of the curves of Fig. 2 differs from that of the Fig. 1 curves in that the intensity of the former increases significantly only several ev above onset. The CH; curve has an intermediary form to these two groups of curves. From Fig. 2 it is clear that the probability for producing these fragment ions and their respective reaction products with minimal internal and kinetic energy is very small. The relative peak height intensities of the ethane ions are given in Table 1 where it is seen that the most abundant ion at 35 ev is the C2H; fragment. Once the low frequency continuum of the Figs. 1 and 2 curves is removed, by using the aforementioned high pass filter, numerous structures can be seen. The filtered curves are presented in Figs. 3 to 5. The data offig. 3 are shown at 20 mev per channel. The energies of the indicated structures are given in Table 2. The intensities of these structures are small compared with the ionization continuum, and therefore are not apparent on the original curves. For example, structure E at ev of the C2HL curve and structure Hz at 18.8 ev of the C2Hi curve have amplitudes of only 0.06% and 0.01% of their respective ionization continuum. Most of the features observed in the

2 PLESSIS AN1 1 MARMET 2005 TABLE 1. Relative peak height intensities of the ethane ions for 35.0 ev electrons Relative Ion intensity Na I I I I I I I Electron energy ( eu> FIG. 1. EIE curves of ethane and of several fragment ions. The zero of the curves have been displaced for clearer presentation. The ion count in the last channel of each curve is given in Table 1 (100 mev per channel). I I I I I I L Electron energy (eu> FIG. 2. EIE curves of the weaker intensity ethane fragments. As in Fig. 1. filtered curves are reported here for the first time. Because of the complexity of the transitions involved and the lack of pertinent experimental and theoretical work on ethane, it is presently not possible to give unequivocal assignments to these features. C2H L X lo9 C2H: x 10" C2H; X 10" C2H: X lo9 C2H: X 10' CzH x lo7 c : x lo5 CH: X lo8 CH: x lo6 CH' X lo6 C x lo5 "N is the number of counts in the last channel of the curves of Figs. 1 and 2. Electron energy ( eu> FIG. 3. C2H2 EIE curve high pass filtered using S = 7 (curve a) and S = 30 (curve b). Energy interval per channel = 20 mev. However, plausible general interpretations for many of the structures are proposed below. These are summarized in Table 3. A remarkable aspect of the Figs. 4 and 5 curves is that a number of features having similar profiles appear at the same energy in several of the curves, as listed in Table 3. It is evident that these structures cannot be attributed to states of the detected ions since such series of coincidences are essentially impossible given the great differences in the detected ions. Each set of features must have a common origin. Previous work on methane (1 l), acetylene (12), and ethylene (13), in which similar sets of structures are observed in the ionization and appearance curves, indicate that negative ion states (Feshbach resonances) and autoionizing states are the major processes responsible for producing these sets of structures as well as most of the features observed in EIE data. In the present case C2Hd * is the source species resulting from the initial collision of an electron on C2H6. During the interaction of the electron with the target molecule, quantum transitions take place in which the number of charges is conserved. Thus, at this stage, only negative ion states are involved which correspond to continua, Feshbach resonances or shape resonances. when transitions occur new states of the

3 CAN. 1. CHEM. VOL. 65, 1987 TABLE 2. Energies (in ev) of the principal features of the ethane and fragment ion curves Structure C2H; C2Hi C2H; C2H: CH: CH: (subscript-+)" CZHt (1) (2) (3) (4) (5) (6) "Subscripts are adjoined to the letters as in Figs. 3 to 5 Electron energy (eu) FIG. 4. C2Hi (curves a and b), C2H; (curve c), and C?H; (curves dand e) EIEcurves filtered using S = 2,10, 10,7, and 1, respectively. negative ion are formed which can either (directly or indirectly) autoionize, dissociate into fragments, or emit photons. The variations in cross section as a function of energy of the electrons interacting with the ethane molecules to form different states of C2H6 * will be reflected in the ionization and various fragment ion cross sections, as seen in the curves of Figs. 4 and 5. It follows that when a structure is common to several ethane ions, it is caused by a configuration of the parent negative ion. Furthermore, the C2H6 * cross section can be perturbed by the probability of forming excited neutral states (C2Hg + e-), consequently the perturbations due to these neutral states will also manifest themselves in the ionization and ionic fragmentation cross sections. Electron energy ( eu) FIG. 5. C2H:, C2Hz, CH;, and CH; EIE curves filtered using S = 18, 25, 30, and 100, respectively. Structures T, A, and B of the C2Hg curve (Fig. 3) have been analysed in detail and at higher resolution in the previous paper on ethane (1). The corresponding interpretations are given in Table 3. All other structures listed in Table 3 are considered to be caused by members of negative ion states series, 2~ nl n'l', or of Rydberg autoionizing states series, 2~ nl, where X represents a particular C2H: state, or to a mixture of these which converge to the different electronic states of the ethane ion. The ethane ground state configuration is generally accepted as being (1 a~,)2(la2u)2(2a,,)2(2a2u)2(leu)4(3al,)'(l ej4, 'Al, (4). The ionization energies for the ethane valence electrons, obtained by photoelectron spectroscopy (14), are

4 PLESSIS AND MARMET TABLE 3. Interpretation of some structures Structure Energy (ev)" Interpretation Ionization energy (1 e,)-' Due to competition with C2H; + Hz production 'A!, nl n'l' and/or 'A~, nl 'For the sets of structures the approximate mean energy is given nl n'l' and/or A?, nl Onset of 'Al, nl n'l' and/or 'AI, nl 'Al, nl n'l' and/or 'Alg nl TABLE 4. Ionization energies for single electron ing the start of a new set of series which must converge to the ejection of ethane (from Bieri and Asbrink (13)) 2~1, state of the ethane ion. The C2H+, Cl, CH+, and C+ filtered curves (not shown here) Electron ejected Adiabatic (ev) Vertical (ev) vary gradually with energy and show few distinct features. Little significant information can be drawn from them. "This state has two maxima due to Jahn-Teller distortion. reproduced in Table 4. It is assumed in the interpretations given in Table 3, that a series limit can occur in the region starting from the adiabatic ionization energy to just above the vertical ionization energy. The basic considerations taken into account in assigning the sets of structures listed in Table 3 were the energies of the expected series limits (ethane ion states) and the general shape of the filtered curves. Evidently, a member of a series must appear at an energy below the series limit. From the work on methane (1 l), members of a series may be found down to about 4eV below the ionization limit. The interpretation of structures D, E, and C2 is somewhat ambiguous as-these could belong to series converging to either the 2~ or the 2E, state of C2H:. However, since the general shape and intensities of the C2H: and C2Hz filtered curves above and below 13.5 ev (2~1, vertical ionization energy) are quite different, this suggests that the structures below 13.5 ev converge to the 2Al, state and not to the 2 ~, state, near 15eV. Structures between 13.5 and 15.8 ev can be confidently assigned to series converging to the 2 ~ state u as this is the only energetically plausible limit in this region. Similarly, structures between 16.0 and 19.6 ev are assigned to series which have the 2A2u state as their limit. It is seen in the C2H:, C2H:, C2Hl, and CH: filtered curves that starting from 19.6 ev, the slope of the curves changes and the amplitude of the structures above this energy increases, suggest- 4. Other results The electron impact works of Suzuki and Maeda (2) and Selim (3) also report structures in the ethane EIE curve and in some of the more intense fragment ion curves. Both detect features in their direct EIE curves. This indicates that the relative intensities of their structures are much larger than those observed in this study. Also, the energies of these structures are not consistent with those reported here. The direct EIE curves of Figs. 1 and 2 show no apparent structures above threshold and it is only when the intense continuum is removed that the weak intensity features are seen. Given that the signal-to-noise ratio of their curves is considerably less than that of our data, it is clear to us that their results are incompatible with ours. The photoionization mass spectrometric study of ethane by Berkowitz (4) yields little pertinent information in the energy region considered in this work. 5. Conclusion Numerous clear structures are observed for the first time in the EIE curves of ethane and ethane fragment ions. Several of the features occur at the same energy in these curves and are certainly caused by the common progenitor C2H; *. The structures of the Figs. 4 and 5 curves are tentatively assigned to series of negative ion states and/or to autoionizing Rydberg states. Our present knowledge of the C2H6 + e- system is limited. However, we trust that these new results will be helpful in stimulating further experimental and theoretical work on this system. 6. Acknowledgements The authors wish to thank Drs. M. Proulx and R. Dutil for their contribution to this work. Measurements were carried out at the Laboratoire de physique atomique et molcculaire at Lava1 University, QuCbec, and supported by grants from the Natural Sciences and Engineering Research Council of Canada.

5 2008 CAN. I. CHEM. VOL. 65, P. PLESSIS and P. MARMET. Can. J. Chem. In press. 2. I. H. SUZUKI and K. MAEDA. Int. J. Mass Spectrom. Ion Phys. 24, 147 (1977). 3. E. T. M. SELIM. Indian J. Pure Appl. Phys. 14, 547 (1976). 4. J. BERKOWITZ. Photoabsorption, photoionization and photoelectron spectroscopy. Academic Press, New York p P. PLESSIS, P. MARMET, and R. DUTIL. J. Phys. B.:At. Mol. Phys. 16, 1283 (1983). 6. E. BOLDUC, J. J. QUBMBNER, and P. MARMET. J. Chem. Phys. 57, 1957 (1972). 7. R. CARBONNEAU, E. BOLDUC, and P. MARMET. Can. J. Phys. 51, 505 (1973). 8. P. MARCHAND and P. VEILLETTE. Can. J. Phys. 54,1309 (1976). 9. H. H. ARSENAULT and P. MARMET. Rev. Sci. Instrum. 48, 512 (1977). 10. M. PROULX and P. MARMET. Int. J. Mass Spectrom. Ion Phys. 50, 129 (1983). 11. P. MARMET and L. BINETTE. J. Phys B:At. Mol. Phys. 11, 3707 (1978). 12. P. PLESSIS and P. MARMET. Int. J. Mass Spectrom. Ion Proc. 70, 23 (1986). 13. P. PLESSIS and P. MARMET. Can. J. Phys. In press. 14. G. BIERI and L. ASBRINK. J. Elec. Spectrosc. Relat. Phenom. 20, 149 (1980).