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The cross section for electron impact excitation of the E 3Σg+ state of N2 in the near-threshold region
Volume 146, number 6
CHEMICAL PHYSICS LETTERS
20 May 1988
THE CROSS SECTION FOR ELECTRON IMPAm EXCITATION OF THE E %; STATE OF N2 IN THE NEAR-THRESHOLD REGION
Mariusz ZUBEK Department of Physics, Technical University,SC952 Gdatisk, Poland Received 3 March 1988
The total excitation cross section for the E 2: state of N2 has been determined in the electronl’energy region from threshold to 15 eV and normalized to give absolute values. A detection technique has been used which can differentiate between different metastablestatesof N2. The cross section shows pronounced structure in the near-threshold region due to the formation of resonance states.
1. Introduction
Total cross sections for electron impact excitation of metastable states of atoms and molecules are usually measured in experiments in which the number of excited species at a given electron energy is determined through direct detection. In the nearthreshold region, the direct detection of metastable species allows us to obtain accurate excitation functions, as it avoids the experimentally difficult transmission of very-low-energy ( < 1 eV) electrons. This method of measurement was previously used for the metastable states of nitrogen by Borst [ 1 ] and more recently for the metastable states of noble gases by Mason and Newell [ 21. In the case of nitrogen, only a few metastable states are excited at a fixed electron energy and the time-of-flight technique was employed [ 11 to separate them. When the metastable states of an atom have very long radiative lifetimes ( > 1 ms) or the lifetimes of different states are similar, a laser-induced fluorescence technique can be used to observe the states independently as was done, for example, for the 2~~3s J=O, 2 metastable states of Ne by Phillips et al. [ 31. In the present work the total excitation cross section of the E ‘Cl state, the highest-lying metastable state of N2, has been measured using a new detection technique developed recently by Zubek [4], which is able to differentiate between molecules in different metastable states. This technique introduced en496
ergy analyses of electrons ejected in the de-excitation process of metastable molecules and it was used to determine the cross section of the E ‘Ep’state in the electron energy range from the excitation threshold at 11.875 eV to 15 eV. The metastable excitation of N2 was studied in high-resolution experiments by Newman et al. [ 5 ] and previously by Brunt et al. [ 6 1, but neither of these works could distinguish between molecules in different metastable states or determine the total excitation cross section for one of the states.
2. Experimental details In the experiment, an electron beam of low-energy spread is crossed at right angles with a molecular beam effusing from a platinum-iridium tube of 0.5 mm bore. The electron beam is produced by an electron monochromator consisting of a 127” electrostatic cylindrical analyzer with a mean radius of 20 mm. The electrons leaving the analyzer are then accelerated and focused by a triple-aperture electrostatic lens. The interaction region is surrounded by a cylindrical molybdenum mesh of 30 mm diameter to prevent any electric field penetration into the region. The incident electron beam is collected by a Faraday cup placed behind the cylindrical mesh. The electron beam energy is adjustable and has an energy spread (fwhm) of 70 meV and a current of typically
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Volume 146, number 6
CHEMICAL PHYSICS LETTERS
1 nA. The detector for metastable states is placed 40 mm above the interaction region. In the present measurements a detector with an adjustable threshold is used of a design similar to that employed by Zubek and King [ 71 in recent studies of metastable excitation in mercury. The molecules eject electrons by an Auger de-excitation process at the detecting surface, which are then detected by a channel electron multiplier. The maximum kinetic energy of the released electrons depends on the excitation energy of the metastable state. In the detector, the kinetic energy of the secondary electrons is analysed by a retarding electrostatic field formed between the detection surface and the retarding grid placed 2 mm below the surface. By applying an appropriate retarding potential only those electrons with an initial kinetic energy greater than a certain value are transmitted and detected. More details on the metastable detector are given in ref. [4]. The detecting surface was a conducting lead silicate glass in the form of a 35 mm diameter disc. This material has a high work function of approximately 7 eV and is appropriate for studies of metastable states with high excitation energies as it reduces the detection efficiency for lower-lying states compared to clean metal surfaces (for example tantalum or molybdenum). The detector has a large detection area (9.6 cm2), so most of the metastables formed in the interaction region can reach the detector giving a yield proportional to the total excitation cross section. The center of the detecting surface is set at an angle of about 5’ with respect to the tube producing the molecular beam to account for the recoil of molecules due to momentum transfer during electron collision. For nitrogen molecules excited to the E ‘Cl state, it is estimated that the recoil angle will change from 5.5” for excitation at threshold to 9” for backward scattered electrons at 15 eV. The detector intercepts the metastable molecules within a cone of half-angle 23.5 ’ and there is no change in the measured yield as a result of the recoil effect. The incident beam current varies with the electron energy due to the change in transmission of the accelerating lens. The current is monitored during data accumulation and the spectra are corrected for changes of the incident current. The electron beam is focused at a point positioned beyond the interaction region rather than at the region itself to min-
20 May 1988
imize the effect of the change of beam size with increasing electron energy. The spectrometer operation is controlled by a microcomputer working in the multiscaling mode. It produces a ramp voltage which is used to vary the incident energy and accumulates synchronously the output pulses from the metastable detector while the metastable yield is measured as a function of electron energy. It also controls the voltage on the retarding grid of the detector working in the retarding mode. The electron energy scale is calibrated against the position of the second resonance peak of band “a” (see below) at 12.782 eV [ 51 and is accurate to 30 meV. The metastable detector employed in the present work is also sensitive to photons of energies greater than the work function of the detecting surface. In a separate experiment in which the detector was placed perpendicular to the incident electron and molecular beams, the photon contribution to the metastable spectra was estimated to be less than 0.5Oh.
3. Results The metastable excitation spectra obtained for Nz in the energy range from 7 to 15 eV are shown in fig. 1. Spectrum (a) corresponds to the detection of all metastable states of NZ excited in this energy region and thus represents the total metastable excitation function. There are three metastable states A ‘E:, a ‘IIp and E ‘Z: with excitation thresholds v=O at 6.169, 8.549 and 11.875 eV, respectively, that directly contribute to the measured excitation function. The overall shape of the spectrum is, however, mainly determined by the excitation of the B311, state with v=O threshold at 7.353 eV and the C ‘II,, with v=O at 11.032 eV. The onset of the spectrum corresponds to the excitation energies of the vibrational levels of the B ‘I& state, while the broad shoulder at about 11 eV is related to the maximum in the excitation cross section of this state. The increase in the metastable yield above 11 eV is a result of the C ‘II” contribution. The B ‘II, state is observed via cascading transitions to the metastable A 3Cz state while the C 311U state via a two-step transition C ?I,+B ‘IIp followed by B ‘&-+A ‘E:. In 497
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CHEMICAL PHYSICS LETTERS
Fig. 1. The metastable excitation function for N2 in the incident energy range from 7 to 15 eV. (a) Total excitation function for all excited metastable and cascading states; (b) total excitation function for a ‘IT,,and E ‘Z: metastable states obtained from an energy analysis of the ejected electrons. The crosses in spectrum (b) show the cross section of ref. [8] fitted to the excitation function of the a ‘II, state. The energy width per channel for both spectra is 19.56 meV.
these cascades vibrational levels of the A ‘Zz state from v=O to 5 and levels up to v=4 (80% transitions) are populated from the B 311eand C 311U states, respectively. In the direct excitation of the A3C: state the v= 8 level attains the highest population as deduced from Franck-Condon factors for the transition from the ground state. The E ‘Z: metastable state is strongly excited above threshold with the formation of a few resonance states. This spectrum is similar to the results of Brunt et al. [ 6 1, although obtained using a detection surface with a lower work function, as can be seen from the comparison of the structures present in both spectra. Spectrum (b) of fig. 1 involves an energy analysis of the ejected electrons and was taken with a potential difference of -0.45 V between the detecting surface and the retarding grid (as compared to +0.95 V for spectrum (a)), which removes low-energy electrons. In this spectrum excitation of the B 311pand C 311Ustates is not observed (as can clearly be seen from a comparison with spectrum (a) ) because these 498
20 May 1988
excitation processes as well as the direct excitation of the A32: state are suppressed in the detector working in the energy analysis mode. These data with a similar spectrum presented previously [ 91 indicate that the increase of the yield above 8.5 eV is due to the excitation of the a ‘II, metastable state. Additional spectra obtained with higher statistics in the limited region of vibrational thresholds of the a ‘II, state did not reveal any clear structures similar, for example, to those observed for the B 31Tpstate [ 5 1, but this is consistent with the recent threshold measurements in N2 of Hammond et al. [ lo], which showed that the a ‘II* state is weakly excited at threshold. Excitation of the E ‘C: state contributes significantly to the total excitation in spectrum (b) and dominates in the 12 eV region. The small metastable yield detected below the v= 0 threshold of the a ‘II, state is a result of the excitation of higher levels ( UZ 12) of the A ‘E: state observed with reduced intensity. From the total cross sections of Cartwright et al. [ 111 and recent studies of the differential cross sections of Cadei et al. [ 121 its contribution in spectrum (b) is estimated to be less than 8% of the total yield at 15 eV. The excitation cross section of the E ‘Xz state was obtained from spectrum (b) of fig. 1 by the following procedure. The contribution of the a ‘l& excitation was subtracted using the total cross sections obtained with the metastable technique by Mason and Newell [ 8 1. This cross section is described by a fourth-degree polynomial in the energy region 9-l 7 eV (excluding 12 and 13 eV because of the E ‘Z:p’ contribution) which was fitted via a least-squares procedure to the a ‘HP yield of spectrum (b) in the energy range 10-l 1.5 eV. The experimental cross section of ref. [ 8 ] is described with an accuracy better than lob by the polynomial and in the fitting, as shown in fig. 1, follows the a ‘HP excitation function to within 6OXThis a ‘TIgcontribution is subtracted yielding the total cross section for the E ‘Zz state in the energy range from threshold at 11.875 eV to 15 eV. As the detection efficiency of the present detector is not known, to obtain the absolute cross sections these results were normalized by taking 0.296 x 1O-22m2 at 15 eV, the cross section obtained in ref. [ 111 as renormalized by Trajmar et al. [ 13 1. In ref. [ 111, by integrating the measured differential cross sections, total cross sections were obtained for
CHEMICAL PHYSICS LETTERS
Volume 146, number 6
of 0.6 eV, by using the metastable detection technique in conjunction with calibration of the Cu-Be-O detection surface. The cross section had a single broad peak with a maximum of (7.0 + 4.0) x 1O-22 m*. Excitation of the E ‘2: state was also observed by Kurzweg et al. [ 161 and Golden et al. [ 17 1, who studied the delayed photon emission for the C 31T,~B 31Tg(0,0) transition due to collisional energy transfer from the E ‘Z.$ state. This study [ 171 carried out with an energy resolution of 100 meV gave an excitation function which has an overall shape similar to the present results. Differential cross sections for the E ‘Cl state were studied by Mazeau et al. [ 141 in the region up to 14 eV to observe resonant phenomena associated with Rydberg states. The total cross sections have also been calculated by Cartwright [ 181, Chung and Lin [ 191 in the Ochkur-Rudge approximation for electron exchange and by Fliflet et al. [20] in the distorted-wave approximation but neither of these calculations account for the resonant excitation above threshold. The results of fig. 2 represent the cross section for the II= 0 vibrational level of the E ‘Z: state and indicate that the lifetime of the v= 1 level is much shorter than that of v=O. The spectra in fig. 1 do not
selected electron energies from 15 to 50 eV with an uncertainty of 50%. The procedure described above may introduce an additional error of about 20% in the threshold energy region, decreasing with increasing energy.
4. Discussion The total cross section for excitation of the E ‘Cc state in the energy range from threshold to 15 eV is shown in fig. 2 and is listed in table 1 for energies including the position of the resonance structures. In the near-threshold region, the cross section is dominated by the presence of resonances designated ‘CT, 2IIu, band “a” (peaks ao, al ) and band “b” (peaks bo, bl, b2) in the recent studies of the resonant structures in N2 [ 51. Although these resonance states can decay to the a lfIp state, their decay rate is low as deduced from the measurements of Mazeau et al. [ 141 supporting the applied subtraction technique. The total excitation cross section of the E “Xl state has been measured previously by Borst et al. [ 15 ] in the energy region 11.8-l 2.7 eV, with an energy spread
I
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20 May 1988
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.
.
.
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Fig. 2. The total cross section for excitation of the E ‘CL state of N2 in the incident energy range from threshold at 11.875 eV to 15 eV. The energy width per channel is 19.56 meV.
499
CHEMICAL PHYSICS LETTERS
Volume 146, number 6
cules, which has accelerated the development of new experimental and theoretical methods.
Table 1 Total cross section u for the E ‘2: state of NZ Energy (eV)
rr (lo-**m*)
N,- ‘)
11.941 12.146 12.342 12.557 12.665 12.782 12.909 12.987 13.144 13.251 13.378 13.486 14.0 14.5 15.0
1.20 il.68 0.26 0.43 0.36 0.41 0.24 0.30 0.21 0.25 0.22 0.25 0.25 0.28 0.296 b,
22: *H” a0 ai bo
Acknowledgement The author is grateful to K. Maciag for technical assistance and to Dr. Cz. Szmytkowski for a critical reading of the manuscript This work was supported by the Ministry of National Education under the programme CPBP 01.06 project 3.01.
b, bz
a) Assignment of the resonance structure as in ref. [ 51.
b, Normalization point.
show any structure at the excitation energy of the v= 1 level at 12.145 eV (the higher levels 0 2 are subject to predissociation). In threshold electron studies of N2 [lo], excitation of the v= 1 level was observed with the cross section at threshold equal to 9O/6 of that for v=O, which is high enough to make the observation of o= 1 in the present spectra possible if its lifetime is comparable to that of v= 0 ( 190 ps). The detection efficiencies for both levels are assumed to be the same as is expected from observations for other metastable states in N2 and CO [ 2 11. From the mean transit time between the interaction region and the detecting surface in the present experiment the lifetime for the ZI=1 level is estimated to be less than 30 ps. Helm and Cosby [22] have recently found the II= 1 lifetime to be 6 ps.
5. Conclusions The total cross sections for excitation of the E ‘C: state have been determined in the energy region from threshold to 15 eV. This corresponds to excitation of the v= 0 vibrational level and indicates that the lifetime of the v= 1 level is much shorter than that of v= 0. A new detection technique has been used which allows differentiation between different metastable states. This arose from the increasing interest
in the excitation cross sections of atoms and mole500
20 May 1988
References [ 1] W.L. Borst, Phys. Rev. A 5 (1972) 648. [2] N.J. Mason and W.R. Newell, J. Phys. B 20 (1987) 1357. [ 31M.H. Phillips, L.W. Anderson and C.C. Lin, Phys. Rev. A 32 (1985)2117. [4] M. Zubek, J. Phys. E 19 (1986) 463. [5] D.S. Newman, M. Zubek and G.C. King, J. Phys. B 16 (1983) 2247. [6] J.N.H. Brunt, G.C. King and F.H. Read, J. Phys. B 11 (1978) 173. [7] M. Zubekand G.C. King, J:Phys. B 20 (1987) 1135. [8] N.J. Mason and W.R. Newell, J. Phys. B 20 (1987) 3913. [ 91 M. Zubek, Contributed Papers of XIII Symposium on the Physics of Ionized Gases, Sibenik (1986) (University of Belgrade) p. 23. [IO] P. Hammond, G.C. King, J. Jureta and F.H. Read, J. Phys. B 20 (1987) 4255. [ 111 D.C. Cartwrigth, S. Trajmar, A. Chutjian and W. Williams, Phys. Rev. A 16 (1977) 1041. [ 12 ] I. Cadei, R.I. Hall, M. Landau, F. Pichou and C. Schermann, Contributed Papers of XIII Symposium on the Physics of Ionized Gases, Sibenik ( 1986) (University of Belgrade) p. 19. [ 131 S. Trajmar, D.F. Register and A. Chutjian, Phys. Rept. 97 (1983) 219. [ 141J. Mazeau, RI. Hall, G. Joyez, M. Landau and J. Reinhardt, J. Phys. B 6 (1973) 873. [ 151 W.L.Borst, WC. WellsandEC. Zipf, Phys.Rev. A 5 (1972) 1744. [16] L. Kurzweg, G.T. Egbert and D.J. Bums, Phys. Rev. A 7 (1973) 1966. [ 17 ] D.E. Golden, D.J. Bums and V.C. Sutcliffe Jr., Phys. Rev. A 10 (1974) 2123. [18] D.C. Cartwright, Phys. Rev. A 2 (1970) 1331; 5 (1972) 1974. [ 191 S. Chung and C.C. Lin, Phys. Rev. A 6 (1972) 988. [20] A.W. Fliflet, V. McKay and T.N. Rescigno, J. Phys. B 12 (1979) 3281. [ 2 11M. Zubek, to be published. [22] H. Helm and P.C. Cosby, Bull. Am. Phys. Sot. 31 ( 1986) 936.
CHEMICAL PHYSICS LETTERS
20 May 1988
THE CROSS SECTION FOR ELECTRON IMPAm EXCITATION OF THE E %; STATE OF N2 IN THE NEAR-THRESHOLD REGION
Mariusz ZUBEK Department of Physics, Technical University,SC952 Gdatisk, Poland Received 3 March 1988
The total excitation cross section for the E 2: state of N2 has been determined in the electronl’energy region from threshold to 15 eV and normalized to give absolute values. A detection technique has been used which can differentiate between different metastablestatesof N2. The cross section shows pronounced structure in the near-threshold region due to the formation of resonance states.
1. Introduction
Total cross sections for electron impact excitation of metastable states of atoms and molecules are usually measured in experiments in which the number of excited species at a given electron energy is determined through direct detection. In the nearthreshold region, the direct detection of metastable species allows us to obtain accurate excitation functions, as it avoids the experimentally difficult transmission of very-low-energy ( < 1 eV) electrons. This method of measurement was previously used for the metastable states of nitrogen by Borst [ 1 ] and more recently for the metastable states of noble gases by Mason and Newell [ 21. In the case of nitrogen, only a few metastable states are excited at a fixed electron energy and the time-of-flight technique was employed [ 11 to separate them. When the metastable states of an atom have very long radiative lifetimes ( > 1 ms) or the lifetimes of different states are similar, a laser-induced fluorescence technique can be used to observe the states independently as was done, for example, for the 2~~3s J=O, 2 metastable states of Ne by Phillips et al. [ 31. In the present work the total excitation cross section of the E ‘Cl state, the highest-lying metastable state of N2, has been measured using a new detection technique developed recently by Zubek [4], which is able to differentiate between molecules in different metastable states. This technique introduced en496
ergy analyses of electrons ejected in the de-excitation process of metastable molecules and it was used to determine the cross section of the E ‘Ep’state in the electron energy range from the excitation threshold at 11.875 eV to 15 eV. The metastable excitation of N2 was studied in high-resolution experiments by Newman et al. [ 5 ] and previously by Brunt et al. [ 6 1, but neither of these works could distinguish between molecules in different metastable states or determine the total excitation cross section for one of the states.
2. Experimental details In the experiment, an electron beam of low-energy spread is crossed at right angles with a molecular beam effusing from a platinum-iridium tube of 0.5 mm bore. The electron beam is produced by an electron monochromator consisting of a 127” electrostatic cylindrical analyzer with a mean radius of 20 mm. The electrons leaving the analyzer are then accelerated and focused by a triple-aperture electrostatic lens. The interaction region is surrounded by a cylindrical molybdenum mesh of 30 mm diameter to prevent any electric field penetration into the region. The incident electron beam is collected by a Faraday cup placed behind the cylindrical mesh. The electron beam energy is adjustable and has an energy spread (fwhm) of 70 meV and a current of typically
0 009-2614/88/$ 03.50 0 Elsevier Science Publishers B.V. ( North-Holland Physics Publishing Division )
Volume 146, number 6
CHEMICAL PHYSICS LETTERS
1 nA. The detector for metastable states is placed 40 mm above the interaction region. In the present measurements a detector with an adjustable threshold is used of a design similar to that employed by Zubek and King [ 71 in recent studies of metastable excitation in mercury. The molecules eject electrons by an Auger de-excitation process at the detecting surface, which are then detected by a channel electron multiplier. The maximum kinetic energy of the released electrons depends on the excitation energy of the metastable state. In the detector, the kinetic energy of the secondary electrons is analysed by a retarding electrostatic field formed between the detection surface and the retarding grid placed 2 mm below the surface. By applying an appropriate retarding potential only those electrons with an initial kinetic energy greater than a certain value are transmitted and detected. More details on the metastable detector are given in ref. [4]. The detecting surface was a conducting lead silicate glass in the form of a 35 mm diameter disc. This material has a high work function of approximately 7 eV and is appropriate for studies of metastable states with high excitation energies as it reduces the detection efficiency for lower-lying states compared to clean metal surfaces (for example tantalum or molybdenum). The detector has a large detection area (9.6 cm2), so most of the metastables formed in the interaction region can reach the detector giving a yield proportional to the total excitation cross section. The center of the detecting surface is set at an angle of about 5’ with respect to the tube producing the molecular beam to account for the recoil of molecules due to momentum transfer during electron collision. For nitrogen molecules excited to the E ‘Cl state, it is estimated that the recoil angle will change from 5.5” for excitation at threshold to 9” for backward scattered electrons at 15 eV. The detector intercepts the metastable molecules within a cone of half-angle 23.5 ’ and there is no change in the measured yield as a result of the recoil effect. The incident beam current varies with the electron energy due to the change in transmission of the accelerating lens. The current is monitored during data accumulation and the spectra are corrected for changes of the incident current. The electron beam is focused at a point positioned beyond the interaction region rather than at the region itself to min-
20 May 1988
imize the effect of the change of beam size with increasing electron energy. The spectrometer operation is controlled by a microcomputer working in the multiscaling mode. It produces a ramp voltage which is used to vary the incident energy and accumulates synchronously the output pulses from the metastable detector while the metastable yield is measured as a function of electron energy. It also controls the voltage on the retarding grid of the detector working in the retarding mode. The electron energy scale is calibrated against the position of the second resonance peak of band “a” (see below) at 12.782 eV [ 51 and is accurate to 30 meV. The metastable detector employed in the present work is also sensitive to photons of energies greater than the work function of the detecting surface. In a separate experiment in which the detector was placed perpendicular to the incident electron and molecular beams, the photon contribution to the metastable spectra was estimated to be less than 0.5Oh.
3. Results The metastable excitation spectra obtained for Nz in the energy range from 7 to 15 eV are shown in fig. 1. Spectrum (a) corresponds to the detection of all metastable states of NZ excited in this energy region and thus represents the total metastable excitation function. There are three metastable states A ‘E:, a ‘IIp and E ‘Z: with excitation thresholds v=O at 6.169, 8.549 and 11.875 eV, respectively, that directly contribute to the measured excitation function. The overall shape of the spectrum is, however, mainly determined by the excitation of the B311, state with v=O threshold at 7.353 eV and the C ‘II,, with v=O at 11.032 eV. The onset of the spectrum corresponds to the excitation energies of the vibrational levels of the B ‘I& state, while the broad shoulder at about 11 eV is related to the maximum in the excitation cross section of this state. The increase in the metastable yield above 11 eV is a result of the C ‘II” contribution. The B ‘II, state is observed via cascading transitions to the metastable A 3Cz state while the C 311U state via a two-step transition C ?I,+B ‘IIp followed by B ‘&-+A ‘E:. In 497
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Fig. 1. The metastable excitation function for N2 in the incident energy range from 7 to 15 eV. (a) Total excitation function for all excited metastable and cascading states; (b) total excitation function for a ‘IT,,and E ‘Z: metastable states obtained from an energy analysis of the ejected electrons. The crosses in spectrum (b) show the cross section of ref. [8] fitted to the excitation function of the a ‘II, state. The energy width per channel for both spectra is 19.56 meV.
these cascades vibrational levels of the A ‘Zz state from v=O to 5 and levels up to v=4 (80% transitions) are populated from the B 311eand C 311U states, respectively. In the direct excitation of the A3C: state the v= 8 level attains the highest population as deduced from Franck-Condon factors for the transition from the ground state. The E ‘Z: metastable state is strongly excited above threshold with the formation of a few resonance states. This spectrum is similar to the results of Brunt et al. [ 6 1, although obtained using a detection surface with a lower work function, as can be seen from the comparison of the structures present in both spectra. Spectrum (b) of fig. 1 involves an energy analysis of the ejected electrons and was taken with a potential difference of -0.45 V between the detecting surface and the retarding grid (as compared to +0.95 V for spectrum (a)), which removes low-energy electrons. In this spectrum excitation of the B 311pand C 311Ustates is not observed (as can clearly be seen from a comparison with spectrum (a) ) because these 498
20 May 1988
excitation processes as well as the direct excitation of the A32: state are suppressed in the detector working in the energy analysis mode. These data with a similar spectrum presented previously [ 91 indicate that the increase of the yield above 8.5 eV is due to the excitation of the a ‘II, metastable state. Additional spectra obtained with higher statistics in the limited region of vibrational thresholds of the a ‘II, state did not reveal any clear structures similar, for example, to those observed for the B 31Tpstate [ 5 1, but this is consistent with the recent threshold measurements in N2 of Hammond et al. [ lo], which showed that the a ‘II* state is weakly excited at threshold. Excitation of the E ‘C: state contributes significantly to the total excitation in spectrum (b) and dominates in the 12 eV region. The small metastable yield detected below the v= 0 threshold of the a ‘II, state is a result of the excitation of higher levels ( UZ 12) of the A ‘E: state observed with reduced intensity. From the total cross sections of Cartwright et al. [ 111 and recent studies of the differential cross sections of Cadei et al. [ 121 its contribution in spectrum (b) is estimated to be less than 8% of the total yield at 15 eV. The excitation cross section of the E ‘Xz state was obtained from spectrum (b) of fig. 1 by the following procedure. The contribution of the a ‘l& excitation was subtracted using the total cross sections obtained with the metastable technique by Mason and Newell [ 8 1. This cross section is described by a fourth-degree polynomial in the energy region 9-l 7 eV (excluding 12 and 13 eV because of the E ‘Z:p’ contribution) which was fitted via a least-squares procedure to the a ‘HP yield of spectrum (b) in the energy range 10-l 1.5 eV. The experimental cross section of ref. [ 8 ] is described with an accuracy better than lob by the polynomial and in the fitting, as shown in fig. 1, follows the a ‘HP excitation function to within 6OXThis a ‘TIgcontribution is subtracted yielding the total cross section for the E ‘Zz state in the energy range from threshold at 11.875 eV to 15 eV. As the detection efficiency of the present detector is not known, to obtain the absolute cross sections these results were normalized by taking 0.296 x 1O-22m2 at 15 eV, the cross section obtained in ref. [ 111 as renormalized by Trajmar et al. [ 13 1. In ref. [ 111, by integrating the measured differential cross sections, total cross sections were obtained for
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Volume 146, number 6
of 0.6 eV, by using the metastable detection technique in conjunction with calibration of the Cu-Be-O detection surface. The cross section had a single broad peak with a maximum of (7.0 + 4.0) x 1O-22 m*. Excitation of the E ‘2: state was also observed by Kurzweg et al. [ 161 and Golden et al. [ 17 1, who studied the delayed photon emission for the C 31T,~B 31Tg(0,0) transition due to collisional energy transfer from the E ‘Z.$ state. This study [ 171 carried out with an energy resolution of 100 meV gave an excitation function which has an overall shape similar to the present results. Differential cross sections for the E ‘Cl state were studied by Mazeau et al. [ 141 in the region up to 14 eV to observe resonant phenomena associated with Rydberg states. The total cross sections have also been calculated by Cartwright [ 181, Chung and Lin [ 191 in the Ochkur-Rudge approximation for electron exchange and by Fliflet et al. [20] in the distorted-wave approximation but neither of these calculations account for the resonant excitation above threshold. The results of fig. 2 represent the cross section for the II= 0 vibrational level of the E ‘Z: state and indicate that the lifetime of the v= 1 level is much shorter than that of v=O. The spectra in fig. 1 do not
selected electron energies from 15 to 50 eV with an uncertainty of 50%. The procedure described above may introduce an additional error of about 20% in the threshold energy region, decreasing with increasing energy.
4. Discussion The total cross section for excitation of the E ‘Cc state in the energy range from threshold to 15 eV is shown in fig. 2 and is listed in table 1 for energies including the position of the resonance structures. In the near-threshold region, the cross section is dominated by the presence of resonances designated ‘CT, 2IIu, band “a” (peaks ao, al ) and band “b” (peaks bo, bl, b2) in the recent studies of the resonant structures in N2 [ 51. Although these resonance states can decay to the a lfIp state, their decay rate is low as deduced from the measurements of Mazeau et al. [ 141 supporting the applied subtraction technique. The total excitation cross section of the E “Xl state has been measured previously by Borst et al. [ 15 ] in the energy region 11.8-l 2.7 eV, with an energy spread
I
I
I
:
I
I
..,. _:
20 May 1988
I
I
I
.T’
.
.
.
___.:
.:.. . ..
,_~_ ,...,,.... .__...,
-... :.
ELECTRON
.
.,...”
ENERGY
..
A.......
. ..I..
.
..
.
.’ . . . .
..,.,......
““..:.’
.
(eV I
Fig. 2. The total cross section for excitation of the E ‘CL state of N2 in the incident energy range from threshold at 11.875 eV to 15 eV. The energy width per channel is 19.56 meV.
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CHEMICAL PHYSICS LETTERS
Volume 146, number 6
cules, which has accelerated the development of new experimental and theoretical methods.
Table 1 Total cross section u for the E ‘2: state of NZ Energy (eV)
rr (lo-**m*)
N,- ‘)
11.941 12.146 12.342 12.557 12.665 12.782 12.909 12.987 13.144 13.251 13.378 13.486 14.0 14.5 15.0
1.20 il.68 0.26 0.43 0.36 0.41 0.24 0.30 0.21 0.25 0.22 0.25 0.25 0.28 0.296 b,
22: *H” a0 ai bo
Acknowledgement The author is grateful to K. Maciag for technical assistance and to Dr. Cz. Szmytkowski for a critical reading of the manuscript This work was supported by the Ministry of National Education under the programme CPBP 01.06 project 3.01.
b, bz
a) Assignment of the resonance structure as in ref. [ 51.
b, Normalization point.
show any structure at the excitation energy of the v= 1 level at 12.145 eV (the higher levels 0 2 are subject to predissociation). In threshold electron studies of N2 [lo], excitation of the v= 1 level was observed with the cross section at threshold equal to 9O/6 of that for v=O, which is high enough to make the observation of o= 1 in the present spectra possible if its lifetime is comparable to that of v= 0 ( 190 ps). The detection efficiencies for both levels are assumed to be the same as is expected from observations for other metastable states in N2 and CO [ 2 11. From the mean transit time between the interaction region and the detecting surface in the present experiment the lifetime for the ZI=1 level is estimated to be less than 30 ps. Helm and Cosby [22] have recently found the II= 1 lifetime to be 6 ps.
5. Conclusions The total cross sections for excitation of the E ‘C: state have been determined in the energy region from threshold to 15 eV. This corresponds to excitation of the v= 0 vibrational level and indicates that the lifetime of the v= 1 level is much shorter than that of v= 0. A new detection technique has been used which allows differentiation between different metastable states. This arose from the increasing interest
in the excitation cross sections of atoms and mole500
20 May 1988
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