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A note on dissociative photoionization of neopentane in the 10–30 e V photon energy range
Chemical Physics ELSEVIER
Chemical Physics 188 (1994) 387-393
A note on dissociative photoionization of neopentane in the lo-30 eV photon energy range C. Cauletti a,* , E. Rachlew-Ksllne b, M.-Y. Adam ‘, S. Stranges a, S. Sorensen b, A. Karawajczyk b, M. Kirm d aDepartment of Chemistry, University of Rome “La Sapienza”, Piauale A. More 5, 00185 Rome, Italy b Department of Physics I, Royal Institute of Technology, 10044 Stockholm, Sweden ’ LURE, Ba^timent 209d. Centre Universitaire Paris&d, 91405 Orsay Cedex, France d Department of Atomic Spectroscopy, Land University, 22100 Land, Sweden Received 21 February
1994; in final form 4 July 1994
Abstract The photofragmentation of neopentane in the photon energy range lo-30 eV with synchrotron radiation is analysed and compared with previous data on mass and photoelectron spectra of this molecule. A broad maximum due to a shape resonance in the ion yield of the most abundant fragment ions is observed around 16 eV photon energy along with Rydberg series converging to the 2a; ’ ionization threshold. The analysis of the branching ratios of the main fragment ions hints at a preferential decay of the shape resonance through the ionization channel 3t; ‘.
1. Introduction Neopentane is an interesting example of a high-symmetry molecule in which Rydberg excitations originating at the 2a1 orbital interact with a d ---)f giant resonance due to le u( C-H) + 2t, u* (C-H) excitation [ 11, producing a well-resolved set of Fano resonances in the photoabsorption cross section energy range 10-35 eV [ 21. In the absorption spectrum [ 2,3], sharp peaks appear on top of a strong continuum with a broad maximum around 16 eV photon energy, which is due to the shape resonance. Koch et al. [2] interpreted these features as vibrationally resolved p-like Rydberg series, precisely 2al -P 3p, 2al + 4p and 2a1 --) 5p. The purpose of the present work is to study whether these Rydberg series could be also observed in the various fragment ions following photoionization, possibly identifying preferential decay channels into specific fragments. Furthermore, we also wished to try to observe Rydberg series below the 3t; ’ and le- ’ ionization thresholds. The experiment consisted of measuring the ion yield as a function of photon energy using synchrotron radiation for the most abundant fragment ions. We found an abundance distribution of the fragment ions in good agreement with the literature data. In particular, the relative intensities measured by Brehm et al. [ 81 at 584 A (21.22 eV) excitation are equal to our findings within experimental error. * Corresponding
author.
0301-0104/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved SSDIO301-0104(94)00237-l
388
C. Cauletti et al. /Chemical
Physics 188 (1994) 387-393
The mass spectrum of neopentane C( CH3)4 has already been studied by different techniques [ 4-101. All these investigations found that the parent ion, m/e = 72, is very short-lived, similar to the isoelectronic CF: ion in which CF,t production occurs from CFd by a direct mechanism [ II], However, the neopentane molecular ion is stable, at variance with CFZ , at a very low internal energy, as shown by photoionization measurements with a photon energy up to the ionization potential of 11.9 eV [ 71, By far, the most dominant fragmentation of neopentane is the loss of CH,, and the formation of tertbutyl cation [ C( CH,) 3] +, m/e = 57: [C(CH,),]
-i-hv+e-+CH,+[C(CH,),]+
,
mle=57, channel (1).
This fragmentation has an average life-time of 3 X 10e2 s, as determined by field ionization measurements E121. Another signi~cantly stable ion fragment is that with m/e = 56, derived from the nuance ion by loss of one CH,:
[C(W).+1 +hv+e-
+ [C(CWd+ CH, + fC,H,I + ,
channel (2) .
m/e=56,
Again by loss of methane (CHJ or ethylene ( H2C=CH2) from the tert-butyl cation, the ions of m/e = 4 1 ( [C,H,I channel (3) ) and m/e = 29 (channel (4) ) are produced: r’ CH, f GH,l+ [C(CH,),]
+hv--+e-
+,
m/e=41, channel (3)
,
+CH,+[C(CH,),]+
L WL
+ W&I + > ml e = 29, channel (4) 16.38 j,
9
II
13
I
15 -17
19
21
23
Photon Energy (eV) Fig, 1. The ion yield of the main fragments of neopentane as a function of photon energy. The spectra are corrected for incident photon flux. The curves are normalized according to the relative intensities measured in the mass spectrum at 16.38 eV photon energy. The experimental data at 21.22 eV from Ref. [ 81 are also indicated (0). Tbe photoionization thresholds shown are from Ref. [ 191. Between 9 and 13 eV photon energy, the measurements are affected by second-order light from the monochromator.
C. Cauletti et al. /Chemical Physics 188 (1994) 387-393
389
We found for all the fragment ions, a general shape of the ion yield as a function of photon energy similar to the photoabsorption spectrum reported by Koch et al. [ 21, with a maximum at ca. 16 eV excitation energy and wellresolved features easily attributable to the Rydberg series previously described. However, differences could be observed which are discussed below.
2. Experimental Ion-fragment spectra were measured at MAX-Lab, the 550 MeV electron storage ring in Lund, Sweden. The photon flux from the l-m normal-incidence monochromator is focussed by a toroidal mirror to the source volume of the mass spectrometer. The storage ring typically provides a flux of 10” ph/s at 20 eV at a band width of 0.2 eV for a storage ring current of 100 mA [ 131. The sample is an effusive gas jet giving a pressure of lop5 mbar in the experimental chamber. The background pressure in the target chamber is lo-’ mbar. The ionic fragments are mass analyzed using a commercial quadrupole mass spectrometer, QMS (VG SXP-300). The experimental geometry has been previously described [ 141.
3. Results and discussion In Fig. 1, the abundances of the various fragment ions as a function of photon energy in the range 9-24 eV are reported for different photon energies. The abundances of the ions with m/e = 72 and m/e = 56 are not reported due to the very low values. The ion signals for different fragments were optimized at the same photon energy by varying the voltage applied to the repelling lens mounted opposite the mass spectrometer entrance. The design of the QMS requires that the ions have approximately 4 eV kinetic energy in order to pass the mass filter. Thus, the focussing and bias potentials are adjusted along with the lens voltage in order to optimize the transmission of a given fragment. The agreement between our results and previously reported data [ 81 obtained with an He1 photon source (2 1.22 eV) is fairly good, as shown in Fig. 1. It is clear that the dominant fragment is in any case the tert-butyl cation (ml e=57), followed by the fragments with m/e=41 and mle=29, in order of decreasing abundance. The general shapes of the curves are substantially similar, showing a broad maximum around 16 eV, strongly resembling the photoabsorption spectrum already mentioned [ 2,3]. An interesting observation is that the fragment with m/e = 41 and m/e = 29 have appearance potentials at ca. 13, respectively 13.5 eV, whilst the ion with m/e = 57 appears already at 11 eV. This is in agreement with literature data on photoelectron-photoion coincidence spectra [8], which show that the ions of m/e= 29 practically do not appear until the ionization channel at 12.76 eV (It;‘) opens. A possible interpretation of this behaviour could be that, at a photon energy higher than 13 eV, the excess energy after the ionization of the 4t, orbital and the consequent formation of the tert-butyl cation is enough to allow the rearrangement of this ion and its further fragmentation, which is impossible at lower photon energy. Another explanation can be found by considering the nature of the orbitals involved in the different ionization processes as shown in Fig. 2 (for the analysis of the electronic structure of neopentane, see the photoelectron spectroscopy studies listed in Refs. [ 15-191). The 4t, orbital is of o-type C-C bond character and the ionization out of this level, at ca. 11 eV, is strongly dissociative in favour of the formation of the tert-butyl cation. The following MOs ( It,, IP ca. 13 eV; le, IP ca. 14 eV; and 3t,, IP ca. 15.5 eV) have a pseudo n-character and are mainly localized on the CH, groups, with some contribution from n-type C-C bonds. The ionization out of one of these orbitals may therefore favour the further elimination of a CH4 or C2H4 molecule from [ C(CH3) j] +. This occurs through some not easily defined metastable intermediate states, whose formation implies in any case the rearrangement of the tert-butyl ion, favoured by the weakening of the C-H and C-C bonds following the ionization. Both effects are most probably determinant in the appearance potentials of the lightest ions.
390
C. Cuuierti et al. /Chemical Physics 188 (1994) 387-393
HC
H
h
I
!I
A,
0
Fig. 2. The atomic co~sition
of the four highest occupied molecular orbit&s of neo~nt~e
(From Ref. [ 191).
For the most abundant ion [C (CH,) 3] + (m/e = 57)) we analyzed the spectral region 15-18 eV photon energy in detail. The result is shown in Fig. 3a. We could identify peaks which can be attributed to the 2at -+ np Rydberg series, vibrationally resolved. Table 1 reports tentative assignments of the observed features. Such a detailed analysis for the ion yield spectra of the [ C,H,] + and [ C,H,] + (m/e = 29) ions is prevented by statistics. However, the overall shape of the spectra, shown in Figs. 3b and 3c, is similar. The resonance features in the photon energy range ca. 16.3-17 eV for all the fragments were analyzed by Eland et al. [lo], who found in this energy interval a substantial similarity in the peak shapes in all channels. The novelty of our results consists in the wider photon-energy range, and their quality is fairly good, considering that we obtained a resolution of 0.03 eV for the fragment with m/e = 57 and 0.06 eV for that with m/e = 29.
C. Cauletti et al. /Chemical
Physics 188 (1994) 387-393
391
I
”
1
7
m/e
b
” q
41
36000 34000 07
E 1 32000 s 30000 28000
t,,,,,,,,,,,,,,i 15
16 17 Photon Energy (evj
18
Fig. 3. The ion yield of the following fragment ions as a function of photon energy in the range 15-18 eV. (a) tert-butyl cation (m/e = 57). The photoionization threshold 2a;’ is from Ref. [ 151, (b) Fragment ion with m/e = 41. (c) Fragment ion with m/e = 29.
A point of interest of the present work may be the possibility of identifying the states through which the shape resonance preferentially decays. Although this is a difficult task with such an experiment, the analysis of Fig. 4 could be of some help. In Fig. 4 we show the branching ratios of the various fragments in the photon energy range 12.5-27.5 eV. Apart from the obvious steep increase of the branching ratios for the ions with mle=41 and ml e = 29 in correspondence to their appearance potential, it is possible to observe for both ions a broad maximum around the very photon energy region where the shape resonance appears, whilst in the same region the curve for the ion of m/e = 57 shows a shallow minimum. Furthermore, the curve of the fragment of m/e = 29 shows a steeper increase in intensity with respect to that of m/e = 41 starting from ca. 15 eV. At the same photon energy, the curve of the fragment of m/e = 57 also displays a small bump. If we consider that at ca. 15.5 eV the 3t, MO of neopentane Table 1 Rydberg states and vibrational
series members for the 2a, state in the ion yield spectrum of [C(CH,),]
Photon energy (eV)
Assignment
Photon energy (eV)
15.46 15.55 15.60 15.71 15.77 15.84 15.95 16.54 16.62
2a, + 3p
16.69 16.78 16.87 16.96 17.13 17.29 17.44 17.23
2a, -+4p
+ as a function of photon energy Assignment
2a, + 5p
2a, -+ 6p
392
C. Cauletti et al. /Chemical
Physics 188 (1994) 387-393
1
m/e = 57
0.8 .-0 3
0 c .c 2 k
0.6
0.4
0.2
I
m/e = 41
I,
0 -T 12.5
15
17.5
20
22.5
25
27.5
Photon Energy (eV) Fig. 4. The branching ratios of the main fragment ions of neopentane in the photon energy range 12.5-27.5 each partial ion yield and the total ion yield. The photoionization thresholds are from Ref. 1191.
eV obtained as the ratio between
does ionize, this could hint at a preferential decay of the shape resonance through this channel. Such a hypothesis must of course be tested by measuring the photoionization cross section as a function of photon energy for the single channels through photoemission experiments with synchrotron radiation. We are planning to perform such an experiment.
Acknowledgement The authors wish to thank the Italian CNR and the Swedish NFR for financial support.
References [l] M.B. Robin, Chem. Phys. Letters 119 (1985) 33. [2] E.E. Koch, V. Saile and N. Schwentner, Chem. Phys. Letters 33 ( 1975) 322. [3] J.C. Person and P.P. Nicole, Fifth International Conference on Vacuum Ultraviolet Radiation Physics, eds. M.C. Caste& M. Poney and N. Poney (CNRS, Mendon, France, 1977) p. 111. [4] P.N. Rylander and S. Meyerson, J. Am. Chem. Sot. 78 (1956) 5799. [5J A. Langer and C.P. Johnson, J. Phys. Chem. 61 (1957) 891. [6] C.P. Johnson and A. Langer, J. Phys. Chem. 61 (1957) 1010. [7] B. Steiner, C.F. Giese and M.G. Inghram, J. Chem. Phys. 34 (1961) 189. [ 81 B. Brehm, J.H.D. Eland, R. Frey and H. Schulte, Intern. J. Mass Spectrom. Ion Phys. 21 ( 1976) 373. [ 91 H.V. Leung, C.W. Tsang and A.G. Harrison, Org. Mass Spectrom. 11(1976) 664. [lo] J.H.D. Eland, J. Berkowitz and J.E. Monahan, J. Chem. Phys. 72 (1980) 253. [ 1 l] B. Brehm, J.H.D. Eland, R. Frey and A. Kiistler, Intern. J. Mass Spectrom. Ion Phys. 13 (1974) 251. [ 121 B.W. Viney, Intern. J. Mass Spectrom. Ion Phys. 8 (1972) 417.
C. Cauletti et al. /Chemical
Physics 188 (1994) 387-393
393
[ 131 S.L. Sorensen, B.J. Olsson, 0. Widlund, S. Huldt, S.-E. Johnnsson, E. K%Ilne, A.E. Nilsson, R. Hutton, U. Litzen and A. Svensson, Nucl. Instr. Methods A 297 ( 1990) 296. [ 141 S.L. Sorensen, T. Aberg, J. Tulkke, E. Rachlew-Kallne, G. Sundstrom and M. Kirm, Phys. Rev. A, in press. [ 1.51 S. Evans, J.C. Green, P.J. Joachim, A.F. Orchard, D.W. Turner and J.P. Maier, J. Chem. Sot. Faraday Trans. 1168 (1972) 905. [ 161 A.E. Jonas, G.K. Schweitzer, F.A. Grimm and T.A. Carlson, .I. Electron Spectry. Relat. Phenom. 1 (1972) 29. [ 171 J.N. Murrell and W. Schmidt, J. Chem. Sot. Faraday Trans. II 68 (1972) 1709. [ 181 A.W. Potts and D.G. Streets, J. Chem. Sot. Faraday Trans. II 70 (1974) 875. [ 191 K. Kimura, S. Katsumata, Y. Achiba, T. Yamazaki and S. Iwata, in: Handbook of the He1 photoelectron spectra of fundamental organic molecules (Japan Scientific Societies Press, Tokyo, Halsted Press, New York, 1981) p. 53.
Chemical Physics 188 (1994) 387-393
A note on dissociative photoionization of neopentane in the lo-30 eV photon energy range C. Cauletti a,* , E. Rachlew-Ksllne b, M.-Y. Adam ‘, S. Stranges a, S. Sorensen b, A. Karawajczyk b, M. Kirm d aDepartment of Chemistry, University of Rome “La Sapienza”, Piauale A. More 5, 00185 Rome, Italy b Department of Physics I, Royal Institute of Technology, 10044 Stockholm, Sweden ’ LURE, Ba^timent 209d. Centre Universitaire Paris&d, 91405 Orsay Cedex, France d Department of Atomic Spectroscopy, Land University, 22100 Land, Sweden Received 21 February
1994; in final form 4 July 1994
Abstract The photofragmentation of neopentane in the photon energy range lo-30 eV with synchrotron radiation is analysed and compared with previous data on mass and photoelectron spectra of this molecule. A broad maximum due to a shape resonance in the ion yield of the most abundant fragment ions is observed around 16 eV photon energy along with Rydberg series converging to the 2a; ’ ionization threshold. The analysis of the branching ratios of the main fragment ions hints at a preferential decay of the shape resonance through the ionization channel 3t; ‘.
1. Introduction Neopentane is an interesting example of a high-symmetry molecule in which Rydberg excitations originating at the 2a1 orbital interact with a d ---)f giant resonance due to le u( C-H) + 2t, u* (C-H) excitation [ 11, producing a well-resolved set of Fano resonances in the photoabsorption cross section energy range 10-35 eV [ 21. In the absorption spectrum [ 2,3], sharp peaks appear on top of a strong continuum with a broad maximum around 16 eV photon energy, which is due to the shape resonance. Koch et al. [2] interpreted these features as vibrationally resolved p-like Rydberg series, precisely 2al -P 3p, 2al + 4p and 2a1 --) 5p. The purpose of the present work is to study whether these Rydberg series could be also observed in the various fragment ions following photoionization, possibly identifying preferential decay channels into specific fragments. Furthermore, we also wished to try to observe Rydberg series below the 3t; ’ and le- ’ ionization thresholds. The experiment consisted of measuring the ion yield as a function of photon energy using synchrotron radiation for the most abundant fragment ions. We found an abundance distribution of the fragment ions in good agreement with the literature data. In particular, the relative intensities measured by Brehm et al. [ 81 at 584 A (21.22 eV) excitation are equal to our findings within experimental error. * Corresponding
author.
0301-0104/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved SSDIO301-0104(94)00237-l
388
C. Cauletti et al. /Chemical
Physics 188 (1994) 387-393
The mass spectrum of neopentane C( CH3)4 has already been studied by different techniques [ 4-101. All these investigations found that the parent ion, m/e = 72, is very short-lived, similar to the isoelectronic CF: ion in which CF,t production occurs from CFd by a direct mechanism [ II], However, the neopentane molecular ion is stable, at variance with CFZ , at a very low internal energy, as shown by photoionization measurements with a photon energy up to the ionization potential of 11.9 eV [ 71, By far, the most dominant fragmentation of neopentane is the loss of CH,, and the formation of tertbutyl cation [ C( CH,) 3] +, m/e = 57: [C(CH,),]
-i-hv+e-+CH,+[C(CH,),]+
,
mle=57, channel (1).
This fragmentation has an average life-time of 3 X 10e2 s, as determined by field ionization measurements E121. Another signi~cantly stable ion fragment is that with m/e = 56, derived from the nuance ion by loss of one CH,:
[C(W).+1 +hv+e-
+ [C(CWd+ CH, + fC,H,I + ,
channel (2) .
m/e=56,
Again by loss of methane (CHJ or ethylene ( H2C=CH2) from the tert-butyl cation, the ions of m/e = 4 1 ( [C,H,I channel (3) ) and m/e = 29 (channel (4) ) are produced: r’ CH, f GH,l+ [C(CH,),]
+hv--+e-
+,
m/e=41, channel (3)
,
+CH,+[C(CH,),]+
L WL
+ W&I + > ml e = 29, channel (4) 16.38 j,
9
II
13
I
15 -17
19
21
23
Photon Energy (eV) Fig, 1. The ion yield of the main fragments of neopentane as a function of photon energy. The spectra are corrected for incident photon flux. The curves are normalized according to the relative intensities measured in the mass spectrum at 16.38 eV photon energy. The experimental data at 21.22 eV from Ref. [ 81 are also indicated (0). Tbe photoionization thresholds shown are from Ref. [ 191. Between 9 and 13 eV photon energy, the measurements are affected by second-order light from the monochromator.
C. Cauletti et al. /Chemical Physics 188 (1994) 387-393
389
We found for all the fragment ions, a general shape of the ion yield as a function of photon energy similar to the photoabsorption spectrum reported by Koch et al. [ 21, with a maximum at ca. 16 eV excitation energy and wellresolved features easily attributable to the Rydberg series previously described. However, differences could be observed which are discussed below.
2. Experimental Ion-fragment spectra were measured at MAX-Lab, the 550 MeV electron storage ring in Lund, Sweden. The photon flux from the l-m normal-incidence monochromator is focussed by a toroidal mirror to the source volume of the mass spectrometer. The storage ring typically provides a flux of 10” ph/s at 20 eV at a band width of 0.2 eV for a storage ring current of 100 mA [ 131. The sample is an effusive gas jet giving a pressure of lop5 mbar in the experimental chamber. The background pressure in the target chamber is lo-’ mbar. The ionic fragments are mass analyzed using a commercial quadrupole mass spectrometer, QMS (VG SXP-300). The experimental geometry has been previously described [ 141.
3. Results and discussion In Fig. 1, the abundances of the various fragment ions as a function of photon energy in the range 9-24 eV are reported for different photon energies. The abundances of the ions with m/e = 72 and m/e = 56 are not reported due to the very low values. The ion signals for different fragments were optimized at the same photon energy by varying the voltage applied to the repelling lens mounted opposite the mass spectrometer entrance. The design of the QMS requires that the ions have approximately 4 eV kinetic energy in order to pass the mass filter. Thus, the focussing and bias potentials are adjusted along with the lens voltage in order to optimize the transmission of a given fragment. The agreement between our results and previously reported data [ 81 obtained with an He1 photon source (2 1.22 eV) is fairly good, as shown in Fig. 1. It is clear that the dominant fragment is in any case the tert-butyl cation (ml e=57), followed by the fragments with m/e=41 and mle=29, in order of decreasing abundance. The general shapes of the curves are substantially similar, showing a broad maximum around 16 eV, strongly resembling the photoabsorption spectrum already mentioned [ 2,3]. An interesting observation is that the fragment with m/e = 41 and m/e = 29 have appearance potentials at ca. 13, respectively 13.5 eV, whilst the ion with m/e = 57 appears already at 11 eV. This is in agreement with literature data on photoelectron-photoion coincidence spectra [8], which show that the ions of m/e= 29 practically do not appear until the ionization channel at 12.76 eV (It;‘) opens. A possible interpretation of this behaviour could be that, at a photon energy higher than 13 eV, the excess energy after the ionization of the 4t, orbital and the consequent formation of the tert-butyl cation is enough to allow the rearrangement of this ion and its further fragmentation, which is impossible at lower photon energy. Another explanation can be found by considering the nature of the orbitals involved in the different ionization processes as shown in Fig. 2 (for the analysis of the electronic structure of neopentane, see the photoelectron spectroscopy studies listed in Refs. [ 15-191). The 4t, orbital is of o-type C-C bond character and the ionization out of this level, at ca. 11 eV, is strongly dissociative in favour of the formation of the tert-butyl cation. The following MOs ( It,, IP ca. 13 eV; le, IP ca. 14 eV; and 3t,, IP ca. 15.5 eV) have a pseudo n-character and are mainly localized on the CH, groups, with some contribution from n-type C-C bonds. The ionization out of one of these orbitals may therefore favour the further elimination of a CH4 or C2H4 molecule from [ C(CH3) j] +. This occurs through some not easily defined metastable intermediate states, whose formation implies in any case the rearrangement of the tert-butyl ion, favoured by the weakening of the C-H and C-C bonds following the ionization. Both effects are most probably determinant in the appearance potentials of the lightest ions.
390
C. Cuuierti et al. /Chemical Physics 188 (1994) 387-393
HC
H
h
I
!I
A,
0
Fig. 2. The atomic co~sition
of the four highest occupied molecular orbit&s of neo~nt~e
(From Ref. [ 191).
For the most abundant ion [C (CH,) 3] + (m/e = 57)) we analyzed the spectral region 15-18 eV photon energy in detail. The result is shown in Fig. 3a. We could identify peaks which can be attributed to the 2at -+ np Rydberg series, vibrationally resolved. Table 1 reports tentative assignments of the observed features. Such a detailed analysis for the ion yield spectra of the [ C,H,] + and [ C,H,] + (m/e = 29) ions is prevented by statistics. However, the overall shape of the spectra, shown in Figs. 3b and 3c, is similar. The resonance features in the photon energy range ca. 16.3-17 eV for all the fragments were analyzed by Eland et al. [lo], who found in this energy interval a substantial similarity in the peak shapes in all channels. The novelty of our results consists in the wider photon-energy range, and their quality is fairly good, considering that we obtained a resolution of 0.03 eV for the fragment with m/e = 57 and 0.06 eV for that with m/e = 29.
C. Cauletti et al. /Chemical
Physics 188 (1994) 387-393
391
I
”
1
7
m/e
b
” q
41
36000 34000 07
E 1 32000 s 30000 28000
t,,,,,,,,,,,,,,i 15
16 17 Photon Energy (evj
18
Fig. 3. The ion yield of the following fragment ions as a function of photon energy in the range 15-18 eV. (a) tert-butyl cation (m/e = 57). The photoionization threshold 2a;’ is from Ref. [ 151, (b) Fragment ion with m/e = 41. (c) Fragment ion with m/e = 29.
A point of interest of the present work may be the possibility of identifying the states through which the shape resonance preferentially decays. Although this is a difficult task with such an experiment, the analysis of Fig. 4 could be of some help. In Fig. 4 we show the branching ratios of the various fragments in the photon energy range 12.5-27.5 eV. Apart from the obvious steep increase of the branching ratios for the ions with mle=41 and ml e = 29 in correspondence to their appearance potential, it is possible to observe for both ions a broad maximum around the very photon energy region where the shape resonance appears, whilst in the same region the curve for the ion of m/e = 57 shows a shallow minimum. Furthermore, the curve of the fragment of m/e = 29 shows a steeper increase in intensity with respect to that of m/e = 41 starting from ca. 15 eV. At the same photon energy, the curve of the fragment of m/e = 57 also displays a small bump. If we consider that at ca. 15.5 eV the 3t, MO of neopentane Table 1 Rydberg states and vibrational
series members for the 2a, state in the ion yield spectrum of [C(CH,),]
Photon energy (eV)
Assignment
Photon energy (eV)
15.46 15.55 15.60 15.71 15.77 15.84 15.95 16.54 16.62
2a, + 3p
16.69 16.78 16.87 16.96 17.13 17.29 17.44 17.23
2a, -+4p
+ as a function of photon energy Assignment
2a, + 5p
2a, -+ 6p
392
C. Cauletti et al. /Chemical
Physics 188 (1994) 387-393
1
m/e = 57
0.8 .-0 3
0 c .c 2 k
0.6
0.4
0.2
I
m/e = 41
I,
0 -T 12.5
15
17.5
20
22.5
25
27.5
Photon Energy (eV) Fig. 4. The branching ratios of the main fragment ions of neopentane in the photon energy range 12.5-27.5 each partial ion yield and the total ion yield. The photoionization thresholds are from Ref. 1191.
eV obtained as the ratio between
does ionize, this could hint at a preferential decay of the shape resonance through this channel. Such a hypothesis must of course be tested by measuring the photoionization cross section as a function of photon energy for the single channels through photoemission experiments with synchrotron radiation. We are planning to perform such an experiment.
Acknowledgement The authors wish to thank the Italian CNR and the Swedish NFR for financial support.
References [l] M.B. Robin, Chem. Phys. Letters 119 (1985) 33. [2] E.E. Koch, V. Saile and N. Schwentner, Chem. Phys. Letters 33 ( 1975) 322. [3] J.C. Person and P.P. Nicole, Fifth International Conference on Vacuum Ultraviolet Radiation Physics, eds. M.C. Caste& M. Poney and N. Poney (CNRS, Mendon, France, 1977) p. 111. [4] P.N. Rylander and S. Meyerson, J. Am. Chem. Sot. 78 (1956) 5799. [5J A. Langer and C.P. Johnson, J. Phys. Chem. 61 (1957) 891. [6] C.P. Johnson and A. Langer, J. Phys. Chem. 61 (1957) 1010. [7] B. Steiner, C.F. Giese and M.G. Inghram, J. Chem. Phys. 34 (1961) 189. [ 81 B. Brehm, J.H.D. Eland, R. Frey and H. Schulte, Intern. J. Mass Spectrom. Ion Phys. 21 ( 1976) 373. [ 91 H.V. Leung, C.W. Tsang and A.G. Harrison, Org. Mass Spectrom. 11(1976) 664. [lo] J.H.D. Eland, J. Berkowitz and J.E. Monahan, J. Chem. Phys. 72 (1980) 253. [ 1 l] B. Brehm, J.H.D. Eland, R. Frey and A. Kiistler, Intern. J. Mass Spectrom. Ion Phys. 13 (1974) 251. [ 121 B.W. Viney, Intern. J. Mass Spectrom. Ion Phys. 8 (1972) 417.
C. Cauletti et al. /Chemical
Physics 188 (1994) 387-393
393
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