- Email: [email protected]
Mechanisms of the charge separation in the phosphorus tetramer dication P42+
Chemical Physics Chemical Physics 188 ( 1994) 38 l-386
Mechanisms of the charge separation in the phosphorus tetramer dication P$+ S. Hsieh, J.H.D. Eland Physical Chemistry Laboratory, South Parks Road, Oxford OX1 3QZ, UK Received 6 June 1994
Abstract The charge separation reactions of Pi’ have been investigated using the PEPIPICO (photoelectron-photoion-photoion coincidence) technique at photon energies between 25 and 48 eV. All four possible ion pairs are formed. The relatively low appearance energy of the ion pair PC + PT provides an estimate of < 25.4 eV for the vertical double ionization energy of Pd. Dissociations producing P+ +P$ or 2P+ are mainly sequential, proceeding via the intermediate ions P:’ or Pg’ respectively.
1. Introduction The phosphorous tetramer P4 and its ionic forms are of interest as prototypes of small clusters and also attract industrial interest from the fields of surface science and catalysis [ 1,2]. These applications have led to studies of P4 by photoelectron spectroscopy and by electron impact [ l-31. While the behaviour of the PC ion has been well characterized, little work has been done on the dication species Pi’ . In this paper, we present a study of the Pz’ ion and its charge separation reactions using the PEPIPICO (photoelectron-photoion-photoion coincidence) technique. This technique allows correlations between the momenta of product ions to be investigated; the correlations give information about the dissociation mechanisms. The tetramer species of phosphorus is a good candidate for a PEPIPICO study for two main reasons. First, the fact that phosphorus has only one isotope eliminates complications due to additional peaks in the spectra. Secondly, P4 is one of a few four-atom molecules to be studied by PEPIPICO [4-6]. The most detailed PEPIPICO studies of basic 0301-0104/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved SSDIO301-0104(94)00249-5
reaction mechanism thus far have concentrated on three-atom molecules for the relative ease of determining fragmentation pathways [ 7,8].
2. Experimental
method
The experimental sample of white phosphorus ( P4) was allowed to sublime directly into the apparatus at ambient temperature. Light in the range of 25 to 48 eV, generated by an electron cyclotron resonance lamp in either neon or helium, was used to doubly-ionize the gaseous Pd. Since the PEPIPICO apparatus has been described in detail previously [ 9,101, we present a diagram of the apparatus in Fig. 1 and the following brief description. Atomic lines are separated by a grating monochromator and pass through a collimator with internal electrodes to suppress unwanted electrons. In the reaction chamber, the light beam intersects an effusive jet of gas at right angles. A uniform electric field (300 V/cm) is applied over the interaction region. The electric field accelerates photoelectrons towards a channelton electron detector and ions through a second
382
S. Hsieh, J.H.D. ElundiChemical
HV
Physics 188 (1994) 3Rl-386
ION DETECTOR
TOROIDAL GRATING MONOCHROMATOR
ACCELERATION
RARE GAS DETECTOR D’s~~~GE
Fig. 1. Schematic diagram of the experimental in Ref. [lo].
PEPIPICO apparatus.
accelerating field [ lo] and then through a field-free flight tube to a multichannel plate multiplier. A time-to-digital converter records the arrival of electrons and ions. An ion’s time-of-flight (TOF) relative to the electron signal in coincidence allows for 500
Recent modifications
to provide second order space focusing are described
the determination of its mass. Two-dimensions coincidence maps, with ti (the first ion’s TOF) on the xaxis and t2 (that of the second ion) on the y-axis, contain peaks for each ion pair produced in the dissociation of the parent dication.
3. Results and mechanistic
interpretation
3. I. Mass spectra 400
z
The mass spectra at all the photon energies used show P4’, P: , Pz , and Pi. Doubly-charged ions are not unexpected; both P:” and Ps’ have been proposed previously as species produced in collision reactions within an electron impact mass spectrometer [ 111. In our mass spectra, the Pg” ion appears at photon energies higher than 38 eV. Unfortunately, any evidence of Pz ’ or P: t ions is obscured by the strong Pg and P + peaks. A sharp spike in the center of the broad P: peak, however, does indicate significant production of the parent dication Pi’, but its wavelength dependence cannot be quantified.
300
s .c .-z B z
200
100
0 2 Photon energy (eV) Fig. 2. Relative yield of ion pairs from P+ Counts are normalized to the intensity of the Pl ion at each wavelength, which is expected to vary only slowly with photon energy in this range.
3.2.1. PC + P: The ion pair with the lowest appearance potential is P + -t P: , as shown in Fig. 2. The lowest energy at
S. Hsieh, J.H.D. Eland /Chemical Physics 18%(1994) 381-386
which we detect this pair is 25.4 eV; this indicates that part of the accessible potential energy surface of Pi+ lies near 25.4 eV, but we cannot tell whether the surface is bonding or repulsive in the area concerned. The measurement also provides 25.4 eV as a crude estimate of the vertical double ionization energy of P+ This experimentally estimated value is the same as that obtained using the rule of Tsai and Eland [ 121. By the rule of thumb, the predicted double ionization potential is (2.8 _t 0.1 )IE, where IE is the first vertical ionization potential. Using 9.08 eV as the first ionization for P4 [ 131, this rule predicts a vertical double ionization potential of 25.4 & 0.9 eV. At 25.4 eV, the kinetic energy release in the PC + Pz ion pair is 2.5 iO.5 eV. Thus, the product energy would appear to be 25.4 - 2.5 = 22.9 rt: 0.5 eV, which agrees, within its error limits, with the thermodynamic threshold of 23.1 eV [ 13,141 for this dissociation reaction. (The thermodynamic limits are quoted here to a precision of 0.1 eV only, even though some are known more precisely.} This comparison proves that the products are formed in their ground electronic states; the same is true at all photon energies below about 30 eV, where the product energy is lower than the energy required (26.3 eV [ 13,151) to produce the linear first excited state of P: with the observed kinetic energy releases. Since the ground state of the PC ion has l&, (equilateral triangle) symmetry [ 51, it seems likely that the P4* + ion retains a configuration similar to the tetrahedral neutral molecule prior to dissociation into P+ + PC . 3.2.2. P: +pz’ The thermodynamic threshold for the production of the PC + P: ion pair is 23.4 CV [ 131. Although the thermodynamic limits for both two-body reactions are nearly equal, the Pzf +P: cross section does not rise steeply until between 27.0 and 30.4 eV. The cross section in Fig. 2 has a foot between 25.4 and 27.0 eV; this may be an artefact due to the decomposition of P:’ or PC metastable ions in the field-free flight tube. Extrapolation of the steep rise indicates an appearance potential roughly near 27 i 1 eV. The kinetic energy release increases with increasing photon energy, from 2.6 i I .O eV at 27.7 eV to 4.0 * 0.2 eV at energies greater than or equal to 38.4 eV. The extrapolated value for the kinetic energy release at 27 eV is around 2.5 eV. This indicates a product energy of 24.5 eV, 1 eV above the
383
thermodynamic limit. Since the first excited state of PC is only 0.27 eV above the ground state [ 161, the possibility of excited state Pz ions cannot be eliminated, even near the appearance energy. 3.3. Three- or four-body reactions 3.3.1. P+ + P: From Fig. 2, the appearance potential for P + + Pz seems to be near 315 1 eV, only 2.5 eV above the thermodynamic limit of 28.4 eV [ 131, From the projections of the peak onto the tl and tz axes, the ions alone appear to carry a kinetic energy of 3.3 f0.5 eV at all photon energies, while from the simulation in Fig. 3, we estimate an upper limit of 4.0 eV for the total energy release. As was the case with Pf + P: , the available energy can barely account for the kinetic energy release, so its seems unlikely that excited state products are formed. Indeed, the first excited states of P+ and P are 1.10 and 1.41 eV above their respective ground states [ 171; thus, they cannot be formed below around 33 eV. As mentioned above, however, the first excited state of P; is so low that, given our experimental uncertainty, we cannot eliminate it as a possible product. The shape of the Pi +Pc peak, shown in Fig. 3, shows no significant variation with photon energy, suggesting that the same mechanism or mechanisms are involved at all wavelengths. The peak slope is approximately - 1, the expected slope for a simple deferred charge separation reaction [ 71 of the form pj+--+p+p;+,
P:” -+p+
+P:
.
Thus, it appears that deferred charge separation of the P:’ ion is the dominant pathway. A closer investigation of the peak shape at 40.8 eV, where we have the best statistics, however, shows some deviation from the expected slope for pure deferred charge separation. The peak is “twisted”, a form of peak shape seen in PEPIPICO spectra of various previously studied compounds [ 181. The “twist” manifests itself as a variation of peak slope on contours of increasing intensity. The slope progresses from - 1.0&0.05 along a low intensity contour (one-sixteenth the maximum intensity) to - 0.9 + 0.05 along the contour at one-half the maximum intensity. This “twist” is also evident at 48.4 eV. Although the statistics are not as good at this higher photon energy, the
384
S. Hsieh, J.H.D. Eland/Chemical
Physics 188 (1994) 381-386
Pi’ retains its tetrahedral geometry, the first step of neutral P release would given the triangular P:’ fragment a momentum pe~ndicul~ to the direction of charge separation. Such a momentum release would broaden the peak instead of changing its slope. A more reasonable explanation for the deviation from the expected peak slope in the case of P4 is suggested by the direction of the “twist.” The slope of the most intense part of the peak approaches the limit ( - 0.67) for a secondary decay reaction [ 71 in which an initially created PC ion decays in a second step into P$ +P. As shown in Fig. 3, a combination of simple deferred charge separation of e’ and of the secondary decay of P: gives a reasonable simulation of the P: + P$ peak shape at 40.8 eV. The increased “twist” effect for the peak at 48.4 eV suggests that the decay ofPi+ into P+ + PC + P is more like a secondary decay reaction at higher energies.
(a
Time for first ion Fig. 3. Experimental (left) and simulated (right) peaks for P-‘+P,f at 40.8 eV (top), P’+P+ at 40.8 eV (middle), and P+ +P+ at 48.4 eV (bottom). Details of the simulations are as follows. (a) Simulation for PC + P; peak contains 30% secondary decay of PC into PC + P (KER of 3.5 eV in charge separation and 0.5 eV, with an exponential distribution, in neutral release) and 60% deferred charge sep~tion of P:’ (KER 2.6 eV in charge sep~ation and 1.3 eV, with a wide Gaussian distribution, in neutrai release). (b) Simulation for P+ + Pf peak at 40.8 eV is of deferred charge separation of P$,’ (KER 3 eV in charge separation and 1.3 eV, with a wide Gaussian distribution, in neutral P2 release). (c) Simulation of P’ +P+ peak at 48.4 eV is for a concerted reaction producing P’ + P+ + P2 with a KER of 5 eV, with a broad spread (half-width of IOeV ) and a fixed angle of 130* between ions.
“twist” appears to approach a shallower slope of - 0.74 + 0.1 along the contour at one-half the maximum intensity. One possible explanation for deviations from the expected peak slopes in sequential reactions is the existence of energy release in the second step producing some momentum release parallel to the direction of charge separation [ 81. This explanation seems inappropriate for the deferred charge separation of Pi’ . If
3.3.2. Pi +p+ The cross section for P -)-+ PC in Fig. 2 shows a foot of low intensity followed by a steeper rise starting from between 34.8 and 38.0 eV. As was the case with the P: +P: peak, the foot may be an artefact due to the decay of metastable ions in the flight tube, in this case, of Pi+ . Extra~lation of the rise leads to a rough estimate of the appearance potential. The appearance potential is near 3 1 + 2 eV, approximately 1.6 % 2 eV above the thermodynamic limit (28.4 eV [ 131) for the three-body fragmentation to 2P+ -t- Pz. The kinetic energy release for the ions alone, based on the peak’s projections onto the ti and r, axes, increases slightly between 34.8 and 40.8 eV, from 3.2 rtO.6 to 5.2t-0.4 eV. Extrapolation of these kinetic energies leads to an estimated minimum kinetic energy release of 1.3 eV near the threshold. Because of the uncertainties in the appearance energy and kinetic energy releases, we cannot eliminate excited state products. It is, however, likely that P, is produced in its ground state near the appearance potential since its first excited state is 2.33 eV above the ground state [ 161. At all photon energies, the peak slope is near - 1. If the reaction occurred via the secondary decay of PC into P’ + Pz, the expected peak shape would be a “V” shape with arms of slopes - 0.3 and - 3. Instead the rectangular (or ovular) shape and - 1 slope of the P” +P+ peak are more consistent with a deferred charge separation mechanism. The main mechanism,
S. H&h. J.H.D. Eland/Chemical
then, appears to be the release of neutral P2 followed by the deferred charge separation of P$’ . As shown in Fig. 3, simple deferred charge separation of Pz’ (after neutral P, release) yields an adequate simulation of the peak at 40.8 eV. Fig. 3 also shows a significant change in peak shape between 40.8 and 48.4 eV. At 38.0 and 40.8 eV, the peak appears rectangular, with the intensity concentrated in the center; at 48.4 eV, the peak appears more oval-shaped, with the intensity more evenly scattered throughout. Although kinetic energies are difficult to determine for peaks on the diagonal (since we do not detect ion pairs with time differences less than 20 ns) , a significant increase in product ion kinetic energy release (from 5.2 eV & 0.4 eV at 40.8 eV to 6.2 + 0.6 eV at 48.4 eV) is evident as well. These changes in peak shape indicate that the steep rise in P+ + P+ cross section in Fig. 2 is due to the addition of a new mechanism. One possible explanation of the oval-shaped peak at 48.4 eV is a concerted reaction producing 2Pf +P, fragments with definite relative angles. Such concerted reactions in non-linear ions produce circular or ovalshaped peaks. Simulations of the concerted three-body reaction require an angle near 120” between ions in order to achieve the elongation in the desired direction. We have simulated the concerted reaction with an initial angle of 130” between ions, as shown in Fig. 3. It is also possible that the reaction at 48.4 eV is a four-body reaction, producing 2P instead of Pz. The thermodynamic limit for this reaction is 33.4 eV [ 131, making it barely energetically possible at 40.8 eV. In contrast, there is more than enough energy available at 48.4 eV for the four-body reaction.
4. Conclusion The Pi’ ion fragments into all possible ion pairs. Fragmentation producing more than two bodies occurs mainly through the deferred charge separation of the ions Pz’ and P:’ . Concerted dissociation or a sequential four-body separation process may be responsible for the production of P+ + P+ at higher (48.4 eV) photon energy. The relatively straightforward dissociation of P:’ can be compared to the dissociations of other four-atom dications. The dications of the molecules C,H, [ 41,
Physics 188 (1994) 381-386
385
HN,, ClN, and IN, [ 5 I appear to decay primarily via secondary decay. In contrast, deferred charge separation mechanisms appear to dominate the decays of the non-linear Pz’ and NF$+ [ 61 ions. Perhaps this preference for deferred charge separation reflects the relative stability of the proposed intermediate ions, P:’ , Pz’ and NFz+ . This is supported by the fact that the intermediate ions Pz’ and NF;+ appear in the mass spectra of P4 and NF, [ 61 respectively. Acknowledgement The authors thank John Turner at the Inorganic Chemistry Laboratory, Oxford University for supplying pure white phosphorus. This research was supported by SERC under grant number GR/H28219. One of us (SH) is supported by the Marshall Aid Commemoration Commission. References 11 1 G. Monnom, Ph. Gaucherel and C. Paparoditis, J. Phys. (Paris) 45 (1984) 77. [2] L.-S. Wang, B. Niu, Y.T. Lee, D.A. Shirley, E. Ghelichkhani and E.R. Grant, J. Chem. Phys. 93 (1990) 6318. [ 31 CR. Brundle, N.A. Kuebler, M.B. Robin and H. Basch, lnorg. Chem. 11 (1972) 20; S. Evans, P.J. Joachim, A.F. Orchard and D.W. Turner, Intern. J. Mass Spectrom. Ion Phys. 9 ( 1972) 41; J.A. Zimmerman, S.B. Bach, C.H. Watson and J.R. Eyler, J. Phys. Chem. 95 (1991) 98. [4] R. Thissen, 1. Delwiche, J.M. Robbe, D. Dutlot, J.P. Flament and J.H.D. Eland, J. Chem. Phys. 99 (1993) 6590. [5] Current work in this laboratory. 161 J.H.D. Eland and V. Schmidt, in press. [7] M. Lavollee and H. Bergeron, J. Phys. B 25 (1992) 3101; T. LeBrun, M. Lavollee and P. Morin, AIP Conf. Proc. 215 (1990) 846; J.H.D. Eland, Mol. Phys. 61 (1987) 725. [8] J.H.D. Eland, Chem. Phys. Letters 203 (1993) 353. [9] SD. Price, J.H.D. Eland, P.G. Foumier, J. Foumier and P. Millie, J. Chem. Phys. 88 ( 1988) 15 11. [ 101 J.H.D. Eland, Meas. Sci. Technol. 4 (1993) 1522. [ 1 l] J.-D. Carette and L. Kerwin, Canad. J. Phys. 39 (1961) 1300. [ 12 ] B.P. Tsai and J.H.D. Eland, Intern. J. Mass Spectrom. Ion Phys. 36 (1980) 143. [ 131 S.G. Lias, J.E. Bartmess, J.F. Liebman, J.L. Holmes, R.D. Levin and W.G. Mallard, J. Phys. Chem. Ref. Data 17 (1988) Suppl. 1; R.K. Yoo, B. Russic and J. Berkowitz, J. Electr. Spectry. Relat. Phen. 66 (1993) 39. [ 141 J. Smets, P. Coppens and J. Drowart, Chem. Phys. 20 (1977) 243.
386
S. Hsieh, J.H.D. Eland/Chemical
[ 151 K. Balasubramanian, 95 (1991) 3494.
K. Sumathi and D. Dai, J. Chem. Phys.
[ 16 I K.P. Huber and G. Her&erg, Molecular spectra and molecular structure, Vol. IV. Constants of diatomic Nostrand, New York, 1970) pp. 520-523.
molecules
(Van
Physics 188 (1994) 381-386 [ 171 C.E. Moore, Atomic energy levels as derived from the analyses of optical spectra, Vol. 1 (National Bureau of Standards, Washington, DC, 1949) pp. 163-165. [ 181 J.H.D. Eland and B.J. Treves-Brown, lnt. J. Mass Spectrom. Ion Processes 113 (1992) 167.
Mechanisms of the charge separation in the phosphorus tetramer dication P$+ S. Hsieh, J.H.D. Eland Physical Chemistry Laboratory, South Parks Road, Oxford OX1 3QZ, UK Received 6 June 1994
Abstract The charge separation reactions of Pi’ have been investigated using the PEPIPICO (photoelectron-photoion-photoion coincidence) technique at photon energies between 25 and 48 eV. All four possible ion pairs are formed. The relatively low appearance energy of the ion pair PC + PT provides an estimate of < 25.4 eV for the vertical double ionization energy of Pd. Dissociations producing P+ +P$ or 2P+ are mainly sequential, proceeding via the intermediate ions P:’ or Pg’ respectively.
1. Introduction The phosphorous tetramer P4 and its ionic forms are of interest as prototypes of small clusters and also attract industrial interest from the fields of surface science and catalysis [ 1,2]. These applications have led to studies of P4 by photoelectron spectroscopy and by electron impact [ l-31. While the behaviour of the PC ion has been well characterized, little work has been done on the dication species Pi’ . In this paper, we present a study of the Pz’ ion and its charge separation reactions using the PEPIPICO (photoelectron-photoion-photoion coincidence) technique. This technique allows correlations between the momenta of product ions to be investigated; the correlations give information about the dissociation mechanisms. The tetramer species of phosphorus is a good candidate for a PEPIPICO study for two main reasons. First, the fact that phosphorus has only one isotope eliminates complications due to additional peaks in the spectra. Secondly, P4 is one of a few four-atom molecules to be studied by PEPIPICO [4-6]. The most detailed PEPIPICO studies of basic 0301-0104/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved SSDIO301-0104(94)00249-5
reaction mechanism thus far have concentrated on three-atom molecules for the relative ease of determining fragmentation pathways [ 7,8].
2. Experimental
method
The experimental sample of white phosphorus ( P4) was allowed to sublime directly into the apparatus at ambient temperature. Light in the range of 25 to 48 eV, generated by an electron cyclotron resonance lamp in either neon or helium, was used to doubly-ionize the gaseous Pd. Since the PEPIPICO apparatus has been described in detail previously [ 9,101, we present a diagram of the apparatus in Fig. 1 and the following brief description. Atomic lines are separated by a grating monochromator and pass through a collimator with internal electrodes to suppress unwanted electrons. In the reaction chamber, the light beam intersects an effusive jet of gas at right angles. A uniform electric field (300 V/cm) is applied over the interaction region. The electric field accelerates photoelectrons towards a channelton electron detector and ions through a second
382
S. Hsieh, J.H.D. ElundiChemical
HV
Physics 188 (1994) 3Rl-386
ION DETECTOR
TOROIDAL GRATING MONOCHROMATOR
ACCELERATION
RARE GAS DETECTOR D’s~~~GE
Fig. 1. Schematic diagram of the experimental in Ref. [lo].
PEPIPICO apparatus.
accelerating field [ lo] and then through a field-free flight tube to a multichannel plate multiplier. A time-to-digital converter records the arrival of electrons and ions. An ion’s time-of-flight (TOF) relative to the electron signal in coincidence allows for 500
Recent modifications
to provide second order space focusing are described
the determination of its mass. Two-dimensions coincidence maps, with ti (the first ion’s TOF) on the xaxis and t2 (that of the second ion) on the y-axis, contain peaks for each ion pair produced in the dissociation of the parent dication.
3. Results and mechanistic
interpretation
3. I. Mass spectra 400
z
The mass spectra at all the photon energies used show P4’, P: , Pz , and Pi. Doubly-charged ions are not unexpected; both P:” and Ps’ have been proposed previously as species produced in collision reactions within an electron impact mass spectrometer [ 111. In our mass spectra, the Pg” ion appears at photon energies higher than 38 eV. Unfortunately, any evidence of Pz ’ or P: t ions is obscured by the strong Pg and P + peaks. A sharp spike in the center of the broad P: peak, however, does indicate significant production of the parent dication Pi’, but its wavelength dependence cannot be quantified.
300
s .c .-z B z
200
100
0 2 Photon energy (eV) Fig. 2. Relative yield of ion pairs from P+ Counts are normalized to the intensity of the Pl ion at each wavelength, which is expected to vary only slowly with photon energy in this range.
3.2.1. PC + P: The ion pair with the lowest appearance potential is P + -t P: , as shown in Fig. 2. The lowest energy at
S. Hsieh, J.H.D. Eland /Chemical Physics 18%(1994) 381-386
which we detect this pair is 25.4 eV; this indicates that part of the accessible potential energy surface of Pi+ lies near 25.4 eV, but we cannot tell whether the surface is bonding or repulsive in the area concerned. The measurement also provides 25.4 eV as a crude estimate of the vertical double ionization energy of P+ This experimentally estimated value is the same as that obtained using the rule of Tsai and Eland [ 121. By the rule of thumb, the predicted double ionization potential is (2.8 _t 0.1 )IE, where IE is the first vertical ionization potential. Using 9.08 eV as the first ionization for P4 [ 131, this rule predicts a vertical double ionization potential of 25.4 & 0.9 eV. At 25.4 eV, the kinetic energy release in the PC + Pz ion pair is 2.5 iO.5 eV. Thus, the product energy would appear to be 25.4 - 2.5 = 22.9 rt: 0.5 eV, which agrees, within its error limits, with the thermodynamic threshold of 23.1 eV [ 13,141 for this dissociation reaction. (The thermodynamic limits are quoted here to a precision of 0.1 eV only, even though some are known more precisely.} This comparison proves that the products are formed in their ground electronic states; the same is true at all photon energies below about 30 eV, where the product energy is lower than the energy required (26.3 eV [ 13,151) to produce the linear first excited state of P: with the observed kinetic energy releases. Since the ground state of the PC ion has l&, (equilateral triangle) symmetry [ 51, it seems likely that the P4* + ion retains a configuration similar to the tetrahedral neutral molecule prior to dissociation into P+ + PC . 3.2.2. P: +pz’ The thermodynamic threshold for the production of the PC + P: ion pair is 23.4 CV [ 131. Although the thermodynamic limits for both two-body reactions are nearly equal, the Pzf +P: cross section does not rise steeply until between 27.0 and 30.4 eV. The cross section in Fig. 2 has a foot between 25.4 and 27.0 eV; this may be an artefact due to the decomposition of P:’ or PC metastable ions in the field-free flight tube. Extrapolation of the steep rise indicates an appearance potential roughly near 27 i 1 eV. The kinetic energy release increases with increasing photon energy, from 2.6 i I .O eV at 27.7 eV to 4.0 * 0.2 eV at energies greater than or equal to 38.4 eV. The extrapolated value for the kinetic energy release at 27 eV is around 2.5 eV. This indicates a product energy of 24.5 eV, 1 eV above the
383
thermodynamic limit. Since the first excited state of PC is only 0.27 eV above the ground state [ 161, the possibility of excited state Pz ions cannot be eliminated, even near the appearance energy. 3.3. Three- or four-body reactions 3.3.1. P+ + P: From Fig. 2, the appearance potential for P + + Pz seems to be near 315 1 eV, only 2.5 eV above the thermodynamic limit of 28.4 eV [ 131, From the projections of the peak onto the tl and tz axes, the ions alone appear to carry a kinetic energy of 3.3 f0.5 eV at all photon energies, while from the simulation in Fig. 3, we estimate an upper limit of 4.0 eV for the total energy release. As was the case with Pf + P: , the available energy can barely account for the kinetic energy release, so its seems unlikely that excited state products are formed. Indeed, the first excited states of P+ and P are 1.10 and 1.41 eV above their respective ground states [ 171; thus, they cannot be formed below around 33 eV. As mentioned above, however, the first excited state of P; is so low that, given our experimental uncertainty, we cannot eliminate it as a possible product. The shape of the Pi +Pc peak, shown in Fig. 3, shows no significant variation with photon energy, suggesting that the same mechanism or mechanisms are involved at all wavelengths. The peak slope is approximately - 1, the expected slope for a simple deferred charge separation reaction [ 71 of the form pj+--+p+p;+,
P:” -+p+
+P:
.
Thus, it appears that deferred charge separation of the P:’ ion is the dominant pathway. A closer investigation of the peak shape at 40.8 eV, where we have the best statistics, however, shows some deviation from the expected slope for pure deferred charge separation. The peak is “twisted”, a form of peak shape seen in PEPIPICO spectra of various previously studied compounds [ 181. The “twist” manifests itself as a variation of peak slope on contours of increasing intensity. The slope progresses from - 1.0&0.05 along a low intensity contour (one-sixteenth the maximum intensity) to - 0.9 + 0.05 along the contour at one-half the maximum intensity. This “twist” is also evident at 48.4 eV. Although the statistics are not as good at this higher photon energy, the
384
S. Hsieh, J.H.D. Eland/Chemical
Physics 188 (1994) 381-386
Pi’ retains its tetrahedral geometry, the first step of neutral P release would given the triangular P:’ fragment a momentum pe~ndicul~ to the direction of charge separation. Such a momentum release would broaden the peak instead of changing its slope. A more reasonable explanation for the deviation from the expected peak slope in the case of P4 is suggested by the direction of the “twist.” The slope of the most intense part of the peak approaches the limit ( - 0.67) for a secondary decay reaction [ 71 in which an initially created PC ion decays in a second step into P$ +P. As shown in Fig. 3, a combination of simple deferred charge separation of e’ and of the secondary decay of P: gives a reasonable simulation of the P: + P$ peak shape at 40.8 eV. The increased “twist” effect for the peak at 48.4 eV suggests that the decay ofPi+ into P+ + PC + P is more like a secondary decay reaction at higher energies.
(a
Time for first ion Fig. 3. Experimental (left) and simulated (right) peaks for P-‘+P,f at 40.8 eV (top), P’+P+ at 40.8 eV (middle), and P+ +P+ at 48.4 eV (bottom). Details of the simulations are as follows. (a) Simulation for PC + P; peak contains 30% secondary decay of PC into PC + P (KER of 3.5 eV in charge separation and 0.5 eV, with an exponential distribution, in neutral release) and 60% deferred charge sep~tion of P:’ (KER 2.6 eV in charge sep~ation and 1.3 eV, with a wide Gaussian distribution, in neutrai release). (b) Simulation for P+ + Pf peak at 40.8 eV is of deferred charge separation of P$,’ (KER 3 eV in charge separation and 1.3 eV, with a wide Gaussian distribution, in neutral P2 release). (c) Simulation of P’ +P+ peak at 48.4 eV is for a concerted reaction producing P’ + P+ + P2 with a KER of 5 eV, with a broad spread (half-width of IOeV ) and a fixed angle of 130* between ions.
“twist” appears to approach a shallower slope of - 0.74 + 0.1 along the contour at one-half the maximum intensity. One possible explanation for deviations from the expected peak slopes in sequential reactions is the existence of energy release in the second step producing some momentum release parallel to the direction of charge separation [ 81. This explanation seems inappropriate for the deferred charge separation of Pi’ . If
3.3.2. Pi +p+ The cross section for P -)-+ PC in Fig. 2 shows a foot of low intensity followed by a steeper rise starting from between 34.8 and 38.0 eV. As was the case with the P: +P: peak, the foot may be an artefact due to the decay of metastable ions in the flight tube, in this case, of Pi+ . Extra~lation of the rise leads to a rough estimate of the appearance potential. The appearance potential is near 3 1 + 2 eV, approximately 1.6 % 2 eV above the thermodynamic limit (28.4 eV [ 131) for the three-body fragmentation to 2P+ -t- Pz. The kinetic energy release for the ions alone, based on the peak’s projections onto the ti and r, axes, increases slightly between 34.8 and 40.8 eV, from 3.2 rtO.6 to 5.2t-0.4 eV. Extrapolation of these kinetic energies leads to an estimated minimum kinetic energy release of 1.3 eV near the threshold. Because of the uncertainties in the appearance energy and kinetic energy releases, we cannot eliminate excited state products. It is, however, likely that P, is produced in its ground state near the appearance potential since its first excited state is 2.33 eV above the ground state [ 161. At all photon energies, the peak slope is near - 1. If the reaction occurred via the secondary decay of PC into P’ + Pz, the expected peak shape would be a “V” shape with arms of slopes - 0.3 and - 3. Instead the rectangular (or ovular) shape and - 1 slope of the P” +P+ peak are more consistent with a deferred charge separation mechanism. The main mechanism,
S. H&h. J.H.D. Eland/Chemical
then, appears to be the release of neutral P2 followed by the deferred charge separation of P$’ . As shown in Fig. 3, simple deferred charge separation of Pz’ (after neutral P, release) yields an adequate simulation of the peak at 40.8 eV. Fig. 3 also shows a significant change in peak shape between 40.8 and 48.4 eV. At 38.0 and 40.8 eV, the peak appears rectangular, with the intensity concentrated in the center; at 48.4 eV, the peak appears more oval-shaped, with the intensity more evenly scattered throughout. Although kinetic energies are difficult to determine for peaks on the diagonal (since we do not detect ion pairs with time differences less than 20 ns) , a significant increase in product ion kinetic energy release (from 5.2 eV & 0.4 eV at 40.8 eV to 6.2 + 0.6 eV at 48.4 eV) is evident as well. These changes in peak shape indicate that the steep rise in P+ + P+ cross section in Fig. 2 is due to the addition of a new mechanism. One possible explanation of the oval-shaped peak at 48.4 eV is a concerted reaction producing 2Pf +P, fragments with definite relative angles. Such concerted reactions in non-linear ions produce circular or ovalshaped peaks. Simulations of the concerted three-body reaction require an angle near 120” between ions in order to achieve the elongation in the desired direction. We have simulated the concerted reaction with an initial angle of 130” between ions, as shown in Fig. 3. It is also possible that the reaction at 48.4 eV is a four-body reaction, producing 2P instead of Pz. The thermodynamic limit for this reaction is 33.4 eV [ 131, making it barely energetically possible at 40.8 eV. In contrast, there is more than enough energy available at 48.4 eV for the four-body reaction.
4. Conclusion The Pi’ ion fragments into all possible ion pairs. Fragmentation producing more than two bodies occurs mainly through the deferred charge separation of the ions Pz’ and P:’ . Concerted dissociation or a sequential four-body separation process may be responsible for the production of P+ + P+ at higher (48.4 eV) photon energy. The relatively straightforward dissociation of P:’ can be compared to the dissociations of other four-atom dications. The dications of the molecules C,H, [ 41,
Physics 188 (1994) 381-386
385
HN,, ClN, and IN, [ 5 I appear to decay primarily via secondary decay. In contrast, deferred charge separation mechanisms appear to dominate the decays of the non-linear Pz’ and NF$+ [ 61 ions. Perhaps this preference for deferred charge separation reflects the relative stability of the proposed intermediate ions, P:’ , Pz’ and NFz+ . This is supported by the fact that the intermediate ions Pz’ and NF;+ appear in the mass spectra of P4 and NF, [ 61 respectively. Acknowledgement The authors thank John Turner at the Inorganic Chemistry Laboratory, Oxford University for supplying pure white phosphorus. This research was supported by SERC under grant number GR/H28219. One of us (SH) is supported by the Marshall Aid Commemoration Commission. References 11 1 G. Monnom, Ph. Gaucherel and C. Paparoditis, J. Phys. (Paris) 45 (1984) 77. [2] L.-S. Wang, B. Niu, Y.T. Lee, D.A. Shirley, E. Ghelichkhani and E.R. Grant, J. Chem. Phys. 93 (1990) 6318. [ 31 CR. Brundle, N.A. Kuebler, M.B. Robin and H. Basch, lnorg. Chem. 11 (1972) 20; S. Evans, P.J. Joachim, A.F. Orchard and D.W. Turner, Intern. J. Mass Spectrom. Ion Phys. 9 ( 1972) 41; J.A. Zimmerman, S.B. Bach, C.H. Watson and J.R. Eyler, J. Phys. Chem. 95 (1991) 98. [4] R. Thissen, 1. Delwiche, J.M. Robbe, D. Dutlot, J.P. Flament and J.H.D. Eland, J. Chem. Phys. 99 (1993) 6590. [5] Current work in this laboratory. 161 J.H.D. Eland and V. Schmidt, in press. [7] M. Lavollee and H. Bergeron, J. Phys. B 25 (1992) 3101; T. LeBrun, M. Lavollee and P. Morin, AIP Conf. Proc. 215 (1990) 846; J.H.D. Eland, Mol. Phys. 61 (1987) 725. [8] J.H.D. Eland, Chem. Phys. Letters 203 (1993) 353. [9] SD. Price, J.H.D. Eland, P.G. Foumier, J. Foumier and P. Millie, J. Chem. Phys. 88 ( 1988) 15 11. [ 101 J.H.D. Eland, Meas. Sci. Technol. 4 (1993) 1522. [ 1 l] J.-D. Carette and L. Kerwin, Canad. J. Phys. 39 (1961) 1300. [ 12 ] B.P. Tsai and J.H.D. Eland, Intern. J. Mass Spectrom. Ion Phys. 36 (1980) 143. [ 131 S.G. Lias, J.E. Bartmess, J.F. Liebman, J.L. Holmes, R.D. Levin and W.G. Mallard, J. Phys. Chem. Ref. Data 17 (1988) Suppl. 1; R.K. Yoo, B. Russic and J. Berkowitz, J. Electr. Spectry. Relat. Phen. 66 (1993) 39. [ 141 J. Smets, P. Coppens and J. Drowart, Chem. Phys. 20 (1977) 243.
386
S. Hsieh, J.H.D. Eland/Chemical
[ 151 K. Balasubramanian, 95 (1991) 3494.
K. Sumathi and D. Dai, J. Chem. Phys.
[ 16 I K.P. Huber and G. Her&erg, Molecular spectra and molecular structure, Vol. IV. Constants of diatomic Nostrand, New York, 1970) pp. 520-523.
molecules
(Van
Physics 188 (1994) 381-386 [ 171 C.E. Moore, Atomic energy levels as derived from the analyses of optical spectra, Vol. 1 (National Bureau of Standards, Washington, DC, 1949) pp. 163-165. [ 181 J.H.D. Eland and B.J. Treves-Brown, lnt. J. Mass Spectrom. Ion Processes 113 (1992) 167.