- Email: [email protected]
On the lifetime of electronically excited acetone molecular ions
Volume 179, number 1,2
12April 1991
CHEMICAL PHYSICS LETTERS
On the lifetime of electronically excited acetone molecular ions S. Fenistein, J. Futrell ‘, M. Heninger, R. Marx, G. Mauclaire and Y.M. Yang Laboratoire
de Physico-Chimie des Rayonnements (associated to CNRS), Universitt Paris&d,
91405 Orsay, France
Received 13 December 1990; in final form 7 January 199I
A triple-cell ICR spectrometer has been used to study the dissociation of electronically excited acetone ions (CHs)$O+* induced by collision with He and to measure their radiative lifetime. The threshold energy for CID is around 0.15 eV, consistent with a small barrier to curve-crossing into the ground state from which dissociation occurs. Endothermic charge transfer with 02 has also been used to probe the internal energy of acetone ions as a function of storage time. The radiative lifetime depends on the internal energy of the ions: 4:: and lb+_3 ms, determined respectively by CID with He and charge transfer with O;, corresponding to E,,> 2.9 eV and E,., > 2.4 eV.
1. Introduction
A recent publication [ 1 ] on the threshold-energy collision-induced dissociation (CID) of acetone molecular ion with helium, CH,COCH,+ tHe+CH3CO++CH3
tHe,
(1)
provided conclusive evidence that a significant fraction of the population of acetone ions formed by 70 eV electron impact is in an electronically excited state which survives more than 38 ps prior to dissociation. It was reported that low-energy collisions of ions in this excited state convert their internal (electronic and possibly vibrational) energy largely into recoil kinetic energy of the acetone ion which then rapidly dissociates into CH$O+ and CH3 on the groundstate potential surface. Fragment ions from the decomposition of superelastically scattered molecular ions are back-scattered in the center-ofimass (c.m.) frame. Reducing the energy of the ionizing electrons in the ion source to 12 eV completely suppressed the superelastic scattering mechanism, demonstrating the excited-state(s) origin of this mechanism. The superelastic scattering mechanism is also suppressed at high collision energies [ 21 where forward-scattered ’ Permanent address: Department of Chemistry and Biachemistry, University of Delaware, Newark, DE 19716, USA.
highly inelastic (endothermic) processes dominate acetone-ion CID. A more extensive crossed-beam study of the superelastic scattering mechanism which dominates the low collision energy (E,,,< 3 eV) CID of the acetone ion has established its key dynamics features [ 3 1. Over the range of collision energies from 0.4 to 3 eV, exactly 2.2kO.3 eV of internal energy is converted into translational energy of the acetone ion and He (or Ar) atom as they recoil from the collision center. This energy release matches the adiabatic X+A electronic excitation energy difference between the ground and first excited states of the acetone ion, suggesting a very efficient E-T energy-transfer mechanism for this system. The observed acetyl-ion velocity distribution is remarkably narrow, indicating that very little kinetic energy is released in the fragmentation step. It was inferred that the internal energy of the acetone ion, after the collision had induced the X&A transition, is only slightly greater than the 0.65 eV threshold energy for dissociation. The location of the intensity maximum for this mechanism on the relative velocity vector (as a fullrebound peak) indicates that small impact-parameter “head-on” collisions triggers this energy-transfer process. A curve-crossing mechanism was suggested to rationalize these results, which have been further elaborated to demonstrate microscopic reversibility, e.g., that ground-state ions can be colli-
0009-2614/91/$ 03.50 Q 1991 - Elsevier Science Publishers B.V. (North-Holland)
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sionally excited to the upper surface with comparable efficiency to the collisional quenching of the excited state [4], in this interesting system. At collision energies of 6 eV and higher, the superelastic scattering mechanism is no longer observed and translationally endothermic forward (but nonzeroangle) scattering takes over as the predominant CID mechanism [ 21. The observed dynamics are consistent with the hypothesis that the endothermic CID channel proceeds on the electronically excited surface(s) and involves the effkient T-PE excitation of ground-state acetone ions. The present experiments were undertaken to characterize the long-lived electronically excited state (s ) of the acetone ions responsible for this exceptional behavior and to measure their lifetime using the Orsay Tricyclotron Ion Cyclotron Resonance (ICR) mass spectrometer.
2. Experimental The ICR spectrometer utilized for these measurements has been described previously [ $61. Basically, it consists of three cells located in three differentially pumped chambers and communicating through two constrictions (ion funnels). This gives a pressure-drop ratio of about 150 between the second cell and each of the two adjacent cells. The first cell served as the ion source where ions are generated by electron-impact ionization and all ions of m/ za 18, except the ions of. interest, are ejected. The second cell is used to store the ions in near-collisionfree conditions for a variable period before entering the third cell where they undergo reaction. It is also used for FIICR detection of the ions by drifting them back to the central cell after reaction in cell three. Two types of acetone-ion reactions have been studied, namely, collision-induced dissociation with He and charge transfer with a monitor molecule, OZ. In both experiments, the ions were generated by a 30 eV electron-beam pulse of 0.5 ms duration. All ions of mass higher than 17, except (CH,),CO+ (5X) and (CH,)2COHf (59), were ejected. The possible role of the most abundant non-ejected ions (CH,+ , H2+) or partially ejected ions ( HzO+ ) will be discussed later. In our experimental conditions (acetone pressure of 10P6 Torr and drift time w0.5 ms ), the ratio 126
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of mass 59 to mass 58 entering the central cell is 0.044. The acetone partial pressure in the storage cell is less than IO-* Torr so that m/z= 59 formed during the storage time is negligible. In contrast, the ions resulting from reaction with the gas leaking from the third cell have to be ejected before the other ions are drifted into the reaction cell. The minimum transit time from the first to the third cell is 1.3 ms. After reaction for 10 ms in the third cell, the ions were migrated back to the central cell for mass analysis and quantification by FI’MS.
3. Results 3.1. CoNision-induced dissociation To characterize the long-lived excited ions, we first examined their collision-induced dissociation into acetyl ions (m/z= 43) as a function of collision energy. In this experiment, the ions were drifted into the third cell 1.3 ms after ion formation, then energized for 0.2 ms with a variable amplitude rf excitation pulse at the acetone-ion cyclotron frequency. After 10 ms of reaction time in the presence of 4~ lo-’ Torr of He, the ions were drifted back and detected in the central cell. The mean kinetic energy imparted to resonant acetone ions is given by Ei,,= (eVt/d)2/8mi
,
(2)
where I/is the (variable) amplitude ofthe excitation pulse, t is the excitation time (0.2 ms in our experiments), d the distance between the two drift plates of the ICR cell (1 cm) and mi the mass of the ion [ 61. If we assume the neutral collider is at rest in the laboratory frame, the c.m. collision energy is calculated from EC.,. = &
Eion9
(3)
where mi is the mass of the ion and WI,is the mass of the neutral. This is the energy plotted as the abscissa in fig. 1. The effect of energy spread on the shape of the excitation function will be discussed later. Fig. 1 plots the ratio of acetyl ion to the initial acetone-ion intensities, 43/( 58j0 as a function of the
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$, O.lOll .. 72 8 ++ 8
0.05.
0
’
l
l
m
0
8 0
+’ 8
l l
8
o.oolc 0
:
, 0.30 0.40 0.10 0.20 (CH,),CO+KINETICENERGYCM kV)
Fig. I. Corrected ratio of acetyl ion to the initial acetone-ion intensities, as a function ofthe center-of-mass collision energy with He. The apparent threshold energy, 0.025 eV, corrected for thermal motion of He (see text) is ~0. I5 eV. Less than 7% of the ions undergo CID.
center-of-mass collision energy calculated from eq. (3). The denominator of this fraction is calculated rather than measured because the transmission of energized m/z= 58 drifted back into the central cell depends on their kinetic energy (i.e. on their cyclotron radii compared to the size of the ion funnels). Therefore, the unreactive (CHj)2COH+ ions were used as an internal standard, the normalized acetone-ion intensity was calculated by multiplying the measured m/z=59 by the ratio l/O.044 observed when no excitation of m/z= 58 is applied. A second correction to the experimental data is applied in plotting fig. 1. At zero excitation of m/ z= 58, the m/z= 43 intensity is 1.25%;this is the relative abundance of mass 43 detected 12 ms after acetone-molecular-ion formation. These acetyl ions originate from unimolecular decay and/or the collisional decomposition at thermal energy of the acetone ions occurring during the time interval from ion formation to detection in this experiment. This intercept value of 1.25Ohis subtracted from the ohserved m/z=43 intensity to generate fig. 1. The ap parent threshold is 0.025 eV and the intensity of the acetyl ions increases monotonically with increasing c.m. energy, levelling off at approximately 0.25 eV in fig. 1 and remaining constant up to 0.4 eV, the upper energy limit in these experiments beyond which ion losses become unacceptably large. The plateau level may be interpreted as proportional to the fraction of electronically excited states present in
12 April 1991
the reactant beam ( x 6.8% compared to the 10%estimated in ref. [ 3 ] ). The true excitation function is much sharper than depicted in fig. 1, which is a convolution of the energy spread in the ion beam with that of the neutral collider. The energy width of the reactant acetone ions is a function of thermal energy of the precursor acetone molecules and is broadened with ion energy by the random initial phase between the ion motion and the irradiating rf field. It is given by [ 7 ] (4j where v. is initial ion velocity, $ is the phase angle between the ion motion and the irradiating resonant rf field, and the other terms have already been defined. Doppler broadening resulting from thermal motion of the neutral has been discussed in great detail by Chantry [ 81 who showed that the energy distribution (for monoenergetic ion beams) is given by W,,,=(ll.lykTE,)“*,
(5)
where W,,, is the full-width half-maximum energy width, y is the mass ratio (mi/(mi+m,)>, k is the Boltzmann constant, T is temperature and E0 is the c.m. collision energy. The present example of heavy ion-light neutral represents an extreme case. At a collision energy of 0.1 eV, the fwhm energy spread arising from this effect is 0.16 eV; at 0.3 eV, it is 0.28 eV. The effect of thermal motion of both target and projectile was discussed by Chantry [ 81, who pointed out that the width of the distribution is given approximately by WL/Z= [ ( W~/Z)kgct + ( Wt/Z)&ojcctilel I” >
(6)
where ( W fz hawetis the fwhm energy spread of the target and ( W 1, 1) p+ojectile is the fwhm energy spread of the projectile (both expressed in the c.m. frame). Over the energy range of fig. 1, the spread in collision energy is dominated by the Doppler-broadening term, eq. (5). The phase-angle term of eq. (4) contributes + (eVt/d)vO and is the dominant factor in ( Wl/2)pmjwtilc.Substituting WW x (eVf/d)v, and converting this to the c.m. frame using eq. (3) leads to the conclusion that ( Wl/Z)pr+eile is about 43% of ( W,,,)target in eq. (6 ). Because of the square-term
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relationship in eq. (6) this will broaden the energy width by about 18% over the Doppler term alone. Although the quality of the data does not warrant a quantitative attempt at deconvolution, it is clear that the fig. 1 cross section follows most closely the case A model considered by Chantry [ 8 1, a step function rising steeply from threshold to a near constant value. In such cases, the “apparent threshold” obtained by linear extrapolation of the appearance energy curve is too low by about 0.6 W,,2(ET) where W,,, is the fwhm evaluated (e.g., using eq. (6)) at the true threshold energy, ET, The resulting value of ET is 0.135 eV. We, therefore, infer that the threshold energy for the excited-state acetone-ion CID reaction is about 0.15 eV and can be adequately represented by a step function. As discussed in our earlier publication [ 11, the thermochemical energy requirements for breaking the acetyl-methyl bond for ground-state acetone ions at 298 K is 0.65 eV, and the threshold energy for observing the lowest energy endothermiccollision-induced dissociation of groundstate acetone ions is of the order of 1.3 eV [ 3 1. Consequently, we may be sure that the excitation function shown in fig. I is characteristic of electronically excited acetone molecular ions rather than groundstate species. Next, we conducted a series of measurements using the CID reaction as the monitor for the composition of the reactant-ion beam as a function of delay time. The excitation level in cell three was set to impart 0.08 eV (c.m. frame) in the acetone ionhelium collisional activation. After a 10 ms reaction time, all ions then present in the third chamber were transferred back to the second chamber and analyzed by ftms. In fig. 2, the ratio of the acetyl ion (43) to total ion (43 + 58) intensity is plotted as a function of relaxation time prior to CID. A small amount of mass 43 independent of storage time appears on the curve. This signal may arise from slow dissociation of vibrationally excited (metastable) groundstate acetone ions or simply represent electronic noise pick-up at the acetyl-ion frequency coming from the source where these ions are continuously ejected. This constant signal has to be subtracted for the fitting of the curves to an exponential. The least-squares fit of the curve plotted in fig. 2 gives a lifetime for this population of 4 ms; repeat measurements gave values of 4:: ms. 128
12 April 1991
P 0.020. 8 ^, 5 0.015. l+ 8 Q 0.010. B ~0.005.
. . , . I r . 10
20
I
.
30
I
40
RELAXATIONTIME (ms) Fig. 2. Ratio of the acetyl ion normalized to the total ion intensities as a function of the relaxation time prior to 0.08 eV (cm.) CID. The least-squares fit of the curve after subtraction of the plateau gives a lifetime of 4 ms.
3.2. Monitor-ion experiments An independent measurement of the acetone-ion excited-state lifetime using the monitor-gas method has been performed using oxygen as monitor gas. The principle of the monitor-ion technique has already been described and used to determine the radiative lifetime of various ions [ $61. The method is based on the assumption that at thermal energies, reactions like charge or proton transfers are much faster for exothermic than for endothermic processes. In the present experiment, the monitor ion is 0: resulting from the charge transfer of excited acetone ions with OZ.The ionization potential of O2 is 12.06 eV [9] while the first ionization potential of acetone is 9.7 eV and the second ionization potential (A state) is 11.9 eV [ lo]. Therefore, oxygen serves as a monitor for acetone ions containing more than 2.4 eV internal energy, possibly the first excited A state with more than 0.2 eV of vibrational and rotational energy. The amount of 0: ions produced in the third cell by the excited acetone ions depends on the storage time as shown in fig. 3. The shape of this curve reflects the overall decay rate of the energy in the excited acetone ions below 12.06 eV the minimum energy required to ionize OZ. If all the experimental conditions remain constant when the relaxation time is varied, and if the excited acetone-ions reaction rate with 0, does not depend on their internal energy
CHEMICAL PHYSICS LETTERS
Volume 179, number 1,2
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1991
4. Dim&on
0.000’~.
I
.
20
8
.
40
.
I
60
I
80
.
I
.
100
RELAXATIONTIME (ms) Fig. 3. Normalized monitor-ion 0: intensity as a function of relaxation time. The least-squares fit of the curve after subtraction of the plateau gives a lifetime of 14 ms. Repeat measurements gave values of 142 3 ms.
provided
[O;]
the
reaction is exothermic, it follows that
[Gl t [(CH,),CO+]
=’ exp(-ts’7)’
(7)
where A is a constant depending on the amount of excited acetone ions entering the relaxation cell and on the reaction conditions in the third cell ( O2 pressure: 7x 10e6 Torr; reaction rate constant equals collision rate constant kc= 6.5 x lo-” cm3 s-’ and reaction time is IO ms), t, is the storage time and l/ r the overall decay rate. Fig. 3 shows the ratio of 0; ion signal to the total ion signal. The ratio is plotted as a function of ion storage time in the second cell prior to injection into the third cell where the reaction takes place. A small amount of 0: independent of storage time shows up as a plateau on the curve. This 0: signal results from the reaction of O2 with a stable ion, probably H,Of (CH: and H: are also present but would not generate 0: ), which is produced by ionization of the background gas in the source and is very difficult to eject completely. This plateau has to be subtracted to lit the curve with an exponential decay. The mean lifetime deduced from fitting the corrected curve in repeat experiments is 14 f 3 ms.
It is noteworthy that the lifetimes deduced from the CID experiment and the monitor-gas experiment are of comparable magnitude but differ from each other by about a factor of four. It is important to note that the two techniques sample ions with somewhat different internal energy content. As pointed out above, the monitor-gas O2 samples ions containing 2.4 eV of internal energy above the ground-state ionization potential, while the CID experiment samples acetone ions containing 2.9 eV of internal energy. This is the sum of the adiabatic XeA energy difference of 2.2 eV plus about 0.7 eV required to dissociate acetone on the ground-state surface. Consequently, the CID experiment (fig. 2) samples ions containing about 0.5 eV more energy than the monitor-ion experiment (fig. 3). It is not unreasonable that a difference in internal energy content of 0.5 eV might change the apparent lifetime by a factor of four. A lifetime of 4 & 1 ms also implies a significant number of unimolecular decays of the more highly excited acetone ions excited to this level during the ionsampling time of the present experiments. Finally, we note that fig. 1 indicates a threshold energy of the order of 0.15 eV for collisionally inducing the dissociation of excited acetone ions to acetyl ion and methyl radical. The model suggested by the CID study [3] and elaborated by the present research is depicted in fig. 4. Vertical ionization of acetone into the excited-state manifold deposits about 0.7 eV of vibrational energy [lo]. The collision of the excited acetone ion and He induces a curve-crossing mechanism which releases the adiabatic energy difference (2.2 eV) as translational energy. This interpretation relies upon the previous study of the reaction dynamics of electronically excited acetone in CID. The barrier height for inducing this transition is estimated from fig. 1 to be about 0.15 eV. Accordingly, this value is marked on the fig. 4 diagram as the barrier height for the curve-crossing transition. The internal energy of the electronically excited state is preserved as vibrational energy on the ground-state surface after the collision-induced curve-crossing occurs and is responsible for the dissociation into acetyl ion and methyl radicals.
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gratefully acknowledged. These measurements were carried out in January 1990 when Jean Futrell was Professeur Associt at the University of Pa& Orsay Campus: their hospitality is gratefully acknowledged.
References
Fig. 4. Schematic potential energy diagram summarizing energetits for collision-induced decomposition of the acetone molecular ion. Collision at and above 0.15 eV induces curve-crossing of electronically excited ions to the ground state with the release of 2.2 eV into translational energy. Some of the ground-state ions subsequently decompose into acetyl ions and methyl radicals, which requires a minimum energy of 0.65 eV.
Acknowledgement The support of this work by the National Science Foundation, CHE 8796291, and by the Air Force Oftice of Scientific Research, AFOSR 87-0390, is
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[I ] K. Qian, A. Shukla, S. Howard, S. Anderson and J. Futrell, J. Phys. Chem. 93 (1989) 3889. [2]A.K.Shukla,K.Qian,S.L.Howard,S.G.AndersonandJ.H. Futrell, Intern. J. Mass Spectrom. Ion Processes 92 ( 1989) 147. [ 31 K. Qiau, A. Shukla andJ. Futrell, J. Chem. Phys. 92 ( 1990) 5988. [4] K. Qian, A. Shukla and J. Futrell, Chem. Phys. Letters 175 (1990) 51. [ 51G. Mauclaire, M. Heninger, S. Fenistein, J. Wronka and R. Marx, Intern. J. Mass Spectrom. Ion Processes 80 ( 1987) 99. [ 61 S. Fenistein, M. Heninger, R. Marx, G. Mauclaire andY.M. Yang, Chem. Phys. Letters 172 (1990) 89. [ 71 J.H. Futrell, in: Dynamic mass spectrometry, Vol. 2, ed. D. Price (Heyden, London, 1972) pp. 97 ff. [S] P.J. Chan’try, J. Chem. Phys. 55 (I 971) 2746. [9] S.G. Lias, J.E. Bartmess, J.F. Liebman, J.L. Holmes, R.D. Levine and W.G. Mallard, J. Phys. Chem. Ref. Data 17 Suppl. I (1988). [lo] R. Bombach, J.P. Stadelman and J. Vogt, Chem. Phys. 72 (1982) 259.
12April 1991
CHEMICAL PHYSICS LETTERS
On the lifetime of electronically excited acetone molecular ions S. Fenistein, J. Futrell ‘, M. Heninger, R. Marx, G. Mauclaire and Y.M. Yang Laboratoire
de Physico-Chimie des Rayonnements (associated to CNRS), Universitt Paris&d,
91405 Orsay, France
Received 13 December 1990; in final form 7 January 199I
A triple-cell ICR spectrometer has been used to study the dissociation of electronically excited acetone ions (CHs)$O+* induced by collision with He and to measure their radiative lifetime. The threshold energy for CID is around 0.15 eV, consistent with a small barrier to curve-crossing into the ground state from which dissociation occurs. Endothermic charge transfer with 02 has also been used to probe the internal energy of acetone ions as a function of storage time. The radiative lifetime depends on the internal energy of the ions: 4:: and lb+_3 ms, determined respectively by CID with He and charge transfer with O;, corresponding to E,,> 2.9 eV and E,., > 2.4 eV.
1. Introduction
A recent publication [ 1 ] on the threshold-energy collision-induced dissociation (CID) of acetone molecular ion with helium, CH,COCH,+ tHe+CH3CO++CH3
tHe,
(1)
provided conclusive evidence that a significant fraction of the population of acetone ions formed by 70 eV electron impact is in an electronically excited state which survives more than 38 ps prior to dissociation. It was reported that low-energy collisions of ions in this excited state convert their internal (electronic and possibly vibrational) energy largely into recoil kinetic energy of the acetone ion which then rapidly dissociates into CH$O+ and CH3 on the groundstate potential surface. Fragment ions from the decomposition of superelastically scattered molecular ions are back-scattered in the center-ofimass (c.m.) frame. Reducing the energy of the ionizing electrons in the ion source to 12 eV completely suppressed the superelastic scattering mechanism, demonstrating the excited-state(s) origin of this mechanism. The superelastic scattering mechanism is also suppressed at high collision energies [ 21 where forward-scattered ’ Permanent address: Department of Chemistry and Biachemistry, University of Delaware, Newark, DE 19716, USA.
highly inelastic (endothermic) processes dominate acetone-ion CID. A more extensive crossed-beam study of the superelastic scattering mechanism which dominates the low collision energy (E,,,< 3 eV) CID of the acetone ion has established its key dynamics features [ 3 1. Over the range of collision energies from 0.4 to 3 eV, exactly 2.2kO.3 eV of internal energy is converted into translational energy of the acetone ion and He (or Ar) atom as they recoil from the collision center. This energy release matches the adiabatic X+A electronic excitation energy difference between the ground and first excited states of the acetone ion, suggesting a very efficient E-T energy-transfer mechanism for this system. The observed acetyl-ion velocity distribution is remarkably narrow, indicating that very little kinetic energy is released in the fragmentation step. It was inferred that the internal energy of the acetone ion, after the collision had induced the X&A transition, is only slightly greater than the 0.65 eV threshold energy for dissociation. The location of the intensity maximum for this mechanism on the relative velocity vector (as a fullrebound peak) indicates that small impact-parameter “head-on” collisions triggers this energy-transfer process. A curve-crossing mechanism was suggested to rationalize these results, which have been further elaborated to demonstrate microscopic reversibility, e.g., that ground-state ions can be colli-
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sionally excited to the upper surface with comparable efficiency to the collisional quenching of the excited state [4], in this interesting system. At collision energies of 6 eV and higher, the superelastic scattering mechanism is no longer observed and translationally endothermic forward (but nonzeroangle) scattering takes over as the predominant CID mechanism [ 21. The observed dynamics are consistent with the hypothesis that the endothermic CID channel proceeds on the electronically excited surface(s) and involves the effkient T-PE excitation of ground-state acetone ions. The present experiments were undertaken to characterize the long-lived electronically excited state (s ) of the acetone ions responsible for this exceptional behavior and to measure their lifetime using the Orsay Tricyclotron Ion Cyclotron Resonance (ICR) mass spectrometer.
2. Experimental The ICR spectrometer utilized for these measurements has been described previously [ $61. Basically, it consists of three cells located in three differentially pumped chambers and communicating through two constrictions (ion funnels). This gives a pressure-drop ratio of about 150 between the second cell and each of the two adjacent cells. The first cell served as the ion source where ions are generated by electron-impact ionization and all ions of m/ za 18, except the ions of. interest, are ejected. The second cell is used to store the ions in near-collisionfree conditions for a variable period before entering the third cell where they undergo reaction. It is also used for FIICR detection of the ions by drifting them back to the central cell after reaction in cell three. Two types of acetone-ion reactions have been studied, namely, collision-induced dissociation with He and charge transfer with a monitor molecule, OZ. In both experiments, the ions were generated by a 30 eV electron-beam pulse of 0.5 ms duration. All ions of mass higher than 17, except (CH,),CO+ (5X) and (CH,)2COHf (59), were ejected. The possible role of the most abundant non-ejected ions (CH,+ , H2+) or partially ejected ions ( HzO+ ) will be discussed later. In our experimental conditions (acetone pressure of 10P6 Torr and drift time w0.5 ms ), the ratio 126
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of mass 59 to mass 58 entering the central cell is 0.044. The acetone partial pressure in the storage cell is less than IO-* Torr so that m/z= 59 formed during the storage time is negligible. In contrast, the ions resulting from reaction with the gas leaking from the third cell have to be ejected before the other ions are drifted into the reaction cell. The minimum transit time from the first to the third cell is 1.3 ms. After reaction for 10 ms in the third cell, the ions were migrated back to the central cell for mass analysis and quantification by FI’MS.
3. Results 3.1. CoNision-induced dissociation To characterize the long-lived excited ions, we first examined their collision-induced dissociation into acetyl ions (m/z= 43) as a function of collision energy. In this experiment, the ions were drifted into the third cell 1.3 ms after ion formation, then energized for 0.2 ms with a variable amplitude rf excitation pulse at the acetone-ion cyclotron frequency. After 10 ms of reaction time in the presence of 4~ lo-’ Torr of He, the ions were drifted back and detected in the central cell. The mean kinetic energy imparted to resonant acetone ions is given by Ei,,= (eVt/d)2/8mi
,
(2)
where I/is the (variable) amplitude ofthe excitation pulse, t is the excitation time (0.2 ms in our experiments), d the distance between the two drift plates of the ICR cell (1 cm) and mi the mass of the ion [ 61. If we assume the neutral collider is at rest in the laboratory frame, the c.m. collision energy is calculated from EC.,. = &
Eion9
(3)
where mi is the mass of the ion and WI,is the mass of the neutral. This is the energy plotted as the abscissa in fig. 1. The effect of energy spread on the shape of the excitation function will be discussed later. Fig. 1 plots the ratio of acetyl ion to the initial acetone-ion intensities, 43/( 58j0 as a function of the
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$, O.lOll .. 72 8 ++ 8
0.05.
0
’
l
l
m
0
8 0
+’ 8
l l
8
o.oolc 0
:
, 0.30 0.40 0.10 0.20 (CH,),CO+KINETICENERGYCM kV)
Fig. I. Corrected ratio of acetyl ion to the initial acetone-ion intensities, as a function ofthe center-of-mass collision energy with He. The apparent threshold energy, 0.025 eV, corrected for thermal motion of He (see text) is ~0. I5 eV. Less than 7% of the ions undergo CID.
center-of-mass collision energy calculated from eq. (3). The denominator of this fraction is calculated rather than measured because the transmission of energized m/z= 58 drifted back into the central cell depends on their kinetic energy (i.e. on their cyclotron radii compared to the size of the ion funnels). Therefore, the unreactive (CHj)2COH+ ions were used as an internal standard, the normalized acetone-ion intensity was calculated by multiplying the measured m/z=59 by the ratio l/O.044 observed when no excitation of m/z= 58 is applied. A second correction to the experimental data is applied in plotting fig. 1. At zero excitation of m/ z= 58, the m/z= 43 intensity is 1.25%;this is the relative abundance of mass 43 detected 12 ms after acetone-molecular-ion formation. These acetyl ions originate from unimolecular decay and/or the collisional decomposition at thermal energy of the acetone ions occurring during the time interval from ion formation to detection in this experiment. This intercept value of 1.25Ohis subtracted from the ohserved m/z=43 intensity to generate fig. 1. The ap parent threshold is 0.025 eV and the intensity of the acetyl ions increases monotonically with increasing c.m. energy, levelling off at approximately 0.25 eV in fig. 1 and remaining constant up to 0.4 eV, the upper energy limit in these experiments beyond which ion losses become unacceptably large. The plateau level may be interpreted as proportional to the fraction of electronically excited states present in
12 April 1991
the reactant beam ( x 6.8% compared to the 10%estimated in ref. [ 3 ] ). The true excitation function is much sharper than depicted in fig. 1, which is a convolution of the energy spread in the ion beam with that of the neutral collider. The energy width of the reactant acetone ions is a function of thermal energy of the precursor acetone molecules and is broadened with ion energy by the random initial phase between the ion motion and the irradiating rf field. It is given by [ 7 ] (4j where v. is initial ion velocity, $ is the phase angle between the ion motion and the irradiating resonant rf field, and the other terms have already been defined. Doppler broadening resulting from thermal motion of the neutral has been discussed in great detail by Chantry [ 81 who showed that the energy distribution (for monoenergetic ion beams) is given by W,,,=(ll.lykTE,)“*,
(5)
where W,,, is the full-width half-maximum energy width, y is the mass ratio (mi/(mi+m,)>, k is the Boltzmann constant, T is temperature and E0 is the c.m. collision energy. The present example of heavy ion-light neutral represents an extreme case. At a collision energy of 0.1 eV, the fwhm energy spread arising from this effect is 0.16 eV; at 0.3 eV, it is 0.28 eV. The effect of thermal motion of both target and projectile was discussed by Chantry [ 81, who pointed out that the width of the distribution is given approximately by WL/Z= [ ( W~/Z)kgct + ( Wt/Z)&ojcctilel I” >
(6)
where ( W fz hawetis the fwhm energy spread of the target and ( W 1, 1) p+ojectile is the fwhm energy spread of the projectile (both expressed in the c.m. frame). Over the energy range of fig. 1, the spread in collision energy is dominated by the Doppler-broadening term, eq. (5). The phase-angle term of eq. (4) contributes + (eVt/d)vO and is the dominant factor in ( Wl/2)pmjwtilc.Substituting WW x (eVf/d)v, and converting this to the c.m. frame using eq. (3) leads to the conclusion that ( Wl/Z)pr+eile is about 43% of ( W,,,)target in eq. (6 ). Because of the square-term
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relationship in eq. (6) this will broaden the energy width by about 18% over the Doppler term alone. Although the quality of the data does not warrant a quantitative attempt at deconvolution, it is clear that the fig. 1 cross section follows most closely the case A model considered by Chantry [ 8 1, a step function rising steeply from threshold to a near constant value. In such cases, the “apparent threshold” obtained by linear extrapolation of the appearance energy curve is too low by about 0.6 W,,2(ET) where W,,, is the fwhm evaluated (e.g., using eq. (6)) at the true threshold energy, ET, The resulting value of ET is 0.135 eV. We, therefore, infer that the threshold energy for the excited-state acetone-ion CID reaction is about 0.15 eV and can be adequately represented by a step function. As discussed in our earlier publication [ 11, the thermochemical energy requirements for breaking the acetyl-methyl bond for ground-state acetone ions at 298 K is 0.65 eV, and the threshold energy for observing the lowest energy endothermiccollision-induced dissociation of groundstate acetone ions is of the order of 1.3 eV [ 3 1. Consequently, we may be sure that the excitation function shown in fig. I is characteristic of electronically excited acetone molecular ions rather than groundstate species. Next, we conducted a series of measurements using the CID reaction as the monitor for the composition of the reactant-ion beam as a function of delay time. The excitation level in cell three was set to impart 0.08 eV (c.m. frame) in the acetone ionhelium collisional activation. After a 10 ms reaction time, all ions then present in the third chamber were transferred back to the second chamber and analyzed by ftms. In fig. 2, the ratio of the acetyl ion (43) to total ion (43 + 58) intensity is plotted as a function of relaxation time prior to CID. A small amount of mass 43 independent of storage time appears on the curve. This signal may arise from slow dissociation of vibrationally excited (metastable) groundstate acetone ions or simply represent electronic noise pick-up at the acetyl-ion frequency coming from the source where these ions are continuously ejected. This constant signal has to be subtracted for the fitting of the curves to an exponential. The least-squares fit of the curve plotted in fig. 2 gives a lifetime for this population of 4 ms; repeat measurements gave values of 4:: ms. 128
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P 0.020. 8 ^, 5 0.015. l+ 8 Q 0.010. B ~0.005.
. . , . I r . 10
20
I
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RELAXATIONTIME (ms) Fig. 2. Ratio of the acetyl ion normalized to the total ion intensities as a function of the relaxation time prior to 0.08 eV (cm.) CID. The least-squares fit of the curve after subtraction of the plateau gives a lifetime of 4 ms.
3.2. Monitor-ion experiments An independent measurement of the acetone-ion excited-state lifetime using the monitor-gas method has been performed using oxygen as monitor gas. The principle of the monitor-ion technique has already been described and used to determine the radiative lifetime of various ions [ $61. The method is based on the assumption that at thermal energies, reactions like charge or proton transfers are much faster for exothermic than for endothermic processes. In the present experiment, the monitor ion is 0: resulting from the charge transfer of excited acetone ions with OZ.The ionization potential of O2 is 12.06 eV [9] while the first ionization potential of acetone is 9.7 eV and the second ionization potential (A state) is 11.9 eV [ lo]. Therefore, oxygen serves as a monitor for acetone ions containing more than 2.4 eV internal energy, possibly the first excited A state with more than 0.2 eV of vibrational and rotational energy. The amount of 0: ions produced in the third cell by the excited acetone ions depends on the storage time as shown in fig. 3. The shape of this curve reflects the overall decay rate of the energy in the excited acetone ions below 12.06 eV the minimum energy required to ionize OZ. If all the experimental conditions remain constant when the relaxation time is varied, and if the excited acetone-ions reaction rate with 0, does not depend on their internal energy
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4. Dim&on
0.000’~.
I
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RELAXATIONTIME (ms) Fig. 3. Normalized monitor-ion 0: intensity as a function of relaxation time. The least-squares fit of the curve after subtraction of the plateau gives a lifetime of 14 ms. Repeat measurements gave values of 142 3 ms.
provided
[O;]
the
reaction is exothermic, it follows that
[Gl t [(CH,),CO+]
=’ exp(-ts’7)’
(7)
where A is a constant depending on the amount of excited acetone ions entering the relaxation cell and on the reaction conditions in the third cell ( O2 pressure: 7x 10e6 Torr; reaction rate constant equals collision rate constant kc= 6.5 x lo-” cm3 s-’ and reaction time is IO ms), t, is the storage time and l/ r the overall decay rate. Fig. 3 shows the ratio of 0; ion signal to the total ion signal. The ratio is plotted as a function of ion storage time in the second cell prior to injection into the third cell where the reaction takes place. A small amount of 0: independent of storage time shows up as a plateau on the curve. This 0: signal results from the reaction of O2 with a stable ion, probably H,Of (CH: and H: are also present but would not generate 0: ), which is produced by ionization of the background gas in the source and is very difficult to eject completely. This plateau has to be subtracted to lit the curve with an exponential decay. The mean lifetime deduced from fitting the corrected curve in repeat experiments is 14 f 3 ms.
It is noteworthy that the lifetimes deduced from the CID experiment and the monitor-gas experiment are of comparable magnitude but differ from each other by about a factor of four. It is important to note that the two techniques sample ions with somewhat different internal energy content. As pointed out above, the monitor-gas O2 samples ions containing 2.4 eV of internal energy above the ground-state ionization potential, while the CID experiment samples acetone ions containing 2.9 eV of internal energy. This is the sum of the adiabatic XeA energy difference of 2.2 eV plus about 0.7 eV required to dissociate acetone on the ground-state surface. Consequently, the CID experiment (fig. 2) samples ions containing about 0.5 eV more energy than the monitor-ion experiment (fig. 3). It is not unreasonable that a difference in internal energy content of 0.5 eV might change the apparent lifetime by a factor of four. A lifetime of 4 & 1 ms also implies a significant number of unimolecular decays of the more highly excited acetone ions excited to this level during the ionsampling time of the present experiments. Finally, we note that fig. 1 indicates a threshold energy of the order of 0.15 eV for collisionally inducing the dissociation of excited acetone ions to acetyl ion and methyl radical. The model suggested by the CID study [3] and elaborated by the present research is depicted in fig. 4. Vertical ionization of acetone into the excited-state manifold deposits about 0.7 eV of vibrational energy [lo]. The collision of the excited acetone ion and He induces a curve-crossing mechanism which releases the adiabatic energy difference (2.2 eV) as translational energy. This interpretation relies upon the previous study of the reaction dynamics of electronically excited acetone in CID. The barrier height for inducing this transition is estimated from fig. 1 to be about 0.15 eV. Accordingly, this value is marked on the fig. 4 diagram as the barrier height for the curve-crossing transition. The internal energy of the electronically excited state is preserved as vibrational energy on the ground-state surface after the collision-induced curve-crossing occurs and is responsible for the dissociation into acetyl ion and methyl radicals.
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gratefully acknowledged. These measurements were carried out in January 1990 when Jean Futrell was Professeur Associt at the University of Pa& Orsay Campus: their hospitality is gratefully acknowledged.
References
Fig. 4. Schematic potential energy diagram summarizing energetits for collision-induced decomposition of the acetone molecular ion. Collision at and above 0.15 eV induces curve-crossing of electronically excited ions to the ground state with the release of 2.2 eV into translational energy. Some of the ground-state ions subsequently decompose into acetyl ions and methyl radicals, which requires a minimum energy of 0.65 eV.
Acknowledgement The support of this work by the National Science Foundation, CHE 8796291, and by the Air Force Oftice of Scientific Research, AFOSR 87-0390, is
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[I ] K. Qian, A. Shukla, S. Howard, S. Anderson and J. Futrell, J. Phys. Chem. 93 (1989) 3889. [2]A.K.Shukla,K.Qian,S.L.Howard,S.G.AndersonandJ.H. Futrell, Intern. J. Mass Spectrom. Ion Processes 92 ( 1989) 147. [ 31 K. Qiau, A. Shukla andJ. Futrell, J. Chem. Phys. 92 ( 1990) 5988. [4] K. Qian, A. Shukla and J. Futrell, Chem. Phys. Letters 175 (1990) 51. [ 51G. Mauclaire, M. Heninger, S. Fenistein, J. Wronka and R. Marx, Intern. J. Mass Spectrom. Ion Processes 80 ( 1987) 99. [ 61 S. Fenistein, M. Heninger, R. Marx, G. Mauclaire andY.M. Yang, Chem. Phys. Letters 172 (1990) 89. [ 71 J.H. Futrell, in: Dynamic mass spectrometry, Vol. 2, ed. D. Price (Heyden, London, 1972) pp. 97 ff. [S] P.J. Chan’try, J. Chem. Phys. 55 (I 971) 2746. [9] S.G. Lias, J.E. Bartmess, J.F. Liebman, J.L. Holmes, R.D. Levine and W.G. Mallard, J. Phys. Chem. Ref. Data 17 Suppl. I (1988). [lo] R. Bombach, J.P. Stadelman and J. Vogt, Chem. Phys. 72 (1982) 259.