5134
J . Phys. Chem. 1991,95, 5134-5146
Electronic-State Chromatography: Application to First-Row Transition-Metal Ions Paul R. Kemper* and Michael T. Bowers* Department of Chemistry, University of California, Santa Barbara, California 931 06 (Received: November 9, 1990)
Ion mobilities for first transition series atomic metal ions in helium have been measured. The mobilities of both groundand excited-electronic-stateions were measured as a function of temperature. Very large differences in mobility (as much as 40%) were found for different electronic states of the same ion. The largest mobility differences were observed between electronic states with different electron configurations (i.e., 3d" and 3dW14s).Electronic states within the 3dW14smanifold of states have slightly differing mobilities, while differences in mobilities between states with 3dnconfigurationscould not be resolved. These results are explained by differences in repulsion between the helium neutral and the ion 4s or 3d orbital electrons. Preliminary metal ion-helium clustering data support the proposed explanation. These large mobility differences, together with existing data showing the presence of certain electronic states, allow the quantitative determination of the principal electronic states present in the atomic metal ions. This, in turn, makes the realization of state-specific chemistry with these ions much easier. Deactivation of excited electronic states is also discussed. Only cases in which the excited electronic state has a Ar 3d" configuration and the ground state an Ar 3dF14sconfiguration are found to deactivate significantly in collisions with helium. This occurs in Mn+ and Fe+ and can be explained with a curve-crossing argument proposed by Armentrout.
I. Introduction Transition-metal-ion chemistry is currently an area of very active study.'V2 The chemistry is complex due to the variety of different electronic states, electron spins, and electron configurations in the different ions. The large number of low-lying electronic states gives rise to experimental complexity as well. Since the different electronic states have radiative lifetimes on the order of seconds3 and can have radically different reactivities, the question of which states are present is critical. The three common methods of producing atomic transition-metalions (laser ~aporization,'.~ surface ionization,2.6v' and electron impact) all produce a mixture of ground and excited electronic states.2 With electron impact ionization (EI), the number of excited states is often large and varies strongly with electron energy. The limited data available indicate that laser vaporization also produces a significant amount of electronically excited ions as ~ell.259~Only with surface ionization (SI)and resonant multiphoton ionization (REMPI)*" are the distributions well characterized. Several investigators have worked to determine the excited states that are present and their populations. The observance of an endothermic reaction is commonly used as a diagnostic. This gives a lower bound for the energy of one or more excited states but little or no information as to the ground- and excited-state populations (due to the unknown reactivities of the states). Translational energy spectroscopy (TES)I2-l5can identify particular ( I ) Gas Phase Inorganic Chemistry; Russel, D. H., Ed.; Plenum: New York, 1989. (2) Armentrout, P. B. Electronic State Specific Transition Metal Ion Chemistry. Annu. Reu. Phys. Chem. 1990, 41, 313. (3) All low-lying electronic states in these ions have either Ar 3d" or Ar 4s3d"' electron configurations. As a result, radiative transitions between these states are parity forbidden with lifetimes estimated to be on the order of seconds. See: Garstang, R. H. Mont. Not. R.Astron. Soc. 1962, 124, 321. Strobel and Ridge have measured the lifetime of the Mn+ % first excited state to be 5.8 i 0.7 s; see ref 53. (4) Cody, R. E.; Burnjer, R. C.; Reents, W. D.; Carlin, T. J.; McCrery, D.A.; Lengel, R. K.; Freiser, E. S. Int. J. Mass Spectrom. Ion Phys. 1980, 33. 37. (5) Loh. S. K.; Fisher, E. R.;Lian, L.; Shultz, R. H.; Armentrout, P. B. J. Phvs. Chem. 1989. 93. 3 159. (6j Sunderlin, L. S.; Armentrout, P. B. J . Phys. Chem. 1988, 92, 1209. (7) Halle, L. F.; Armentrout, P. B.; Beauchamp, J. L. J . Am. Chem. Soc. 1981. 103. 962. ( 8 ) von Heiden, G.; Kemper, P. R.; Bowers, M. T. Experiments in prog-
ress.
(9) Sanders, L.; Sappy, A. D.; Weisshaar, J. C. J . Chem. Phys. 1986,85, 6952. (IO) Sanders, L.; Sappy, A. D.; Weisshaar, J. C. J . Phys. Chem. 1987.91, 5145. (11) Hanton, S.; Sanders, L.; Weisshaar, J. C. J . Phys. Chem. 1989, 93, 1963.
0022-3654/91/2095-5134$02.50/0
excited states present but may be limited in the type of state observed. It also usually gives no information about populations. Elkind and Armentrout have reported a large number of experiments2Jb22aimed at characterizing the production (by SI and EI) and reactivity of ground-state and excited atomic metal ions. By determining the cross section vs kinetic energy function for ions formed by SI, where the excited-state populations presumably follow a Maxwell-Boltzmann distribution: state-specific cross sections can be obtained through a fitting procedure. The states that can be formed by SI are limited to rather low energy (50.5 eV)? however, and to investigate higher energy states, ions must be formed by electron impact (EI). In these experiments, reactive states are detected by their thresholds in the cross section vs kinetic energy curve (excitation function); however, the effect of a state on the excitation function depends on the state's population and reactivity, both of which are unknown. Furthermore, any nonreactive states present do not appear, except as an overall reduction in cross section. Populations of low-energy states in E1 experiments can be estimated by comparing E1 and SI data. Weisshaar has used REMPI to form essentially pure populations of vanadiume1' and iron23ground-state and excited ions, even to the point of producing a particular J level. The drawback to the REMPI technique is its complexity and difficulty. The need remains then for a simple method of producing and characterizing populations of electronically excited metal ions. It is known that ground and excited electronic states of the same ion often have significantly different mobilities. This has been shown for C+ and O+.% The differences are typically 5-20%,. 24325
~~
(12) CollisionSpectmcopy; Cooks, R.G., Ed.; Plenum: New York, 1978. (13) Illies, A. J.; Bowers, M.T. Chem. Phys. 1982, 65, 28. (14) Hanratty, M. A.; Carter, E.A.; Beauchamp, J. L.; Goddard, 111, W. A.; Illies, A. J.; Bowers, M. T. Chem. Phys. Lett. 1986, 123, 239. (15) Hanratty. M. A.; Beauchamp, J. L.; Illies, A. J.; van Koppen, P.A. M.; Bowers, M.T. J. Am. Chem. Soc. 1988,110, 1. (16) Elkind, J. L.; Armentrout, F. E. J . Phys. Chem. 1985, 89, 5626. (17) Elkind, J. L.; Armentrout, P. B. J . Chcm. Phys. 1986, 84, 4862. (18) Elkind, J. L.;Armentrout, P. B. J. Phys. Chem. 1986, 90,5736. (19) Elkind, J. L.; Armentrout, F. E. J . Phys. Chem. 1986, 90, 6576. (20) Elkind, J. L.; Armentrout, F. B. J . Chem. Phys. 1987, 86, 1868. (21) Elkind, J. L.; Armentrout, P. B.Int. J . Mass Spectrom. Ion Processes 1988,83, 259. (22) Elkind, J. L.; Armentrout, P. E. J . Phys. Chem. 1987, 91,2037; this is a review which summarizes much of refs 16-21. (23) Weisshaar, J. C. Proc. 38th Am. Soc. Mass Spectrom. Conj.,Tucson, AZ. 1990. (24) Twiddy, N. D.; Mohebati, A.; Tichy, M. Inr. J . Mass Spectrom. Ion Processes 1986, 74, 25 1. (25) Grice, S. T.; Harland. P. W.; Maclagan, R.;Simpson, R. W. Inr. J. Mass Spectrom. Ion Processes 1989, 87, 18 1.
0 1991 American Chemical Society
The Journal of Physical Chemistry, Vol. 95, No. 13, 1991 5135
Electronic State Chromatography We have found that different electronic states in a given transition-metal atomic ion often have mobilities as much as 50% differenta2’ The valence electronic configurations in these ions are either 3d” or 4~3d”’.~*States with different configurations exhibit the largest mobility differences; however, mobilities of electronic states within the 4s3dw1 configuration can also be significantly different. When mobilities of electronic states are significantly different, the spatial and temporal distributions of the states separate as they diffuse through the buffer gas. Consequently, if all of the electronic states enter the drift cell at the same point in time, they will arrive at the exit slit, and hence the detector, at different times. Since it is the differing interactions of the states with the buffer gas that cause the temporal separation, we coin the phrase “electronic-state chromatography”. The result is a greatly improved and, in many cases,an absolute determination of ground- and excited-state populations. A better determination of excited-state populations in turn allows further investigation of state-specific reactions. Further, the mobilities of these ions are interesting in their own right. Only one transition-metal ion mobility has been measured previously (Ti+),29 and the dependence of mobility on temperature or field strength (effective t e m p e r a t ~ r e ) ’ ~can ’ ~ be used to determine the interaction potential between the ion and buffer gas.25.32 In this paper we will describe the experimental method and the physical basis for the temporal resolution we observe between differing electronic states of the same ion. These techniques will be applied to the first-row transition-metal series. Examples of two of the applications of the technique will also be given: deactivation of electronically excited states via collision with He and state-specific clustering equilibria with He. 11. Experimental Section A . The Instrument. A detailed description of the apparatus and experimental technique has appeared elsewhere,” and only an overview is given here. The atomic metal ions are formed by electron impact on high vapor pressure, metal-containing compounds. Ion source pressures and residence times are low enough to reduce the fraction of ions that undergo collisions in the source to 55%. Electron-accelerating voltages are measured from the center of the filament to the source body. The interior of the source is field-free except for minor penetration of the ion-accelerating voltage. The resulting ions are mass-selected in a double-focusing mass swrometer, decelerated to a kinetic energy of 2-3 eV, and focused into a reaction cell containing 1-5 Torr of buffer gas (in this case helium). Cell entrance and exit holes were 0.5 mm, and the drift length was 4.00 cm. The ions are quickly thermalized by collisions with He both outside and immediately upon entering the cell.33 The ions are then drifted through the cell with a uniform electric field. The effect of the field on the ion energy is discussed below. Ions that exit the cell are quadrupole mass analyzed and collected. The cell temperature is variable from -80 to 600 K; temperatures of 150 and 305 K were used in these experiments. B. Mobility Experiment. To measure mobilities, the massselected ions are pulsed into the drift cell (pulse width 1-3 ps).
-
-
(26) Rowe, B. R.; Fahey, D. W.; Fehsenfeld, F. C.; Albritton, D. L. J.
Chem. Phys. 1980, 73, 194.
(27) Kemper. P.R.; Bowers, M. T. J . Am. Chem. Soc. 1990.112. 3231. (28) (a) Moore. C. E. Afomic Enerpv Leuels: US. National Bureau of Standardsf Washington, DC, 1952; Cirz 467 (UiS. Natl. Bur. Stand.). (b) Sugar, J.; Corliss, C. J . Phys. Chem. Ref. Data 1977,6, 317. (c) Ibid. 1978, 7, 1191. (d) Ibid. 1979.8, 1. (e) Ibid. 1980, 9,413. (f) Ibid. 1981, 10, 197, 1097. (g) Ibid. 1982, 1 1 , 135. (29) Johnsen, R.;Castell, F. R.; Biondi, M. A. J . Chem. Phys. 1974, 61, 5404. (30) (a) Ellis, H.W.; et al. Transport Properties of Gaseous Ions Over a Wide Range. At. Nucl. Dafa Tables 1976, 17, 177; 1978,22, 179; 1984, 31, 113. (b) Lindinger, W.; Albritton, D. C. J . Chem. Phys. 1975, 62, 3517. (31) McFarland, M.; Albritton, D.L.; Fehsenfeld, F. C.; Ferguson, E. E.; Schmeltekopf, A. L. J. Chem. Phys. 1976,59, 6610. (32) MeDaniel, E. W.; Mason, E. A. The Mobilify and Dij/usion of Ions in Gases: Wiley: New York, 1973. (33) Kemper, P.R.; Bowers, M. T. J. Am. Soc. Mars Spectrom. 1990, I , 197.
TABLE I: Measurement of the Temperature Dependence of R e d d Low-FieldAr+ Mobilities in Helium ____
temp, K 17 82 180 302
KO(this work)‘ 17.9 f 1.4
*
19.0 1.5 20.9 f 1.5
K,,(literature)‘b 18.8
1.3
20.8
1.4, 20.5
* 1.O
“In units of cm2/(V.s). *Reference 30.
The pulse simultaneously triggers either a multichannel scalar (MCS) scan or a time-to-pulse-heightconverter (TPHC) ramp. Ions that exit the cell are then collected as a function of time, giving an arrival time distribution (ATD). Scans collected by using the TPHC have better resolution (-0.4 @/channel), but collection rates are limited by sampling statistics to S5% of the ion input pulse frequency of 1-5 kHz (depending on the TPHC scale). The MCS scans have less resolution (22 ps/channel), but collection rates up to -3 X lo6 counts/s are allowed. Ions that have different mobilities have different drift times through the cell and appear as different peaks in the arrival time distribution. Mobilities are determined in our experiment by plotting the arrival times of the various peak centers vs 1 / V (V is the drift voltage a c r m the cell).M This gives an extremely linear plot with a slope inversely proportional to the ion mobility ( K )
where ud is the ion drift velocity, E is the drift field, td is the drift time (arrival time), and z is the cell length. The intercept of the td vs 1/Vplot is the time spent in the quadrupole. These mobilities are converted to reduced mobilities (KO)to remove the first-order effects of helium density (N)”s’~ P 273.15 KO=K-760 T where P is the He pressure in Torr. Our method of analysis, using the slope of the arrival time vs 1 / V plot, makes three main assumptions: (1) K is independent of the drift field, (2) the center of the ATD peak accurately represents the drift time of the Gaussian ion packet, and (3) the ion packet does not penetrate significantly into the cell before thermalization. Regarding (l), it has been shown that the effect of the ion drift velocit on K can be represented by a change in an “effective t e m p e r a t ~ r e ” ’ ~ ’ ~
(3) where Tis the thermodynamic temperature, M Bis the buffer gas mass, and k is Boltzmann’s constant. At the highest E / N values used in our experiments ( E / N I 8 X lo-’’ cm2.V = 8 Td at 305 K and I 4 Td at 160 K) Teffis calculated to be 348 and 172 K (respectively), about a 10% increase. It was necessary to use these high E / N values to resolve the ATD peaks. Changes in thermodynamic temperature ( T ) from 150 to 305 K produced changes in mobility of 97% of the excited Fe+. Perhaps more interesting is the reason why deactivation only occurs in the Fe+ and Mn+ systems. These are the only cases where an excited state of 3d" configuration is deactivated to a 4s3d"' ground state. Excited 4s3d"' states do not collisionally deactivate at thermal energies. This finding is nicely explained by a model proposed by Loh et aL5 Consider the difference in
-
Kemper and Bowers potential curves for a M+-He collision when the M+ ion has a 4s3dwl configuration and when it has a 3d" configuration. In the 4s3d"l case, the potential energy curve will become repulsive at larger internuclear distance than with the 3d" configurations, due to the repulsion between the 4s electron and the filled ls2 shell of He. Thus, if the M+ ion has a 4s3d-I ground state and a 3d" excited state, the potential energy curves for the two states in the collision will cross, perhaps a t a low collision energy, providing a means for deactivation in the collision. For 3dn ground states and 4s3d"I excited states, however, the curves will not cross until much higher collision energies and consequently excited 4s3d"' configurations should not easily deactivate, as we observe experimentally. This model suggests that higher collision energies might promote deactivation of excited 4s3d"' states. This was investigated in our experiment by using very high drift fields. Both V+ and Co+ showed substantial increases in deactivation at high drift fields. We are investigating the other M+-He systems for kinetic energy dependent deactivation as well as trying to quantify the observed energy dependencies. D. Clustering of M+ and He. We have examined the clustering equilibrium between the metal ions and He, Ne, and Ar. The study is not complete, but we report here the results with He since they provide a further example of the effect of electron configuration on the M+-He interaction. Values of the equilibrium constant Kp for the process M+ + He
F?
M.He+
(6)
were measured at temperatures between 150 and 400 K for those M+ which formed clusters. In order to calculate Kp, we need to know what fraction of the M+ ions are actually clustering, Le., which M+ states form clusters. This is determined by comparing the ATD spectra of the M.He+ cluster and the parent M+ (see Experimental Section). At equilibrium, the parent and product spectra are superimposable (not in size, but in shape and apparent mobility). Thus, the cluster ATD peak will coincide with the parent M+ ATD peak, giving direct experimental confirmation of the M+ state forming the cluster. Our experiments show that only the states with 3dn configuration clustered with He; no clustering attributable to 4s3d"' states was found. Clusters were observed for V+, Cr+, Co+, and Ni+. Not enough signal was available to check for Cu+ clustering. Clusters were not found for Ti+, Mn+, and Zn+. Again, this strong division of a basic chemical property of M+ based on electronic configuration must be due to the 4s electron. The M+ species with 4s3d"' configurations have an increased repuslive interaction between the M+ and He electron clouds, with average van der Waals radii overlapping at -2.5 A. This overlap will reduce the M+-He binding by greatly increasing the equilibrium internuclear distance and thereby decreasing the charge-induced dipole attraction between M+ and He. Thus, the equilibrium results parallel the mobility results presented earlier. These results are also in agreement with the theoretical studies of Hammond et al." and Bauschlicher and L a n g h ~ f f .Ham~~ mond et al.'s calculated binding energies for M+-Ar ( M = Cr, Mn, Fe, Co, Ni, and Cu) showed that the 3d" configurations were bound 3-3.5 times stronger than the corresponding 4s3d"l configurations. Further, they found that the internuclear distances were much greater in the 4s3d"' configuration ion clusters. They and Bauschlicher and Langhoff also note that the M+-Ar binding is almost entirely electrostatic. The case of Fe+ is interesting: Fe-He+ clusters were observed, in apparent contradiction to the above argument. We believe, however, that these clusters are due to the excited Fe+ 3d' state(s). We have been unable to collect ATDs in this case due to low signal levels. The excited Fe+ is almost nonexistent in these experiments ~
~~~~~~
(44)(a) Hammond, B. L.; Lester, W. A.; Braga, M.; Taft. C. A. Phys. Reu. E 1990, 41 (IS), 10447. (b) Braga, M.;Almeida, A. L.; Taft, C. A.; Hammond, B. L.; Lester Jr., W. A. J . Chem. Phys. 1988,89, 4867. (45)Bauschlicher, C.W.;Langhoff, S. R. Chem. Phys. Lerf. 1988, 149, 10.
Electronic State Chromatography
The Journal of Physical Chemistry, Vol. 95, No. 13, 1991 5143
TABLE V Summrn of Existing Tnnritioa-Metal-Ioa Excited-State PowlrHon Data (See Appeadix B for Dbcusclh) 5% of ions ion state configuration energy" at 30 eV Ti+ (TiCI4) a4F 4s3d2 0.028 I 73 1 b J b4F 3d3 0.135 9 3" a2F 4s3dZ 0.593 stat&) with 1.1 C E C 1.24 eVb: 4s3d2 1.082 a2D a% 3d3 1.124 18 & 4" a4P 3d3 1.172 a+ 3d3 1.232 b4P 4s3d2 1.236 stat&) with E >, 2 eV obs, smallb stat+) with E > 3.6 eV obs, smallb
at 50 eV
* *
1
v+ (VOCI,)
Cr+ (CrO2CI2)
Ni+ (Ni(CO),)
Cu+ Cu(1)Ac
Zn+ (Zn)
a5D 3d4 a'F 4s3d3 a3F 4s3d3 states with 1.48 IE C 1.9 eV: 3d4 a3P a3H 3d4 b3F 3d4 asp 4s3d3 a3G 3d4 states with E > 2.4 eV: a'G 4s3d3 b3P 4s3d3 all 3d4 a% 3d4 c3P 4s3d3
0.0215 0.363 1.104 1.452 1.566 1.68 1 1.692 1.807 2.370 2.314 2.319 2.468 2.509
1
1 I
40-
-0-
45,dJI f 4cd
75,dJ 4
3"
15,dJJ 12 f 4W
0.2,dJ 0.13'5
80,"h 70-8oCJ" 18,"J obs'Y
6WA 38,"J ob&
a6S a6D a4D a4G ?
3d' 4s3d4 4s3d4 3d5 ?
0.000
a% a6D a4D a4G
0.000
203* 30
1.522
obs: 76hJ
?
3d' 4s3d4 4s3d4 3d' ?
a7S ass a5D
4s3d5 4s3d5 3d6
0.0
7oc"
1.175 1.808
5*fl
IW"
1.78"
3.4,g" obso
a6D a4F a4D a4P a%
4s3d6 3d7 4s3d6 3d7 3d7
0.052 0.300 1.032
a3F a'F b3F a3P a5P b3P
3d8 4s3d7 4s3d7 3d8 4s3d7 4s3d7
0.086 0.515 1.298 1.655 2.228 2.998
a2D a4F a2F b2D
3d9 4s3d8 4s3d8 4s3d8
0.075 1.160 1.757 2.899
obs' obs'd obs, very small'
a's a3D alD
3d1° 4s3d9 4s3d9
0.00
N
1.522 2.458 2.543 23.24
2.458 2.543 23.24
1.993
2.808 3.251
1
l.3,8*hJ
1.4 f 0.5,"'.'
2ghJ
obs (small)hJJ"
1
* 6'sW
4,g*h*robs'
obs (sma1Qha 5oc"
4oc' 16c'
1
-
50%:'
obs'
50%:'
obsx
obs, very small'
-
1
2oc'
6OCI.f
obs'
4s3d'OY 0.000 100' 1 00' 4s23d9 7.909 4 P 4s4p3d9 12.978 ORefcrcnce 28; in eV; averaged over J levels. bReference 52. CFromcomparison of E1 and SI cross-section data for this state; should give absolute population. dReferencc 16. 'Reference 51. 'Assuming equal reactivities for all states-better viewed as a reaction probability times a population. 'Assuming 10046 reaction efficiency (Le., Langcvin rate), thus these are minimum populations. hReference 20. 'Reference 50.70 cV. )Not observed directly; % = 100 - I:(all other states). kReference 14. 'Reference 2. mRefercncc48. "Reference 17. ORcference 53. PReference 18. qReference 5. 'Reference 19. 'Reference 15, Co+ from Co(CO),NO. 'Reference 54. "Only metastable states are listed. 'From comparison of E1 and SI data assuming identical excited-state populations are produced with El and SI. Unknown error. WFromfitting ICR reaction data, should give absolute population. From comparison of apparent reaction rate with known rate, should give absolute population. Y Reference 49. 2s
2D
due to deactivation, and the clusters are correspondingly small. W e hope to repeat the experiment and determine the Fe+ state(s) responsible for the cluster.
The values of W 0 and ASo for the various clustering reactions a r e given in Table 111. Reaction enthalpies are typically 1-4 kcal/mol. For comparison, the pure electrostatic (point charge
5144 The Journal of Physical Chemistry, Vol. 95, No. 13, 1991 induced dipole) attraction between M+ and He at a separation of 2 A is 2 kcal/mol. Although this is only a very rough estimate of the true value, it indicates that the binding between the M+ and He is largely electrostatic, as expected. By comparison, Lessen and Brucat" have measured the M+-Ar binding energies for Ni+ and Co+ to be 12.7 and 11.7 kcal/mol, respectively. The reaction entropy can be calculated from statistical mechanics. Assumptions must be made as to internuclear distance and vibrational spacing which introduce some uncertainty. However, for a bond length of 2 A and a vibrational spacing of 500 cm-', AS" is calculated to be -15.4 cal/(mol.K). This is in agreement with our range of experimental values from 13.1 to 17.5 cal/(mol.K). Accurate experimental values for AS" will allow evaluations of molecular rotational constants and vibrational frequencies calculated from theory. The moo values measured in our equilibrium experiments are for 3d" configurations clustering with He. The corresponding A H " 0 for the 4s3d"' configurations must be much smaller, and the clusters may not even be bound at the zero-point energy. Consequently, these very small binding energies may be impossible to measure via equilibrium studies. In this case, an alternate technique may be to measure the mobilities as a function of temperature and fit the results with calculated mobilities using different binding energies.'Z4' The mobilities of the 4s3d"' ions generally show large variations with temperature, and examining the temperature dependence should yield important information on the interaction potential.
Note Added in Proof. More detailed analysis of the M+-He data supports the general conclusions given here, but indicates the actual values of Moo are 0.2-0.5 kcal/mol less negative than those reported in Table 111. These results will be reported elsewhere.56 V. Summary and Conclusion 1. The mobilities in He of atomic transition-metal ions can be used to separate electronic states with different electron configurations (4s3d"l and 3d") and, in many cases, states within the 4s3d"' configuration. Consequently, we coin the term 'electronic-state chromatography". The large difference in mobility between configurations is due to the repulsion between the 4s electron and the filled He Is2 electron cloud. The result is a reduced attraction between the metal ion and helium which leads to an increase in mobility for the states with a 4s3dW1configuration. 2. Separation of the metal ion electronic states (using their different mobilities) coupled with previous work determining the electronic states present in these ions allows a quantitative determination of the populations of the major states present in most of the ions. The present data consider only ions formed by electron impact, but the technique is applicable to all forms of ionization. 3. Also determined (for the first time in most cases) were the reduced, zero-field mobilities of the different electron configurations/states. The states with 3d" configuration had in all cases mobilities very close to the Langevin mobility. These mobilities showed little temperature dependence. States with 4s3d"' configurations had mobilities 4 M O % higher than Langevin. There was considerablevariation in these mobilities from metal to metal, and a large temperature dependence was usually observed. 4. Collisional deactivation of excited metal ions was observed only for Mn+ and Fe+ between excited states with 3d" configurations and 4s3d"' ground states. This was explained in terms of a model proposed by Armentrout and co-~orkers.~ 5 . Clustering between M+ and He was observed only for ions with 3d" configurations. This observation was also explained in terms of repulsion between the M+ 4s electron and He. Equilibrium studies at different temperatures showed that cluster binding energies ranged from 1.3 kcal/mol (Cr+) to 3.68 kcal/mol ((20'). The corresponding entropies ranged form -13.1 cal/ (46) Lessen, D.;Brucat, P. J. Chem. Phys. Lett. 1988,152,473; J . Chem. Phys. 1989, 90,6296. (47) Takebe, M.J . Chrm. Phys. 1983, 78, 7223.
Kemper and Bowers (mo1.K) (Cr+) to -17.5 cal/(mol-K) (Co+).
Acknowledgment. The support of the National Science Foundation under Grant CHE88-17201 is gratefully acknowledged.
-
Appendix A The metal ion mobilities were measured at 160 as well as 305 K for two reasons: (1) to examine the temperature dependence of the mobilities and (2) to increase the resolution in the ATD spectra. This resolution can be calculated in the limit of zero input ion pulse width
where At, is the time width of the exiting ion packet and Az is the one-dimensional spatial width of the packet as it exits. The root-mean-square, one-dimensional displacement is easily calculated32to be
Az = 2(7)'12 = 2(2Dr)'12
(A4
where D is the diffusion coefficient and t is the time in the cell. If one uses the Einstein relation between mobility and LP2 (applicable at low E / N ) K = qD/kT
('4.3)
where q is the electron charge, and remembering vd = K E , the resolution becomes resolution = (qV/8kT)'12
('4.4)
Equation A.4 indicates the path to higher resolution: higher extraction voltage and lower temperature. The input ion pulse time width must also be much less than the time width of the arrival time distribution. Unfortunately, some limitations apply. The above calculation assumes that mobility is not a function of temperature. In the absence of dipole or quadrupole potential terms (proportional to l / S and 1/9), the charge-induced dipole potential (1/#) becomes dominant as the temperature approaches 0 K. Thus, mobilities tend to converge to the charge-induced dipole value (Langevin mobility) with decreasing temperat~re,'~ and the different mobilities become harder to resolve. (This was clear in data we took on Fe+ at 77 K.) Consequently, 160 K temperature was chosen as a compromise temperature. Second, the drift voltage (V) cannot be made arbitrarily large. If the cell pressure is not increased proportionally, the resulting ion drift energy will perturb the approximately thermal energy conditions in the cell. The mobility will then strongly depend on the ion energy, and our mobility analysis breaks down. Thus, in order to increase resolution via higher drift voltages greater cell pressures are required, and the limits of the vacuum system are won reached.
Appendix B. State Assignments In this appendix a discussion is given of how we assigned the state populations given in Table IV. A summary of useful literature data is given in Table V. 1. Chromium. The ATD data (Figure 5 ) show three peaks; the lowest mobility peak contains the a% 3dSground state. Peaks I and I1 have a 4s3d4 configuration. Peak I1 (the smallest) was the highest in energy. Armentrout and cc-workers have identified reactions of the a6S 3d5 ground ~ t a t e , ~ ~the' *a6D ~ ~4s3d4 * ~ first excited ~ t a t e , the ~ . a4D ~ ~ 4s3d4 second excited state (or possibly the 4G 3d5 third excited and a small amount of a state(s) with energy -3.2 eV,2*48 all in Cr+ formed by electron impact on Cr(C0)6 and CrO2Cl2. By comparing SI and E1 data, they determined the fraction of ground state present. The individual excited-statepopulations could not be determined because (48) Georgiadis, R.;Armentrout, P. B. J. Phys. Chrm. 1988, 92, 7067. (49) Fisher, E.; Armentrout, P. 9. J. Am. Chem. Soc., submitted for
publication.
The Journal of Physical Chemistry, Vol. 95, No. 13, 1991 5145
Electronic State Chromatography of their unknown reactivity. Instead, ranges of possible populations were given (see Table V). Hanratty et have identified the a6D in TES studies of Cr+. Finally, Reents et al.so and Halle et al.' have seen endothermic reactions of electron impact formed Cr+ with CH4 and ascribed them to the a4D state. Reents also determined that -30% ground-state Cr+ was formed with 70-eV E1 ionization on Cr(C0)6. From these previous experiments it appears there are three major states formed by EE on Cr(C0)6 and Cr02C12. There are also three peaks present in our ATD spectra. The slowest (peak 111) must contain the a6S 3dS ground state plus possibly small amounts of highly excited states with 3dS configurations. The intermediate-energy peak (peak I) is then the a6D 4s3d4 first excited state, and the small (-7%) high-energy peak (peak 11) is the 4D 4s3d4 second excited state. A r m e n t r o ~ t ~ also * ~ ~es*~* timates the a4D population to be small (22%. although this is based on an assumption of unit reaction efficiency; see Table V). The derived state populations at 30 and 50 eV are summarized in Table 1V. 2. Vanadium. Again we see three peaks in the ATD spectrum (Figure 3). The asD 3d4 ground state is contained in the lowest mobility peak (111). Peaks I and I1 have 4s3d3 configurations, and peak I is highest in energy. Armentrout and co-workers have identified reactions of the aSD 3d4 ground state,'6J' the a3F 4s3d3 second excited state,l6J1an unknown state with energy 21.48 eV" (energy probably 1.65 eV),I6 and a small amount of highly excited state(s)'"' with energy 22.4 eV (Table V). Any reactivity due to the aSF4s3d3first excited state could not be distinguished from the ground state in their experiment. The population estimates for the excited states depend greatly on the reacting system, presumably due to different reaction efficiencies; the estimates from the V+ CH4 study5' are derived by comparing SI and E1 data and should be more reliable. As noted, peak 111 must contain the aSD3d4 ground state. Two other states with significant populations are known to exist (the a3F 4s3d3 second excited state and a state with -1.65-eV energy). It ap ars reasonable to assign peak I (4s3d3 configuration) as the a p" P state (4s3d3, 1.69 eV) and peak I1 (4s3d3)as the a3F4s3d3 state. Armentrout could not assign the state at 1.65 eV since his measurement of the energy was not sufficiently precise. However, the aSP is the only state in that range with a 4s3d3 configuration (required to match the observed mobility). The ATD peak from the small amount of highly excited (>2.4 eV) state either is too small to detect or is superimposed on one of the other peaks. This assignment gives state populations in serious conflict with Aristov and Armentrout's a~signment,~' however. They estimate that at 50-eV ionizing energy the fraction of ground state is -0% the fraction of SF(second excited state) is 4 f 36, and the fraction of the E 1 1.48 eV state is 32 f 11%. Our initial assignment would give ground state -20%,3F -30% and E 1 1.48 eV state -50%. In addition, Aristov and Armentrout believe the 1.48-eV state is one or more of the 3P, 3H, and/or b3F states (all 3d4 configuration) rather than the 5P (4s3d3configuration) required in the above assignment. An alternative (more complicated) assignment of our ATD peaks can resolve much of the conflict. Peak I11 (20% at 50 eV) is composed of the 5D 3d4 ground state (-0% at 50 eV) plus one (or more) of the 3P, 3H, or b3F states ( - 3 2 f 11%at 50eV). PeakII(-30%at 50eV)iscomposed of the 'F 4s3d3 second excited state (-4%) plus a substantial amount of the SF4s3d3 first excited state. Peak I (-50% at 50 eV) is then a high-energy, 4s3d3 state, perhaps the E > 2.4 eV state found by A r m e n t r ~ u t . ' ~This . ~ ~ assignment is considerably more complicated than the one first presented. It also assumes the existence of large amounts of the SFfirst excited state which has not been observed (perhaps due to low reactivity). It is, however, consistent with the data of Aristov and Armentrout, and this assignment was used to derive the populations in Table IV.
-
+
-
(50) Recnts, W. D.; Strobel, F.; Freas, R. B.; Wronka, J.; Ridge, D. P. J . Phys. Chem. 1985,89, 5666. (51) Aristov, N.; Armentrout, P. B. J . Phys. Chem. 1W7,91, 6178.
Very recent measurements by Clemmer and Armentrouts5of the 3Fand )P populations are in very good agreement with those given in Table IV. Despite this, the excited-state populations in vanadium are probably the least well characterized of all of the first-row transition-metal ions. 3. Titanium. The ATD spectrum of Ti+ again shows three features (Figure 1) with peak I (highest mobility) containing the a4F 4s3d2 ground state. Peak I formed 80-908 of the ionization at 30 eV and -45% at 50 eV. The relative proportions of peaks I1 and 111 are less certain. As noted above, apparent peak heights of I1 and 111 in the lower resolution 300 K spectra were assumed to represent their relative populations. Peak I11 appears highest in energy. The mobilities of peaks I and I1 correspond to 4s3d2 configurations. Peak 111 probably does as well; however, its mobility (19.7 cm2/(V.s)) is the lowest of the 4s3d" configuration mobilities, and it is conceivable that peak 111 represents a 3d3 configuration. This would make its mobility the largest (by far) of the 3dn mobilities, and as discussed above, we do not expect such large variations in these 3d" mobilities. Our analysis assumes, however, that peak 111 could represent a state either in the 4s3d2 or in the 3d3 manifold. Armentrout and c o - ~ o r k e r s ~see , ~reactivity ~ , ~ ~ due to the a4F 4s3d2 ground state (possibly together with the b4F 3d3first excited state at 0.135 eV), to the a2F 4s3d2 second excited state (0.593 eV), to one or more of a group of states between 1.1 and 1.24 eV:J2 to a small amount of a state(s) with E > 2 and finally to a small amount of a state(s) with greater than 3.6 eVS2(see Table V). We can exclude the presence of significant amounts of the first excited state since it would have to correspond to peak I1 (next in energy above the ground state). Peak I1 definitely corresponds to a 4s3d2configuration-not the b4F 3d3 first excited state. This leaves three main reacting states to match with our three ATD peaks. Peak I contains the ground state and possibly small amounts of higher energy states (>2- and/or >3-eV states). Peak I1 is then the a2F 4s3d2 second excited state since it is next in energy to the ground state and cannot be the first excited state. Peak 111 then is due to a state or states between 1.1 and 1.24 eV: most probably either the a2Dor b4P (4s3d2) but possibly the a%, a4P, or a2P (3d'). The very high energy states (>2, >3 eV) seen by Armentrout either are contained within one of these peaks or are too small to observe. Sunderlin and Armentrouts2 estimate the population of the 2F (second excited state) to be 9 f 4% at 30-eV ionizing energy. This was done by comparing the E1 data with SI data where the populations should be known. Our fraction of peak I1 at 30 eV is -22%. However, this fraction is almost certainly too large. Peak I1 is not well resolved from the trailing edge of the much larger peak I; thus, part of the apparent peak height of I1 is due to peak I. The disagreement between our data and Sunderlin's data is probably not severe. Sunderlin and Armentrout's estimate of the 1.1-1.2-eV excited-state population is based on an assumption of equal reactivity between the E1 and SI generated excited states.s2 Since completely different states may be populated with the two techniques, this estimate has considerable uncertainty. Their estimate of the ground-state populations is simply the difference between the excited-state estimates and the total populations2 thus and has comparable uncertainty. 4 . Mungunese. The ATD spectrum of Mn+ (Figure 7) shows two peaks clearly corresponding to 4s3d5 and 3d6 configurations. The a7S 4s3dSground state is contained in the higher mobility peak. The fraction of 3d6 excited state(s) observed varies from 2 to 12% with 25-100-eV ionizing energy. Elkind and ArmentroutI7 see reactivity due to the a7S 4s3dS ground state as well as the aSS 4s3ds first excited state and the aSD 3d6 second excited state. They estimate the ratio of sS/SD population to be -3 for ionizing energies greater than 30 eV. This assumed equal sSand 5D reactivities, which was consistent with their SI data. They also note that higher energy unreactive states may also be present. Strobe1 and Ridges3 have also identified (52) Sunderlin, L. S.; Armentrout, P. B. J . Phys. Chem. 1988, 92, 1209.
5146 The Journal of Physical Chemistry, Vol. 95, No. 13, 199 1
reactions of the aSSstate (and measured its radiative lifetime). On the basis of this work, we assign peak I in our ATD spectrum to the a7S ground state plus the a5S first excited state. Peak I1 is then the a5D second excited state. Our resolution is not good enough to directly determine the amount of a s s excited state in peak I; however, using Armentrout's sS 5D ratio of -3.0 gives -52% ground state, 36% 5S, and 12% D at 95 eV (Table IV). This is in good agreement with Elkind and Armentrout's value of 50% ground state at 50 eV (derived by comparison of E1 and SI cross sections). Their estimates of the 5S and 5D populations are based on unit reaction efficiency and thus represent minimum values. 5. Iron. The ATD spectrum of Fe+ (Figure 8) shows two widely separated peaks with mobilities typical of 4s3d"' and 3dn configurations. The a6D 4s3d6 ground state is contained in the higher mobility peak I. Armentrout and co-workers have observed reactions due to the a6D 4s3d6 ground state, the a4F 3d7first excited state, and a state or states at -1.85 eV (probably the a% 3d7).z5J8Reactivity (with H,) of the a4D 4s3d6 second excited state and the a4P 3d7 third excited state was not directly observed but could be hidden under the onsets of the a% and a4Fstates.I8 ReentsMhas also observed state(s) with energy greater than the a4Fin reactions with CH30H (Table V). Comparison of E1 (50 eV) and SI data for the Fe+/H2 systemla indicated that -44% of the Fe+ was formed in the 4D or higher states. The same comparison in the Fe+/02 system5 indicated that 1 4 % of the Fe+ was formed in states higher than the 4D. These results suggest that substantial amounts of 4D (second excited state) may be formed. On the basis of this data, we assign peak I to the a6D ground state with probably some contribution from the a4D second excited state. These are the only two low-lying states with the 4s3d6 configuration required to match the observed peak I mobility. Peak I1 is composed of the a4F and a% states and possible some a4P, all with 3d7 configurations (Table IV). The existence of at least two 3d7 excited states is consistent with the observations of Loh et aLI5who found that at least two states with different deactivation rates were present. For reasons presented in the section on deactivation, we do not expect quenching of 4s3d"' configurations. Thus at least two reactive, excited 3d" states are expected. 6. Cobalt. The ATD spectrum of Co+ (Figure 10) is similar to Mn+ and Fe+: two well-separated peaks with mobilities typical of 4s3d"' and 3d" configurations. The a3F 3d8 ground state contained in peak 11. Using translational energy spectroscopy (TES), Hanratty et aLI5established the presence of the b3F 4s3d7second excited state in Co+ formed by electron impact on Co(CO),NO. In reactions with H,, Elkind and Armentrout19saw a weak threshold due to the b3F state as well as a very small component due to the b3P 4s3d7 state. On the basis of the observed change in cross section between SI and E1 formed Co+, they estimated that -50% ground state was formed in 50-eV electron impact on C O ~ ( C O(Table )~ V). Reactions due to the a5F 4s3d7 first excited state have not been reported, although A r m e n t r o ~ t refers ~ . ~ ~ to 'low-energy features" in the excitation function of the Co+ + 0,reaction. Based on the above, peak I in the ATD spectrum is primarily due to the b3F 4s3d7 second excited state. There is no evidence
1
(53) Strobel, F.; Ridge, D. P. J . Phys. Chem. 1989, 93, 3635.
Kemper and Bowers for any aSF 4s3d7 first excited state. Peak I1 is then composed of ground-state Co+ (SF3d8). As usual, small amounts of highly excited states could be present. 7. Nickel. The ATD spectrum of Ni+ (Figure 11) contains two well-resolved peaks and is almost identical with that of Co'. The a2D 3d9 ground state is contained in the lower mobility peak 11.
Hanratty et a1.ls see a peak corresponding to the a2F 4s3d8 second excited state in the TES spectrum of Ni+. Elkind and Armentrout19 see reactivity with H2 due to the a2D ground state as well as the a4F and a2F first and second excited states (both 4s3d8). A small amount of b2D 4s3d8or higher energy state was also observed (Table V). The assignment of peak I1 in this case is trivial since only one state with a 3d9 configuration exists: the a2Dground state. Thus, the fraction of ground-state Ni+ is known unequivocally. Peak I is composed of the 4s3d"l excited states: primarily the a4F and a2F first and second excited states with a small amount of b2D or higher energy state@). Unfortunately, no estimate can be made as to their relative populations since these excited states are not yet resolved in our ATD spectra. 8. Copper. Only one ATD peak was found in the ATD spectrum of Cu+. The mobility corresponded to a state with a 3d" configuration. This must be the a% 3d1° ground state since only one 3dI0 state exists. Excited states with 4s3d9 configurations were
