J . Phys. Chem. 1985, 89, 4501-4517 What may be the nature of the crystal structure of phase III? The structure of C 0 2 phase I is largely determined by its large quadrupole moment as shown by Kihara.I2 Presumably the new phase of C 0 2has a structure which is also dominated by this large quadrupole moment in some as yet to be determined way. It should be a phase closely related to the Pa3 structure such that the three strong diffraction lines observed by L i d would also be strong lines for phase 111. The phases of solid N 2 might be a place to look for a suggestion of what is happening here. The Pa3 structure of solid N 2 is stable up to 0.4 GPa at which pressure N 2 transforms to the tetragonal P42/mnm phase. The Raman scattering pattern of this phaseI3 is very different from our observations and is thus ruled out. Other higher pressure structures (12) Kihara, T.; Koide, A. Adu. Chem. Phys. 1975, 33, 51-72.
4501
of N 2 have been reported recently, but these are disordered structures and are unlikely for C 0 2because of its large quadrupole moment. Detailed structural information on the new phase 111 of solid C 0 2 will await a more detailed high-pressure powder X-ray diffraction study. Improving on Liu's3 study will be difficult because of the weak X-ray scattering of first-row elements coupled with the small sample volume in a diamond anvil cell. Acknowledgment. Acknowledgments are made for help in the experiments and discussions with R. L. Mills, D. Schiferl, and B. Olinger. Registry No. C 0 2 , 124-38-9. (13) Medina, F. D.; Daniels, W. B. J . Chem. Phys. 1976, 64, 150-161.
Branching Fractions for Penning Ionization in Quenching of He(2%), Ar('P,,,), Ne(3P2,,) Atoms'
and
Michael T. Jones, T. D. Dreiling, D. W. Setser,* and Richard N. McDonald Department of Chemistry, Kansas State University, Manhattan, Kansas 66506 (Received: October 17, 1984)
A cold-cathode discharge was used to produce metastable He(2%), Ne(3P2,0),and Ar(3P2,0)atoms in a flowing-afterglow apparatus with He as the carrier gas, and a mass spectrometer was used to detect the relative intensities of the positive ion signals from the reactions of various added reagents. The branching fractions for Penning ionization of 15 compounds with He(2%), Ar(3P2,0),and Ne(3Pzo) have been measured relative to standard reference reactions, which were CO for He(2%) and Ne(3P2,0)and NO for Ar(fP2,0).For each reaction the primary distribution of ions from the Penning ionization event was recorded. For both He(2%) and Ne('P2,,), Penning ionization appears to be the dominant quenching channel: however, Penning ionization branching fractions of less than unity were found in most cases for Ar(3Pz,o)atoms. In addition to measurements for the ionization channels, branching fractions for neutral channels that give emission are reported in an appendix for some Ar(3Pz,o)and Kr(3P2)reactions with bromine and iodine containing reagents. The ionization and neutral product branching fractions can be combined to obtain a general viewpoint for quenching of metastable rare gas atoms for several reagents. Penning ionization by Ar(3P2,0)is a useful way for generating parent ions with a minimum of fragmentation for subsequent studies of ion-molecule reactions in a flowing-afterglow apparatus.
I. Introduction Penning ionization (PI) reactions of electronically excited He(2%; 19.8 eV), Ne(3P2,0;16.6 and 16.7 eV), and Ar(3P2,0;11.5 and 1 1.7 eV) atoms have been actively studied in recent The branching fractions for ionization with He(%) have been studied for several small molecules; but, less is known regarding exit channel distributions for quenching of Ar(3P2,0)and Ne(3P2,0) atoms. While our work was in progress, some PI branching fractions were reported for Ar(3P2,0)with several small molecules using an indirect technique."' Branching fractions do not appear (1) This work taken in part from the M.S. thesis of M.T.J., Kansas State University, 1983. (2) Hotop, H.; Neihaus, A. Int. J . Mass Spectrom. Ion Phys. 1970,5,415. ( 3 ) Cermak, V. J. Electron. Spectrosc. Relat. Phenom. 1976, 9, 419. (4) Hotop, H. Electron. At. Collisions, Proc. Int. ConJ, l l t h , 1979, 1980, 271. ( 5 ) Niehaus, A. Adu. Chem. Phys. 1981, 45, part XI, 399. (6) Kolts, J. M.; Setser, D. W. In "Reactive Intermediates in the Gas Phase"; Setser, D. W., Ed.; Academic Press: New York, 1979; Chapter 3. (7) Golde, M. F. "Gas Kinetics and Energy Transfer"; The Chemical Society: London, 1977; Vol. 2. (8) King, D.; Setser, D. W. Annu. Reu. Phys. Chem. 1976, 27, 407. (9) Golde, M. F.; Ho, Y.-S.; Ogura, H. J . Chem. Phys. 1982, 76, 3535. (10) Balamuta, J.; Golde, M. F. J. Chem. Phys. 1982, 76, 2430. (1 1) Balamuta, J.; Golde, M. F.; Ho, Y . 4 . J . Chem. Phys. 1983, 79, 2822.
0022-3654/85/2089-4501$01.50/0
to have been reported for Ne(3P2,0)atom reactions, although N, and CO probably quench mainly by PI.'2-'4 In this work, the branching fractions for ionization, rQ+= kQ+/kQ,have been measured at room temperature by using a Rg*
+Q
- + kQ+
Rg kQ
Q'
+ e-
kQ = kQt
other products
+ kQ*
(la) (1b)
flowing-afterglow apparatus, which employed a quadruple mass spectrometer to monitor the product ions. A cold-cathode discharge located near the entrance to the flow reactor generated the metastable rare gas atoms (Rg*).6 The rQ+ were measured by observing the ion signals for a given reagent molecule relative to that of a reference reaction with a known branching fraction for a common metastable atom concentration. By changing the reagent concentration the primary and secondary ion channels could be identified and the primary ions from Rg* quenching are results, some electronically excited reported. In addition to the rQ+ (12) West, W. P.; Cook, T. B.; Dunning, F. B.; Rundel, R. D.; Stebbings, R. F. J. Chem. Phys. 1975, 63, 1237. (1 3) Brom, J. M.; Kolts, J. H.; Setser, D. W. Chem. Phys. Lett. 1978, 55. 44. (14) Bruno, J. B.; Krenos, J. J . Chem. Phys. 1983, 78, 2800.
0 1985 American Chemical Society
4502
The Journal of Physical Chemistry, Vol. 89, No. 21, 1985
hlC0 INLET
-
Jones et al.
ROOTSYcnANIcAL VACUUM
I
-
Figure 1. Schematic diagram of the flow reactor and ion sampling portions of the apparatus. The cold-cathode discharge tube is shown in the expanded section of the diagram. The potential across the electrodes was 260, 280, and 300 V for Ar, Ne, and He, respectively. The rare gas flow rates through the cold cathode were -20 atm cm3 s-l, and the He buffer gas flow was 195 atm cm3 s-' for a flow tube pressure of 0.5 torr. In the present experiments the reagents were added through the first ion-molecule neutral inlet, which is 71 cm from the end of the flow reactor.
neutral product channels from quenching of Ar(3P2,0)and Kr(3Pz) atoms by bromine and iodine containing reagents are summarized in the Appendix. The main interest in these data, which were compiled as part of a systematic study of halogen containing molecule^,^^^'^ is the branching fraction, rQ*, for excitation and the Br* and I* state distribution from the predissociation of ArBr*, ArI*, and KrI*. In the present work, the 3P0and 3Pzstates for Ne* and Ar* are both present, roughly in statistical proportions; however, only He(2%) and Kr(3Pz)are present for the He* and Kr* studies. Because of cost, Ne was used sparingly in these experiments. This PI study was motivated by two factors. The first was the desire to acquire a set of ionization branching fractions for advancing our understanding of Rg* atom reactions. The second factor was the need to develop a method for generating positive ions with low vibrational energy so that fragmentation of the parent ion could be minimized. This latter factor is desirable for subsequent studies of ion-molecule reactions in the flow reactor. Since PI is a Franck-Condon controlled ionization in which the electron can remove the excess energy, the parent ion usually is produced in the ground or electronically excited ionic states with little vibrational energy. The second paper" of this series will utilize the Ar(3Pz,o)reaction as the source for studies of the reactions of phenylnitrene radical cation. Another group also has utilized PI by Ar(3P2,0)for generation of organic positive ions in a flowing-afterglow reactor.18 All molecules have ionization channels below the excitation energy of He(23S) and ionization always seems to be the major product, although dissociative channels have been observed for some cases by emission spectroscopy.1e2z The work to be reported (15) (a) Gundel, L. A.; Setser, D. W.; Clyne, M. A. A,; Coxon, J. A,; Nip, W. J . Chem. Phys. 1976,64,4390. See: Piper, L. G . J . Chem. Phys. 1977, 67, 1795 for a comment on the Kr reference reaction. (b) Kolts, J. H.; Brashears, H. C.; Setser, D. W. J. Chem. Phys. 1977, 67, 2931. (16) (a) Setser, D. W.; Dreiling, T. D.; Brashears, H. C., Jr.; Kolts, J. H. Faraday Discuss. Chem. SOC.1979,67, 255. I'g2,.should be 0.1 rather than 0.01. (b) Tamagake, K.; Setser, D. W.; Kolts, J. H. J . Chem. Phys. 1981, 74, 4286. (c) Kolts, J. H.; Velazco, I. E.; Setser, D. W. J. Chem. Phys. 1979, 71, 1247. (d) Velazco, J. E.; Kolts, J. H.; Setser, D. W. J . Chem. Phys. 1976, 65,3468. (e) Lin, D.; Yu, Y. C.; Setser, D. W. J. Chem. Phys. 1984,81, 5830. This note calls attention to a correction to the branching fraction for Xe('P2) + NF, initially reported in 16d (and summarized in 16a). (17) Jones, M. T.; McDonald, R. N.; Setser, D. W., to be published. (18) Anderson, D. R.; Bierbaum, V. M.; DePuy, C. H.; Grabowski, J. J. Int. J . Mass Spectrom. Ion Phys. 1983, 52, 65. (19) Chang, R. S . F.; Setser, D. W.; Taylor, G . W. Chem. Phys. 1978, 25, 201. (20) (a) Coxon, J. A.; Clyne, M. A. A,; Setser, D. W. Chem. Phys. 1975, 7, 255. (b) Coxon, J. A.; Marcoux, P. J.; Setser, D. W. Chem. Phys. 1976, 17, 403. (c) Yencha, A. J.; Wu,K. T. Chem. Phys. 1980, 19, 127.
here shows that the Ne(3Pz,o)reactions resemble those for He(23S) in that PI is the dominant exit channel. In contrast, ionization frequently is not the dominant product from quenching of Ar(3P2,0) and neutral dissociation with or without excitation can be imp ~ r t a n t , ~ - " *even ~ ~ if- ~ionization ~ is energetically allowed. For halogen containing molecules, reactive quenching yielding rare gas halide molecules (or excited halogen atoms from predissociation of RgX*, see Appendix) can be a major exit ~ h a n n e l . ' ~ * ' ~ Preliminary work also has suggested that reactive quenching may be important for some He(2jS) and Ne(3Pz,o) and this point is investigated more thoroughly here. The halogen containing molecules, which are taken as representative of quenching by molecules with significant electron affinities (EA), present the opportunity to study the competition between PI and reactive quenching by monitoring the ion and rare gas halide derived products, respectively. The RgX* product arises as a consequence of curve crossing between V(Rg*,RX) and the ionpair potential, V(Rg+,RX-), in the entrance channel, whereas, simple PI occurs mainly via electron tunneling from V(Rg*,RX) for simple PI is a measure of at short range.2-5 Thus, the rQ+ the collisions that do not follow the ion-pair potential. If V(Rg+,RX-) is above the ionization threshold, a more complex PI mechanism from this potential must be included as a process yielding ion^.^^.^' Thus, consideration of the ionization and the neutral products provides some insight about the exit channel couplings to the V(Rg*,RX) entrance channel. This point of view also was the basis of a recent molecular beam studyz6 of the velocity dependence of the ionization channels for Ne* and Ar* with Oz and C12. 11. Experimental Techniques
The flowing-afterglow (FA) apparatus was similar to that previously reported by this laboratory for negative ion molecule reactions.z8 The FA reactor (Figure 1) consisted of a region to (21) (a) Richardson, T. H.; Setser, D. W., unpublished work. (b) Wu,K. T.; Morgner, H.; Yencha, A. J. Chem. Phys. 1982, 68, 285. (22) Baltayan, P.; Pebay-Peyroula,J. C.; Sadeghi, N. J . Chem. Phys. 1982, 78, 833; Chem. Phys. Lett. 1984, 104, 168. (23) Hartmann, D. C.; Hollingsworth, W. E.; Winn, J. S . J . Chem. Phys. 1980, 72, 833. (24) (a) Yencha, A. J.; Ozaki, T.; Kondow, T.; Kuchitsu, K. Chem. Phys. 1980.51, 343. (b) Duewer, W. H.; Coxon, J. A,; Setser, D. W. J. Chem. Phys. 1972.56.4355; 1973,58,2244. (c) The reactions of He(2%) and Ne(3P2,0) with CN containing compounds also give CN emission, ref 21a. (25) Forman, P. B.; Parr, T. P.; Martin, R. M. J . Chem. Phys. 1977, 67, 5591. (26) Alvarino, J. M.; Hepp, C.; Kreiensen, M.; Staudenmayer, B.; Vecchiocattivi, F.; Kempter, V. J . Chem. Phys. 1984, 80, 765. (27) (a) Kischlat, W.; Morgner, H. Z . Phys. A 1983, 312, 305. (b) Leisein, 0.;Morgner, H.; Muller, W. Z . Phys. A 1982, 304, 23.
Branching Fractions for Penning Ionization
The Journal of Physical Chemistry, Vol. 89, No. 21, 1985
prepare the ion (metastable atom) of interest followed by a region in which to examine the interactions of that ion (metastable atom) with neutral reactants. The other major components of the FA are a fast pumping system, a quadrupole mass spectrometer to monitor the ion composition, and a gas handling system for metering reagents. Helium, the buffer gas, is introduced into the upstream end of a stainless steel flow tube, and the pressure ( P H ~ ) and flow velocity (a) are maintained by a Roots blower/mechanical pump system. Our standard operating conditions are PHe= 0.5 torr and ( u ) = 80 m s-l. The flow is sampled through orifices (- 1-mm diameter) in two molybdenum nose cones (see Figure l ) , which separate the flow reactor from the mass spectrometer. The ions passing through the nose cones are focused by a series of ion lenses into the quadrupole mass spectrometer. For the present work a Rg* source replaced the conventional electron impact source for the generation of ions; however, both sources can be placed simultaneously in the flow tube if desired. Flow Tube and Metastable Atom Source. The flow tube was a 7.15-cm4.d. stainless steel pipe of 124 cm length. As shown in Figure 1, a window located after the neutral reagent inlet allowed measurement of light emission from excited-state products. A series of other ports on the flow tube allow for installation of the cold-cathode discharge and ion gun, introduction of reagents, and measurements of pressure. The He flow was measured with a precalibrated tri-flat flowmeter, and the bulk flow rate was controlled by the gate valve leading to the Roots blower. The He was purified by passage through two molecular sieve (Davidson Type 4A) packed traps cooled to 77 K. The metastable atom source was a cold-cathode discharge, which produces Rg* in the absence of Rg". The rare gas flow (lo-15% of the helium buffer gas flow, 200 STP cm3 9')for the desired metastable (including He*) was introduced through the discharge. The discharge tube was constructed of two 8-mm4.d. glass tubes connected by an O-ring joint. Both electrodes were made of rolled tantalum foil and were spot-welded to tungsten wire metal-to-glass feed-throughs. The cathode was placed downstream and the lead wire was covered with a glass sleeve (3-mm glass tubing) to prevent arcing between it and the anode lead wire. The best results were for flows of -20 cm3 s-l with 260 V for Ar*; the voltage was increased to 280 and 300 V for Ne* and He*, respectively. A 20 kil resistor was used in series with the discharge. N o purification was done for N e or Ar; however, the He flow for the cold-cathode discharge was purified by passage through two cooled (77 K) molecular sieve traps. The pressure in the discharge tube was not measured, but must be somewhat higher than the 0.5-torr pressure in the FA reactor. The discharge was operated below the threshold for formation of rare gas ions, Le., I290 V for Ar, I300 V for Ne, and I330 V for He. The [Rg*] was very constant in this apparatus. For example, the N,+ signal from He* or Ne* (with N,) was constant to better than 1.O% for several hours of operation. In the absence of reagent, the [Rg*] decay rates are slow and consist of diffusion to and deactivation at the walls of the flow tube and reaction with the impurities in the buffer g a s 6 The relative He* and Ne* concentrations were measured from the N2+ or CO+ signals by adding enough N, or C O to totally quench the metastable atoms. The relative He* and Ar* concentrations were measured from the relative emission intensities from the reactions with N2, as will be described in the Reference Reaction section. The relative concentrations of the He*:Ne*:Ar* for standard operating conditions were 1.01.1:1S,and the absolute concentrations are in the 109-1010-cm-3range. The excited-state neutral products from Ar(3P2,0)and Kr(jP2) reactions were observed from the product emission intensities. These experiments were done in a smaller scale flowing-afterglow apparatus6using Ar carrier gas. These experiments are described in the Appendix. Mass Spectrometer Calibration. The chamber with the mass spectrometer was maintained at -5 X lO-' torr. The molybdenum (28) McDonald, R. N.; Chowdhury, A. K.; Setser, D. W. J . Am. Chem. SOC.1980, 102, 6491.
4503
TABLE I: Calibration of Mass Spectrometer Response concn in mixture, molecules cm-3
mass
kQ," cm3 predicted exptl molecule-I s-l mass ratio signal ratio
Ar (40) Kr (78-86)
2.4 X 10l2 2.0 X lo'*
7.04 X lo-" 9.94 X lo-"
1.00 1.18
1.oo 1.24
Xe
1.5 X 10l2
12.40 X lo-"
1.10
1.23
(1 29- 136) Ar (40) N2 (28)
1.1 X 10I2 1.1 X 10I2
7.04 X lo-" 7.00 X lo-"
1.00 1.00
1 .oo 1.oo
" Reference 29. TABLE 11: PI and Quenching Rate Constants (lo-" cm3 molecule-' S-1)
He(23S) N2
CO NO
Ne(3P2,0)
kp'
kQ+b kQ'
kQ+'
kQd
7.0 9.8 24.2
7.0 9.6 18.8
6.6 9.5 2lC
3.6 1.4 22.0
6.6 9.5 21.0
low [NO]'
high [NOIe
kQ+
kQ+
8.8
* 3.4'
8.2
* 1.lC
"Reference 32. bReferences 19 and 32. CReferences 13 and 31; we assumed that the kQ+values were the same as the kQ values; ref 12-14. dReference 33. CThis work; see text for correction of the measured kg0+ to 8.8 X lo-" cm3 molecule-I s-l.
nose cones separating the chambers were electrically isolated by Teflon spacers from the stainless steel main housing; the bias potentials, relative to a common ground, that gave the best ion signals were 0.2-1.0 and -15 V for the first and second nose cones, respectively. The quadrupole mass spectrometer (Electron Associates Inc.) had on-axis ionizer and ion lenses; the optimum bias potentials were -1 5 V on all lenses except the extractor, which was adjusted for each reagent. The sensitivity for ions of different masses was determined from product ions signals from a preprepared argon/krypton/xenon mixture reacting with He(2)S) to produce Ar+, Kr+, and Xe". The rate constants, k?, for the quenching of He(23S) by these rare gases are ~ e l l - k n o w n and ,~~ the relative concentrations are given by eq 2. The anticipated Xe+:Kr+:Ar+ = k&Xe] : k g [Krl :k$ [Arl
(2)
and observed ion signals were compared to obtain the calibration factors. A sample calibration, which shows the typical magnitudes of the corrections, is given in Table I. The calibration was done after each series of reactions; only slight changes occurred over a period of 6 months. The calibration was extended to lower mass range by comparing the Ar+/N2+ ratio from He(23S) reacting with a known Ar/N2 mixture. Reagents. The following reagents (with suppliers) were used without purification, except for vacuum distillation of compounds that could be condensed (77 K): N2, Ar, O2(Welder Products), Kr and Xe (Cryogenic Rare Gas), Ne (Union Carbide), CHF3, CH3C1, C2N2,NO, CO, C2H2,C2F4,CS,, C12, (CH3)3CH,and HCl (Matheson), (CH3),C (Fluka), C6H6 (Aldrich), and NF3 (Pennwalt). The C H 3 0 H , CH2C12,S0Cl2, and CHCl:, samples were purified by standard methods. The other liquid reagents (CCl,, (C2H5),0, C6H6,and C6F6) were fractionally distilled and the center-cut constant boiling fraction was used. Several special compounds PhN3, c-CSH4N2,and (CF3)2CN2were prepared in our laboratory by standard methods. All reagents were subjected to freezepumpthaw degassing cycles before being stored in large Pyrex bulbs as dilute samples in He prior to being added to the flow reactor. Reference Reaction Technique. In measuring rq+, a reference reaction method was adopted. Thus, the ion signal from a ref(29) (a) Lindinger, W.; Schmeltekopf, H. C.; Fehsenfeld, F. C. J . Chem. Phys. 1974,61,2890. (b) Ueno, T.; Yokoyama, A.; Takao, J.; Hatano, Y. Chem. Phys. 1980, 45, 261.
4504
The Journal of Physical Chemistry, Vol~89, No. 21, 1985
erence reaction with a known total rate constant and ionization branching fraction was compared to the ion signals from other reagents. In principle N2 and C O should be ideal references for both He(%) and Ne(3P2,0)because the rate constants and product distributions are (see Table 11). At low N, and CO concentrations the relative ion yields, within experimental error, were in accord with unit branching for ionization, although dissociative excitation giving C* may account for -2% of the He* + CO quenching.I9 However, for high concentrations the C O reaction was found, in fact, to be the better standard because lower I'N2+ were recorded, relative to rCO+, than expected. For example, ~ , was 0.9 rather than with [N,] 2 1 X 10l2molecule ~ m - rN2+ 1.0, which probably is a consequence of secondary reactions producing N4+followed by electron recombination.19 Although reo+ and ry2+ were equal to within the N2+ N2 He N4+ H e (3)
Jones et al. 900r
-
+
N4+
-
+
+ e-
N,(B)
+
+ N,
experimental uncertainty, the difference was systematic and C O was selected as the reference for He(3S) and Ne(3P2,0)reactions. Prior to this work there was no reaction for which = 1 was established. We selected N O as a reference, since the reported rf;+ was significant (0.28 f 0.10)9 and since N O is a stable gas to handle and meter. The I$$+ was assigned by a method which utilized the relative photon emission signals from the reaction of He* and Ar* with N, to obtain [Ar*]:[He*] and the relative ion signals from reaction of He* and Ar* with N O to assign k t i + from comparison to kE+. He(%) + N, N2+(B) + He + e(4)
-- ++ + N,+(A) N,+(X)
Ar(3P2,0) N 2
+ eHe + eN,(C) + Ar He
(5)
other channels The N,+(B-X) and N2(C-B) relative emission intensities for low [N2] give the local concentration ratio of He* and Ar*. In eq 6, and I N 2 ( c ) are the emission intensities, kqHe*and kQAr' ") -[He*] - - h , + d 0 . 8 0 k ~ ~[N21 [Ar*I
Z,,cc,(0.42kQHe')[Nz]
(6)
are the quenching rate constants (7 X lo-" and 3.8 X cm3 re~pectively),*~*~~ and the numbers are the branching molecule-I SKI, factors for N2+(B) and N2(C) formation. The branching fraction for N2(C) has been repeatedly s t ~ d i e d , 'and ~ ~ ,we ~ ~used 0.8 f 0.2 because the dissociative channel, although relatively minor, is still uncertain. The small Ar(3Po) concentration relative to Ar(3P2) permits the use of kQ for Ar(3P2) without serious error, given the similarity of the 3P0and 3P2reactions.35 The first-order dependence of the emission intensities on [N,] is demonstrated in Figure 2 and the N2(C)/N2+(B)intensity ratio of 1.45 gives He*:Ar* = 1:1.5. For low [Qj the ion signals from He* and Ar* + N O are first order and kc0+ may be inferred from eq 7. The kFfed+[He* ] [NO]Igd+ kAr* YO+ = (7) [Ar*] [NO]Z;$+ NO' signals were measured from He* and Ar* with the same [NO], and k$+ = 9.8 f 3.4 X lo-'' cm3 molecule-' s-I was obtained from the previously determined [He*]/[Ar*] ratio and the kE value (see Table I1 and below). Our measurement of [He*]/[Ar*] pertains to the ratio at the position of the observation window; however, the ratio that is needed in eq 7 is the mean ratio (30) Richardson, W. C.; Setser, D.W. J. Chem. Phys. 1973, 58, 1809. (31) Yokayama, A.; Hatano, Y. Chem. Phys. 1981, 63, 59. (32) Schmeltekopf, A. L.; Fehsenfeld, F. C. J. Chem. Phys. 1970,53,3173. ( 3 3 ) (a) Velazco, J. E.; Kolts. J. H.; Setser, D. W. J . Chem. Phys. 1978, 69,4357. (b) Bourne, M.; LeCalve, J. J . Chem. Phys. 197, 58, 1452. (34) Setser, D. W.; Sadeghi, N. Chem. Phys. Leu. 1982, 82, 44. (35) (a) Sadeghi, N.; Nguyen, T. D. J . Phys. 1977, 38, L-283. (b) Nguyen, T. D.; Sadeghi, N. Chem. Phys. 1983, 79, 41.
10 20 3.0 4'0 (Nz), IO" M O L E C U L E S C M - ' Figure 2. First-order plots of the N2+(B)and N2(C)integrated emission and Ar* (0) vs. N2 concentration intensities from reactions of He* (0) for addition of N2just in front of the quartz window.
over the length of the flow tube. Since the diffusional loss of He* is larger than for Ar*,see next section, the [He*]/[Ar*] measured from eq 6 is an upper limit. Using the equations and the diffusion constants developed in the next section, we estimate that the ka:+ should be reduced to 8.8 X lo-'' cm3 molecule-l s-l, which corresponds to l$$+ = 0.40 f 0.15 for a quenching rate constanP of 22 x lo-" cm3 molecule-' s-]. Chang, Taylor, and Setser19reported a product formation rate from constant for He* + N O of 19.9 X lo-'' cm3 molecule-] SKI the emission intensities of NO+(A-X), N*, and O* with scaling for the NO+* channels that gave no emission using PI electron spectroscopy data. A subsequent identified two more NO+* states that were not included in the scaling, and addition of these channels would give a product formation rate constant of 33 X lo-" cm3 molecule-' s-I. This value exceeds kp' and there must be error in some of the measurements. We subtracted the N * and O* channels from the kQHe' value reported by Schmeltekopf and Fehsenfeld3, and used this result for kE+in eq 7 and Table 11. The ZN2+from reaction 4 also was monitored simultaneously with the N,+(B-X) emission intensity. At sufficiently hifh [N,], ,::Z'" can be all [He*] is converted to [N2+] and the ion signal, that would correspond to the total used to predict the quenching of [Ar*].
"65
Comparing "'I$* to the actual signal observed from adding excess [NO] to Ar* gives another measure of I?@;+, and this result was 0.37. The different diffusional rates of Ar* and He* do not affect this high [Q] measurement because the He* and Ar* are converted to ions near the window position in the reactor. The mean rtit value from the two types of measurement becomes 0.38 f 0.15. Using the same Ar* vs. He* comparison, but without mass selection of the ions, Golde and co;workers reported Ig ' :+ = 0.28 f 0.10. However, they used = 0.60 rather than 0.80 in their calibration and, in fact, the two measurements are in moderately good agreement. Dissociation is the other major exit channel from Ar* NO, and the independently measured branching fraction for dissociation is 0.69 & 0.10.9310This is in good agreement with the value for dissociation which would be assigned from our data, namely 0.62. The reference reactions for assignment of product rate constants from relative emission intensities are and Kr(3P,) with Cl,. The rate constants for ArCl* and KrCl* formation have been assigned,6-16dand comparison of the ArC1* or KrCl* intensities to the product intensities from other reagents for the low [ Q ] first-order regime gives the product formation rate constants. The
+
~
~~
~~
~
(36) Hotop, H.; Kolb, E.; Lorenzen, J J Electron Specfrom Relar. Phenom. 1979, 16, 213
The Journal of Physical Chemistry, Vol. 89, No. 21, 1985 4505
Branching Fractions for Penning Ionization
+
Kr(3P2) C12 reaction is well estab1ished;la however, there still is some uncertainty for Ar(3P2,0) C12 (see Appendix).
[Co]; X1010MOLECULES.CM-3
+
10
IV. Experimental Results A. Experimental Method for Assigning rQ+. Model calculations were compared with experimental results to evaluate the reliability of the flowing-afterglow/mass spectrometer technique Rate equations were developed for for the measurement of rQ+. the following set of reactions, which included diffusion to and quenching at the walls. Rg*
+Q
kQ+
Q+
+ Rg + e-
5Xi + Rg
-
(14 (1b)
kw
Rg* Q+
',k
wall
(9)
wall
(10)
The products from all other quenching channels are represented by Xi, and k, and k,' are the rate constants for diffusion to the wall where destruction is assumed to occur for Rg* and Q+. The rate equation for the formation of Q+ is given by eq 11, providing there are no secondary reactions with Q.
IQ+l
v 0 5 &I
Figure 3. Comparison of the calculated (solid lines) by eq 11 and experimental results for the He* + C O reaction for 1 = 5.7 ms. For the calculation, k , = k,' = 140 s-I, kQ+ = kQ = 9.8 X lo-'' cm3 molecule-' s-l, and [He*] = 1 X l O l o molecules ~ m - ~The . high [CO] regime (A) us& the left [CO'] and bottom [CO] concentration scales, while the low [CO] regime (0)uses the right [CO'] and top [CO] scales. The exponential curve is the calculated [He*] decay. The low [CO] data demonstrate the linearity of the [CO'] yield for the appropriate [CO] range.
=
We wish to measure the relative ion signals from two different reagents for a range of [Q]. For the low [Q] regime eq 12 holds.
The diffusion rates of Q1+and Q2+ in He are nearly equal since the reduced masses are similar and eq 12 reduces to the following desirable result since kwl' = kw2/.
-[QI'] - -
kq,+[Rg*lo [QI1
[Q2+]
kq,+[Rg*lo[Q21
(13)
For sufficiently large [QJ, all Rg* are quenched and the [Q1']/[Qz+] ratio reduces to eq 14. Providing that kwl'= kw2/,
eq 14 will reduce to the simple result given in eq 15 Figure 3
+
shows some data for He* CO and the satisfactory fit to model calculations using reo+ = 1.O and the known diffusion coefficients ) ~He*(1.44 ~ ~ X lOI9 for CO'(1.35 X lOI9 molecule cm-I s - ~ and molecules cm-' s - ~ ) .Of ~ ~special ~ interest is the first-order range ([Q] 1 3 X 10" molecule ~ m - and ~ ) the saturated range ([Q] 1 3 (37) (a) Fitzsimmons, W. A.; Lane, N. F. Phys. Rev. 1968,174, 193. (b) Lindinger, W.; Albritton, D. L. J . Chem. Phys. 1975, 62, 3517. (c) Phelps, A. V. Phys. Rev. 1959,114, 1011. (d) Lin, C. L.; Kaufman, F. J. Chem. Phys. 1971, 55, 3760. (e) Sadeghi, N., private communication. The presence of exchange interactions for Ar('PO2) with Ar leads to an anomalously low diffusion coefficient; therefore, the diffusion coefficient for Ar(3Po,2)in He will be even larger than implied by the ratio for Cu, Zn,and Ni in He and Ar.
Figure 4. Comparison of the calculated (eq 11) and experimental results for Ar* + NO (0),CCI, ( U ) , and CH3CI (A)for t = 5.7 ms. The secondary ions from the CH3C1system have been summed in making the experimental plot. The calculations, shown as the solid lines, include diffusion, and the values of the rate constants used in the calculation are given in Table 111. The maximum shown for the CH3Cl curve also exists for CC1,; but, it is less obvious because of the scale of the CCl, plot. The dashed lines are calculated results without diffusion from eq 11. All calculated plots have been scaled relative to re$. X 10l2molecule ~ m - ~ Since ) . k,H"' = k / , the kinetics are relatively straightforward for He* in H e buffer gas. However, the more rapid diffusion of the ions relative to Ar* in He requires more consideration. The diffusion coefficient of Ne(3P2,0)in He is 2.0 X 1019molecule cm-l s-1,37cand the kinetics for Ne* can be treated in the same way as for He*. DePuy et a1.18observed that the diffusion rate of k r * was to of the diffusion rate of organic ions in helium; however, a reliable measurement of the diffusion constant of Ar* in He apparently has not been made. The diffusion constant6 of Ar* in Ar is 1.8 X 10l8 molecule cm-I s-l, and a lower limit to the diffusion constant of Ar* in He can be estimated by comparing this value to that of other species which has been studied in both H e and Ar. The diffusion coefficients of N(2D) in He and Ar are 37d 26 X 10l8and 6.7 X 10l8 molecule cm-I s-l, respectively, and the ratio of the diffusion coefficients for Cu, Zn, and Ni in He and Ar are 3.7,3.5, and 3.5, respectively.37eFrom these values, the diffusion coefficient of Ar* in H e would be 27.0 X 10" molecule cm-I S-I, which is about half the value for most ions;38
(38) (a) MkFarland, M.; Albritton, D. C.; Fehsenfeld, F. C.; Ferguson, E. E.; Schmeltekopf, A. L. J . Chem. Phys. 1973, 59, 6610. (b) Shaw, M. J.; Stock, H. M. P. J. Phys. B 1975, 8, 2752. (c) McDaniel, E. W.; Mason, E. A. "The Mobility and Diffusion of Ions in Gas"; Wiley: New York, 1973. (d) Patterson, P. L. J. Chem. Phys. 1972, 56, 6610. (39) (a) Rosenstock, H. M.; Draxl, K.; Steiner, B. W.; Herron, J. T. J . Phys. Chem. Ref.Data, Suppl. 1977, 6. (b) Levin, R. D.; Lias, Sharon G. 'Ionization Potential and Appearance Potential Measurements 1971-1981"; Natl. Stand. ReJ Data Ser. (US., Natl. Bur. Stand.) 1982, NSRDS-NBS 71.
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The Journal of Physical Chemistry, Vol. 89, No. 21, 1985
TABLE III: Comparison of Experimental and Model Results" for Ar* NO, CCl,, and CH$l
+
Jones et al.
ooi
1
calcdb
no with diffusion' diffusiond exDtl High [Q] Regime apparent 1.84 1.76 1.85 f 0.46 (k~+/kQ)~~"'/(k~+/k~)~' apparent 0.76 0.74 0.76 f 0.15 ( ~ Q + / ~ Q ) ~ ~ " / ( ~ Q + / ~ Q ) ~ O rCH3CI+ 0.70
apparent apparent
rcCl4+
0.29
Low [Q] Regime apparent ( ~ c H , c I + / ~ N o + ) 8.1 apparent (kcci4+/kNo+) 3.2 apparent rCH3CI+ 0.64 apparent rcCl4+ 0.27
10" MOLECULESCM-3
0.67 0.28
0.70 f 0.20 0.29 f 0.06
8.0 3.2 0.64 0.27
8.16 f 0.89 3.67 f 0.36 0.65 f 0.08 0.30 f 0.03
"k;O = 2.2 X k p 3 c 1 = 10.5 X and k? = 10 X cm3 molecule-i s-l; the kQ+values were chosen such that (kQ+/lfQ)No = 0.38, ( k ~ + / k ~ ) ' ~=~0.70, ' ' and ( k ~ + / k ~ )="0.29. ~ bThe ratios were taken from the graphs of calculated results, see Figure 3, in the same way as the experimental data were analyzed. k , = k , = 0. k, = 2 k , = 140 s-l.
50
t
'O0I
20
10
10" MOLECULES CM-3
Figure 6. Representative plots of total ion signals for CO (n),NO (O), and N, (A)reactions with He* for low (a, top) and high (b, bottom) reagent concentration for t = 5.7 ms. The ions signals have been corrected for mass discrimination.
10" MOLECULES CM-3 V '
'
2'0
10" MOLECULES C W 3
10'' MOLECULES CM-3
Figure 5. Representative plots of the total ion signals for reaction of NO (0),C6F6 ( O ) , C2F4 ( O ) , and C6H6 (V)with Ar* for low (a, top) and high (b, bottom) low reagent concentration for t = 5.7 ms. The ion signals have been corrected for mass discrimination.
this is consistent with DePuy's observations.'* According to the approximate results developed for the limiting [ Q ]regimes, Le., eq 13 and 15, the difference between k , and k,' should not be important providing that kwl' = kw2'. Some model calculations with and without diffusion for CH3C1, CCl,, and NO are compared with experimental results in Figure 4; Table I11 summarizes the rate constant values used in the calculations, as well as the experimental and calculated ion intensity ratios for low and high [ Q ] . The main consequence of diffusion is the reduction of [Q'], which is not apparent in the relative plots shown in Figure 4. The slow decline in [Q'] at very high [Q] is a consequence of Q' being formed further upstream in the reactor and hence having a greater opportunity for diffusional loss. From the summary of the model calculations given in Table 111, we conclude that a factor of 2 difference in the diffusion rates of Ar* vs. Q' will have no significant effect, relative to our *20% general uncertainty, for the relative rQ+obtained from either the high [Q] or the low [Q] measurements, providing that kwl' kw2'. In doing the measurements, preliminary experiments were done for each reagent with Ar*, Ne*, and He* to establish (1) that a stable (=k5%) ion signal could be maintained and reproduced,
lo'* MOLECULES CM-3 Figure 7. Representative plots of total ion signals for CO ( O ) , NO (O), N2 (V),C C 4 (V),and CH3CI (0)for reactions with Ne* for low (a, top) and high (b, bottom) reagent concentration for t = 5.7 ms. The ion signals have been corrected for mass discrimination.
(2) that the concentration ranges for both the first-order and saturated concentration regimes could be reached, and (3) that corrections could be made for the effects of mass discrimination. In many, but not all, cases the low and high [Q] regimes were studied. Four points for [Q] = 5-10 X 10'O molecule cm-3 were taken to obtain the linear low [Q] plot, and several points then . data were taken for [ Q ]= 1-10 X lo1*molecule ~ m - ~These were then compared to the signals from low and high concentrations, respectively, of the reference reaction. For each reagent the measurements were done under the same conditions, e.g., PHe, rare gas flow, At = 5.7 ms, lens potentials, and cold-cathode operation, as for the reference reaction. Figures 5-7 show typical plots of the ion signal vs. [Q] for several reagents with Ar*, Ne*, and He*. In practice, the small maximum expected for the high [Q]regime was usually not observed and the constant ion signal was taken as the saturated ZQ+.The results from 3 to 5 experiments were averaged t o obtain the reported branching fractions given
The Journal of Physical Chemistry, Vol. 89, No. 21, 1985 4507
Branching Fractions for Penning Ionization TABLE I V Branching Fractions" for Ar* Penning Ionization
reagent ( k d b NO (2.2) CC14 (10.0) CHC13 (10.5) CH2C12 (8.5) C H 3 C I (10.5) CF31 (4.7) C2Hz (4.7) C12 (7.1) CH30H(6.0) S0Cl2 (CH314C (CF3) (C F3)2C=O C6H6 (7.9) C6F6
cs, (10.0) C2F4 (10.0)
kQ+/ kNO+C
1.oo 3.57 3.45 4.35 8.16 1.40 4.17
f 0.36 f 0.35 f 0.44 f 0.89 f 0.09 f 0.42
3.0 f 0.60 1.79 f 0.18
2.13 f 0.21 1.39 f 0.14
rQce
~ Q + / ~ N O + ~
1.oo 0.76 f 0.08 1.03 f 0.11 0.80 f 0.08 1.85 f 0.20 0.51 f 0.04 1.61 f 0.16 0.08 f 0.03 0.87 f 0.18 0.23 f 0.05 1.00 f 0.09 1.39h 0.04* 1.41 f 0.14 2.13 f 0.22 1.96 f 0.20 0.57 f 0.06
0.38 0.29 f 0.06 0.39 f 0.06 0.30 f 0.06 0.70 f 0.20 0.19 f 0.04 0.61 f 0.14 0.03 f 0.01 0.33 f 0.05 0.09 f 0.04 0.38 f 0.05 0.53h Br, > CH2Br2> HBr. The Br2 reaction appears anomalous, since it has the largest exothermicity, as is evident from the short wavelength limit of the KrBr(B-X) emission and the presence of the KrBr(D-X) and the Br* emissions, and yet neither the B-X nor C-A spectra show extensive oscillations. Although discussion of Kr(3P2) IBr will be delayed until the Discussion section in this Appendix, the Kr* IBr reaction also gives an anomalously low KrBr* vibrational distribution, relative to Kr* CBr,. The Kr* Br2 reaction also produced substantial amounts of Brz*; most Br2* emission is in the 250-300-nm range, see Figure 11, but some also overlaps the KrBr(C-A) spectrum. Much of the strong emission is from D’-A’; however, other Br2* ion-pair states also are product states. At the present time these spectra cannot be firmly identified.4s The pressure dependence of the KrBr* spectra are shown in Figures 10 and 11 for CBr4 and Br,. Increasing the pressure enhances the KrBr(B-X) intensity at the expense of the (C-A) intensity, as well as giving vibrational relaxation as shown by the reduction of oscillatory structure in the spectra. A detailed analysis of the XeCI(B,C) relaxation has shown that molecules in the C state are collisionally transferred to the B states (with some loss in vibrational energy) and, in addition, vibrational relaxation occurs within each state as the pressure is increased.& The KrBr* spectra
+
+
-
-
+
+
+
+
+
+
(45) Yu, Y. C.; Setser, D. W. J. Phys. Chem., submitted for publication. These Br2* emissions will be considered more fully in a paper dealing with excitation of Br2 at 147.0 nm. (46) Dreiling, T. D.; Setser, D. W. J. Chem. Phys. 1981, 45, 4360.
Branching Fractions for Penning Ionization
The Journal of Physical Chemistry, Vol. 89, No. 21, 1985
4515
in Figure 10 are consistent with the XeCl* results. Since the C-A and B-A emission overlap, the relative amounts of KrBr(B) and (C) initially formed cannot be measured with reliability. Therefore, only the relative intensities, 1/11, of the two broad bands have been tabulated in Table X. The 1/11 ratios were extrapolated to zero pressure for Br,, IBr, and CBr,, and the result was -0.6. Since the B-A transition is expected to comprise 0.2-0.3 of the band labeled 11, the 1/11 low-pressure values suggest that the B and C states are formed in approximately equal amounts. This is consistent with results for Xe(3P2)and Xe(3Pl) reacting with halogen containing reagents and with K$P2) and K@P1) reacting with R F and RC1.I6 The reaction of Ar(3P2,0)atoms with RBr gives mainly Br* emission, although discernible ArBr* emission was observed at 165 nm for HBr, CF3Br, and CH2Br2. The ArBr* emission has been characterized by Golde and K ~ a r a n , , and ~ we will not discuss it further. The distribution of atomic Br* states are tabulated in Table X and compared, where possible with the results of Golde and K ~ a r a n and ~ , ~Clyne and Smith.47 The Br* distributions from the three studies are well within the combined experimental uncertainties, which demonstrate that the monochromator calibrations were satisfactory. In addition to Br(5s) states, the Br(5p) states are energetically accessible for some reagents. These states emit in the far red to the Br(5s) states. W e examined the 600-900-nm region; but, the Br*(Sp) quartet state emission intensity (which was compared with the Ar(3P2,0) + Kr reference reaction) was weak and comprised only 2-5% of the total Br* emission. Evidently, the ArBr* predissociation strongly favors the Br(5s) states and the rate constants of Table XI can be assigned to direct Br(5s) formation without a significant radiative cascade component from the Br(5p) states. The reaction of Ar(3P2,0)with all iodine donors produces strong I* emission. N o emission was observed which could be attributed to ArI* and evidently ArI(B and C) are completed predissociated. Table XI1 summarizes the I* state distribution for several reagents. A large number of I* states are energetically allowed and numerous lines were observed. Although some I(7s and 5d) emission was observed, the preferred states were the I(6s) states for all Ar(3P2,0) R I reactions. N o attempt was made to study the I(6p-6s) radiative cascade; however, a scan of the Ar* + I2 emission from 500 to 650 nm showed that I* emission in this range is not strong. The reactions of Kr(3P2) with I,, CH212,and CFJ produced only I*(6s) emission, and the observed states are summarized in Table XII. In addition to I*(6s), small amounts of KrI* were observed from Kr(3Pz) H I and CH31. These are reagents which gave low XeI* vibrational distributions, and presumably the same trend explains the smaller degree of KrI* predissociation. Figure 12 shows the KrI* spectrum from HI, which appears identical with that obtained by Casassa et Tables X and XI show the excited-state product distributions and the branching fractions for KrBr*, KrI*, Br*, and I* formation. The neutral excited-state branching fractions are large only for Br,, IBr, and 1,. Since the [I,] and [IBr] were not measured, the absolute branching fractions cannot be given, but rRgX* I'x. certainly is significant and probably close to unity. Formation of Br,* and IBr* (various ion-pair states) also is significant for Kr@,) reactions. For the polyatomic molecules, rRgX. + rX.are much smaller than for the molecular halogens and only + rX.2 0 . 1 0 . CFJ, CHJ,, and perhaps CBr, have rRgX. Discussion. A . Branching Fractions for Reactive Quenching. The reliability of the reported branching fractions depends upon the validity of the reference reaction, the calibration of the response of the detection system, and the measurement of the relative reagent flow rates. Assuming the reference reaction data are correct, we expect uncertainties of f30% and f50% for branching fractions 20.1 and 1.O. One should remember that errors also may exist in the k , values which would affect the branching fractions. Final resolution of the problem probably will require new systematic experimental measurements for the rate constants of the product channels, as well as the total quenching rate constants. At the present time r:ii 2 0.6 and r;Z 1 0.85 we favor I'prF.1 0.53, r&,, I'FF. (I'Br2* = 0.07). Another factor inhuencing our thinking is that rE$. :I& '. was 0.67;50 a small increase in the reference rek~~cl(B would ) ) make the chemiluaction rate constant (@ minescence branching fractions for Ar(3P2) + F2, C12, and ClF all nearly unity. A very recent studySZaof the Ar(3Po) + Kr
(47) Clyne, M. A. A,; Smith, D. J. J . Chem. Soc., Faraday Tram. 2 1978, 74, 263. (48) Casavecchia, P.; Guozhong, H.; Sparks, R. K.; Lee, Y . T. J . Chem. Phys. 1982, 77, 1878.
(49) Clyne, M. A. A.; Nip, W. S . J . Chem. Soc., Faraday Trans. 2 1977, 73, 161. (50) Kolts, J. H. Ph.D. Thesis, Kansas State University, Manhattan, KS, 1978. (51) Kolts, J. H.; Setser, D. W. J . Chem. Phys. 1978, 82, 1766.
-
+
+
+
+
+
+
+
+
+
+
+
+
+
+
4516
The Journal ofPhysica1 Chemistry, Vol. 89, No. 21, 1985
reaction shows that Ar('Po) contributes even less to the Kr(5p[3/2],) excitation than previously thought. The C ~ ( ~ S , ~ P ~ , , , emission generally has not been included in Fa. and the difficulties assigning the branching fractions for Ar(3P2)+ ClZare not associated with the presence of Ar(3Po). In summary, the reported branching fractions in this work for Ar(3Po,z)+ RBr and RI may be systematically low by 30%, if the rate constants for the reference reaction prove to be in error in accord with the above discussion. The present data together with previous results allow a comparison to be made of the RgX* formation channel via reactive quenching for Ar(3P2),Kr(3Pz),and Xe(,PZ). Since the excited X* states are formed by predissociation of the initially formed RgX*, the sum of rRa*+ rX.is used in the comparison. The results given in Tables X-XI1 follow the same general trends as noted before in that only molecular halogens have large branching fractions for RgX* formation. The r R g X ' for Kr(3P2) are somewhat smaller than the corresponding values for Xe(3P2) values are appreciable for the former. With because the rX2* the sum of the branching fractions is nearly unity inclusion of rX2., and the chemiluminescence exit channels probably account for all Kr()P,) quenching by F2, Clz, Br,, and IBr (and perhaps for Iz also). As noted in the above discussion, the branching fractions for Ar(3P2) reactions with molecular halogens is not yet firmly and especially rArx. established. Our previous claimI6" that I'KrX. declined relative to rXeX., even for molecular halogens, may have been an overstatement. Simonss3 has discussed the tendency for Kr(3Pz) to give X2* exit channels; but, at the present time there is not a generally accepted explanation of why Kr* differs from the other cases. Since many of the observed Xz* product states are now known to react with Rg to give RgX*, detailed balance arguments suggest that the V(Rg+,X
