J. K. Northrop and F. W. Lampe
30
8 IP-Mo l e x ule
React ions in Monoge rma ne
J. K. hlarthrop and F. W. Lampe* W/?itmofe Laboratory, Deparfment of Chemistry, The Pennsylvania State University, University Park, Pennsylvania 7 6802 (Received July 14, 1972) Publication costs assisted by the U.S. Atomic Energy Commission
Ion--molecule reactions occurring in ionized monogermane have been studied by high-pressure mass spectrometry a t pressures up to 200 p and rate constants for second- and third-order reactions of all primary ions have been measured. As expected the .reactions observed and,the specific reaction rates are much niore similar to those occurring in monosilane than to those occurring in methane.
Recent interest in the ion-molecule reactions occurring in ionized monosikane2-4 has developed in an effort to elucidate possible ionic mechanisms in the radiolysi~s-~ and other high-energy reactionss-10 of monosilane, in view of the fact that ionic mechanisms are well established in methane systems. 11 ~2 Since ion-molecule reactions in monosilane differ significantly from those in methane,l3 it is of interest to determine those in monogermane to uncover any correlations of the germane system with its lighter group TV analogs and for their possible importance in high-energy systems where ions are present.
Experimental Section Appearance potential measurements and low-pressure (%lo p ) kinetic studies were carried out on a Bendix Model 14-101 time-of-flight mass spectrometer, the basic design of which has been described previously.2,14 The ionizing electron beam was pulsed at 10 kc/sec with pulse width 1.4 psec during appearance potential measurements and 0.25 psec during the kinetic studies at 75-eV electron energy. A. 280-V ion drawout pulse of 2.5 psec was imposed after a variable delay time of 0.00 to 1.00 psec. Source pressures were measured with a calibrated McLeod gauge. Higher pressure (10-200 p ) kinetic studies were carried out on a Nuclide 12-90-G magnetic apectrometer which also has been described e1~ewhere.l~ The electron beam energy was 70 eV, the repeller potential was 6.25 V,/crr* leading to an ion-exit energy of 2.1 eV, and the accelerating voll age was 2500 V. Source pressure, as a function of the measured reservoir pressure, was determined using the rate constant for the disappearance of GeH2- obtained from the low-pressure studies. Mono- and digermane were prepared by reduction of GeOz by KBP& in acid solution under 100 Torr of Na according to the method of Jolly and Drake.l' A cold trap a t - 108" and sucrcwme traps of ascarite and anhydrous magnesium perchlorate removed the solvent and higher germanes. Digermane was separated from monogermane by two cold traps at - 117". Thermal drift effectsl7 in the time-of-flight spectrometer were determilied by measuring the dependence of 132Xe+ and s4Kr+ intensities on time delay. As expected, for heavy ion&the effect was negligible in the range 0.000.60 psec; thus only this range was used to determine rate constants and no correction factors were employed. All experimental ion currents as a function of m / e were converted i o monoiuotopic ion currents using the natural abundances of germanium (?OGe, 20.5; 72Ge, 27.43%; The Journai of Physicai Chemistry, Voi. 77, No. 7, 1973
;3Ge, 7.76%; 74Ge, 36.5; 76Ge, 7.76%) in a system of simultaneous equations. The matrix of coefficients involved was inverted by the Gauss-Jordan method.
Results and Discussion A . Low-Pressure Studies. The variation of monogermane ions with time delay at two source pressures is shown in Figure 1. Gef and GeH+ are essentially independent of pressure and time delay and would appear to be neither significant reactants nor products in secondary reactions; however, high-pressure studies, to be discussed later, show both to be reactants. GeH3+ increases both with pressure and time delay, while the only primary ion which shows a marked decrease in ion fraction is GeH2+. Thus, GeH2+ is the major, if not the only, reactant ion and GeH3f is a secondary as well as a major primary ion. The variation of digermanium ions with time delay, shown in Figure 2, indicates they are all. products of secondary reactions. Appearance potentials of the monogermanium ions and those digermanium ions which are accessible with a minimum of isotopic interference are given in Table I. Of these digermanium ions it appears all result from reaction of GeH2+ with GeH4. Formation of GezWe+ from bimolecular reaction of GeH3+ can be eliminated, since this process would require an absurdly low ionization potential of digermane. While GeaHe+ formation from reaction of (1) United States Atomic Energy Commission Document No. COO3416-4. (2) G. G. Hess and F. W. Lampe, J. Chem. Phys., 44,2257 (1966). (3) J. M. S. Henis. G. W. Stewart, M. K. Tripodi, and P, P. Gaspar, J. Chem Phys , in press (4) Tung-Yang Yu, T. M. H. Cheng, V. Kempter, and F. W. Lampe, J. Phvs. Chem. 76,3321 (1972) (5) W.'Ando and S.Oae, &il.Chem. SOC.Jap., 35, 1540 (1962). (6) G. J. Mains and T. Tiernan, USAEC Rept. No, NYO-2007-8 (1965). (7) J. F. Schmidt and F. W. Lampe, J. Phys. Chem.?73,2706 (1969). (8) P. P. Gaspar, 8 . D. Pate, and W. Eckelman, J. Amer. Chem. Soc., 88,3878(1966). (9) P P Gaspar, S. A Boch, and W L Eckelman J Pmer Chem Soc , 90.6914 11968) ~,.. ~
0.K. Snediker &W.
W. Miller, Radiochim. Acta, IO, 30 (1968). (11) F. W. Lamoe. J. Amer. Chem. Soc.. 79. 1055 !19571. (12j G. G. Meisels, W. H. Hamill, and R. 'R.~WilGams.'J. Phvs. Chern.. 61, 1456 (1957). (13) F. P. Abramson and J. H. Futrell, J. Chem. phys., 45, 1925 (1966), and references cited therein. (14) W. C. Wiley and I. H. McLaren, Rev. Sci. Instrum., 26, 1150 (1 955), (15) P. Potzinger and F. W. Lampe, J. Phys. Chem., 74,587 (1970). (16) W. J. Jolly and J. E. Drake, USAEC Rept. No. UCRL 9615,University of California. Los Angeles, Calif. (1961). (17) P. M. Becker and F. W. Lampe. J. Chem. Phys., 42, 3857 (;965). (10)
Ion-Molecule Reactiorls in Monogermane
31 TABLE I: Appearance Potentials of Monogermanium and Some Digermanium tons from GeH4a I _
Ap, eV
Ion
7oGe+ 70GeH 76GeH2+ 76GeH3+ a
10.4 3z 0.2
70Ge2
1 1 2 f 0.2 11.9 f 0.2 10.9 f 0.2
'OGe2H -
+
'OGe2 H2+ 70Ge2t17+ "Ge2W,+
f GeH,
Ge2B,'GeII, -%- Ge&+ -1.
Ge&+ I
12.0 3z 0.2 0.2 11.9 i 11.9 i 0.3 12.0 f 0.3 11 .9 i 0.3
Argon used as internal standard
GeH; GeH,'
.c
AP, eV
Ion
+ GeH4
-
+
2H2
(4)
H, I - H
(5)
GezW4+ f HZ
GeH3+ C Ge1-L- GezH5-I -t-
GI
ions as a
(7)
The rate of change of [GeH2+-]can be written as
Delay Time, microsec
Figure 1. Ion fpactions of primary time U , 4 9 u 0 , 8 7 ~ ~
(6)
function of
delay
-d[GeHz+]/dt = Ziki[GeH2i-][GeH4]
($1
where i indexes ai! reactions involving disappearance of GeH2+. Assuming [GeH4] is constant, integrating and replacing ion concentration by ion fraction, x, leads to
DOSp
X(GeHZ+) = XO(Ge&+) exp( --2:,k,[GeH4]t)
(9)
where XO is the initial fraction formed in the electron beam. Expanding the exponential in (9) with retention of only the linear term introduces a maximum error of L%. To the same order of approximation the rate equations of the other ions give
+
X(GeH3+) = Xo(GeH3+) ( klXo(GeH2+)- k7Xo(GeH3+i[GeH4]t (10) X(Ge2Hy7) = k,+2Xo(GeH~+)[GeH~lt; X(Ge2Hs-b) = F27Xo(GeH3+)[GeH.+]tf iy([C+eH4]2t2) (12)
D e l o j Time, microsec
Figure 2. Ion frsctinis of
time
secondary
ions
as a function of delay
Ge&+ is energetically feasible, it is unlikely that the lifetime of the excited Ge&+ so formed is sufficiently long for its detection in ii bimolecular process. As will be discussed later the high-pressure studies indicate the formation of Ge& i- to be a termolecular process and also that Ge2H5+ arises from reaction of GeH3+. Thus, we conclude that the important himolecular reactions in monogermane are
where the higher order terms in (12) denoted by iy can be neglected on the basis of the linearity of the plots in Figure 2. Thus, ZiFzi and the individual ki are easily determined by appropriate combination of the slopes and intercepts of Figures 1 and 2. B. High-Pressure Studies. The variation of mono- and digermanium ions with source pressure is shown in Figures 3 and 4. GezH3+ was also observed but the intensity is constant beyond 50 and is not shown. It is clear from Figure 3 that G e t and GeH- also react with GeH4; also, a material balance indicates that reaction of Ge&+ does not account for the formation of all product ions. Since the trigermanium ions do not contribute significantly to the sum of ion fraction increases until after 120 p, Ge+ and GeH-'- m.ust be involved in digermanium ion formation. Inasmuch as these reactions coultd not be identified by appearance potential correspondence, an attempt was made to eliminate reactions on energetic grounds. The ionization and. appearance potentials and corresponding A&' of primary ions frorn mono- and digermane measured by Saalfeld and Svec18 are given in 'Table I:I with the upper limits to A&" obtained in this study. The latter were calculated from calorimetric data,lg the exother(18) F E Saalfeld and H J Svec lnorg Chem 2, 46 50 (1963) (19) S R Gunn and L G Green, J Phys Cqem 6 5 , 7 7 9 (1961) The Journai of Physicai Chemistry, Voi. 7 7 , No. 7 , 1973
J. K. Northrop and F. W. Lampe
32 TABLE II: ionic: Heats of Formation -__I_
I_____
A H f , kcal/mol Ion
Ge+ GeH+ GeH*+ Get&+
AP from GeH4, eVa
AHr, Ion
kcal/mol
10.7 f 0.2 11.3 f 0.3 11.8 f 0 . 2 10.8 f 0.3
Gen+ Ge2H+ Ge*H2+ Ge2H3+ Ge2H4+ Ge2H5+ Ge2H6+
269 253 294 21 9
AP from Ge2H6, eVa
From A P
13.1 f 0.3 13.0 f 0.3 12.9 f 0.3 12.8 f 0.3 12.7 f 0.3 12.6 f 0.3 12.5 f 0.3
341 286 336 282 332 277 327
This work
-5314 5 262 5314 5262 4314 5 239
a Reference 18.
Get
I
0
GeH:
0 A
of formation of digermanium ions indicate no ion-molecule reactions to be possible. In the absence of accurate thermodynamic data, indirect energetic considerations and material balances were used to eliminate the following reactions. Ge+
+ GeH4
+ GeH4 GeH+ + GeH4 Ge+
GeH+ GeH+
GeH+ + GeH4
Source Pressure, Microns
Figure 3. Ion fractions of tion of source pressure.
primary ions and Ge2H7+ as a func-
e
+ GeH4
+ GeH4
Ge2+ + 2
-+
(13)
+ H2 + H Ge2+ + 2Hz + €3
Ge2H+
(14)
(15)
Ge2H+ -f- 2K2
(16)
GezHz+ + %312+ H
(17)
Ge2H4+ -i-W
(18)
Using the known heats of formation of GeH+ and GeH4 and the exothermicity condition, the upper limits to the heats of formation of digermanium ions were calculated assuming they were formed by reaction of GeHf. For (IS), (17), and (18) to be energetically possible the appearance potentials of the digermanium ions from Ge& would be necessarily 57.9 eV, which is absurdly low. The initial slopes of the ion fraction curves of Ge2+ and Ge2H+ in Figure 4 in the range 0-80 p are identical with those predicted, assuming that these ions come only from reactions of GeHz+ having rate constants determined in the lowpressure study. This observation eliminates (13), (14), and (16). At pressures beyond 25 p termolecular reactions are sufficiently probable for detection and possible reactions of Ge+ and GeH+ can be described by
05
I+ C GeH, 0
1'
'Source Pressure, Microns
Figure 4. Ion fractions of digermanium ions as a function of source pressure.
micity requirement, and the observed reactions at low pressure. We may first observe that the appearance potentials measured in our work €or monogermanium ions (Table I) are in good agrieement with those of Saalfield and Svec.ls The heats of formation of ions from monogermane shown in Table I1 are probably the most accurate values available, since extreme care was taken to eliminate excess kinetic energy from fragments and monoisotopic monogermane was used.l8 For ions from digermane it was not possible to eliminate this excess kinetic energy and, as Saalfeld and Svec point out,ls the appearance potentials may be high. Comparison with_ the upper limits obtained in this study indicates this to be the case since the heats The Journal of Physical Chemistry, Voi. 77, No. 1, 7973
+
2GeH,
Ge2H,+ li-
F
(19)
.%- GeyHzf C F
(20) where I+ is either primary ion and F is a neutral fragment. These reactions lead to an ion fraction pressure dependence of In [Xo(I+)/X(I+)I= kls[GeH4]t
+ k20[GeH&
(21)
Experimental X(Ge+) and X(GeH+) were fitted to (21) and in both cases K19 is very small, accounting for the fact that neither ion is an observed reactant at low pressure. This is shown in Figure 5a in which (21) is plotted for [I+] = [GeHf], neglecting the linear term. The analogous curve for Ge+ is nearly identical. The second- and thirdorder rate constants determined from the fitting procedure in conjunction with the low-pressure rate constants allow the prediction of digermanium ion fraction curves. The digermanium ions whose experimental pressure dependences agree most closely with these predicted curves are taken to be the product ions. By this method the loss of Ge+ and GeH+ in the pressure range 0-110 p can best be
Ion-Molecule Reactions in Monogermane
33
anel3 and silane4 systems as well, is ( 7 ) . As mentioned above, it is the high-pressure dependence of X(GezHs+) in Figure 4 which suggests its formation by a termolecular rather than a second-order process as Figure 2 would seem to indicate. If one assumes the secondorder process
+
GeH,' GeH, %. Ge,H6" (29) and determines hzs from Figure 2 and an equation analogous to (11),the resulting rate constant predicts a highpressure curve inconsistent with that obtained experimentally both in general shape and in the value of X at the plateau. If instead one assumes that K29, determined from Figure 2, is actually k3o/[GeH4] where h30 is the third-order rate constant corresponding to GeH,"
I
0
10
20
30
(Source P u e s ~ u r e x) ~IO-',
Micron'
Figure 5 . Third-order kinetic plots: (a) reaction of GeH + ; (b) formation of iGe:,l-iT-l-from GeW3+.
accounted for hy the following reactions. Ge+ 4- GeH, Ge'.
-I- 2GeM,
GeH- -t GeH,
-
Ge,H,+
+
H2
(22)
Ge2H4+
+
GeH,
(23)
-% Ge,H,+ % Ge,H,+
C H,
(24)
+
GeH+ 4- 2GeI3, GeH, (25) eyond this pressure several competing reactions make it impossible to fit experimental ion fraction curves with a unique set of rtite constants. Figure 3 shows that a t higher pressures GeH3f becomes an important reactant ion. A material balance indicates the major reacidiondepleting GeH3+ to be GeH,'
h
-t- E e H , A Ge,H-+
+
GeH,
(26)
since, in the pressure region where X(GezH7+) increases, GeH3+ is the only ion present in sufficient abundance to account for GezN[7+ formation. In addition, the form of the X(Gez€17' ) curve suggests GeH3+ as the precursor to Ge221[5+ The basis for this is as follows. If GezH7+ were the only ion produced by reaction of GeH3+ as in (26), the rate of change of IGezM7+] would be described by d[GeZH-,-],'dt
-I
hzs[Ge 4]2{[GeH3+]0- [GezH7+ll ( 2 7 )
in the presstare region where GeHz+ makes negligible contribution to Gel33 k formation by (1). From Figure 3, (27) and its integrated form, namely In { I- [X(Ge&IT +)/Xo(GeH3t)]]= -hzs[GeH&
(28)
should be vdid beyond -100 p. The left-hand side of (28) is plotted iis. the q u a r e of the pressure in Figure 5b where the predicted linear relationship may be recognized but only a t much higher pressure (5165 p) than expected if there were EO competing reaction of GeH3+ of lower order. The most probable competing second-order reaction, analogouis to (131, (221, and (24), and to the meth-
+
t- GeH,
ZGeH, -!-% Ge,H,'
(30)
one obtains a functional dependence consistent within experimental error with the experimental curves at all pressures. It should be pointed out that the appropriate value of t in the pressure region corresponding to termolecular processes is not the collision-free residence time but depends on the pressure. Using the E / P value at 150 p, our source dimensions, and the drift velocity rebults of Warneckz0 adjusted to the average monogermanium ion mass, the average residence time over the pressure range was calsec. culated to be 1.2 x The rate constants obtained as described above are summarized in Table I11 with the rate constants for analogous reactions in monosilane. Figure 6 shows the intensities of the trigermanium ions observed as a function of source pressure cubed, with the exception of Ge3H4+ whose curve is nearly identical with that of Ge3Hz+. In the region of significant trigermanium ion intensity increase, the primary ions are essentially depleted (Figure 3) and thus the precursors must be digermanium ions. Of the trigermanium ions only Ge3+ and Ge3H3+ are cubic in the pressure over a large range (0-165 p). Contrasted with this cubic pressure dependence froq zero pressure, Ge3Hz+, Ge3H4+, and Ge3H5' appear at a much higher pressure (-135 1) and increase with at least a fourth-order pressure dependence. The intensities of the remaining trigermanium ions, Ge3Na+, Ge3H7+, Ge3 and Ge3H9+, have essentially the same pressure dependence, the order of which increases gradually as the pressure is increased, indicating that these ions or their precursors are formed by more than one process depending on the pressure region. Since GezH4+ and Ge?Hs+, which are formed in two processes, are the only digermanium ions whose intensity decreases in the pressure region corresponding to significant trigermanium ion intensity increase, it is reasonable that these are the reactant ions involved. The similarity of both reactant ion curves as well as that of product ions, coupled with a lack of accurate thermodynamic data for both di- and trigermane, preclude further reaction identification. From Table I11 four react ions can be placed into the following form at low pressure IGeH,'
+
GeH,
e ha k
GeH;GeH,+
-* 12 I
(20) P. Warneck.J. Chem. Phys., 46, 502 (1967) The Journaiof PhysicalChemistry. Voi. 77. No. 1, 1973
J. K. Northrop and F. W. Lampe
34 TABLE II I: Cornparison of Reactions in Monogermane and Monosilane
__---
_ _ I _ I _ _
k X l o l o . cm3/molecule sec
Product ion
Reactant ion
Eq no.
M = Sia
M =: Ge _ _ l _ s _
Bimolecular Reactions 22
M+
M2H2'
MH+
M2H+ M2H3'
4.8 f 0.6; 3.2 f 0.3 0.7 f 0.2; 2.8 f 0.07; 2.0 f 0.3
M H2+
MH3+
10.7
24
1 2 3 4
5 6 7
MH3+ ~~
1.0 f 0.2 Not observed
