T H E
J O U R N A L
O F
PHYSICAL CHEMISTRY Registered in US.Patent Office
0 Copyright, 1978, by the American Chemical Society ~
VOLUME 82, NUMBER 25
~~~~~~~~
DECEMBER 14, 1978
Rate Constants for the Reactions of Atomic Chlorine with Group 4 and Group 5 Hydridest D. J. Schlyer," A. P. Wolf, Department of Chemistry, Brookhaven National Laboratory, Upton, Long Island, New York 11973
and P. P. Gaspar Depariment of Chemktry, Washington University, Saint Louis, Missouri 63 130 (Received March 9, 1978; Revised Manuscript Received September 18, 1978) Publication costs assisted by Brookhaven National Laboratory
The rate constants for the reaction of atomic chlorine with silane, germane, arsine, and phosphine have been determined using the technique of discharge flow resonance fluorescence. The values are compared to the rate constants for the reactions of methane and ethane with chlorine atoms.
Introduction Although reactions of atomic chlorine with various organic substrates have been extensively studied,l the reactions of atomic chlorine with second and third row hydrides have been the subject of very few investigations.2 The kinetics of these reactions are of importance for the possible utilization of these methane and ammonia analogues in the HC1 chemical laser. The system H2-C1, has been thoroughly studied with respect to infrared chemical laser^,^ but this system has the disadvantage that the step C1. Hz --* HC1 H
+
+
is nearly thermoneutral and therefore the HC1 produced in this step is not vibrationally excited. Utilization of the low bond energies of the second and third row hydrides might increase the efficiency of the laser system by increasing chain lengths and minimizing concentrations of radical intermediates (efficient deactivators of HCl*), The kinetic technique being utilized to study these reactions is that of discharge flow-resonance fluorescence Research carried out at Brookhaven National Laboratory under contract with the U.S. Department of Energy and supported by its Division of Basic Energy Sciences. 0022-3654/78/2082-2633$01 .OO/O
spectroscopy. The technique of resonance fluorescence has several advantages as an analytical technique for fast bimolecular r e a ~ t i o n s .These ~ are (1)it is inherently more sensitive than absorption spectrometry, (2) at low optical depths the fluorescence intensity is linear with the atom concentration, and (3) statistical errors in counting photons is generally the limiting factor in sensitivity as opposed to source instabilities as in absorption spectrometry. There are some disadvantages to this technique which must be carefully minimized in order to obtain reliable results. The most serious of these is that the fluorescence intensity is not linear with atom concentration a t high atom concentrations and care must be taken to ensure a linear relationship exists under the conditions of the experiment. Other considerations include quenching of atomic fluorescence lines by added gases and interference with fluorescence intensity measurements by atomic or molecular resonance lines excited by atomic emission lines.
Experimental Section A schematic diagram of the discharge flow-resonance fluorescence (DFRF) system is shown in Figure 1. Apparatus. The flow line consisted of 12-mm i.d. cylindrical tubing fitted with three reagent inlet jets a t 10-cm intervals along the flow path. The distance from 0 1978 American Chemlcal Society
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The Journal of Physical Chemistry, Vol. 82, NO. 25, 1978 LIP
Figure 1. Experimental apparatus for discharge flow resonance fluorescence.
the discharge to the first inlet jet was 10 cm while the distance to the observation port was 40 cm. The entire flow tube was treated with orthophosphoric acid to reduce the wall recombination of chlorine atomsa5 The leak rate of the system is less than 1 mtorr L-l h-l. The pressure in the line was measured a t a point just past the observation port of the monochromator utilizing a Barocel electronic manometer (range 0-1000 torr). The flows of the helium and chlorine were monitored and controlled by Matheson digital mass flow controllers calibrated a t the factory and checked in this laboratory using a water displacement method. The flow velocity was determined from the flow rates and pressure in the system during operation. The reagent flow was regulated with a needle valve and ball-type flow meter (Manostat Co.) while the actual reagent flow was calculated by monitoring the pressure drop in the substrate reservoir. The microwave discharges of 2450 MHz, maintained with Evenson aircooled cavities, were used in both the main flow line and the flow emission lamp. The concentration of atomic chlorine is highly dependent on microwave input power, pressure, and flow of molecular chlorine and therefore these parameters were always kept constant during the reaction, The microwave input power was monitored with a bidirectional power meter (Opthos Instrument Co.) which also allowed the cavities to be fine tuned to obtain highest power input to the gas vs. power reflected back to the microwave generator. Optical System. (i) Emission Lamp. The lamp consisted of a plasma containing 0.5% Clz and 99.5% He a t a pressure of approximately 0.8 torr flowing a t a velocity of 1000 cm/s. The lamp was isolated from the system with LiF windows which transmit light of wavelengths greater than 1050 A. Self-reversal was a severe problem in the lamp but is not as critical as in absorption spectrometry. The flow in the lamp was regulated with a ball-type flowmeter and adjusted to obtain the highest intensity of the fluorescence line. Self-reversal in the main flow line is a much more serious problem and will be discussed later. (ii) Monochromator and Detector. The individual lines of the atomic chlorine resonance fluorescence multiplet
D. J. Schlyer, A. P. Wolf, and P. P. Gaspar
between 1336 and 1390 A were isolated with a 0.3-m McPherson Model 218 monochromator with 100-pm entrance slit and a 2400 oove/mm grated coated with MgFz The pressure inside the monoand blazed a t 1500 chromator was always maintained a t less than torr. At the exit slit a Model 650 McPherson photomultiplier detector, equipped with a EM1 Type 9514 photomultiplier tube, was attached to the monochromator with a vacuum window coated with sodium salicylate phosphor. This arrangement is reported to have virtually invariant quantum efficiency in the vacuum ultraviolet region! The photomultiplier output was preamplified and tuned to the frequency (100 Hz) of the electrical chopper (Bolova Co. Type L40C) which was positioned next to the entrance slit. Materials. The helium-chlorine mixture (0.5% Clz) used in both discharges was prepared by Matheson Gas Products using UHP helium. The silane, germane, phosphine, and arsine were semiconductor grade gases (99.9%) also supplied by Matheson Gas Products. The methane and ethane were research grade (99.98%) supplied by Phillip's Petroleum Co. The purity of the compounds was checked by gas chromatography (CH4and CZHs), infrared spectroscopy (SiH,, GeH,, ASH,, PH,, CH4, and CZHs), and mass spectrometry (SiH4,GeH,, PH3, and ASH,). Kinetic Procedure. Flows of chlorine atoms were first established in both the main flow line and the emission lamp. Just beyond the microwave discharge in the main flow line the gas stream was further diluted with a tenfold excess of He which had been passed t,hrough an inert gas purifier (Mathis Co.). This gas stream was then pumped a t low pressure (-3 torr) through the flow tube at velocities of about 2000 cm/s. The total flow in the system was carefully monitored during the course of the reaction and was found to vary by less than 2% over the course of a series of experiments (1h). When using resonance fluorescence as an analytical tool, one must ensure that the observed fluorescence intensity is a linear function of the atom concentration. In these studies the 1379-, 1396-, and 1390-A lines of the chlorine multiplet were used. These particular lines have the lowest oscillator strengths and should have the weakest self-reversal. At the beginning of each experiment, the chlorine flow was adjusted and the fluorescence intensity recorded to verify that a linear relationship between intensity and atom concentration did exist under the conditions of the experiment. Once all the flow parameters had stabilized (10 min), substrate was added a t a specified flow rate from the reservoir through one of the inlet jets. The fluorescence intensity was recorded for a period of about 5 min and the substrate flow was stopped. After the fluorescence intensity had returned to its original value and stabilized, the procedure was repeated using different substrate flows and/or a different inlet jet. The scatte*d light from the lamp was always less than 5% of the chlorine atom fluorescence intensity with no substrate flow in the system.
Results Calibration of Atomic Resonance Fluorescence Intensity. The accuracy of rate constants determined by the technique of resonance fluorescence is dependent on the assumption that a linear relationship exists between atom concentration and fluorescence intensity. Several preliminary studies were undertaken to ensure that the parameters which determine the optical properties of this system were well understood. The first experiment was to measure the emission spectrum of the lamp used (see Figure 2). It was found
The Journal of Physical Chemistry, Vol. 82, No.
Atomic CI Reaction with Group 4 and 5 Hydrides
I
32 -
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A
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5
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+
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1363i 1379A 13908 1396h
I
25, 1978 2835 I
l
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/ c
E c
m
r;
rr
[cI.] ( X I O ' ~otoms/cm3)
Figure 3. Curves of growth for some transitions in the chlorine atom spectrum.
Flgure 2. Emission spectrum of the chlorine atom lamp.
TABLE I transition (3p5-3p44s')
A,
a
1389.69 1379.53 1373.12 1347.24 1335.72 1396.53 1363.45 1351.66
Aki(x l o 8
fik 1 x 10-4 0.0031 4.1 x 10-5 0.114 0.0233 8.8 x 10-4 0.0418 0.088
S-1)
0.0023 0.11 0.0029 4.19 1.74 0.015 0.75 3.23
that even a t low atomic chlorine concentrations the fluorescence intensities of the transitions from the upper 2P3,2and 2P1,2states to the ground 2P3,2and 2Plj2states were not in accordance with the published transition probabilities (see Table I). These transitions have the highest oscillator strengths of the chlorine multiplet and as a result should show the highest self-reversal. It can therefore be concluded that the geometry of our lamp is such that the high intensity transitions will always be self-reversed. The emission spectrum from our lamp was very similar in appearance to that measured by Bemand and ClyneU8 In a second series of experiments the curves of growth of the fluorescence spectra were measured while varying chlorine concentration in the main flow while the pressure and flow rate in the lamp were held constant. It was found that a plot of the fluorescence intensity vs. concentration of atomic chlorine was linear a t low atomic chlorine concentrations. The positions of the maxima were different for each transition but the qualitative behavior of the fluorescence intensity as a function of concentration was the same for all the transitions. It was also determined that in the linear portion of the plot, the ratio of transitions from a single upper energy level to the two ground states was in agreement with the transition probabilities. This implies that the resonant lines were not self-reversed. In this system the isotope and hyperfine structure of the lines is unresolved. T o determine if the curves of growth measured in this study coincided with those obtained by other^,^^^ an ab-
solute measurement of the chlorine atom concentration was undertaken using the ClNO titration technique.1° The curves of growth for some of the atomic transitions are given in Figure 3. The concentration range over which a linear relationship exists between atom concentration and fluorescence intensity is from approximately 1O1O to W2 atoms/cm3. Two other possible influences on the fluorescence are those of quenching of the reactant by the substrate and third body recombination processes with the substrate acting as a third body. T o verify that these processes are slow, perfluorobutane, benzene, or HC1 was added at the substrate inlet and the fluorescence intensity was monitored. The intensity was invariant a t all but extremely high concentrations of substrate. It can therefore be concluded that reaction is much faster than quenching or recombination in this system. Rate measurements were performed utilizing the 1379-, 1390-, and 1396-A transitions. There was no systematic variation of rate with transition thereby supporting the assumption that the quenching rate was slow in comparison to the reaction rate. Estimates of the importance of pressure drop, radial diffusion, and wall recombination along the flow tube have been made for these experimental conditions using the method of Kaufman.ll The pressure drop may be approximated by the relation A p / l = 1.18 X 10-%/r2 where u is the average velocity in cm/s, r is the radius of the flow tube, 1 is the length of the tube, and Ap is the change in pressure. For our experimental parameters Ap is approximately 0.03 torr or 1%of the total pressure. The radial concentration gradient may be estimated from the equation
where co is the concentration a t the center of the tube, c, is the concentration a t the wall, c is the average atom concentration, r is the radius of the flow tube, D is the diffusion coefficient of chlorine atoms in helium gas, k, is the rate constant for the gas phase recombination of the chlorine atom, and k, is the rate constant for the atom recombination on the walls. Literature values of 5.5 X lo-% cm6 molecule-' 5-l for k:2 and 1.4 s-l for hwl3were used in the calculation. A value of 200 cm2/s was calculated for D. Using these values, the fractional radial concentration gradient was determined to be on the order of The analysis of the data was carried out assuming a pseudo-first-order reaction where the concentration of the
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The Journal of Physical Chemistry, Vol. 82, No. 25, 1978
P. Wolf,
D. J. Schlyer, A.
and P. P. Gaspar
TABLE 11 bond disscn energy, kcal/mol
substrate
CH, CZH, SiH, GeH, PH3 ASH, HZ See ref 15. cm6/molecule2s.
rate constant, cm3/molecule s this work 1.3 i 0.2 x 4.0 k 1.2 x 9.2 2.0 x
104a 9 8a 89-94d
*
1 0 4 3
lo-"
substr concn, molecules/cm3
lit. 1.2 c 0.3 5.9 ?: 1.3
x 10-13b x
2.2 c 1.0x 1 0 - 2 3 " > 2 . 0 x 10-'0'
85-90f
58h 104a
1-10 x 10" 2 - l o x 10"
0.7-4.6 X 10" 0.7-4.7 X 1 0 ' ' 0.7-4.7 X 0.5-2.7 X 1 0 "
