J. Phys. Chem. 1992, 96, 5685-5687
5685
Measurement of Rate Constants for CH(a4Z) Zhenglin Hod and Kyle D. Bay@* Department of Chemistry and Biochemistry, University of California. Los Angeles, Los Angeles, California 90024 (Received: April 28, 1992)
A method has been developed to follow the concentration of CH(a4Z) by measuring the chemi-ions formed in its reaction with oxygen atoms. The CH(a4Z) is made by multiphoton dissociation of bromoform. Excess methane is present to act as a scavenger of ground state CH. The rate constant for the reaction of CH(a42) with O2is (2.6 f O S ) X 10-" an3molecule& s-l. No reaction could be observed between CH(a4Z) and CH4, H2, or D2.These rate constants are at least lo2-lo3 times smaller than the corresponding values for CH(X2*). The low reactivity of CH(a42) with H2 and D2 confirms a prediction by Brooks and Schaefer.
Introduction Little is known about the kinetics or chemical reactivity of CH(a4Z). Although this low-lying metastable state of methylidyne has been predicted by theory for many years,'s2 it has only recently been observed directly in the gas phase by laser magnetic rmnance (LMR).3 The electron photodetachment spectrum of CH- places the a42 state of CH 0.74 eV above the ground X2* state,4 in agreement with theorya2 Calculations predict that CH(X2?r)will react readily with H2, while CH(a42) will not.5 Strong LMR signals attributed to CH(a4Z) occur during the reaction of oxygen atoms with acet~lene,~ which suggests that CH(a4Z) is formed during hydrocarbon combustion. However, CH(a42) is not included in combustion models because its mechanism of formation and its rate constants are completely unknown. Evidence has been presented6that both CH(X2a) and CH(a42) are involved in the Calcote chemi-ionization reaction:
+
0 + CH -,HCO' e(1) that occurs in hydrocarbon flames. Using laser photolysis, we have formed CH(a42) in the presence of oxygen atoms and excess methane. The time dependence of chemi-ion formation is used as a surrogate for the time-dependent CH(a42) concentration. The excess methane acts as a scavenger for ground-state CH(X2r),allowing the rate constants of CH(a4Z) to be measured without interference by ground state CH.
Description of Experiments The CH(a4Z) was formed in a flow system by multiphoton decomposition of bromoform at 193 nm. Typical flows contained 3 m T m of CHBr3,approximately2 mTorr of O(3P) plus an equal amount of CO, 18 mTorr of C02, 100 mTorr of CH4, with He as a carrier gas at a total pressure of 2.5 Torr. Absolute flows were determined by measuring pressure d r o p in known volumes. Total flows were approximately 2 cm3 atm s-I. The reaction cell was a Pyrex cylinder, 4.5 cm in diameter, with gold-plated metal electrodes spaced 1.95 cm apart forming the ends of the cylinder. In the center of one of the electrodes was a conical pinhole 0.2 mm in smallest diameter. Approximately 20 mJ of laser radiation passed through the reaction cell perpendicular to the cylinder axis. The light was brought to a focus in front of the pinhole, midway between the electrodes, by a quartz lens which had a focal length of 20 cm. An electric potential of 157 V was applied between the end plates, with the pinhole electrode negative. This electric field is still within the saturation current region.' Using the mobility of HCO+, one can calculate that for these conditions, ions formed in the middle of the cell should reach the pinhole within 2 ~ s , allowing little time for ion-molecule reactions.8 Positive ions passing through the pinhole were accelerated by 45 V into a quadrupole mass spectrometer. Transmitted ions were detected by a Daly doorknob-scintillator-photomultiplier and the pulses were accumulated on a multichannel scaler as a function 'Permanent address: Chemistry Department, Shandong University, Jinan, Shandong, PRC.
of time after the laser pulse. The laser repetition rate was 40 Hz, and signals from lo4 laser pulses were summed. Oxygen atoms were made by passing a flow of 1% C 0 2 in He through a microwave discharge (2450 MHz). Using a discharge in 0 2 / H e gave similar results, although the signal decays were somewhat faster due to the presence of undissociated 02.The bromoform and other gases were added to the oxygen atom stream just before the reaction cell. All experiments were done at 294 f 2 K.
Observations The time dependence of the signal at mass 29 is shown in Figure 1, both with and without oxygen atoms being present. The signal without oxygen atoms was typical of that observed at several masses and is attributed to ions made by multiphoton ionization (MPI). The maximum signal occurs at 35 ps, which is comparable to the sum of the calculated drift time within the cell and the flight time for a mass 29 ion through the quadrupole mass spectrometer (25 ps). This MPI signal decays rapidly, approaching background levels by 100 ps after the laser pulse. With oxygen atoms present, a stronger signal is observed at mass 29 that lasts to at least 400 ps. The following observations lead us to assign this signal to a chemi-ionization involving CH(a42). (1) Only the mass 29 signal was enhanced when oxygen atoms were added. (2) The slow exponential decay seen in Figure 1 occurred only for mass 29 and only in the presence of oxygen atoms. Increasing the oxygen atom concentration increased the rate of decay of this signal. (3) Adding 100 mTorr of methane to the system did not change the rate of decay of the mass 29 signal. The amplitude was observed to decrease somewhat with added methane, probably due to ion-molecule reactions of HCO+. (4) Adding 02,which is known to quench chemi-ionization in the 0 + C2H2system? increased the decay rate of the mass 29 signal (Figure 2). The "chemi-ion signal" in Figure 2 is the difference in the mass 29 signal with and without oxygen atoms. When the measured decay rate (-d[ln(chemi-ion signal)]/ d t ) is plotted against the partial pressure of 02,a straight line results (Figure 3). (5) The addition of large amounts of hydrogen or deuterium had no effect on the decay rate of the chemi-ion signal (Figure 4). Discussion Any ions formed directly by the laser pulse should be swept out of the cell in less than 5 ps by the uniform applied electric field. The actual lifetime of the signal in Figure 1 is about 65 ps, which means that the mass 29 ions are being formed by the reaction of neutral particles, Le., they are chemi-ions. If oxygen atoms (mass 16) are needed to form a mass 29 chemi-ion, the collision partner must contribute mass 13. The simplest interpretation is that O(3P) is reacting directly with CH to form HCO+. But this CH cannot be in its electronic ground
0022-365419212096-5685.$03.00/0 0 1992 American Chemical Society
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Letters
5686 The Journal of Physical Chemistry, Vol. 96, No. 14, 1992
8 L ' " ' ~ ' " ' ' " ' " " ' a 200 400 600 800 1000
10'
1200
Partial Pressure / mtorr I "
0
100
300
200
400
500
Time / microsecond
Figure 1. Accumulated signal at mass 29 as a function of time after the
laser pulse. Open squares are for no oxygen atoms present (no microwave discharge),and filled squares are with oxygen atoms. Total pressure was 2.5 Torr, and 100 mTorr of CH4 was also present.
Figure 4. Decay rate of the mass 29 signal as a function of partial pressure of CH4(A),H2( O ) ,and D2(0). Error bars as in Figure 3. The total pressure increased as these gases were added, from 2.1 to 2.7 Torr. All of the H2 and D2runs had 100 mTorr of CHI present.
molecule, the radiative lifetime of CH(a42) is probably much greater than 65 ps. Since the oxygen atoms are present in large excess, the decay of chemi-ions formed in reaction l a will mimic the decay of CH(a42). Thus the chemi-ion signal is a surrogate for the CH(a42) concentration. The threebody reaction 0 + O2is slow at these low pressures,12 so addition of O2to the cell will not affect the concentration of O(3P). Therefore the increased decay rates shown in Figure 2 are attributed to reaction 2. The slope of the weighted least O2
"Id
'
100
'
200
'
300
'
400
'
Time / p Figure 2. Chemi-ion signal at mass 29 as a function of time. The filled squares are for no added O2(data from Figure 1). The open squares had 4.3 mTorr of O2 and the triangles had 20.2 mTorr of O2 present.
+ CH(a42)
-
Oxygen Pressure / mTorr
state; with 100 mTorr of CH4and 2 mTorr of O(3P) present, one can calculatelo that 98% of any CH(X2?r) formed should react with CHI instead of with O(3P),and the CH(X2?r) lifetime should be -3 ps. Adding even more methane does not alter the decay rate of the chemi-ion signal (Figure 4). While CH(X2?r) is undoubtedly being formed by the laser pulse, as shown by LIF measurements," it will react rapidly with methane in the present experiments and thus cannot be responsible for the slowly decaying chemi-ion signal in Figure 1. Since CH(a42) has been observed in the 0 C2H2system by LMR3 and since some of the chemi-ionization in that system is not quenched by methane: we attribute the slowly decaying chemi-ion signal in the present study to reaction la. Reaction
+
O(3P)
+ CH(a42)
-
HCO'
+ e-
(la)
l a is spin allowed and exoergic by 0.94 eV. Since this is a light
(2)
squares line in Figure 3 gives a value for k2 of (2.6 f 0.5) X 10-" cm3 molecule-' s-I. The error limits come from 95% confidence limits based on the scatter of points in Figure 3 and a possible 15% systematic error in absolute concentration measurements. The reaction of ground-state C H with O2has been proposed to explain the chemiluminescence of OH that is observed in hydrocarbon flames.I3 Reaction 2a should now be considered as
O2 + CH(a42)
-
OH
+ CO
(24
another source of OH(A22). Reaction 2a is also sufficiently exoergic to form several of the triplet states of CO. Other products of reaction 2 could include C02 H, or C O + 0 + H. Addition of H2or D2 at partial pressures up to 1 Torr did not affect the decay rate of CH(a42). A least-squares fit of the H2 data in Figure 4 gives a rate constant of (0.0 f 4.9) X cm3 molecule-' s-l. Using the upper 95% confidence limit, the rate constant for the removal of CH(a42) by H2 is
