6816
J. Phys. Chem. 1993,97, 6816-6821
Isotopic Branching Ratios and Translational Energy Release of H and D Atoms in Reaction of O(lD) Atoms with Alkanes and Alkyl Chlorides Yutaka Matsumi, Kenichi Tonokura, Yousuke Inagaki, and Masahiro Kawasaki’ Institute for Electronic Science and Graduate School of Environmental Earth Science, Hokkaido University, Sapporo 060, Japan Received: January 13, 1993; In Final Form: April 8, I993
H and D product atoms from the reaction of O(lD) with alkanes and alkyl chlorides have been measured by multiphoton ionization as well as by laser-induced fluorescence. Measured isotopic branching ratios [HI/ [D] in the reaction of O(lD) with CH3CD2CH3, C D ~ C H ~ C DCD3CHClCD3, B, and CHsCDClCH3 are almost statistical at collision energies, 8.1-9.1 kcal/mol. The Doppler profiles of the H atoms from the reactions of O(1D) hydrocarbons (CH4, C2H6, and C3Hg) suggest that the translational energy released to the RO H products is 8 f 1 kcal/mol and almost independent of the size of the alkyl groups. The reaction rate constants between O(lD) and H2, CH4, C2H6, C3H8, and CHpCl are evaluated from the measurement of the time evolution of the H atom signal at thermal collision energies (300 K). Relative quantum yields are estimated for the H formation process over the total reaction processes.
+
+
Introduction
0 (‘Dl t R H
The reaction dynamics of O(1D) with alkanes is characterized by a unimolecular decomposition of the hot molecules due to their large exothermicity.’ Figure 1 shows a deep potential minimum corresponding to ROHt on the ground electronicsurface lA’.2 When vibrational states are energized sufficiently, for example, in the intermediate CH3OHt of the reaction of O(lD) with methane, the CH3OHt can dissociate to form largely an OH radical as well as an H atom to a lesser extent.” When O(lD) reacts with more complex hydrocarbons, ethane and propane, the C-C bond cleavage appears to dominate dissociation of the activated intermediatecomplex.5 The energetics for the reactions of O(1D) with C2H6 are as follows (AHin kcal/mol6):
O(’D)
+
C2Hs
+ H,
7
-20
0
0 Y
0, L
a -100 c
w
(la)
AH=-51
(lb)
C2H5 + OH, A H = -50
(IC)
+ CH3, A H = -46 o ( ~ P+)c ~ H ~AH , = -45
(14
CHaO
-
-L
r
-120
CZHSO+ H, A H = -39 CH3CHOH
O
-140
Figure 1. Schematic potential energy diagram for the O(lD) + RH reactions. OCH,CI
(le)
Casavecchia et ale4observed the production of CH30 in the crossed beam reaction of O(1D) + CH4 CH3O + H. A recent determination of the H atom yields following the O(1D) CH, reaction studied by Satyapal et al.’ suggests that 24% of reactive collisions proceed via H atom formation. Park and Wiesenfelds measured internal energy distributions and some selectivity in A-doublets of the OH(2II) products in the reaction of O(lD) with CH,, C2H6, C3H8, and C(CH3),. The OH product from C3Hs displayed a much cooler rotational distribution than was seen from CH,. As the size of RH increases, the A-doublet selectivity diminishes gradually. That observation seems reasonable in view of the increasing molecular size of the ROHt intermediate and hence a more poorly defined plane of dissociation. They concluded that this fact largely supports an insertion mechanism for these reactions. The reaction of O(1D) with RCl produces H and C1 atoms along with OH radicals. Energetics of the reaction processes are listed for CH3Cl (AHin kcal/molq: In this paper, we report the dynamics of H atom formation from the reaction of O(lD) with alkanes or alkyl chlorides at reagent center-of-mass collision energies of 8-9 kcal/mol,
CH30 O(’D)
+ CHJCI
+
0022-3654/93/2097-68 16$04.Oo/O
+ H.
+ CI,
A H E -38 A H = -53
(2a) (2b)
CHzCl+ OH, A H = -46
(2c)
CHzO + HCI, A H = - 134
(2d)
+ CHsCI,
A H = -45
(2e)
measuring (a) Doppler profiles of the product H atoms from which average kinetic energies can bc calculated and (b) [HI / [D] ratios from the reactions of O(lD) with partially deuterated propanes and 2-chloropropanes that provides information on the isotope- and site-specific nature of the reactions. We also report the total reaction rate constants and quantum yields for the H atom formation proccss from the measurements of the time evolution of the H atom concentration for the reactions of O(lD) with H2, CH,, C2H6, C3Hs. and CH$l at thermal collision energies (300 K). ExperimenW Section
Reaoname-Enbmeed Moltipboton Ionization Measument. The experimental setup used was almost the same as in our @ 1993 American Chemical Society
The Journal of Physical Chemistry, Vol. 97, No. 26, 1993 6817
Reaction of O(lD) with Alkanes and Alkyl Chlorides previous studies.* O3(2 Torr)/RH (2Torr) was mixed just before passing through a pulsed nozzle (General Valve, time width 0.8 ms) into a vacuum chamber. Pressures of the reaction and detection chambers pumped separately were 5 X l e 5 and 1 X 10-6 Torr, respectively. KrF excimer laser light at 248 nm (-2 mJ/pulse, 10 Hz) dissociated O3 and produced translationally hot O(1D) atoms. About 100 ns after the excimer laser pulse, a probe UV laser (-0.2 mJ/pulse) was fired to detect H and D atoms produced from the reaction. An exponential rise of the signal intensity was observed typically with a rise time of submicroseconds. For the probe light the output of a tunable dye laser was frequency doubled with a BBO crystal and focused with a lens (f=200 mm). The delay between the photolysis and probe light pulses was controlled with a time jitter of about 10 ns. The spectra of H and D atoms were measured by (2 1) REMPI at 243.135 and 243.069 nm, respectively. The resulting H and D ions were detected by an electron multiplier. Detection sensitivity of our REMPI system was checked by the measurements of H and D atoms from a microwave discharge of a 1:l mixture of H2 and D2, which was 1.0 f 0.1. The probe laser power was almost constant over the H and D resonance wavelengths (&lo%). These signal intensities showed quadratic dependence on the probe laser intensity. The isotopic [H]/[D] branching ratios in the reaction of O(1D) with partially deuterated compounds were calculated from the measured areas under the peaks in thespectra after the correctionfor quadraticdependence on the probe laser power. Chlorine atoms in the 2P1/2and 2P3/2 level were detected by REMPI at 235.336 and 237.808 nm, respectively. The REMPI sensitivity of C1*(2P1/2) was larger than that of Cl(2P3/2)by a factor of 2.5 f 0.1, which was tested by laser-induced fluorescence of product chlorine atoms from photodissociation of hydrogen chloride and alkyl chlorides.9 The relative REMPI sensitivity of hydrogen atoms to Cl(zP312) atoms was found to be 4 f 1 from the photodissociation experiment of HCl at 193 nm. The photolysis and probe laser beams were usually collinearly counterpropagated at right angles to the molecular beam. However, for measurement of the polarization effect on Doppler profiles, two linearly polarized laser beams were perpendicularly crossed with eacb other. Two different polarization configurations of & (1 1, and Ed I 1, were used where & is the polarization vector of the dissociation laser and 1,is the propagation direction of the probe laser. Since the beam diameter of the photolysis laser was large enough (2 mm), within a 100-ns delay the reactions were presumed to proceed homogeneously in the probing region and escape of the products from the viewing zone could be ignored. Therefore, the measured concentration ratios [H]/[D] are equal to the channel branching ratios of the reaction @ (RO+H)/@(R'O+D) for the reaction. Laser-Induced FluorescmceMeasuremt. The [HI / [D] ratios and Doppler profiles from O(lD) R H were also measured with a LIF method at the Lyman a wavelength (121.6 nm)? 0 3 (8 mTorr) and RH (60 mTorr) gas was mixed and introduced into a reaction cell. The size of the cell was 60 X 60 X 60 mm3 and pumped by a rotary pump with a liquid N2 trap. O(lD) atoms were produced in the photolysis of O3at 248 nm. The probe light for H and D atoms around 121.6 nm was generated by four-wave mixing using a 2wl-w~scheme in a Kr gas cell ( 1 5 Torr) with two tunable dye lasers pumped by a XeCl excimer laser (308 nm, 200 mJ/pulse).10 The output of the vacuum-UV light going through the reaction cell was monitored with a vacuum-UV monochromator and a solar-blind photomultiplier. The laser-induced fluorescencewas observed by another solar-blindphotomultiplier at right angles to both the photolysis and probe laser through a LiF window and a band-pass filter (Acton Research, Model 122VN, X = 122 nm, AX = 12 nm). The delay (20 ns-1000 ps) between the photolysis and probe light pulses was controlled with
L
+
+
I
1
243.05
I
243.1
I
243.15
1
Wavelength I nm F i p e 2. H atom and D atom Doppler profiles from the reaction of O(1D)with CH3CD2CHsor CD~CHZCD~, using the resonancc-enhanced multiphoton ionization technique. O(lD) is formed by the photolysis of 0 3 at 248 nm.
a time jitter of about 10 ns. CH3CDzCH3 and CDsCH2CD3 were supplied by Merck, Sharp and Dohme of Canada. CH3CDClCH3 and CD3CHClCD3 were synthesized from the corresponding acetone," and their purity (>99%) was checked by NMR and mass spectroscopy. The kinetic study for production of H atoms was performed by the vacuum-UV LIF method by changing the delay up to 50 ps and pressure of RH (20-200 mTorr) with a constant pressure of Ar (1-3 Torr) and O3 (
