Lifetime of the CH3OH* intermediate in the reaction CH4+O(1D2) CH3OH* CH3+OH

Article abstract of DOI:10.1063/1.469132

Subpicosecond lasers measured the appearance rate of OH X(v=0) following 267 nm photolysis of the CH4*O3 van der Waals complex.The rise of the OH A <*> OH X laser-induced fluorescence with respect to the photolysis/probe delay time, t_D, was LIF(t_D) = 1-exp(-t_D/τ) with τ approximately 3 ps, indicating that the reaction CH4+O(¹D₂) <*> CH3+OH involves a CH3OH^* intermediate with that lifetime.No prompt OH(v=0) from a direct or fast reaction was observed.

Full text of DOI:10.1063/1.469132

Lifetime of the CH3OH* intermediate in the reaction CH4+O(1 D 2)→CH3OH*→CH3+OH
Roger D. van Zee and John C. Stephenson
Citation: The Journal of Chemical Physics 102, 6946 (1995); doi: 10.1063/1.469132
View online: http://dx.doi.org/10.1063/1.469132
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LETTERS TO THE EDITOR
The Letters to the Editor section is divided into four categories entitled Communications, Notes, Comments, and Errata.
Communications are limited to three and one half journal pages, and Notes, Comments, and Errata are limited to one and
three-fourths journal pages as described in the Announcement in the 1 January 1995 issue.
COMMUNICATIONS
*
Lifetime of the CH3OH intermediate in the reaction
1
*
CH4؉O( D2)˜CH3OH ˜CH3؉OH
Roger D. van Zee and John C. Stephenson
National Institute of Standards and Technology, Gaithersburg, Maryland 20899
͑Received 6 February 1995; accepted 22 February 1995͒
Subpicosecond lasers measured the appearance rate of OH X( ϭ0) following 267 nm photolysis of
v
the CH4•O3 van der Waals complex. The rise of the OH A←OH X laser-induced fluorescence with
respect to the photolysis/probe delay time, tD , was LIF(tD)ϭ1Ϫexp(ϪtD/␶) with ␶ approximately
1
*
3 ps, indicating that the reaction CH4ϩO( D2)→CH3ϩOH involves a CH3OH intermediate with
that lifetime. No prompt OH( ϭ0) from a direct or fast reaction was observed. © 1995 American
v
Institute of Physics.
Because of its importance in combustion and atmo-
spheric chemistry, the reaction between CH4 and O(1D2) has
been the subject of numerous investigations, including
chemical quenching1–3 and molecular beam4 experiments,
spectroscopic measurements of the energy content of the
H,5,6 OH,7–14 and CH3 15,16 products, and ab initio studies.17
The chemical and beam results show that the formation of
CH3ϩOH ͑ϳ75%͒ and CH3OϩH ͑ϳ25%͒ dominate. Very
small yields, if any, are found in CH2OϩH2, CH2ϩH2O, and
COϩ2H2. Despite this large body of work, the nature of the
transition state for the dominant channel, CH4ϩO(1D2)
→CH ( ,J)ϩOH( ,J) ͓⌬H͑0 K͒ϭϪ182.0 kJ molϪ1͔ re-
ϩO2→CH3ϩOHϩO2, was determined by laser-induced
probe pulse. This approach was first used18–20 to determine
fluorescence ͑LIF͒ spectroscopy of OH using a time-delayed
*
the lifetime of the HOCO intermediate in the reaction
*
CO2•HIϩh␯uv→HOCO ϩI→OHϩCOϩI and has also been
applied to other reactions.21
Our laser apparatus consisted of a passively mode-
locked titanium doped sapphire ͑Ti:S͒ laser, which produced
90 fs pulses, approximately transform limited in spectral
content and centered at 800 nm, at 100 MHz repetition rate.
These pulses were stretched to 200 ps, amplified to 2 mJ at
20 Hz repetition rate in a home-built Ti:S regenerative am-
plifier, amplified to 20 mJ in a second Ti:S rod, and com-
pressed to 110 fs. Part of this light was frequency tripled to
give 200 ␮J pulses at 267 nm for the photolysis laser. Part of
the compressed 800 nm light generated a white light con-
tinuum, which was passed through a notch filter to select
light near 500 nm. This 500 nm seed pulse was amplified in
a series of dye cuvettes and summed with 800 nm to give 20
␮J pulses near 308.0 nm ͑⌬␭FHWMϳ2.5 nm͒ to probe the OH
A(2⌺, ϭ0)←OH X(2⌸, ϭ0) transitions. The cross-
v
v
3
mains controversial. It has been hypothesized1–17 that some
*
OH is formed from an excited methanol ͑CH3OH ͒ transi-
tion state, in which energy is randomized among the vibra-
tions before fragmentation into CH3 and OH, with a unimo-
lecular decay rate consistent with statistical theories. Other
OH may be formed from a very short-lived transition state
from either an H-stripping type trajectory or an insertion
complex that decays before energy randomization. Evidence
*
for a long-lived CH3OH transition state comes from chemi-
v
v
cal quenching studies1,2 of the CH4ϩO(1D2) reaction, in
correlation between the photolysis and probe was measured
as the difference frequency ͑ϳ2.0 ␮m͒ in a 0.5 mm KDP
crystal and found to have a Gaussian full-width at half maxi-
mum of 0.5 ps. The full energy of the laser beams was not
used in order to avoid multiphoton processes and to ensure
that the LIF signal was linear in photolysis and probe pulse
energy. The supersonic expansion in which the van der Waals
complexes were formed and the LIF detection apparatus
were the same as in our earlier experiments,14 in which the
distribution of rovibrational energy in the nascent OH was
measured using high resolution, 10 ns pulse duration lasers.
Also discussed in Ref. 14 is the evidence that the OH is
formed in CH4•O3 clusters rather than larger clusters.
which the quantum yield of CH3OH increased with pressure
*
as the CH3OH was collisionally stabilized before dissocia-
tion. Prompt OH production via a different mechanism was
also suggested because significant OH was formed even at
*
pressures where all CH3OH should have been stabilized.
On the other hand, sub-Doppler spectroscopy10 showed that
OH( ϭ4, Nϭ8) formed in the reaction was scattered nearly
v
isotropically in the center of mass, which is consistent with a
transition state with a lifetime of at least one rotational pe-
riod.
To obtain more direct evidence about the lifetime of the
transition state involved in this reaction, we performed
pump/probe experiments using subpicosecond lasers. The re-
action was initiated when a pump laser photolyzed ozone in
the precursor CH4•O3 van der Waals complex14 to form
Figure 1 shows the OH LIF signal as a function of
photolysis/probe delay time, tD . In the upper trace, the probe
laser at 308.0 nm excited predominantly the OH( ϭ0, low-
v
O(1D ). The formation rate of OH( ,J) from the ensuing
J͒ lines; in the lower trace, the probe laser at 306.1 nm
v
2
reaction between the oxygen atom and the adjacent methane
probed predominantly OH( ϭ0, high-J͒ states.22 In both
v
1
*
molecule, CH4•O3ϩh␯267 nm→CH4•O( D2)•O2→CH3OH
cases the signal increases as LIF(tD)ϭ1Ϫexp(ϪtD/␶). A
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Letters to the Editor
6947
are a fair model for those of the analogous ‘‘free’’ or bimo-
lecular reaction, CH4ϩO(1D2). The observed14 rovibrational
distribution, P( ,J), for nascent OH formed from the
v
CH •O clusters is almost identical to the P( ,J) of the free
v
4
3
reaction.9 This suggests that the presence of the O2 as a third
body in the complex does not radically influence the energet-
ics and that there is a similar reaction mechanism for the
cluster and free reactions. Since almost all of E† arises from
the reaction exothermicity, not the kinetic energy from ozone
photolysis, reduction of E† due to a ‘‘squeezed atom’’
effect19,20 is unimportant. Additionally, P( ϭ0,J) was mea-
v
sured using the fast photolysis laser and a high resolution
probe laser and found to be indistinguishable from that mea-
sured using a ns photolysis laser.14
From the classical chemical quenching experiments1,2
a
*
lifetime for the CH3OH intermediate of 0.8 ps was deduced.
This value was the slope-to-intercept ratio of N2/CH3OH vs
CH4 pressure following photolysis of nitrous oxide,
N2Oϩh␯185 nm→N2ϩO(1D2). A significant correction for
the putative unquenchable fast channel was made to the data,
and an E†-independent quenching rate constant of 2ϫ1011
MϪ1 sϪ1 ͑3ϫ10Ϫ10 cm3 sϪ1͒ was assumed to deduce the 0.8
ps value. As discussed by Tsang,2 these experiments1 did not
determine whether the O(1D2) was thermalized prior to re-
action, thereby creating an uncertainty up to 7800 cmϪ1 in
FIG. 1. Laser-induced fluorescence signal from OH( ϭ0) as a function of
v
photolysis/probe time delay for CH4•O3ϩh␯ nm→CH3ϩOHϩO2 . For the
267
lower trace, the probe wavelength was 306.1 nm, probing mostly OH X(
v
ϭ0, high-J͒ states. For the upper trace the probe wavelength was 308.0 nm,
probing mostly low-J states.22 The smooth curves through the data are fits to
the functional form, LIF(tD)ϭ1Ϫexp(ϪtD/␶) for ␶ϭ3 ps. Shown near
tDϭ0 in the upper trace is the integral of the laser cross-correlation.
nonlinear least-squares fit to many data sets yielded ␶ϭ3 ps,
with no significant differences between the two probe wave-
lengths. Values of ␶ less than 2.2 ps or greater than 3.8 gave
fits that looked poor, and the goodness-of-fit parameter, ␹2,
became much larger than its minimum value. Fitting with a
sum of exponentials, LIF(tD)ϭ⌺ici͓1Ϫexp (ϪtD/␶i)], did
*
the energy content of the CH3OH . Because of these uncer-
tainties and the different nature of the two experiments, the
difference between 0.8 ps and the 3 ps reported here is not
surprising.
2
not decrease ␹ by more than two percent, so given the
*
In addition to OH from the long-lived CH3OH com-
present signal-to-noise ratio, the data were fit by a single
risetime. The laser pulse durations in this experiment are
much shorter than the reaction time, so deconvolution of the
pulse duration is unimportant. Also shown in Fig. 1 is the
numerical integral of the experimental cross correlation,
which would be the LIF time dependence if the transition
state were very short lived.
plex, in the free reaction a major fraction of the OH was
hypothesized to be formed via a short-lived intermediate that
is not quenched in the classical chemical studies. In the laser
studies7–13 of OH product P( ,J), this prompt channel sup-
v
posedly yields OH in high rovibrational states and accounts
for most of OH formed. OH( ϭ0) even in low rotational
v
states is from the RRKM-like reaction channel.9 While the
spectral breadth of a fast laser does not permit probing a
single rotational level, the OH rise times at 308.0 m ͑mostly
low-J͒ and 306.1 nm ͑mostly high-J͒ are the same. No evi-
dence was seen for a fast OH component. Given the time
resolution and signal-to-noise of these experiments, any
prompt OH channel must produce less than 13% of the OH
probed at 308.0 nm and less than 20% of that probed at
306.1 ns.29 It is conceivable that vibrationally excited OH
produced in rovibrational states not yet probed is formed
through a short-lived intermediate.
It seems reasonable to interpret the observed 3 ps rise-
*
time of OH as being the decay time of a CH3OH interme-
diate ͑cf. also Refs. 18–21͒. The dissociation,
O3ϩh␯267 nm→O(1D2)ϩO2(1⌬g) is prompt23–25 ͑␶DISSϽ15
fs͒ and the average kinetic energy release26,27 is ϳ4550 cmϪ1
͑O atom velocity of ϳ2.1 nm psϪ1͒, so a direct process, not
involving a long-lived intermediate, should be very fast. Fur-
thermore, the value of 3 ps is comparable to the lifetime
calculated
from
Rice–Ramsburger–Kassel–Marcus
͑RRKM͒ theory for the dissociation of methanol at the level
of excitation of these experiments. The RRKM lifetime,
calculated28 using the vibrational frequencies and rotational
constants for the CH3OH activated complex determined by
Tsang2 to reproduce the experimental thermal rate constant
parameters for the reaction CH3OH→CH3ϩOH, is ϳ2 ps for
an energy, E†, 16 500 cmϪ1 above the thermodynamic
threshold (E†ϭ⌬HϩEtransϭ14 980ϩ1515 cmϪ1͒. Agree-
ment with an RRKM calculation at a single energy is not
definitive, and until ␶ has been measured for many energies
and H/D isotopic substitution experiments are completed,
one may only claim that the present results are consistent
with statistical theory.
We thank the U.S. Air Force Office of Scientific Re-
search for support and M. P. Casassa, E. J. Heilweil, and D.
S. King for valuable discussions and suggestions. R.D.v.Z.
acknowledges receipt of an NRC/NIST. In fact, supposedly
only a minority of postdoctoral associateship. E-mail:
vanzee@micf.nist.gov or jcs@enh.nist.gov
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