Production processes of H(D) atoms in the reactions of NO(A2Σ+) with C2H2, C2H4, H2O, and their isotopic variants

Article abstract of DOI:10.1016/S0301-0104(00)00184-1

The production of H(D) atoms was identified in the reactions of NO(A²Σ⁺) with C₂H₂, C₂H₄, H₂O, and their isotopic variants. By measuring the Doppler profiles, the translational energies of H(D) atoms were determined. As for C₂H₂ and H₂O, around 1/4 of the NO(A²Σ⁺) energy is partitioned into the translational mode, while the ratio is around 1/7 for C₂H₄. Such a large partitioning of the available energy into the translational mode cannot be explained by a C-H(O-H) bond rupture after an energy transfer. Nitrogen compounds, such as ONC₂H and HONO, must be produced as pair products. The measured translational energies are much larger than those statistically expected, suggesting that the intermediate species are short-lived. No large isotope effect was observed in the average translational energies or in the relative yields. Quantum effects, such as tunneling, do not play important roles. Ab initio molecular orbital calculations were carried out to characterize these reactive processes. (C) 2000 Elsevier Science B.V.

Full text of DOI:10.1016/S0301-0104(00)00184-1

Evaluation Only. Created with Aspose.PDF. Copyright 2002-2021 Aspose Pty Ltd.
Chemical Physics 259 (2000) 39±47
www.elsevier.nl/locate/chemphys
Production processes of H(D) atoms in the reactions of
‡
NO(A2R ) with C2H2, C2H4, H2O, and their isotopic variants
Hironobu Umemoto a,*, Naoki Terada a, Kunikazu Tanaka a,
Toshiyuki Takayanagi b, Yuzuru Kurosaki c, Keiichi Yokoyama d
a
Department of Chemical Materials Science, Japan Advanced Institute of Science and Technology, Asahidai, Tatsunokuchi, Nomi,
Ishikawa 923-1292, Japan
b
Advanced Science Research Center, Japan Atomic Energy Research Institute, Tokai, Ibaraki 319-1195, Japan
Advanced Photon Research Center, Japan Atomic Energy Research Institute, Umemidai, Kizu, Soraku, Kyoto 619-0215, Japan
c
d
Department of Materials Science, Japan Atomic Energy Research Institute, Tokai, Ibaraki 319-1195, Japan
Received 9 May 2000
Abstract
‡
The production of H(D) atoms was identi®ed in the reactions of NOꢀA2R † with C2H2, C2H4, H2O, and their
isotopic variants. By measuring the Doppler pro®les, the translational energies of H(D) atoms were determined. As for
2
‡
C2H2 and H2O, around 1/4 of the NO(A R ) energy is partitioned into the translational mode, while the ratio is around
1/7 for C2H4. Such a large partitioning of the available energy into the translational mode cannot be explained by a
C±H(O±H) bond rupture after an energy transfer. Nitrogen compounds, such as ONC2H and HONO, must be produced
as pair products. The measured translational energies are much larger than those statistically expected, suggesting that
the intermediate species are short-lived. No large isotope e€ect was observed in the average translational energies or in
the relative yields. Quantum e€ects, such as tunneling, do not play important roles. Ab initio molecular orbital cal-
culations were carried out to characterize these reactive processes. Ó 2000 Elsevier Science B.V. All rights reserved.
1. Introduction
few studies have contributed to the identi®cation
of the exit channels. Mauldin and Ravishankara
observed the production of O(3P) in the reaction
Nitric oxide is sometimes called a ``pet'' mole-
cule among the spectroscopists [1]. This may not
be the case for researchers on chemical reaction
dynamics. There have been numerous studies on
the spectroscopy of NO molecules from vacuum
ultraviolet to far infrared [2]. There have also been
many studies on the determination of the rate
constants for the overall quenching of the ®rst
‡
of NOꢀA2R † with O2 [12]. The production of
‡
N2ꢀA3Ru † has been suggested in the quenching of
‡
NOꢀA2R ; v0 ˆ 4† by N2, very recently [13]. These
should be the only systems in which the exit
channels are identi®ed, except for a few ®nal
product analysis studies [14,15]. Theoretical stud-
ies have not been carried out extensively, either.
Furlanetto et al. measured the cross-sections for
‡
excited NOꢀA2R † molecules [3±11]. However,
‡
the quenching of NOꢀA2R † by some saturated
and unsaturated hydrocarbons over a wide tem-
perature range [8]. The cross-sections by CH4 and
C2H6 are small even over 1000 K, while those by
* Corresponding author. Fax: +81-761-51-1655.
E-mail address: umemoto@jaist.ac.jp (H. Umemoto).
0301-0104/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved.
PII: S0301-0104(00)00184-1

Evaluation Only. Created with Aspose.PDF. Copyright 2002-2021 Aspose Pty Ltd.
40
H. Umemoto et al. / Chemical Physics 259 (2000) 39±47
C2H2 and C2H4 are large and show moderate de-
creases with the increase in temperature. In order
to explain such a temperature dependence, they
have proposed that the quenching proceeds via
resonant or near-resonant electronic energy trans-
fer. However, it is not clear what happens after the
energy transfer. Since the electronic energy of
®xed at 226.3 nm in most cases. This wavelength
corresponds to the head of the Q1 branch of the
(0,0) band [3]. Some experiments were carried out
at 226.1 nm to excite R1(8). The typical pulse en-
ergy was 0.7 mJ. Excited NO molecules react with
hydride(deuteride) molecules, such as C2H2 and
C2D2, to produce H(D) atoms. After a short time
delay, typically 100 ns, H(D) atoms were detected
with a frequency double output of another YAG
laser pumped dye laser (Lambda Physik,
LPD3000E and Quanta-ray, GCR-170). The typ-
ical pulse energy of the doubled output of the
probe laser was 0.3 mJ. This laser pulse was fo-
cused into the interaction region by using a 200
mm focal-length lens. The pump and probe laser
beams were aligned collinearly. Ground state
H(D) atoms were excited to the 22S state with the
two-photon scheme at 243.2 (243.0) nm. These
excited H(D) atoms were then collisionally relaxed
to the near-by 22P state that ¯uoresces vacuum
ultraviolet light at 121.6 (121.5) nm. The vacuum
ultraviolet ¯uorescence was detected with a solar-
blind photomultiplier tube (Hamamatsu Photon-
ics, R1459) through an MgF2 window and an
interference ®lter (Acton Research, 122-N). The
photomultiplier signal was ampli®ed by a pream-
pli®er (Stanford Research Systems, SR240) and
processed with a boxcar averager-gated integrator
system (Stanford Research Systems, SR280/
SR250). Doppler pro®les of H(D) atoms were
measured by scanning the wavelength of the probe
laser. The bandwidth (0.5 cm 1) of the doubled
output of the probe laser was determined by
measuring the Doppler pro®les of H atoms under
completely thermalized conditions. The typical
total pressure was 40 Pa. There was no change in
the Doppler pro®les when this pressure was
halved.
‡
NOꢀA2R † is larger than or comparable to the
C±H bond strength, the quenchers may decom-
pose after energy transfer. Internal conversion
from the surface correlating to NO ‡ triplet state
hydrocarbons to the electronic ground state may
also take place. In such a case, some closed-shell
nitrogen compounds, such as ONC2H, may be
produced. It is desired to make clear the latter half
of the reaction.
The lack of the experimental studies on the
identi®cation of the exit channels may partly be
‡
because the ¯uorescence range of NOꢀA2R † is
very wide from ultraviolet to visible. It is dicult
to employ a usual laser induced ¯uorescence
technique to identify the product species. The
resonance enhanced multiphoton ionization tech-
nique may be applied, but since NO can easily be
ionized, the mass separation of the ions is critical.
These problems can be overcome by monitoring
the emission in the vacuum ultraviolet region.
Atomic hydrogen can easily be detected by ob-
serving the Lyman-a ¯uorescence after two-
photon excitation [16,17]. Then, the detection of
atomic hydrogen produced in the reaction of
‡
NOꢀA2R † should be a break-through for the
study of the reactions of excited NO molecules.
In the present study, the production of H(D)
‡
atoms was identi®ed in the reactions of NOꢀA2R †
with C2H2, C2H4, H2O, and their isotopic variants.
Quenching processes by these species have been
reported to be very ecient [3,7±10].
In the quenching rate constant measurements,
just one laser was used. NO was excited via the
(0,0) band of the A±X system, while the temporal
pro®les of the (0,3) band emission were moni-
tored with a photomultiplier tube (Hamamatsu,
R106UH) through a monochromator (JASCO,
CT-25CP). The (0,3) band was chosen in order to
avoid any problems caused by radiation trapping
[8]. The slits of the monochromator were opened
widely, 1 mm. Then, the collisional rotational re-
2. Experimental
The experimental apparatus and the procedure
were similar to those described elsewhere [18,19].
Ground state NO molecules were excited to the
‡
A 2R , v0 ˆ 0 state with a frequency double output
of a YAG laser pumped dye laser (Quanta-ray,
PDL-3/GCR-170). The excitation wavelength was

Evaluation Only. Created with Aspose.PDF. Copyright 2002-2021 Aspose Pty Ltd.
H. Umemoto et al. / Chemical Physics 259 (2000) 39±47
Table 1
41
laxation does not appear as electronic quenching.
A digital oscilloscope (LeCroy, 9450A) was used
to average the decay pro®les. All the measure-
ments were carried out at 293 Æ 2 K.
‡
Rate constants for the deactivation of NOꢀA2R † at room
10
1
temperature in units of 10
cm3 s
This work
Literature values
C2H2
C2D2
C2H4
C2D4
H2O
1:64 Æ 0:05
1:47 Æ 0:03
3:04 Æ 0:07
2:95 Æ 0:06
7:51 Æ 0:15
1:89 Æ 0:03a
NO (Sumitomo Seika, 99.999%) and C2H4
(Sumitomo Seika, 99.9%) were used from cylinders
without further puri®cation. C2D4 was the product
of ICON Stable Isotopes (isotopic purity 99%).
Distilled and deionized H2O was used after being
degassed under vacuum. D2O was the product of
Aldrich (isotopic purity 99.9%). C2H2(C2D2) was
synthesized from CaC2 (Katayama Kagaku) and
H2O(D2O).
4:16 Æ 0:26a
7:58 Æ 0:71b,
8:00 Æ 0:37c,
8:08 Æ 0:75d,
8:97 Æ 0:46e
D2O
6:22 Æ 0:23
a Furlanetto et al. [8].
b McDermid and Laudenslager [3].
c Drake and Ratcli€e [7].
d Zhang and Crosley [9].
e Paul et al. [10].
3. Experimental results
The rate constants for the deactivation of
‡
NOꢀA2R † were determined under pseudo-®rst-
order conditions. NO pressure was kept constant,
13 Pa, while the quencher pressure was changed
between 0 and 90 Pa. Measurements with Ar bu€er
gas were also carried out but there was no di€er-
ence in the decay rates. The emission intensity
‡
from NOꢀA2R † decreased exponentially against
time. The decay rate increased linearly with the
increase in the reactant gas pressure. From the
slopes, it is possible to determine the absolute rate
constants for the deactivation. The results are
summarized in Table 1. The error limits are the
standard deviations ꢀ1r†. The result for H2O is in
good agreement with the former literature values
[3,7,9,10]. The results for C2H4 and C2H2 are also
in fair agreement with those reported by Furla-
netto et al. [8]. All the rate constants are large and
gas kinetic, suggesting the absence of activation
barriers. The lack of large isotope e€ects is also
consistent with this model.
Fig. 1. Doppler pro®le of H atoms formed in the reaction of
‡
NOꢀA2R † with C2H2. The pressures were 30 Pa for NO and
10 Pa for C2H2. The photolysis-probe delay time was 100 ns.
‡
Figs. 1 and 2 show the Doppler pro®les of H(D)
‡
not be important because NOꢀA2R † molecules
atoms produced in the reaction of NOꢀA2R † with
may rotate many times during its lifetime. The
Doppler pro®les showed no dependence on the
C2H2 and D2O, respectively. Similar results were
obtained for other species. The pro®les for
C2H2(C2D2) and C2H4(C2D4) can be approxi-
mated with gaussian functions, while that for
H2O(D2O) cannot. In other words, the velocity
distribution for H2O(D2O) cannot be represented
by a Maxwell distribution at one temperature.
Here, anisotropy in the velocity distribution can-
‡
rotational energy of NOꢀA2R †. H(D) atoms ob-
served in the present system are not the direct
photolysis products, because no H(D) atoms could
be detected when the pump laser was detuned to
the NO c system. McGee and Heicklen have ob-
served polymerization when C2H2 was exposed to
the radiation from a low-pressure cadmium arc

Evaluation Only. Created with Aspose.PDF. Copyright 2002-2021 Aspose Pty Ltd.
42
H. Umemoto et al. / Chemical Physics 259 (2000) 39±47
the yields for C2H2 and C2D2 are assumed to be
unity. Similar results were obtained for H2O and
D2O. The relative yields were 0:9 Æ 0:2 and
0:8 Æ 0:3, respectively.
4. Calculations
In order to understand the quenching mecha-
nisms and reaction pathways of hydrogen atom
production, we have carried out ab initio molec-
ular orbital calculations. The adiabatic potential
energy surfaces for NO ‡ C2H2, C2H4, and H2O
were calculated. The computational procedure is
as follows: The N±C or N±O internuclear distance
was ®xed to a certain value in the range of 0.13±
0.30 nm. Other internal coordinates were opti-
mized with respect to the total electronic energy of
the ground state under Cs symmetry constraint.
The electronic energies of the excited states were
calculated by assuming the same geometries. The
geometry optimization was carried out at the un-
restricted Hartree±Fock (UHF) level with Dun-
ning's correlation-consistent polarized valance
double-zeta (cc-pVDZ) basis set [21] using the
GAUSSIAN 94 program [22]. At each point of
the N±C or N±O distance, the state-averaged
complete-active-space self-consistent-®eld (SA-
CASSCF) calculations were performed with the
MOLCAS program [23]. The active space was
carefully chosen so as that suciently smooth
potential energy curves can be obtained but
with less computational costs. In the case of
NO ‡ C2H2, for example, nine electrons were dis-
tributed among nine active orbitals ꢀ5a0 ‡ 4a00†
which include NOp, NOpÃ, O2p, N2p, N3s, two
CCp, and two CCpÃ. The lowest 10 states were
included in the averaging for both A0 and
A00 symmetry of electronic states. Because the
Fig. 2. Doppler pro®le of D atoms formed in the reaction of
‡
NOꢀA2R † with D2O. The pressures were 10 Pa both for NO
and D2O. The photolysis-probe delay time was 100 ns.
Table 2
Average translational energies calculated from the second mo-
ments of the Doppler pro®les and the statistically expected
1
values in kJ mol
Reactant
Measured
Statistical
(prior)
Exothermicity
C2H2
C2D2
C2H4
C2D4
H2O
D2O
133 Æ 6
130 Æ 5
73 Æ 4
39
34
49
246
297
238
78 Æ 3
113 Æ 7
108 Æ 8
[14]. This must be caused by the irradiation at
214.4 nm rather than at 226.5 nm. Table 2 sum-
marizes the translational energies of H(D) atoms
calculated from the second moments of the
Doppler pro®les. The computational procedure is
the same as that employed by Matsumi et al. [20].
Since the pro®les for C2H2(C2D2) and C2H4(C2D4)
are near gaussian, there was no change when the
translational energies were calculated from the
half-widths.
The relative yields of H(D) atoms were also
measured. The H=D signal ratios were measured
for the 1:1 mixtures of C2H2=C2D2, C2H4=C2D2,
C2H2=C2D4, and C2H4=C2D4. Since the overall
rate constants are known, it is possible to convert
these ratios into relative yields. The results were
1:4 Æ 0:3 for C2H4 and 1:3 Æ 0:3 for C2D4, when
‡
NOꢀA2R † state is Rydberg in character (3sr), it
is necessary to supplement the cc-pVDZ basis set
with di€use functions. In order to describe the
‡
NOꢀA2R † Rydberg state properly, 3s and 3p
Rydberg basis functions were included with the
following exponents [24]: Nꢀ3s† ˆ 0:028, Nꢀ3p† ˆ
0:025, Oꢀ3s† ˆ 0:032; Oꢀ3p† ˆ 0:020.
The potential energy curves thus calculated are
plotted in Figs. 3±5. Since the CASSCF method