Relaxation of H2O from its |04〉- vibrational state in collisions with H2O, Ar, H2, N2, and O2

Article abstract of DOI:10.1063/1.1649726

Collisional relaxation of water molecules from the highly excited |04^- vibrational states in collisions with water, argon, hydrogen, nitrogen and oxygen was analyzed. It was found that delay between the pulse from the pump laser and those from

Full text of DOI:10.1063/1.1649726

Evaluation Only. Created with Aspose.PDF. Copyright 2002-2021 Aspose Pty Ltd.
Relaxation of H 2 O from its |04 vibrational state in collisions with H 2 O , Ar, H 2 , N 2 ,
and O 2
Peter W. Barnes, Ian R. Sims, and Ian W. M. Smith
Citation: The Journal of Chemical Physics 120, 5592 (2004); doi: 10.1063/1.1649726
View online: http://dx.doi.org/10.1063/1.1649726
View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/120/12?ver=pdfcov
Published by the AIP Publishing
Articles you may be interested in
CuF 2 Structural Changes in TwoDimensional Quantum Magnet ( H 2 O ) 2 ( pyz ) Under Pressure: Raman
Study
AIP Conf. Proc. 1267, 597 (2010); 10.1063/1.3482697
Structure, stability, and spectra of C 9 H 3 , C 11 H 3 , and C 13 H 3 radicals
J. Chem. Phys. 121, 8212 (2004); 10.1063/1.1804176
Gas-phase hydrogen atom abstraction reactions of S with H 2 , CH 4 , and C 2 H 6
J. Chem. Phys. 119, 8996 (2003); 10.1063/1.1614772
Influence of rotational motion of molecules on the thermal conductivity of solid SF 6 , CHCl 3 , C 6 H 6 , and CCl
4
Low Temp. Phys. 29, 771 (2003); 10.1063/1.1614189
Accurate nonadiabatic couplings for H 3 : Application to predissociation
J. Chem. Phys. 111, 5873 (1999); 10.1063/1.479882
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
134.117.10.200 On: Mon, 23 Jun 2014 09:01:50

Evaluation Only. Created with Aspose.PDF. Copyright 2002-2021 Aspose Pty Ltd.
JOURNAL OF CHEMICAL PHYSICS
VOLUME 120, NUMBER 12
22 MARCH 2004
À
Relaxation of H2O from its
with H2O, Ar, H2 , N2 , and O2
ͦ
04‹ vibrational state in collisions
Peter W. Barnes, Ian R. Sims,a) and Ian W. M. Smithb)
School of Chemistry, University of Birmingham, Edgbaston, Birmingham B15 2TT, United Kingdom
͑Received 16 September 2003; accepted 31 December 2003͒
We report rate coefficients at 293 K for the collisional relaxation of H2O molecules from the highly
excited ͉
04 Ϯ vibrational states in collisions with H2O, Ar, H2 , N2 , and O2 . In our experiments, the
͘
Ϫ
͉
04 state is populated by direct absorption of radiation from a pulsed dye laser tuned to ϳ719 nm.
͘
Evolution of the population in the (
͉
04 Ϯ) levels is observed using the combination of a
͘
frequency-quadrupled Nd:YAG laser, which selectively photolyses H2O(
frequency-doubled dye laser, which observes the OH( ϭ0) produced by photodissociation via
͉
04 Ϯ), and
a
͘
v
laser-induced fluorescence. The delay between the pulse from the pump laser and those from the
photolysis and probe lasers was systematically varied to generate kinetic decays. The rate
coefficients for relaxation of H2O(͉
04 Ϯ) obtained from these experiments, in units of
͘
cm3 moleculeϪ1 sϪ1, are: k(H2O)ϭ(4.1Ϯ1.2)ϫ10Ϫ10, k(Ar)ϭ(4.9Ϯ1.1)ϫ10Ϫ12, k(H2)ϭ(6.8
Ϯ1.1)ϫ10Ϫ12, k(N2)ϭ(7.7Ϯ1.5)ϫ10Ϫ12, k(O2)ϭ(6.7Ϯ1.4)ϫ10Ϫ12. The implications of these
results for our previous reports of rate constants for the removal of H2O molecules in selected
vibrational states by collisions with H atoms ͑P. W. Barnes et al., Faraday Discuss. Chem. Soc. 113,
167 ͑1999͒ and P. W. Barnes et al., J. Chem. Phys. 115, 4586 ͑2001͒.͒ are fully discussed. © 2004
American Institute of Physics. ͓DOI: 10.1063/1.1649726͔
I. INTRODUCTION
transferred by collisions to vibrational levels of similar en-
ergy but with a different distribution of the vibrational en-
ergy amongst the modes of the molecule and this process can
be rather efficient. Studies of the relaxation of small poly-
atomic molecules at moderate levels of vibrational excita-
tion, therefore, provide a bridge between the many studies of
the vibrational relaxation of diatomic molecules at low levels
There are few data in the literature pertaining to the col-
lisional relaxation of small polyatomic molecules from vibra-
tional levels corresponding to ‘‘moderate’’ internal energies;
that is, with more than two quanta of excitation but below a
level where the vibrational levels start to form a quasi-
continuum. The paucity of such data reflects the general dif-
ficulty of observing the small concentrations of excited spe-
cies that can be created in high vibrational levels either by
direct absorption or by stimulated emission pumping. The
data on relaxation of C2H2 , obtained chiefly by Crim,1 Orr2
and their respective co-workers, constitute one exception.
These results for C2H2 , and others at lower levels of excita-
tion for species such as HCN3 and D2CO4 as well as C2H2 ,5
demonstrate that the collisional relaxation of small poly-
atomic molecules from vibrational levels higher than the first
excited state is qualitatively different from that of diatomic
molecules, especially with atomic and chemically inert col-
lision partners. For diatomic molecules, where there is only a
single manifold of well-spaced vibrational levels, collisional
relaxation is inefficient in the absence of near-resonant
vibration–vibration energy exchange, because of the need to
transfer large amounts of energy from the molecular vibra-
tion to the relative translational motion of the collision part-
ners. On the other hand, for small polyatomic molecules the
general mode of vibrational relaxation is via intramolecular
vibration–vibration energy transfer. That is, molecules are
of vibrational excitation, usually ϭ1, and those where
v
polyatomic molecules are provided with ‘‘chemically signifi-
cant’’ amounts of internal energy.6
In this paper, we describe the results of some novel ex-
periments from which we have obtained data about the col-
lisional relaxation of H2O molecules following their selec-
tive excitation to the (
contain internal energy corresponding to ϳ13 830 cmϪ1. The
04 Ϫ) symbol reflects the predominantly local mode char-
͉
04 Ϫ) state. Molecules in this state
͘
(
͉
͘
acter of this vibrational state. ͉04͘ signifies a ‘‘stretching’’
vibrational state of H2O containing zero quanta of excitation
in one O–H bond and four quanta of excitation in the other
O–H bond. However, because the vibrational wave functions
must reflect the symmetry of the molecule, the eigenfunc-
tions of the zeroth-order Hamiltonian for the local mode
Ϯ
stretching states must be the symmetrized functions:
͉04
͘
ϭ2Ϫ1/2
(
͉
04 Ϯ
͘
͉
40 ). The local mode character of these and
͘
other vibrational states of H2O have been widely
discussed.7–9 The splitting between the zeroth rotational lev-
els of the two states, i.e., E(͉ ͉
04 Ϫ)ϪE( 04 ϩ), is only 2.6
͘ ͘
cmϪ1 8,10,11 Detailed spectroscopic measurements12,13 in the
.
range 13 500–15 000 cmϪ1 reveal five states arise from only
local mode stretching states, in ascending order of energy:
a͒
Present address: PALMS—UMR 6627 du CNRS, Equipe: ‘‘Astrochimie
´
´
Experimentale,’’ Bat. 11c, Universite de Rennes 1, 35042 Rennes Cedex,
France. Electronic mail: ian.sims@univ-rennes1.fr
ϩ
Ϫ
ϩ
͉
(
04
͉
,
͉
04
,
͉
͘
13
, ͉
13 Ϫ, and ͉22͘. Of these, the states
͘ ͘
͘
͘
Ϫ
Electronic mail: i.w.m.smith@bham.ac.uk
04 and
͉
13 Ϫ) give rise to the strongest overtone bands
b͒
͘
0021-9606/2004/120(12)/5592/9/$22.00
5592
© 2004 American Institute of Physics
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
134.117.10.200 On: Mon, 23 Jun 2014 09:01:50

Evaluation Only. Created with Aspose.PDF. Copyright 2002-2021 Aspose Pty Ltd.
J. Chem. Phys., Vol. 120, No. 12, 22 March 2004
Relaxation of H2O
5593
from the zeroth vibrational state, with the intensity of the
As well as general fundamental interest, there are two
specific reasons for interest in the vibrational relaxation of
high levels of H2O. First, the work reported here is closely
connected with earlier work, both from Crim and
co-workers18 and from our own group,19,20 on the bimolecu-
lar reactivity towards radical atoms of H2O in high vibra-
tional levels of the ground electronic state. In particular, our
own work has focused on the measurement of rate coeffi-
cients for the removal,19 and especially the removal by
chemical reaction,20 of H2O molecules from specific highly
excited vibrational levels in collisions with H atoms. In that
work, rate coefficients were derived indirectly for the vibra-
tional relaxation of H2O from several excited levels in colli-
sions with unexcited H2O molecules.19 However, in arriving
at the results for removal by H atoms, it was implicitly as-
sumed that the relaxation of the excited H2O molecules by
the small amount of undissociated H2 and by the Ar carrier
gas was unimportant. The present work, to a degree, tests
that assumption.
A second reason for interest in the relaxation from high
vibrational levels of H2O is the proposal that two-stage pho-
tolysis of H2O might provide a source of OH radicals in the
atmosphere21 that has not been accounted for in atmospheric
models. The hypothesis is that visible solar radiation could
promote water molecules to high vibrational levels in the
electronic ground state and that these molecules might then
be photodissociated by light of longer wavelength than is
required to dissociate H2O from its lowest vibronic level.
Obviously, for this mechanism to be significantly operative,
it would be necessary for photolysis from high vibrational
levels to occur at a rate that is competitive with relaxation
from those levels in collisions with atmospheric gases, espe-
cially N2 and O2 .
Ϫ
absorption to
than that to
͉
04 almost two orders-of-magnitude greater
͘
13
ϩ
͉
04
.
͘
The strong local mode character of H2O(
͉
04 Ϫ), com-
͘
bined with its relative ease of preparation via absorption in
the high overtone band at around 719 nm, has led to a num-
ber of interesting dynamics experiments being performed on
this selectively excited species.14 These experiments fall into
two groups: Those which examine the OH produced by pho-
todissociation of H2O(
͉
04 Ϫ), in what is frequently referred
͘
to as vibrationally mediated photodissociation,15–17 and
those in which the results of collisions of H2O(
radical atoms are examined.14,18–20
͉
04 Ϫ) with
͘
Comparisons between the OH produced when
H2O(͉ ͉
04 Ϫ) and H2O( 13 Ϫ) are photolyzed at wavelengths
͘ ͘
р266 nm15–18 yielded two results of particular importance to
our experiments. First, the former process yields, in effect,
only OH( ϭ0), whereas the photodissociation of
v
H2O(͉
13 Ϫ) overwhelmingly produces OH( ϭ1).15 Sec-
v
͘
ond, at 266 nm, van der Waals et al.15 were only able to
detect OH product from the photodissociation from
H2O(͉ ͉
04 Ϫ), not from H2O( 13 Ϫ). These results have been
͘ ͘
rationalized by Weide et al.17 They showed that efficient
photodissociation from these vibrationally excited states
arises because of overlap between the lobe on the wave func-
Ϫ
Ϫ
tions in the
͉04 and ͉13 levels of the electronic ground
͘ ͘
˜
state at large O–H separations and those on the A dissocia-
tive potential energy surface. The fact that the ͉04 wave
͘
Ϫ
function extends to large O–H separations allows appre-
ciable overlap at lower energies, and therefore, longer wave-
Ϫ
lengths, than in the case of
͉
13
͘
.
It should be noted that the experiments of Crim’s group
on the vibrationally mediated photodissociation of H2O were
done under essentially collisionless conditions. Conse-
quently, they could be confident that the rovibrational distri-
butions of the OH that they detected result from selective
The two motivations identified in the two previous para-
graphs explain our choice of collision partners in the present
Ϫ
work. We have studied collisional relaxation of H2O(
͉
04
͘
)
by ͑a͒ H2O, Ar, and H2 , the main constituents of the mix-
tures in our previous experiments,19,20 and ͑b͒ N2 and O2 ,
the main constituents of the Earth’s atmosphere. There have
been only two previous studies of the vibrational relaxation
of H2O. Finzi et al.,22 using the laser-induced vibrational
fluorescence technique, measured rate coefficients from the
photodissociation of the initially prepared vibrational state.
ϩ
Because of the difficulty of preparing H2O in the (
͉
04
͘
)
state, there are no experimental data, or indeed theoretical
calculations, on photodissociation of this state. Nevertheless,
it seems reasonable to suppose that the cross sections for
photodissociation from this state will be similar to those for
coupled ͑100͒ and ͑001͒, or ␯ and ␯3 , normal mode states
04 Ϫ).
1
photodissociation of H2O(͉
͘
in collisions with the same range of collision partners that we
have employed. The temperature range of these measure-
ments was extended by Zittel and Masturzo.23 Both sets of
workers concluded that populations in the ␯ and ␯ states
The method that is used in the present work to examine
the relaxation of highly vibrationally excited H2O relies on
the fact that H2O molecules with four quanta of O–H
stretching vibration are selectively photolyzed at 266 nm;
that is, neither unexcited H2O molecules nor H2O molecules
with fewer than four quanta of excitation in a local O–H
stretching vibration are significantly photolyzed at this wave-
length. We utilize this property to follow the decay of the
1
3
rapidly equilibrated and that these excited molecules then
relaxed via the 2␯ level.
2
II. EXPERIMENT
excited H O population by observing the OH( ϭ0) that is
v
2
produced when H2O(
photolysed at various delays, after an initial population of
H2O(
04 Ϫ) has been produced by exciting H2O molecules
͉
04 Ϫ), and probably H2O(
͉
04 ϩ), are
The general method employed in the present experi-
ments is similar to that used previously to determine the
branching ratio between reaction and relaxation in the re-
͘
͘
͉
͘
from their ground vibrational level using a pulsed tunable
moval of H2O from its (
atoms,19,20 in that it relies on the selective photolysis of
H2O(
04 Ϯ) by laser radiation at 266 nm and the detection
of the OH( ϭ0) that is formed using LIF.20
͉
04 Ϫ) level in collisions with H
͘
dye laser. Later, we discuss further whether our technique is
Ϫ
likely to monitor only the population in the
͉
04 state, or
͉
͘
͘
Ϫ
ϩ
the sum of the populations in the ͉04 and ͉04 states.
v
͘ ͘
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
134.117.10.200 On: Mon, 23 Jun 2014 09:01:50

Evaluation Only. Created with Aspose.PDF. Copyright 2002-2021 Aspose Pty Ltd.
5594
J. Chem. Phys., Vol. 120, No. 12, 22 March 2004
Barnes, Sims, and Smith
In the present experiments, gas mixtures containing H2O
at 293 K were passed slowly through a tubular cell made of
Pyrex. Mixtures of known composition were formed as fol-
lows: The chosen carrier gas ͑the relaxant͒ was passed
through a bubbler apparatus such that the known vapor pres-
sure of water contributed a known fraction of the total pres-
sure in the bubbler apparatus. The total flow of gas through
the bubbler was controlled with a mass flow controller. More
relaxant gas was introduced to the main cell using a second
mass flow controller, thus allowing the composition of the
gas mixture to be varied as required. Pressure in the main
cell was monitored with a capacitance manometer and was
varied as required by applying a throttle tap before the
vacuum pump. Complete mixing was ensured by using
backward-facing gas injectors that combined the gas flows
ϳ1 m upstream of the observation zone. Gas flows were
allowed to equilibrate for 30–60 minutes before measure-
ments were taken to avoid the potential problem of H2O
adsorbing onto the cell walls.
FIG. 1. The main diagram shows the variation of first-order rate coefficients
measured at 293 K for the removal of H2O(͉
04 Ϯ) as the partial pressure of
͘
H2O in the gas sample is varied, at a constant partial pressure of 5.4
ϫ1016 molecule cmϪ3 Ar. The gradient of the line yields a value for the
A very small fraction of the H2O molecules were excited
second-order rate coefficient for the self-relaxation of H2O(
insert is an example LIF trace ͑corresponding to the open circle on the linear
͉
04 Ϯ). The
͘
Ϫ
to the
͉
04 vibrational level using the output from a pulsed
͘
tunable dye laser, the ‘‘pump’’ laser, operating at ϳ719 nm.
This laser was tuned into resonance with lines in the water
overtone band using a home-built photoacoustic cell coupled
to an AC amplifier with band pass filters corresponding to
the resonant frequency of the cell. At variable times after the
plot͒ which shows signals from OH( ϭ0) generated by photolysis of H O
v
2
molecules with 4 quanta in a local OH stretching mode at different delays
Ϫ
following excitation to the
͉
04 vibrational state. The marked ‘‘variable
͘
delay’’ is between the pump and the photodissociation laser pulses; the
probe laser always fires 4 ␮s after the photodissociation laser. These data are
fitted to a single exponential yielding the first-order rate coefficient for re-
laxation that is indicated by an open circle on the main figure.
pulse from the pump laser, a fraction of the H2O molecules
Ϯ
that remained in the
͉
04 vibrational levels was photolyzed
͘
by a 266 nm pulse ͑ϳ20 mJ͒ from the ‘‘photodissociation
laser,’’ a frequency-quadrupled Nd:YAG laser ͑Continuum
Surelite͒. The OH radicals that are produced by this photoly-
sis process are known to be overwhelmingly in their ground
vibrational level15 and they were detected using a pulsed,
frequency-doubled, dye laser, the ‘‘probe’’ laser, operating at
III. RESULTS
Examples of typical experiments are shown in the inserts
to Figs. 1 and 2. In each experiment, the LIF signals from
OH( ϭ0) generated by photolysis of vibrationally excited
v
ϳ282 nm in order to excite OH( ϭ0) in the ͑1,0͒ band of
v
the A 2⌺ϩ –X 2⌸ system. Fluorescence was observed
through a narrow band filter, centered at 312 nm, which
transmits emission lines in both the ͑1,1͒ and ͑0,0͒ bands.
The delay time between the pulses from the photodissocia-
tion and probe lasers was kept constant at 4 ␮s, long enough
to allow for rotational relaxation of the photolytically pro-
duced OH.24 On the other hand, the time delay between the
pulses from the pump and photodissociation lasers was var-
ied to obtain kinetic data on the changing concentration of
Ϯ
H2O in the
͉
04
͘
vibrational levels. In this way, the LIF
signals from OH( ϭ0) map the decay of water molecules
v
from the (͉
04 Ϯ) states.
͘
Experiments of the kind just described were repeated for
gas mixtures of different composition. In general, series of
experiments were carried out with the concentration of one
component of the mixture varied systematically, as the con-
centration of the other component was kept constant. The
variation of the pseudo-first-order rate constant for decay of
the LIF signal with composition yielded a second-order rate
coefficient for relaxation by the particular species whose
concentration was being varied.
FIG. 2. This diagram shows similar data to that in Fig. 1 but the main
diagram now shows the variation of first-order rate coefficients at 293 K as
the partial pressure of Ar in the gas sample is varied, the partial pressure of
H2O being kept constant. The gradient of the line yields a value for the
second-order rate coefficient for the relaxation of H2O(͉
04 Ϯ) by Ar.
͘
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
134.117.10.200 On: Mon, 23 Jun 2014 09:01:50