Direct measurement of the binding energy of the NO dimer

Article abstract of DOI:10.1063/1.1459702

Binding energy of NO dimer was measured using velocity-mapped ion imaging following nonresonant ionization. (NO)₂ was photolyzed with a ～222 nm laser pulse energy of <1 mJ to produce NO(A) and NO(X). NO(A) was ionized by the same laser pulse to produce NO⁺. Binding energy was measured at 696±4 cm^-1.

Full text of DOI:10.1063/1.1459702

Direct measurement of the binding energy of the NO dimer
Elisabeth A. Wade, Joseph I. Cline, K. Thomas Lorenz, Carl Hayden, and David W. Chandler
Citation: J. Chem. Phys. 116, 4755 (2002); doi: 10.1063/1.1459702
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JOURNAL OF CHEMICAL PHYSICS
VOLUME 116, NUMBER 12
22 MARCH 2002
COMMUNICATIONS
Direct measurement of the binding energy of the NO dimer
Elisabeth A. Wadea)
Department of Chemistry and Physics, Mills College, Oakland, California 94613
Joseph I. Cline
Department of Chemistry and Chemical Physics Program, University of Nevada, Reno, Nevada 89557
K. Thomas Lorenz,b) Carl Hayden, and David W. Chandlerc)
Combustion Research Facility, Sandia National Laboratories, Livermore, California 94551-0969
͑
Received 20 November 2001; accepted 23 January 2002͒
The binding energy of the NO dimer has been measured directly using velocity-mapped ion
imaging. NO dimer is photodissociated to produce NO(X) and NO(A), and the NO(A) is then
ϩ
ϩ
nonresonantly ionized to NO . The threshold for production of NO ions is measured at 44 893
Ϫ1
Ϫ1
Ϯ2 cm , which corresponds to a binding energy of 696Ϯ4 cm . © 2002 American Institute of
Physics. ͓DOI: 10.1063/1.1459702͔
NO is an important molecule in a wide range of atmo-
spheric and biological processes, and as a result the reactivity
with a backing pressure of 1.0–1.5 atm. It is then photolyzed
with a ϳ222 nm laser pulse, with a pulse energy of Ͻ1 mJ,
to produce NO(A) from the photodissociation reaction:
of the NO dimer, ͑NO͒ , has been of interest since the
2
1
–3
1
950s. NO is an open shell molecule, so the formation of
͑
NO͒ →NO͑A͒ϩNO͑X͒.
2
the dimer involves the formation of a bond from the unpaired
electron of each NO monomer. This bond is stronger than is
usual for a typical van der Waals molecule.
The frequency of the laser beam is monitored with a New
Ϫ1
Focus Fizeau 7711 wavemeter, to Ϯ0.5 cm , and the cali-
The dissociation and reactivity of ͑NO͒ in the gas phase
bration was confirmed by comparison of the wavemeter cor-
rection and the assignment of the REMPI spectrum of NO
monomer. The NO(A) is ionized nonresonantly by the same
2
has been investigated using a variety of experimental spec-
troscopic techniques.1 Several of these studies have in-
–9
ϩ
laser pulse. The NO ions are accelerated toward the detec-
cluded indirect measurements of the binding energy of
5
tor, which is time-gated so that only ions with the same mass
as NO are detected, and velocity-mapped onto a two-
dimensional position-sensitive detector. The resulting ion im-
ages just below, at, and above the threshold for production of
͑
NO͒ , D ͓͑NO͒ ͔, through analysis of its ultraviolet and
2
0
2
6
photoelectron spectra, analysis of its ␯ and ␯ vibrational
bands, or direct study of vibrational predissociation of
1
5
7
,8
9
,10
͑
NO͒ in infrared photodissociation experiments.
These
2
NO(A) from ͑NO͒ are shown in Fig. 1. The outer ring in all
previous experiments indicate that the binding energy of
2
Ϫ1
three images is due to NO(A) from the dissociation of the
͑
NO͒ is in the range of 560–800 cm , which is larger than
2
NO•Ar clusters, which photodissociates at a much lower en-
a typical van der Waals interaction and smaller than a typical
chemical bond. Several of the previously determined values
Ϫ1 12
ergy (ϳ100 cm
)
and has much greater translational en-
ergy at this laser frequency. In the first image, no center spot
is observed. In the second image, a faint center spot can be
seen, and it grows brighter and slightly larger in the third
image. This central feature is also observed at the same
threshold wavelength when the carrier gas is changed to He,
and the threshold wavelength is also independent of stagna-
for the binding energy of ͑NO͒ are given in Table I. There is
considerable disagreement between these previously mea-
2
sured values. The two most recent measurements differ from
Ϫ1
each other by more than 100 cm
.
In this work, we have applied velocity-mapped ion im-
aging following nonresonant ionization to directly observe
ϩ
tion pressure. We therefore assign this central feature to NO
the threshold for dissociation of ͑NO͒ . Ion imaging has
2
been applied to a wide range of photodissociation
11
processes,
and is typically coupled with resonance-
TABLE I. Measurements of the (NO)2 binding energy.
enhanced multi-photon ionization ͑REMPI͒ to detect state-
selected products.
In this experiment, ͑NO͒2 is produced in a doubly
skimmed molecular beam by expanding 3%–5% NO in Ar,
D ͓(NO) ͔ (cmϪ1)
Reference
This work
Dkhissi et al. ͑8͒
Howard et al. ͑7͒
Fischer et al. ͑6͒
Hetzler et al. ͑10͒
Casassa et al. ͑9͒
Billingsley et al. ͑5͒
Year published
0
2
2002
1997
1993
1992
1991
1988
1971
696Ϯ4
764Ϯ18
639Ϯ35
787
710Ϯ40
800Ϯ150
560Ϯ33
a͒
Electronic mail: ewade@mills.edu
Current address: Lawrence Livermore National Laboratory, Livermore,
b͒
California 94550.
Electronic mail: chandler@california.sandia.gov
c͒
0021-9606/2002/116(12)/4755/3/$19.00
4755
© 2002 American Institute of Physics
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4756
J. Chem. Phys., Vol. 116, No. 12, 22 March 2002
Wade et al.
FIG. 1. Images collected following ionization of NO at ϳ222 nm, showing
the threshold of photodissociation of ͑NO͒ to NO(X)ϩNO(A). The images
2
are scaled so that the brightest pixels are full white. The first image is below
the photodissociation threshold, at 222.768 nm, so no central spot is ob-
served. The second image is just at threshold, at 222.745 nm, and a faint
ϩ
central spot, due to NO from the photodissociation of ͑NO͒2 can be ob-
Ϫ1
served. The third image, at 222.718 nm, is ϳ10 cm over the threshold and
the central spot is becoming larger and more intense. The outer ring in all
three images is due to NO(A) produced by the photodissociation of
NO•Ar, which is much more weakly bound than ͑NO͒ and so has more
2
recoil energy at the same laser frequency.
from the photodissociation of ͑NO͒ , which is being pro-
2
duced from nonresonant ionization of NO(A) just above the
photodissociation threshold and therefore has very little
translational energy.
Since a photon of ϳ222 nm light has more than enough
energy to ionize NO(A), the amount of recoil energy, Erec
,
FIG. 2. Schematic of energies involved in the photodissociation of ͑NO͒ to
NO(X)ϩNO(A), as described in Eq. ͑1͒.
2
observed in the ͑NO͒ image is the sum of the initial internal
2
energy of ͑NO͒ , E
, and the energy of the photon, h␯,
initial
2
minus the sum of D ͓͑NO͒ ͔, the energy difference between
0
2
Ϫ1
the NO(X) state and the NO(A) state, T ͓NO(A)
that D ͓͑NO͒ ͔ is 696Ϯ4 cm . However, it is likely that, if
0
0
2
Ϫ1
�
NO(X)͔ and the internal energy of the NO products, ENO
͑NO͒ were rotating with 3 cm of rotational energy, the
2
rotation would result in centrifugal acceleration of the disso-
ciation products which would in turn result in translational
energy. Since we can directly observe NO(A) produced just
above threshold with Ͻ2 cm-1 of translational energy, it is
likely that at threshold we are observing NO(A) produced
ErecϭEinitialϩh␯Ϫ
͕
0
D ͓͑NO͒ ͔
2
ϩT ͓NO͑A͒� NO͑X͔͒ϩENO͖
.
͑1͒
0
These energy levels are shown schematically in Fig. 2.
By examining the ion images to determine the wave-
length for which Erec approaches zero, where the central spot
in the images disappears, we find that the threshold energy,
from ͑NO͒ with very little initial rotational energy. If that is
2
the case, then D (͑NO͒ ) equals our directly measured
0
2
Ϫ1
threshold energy and is 693Ϯ2 cm
.
Ϫ1
ϩ
or the laser frequency associated with Erecϭ0 cm , is
We also assume that the NO we observe at the thresh-
old wavelength is due to NO products in their rotational and
vibrational ground state, so that ENOϭ0. It is possible that
dynamic constraints make it impossible for the dissociation
Ϫ1
4
4893Ϯ2 cm or 222.75Ϯ0.01 nm. T ͓NO(A)� NO(X)͔
0
Ϫ1 13
is very well established at 44 200.2 cm
D ((NO) )ϪEinitial͖ is 693Ϯ2 cm . Einitial is the sum of
,
so
Ϫ1
͕
0 2
vibrational and rotational energy for ͑NO͒ . The smallest
of ͑NO͒ to produce NO in its ground rotational state. Since
2
2
Ϫ1
frequency normal mode of ͑NO͒ is ␯ , which is 117 cm
this would mean ENOϾ0, it would mean that our value of
D (͑NO͒ ) is larger than the true value. However, we ob-
2
4
for the cis isomer1 and 79 cm for the trans isomer. If we
were observing dissociation from vibrational hot bands, we
would expect to see separate rings for each vibrationally ex-
cited state, as the detection system can easily resolve rings
4
Ϫ1
15
0
2
ϩ
Ϫ1
serve NO with Ͻ2 cm
of translational energy, which
suggests that there is at most a very small barrier to disso-
ciation, and which in turn suggests that just at the dissocia-
tion threshold there is no impulse that would force the NO
products to be rotationally excited.
Ϫ1
corresponding to an energy spacing of Ͼ10 cm . Since we
do not observe such rings, the only remaining source of in-
ternal energy for ͑NO͒ is rotational excitation. It is not pos-
Hetzler et al. observed product rotation distributions of
the NO fragments following dissociation of overtone-excited
͑NO͒2 .10 They accounted for the observed distributions by
2
sible for us to directly measure the extent of rotational exci-
tation, but we can estimate it. REMPI measurements of the
spectrum of NO monomer in our molecular beams give us a
rotational energy of 3Ϯ1 K. Using the available rotational
Ϫ1
postulating a 900 cm barrier to the association of NO frag-
ments to form the dimer. Our results strongly suggest that
7
Ϫ1
constants for ͑NO͒ , and approximating ͑NO͒ as a prolate
there is at most a very small (Ͻ2 cm ) barrier to the re-
2
2
top, it is possible to calculate a Boltzmann distribution for
NO͒ rotational states at 3 K. If we assume that we do not
verse reaction.
ϩ
͑
Similarly, we assume that the first NO we observe is
2
detect signal due to rotationally excited states that are less
than 5% of the total population, we find an upper limit of
rotational excitation of 3Ϯ2 cm , which would indicate
due to the dissociation of the dimer to produce NO(A)
ϩNO(X,⌸1/2), not NO(A)ϩNO(X,⌸3/2). If for some rea-
son, the NO(X) is produced exclusively in the spin-orbit
Ϫ1
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J. Chem. Phys., Vol. 116, No. 12, 22 March 2002
The binding energy of the NO dimer
4757
Ϫ1 13
under Grant No. DE-FG02-98ER45708. This work was
funded by the U.S. Department of Energy, Office of Basic
Energy Sciences, Division of Chemical Physics.
excited state, then ENOϭ119.7 cm
,
and our observed
threshold would correspond to a dissociation energy of 573
Ϯ5 cm . This value is substantially lower than previous
Ϫ1
measurements. It seems unlikely that this dissociation would
produce no NO in the spin-orbit ground state, since statistical
dissociation mechanisms would favor formation of the spin-
orbit ground state rather than the spin-orbit excited state.
This threshold measurement was also confirmed by our
1
W. N. Lipscomb, F. E. Wang, W. R. May, and E. L. Lippert, Jr., Acta
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J. Granier-Mayence and J. Romand, C. R. Acad. Sci. 236, 1148 ͑1953͒.
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͑
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4
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2
Ϫ1
13
5
6
NO is less than 1500 cm above the A-state. By setting
the laser frequency to the predicted energy, we were able to
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J. Billingsley and A. B. Callear, Trans. Faraday Soc. 67, 589 ͑1971͒.
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͑1988͒.
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7
8
Unfortunately, the threshold of the ͑NO͒ →NO(B)
2
ϩNO(X) channel overlaps with a 1ϩ1 REMPI transition in
9
ϩ
NO monomer, producing an NO signal which overwhelms
ϩ
the NO signal rising from the photodissociation of the
10
dimer. As a result, we were not able to directly observe this
channel’s threshold. However, the observed recoil energy
distribution for the new channel is consistent with our value
of D ͓͑NO͒ ͔.
In summary, we have directly observed NO produced by
the photodissociation of NO dimer, and have determined a
͑
1991͒.
1
1
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1995͒.
͑
12
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0
2
13
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prepared by J. W. Gallagher and R. D. Johnson III͒ in NIST Chemistry
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measurement of the binding energy of the NO dimer at 696
Ϫ1
Ϯ4 cm
.
14
15
09, 4378 ͑1998͒.
The authors thank Mark Jaska for technical assistance.
J.I.C. acknowledges support from the Department of Energy
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