Photodissociation of NO2 in the region 217-237 nm: Nascent NO energy distribution and mechanism

Article abstract of DOI:10.1021/jp013966l

The NO₂ molecule is an important component of combustion systems, atmospheric chemistry, and energetic materials decomposition. NO photodissociation at 217-237 nm was studied by proving the nascent NO generated utilizing a one-laser photofragmentation/fragment-detection method. The nascent NO spectrum changed depending on the intensity of the laser beam. For low laser intensity, NO₂ dissociation produced rotationally and vibrationally cold NO through the NO₂ 2²B₂ excited state. Excess energy for this photofragmentation was ～ 0.4 ev at 226 nm. This excess energy was distributed almost entirely to the kinetic energy of NO(X²A₂) and O(¹D) products. The spectrum of photofragmented NO became very crowded with increasing laser intensity because higher rotational and vibrational levels of NO became populated through multiphoton excitation and eventual photofragmentation of NO₂ from higher-energy electronic states. Rotational temperature of NO was ～ 200 and < 30 K for high and low laser intensity, respectively. A rovibronic spectrum was observed near 230 nm, which could not be ascribed to NO. The time-of-flight MS line width for photofragmented NO in this photolysis region increased by a factor of 2 to ～ 40 nsec for high laser intensity. A higher electronic state of NO₂ was accessed to generate highly excited NO through multiphoton absorption processes for the high laser intensity studies. Such intermediate states could be Rydberg, ion pair, or other high-energy states of NO₂ accessed by multiphoton absorption.

Full text of DOI:10.1021/jp013966l

J. Phys. Chem. A 2002, 106, 7565-7572
7565
Photodissociation of NO2 in the Region 217-237 nm: Nascent NO Energy Distribution and
†
Mechanism
H.-S. Im and E. R. Bernstein*
Department of Chemistry, Colorado State UniVersity, Fort Collins, Colorado 80523-1872
ReceiVed: October 25, 2001; In Final Form: May 22, 2002
Photodissociation of NO
using a one-laser photofragmentation/fragment-detection technique. By mixing O
00%) and by setting proper detection timing, only mass-resolved excitation spectra (MRES) of photo-
fragmented NO are obtained and examined. The nascent NO spectrum changes depending on the intensity
2
in the region of 217-237 nm is investigated by probing the nascent NO generated
2
in the sample (20%-
1
2
(
W/cm ) of the laser beam. For low laser intensity, the dissociation of NO
2
produces rotationally and
2
vibrationally cold NO through the NO
2
0.4 eV at 226 nm. This excess energy is distributed almost entirely to the kinetic energy of NO(X A ) and
O( D) products. The spectrum of photofragmented NO becomes very crowded as the laser intensity is increased
because higher rotational and vibrational levels of NO become populated through multiphoton excitation and
2 2
2 B ( B˜ ) excited state. Excess energy for this photofragmentation is
2
∼
1
2
eventual photofragmentation of NO from higher-energy electronic states. The rotational temperature of NO
is ca. 200 K for high laser intensity and less than 30 K for low laser intensity. Near 230 nm, a rovibronic
spectrum is observed that cannot be attributed to NO even though it is detected in the NO mass channel. The
time-of-flight mass spectrum line width for photofragmented NO in this photolysis region increases by a
factor of 2 to about 40 ns (estimated NO kinetic energy is 0.40 ( 0.04 eV) for high laser intensity. These
2
observations lead to the conclusion that a higher electronic state of NO has been accessed to generate highly
excited NO through multiphoton absorption processes for the high laser intensity experiments. Such intermediate
states can be Rydberg, ion pair, or other high-energy states of NO accessed by multiphoton absorption.
2
1
I. Introduction
(40 995 cm- or 244 nm). At lower energies than this latter
value, the excess reaction energy must go into NO rotations,
vibrations, product translational energy, spin-orbit levels (Ω)
The NO2 molecule appears as an important component of
combustion systems, atmospheric chemistry, and energetic
materials decomposition. Spectroscopy and photofragment
dynamics studies of NO2 have revealed a very rich and varied
set of molecular properties for this triatomic molecule. Suf-
ficiently strong intramolecular vibration, rotation, and electronic-
state interactions exist in this molecule that the visible region
of the spectrum of NO2 displays apparently chaotic dynamics
2
3
of NO( ΠΩ; V, J), or internal states of O( Pj) or a combination
thereof. Experimental conditions, such as laser intensity, laser
wavelength, and detection method, can have significant conse-
quences for NO2 photodissociation studies. For example,
photolysis of NO2 at 248 nm (40 323 cm ) monitored by laser-
induced fluorescence (LIF) of NO is reported to yield nascent
NO with a strongly inverted population with a maximum at V′′
-1
3
1
and over 10 resolvable features. Photodissociation of NO2 to
give NO + O has been studied as a model of unimolecular
fragmentation dynamics. As new spectroscopic and photo-
dissociation methods have become available in the last 30 years,
5
a
)
6-8 and no population in V′′ ) 0. Another study under
ostensibly the same experimental conditions but employing
resonance-enhanced multiphoton ionization (REMPI) for detec-
tion of nascent NO, however, determined that the vibrational
distribution of NO peaks at V′′ ) 5 and has a large V′′ ) 0
2
they have systematically been applied to this prototypic system.
Such NO detection of nitro compound (RNO2) photofragmen-
tation has been employed for an analysis/detection technique
for fuels and explosives.
The photochemistry of NO2 has been studied extensively for
many years, with particular attention given to the first dissocia-
5
b
population.
Photodissociation of NO2 at higher energies has also been
studied. The 2 B2 ( B˜ ) origin is at T0 ) 40 126 cm , and at
2
-1
-
1
-1
1
4
0 972 cm (T0 + ∼850 cm ), the NO2 f NO(X) + O( D)
4
-1
2
7
tion threshold (NO2 + hν (∼2.5 × 10 cm ) f NO(X Π) +
channel opens. Morrison and Grant studied the dynamics of
NO2 dissociation via this high-energy B˜ state using two-photon
photodissociation with visible light. They concluded that the
internal energy distribution in the nascent NO produced by this
3
3
4
O( P)) , and various photon energies have been utilized. A few
5
4
-1
studies are carried out near 4.0 × 10 cm . These studies have
focused on the characterization of NO internal and external
energy distributions. Recently, femtosecond laser excitation at
1
two-photon O( D) pathway qualitatively resembles that observed
for the one-photon dissociation via the O( P) pathway for similar
excess energies. Bigio et al. have studied single-photon
photodissociation of NO to compare the dynamics of one-
4
-1
3
2
.7 × 10 cm photon energy has been employed to study the
6
8
dissociation and competitive ionization of NO2. These various
1
excitation energies are not high enough to open the O( D)
dissociation channel: O( D) can only appear above 5.08 eV
2
1
photon vs isoenergetic two-photon photodissociation. They
report the spectrum of photofragmented NO using a focused
high-intensity laser beam in the region 227-220 nm. A
†
Part of the special issue “G. Wilse Robinson Festschrift”.
1
0.1021/jp013966l CCC: $22.00 © 2002 American Chemical Society
Published on Web 07/11/2002

7
566 J. Phys. Chem. A, Vol. 106, No. 33, 2002
Im and Bernstein
simulation of rotational distribution characterizing the spectra
II. Experimental Methods
of ref 8 yields a rotational temperature of ca. 200-300 K.
A. Apparatus and Procedures. The experimental apparatus
employed for these photofragmentation studies of NO2 has been
fully described in previous papers from our laboratory.15 NO2
is introduced into the ionization region of a time-of-flight mass
spectrometer (TOFMS) through a pulsed supersonic nozzle. A
laser beam intersects the molecular beam created by this
supersonic expansion at right angles to both the molecular beam
flow axis and the TOFMS axis. At this intersection, NO2 absorbs
a photon (or photons) from the laser and dissociates to NO +
O. The fragmented NO is subsequently probed using a (1 + 1)
9
Marshall et al. demonstrate that the spectrum of NO obtained
from NO2 is different than that found for NO gas itself. At 232
nm, they observed a band in the NO-detected NO2 photo-
fragmentation spectrum that is not present in the NO spectrum.
Other experiments explore NO2 fragmentation behavior
through observation of the O atom states using one- or two-
1
0
11
photon laser-induced fluorescence (LIF), REMPI, and ion
imaging.1
2,13
Most of these studies focus attention on the state
distribution of O( Pj) following the photodissociation of NO2
near 225 nm.
3
2
+
2
+
REMPI scheme through its A( Σ ) r X( Π) transition. NO
Translational energy and angular distribution of the fragments
ions are detected at the end of a 1.5 m flight tube by a
microchannel plate detector, and the output voltage from the
detector is averaged in a boxcar averager and recorded on a
computer through an ADC card.
(NO and O) have also been determined using Doppler profile
measurements and velocity map ion-imaging techniques. From
1
the study of the O( D) Doppler profile in the photodissociation
of NO2 at 205.47 nm,11 rotational excitation in the NO fragment
is determined to be small because only a small amount of energy
The rovibronic excitation spectrum of fragmented NO (γ
bands) is obtained by scanning the laser wavelength from 217
to 237 nm. For comparison, the (0-0) transition spectrum of
NO gas itself is also recorded. Results presented below are
generated by a single laser, so both NO2 photodissociation and
the NO fragment spectrum are generated by the same photons.
The rotational temperature of the nascent NO is determined from
simulation of the NO (0-0) band rotational spectrum using
rotational spectrum simulation software for diatomic molecules
1
is released into the O( D) atom translational degrees of freedom.
Most available energy appears in the vibrational degrees of
freedom for NO at 205.47 nm photodissociation. A velocity
+
+
13
map ion-imaging study of O and NO by Ahmed et al. at
26 nm excitation/dissociation of NO2 finds that nearly all of
2
1
the excess energy for NO2 photodissociation into O( D) and
NO(X) appears as product translational energy;1 that is, the
NO(X,0) product is not highly rotationally excited. Additionally,
they report that the product energy distribution is quite sensitive
to the exact wavelength of the excitation/detection laser.
2,13
16
developed at Sandia National Laboratory. The vibrational
temperature of NO-fragmented from NO2 can be estimated based
on the Franck-Condon-corrected intensity ratio I(0-1)/I(0-0)
and the Boltzmann population formula. The TOFMS line shape
for the NO mass channel signal at a given laser wavelength is
recorded on a digital oscilloscope for a 256 shot average. The
full width at half-maximum (fwhm) of the feature is measured
to determine the line width of the TOFMS peak at a specific
NO transition. The features always appear to be symmetric.
Information on the line width of this feature reflects the
dissociation dynamics; in particular, it gives an estimate of the
translational energy for the fragmented NO. The instrumental
resolution of the TOFMS for cooled NO gas is ca. 22 ns for an
unfocused beam and ca. 18 ns for a focused beam. These widths
depend, in part, on the laser beam position, mode structure, and
size at the ionization region of the TOFMS.
In a very recent paper, Richter et al.14 discuss NO2 fragmen-
tation at 212.8 nm. They determine the translational energy
released in the fragmentation and characterize the states of both
NO and O fragments. They find that, if NO is formed in a
1
rovibrational level that permits the formation O( D) energeti-
cally, then this state of oxygen is the exclusive coproduct. NO2
multiphoton processes are a factor for their experiments as well,
just as we have found in the present study. They conclude that
1
the NO from the O( D) channel is found to be mostly in the V′′
)
3 for 212.8 nm photodissociation.
In this present study, the dissociation of NO2 in the region
217-237 nm is examined by probing photofragmented nascent
NO using a one-laser photofragmentation/fragment-detection
technique. The nascent NO spectrum changes dramatically
The frequency-doubled output (532 nm) of the fundamental
(1064 nm) of a Nd:YAG laser is used to pump a dye laser. To
obtain the required UV photons, the dye laser output is doubled
and mixed with the Nd:YAG fundamental. A series of seven
dyes is used to cover the fragmentation/excitation range from
237 to 217 nm: R640, R640/KR620, KR620, R610, R610/R590,
R590, and R575 (Exciton). The UV output energy of this laser
is between 300 and 500 µJ/pulse depending on the dye medium
and exact wavelength in the output range. The laser-beam
2
2
depending on the intensity (W/cm ) or fluence (J/cm ) of the
laser beam; the spectrum of NO becomes very crowded as the
laser intensity is increased because higher rotational and
vibrational levels of NO become populated. Additionally, the
line width of the mass spectrum signal increases at laser
wavelengths in the 227-237 nm region. This observation is
interpreted as an increase in the kinetic energy of the photo-
fragmented NO due to the multiphoton nature of the NO2
dissociation process at high laser intensity. Near 230 nm, a
spectrum is observed that cannot be attributed to NO; it has a
progression in a 35 cm-1 energy spacing, it has multiple features
with different line widths, it has an intensity pattern inconsistent
with a NO rotational distribution based on a given temperature,
but it is nonetheless detected in the NO mass channel. From
these observations, we conclude that a higher electronic state
of NO2 has been accessed through multiphoton processes for
the high laser intensity experiments. This intermediate state can
5
2
intensity is (1.1-1.9) × 10 W/cm for the unfocused beam,
and for the focused beam (360 mm focal length lens), the beam
8
2
intensity is (1.3-2.1) × 10 W/cm . The time spread (full width
at half-maximum) of this UV pulse is typically ca. 6 ns measured
for the pulse envelope but this envelope typically represents a
random, time-varying distribution of ca. 100 ps pulses.
NO2 gas (99.5+% pure, Aldrich) and NO gas (98.5+% pure,
Aldrich) are employed without additional purification. To
1
suppress the reaction NO2 f NO + ( /2)O2, the expansion gas
+
-
be a Rydberg state, an ion-pair state (NO ‚‚‚O ), or other
higher-electronic state of NO2 reached by multiphoton absorp-
tion. Rotational, vibrational, and translational energy distribu-
tions of NO are estimated for the NO2 photodissociation reaction
on the basis of the observed spectra.
for NO2 experiments includes at least 20% O2. A typical mixture
of 0.5% NO2, 20% O2, and 80% (10% Ar in He), is used for
these experiments. The gas mixture of NO2/O2/Ar/He is held
in a steel mixing vessel for at least 1 day to ensure thorough
sample mixing. Additionally, the vessel is filled through a

Photodissociation of NO2 in the Region 217-237 nm
J. Phys. Chem. A, Vol. 106, No. 33, 2002 7567
stainless steel tube plugged at one end and welded to the inlet/
outlet valve at the other, which has more than 100 small holes
drilled over its length. The sample gas is held in this vessel for
a number of weeks over the course of these experiments to
facilitate the reaction 2NO + O2 f 2NO2. The expansion gas
mixture is thus thoroughly equilibrated and homogeneous before
use in these experiments. This mixture is delivered through PVC
or Teflon tubing to the pulsed nozzle and expanded into the
vacuum chamber at a total backing pressure of 100 psig. For
NO experiments (no NO2 fragmentation), 10 ppm of NO is
prepared in a premixed 10% Ar/He backing gas as indicated
above.
For velocity distribution measurements, the chamber working
pressure and the timing of the probe laser are kept constant and
unchanged as the timing of the pulsed valve is swept. As has
been reported a number of times, velocity slip occurs for these
supersonic expansions of a carrier gas (He/Ar) with seeded
17
molecules (NO2, NO, CH3NO2, etc.). That is, light molecular
species can achieve a higher speed in the beam than the heavy
ones and thereby arrive at the source region of the TOFMS
earlier than the heavy ones. Thus, each molecular species in
the expansion can achieve a different terminal velocity and thus
a different arrival time at the ionization region of the TOFMS,
Figure 1. Mass-resolved excitation spectra (MRES) of NO gas (0.01%
NO/10% Ar/90% He) at the A r X (0-0) transition obtained using (a)
5
2
low laser intensity (1.9 × 10 W/cm ) and (b) high laser intensity (2.1
8
2
-1
×
10 W/cm ). The peak at 44 202 cm is the (Q11 + P21) band of
depending on its mass, collision cross section, concentration,
temperature, and relaxation behavior.17 This means that each
each NO spectrum. The fwhm of the TOFMS signal for trace a is 22
ns, while that for trace b is 18 ns as indicated in the figure. Narrowing
of the TOFMS line width can occur, because the interaction volume
between the laser beam and NO gas becomes smaller by focusing the
laser beam. Note the Stark broadening of the NO spectral features at
high laser intensity.
precursor species in the supersonic expansion beam has its own
velocity (or arrival time) distribution at the ionization region
of the time-of-flight mass spectrometer. By comparing these
distributions in arrival time, one can determine the particular
parent species associated with the photofragmented species as
shown in ref 17 for many other molecular and cluster systems.
The arrival time distribution for the observed photofragment
signal can thus be related to a specific precursor molecule in
the supersonic expansion by direct comparison with arrival time
distributions of possible precursors. Therefore, the carrier
molecule for the detected NO can be readily distinguished.
Modeling of this velocity slip for NO/NO2, employing the
approximations of ref 17b, gives rough agreement with the
observed arrival time differences for these species.
expansion are identical to those for a full (∼100%) O2
expansion. This observation suggests that the spectra presented
in this report are not associated with a NO concentration present
in the sample gas prior to photodissociation of NO2. One can
compare the spectra obtained for NO that arise from two
different samples: (1) a mixture of 10 ppm NO/10% Ar/90%
He and (2) 0.1% NO2 expanded with 10% Ar/90% He, 20%
O2/80% He, or 100% O2. Such a comparison yields several
observed differences. NO spectra feature Stark broadening as a
function of increased laser intensity for a pure NO sample
B. Source of NO Spectra. NO2 is unfortunately a very
complex “triatomic” molecule. It can be viewed, in general, as
four systems: NO2, (NO2)2(tN2O4), NO, and (NO)2. To
confirm the source of the detected NO (i.e., whether it comes
from the photodissociation of NO2 or some other source or
process), several different experiments and approaches have
been pursued: velocity/timing measurements, comparisons of
NO spectra from different samples, concentration studies,
expansion gas changes, laser-intensity studies, and spectral
analysis. Below, we present several results that strongly support
the conclusion that the parent molecule for the photodissociation
processes that we investigate is truly NO2, that is, only
photofragmented NO from NO2 is studied in the present work.
(
number 1 above) but do not for a NO2 sample (number 2
above). See Figures 1 and 2. The observed NO (0-0) rotational
structure and extent do not change with increasing laser intensity
for a NO sample but do change with increasing laser intensity
for a NO2 sample (see Figure 2). At the same low laser intensity
and under the same expansion conditions, a NO sample gives
a Trot ≈ 11 K, while a NO2 sample gives a NO Trot ≈ 30 K (see
Figure 3). At low laser intensities, the NO signal in mass channel
3
0 amu has a laser intensity dependence of ca. 1.5 for an
expansion of 10 ppm NO/10% Ar/90% He. Under these same
experimental conditions, a sample of ∼0.1% NO2/20% O2/80%
He or 0.1% NO2/100% O2 has a NO signal dependence on laser
intensity that is greater than 2.0. Saturation effects for these
two different processes (NO (1 + 1) ionization and NO2
photodissociation/NO ionization) cause these laser energy
dependences for the NO mass signal to be lower than their
predicted values of 2 and 3, respectively.
As seen in Figure 4, the velocity or arrival time distribution
for each NO-containing sample (1 or 2 above) is quite differ-
ent. The TOFMS maximum signal in mass channel 30 (NO )
arrives 30 µs earlier for a NO sample than for a NO2 sample,
due to velocity slip in the supersonic molecular beam.17 The
NO (0-0) spectrum is not changed in timing for the various
features for NO2 concentrations, 0.5%, 0.1%, or 0.01%, in the
expansion. All spectra are identical for these samples under the
1
The equilibrium constant for the reaction NO + /2O2 f NO2
9
is 1.3 × 10 on the basis of the Gibbs free energies for the
+
reactants and products. In an expansion with significant cooling,
the collision rate is very high at the nozzle throat in the gas
phase, and therefore, an expansion with 100% O2 at 20 psig
should have a subpart per billion NO concentration. This
concentration is an order of magnitude below the detection
sensitivity for our experiment.18 Therefore, the NO spectrum
obtained from a sample of 0.5% NO2/99.5% O2 expanded at
ca. 20 psi can be assumed to be derived only from NO
fragmented from NO2 and to be free of NO contamination.
Additionally, the NO spectra from a partial (∼20%) O2
+

7568 J. Phys. Chem. A, Vol. 106, No. 33, 2002
Im and Bernstein
Figure 4. Velocity distributions for (2) NO gas and (9) fragmented
NO from NO . Both NOs are detected at mass 30.
2
line width measurements given below) is too small by a factor
of at least 4 for the NO to arise from N2O4 photodissociation.
Figure 2. MRES of nascent NO from photodissociation of NO
2
19
5
2
obtained using (a) a low-intensity (1.9 × 10 W/cm ) laser beam and
Given the above experimental observations, we conclude that
8
2
(
b) a high-intensity (2.1 × 10 W/cm ) laser beam. The most intense
+
-1
the signals observed in the 30 amu (NO ) mass channel for
peak (arrows) at 44 202 cm is the (Q11 + P21) band of each NO
samples with the NO2 parent molecule photodissociated to
generate NO are derived from the isolated, cooled NO2 molecule
and not from NO, (NO)2, (NO2)2(dN2O4), etc.
spectrum.
III. Results
The spectrum of NO from fragmented NO2 in the NO (0-0)
2
+
2
+
region of the A( Σ ) r X( Π) transition detected in the NO
mass channel for two different laser intensities is presented in
Figure 2. Both spectra are well-resolved, and various bands can
be clearly identified in both spectra. Even though NO is
fragmented from NO2, rotationally cold NO is detected for an
2
unfocused laser beam (Figure 2a) with 700 µJ/cm fluence or
5
2
∼
10 W/cm intensity. The most intense rotational feature in
-
1
the spectrum, at 44 202 cm , is assigned to the (Q11 + P21)
band, which is the origin of this transition. The P11 band at
2
0
-
1
4
4 197 cm , which is a hot band of the (Q11 + P21) rotational
transition, is considerably weaker: other bands, (R11 + Q21) at
-
1
-1
4
4 207 cm and R21 at 44 215 cm , are also weak. These
features indicate a low rotational temperature for the fragmented
2
NO at the X( Π) V′′ ) 0 vibrational level: simulation of this
rotational structure (see Figure 3) suggests a Trot ca. 30 K.
On the other hand, when a focused laser beam is used to
8
2
2
fragment NO2 (ca. 10 W/cm intensity or 700 mJ/cm fluence),
NO is born with a substantial degree of rotational excitation as
shown in Figure 2b. P11 is now clearly much more intense and
2
many X( Π) V′′ ) 0 rotational levels are populated. In addition,
2
significant rotational population can be observed in the Π3/2
-
1
2
spin state at 120 cm to higher energy than the Π1/2 (X) state.
The estimated rotational temperature for NO under these fluence
conditions is Trot ≈ 200 K. Figure 5 shows how the rotational
transition intensity (Trot) increases with laser beam intensity.
Figure 3. Simulation of the NO rotational spectrum at the (0-0)
transition obtained by (a) NO gas and (b) fragmented NO from NO .
2
The simulated spectrum for each sample is shown by the dotted line
fit to the experimental spectrum (solid line). The rotational temperature
is determined to be ∼11 K for NO gas (panel a) and ∼30 K for
The same NO (0-0) spectral pattern for a focused laser beam
and a NO2 sample can be observed for the (1-1) and (2-2)
transitions as shown in Figure 6. The TOFMS line width for
these peaks is the same as the high intensity (0-0) spectrum,
ca. 20 ns. The (1-1) and (2-2) transitions can only be observed
for a focused, high-intensity laser beam. No significant signal
is observed in the (3-3) region; very weak signals, less than 10
2
fragmented NO from NO .
same experimental conditions (i.e., laser intensity, expansion
conditions, etc.), thereby showing that neither NO nor NO2
complexes ((NO)n, (NO2)n, etc.) are involved in the NO2
photodissociation. A final point to note is that the kinetic energy
released for the photofragmented NO in our studies (see TOFMS

Photodissociation of NO2 in the Region 217-237 nm
J. Phys. Chem. A, Vol. 106, No. 33, 2002 7569
Figure 5. (0-0) transition of fragmented NO as a function of laser
6
7
8
2
intensity: (a) 9.5 × 10 ; (b) 3.0 × 10 ; (c) 2.1 × 10 W/cm . This
observation indicates the multiphoton behavior of the spectrum.
Figure 7. MRES for mass channel 30 from the photodissociation of
8
2
NO
2
obtained using high-intensity (1.3 × 10 W/cm ) laser excitation
in (a) the (0-1) region, (b) the (1-2) region, and (c) the (2-3) region.
Most of the peaks show a fwhm TOFMS line width of about 40 ns.
The feature marked with / in panel a has a fwhm TOFMS line width
of 30 ns.
TABLE 1: The NO Vibrational Frequencies in the Ground
and Excited States
2
2 +
vibrational
level
X( Π)
A( Σ )
-
1
-1
(cm
)
(cm )
1
2
3
1870
3723
5543
2337
4651
a
a
The energy of vibrational level 3 is obtained from ref 21.
transition is actually observed in this spectral range. Instead of
observing the (1-2) and (2-3) NO transitions near 42 800 ( 200
-
1
-1
cm and 43 400 ( 200 cm , respectively, much different
-
1
spectra are found, consisting of doublet peaks spaced 28 cm
apart in the (1-2) region and another set of doublet peaks spaced
5 cm apart in the (2-3) region. The NO (0-1) transition
-
1
3
Figure 6. MRES of (a) the (1-1) transition and (b) the (2-2) transition
spectrum shown in Figure 7a results from rotationally hot NO
because the spectrum appears similar to the one in Figure 6b
for the high-intensity focused laser beam. The two other spectra
of Figure 7b,c do not display any rotational bands indicative of
expected NO (A r X) transitions. The line widths of the
2
of the nascent NO from the photodissociation of NO obtained using
8
2
high laser intensity (2.1 × 10 W/cm ). These peaks show a fwhm of
about 20 ns for a TOFMS line width. A few of them have the fwhm
of about 30 ns.
+
mV with about a 40 ns fwhm, are found in this spectral region
TOFMS features for NO (mass channel 30) generated by
+
-1
for the NO mass channel.
photofragmentation of NO2 in the range 42 200-44 050 cm
The NO spectrum between 42 200 and 44 050 cm- has also
been obtained through NO2 photodissociation. These features
can only be observed with a focused laser beam (high intensity).
Spectra observed in the NO (2-3), (1-2), and (0-1) regions are
displayed in Figure 7. The experimentally determined NO
1
are also different from those generated in the range 44 050-
45 200 cm . The NO peak just below 44 050 cm has an
associated TOFMS line width of about 40 ns, and the peak at
-
1
-1
-
1
2
44 055 cm (P11 of Π3/2 (0-0)) has a line width of about 20
ns, as shown in Figure 8. Most of the spectral features in the
2
2
+
-1
vibrational levels for both the X( Π) ground state and the A( Σ )
42 200-44 050 cm show a TOFMS peak with a line width
excited state are presented in Table 1. Only the (0-1) NO
of ca. 40 ns. Some features in the (0-1) transition region show

7570 J. Phys. Chem. A, Vol. 106, No. 33, 2002
Im and Bernstein
energy distributions would need to be much different than
observed. The amount of translational energy available to NO
under these conditions is insufficient for fragmented NO in this
-
1
process to occupy the V′′ ) 1 vibrational level at 1870 cm
see Table 1). This is the reason that with the unfocused laser
beam (low intensity to ensure only single-photon NO2 excitation,
(
2
+
2
B˜ r X˜ ), only the (0-0) NO A( Σ ) r X( Π) transition is
observed. These observations are consistent with the NO2
molecule dissociating from a linear configuration on a repulsive
part of the potential surface.
The additional kinetic energy for photofragmented NO means
that the NO ion has an additional velocity that either assists or
hinders the acceleration given the ion by the ca. 250 V/cm
electric field in the ion extraction region (4000-3750 V) of
the TOFMS ion source. This additional NO kinetic energy from
NO2 photofragmentation should generate an additional TOFMS
+
line width for the 30 amu (NO ) mass channel signal.
23
Employing the TOFMS equations for the ion flight time, we
+
calculate the TOFMS signal line width for the NO photo-
fragment with Etrans(NO) ) 0.137 eV kinetic energy to be ca.
2
0 ns. This line width is comparable to the instrumental line
width.
8
2
When the photolysis/probe beam is focused (∼10 W/cm ),
the (0-0), (1-1), and (2-2) vibronic transitions of NO A r X
are observed with an extensive rotational structure (Trot ca. 200
K) as shown in Figures 5 and 6. Spectra in these figures can
only be observed with high-intensity laser excitation.
Figure 8. TOFMS line width of various NO peaks obtained with high
8
2
laser intensity ((1.3-2.1) × 10 W/cm ). The bottom signal (trace a)
-
1
is for the peak at 44 055 cm (marked with / in the inserted figure),
for which fwhm is 18 ns. This peak is the lowest-energy feature in the
The focused laser beam produces a much more highly excited
NO (both rotationally and vibrationally) than does the unfocused
beam. This circumstance implies more excess energy in NO2
for the focused beam than for the unfocused beam photolysis
conditions. Note that from the above energy balance discussion,
single-photon excitation of NO2 to the B˜ state in this energy
(
4
0-0) transition of NO. The middle trace (trace b) is for the peak at
4 008 cm (marked with an arrow in the inserted figure), for which
-
1
fwhm is 42 ns. This peak is at the high-energy end of the spectrum of
-
1
trace c. The top dashed trace (trace c) is for the peak at 42 405 cm
(
marked with / in the inserted figure), for which fwhm is 30 ns. See
the text for the details. Note that the peak in trace c has a step change
in its width at roughly one-half its height.
-
1
region (44 200 cm , (0-0) transition for NO A r X) is not
energetic enough to dissipate more energy into vibrational and
rotational degrees of freedom with O( D) production. Thus,
a superposition of 40 and 20 ns line widths as presented in
Figure 8c. Trace c of this figure shows a line shape not readily
characterized by a single-width parameter at half-height.
1
multiphoton excitation of NO2 must occur to generate the
required NO energy balance and summation. The (0-0) NO
structure of Figure 5 clearly shows a laser-intensity depend-
ence: a multiphoton process for NO2 photodissociation is
indicated. Under single-photon NO2 excitation to the B˜ state,
the excess energy is not sufficient for X (V′′ ) 2) occupation if
IV. Discussion
A. Dissociation Mechanism for NO2 near 226 nm and
below. A single photon of wavelength 226 nm will excite NO2
1
2
2
2
O( D) is also produced. If one assumes that the dissociation
to its 2 B2 state. This transition ( B˜ B2 r X˜ A1) shows discrete
rotational structure near its origin at ca. 249.1 nm, but the
transition becomes diffuse below 245.9 nm because of vibra-
3
occurs through production of O( P) ground state, then (3-3)
-1
should also be observed: no (3-3) (ca. 45 700 cm or 219 nm)
could be detected for a focused laser beam in the appropriate
spectral region. These arguments lead to the conclusion that
NO production for focused and unfocused laser beam excitation
of NO2 is through different pathways.
tional predissociation.22 At 226 nm excitation, the NO + O(1D)
channel for dissociation is open; the threshold for this channel
is at 243.9 nm, and thus,
2
1
From laser intensity studies, the fragmentation of NO2 and
the excitation and ionization of NO (for the (0-0), (1-1), and
NO + 1hν (226 nm) f NO * (2 B ) f NO(X) + O( D)
2
2
2
(
2-2) NO A r X transitions) require four or more photons: at
The mass-resolved excitation spectrum (MRES) of frag-
mented NO ca. 226 nm yields a low rotational temperature (Trot
ca. 30 K) with no vibrational excitation (A(0) r X(0)). On the
basis of these results, most of the excess energy for the reaction
least two to excite the NO2 and two to excite the NO(X)
photoproduct. Saturation for the second photon absorbed for
NO2 or for NO or for both probably occurs. A two-photon
dissociation of NO2 puts about 11 eV into NO2. Two possible
excited states of NO2 in the 11 eV region can be suggested as
candidates for the NO2 dissociative states reached at high laser
intensity (i.e., by two-photon absorption in the 226-200 nm
region): a Rydberg state at 10.85 eV and an ion-pair state
1
NO2 + 5.480 eV (∼226 nm) f NO(X) + O( D) + 0.400 eV
should be released into the translational degrees of freedom of
NO and O products for 226 nm photodissociation. Thus, E(∼226
nm) ) 5.480 eV ) 5.08 eV + 0.005 eV (Trot ≈ 30 K) + 0.137
eV + 0.257 eV ) Ediss + Erot + Etrans(NO) + Etrans(O). This
+
-
+ -
1
(NO ‚‚‚O ). The energy of the ion-pair state (ENO O ) can be
estimated as follows:
translational energy distribution for both NO and O( D) has been
2
5
observed by velocity map ion-imaging studies recently re-
ported.1 If V′′ ) 1 were populated under these conditions of
2,13
1
ENO+O- ) D (ON-O) + IP(NO) - EA(O)
low laser intensity, the NO and O( D) rotational and translational
0

Photodissociation of NO2 in the Region 217-237 nm
J. Phys. Chem. A, Vol. 106, No. 33, 2002 7571
hν
in which D0 is the dissociation energy of NO2 to NO + O,
IP(NO) is the ionization energy of NO, and EA(O) is the
electron affinity of O. This equation gives 10.92 eV for the
threshold of the NO2 ion-pair state. Thus, NO2 with the
absorption of two photons at ca. 225 nm (∼11 eV) can be
excited to either a Rydberg state or an ion-pair state. For either
state, the dissociation dynamics can produce NO(X) predomi-
nantly, NO2 + 2hν f NO2* f NO(X) + O. NO(X) must be
generated because the (0-0), (1-1), and (2-2) transitions of the
NO A r X transition are detected. An energy balance must be
achieved through oxygen atom excitation for this 226-220 nm
excitation of NO2 and probe of NO.
+
-
+
-
NO + 2hν f (NO ‚‚‚O )*
9
8 NO + O
2
hν
+
-
NO + 3hν f NO * f NO* + O
98 NO + O + e
2
2
+
-
NO + 3hν f NO * f NO + O + e
2
2
+
The production of NO from NO2 requires at least 12.4 eV
D0(ON-O) + IP(NO)) of energy deposited in the NO/NO2
system. The two-photon energy available in the wavelength
region 230-237 nm is less than 10.9 eV and thus is not high
enough to produce NO through NO dissociation directly: to
produce NO from NO2 dissociation, the pathway should
(
+
2
+
B. Dissociation Mechanisms for NO2 between 237 and 227
nm. The fragmentation of NO2 by excitation in the region 237-
involve absorption of at least three photons. Thus, the two NO2
to NO mechanisms require three photons absorbed by NO2
with a resonance at the second or third absorption steps.
Generation of electronically excited neutral NO can occur
through a three-photon NO2 absorption with resonance and final
ionization of NO by an additional photon absorption.
-1
+
2
7
2
27 nm (42 195-44 050 cm , A r X (0-1), (1-2), (2-3), Figure
) is different than that described above for the region 226-
20 nm (A r X (0-0), (1-1), (2-2)) on the basis of both literature
9
reports and the present studies. First, most features in the
spectral range 227-237 nm generate a TOFMS line width for
+
the NO signal of about 40 ns. This value is twice that observed
In the region of the (0-1) NO transition (235-237 nm), both
NO A r X (0-1) features and NO2 multiphoton resonances are
observed (see Figure 7a). The TOFMS line width in this (0-1)
region is a composite of 20 and 40 ns widths (see Figure 8c).
These observations imply that NO is generated at these
wavelengths by two parallel mechanisms, one producing NO(X)
for the high-intensity laser studies below 226 nm and that
observed for the low-intensity A r X (0-0) NO transition.
-1
Second, in the range 231-233 nm, doublet peaks with 28 cm
spacings are observed (Figure 7b), and at 230 nm (Figure 7c),
these spacings become 35 cm-1 and the spectrum seems to
consist of a few different vibronic progressions. These two
observations lead us to conclude that these spectra do not arise
from NO excitation but rather that these features are NO2
transition peaks that are detected in the NO mass channel. NO
dissociates from NO2 after the resonance in the NO2 vibronic
energy levels accessed in a multiphoton process. Results from
+
and one producing electronically excited NO or directly NO ,
with resonant NO2 absorptions.
The main point of the above discussion of the photodisso-
ciation of NO2 between 237 and 217 nm is that the process
2
does not simply involve the B˜ (2 B2) state of NO2 accessed by
a single photon present in this wavelength region. The NO2
behavior is demonstrably quite complex, depending on both the
single-photon energy and the number of photons present in the
active volume for NO2 absorption. This combination yields both
NO and NO2 spectra in the region 237-217 nm that are laser-
intensity-dependent.
9
other studies and our NO gas-phase experiments support this
conclusion: no NO features are observed around the expected
NO (1-2) and (2-3) transitions. We have attempted to fit these
spacings to transitions involving two NO or NO2 excited
electronic states and to generate such spectral spacing using a
two-color excitation scheme for both NO and NO2. We are not
able to fit the observed pattern of transitions, employing known
states of either molecule. Note, however, that in the (0-1) region,
both NO and NO2 features appear. The TOFMS line width for
the 227-237 nm region gives a much larger kinetic energy for
the NO fragment than that found in the 227-220 nm region.
The TOFMS instrumental resolution is approximately 20 ns at
V. Conclusions and Summary
Photodissociation of NO2 is studied by detection of MRES
of NO and by observation of resonances in the NO2 multiphoton
+
excitation process also detected in the NO mass channel.
Possible dissociation pathways are suggested for this complex
photodissociation that depend on the experimental conditions,
in particular, the laser intensity and laser pump/probe wave-
length. The intensity of the laser beam (focused vs unfocused)
has a major impact on the spectra and TOFMS line width
detected in the spectral region 237-217 nm. Some general
+
mass channel 30 amu, and NO signals with larger line widths
than this should arise from photofragmented NO with additional
kinetic energy due to NO2 fragmentation. The kinetic energy
of NO generated by NO2 photodissociation in the 237-230 nm
region is ∼0.4 ( 0.04 eV; this gives a NO velocity of ∼1600
m/s in the direction of the electric vector (parallel to the flight
tube axis) of the laser beam (a 40 ns line width for the TOFMS
feature). Because no signal in this region is observed for an
unfocused beam, we conclude that the excited electronic state
from which NO2 dissociates must be accessed by two- or three-
photon excitation in this wavelength region (227-237 nm).
conclusions can be stated as follows:
2
(1) Single-photon dissociation of NO2 through the 2 B2 state
occurs (low laser intensity) to produce NO(X) at low rotational
2
+
2
and vibrational excitation [A( Σ , V′ ) 0) r X( Π, V′′ ) 0)]
with NO kinetic energy of ca. 0.137 eV.
(2) Multiphoton dissociation of NO2 through a Rydberg
excited state or an ion-pair state occurs to produce translation-
Three possible pathways (NO2 excited electronic states from
which dissociation occurs) can be suggested that would lead to
these NO2-related spectra detected in the NO mass channel: (1)
ally, rotationally, and vibrationally hot NO [Trot ca. 200 K,
2
+
2
A( Σ , V′′ ) 0, 1, 2) r X( Π, V′′ ) 0, 1, 2)]. For NO2
multiphoton dissociation, both neutral and ionic NO can be
produced.
+
NO can be produced through a NO2 ion-pair state, as discussed
above; (2) neutral NO fragments can be produced in a highly
excited electronic state through NO2 multiphoton absorption and
can be ionized subsequently by one-photon absorption, as
reported in a previous study;26 (3) NO can be produced through
the accessible NO2 excited states during NO2 dissociation.
Explicitly,
(a) In the range 220-227 nm, mostly neutral NO fragments
are detected through NO (A r X) transitions with an instrument
resolution limited 20 ns NO mass channel line width consistent
with a NO kinetic energy of ca. 0.137 ( 0.01 eV (935 m/s).
(b) In the range 227-235 nm, ionic and electronically excited
NO fragments are detected that are probably generated through
+
+

7572 J. Phys. Chem. A, Vol. 106, No. 33, 2002
Im and Bernstein
a NO2 Rydberg or ion-pair state. This mechanism generates a
TOFMS line width of 40 ns corresponding to a NO+ kinetic
energy of 0.4 ( 0.04 eV (1600 m/s). Spectra detected in the
(8) Bigio, L.; Tapper, R. S.; Grant, E. R. J. Phys. Chem. 1984, 88,
1271.
9) Marshall, A.; Clark, A.; Ledingham, K. W. D.; Sander, J.; Singhal,
R. P. Int. J. Mass Spectrom. Ion Processes 1993, 125, R21.
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(
+
NO mass channel in this wavelength range reflect resonances
(
in the NO2 multiphoton excitation.
M. J.; Zare, R. N. Chem. Phys. Lett. 1991, 186, 154. (b) Miyawaki, J.;
Tsuchizawa, T.; Yamanouchi, K.; Tsuchiya, S. Chem. Phys. Lett. 1990,
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two mechanisms for NO2 photodissociation can occur and a
complex spectrum in the NO [A(V′ ) 0) r X(V′′ ) 1)] region
is observed, composed of both NO (0-1) and NO2 transitions.
(11) Shafer, N.; Tonokyra, K.; Matsumi, Y.; Tasaki, S.; Kawasaki, M.
J. Chem. Phys. 1991, 95, 6218.
(12) Ahmed, M.; Peterka, D. S.; Bracker, A. S.; Vasyutinskii, O. S.;
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(
13) Ahmed, M.; Peterka, D. S.; Suits, A. G. In Atomic and Molecular
Acknowledgment. These studies are supported by the
USARO. We gratefully acknowledge very useful conversations
about NO/NO2 photodissociation reactions with Professors C.
Wittig and E. R. Grant and A. G. Suits, and Drs. M. Ahmed,
C. C. Hayden, and D. W. Chandler. Dr. Chandler also provided
us with very helpful ion-imaging data that are consistent with
our overall mechanism for NO2 dissociation.
Beams. The State of the Art 2000; Compargue, R., Ed.; Springer: New
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NM, 1992.
(
2
(
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(