Photodissociation dynamics of HN3 and DN3 at 157 nm

Article abstract of DOI:10.1021/jp800062t

Photodissociation dynamics of HN₃ at 157.6 nm have been studied using the H-atom Rydberg tagging timeof-flight technique. Product translational energy distributions and angular distributions have been measured. From these distributions, three H-atom channels are observed. The vibrational structure in the fast-H channel could be assigned to a progression in the N₃ symmetric stretching mode (v₁00), together with a progression of the symmetric stretching mode with one quantum of bending motion (V₁10). The broad translational energy distribution of the slow-H channel is energetically consistent with the cyclic-N₃ formation process or a triple product dissociation channel. Photodissociation of DN₃ was also investigated using the same technique. Isotope effect on the product translational energy distribution has been observed, in which the slow H-atom is clearly more pronounced.

Full text of DOI:10.1021/jp800062t

5
332
J. Phys. Chem. A 2008, 112, 5332–5337
Photodissociation Dynamics of HN3 and DN3 at 157 nm
Kaijun Yuan, Yuan Cheng, Fengyan Wang, and Xueming Yang*
State key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy
of Sciences, 457 Zhongshan Road, Dalian 116023, China
ReceiVed: January 4, 2008; ReVised Manuscript ReceiVed: April 3, 2008
3
Photodissociation dynamics of HN at 157.6 nm have been studied using the H-atom Rydberg tagging time-
of-flight technique. Product translational energy distributions and angular distributions have been measured.
From these distributions, three H-atom channels are observed. The vibrational structure in the fast-H channel
could be assigned to a progression in the N
of the symmetric stretching mode with one quantum of bending motion (ν
distribution of the slow-H channel is energetically consistent with the cyclic-N
product dissociation channel. Photodissociation of DN was also investigated using the same technique. Isotope
3
symmetric stretching mode (ν
10). The broad translational energy
formation process or a triple
1
00), together with a progression
1
3
3
effect on the product translational energy distribution has been observed, in which the slow H-atom is clearly
more pronounced.
Introduction
Photodissociation of hydrazoic acid (HN3) has attracted much
1
attention over the last two decades. The hydrazoic acid
molecule has been sought as the material capable of storing
large amounts of energy in minimal mass or volume, a possible
“
high-energy-density-material”, due to the fact that it is so
2
unstable. Because of its accessibility by various experimental
techniques and high-level quantum chemical methods, this
system has also been referred as a benchmark system for the
study of polyatomic photochemical dynamics.
The UV absorption spectrum of HN3 (Figure 1) is composed
of at least four absorption systems between 150 and 300 nm: a
weak one peaking at about 264.3 nm, two strong ones peaking
around 204 and 190 nm, and an even stronger system around
1
57 nm, respectively.3,4 These absorption features appear broad
and nearly structureless. The weak feature at λ >220 nm has
˜
1
been assigned in terms of excitation from the X A′ ground state
˜
1
to the A A′′ first excited singlet electronic state. At shorter
˜
1
˜ 1
wavelengths, the B A′ and C A′′ electronic states are optically
accessible, and the resulting UV absorption spectrum is strong,
but no discrete structure is present. The Rydberg state transition
3
Figure 1. Absorption spectrum of HN at room temperature (adapted
˜
1
˜ 1
from refs 3 and 4). The position of the photolysis excitation wavelength
D A′ r X A′′ is prominent at λ < 170 nm.
It was known that the spin and thermodynamically allowed
dissociation channels at λ > 200 nm are
-
18
2
in this work is indicated by arrows (1 Mb ) 1 × 10 cm ).
mainly in its vibrational ground state with high rotational
excitation, whereas the rotational excitation of the NH (a ∆)
fragment was quite small.
2
1
˜
˜
HN (X1A′) f H(2S) + N (X Π )
(1)
(2)
3
3
g
1
1
+
˜
fNH(a ∆) + N (X Σ )
2
g
Photodissociation dynamics of channel 1 have been investi-
gated by Comes and co-workers by using H-atom laser-induced
The detailed dynamics of channel 2 have been extensively
studied at several different wavelengths. Measurements of
the NH (a ∆) fragment vibrational state population distribution
resulting from HN3 photolysis in the range 220-290 nm reveal
energetic onsets for products with V ) 1 and V ) 2. The
observed population ratio NV)1/NV)0 is at maximum at λ ) 240
nm. Chu et al. investigated the internal energy distributions
in both the N2(X Σg ) and NH (a ∆) fragments from the 283
1
0,11
5
–9
fluorescence (LIF) at 266, 248, 222, 193 and 122 nm.
1
Comparisons of the H atom LIF intensities with those resulting
from H2S photolysis led to estimates of the respective quantum
yields for forming H atom products: Φ266 ) 0.04, Φ248 ) 0.20,
1
2
Φ193 ) 0.14. Using H-ion TOF spectroscopy, the H-N bond
-
1
9
5
energy was determined to be D0(H-N3) ) 30850 ( 400 cm .
Because of the limited energy resolution of this technique, the
resulting H-ion TOF profile was broad and poorly resolved. The
result suggested that the N3 internal energy distribution is very
˜
1
+
1
nm photolysis. They found that the N2 fragment was produced
-
1
broad with its width of ∼6000 cm . The dominant features in
*
To whom all correspondence should be addressed. E-mail: xmyang@
dicp.ac.cn.
this profile were assigned to a progression in the ν1 symmetric
1
0.1021/jp800062t CCC: $40.75  2008 American Chemical Society
Published on Web 05/24/2008

Photodissociation Dynamics of HN3 and DN3
J. Phys. Chem. A, Vol. 112, No. 24, 2008 5333
stretching mode of N3 and the relative population of the (000):
(
100):(200):(300):(400) product states was found to be 0.09:
0
.19:0.28:0.29:0.15.
Recently, the high-n Rydberg H-atom time-of-flight (HRTOF)
technique has also been employed to investigate the H-atom
1
3–17
13,14
channel.
Cook et al.
reported the translational energy
˜
spectra of the H + N3(X) products for photolysis wavelengths
between 240 and 280 nm. On the basis of their experimental
results and ab initio potential energy surface calculations, they
concluded that the N3 fragments were formed in the progression
involving both the symmetric stretch mode (ν100) only and the
symmetric stretch mode combined with one and two quanta of
bending motion, ν2. They used an impulsive model to reproduce
˜
the deduced N3(X) product rotational energy disposal, obtained
an impact parameter, b ) 1.26 ( 0.05 Å, and a refined measure
-
1
of the H-N3 bond strength, D0(H-N3) ) 30970 ( 50 cm .
Figure 2. Time-of-flight spectra of the H atom product from the
photodissociation of HN at 157.6 nm with the rotating detector
3
direction perpendicular (solid line) and parallel (dashed line) to the
photolysis laser polarization.
1
5
Zhang et al. have also used HRTOF to investigate the HN3
photodissociation at 248 and 193 nm. They found that photolysis
at 193 nm clearly leads to more internally excited N3 radicals
than does photolysis at all other wavelengths that have been
studied. The observed product anisotropy parameter is clearly
energy-dependent, implying multiple dissociation pathways are
then focused into a cell with Kr/Ar mixing gas where four-
wave mixing at 121.6 nm is generated. The remainder of the
1
6,17
involved. More recently, Zhang et al.
reinvestigated the
5
32 nm source is used to pump a third dye laser (Radiant Dye
photodissociation of HN3/DN3 using the HRTOF method,
spanning the range of photolysis wavelengths between 188 and
Laser-Jaguar, D90MA), operating at ∼732 nm, the output from
which the frequency is doubled to ∼366 nm, and used to excite
the H/D atoms from the n ) 2 level to a Rydberg state with n
2
80 nm in roughly 5 nm steps. They have observed subtle yet
striking changes in the photodissociation dynamics as the
photolysis energy passes through ∼5.6 eV. Their results suggest
that a cyclic-N3 formation channel is present, in addition to the
linear-N3 fragment channel.
)
30-80, lying just below the ionization threshold. Any charged
species formed at the tagging region by initial laser excitation
are extracted away from the TOF axis by a small electric field
(
∼30 V/cm) placed across the interaction region.
The photolysis of HN3 in the UV region has been studied in
many experiments as shown above; however, little work has
been done on the dynamics of the HN3 photodissociation after
excitation in the vacuum ultraviolet (VUV) region. In this work,
we investigated the photodissociation dynamics of HN3/DN3
molecules using HRTOF technique at 157.6 nm. Product
translational energy distribution and angular distribution have
been measured by using polarized photolysis laser radiation in
this study. Photodissociation mechanisms and the excited
electronic states involved are discussed.
The neutral Rydberg H/D atoms then fly a certain distance
∼333 mm) to reach a MCP detector with a fine metal grid
grounded) in the front. After passing through the grid, the
(
(
Rydberg tagged atoms are then immediately field-ionized by
the electric field applied between the front plate of the Z-stack
MCP detector and the fine metal grid. The signal detected by
the MCP is then amplified by a fast preamplifier, and counted
by a multichannel scaler.
HN3 was prepared by heating sodium azide (NaN3) in excess
stearic acid under vacuum for 6-7 h at ∼90-100 °C. DN3 was
produced by reacting NaN3 with an excess of deuterated
phosphoric acid, which is generated by reacting D O with P O
2
. Experimental Section
2
2
5
under vacuum conditions. The HN3/DN3 sample was stored in
a stainless steel container and He gas was immediately filled to
produce a mixture of 2.5% HN3 (DN3) in He. Purity was
checked by mass spectrometry (SRS, RGA200).
The high-n Rydberg H-atom time-of-flight (HRTOF) tech-
nique utilized in this study has been described in detail elsewhere
and only a brief description is presented here.18,19 A skimmed
pulsed molecular beam of HN3, seeded in helium (mixing ratio
∼
2.5%, total pressure ∼760 Torr), is crossed perpendicularly
with the output of the 157.6 nm photolysis laser (VUV power
-4 mJ, with the beam diameter ∼10 mm), which was produced
3
. Results and Discussions
2
3.1. Photodissociation of HN3. The time-of-flight spectra
by a Lambda Physik LPX210I F2 laser. The 157.6 nm radiation
was polarized by a special 157.6 nm polarizer and focused by
an MgF2 lens. The H/D products from photodissociation were
then excited to a high-n Rydberg level using a two-step
excitation scheme: coherent VUV excitation at the Lyman-R
wavelength (121.6 nm) followed by UV photon excitation at
about 366 nm. VUV coherent radiation at the Lyman-a
wavelength is generated by four-wave mixing of two 212.5 nm
photons and one 845 nm photon in a cell filled with a 3:1 ratio
Ar-Kr mixture. Photons at 212.5 nm are produced by doubling
the output of a tunable dye laser (Sirah, PESC-G-24) operating
at ∼425 nm, pumped by the third harmonic output of a Nd:
YAG laser (Spectra Physics Pro-290). A portion of the 532 nm
output of the YAG is used to pump another dye laser
(TOF) of the H-atom product from the HN3 photodissociation
at 157.6 nm were measured using the HRTOF technique
described above. Two TOF spectra were obtained with the
rotating detector direction parallel and perpendicular to the
photolysis laser polarization. The TOF spectrum at the magic
angle was also measured to check the accuracy of the polariza-
tion direction.
3.1.1. Product Translational Energy Distribution. Figure 2
shows the TOF spectra at the parallel and perpendicular
directions. The perpendicular direction is the direction of the
molecular beam, whereas the parallel direction is perpendicular
to the molecular beam. The velocity of the parent molecular
beam leads to the shift of the TOF spectra in Figure 2. The two
TOF spectra have been converted to the total product transla-
tional energy distributions in the center-of-mass frame, as shown
(
Continuum ND6000) operating at ∼845 nm. These beams are

5
334 J. Phys. Chem. A, Vol. 112, No. 24, 2008
Yuan et al.
Figure 3. Product translational energy distribution derived from the
experimentally measured H atom TOF spectra recorded at perpendicular
and parallel direction.
Figure 4. Anisotropy parameter ꢀ as a function of the total translational
energy of HN photodissociation at 157.6 nm.
3
vector of the H-atom product (the direction of detection), and
P2(cos θ) is the second Legendre polynomial. Figure 4 shows
the translational energy dependent product anisotropy parameter
obtained at 157.6 nm photolysis. Clearly, the anisotropy ꢀ
parameter changes considerably from low to high translational
in Figure 3. Obviously, the translational energy spectra are
composed of three features: a slow and broad H-atom feature
-1
peaking at ∼2400 cm , a fast and broad H-atom feature peaking
-
1
at ∼15,000 cm and a very fast H-atom feature with some
vibrational structure near the energetic limit.
-1
energy. In the translational energy range of 1000-25000 cm ,
Because the momentum and energy are conserved in the
photodissociation process, the product internal energy (Eint)
deposited into the N3 product can be determined:
the anisotropy parameter goes down from about -0.1 to -0.4
-1
slowly and in the range 25000-32000 cm , the product
anisotropy parameter increases to about 0.2. The negative ꢀ
-1
value in the region 1000-25000 cm indicates a perpendicular
E (N ) ) hν + E (HN ) - D (H-N ) - E
trans
(3)
int
3
int
3
0
3
electronic transition. The dissociation dynamics in the range
-
1
where hν is the photon energy of the photolysis laser and
Eint(HN3) is the internal energy of the HN3 molecule, which
was set to zero because the HN3 molecule is cooled down to
very low temperature in the supersonic expansion. Using this
equation, the product translation energy distribution can be
converted to the N3 product internal energy distribution.
25000-32000 cm are clearly different from that in the lower
energy region. In addition, there are some oscillations at high
translational energy near the energetic limit. The oscillation
structure in ꢀ seems to be coincident with the peaks in the
translational energy distribution. Comparing with other wave-
lengths, where the anisotropy parameter (ꢀ) was found to be
negative (-0.56 at 266 nm, -0.72 at 248 nm and ∼0 at 193
nm, respectively), the anisotropy parameter at 157 nm is more
complicated. The structure in the energy distribution as well as
the varying ꢀ parameter suggests that the 157.6 nm photodis-
sociation proceeds via multiple dissociation pathways.
max
From eq 3 if the maximum translational energy limit (Etrans)
in the product translational energy distribution corresponds to
the N3 ground ro-vibrational state, i.e., Eint(N3) ) 0, D0(H-N3)
max
should be equal to hν - Etrans. The extrapolated onset of the
-
1
maximum in Figure 3 yields D0(H-N3) ) 30920 ( 100 cm .
The error limit is due to the uncertainty in determining the
threshold, the photolysis laser line width and the resolution of
the TOF spectrometer. Our present result is very close to the
3.1.3. Dissociation Channels. From the translational energy
distribution obtained in this experiment, it appears that there
are three possible channels: the slower H atom channel peaking
14
-1
-1
recent experimental value obtained by Cook et al., D0(H-N3)
at 2400 cm , the fast broad channel peaking at 15000 cm ,
and the fast structured channel near the energetic limit. In the
-
1
16,17
)
3
30970 ( 50 cm , and by Zhang et al.,
D0(H-N3) )
-
1
-1
0910 ( 100 cm .
translational energy range of 25000-32000 cm , the several
From the translational energy distributions shown in Figure
peaks are observed and can be assigned to the vibrational levels
2
3
, the average translational energy, 〈Etrans〉 is determined to be
of the ground electronic state N ( X˜ Π ) product. The peaks in
3
g
-
1
about 35%. By comparison of this value with the recent
experimental results at other photolysis wavelengths (73% at
the E (N ) distribution in the range of 25000-32000 cm ,
int
3
-
1
shown in Figure 3 are separated by ∼1300 cm and have
-1
2
2
48 nm, 64.6% at 230 nm, and 49% at 193.3 nm),15,17 it is
widths of ∼700 cm . The two spin-orbit states, X˜ Π and
/2
2 -1 21
1/2 3
3
obvious that more available energy is deposited into the N3
product internal energy as photolysis energy increases.
X˜ Π , of the N product are only separated by 71.3 cm ,
which are not resolvable in this experiment.
2
3.1.2. Product Anisotropy Distribution. The product angular
The electronic transition from X˜ Π to the low-lying excited
2
21,22
distribution has also been determined in this experiment using
linearly polarized 157.6 nm photolysis laser radiation. Linearly
polarized light preferentially excited those HN3 molecules with
their electronic transition dipole parallel to the electric vector
E of the polarized laser radiation. The photofragment angular
Σ state of the N product was studied using LIF,
and the
3
recommended vibrational frequencies of symmetric stretching
ν , bending ν , and asymmetric stretching ν are 1320, 457,
1
2
-
3
1
and 1645 cm , respectively. According to these vibrational
frequencies, the observed vibrational structure in the translational
energy distribution could be assigned to a progression involving
the N3 symmetric stretching mode ν1. In one of the simulations,
we have included only the progression of the (ν100) vibrational
series, and the simulation is not satisfactory. However, by
including the series of (ν110), the fitting to the translational
2
0
distribution is given by
I(θ) ) (1/4π)[1 + ꢀP (cos θ)]
(4)
2
where ꢀ is the anisotropy parameter (-1 e ꢀ e 2), θ is the
angle between the electric vector E and the recoiling velocity

Photodissociation Dynamics of HN3 and DN3
J. Phys. Chem. A, Vol. 112, No. 24, 2008 5335
Figure 6. Broad and structureless translational energy distribution for
photolysis of HN
translational energy channel (dashed line) is consistent in energy release
with formation of cyclic-N , the downward pointing arrows show the
predicted maximum translational energy for the H + cyclic-N , H +
+ N( P), respectively; the high translational
energy channel (dotted line) produces linear-N products.
3
at 157.6 nm. Two channels are observed: the low
Figure 5. Experimentally derived internal energy profiles together with
3
1 1
simulations including progressions in the (ν 00) and (ν 10) modes. The
3
detector direction is (a) perpendicular and (b) parallel to the photolysis
laser polarization.
2
2
N
2
+ N( D), and H + N
2
3
The slow product channel peaks at about 2400 cm- in the
translational energy distribution and its anisotropy parameter
changes from -0.1 to -0.4 as the translational energy increases.
Considering the theoretical predicted energetic values of cyclic-
N3, which lies about 30 kcal/mol above the ground state of
1
energy distribution in that region is much improved, as shown
in Figure 5. This suggests that the N3 products with bending
excitation are also likely present. It is interesting to point out
that such low vibrational states of the linear N3 product are
observed so clearly. This structure is quite similar to the
dissociation structure observed at about 240 nm excitation,
which is clearly due to the A A′′ state dissociation. Therefore,
it is likely that this structured feature observed in this work is
due to an indirect dissociation process: a fast conversion to the
2
3–25
linear-N3 with a 32 kcal/mol barrier to the linear structure,
˜
1
this slow channel observed here could be the formation of cyclic-
N3 in the HN3 photodissociation. It is necessary to point out
that this slow feature is very similar to that in the 193 nm
photodissociation, which is also assigned to the cyclic-N3
formation channel with a branching ratio of less than 10%.
Cyclic-N3 formation has also been reported in the 157 nm
photolysis of ClN3. Photoproduct translational energy measure-
ments by Samartzis and Wodtke revealed a single-peaked
distribution for an N3-formation channel with maximum and
minimum translational energies matching the theoretically
predicted minimum and maximum binding energies of cyclic-
N3, respectively. Obviously, the slow product channel peaks in
the 157 nm photodissociation of HN3 is similar to that for ClN3.
From the simulation shown in Figure 6 assuming the energetic
limit for this channel is the same for the cyclic-N3 formation,
the percentage of the slow-H channel is determined to be ∼50%.
This measured ratio is considerably larger than the branching
ratio of the slow channel of HN3 photolysis from 193 to 220
nm.
In addition to this cyclic-N3 formation channel, there are also
possible triple product channels that might contribute to the slow
H atom channel. The energetically allowed triple product
channels that generate H atoms in the photolysis of HN3 at 157.6
nm are
1
7
˜
1
˜ 1
A A′′ state from the D A′ state and then a direct dissociation
˜
1
to the H + N3(X) channel from the A A′ spmstate. Another
interesting observation of this process is the large change of
the anisotropy parameter from -0.4 to +0.2. This is very
2
6
˜
different from the A state dissociation at about 240 nm, in which
the anisotropy parameter remains almost constant. It indicates
˜
1
that the excited molecules in the D A′ state live at least for a
short period of time (likely a few vibration periods). The
dynamical origin of such behavior is, however, not immediately
clear. This large change of the anisotropy parameter could be
induced by the suggested fast conversion process between the
˜
1
˜ 1
D A′ state and the A A” state if conical intersections are
involved.
In addition to the structured feature, the fast broad feature
peaking at 15000 cm- is important. It also appears that its
translational energy distribution goes up to the energy limit of
the ground electronic state of the N3 product, and therefore the
dissociation product of this feature should be the ground state
N3. It appears that the dissociation process has an anisotropy
parameter of -0.4, which is nearly independent of the trans-
lational energy, indicating that this is likely a rather direct
1
˜
1
dissociation process from the D A′ state to form the H + l-N3(X)
channel. And the time scale for the dissociation process likely
not much longer than one rotational period of the parent
molecule. This feature is very similar to that observed at 193.3
˜
1
2
4
-1
HN (X A′) f H( S) + N + N( S)
∆E ) 32960 cm
3
2
(5)
2
2
-1
1
5,17
16
H ( S ) + N + N( D )
∆E ) 13780 cm
(6)
(7)
nm
and also at 199 nm, in which the dissociation processes
2
˜
1
1
are almost from the B A′ and C A′′ state to form the H +
l-N3(X) channel. It is not clear, however, whether this channel
occurs directly on the D A′ state or converts to another lower
2
2
-1
H( S) + N + N( P)
∆E ) 4180 cm
2
˜
1
Among these channels, channel 5 is spin forbidden. The
energetic limit of the product translational energy for channel
electronic state then dissociate.

5
336 J. Phys. Chem. A, Vol. 112, No. 24, 2008
Yuan et al.
Figure 8. Anisotropy parameter ꢀ as a function of the total translational
energy of DN photodissociation at 157.6 nm.
3
Figure 7. Product translational energy distribution derived from the
experimentally measured D atom TOF spectra recorded at perpendicular
and parallel direction.
to close the ring for the cyclic-N3 formation channel in the case
of DN . This isotope effect is similar to that observed previously
by Zhang et al. in the HN3 photolysis around 200 nm.
-
1
-1
6
is 13780 cm , whereas that for channel 7 is 4180 cm (as
3
1
6
down pointing arrows in Figure 6). Clearly, the energetic limit
for channel 7 is much too low for the slow H-atom channel
which peaks at about 2400 cm- and seems to extend toward
much higher energy. Therefore, contribution to the slower
H-atom channel from the channel 7 should be not significant.
1
4. Conclusions
The 157.6 nm photodissociation of HN3/DN3 has been
investigated using HRTOF technique. Time-of-flight spectra of
the H/D atom product were measured at the detection direction
both perpendicular and parallel to the photolysis laser polariza-
tion. Total product translational energy distributions and the
product angular anisotropy parameters were also determined.
Three H atom channels have been observed from the transla-
tional energy distributions. The vibrational structure in the fast
H-atom channel have been assigned to the progressions of the
2
Contribution to the slow H-atom channel from the H( S) + N2
2
+
N( D) channel, however, cannot be ruled out because the
energetic limit of the slow H-atom channel cannot be determined
exactly and it seems that the energetic limit of this channel is
more or less within the range of that limit. The slow H-atom
channel seems to have a small changing anisotropy parameter
from -0.1 to -0.4 in the lower energy region. This might be
caused by the two possible channels in which the two channels
have different anisotropy parameters and their translational
energy distributions are different.
(
ν100) and (ν110) vibrational series. This channel likely occurs
˜ 1
˜ 1
via a fast conversion from the D A′ state to the A A′′ state,
and then dissociates on the A A′′ surface. The translational
˜ 1
3
.2. Photodissociation of DN3. To understand the isotope
energy distribution of the slow H-atom channel is energetically
consistent with the cyclic-N3 formation channel and the three-
effect on the photodissociation of HN3, photodissociation of DN3
at 157.6 nm has also been investigated. Time-of-flight spectra
of the D-atom product from DN3 photolysis were measured at
two laboratory angles in a similar way to HN3 photolysis except
using the D-atom Rydberg tagging TOF technique. The
experimental TOF spectra are then converted to the product
translational energy distribution. Figure 7 shows the total product
translational energy distributions of the DN3 photodissociation
at 157.6 nm. Two broad peaks are observed, similarly to that
in HN3. The vibrational structure near the energetic limit is,
however, not well resolved. It appears that isotope substitution
has a large effect on this channel. From the anisotropy parameter
plot shown in Figure 8, the high energy section from 25000 to
2
2
body dissociation channel: H( S) + N2 + N( D). The fast broad
-1
channel peaked at 15000 cm is clearly a linear-N3 formation,
similar to that at 193.3 nm. Photodissociation dynamics of DN3
have also been investigated. Noticeable isotope effects on the
dissociation dynamics have been detected, in which the slow
H-atom products are more pronounced.
Acknowledgment. This work was supported by Chinese
Academy of Sciences via the grant of the CAS international
collaboration group, and also by the Ministry of Science and
Technology of China and the National Natural Science Founda-
tion of China.
-1
3
2000 cm still shows a large change from -0.2 to +0.3. This
indicates that this fast channel might still exist but with much
smaller contribution in the case of DN3. We have suggested
that this fast feature with vibrational structure might occur via
References and Notes
(
1) Okabe, H. Photochemistry of Small Molecules; Academic Press:
New York, 1978.
˜
1
˜ 1
(2) Friedmann, A.; Soliva, A. M.; Nizkorodov, S. A.; Bieske, E. J.;
Maier, J. P J. Phys. Chem. 1994, 98, 8896.
coupling between the D A′ state and the A A′′ state. If this is
the case, the D atom substitution could affect this coupling
considerably. The two broad features in the DN3 photodisso-
ciation at 157.6 nm is, however, very much similar to that of
HN3 in terms of the translational energy distributions and the
anisotropy parameter distributions. However, the branching ratio
of the slower channel in the DN3 photolysis, which is determined
to be 64%, is significantly larger than that of HN3 (50%).
Because velocities of the D-atom passing through the transition
state will be approximately half-that of H-atom, it is possible
that more time is available for the N-atom motion that is required
(
(
3) Okabe, H. J. Chem. Phys. 1968, 49, 2726–2733.
4) Orlando, J. J.; Tyndall, G. S.; Betterton, G. A.; Lowry, J.; Stegall,
S. T. EnViron. Sci. Technol. 2005, 39, 1632.
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