Ultraviolet photodissociation dynamics of HN3: The H+N3 channel

Article abstract of DOI:10.1016/S0009-2614(98)01279-2

The H+N₃(X ²Π_g) channel in the ultraviolet photodissociation of HN₃ has been studied at 248.3 and 193.3 nm using the high-n Rydberg H-atom time-of-flight technique. In 248.3 nm photodissociation, center-of-mass translational energy distribution reveals a propensity toward product translation (〈f_trans〉=0.71) and N₃ bending and symmetric stretching excitation. An anisotropic product angular distribution (β≈-0.8) indicates a perpendicular electronic transition and rapid dissociation on the excited-state surface. H-N bond energy is estimated: D₀(H-N₃)≤88.7±0.5 kcal/mol. At 193.3 nm, 〈f_trans〉 is smaller and product angular distribution is energy-dependent, suggesting multiple dissociation pathways via different electronic states.

Full text of DOI:10.1016/S0009-2614(98)01279-2

1
1 January 1999
Chemical Physics Letters 299 Ž1999. 285–290
Ultraviolet photodissociation dynamics of HN : the HqN
3
3
channel
)
Jingsong Zhang , Kesheng Xu, Gabriel Amaral
Department of Chemistry, UniÕersity of California, RiÕerside, CA 92521, USA
Received 12 October 1998; in final form 5 November 1998
Abstract
2
The HqN Ž X˜ P . channel in the ultraviolet photodissociation of HN has been studied at 248.3 and 193.3 nm using
3
g
3
the high-n Rydberg H-atom time-of-flight technique. In 248.3 nm photodissociation, center-of-mass translational energy
distribution reveals a propensity toward product translation Ž² ftrans:s0.71. and N bending and symmetric stretching
3
excitation. An anisotropic product angular distribution Ž bfy0.8. indicates a perpendicular electronic transition and rapid
dissociation on the excited-state surface. H–N bond energy is estimated: D ŽH–N .(88.7"0.5 kcalrmol. At 193.3 nm,
0
3
²
f
: is smaller and product angular distribution is energy-dependent, suggesting multiple dissociation pathways via
trans
different electronic states. q 1999 Elsevier Science B.V. All rights reserved.
1
. Introduction
It is known that the UV photodissociation of HN3
occurs primarily via two spin-allowed channels: w1–
4x
1
The ultraviolet ŽUV. absorption spectrum of hy-
drazoic acid ŽHN . is composed of at least three
1
X
1
1
q
˜
3
HN3
Ž
X A
.
qhn™NH
Ž
a D
.
qN X S
Ž
Ž
1
2
.
.
2
ž
g
/
absorption systems between 180 and 300 nm: a weak
one peaking at 270 nm and two strong ones peaking
at 204 and 190 nm, respectively w1–4x. These absorp-
tion features are broad and nearly structureless w1–4x.
2
2
˜
™
H
Ž
S
.
qN X P
.
3
ž
g
/
The energy level diagram relevant to this system is
shown in Fig. 1.
The weak feature at l)220 nm has been assigned
1
Y
1 X
˜
˜
Ž
Channel Ž1. has been extensively studied at sev-
to A A §X A i.e., to the lowest excited singlet
Y
1
X
1
Y
2
eral ultraviolet wavelengths w4,9–14x. NH is pro-
via Ž2a . Ž10a . §Ž2a . . w2,3,5x.
1
duced primarily in the lowest singlet Ža D. instead of
3
y
the X S ground state, as the lowest spin-allowed
1
1
q
channel is NHŽa D.qN ŽX S .. Vibrational and
2
g
rotational state distributions of NH and N were
2
)
measured w4,9–14x. These distributions can be better
Corresponding author. Also at Air Pollution Research Center,
described by an impulsive rather than by a statistical
University of California, Riverside. Fax: q1 909 787 4713;
e-mail: jszhang@ucrac1.ucr.edu
model, indicating that Channel Ž1. proceeds via di-
0
009-2614r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved.
PII: S0009-2614Ž98.01279-2

2
86
J. Zhang et al.rChemical Physics Letters 299 (1999) 285–290
distributions. The product angular distribution has
been measured by using polarized photolysis laser
radiation. Photodissociation mechanisms and the ex-
cited electronic states involved are discussed.
2
. Experimental
The high-n Rydberg-atom time-of-flight ŽHRTOF.
technique utilized in this study has been described
previously w15–17x. A pulsed molecular beam was
produced by expanding 5% HN rHe gas mixture at
3
;
100 kPa. In a differentially pumped main cham-
ber, the molecular beam was crossed with the UV
photolysis radiation from an ArFrKrF excimer laser
ŽLambda Physik EMG 101.. Typically, 0.5–20 mJ of
Fig. 1. Energy level and state correlation diagram of the HN3
system. Energies are from Refs. w2,5,8,10x, and this study.
1
93.3 or 248.3 nm radiation, focused with a quartz
lens Ž fl ;65 cm., was used. This radiation could be
polarized with a 10-plate stack of quartz slides placed
at the Brewster angle, resulting in )95% polariza-
tion. H-atom products from photodissociation were
probed by sequential excitation to a high-n Rydberg
level via Lyman-a radiation at 121.6 nm and UV
radiation at ;366 nm. These high-n Rydberg H-
atoms are radiatively metastable. Upon arrival at an
ion detector after drifting with their nascent veloci-
ties, the Rydberg atoms are efficiently field-ionized
and detected. The nominal flight length was 37.1 cm.
TOF spectra were recorded and averaged with usu-
rect dissociation on the repulsive part of the excited-
state potential energy surfaces ŽPES..
Photodissociation dynamics of Channel Ž2. has
been investigated by Comes and coworkers by using
H-atom laser-induced fluorescence ŽLIF. w6,8x and
H-ion TOF spectra w7x. The H-atom quantum yield
has been measured at several wavelengths: f Ž266
H
nm.s0.04, f Ž248.3 nm.s0.20, and f Ž193.3
H
H
nm.s0.14 w6–8x. Product translational energy distri-
butions have been determined from Doppler broad-
ened H-atom LIF spectra w6,8x and H-ion TOF spec-
tra w7x. The average center-of-mass ŽCM. transla-
3
ally Ž5–100.=10 laser shots.
HN3 was prepared by heating sodium azide
tional energy release has been found to be 68% and
ŽNaN . in excess stearic acid under vacuum for
3
5
5% of the total available energy at 248.3 and 193.3
;
1–2 h at ;70–908C w18x. The HN sample was
3
nm, respectively. Due to the limited resolution in the
stored in a stainless steel container and He gas was
H-atom LIF and H-ion TOF spectra w6–8x, little
filled immediately to produce 5% HN rHe mixture
3
information is available for N vibrational state dis-
3
that was used for the molecular beam. Purity was
checked by mass spectrometry.
tributions. N3 symmetric stretching excitation was
believed to be observed w7x, although a large amount
of bending excitation was anticipated w6x. At 266 and
2
48.3 nm, product angular distributions have a
3
. Results and discussion
strongly negative anisotropy parameter, indicating a
1
Y
1 X
perpendicular electronic transition Ž A § A.. At
93.3 nm, the anisotropy parameter is near zero, and
3
.1. Photodissociation at 248.3 nm: dissociation dy-
1
namics in the first absorption band
contributions from multiple excited electronic states
of HN have been suggested w8x.
The CM translational energy distribution for the
3
In this work, the HqN channel has been studied
H and N photofragments at 248.3 nm is shown in
3
3
with higher resolution and in greater detail. The
low-frequency bending mode of the N fragment has
been resolved in the CM product translational energy
Fig. 2. This distribution was obtained from direct
conversion of the accumulated H-atom TOF spec-
trum w15–17,19x. Product CM translational energy
3

J. Zhang et al.rChemical Physics Letters 299 (1999) 285–290
287
molecule and clusters., the photolysis laser linewidth,
and the resolution of the TOF spectrometer. Our
present result is very close to the recent experimental
value of 88.3 " 1.1 kcalrmol by Comes and
coworkers w7,8x. By combining our D ŽH–N . value
0
3
0
f,0
and a recent experimental value of D H ŽN . Ž113.7
3
"
2.3 kcalrmol. by Continetti et al. w20x, the heat of
0
f,0
formation of HN can be calculated: D H ŽHN .s
3
3
7
6.6"2.3 kcalrmol. However, this is not in good
agreement with a recent ab initio value of 71.5
kcalrmol by Rogers and McLafferty w21x.
The H-atom product angular distribution has also
been studied by using linearly polarized UV photoly-
sis radiation. Linearly polarized light preferentially
excites those HN3 molecules with their electronic
transition dipole parallel to the electric vector E of
the polarized laser radiation. The photofragment an-
gular distribution is given by w22x:
Fig. 2. CM translational energy distribution of the H and N3
photofragments at 248.3 nm. The distribution is converted from
the H-atom TOF spectrum accumulated by using 20 mJ unpolar-
ized 284.3 nm photolysis laser radiation.
release is quite large, and its distribution peaks to-
wards the maximum available energy. The fraction
of the average translational energy of the total avail-
able energy ² ftrans: is 0.71, which is in good agree-
I
Ž
u
.
s
Ž
1r4p
.
1qbP2
Ž
cos u
.
4
Ž .
where b is the anisotropy parameter y1(b(2 ,
Ž
.
u is the angle between the electric vector E and the
ment with the result by Comes and co-workers
Ž² ftrans:s0.68. w6–8x. This result is similar to that
of 193.3 nm photodissociation of HNCO Žan isoelec-
Ž
recoiling velocity vector of the H-atom product the
direction of detection., and P Žcos u. is the second
2
Lengedre polynomial. A significant anisotropy in the
angular distribution has been observed. After the
correction for imperfect polarization of the polarizer,
b was found to be y0.8"0.1, in agreement with
tronic molecule to HN . on its first excited singlet
3
state w17x. Such a large propensity toward CM trans-
lation, i.e., large CM translational energy release,
suggests direct dissociation on a repulsive part of the
PES.
the y0.72"0.16 value by Comes et al. 8 . The
strongly negative b value indicates that the transi-
tion dipole moment is perpendicular to the recoiling
velocity vector of the H-atom fragment from the
N–H bond dissociation, and that the timescale for
the dissociation process is shorter than one rotational
period of the parent molecule. It has been shown that
w x
The upper limit of the bond energy D ŽH–N .
0
3
can be obtained by utilizing conservation of energy
w17,19x:
hnqEint
Ž
HN3
.
sD0
Ž
H–N3
.
qEtrans qEint
Ž
N3
.
3
Ž .
1
Y
1 X
˜
˜
the first absorption band is due to the A A §X A
where Etrans is the CM translational energy, and
E ŽHN . and E ŽN . are the internal energies of
transition w2,3,5x and thus by 248.3 nm excitation the
1
Y
first excited singlet state A˜ A is accessed. The
int
3
int
3
the parent and fragment, respectively. E ŽHN . is
strongly negative b parameter observed at 248.3 nm
int
3
negligible due to supersonic cooling of the HN3
sample. From Eq. Ž3., if the maximum translational
energy corresponds to the N3 ground state, i.e.
is consistent with a transition moment perpendicular
1
Y
1 X
to the molecular plane in the A˜ A § X˜ A transi-
tion. The anisotropic angular distribution confirms
1
Y
E ŽN .s0, D ŽH–N . achieves its upper limit in
that after excitation to A˜ A by a 248.3 nm photon,
the HN3 molecule undergoes prompt N–H bond
int
3
0
3
max
trans
the inequality: D ŽH–N .(hnyE . The extrap-
0
3
1
Y
olated onset of the maximum in Fig. 2 yields D ŽH–
dissociation on the short-lived A˜ A excited state
w8,23x.
0
N .(88.7"0.5 kcalrmol. The error limit is due to
3
the uncertainty in determining the threshold Že.g.,
By using Eq. Ž3. and assuming a negligible
E ŽHN ., the internal energy distribution of the N
interference from multiphoton dissociation of parent
int
3
3

2
88
J. Zhang et al.rChemical Physics Letters 299 (1999) 285–290
characterized by infrared spectroscopy w28x, and the
recommended vibrational frequencies of symmetric
stretching n1, bending n , and antisymmetric
2
y1
stretching n are 1320, 457, and 1645 cm , respec-
3
tively w26,28x. Because of the large Renner–Teller
2
˜
and spin–orbit interactions, spacings of X P vi-
bronic levels are irregular and complex w27x. With an
experimental resolution better than in the early study
w6–8x, the previously unobserved low-frequency
bending motion is clearly identified in this work. A
significant amount of bending excitation is evident.
The intensities may not be rationalized uniquely on
the basis of a bending progression, and both symmet-
ric and antisymmetric stretch vibrations could make
some contribution. Possible assignments for the lower
Fig. 3. N internal energy distribution of HN3 photodissociation at
3
2
48.3 nm. Possible assignments for the lower vibrational levels of
the N3 product are labeled.
vibrational levels of the N product, based on the
3
fragment can be derived ŽFig. 3.. As shown in Fig. 1,
theoretical calculation w27x, are labeled in the internal
energy distribution ŽFig. 3.. Energy partitioning is
listed in Table 1.
2
only the ground electronic state of N Ž X˜ P. is
3
accessible in our experiments. The two spin–orbit
2
2
˜
˜
states X P
and X P are separated by 71.3
Many aspects of the photodissociation dynamics,
3
r2
1r2
y1
cm w24x. The peaks in the E ŽN . distribution are
separated by ;600 cm and have widths of ;400
cm . These features reflect N3 vibrational state
e.g., the N vibrational excitation and product CM
int
3
3
y1
translational energy release, are determined by the
excited state and its PES. The first excited singlet
y1
1
Y
distribution.
state A˜ A of HN , on which the 248.3 nm photodis-
3
Y X
1
1
The peak widths ŽFig. 3. are primarily due to N3
rotational excitation. After excluding the excimer
sociation occurs, is accessed via Ž2a . Ž10a . §
Y
2
Y
Ž2a . excitation. The HOMO of HN , 2a , is a
3
laser linewidth and the TOF spectrometer function,
nonbonding out-of-plane p orbital with weak bond-
the average rotational energy is found to be ²E :f
ing character in the N –N region in the H–N –
rot
Ž1.
Ž2.
Ž1.
X
0
.04E . Angular momentum conservation requires
N –N molecule, while the LUMO, 10a , is in-
avail
Ž2. Ž3.
JsLqj, where J is the total angular momentum,
plane with antibonding character in the N –N
Ž1. Ž2.
L is the product orbital angular momentum, and j is
region and weak antibonding character in the N –H
Ž1.
1
Y
the N rotational angular momentum. Since the ini-
region w3,5x. Therefore, a HN molecule in the A˜ A
3
3
tial < J < is small for the supersonically cooled HN3
state should have weakened N –N and N –H
Ž1.
Ž2.
Ž1.
molecule, it follows that < j<f< L<smbÕ , where m
bonds. Furthermore, this excited-state HN has an
equilibrium geometry with a bent N–N–N group Žin
either cis or trans configuration with a ;1308
rel
3
is the reduced mass of the products, b is the exit
channel impact parameter, and Õrel is the relative
velocity of the products. Because m is small Ždue to
the light H-atom., < j< and thus rotational excitation
Table 1
of N are expected to be modest.
3
Product energy partitioning in the UV photodissociation of HN3
The spectroscopy of N3 is complicated due to
1
X
2
2
HN Ž X˜ A .qhn ™HŽ S.qN Ž X˜ P .
several interactions ŽRenner–Teller, spin–orbit, etc..
3
3
g
2
w24–27x. Electronic transition from X˜ P to the
l Žnm.
² ftrans
:
² fint:N3
2
low-lying exited S state was studied w24,25x, and
a
0.32a
2
48.3
248.3
93.3
193.3
0.68
0.71
0.55
0.49
2
b
b
˜
0.29 Ž fvib s0.25, frot s0.04.
the lower X P vibronic levels are characterized.
a
a
1
0.45
0.51
However, due to the fast predissociation of exited-
b
b
2
˜
Ž
state N , the higher X P vibronic levels especially
3
those with bending. were only studied by theoretical
calculation w27x. The electronic ground state was also
a
Ref. w8x.
This study.
b

J. Zhang et al.rChemical Physics Letters 299 (1999) 285–290
289
These gradients create a significant bending motion
in the excited HN . This bending motion is carried
3
into the N product while the exited HN dissociates
3
3
along the N–H coordinate to the HqN products.
3
Note that N has a linear equilibrium configuration.
3
The bending excitation in N that was observed in
3
our experiment demonstrates the important features
of the excited-state PES. Extensive bending excita-
tion of NCO has also been observed in the photodis-
sociation of the first excited singlet state of HNCO
w17x, which can be rationalized by the similar fea-
tures of the calculated PES of HNCO w30x. Similarly,
symmetric stretching in N can be understood in the
Fig. 4. N internal energy distribution of HN3 photodissociation at
3
3
1
ized.
93.3 nm. The photolysis laser radiation was 0.6 mJ and unpolar-
same way as for the N bending. The N–N bond
3
lengths in HN ground state are 1.13 and 1.24 A˚ ,
3
respectively, and they are elongated in the excited
state, as suggested in the absorption spectrum w2x.
N–N–N angle w5x and elongated N–N bond lengths,
as suggested in the absorption spectrum w2x. This
Both N–N bond lengths in N3 are 1.18 A˚ . N
3
symmetric stretching excitation can be rationalized
from these changes of N–N bond lengths. Finally,
the calculated N–N–H bending-angle dependence on
excited-state geometry is consistent with Walsh’s
rule w29x and quantum mechanical calculations
1
Y
1
Y
1 X
w3,5,23x. The A˜ A state correlates with both the
the A˜ A surface is comparable to that on the X˜ A
ground state, and because of the small H-atom mass,
1
1
q
2
2
NHŽa D.qN ŽX S . and the HŽ S.qN Ž X˜ P .
2
g
3
g
fragments ŽFig. 1. and is responsible for both disso-
N rotational excitation is modest w5,23x, as observed
3
1
1
q
ciation channels. For the NHŽa D.qN ŽX S .
in our experiment.
2
g
1
Y
˜
channel, ab initio calculations of the A A PES
1
Y
˜
demonstrated that the A A state is repulsive along
3.2. Photodissociation at 193.3 nm
the N –NH coordinate, and in the Franck–Condon
2
region there are pronounced gradients toward bent
With 193.3 nm photolysis radiation, two more
1
X
1 Y
N–N–N. This could account for N rotational excita-
singlet excited states of HN Ž B˜ A and C˜ A . can be
2
3
tion in the NHqN channel w5,9–14x.
reached ŽFig. 1. w5x. The N internal energy distribu-
2
3
2
2
For the HŽ S.qN Ž X˜ P . channel, ab initio cal-
3
g
1
Y
1 Y
˜
˜
culations of the A A surface showed that the A A
y1
state has a small barrier of ;2300 cm at a H–N
distance of 1.3 A, and after this barrier the surface is
˚
purely repulsive along the H–N coordinate w5,23x.
This feature is similar to those along the H–N
coordinate on the first excited singlet state of HNCO
in recent theoretical studies w30,31x. This feature is
also consistent with the repulsive product transla-
tional energy release observed in our experimental
study. Note that the 248.3 nm photon energy is
above the barrier height. The ab initio calculations of
1
Y
˜
the A A state also showed that in the Franck–Con-
don region there are pronounced gradients toward
bent N–N–N w5,23x, which are due to a nearly linear
N–N–N equilibrium geometry in the ground-state
HN and a bent N–N–N in the excited-state HN .
Fig. 5. Anisotropy parameter b as a function of N3 internal
energy for 193.3 nm photodissociation.
3
3

2
90
J. Zhang et al.rChemical Physics Letters 299 (1999) 285–290
w2x J.R. McDonald, J.W. Rabalais, S.P. McGlynn, J. Chem.
Phys. 52 Ž1970. 1332.
tion of 193.3 nm photodissociation is shown in Fig.
. Severe interference of the H-atom signals from
multiphoton dissociation processes limited photolysis
laser power to ;1 mJ. The N3 internal energy
distribution at 193.3 nm is very different from that at
4
w3x J.W. Rabalais, J.R. McDonald, V. Scherr, S.P. McGlynn,
Chem. Rev. 71 Ž1970. 73.
w4x A.P. Baronavski, R.G. Miller, J.R. McDonald, Chem. Phys.
30 Ž1978. 119.
w5x U. Meier, V. Staemmler, J. Phys. Chem. 95 Ž1991. 6111.
w6x K.H. Gericke, M. Lock, F.J. Comes, Chem. Phys. Lett. 186
Ž1991. 427.
2
48.3 nm, peaking at much higher internal energy
and with few structures resolved. Translational en-
ergy release is less repulsive, with ² ftrans:s0.49
Žclose to Comes’ value of 0.55. w8x. The anisotropy
parameter from our polarization study is energy-de-
pendent ŽFig. 5.. The overall average b parameter is
about y0.4, compared to the q0.08 value from
Comes’ study. The dynamic information suggests
that 193.3 nm photodissociation proceeds via multi-
ple dissociation pathways on different excited elec-
7
w x
T. Haas, K. Gericke, C. Maul, F.J. Comes, Chem. Phys. Lett.
02 Ž1993. 108.
2
w8x M. Lock, K.-H. Gericke, F.J. Comes, Chem. Phys. 213
Ž1996. 385, and the references therein.
w9x J.-J. Chu, P. Marcus, P.J. Dagdigian, J. Chem. Phys. 93
Ž1990. 257.
w10x F. Rohrer, F. Stuhl, J. Chem. Phys. 88 Ž1988. 4788.
w11x K.-H. Gericke, T. Hass, M. Lock, R. Thienl, F.J. Comes, J.
Phys. Chem. 95 Ž1991. 6104.
tronic states of HN w8x. This possibility is supported
w12x K.-H. Gericke, M. Lock, R. Fasold, F.J. Comes, J. Chem.
3
Phys. 96 Ž1992. 422.
by the two UV absorption features at 204 and 190
nm w1–3x and by the ab initio calculations of the
excited states w5x, and it is also consistent with the
w13x H.H. Nelson, J.R. McDonald, J. Chem. Phys. 93 Ž1990.
8
777.
w14x M. Hawley, A.P. Baronavski, H.H. Nelson, J. Chem. Phys.
1
q
fact that at 193.3 nm product channels NHŽb S .q
99 Ž1993. 2638.
1
Y
1
N Žcorrelating to B˜ A . and NHŽc P.qN Žcorre-
w15x L. Schneider, W. Meier, K.H. Welge, M.N.R. Ashfold, C.
2
2
1
Y
Ž
.
Western, J. Chem. Phys. 92 1990 7027.
˜
.
w
x
lating to C A have been observed 8,10 . Further
investigation of the 193.3 nm photodissociation is
being carried out.
w16x M.N.R. Ashfold, I.R. Lambert, D.H. Mordaunt, G.P. Morley,
C.M. Western, J. Phys. Chem. 96 Ž1992. 2938.
w17x J. Zhang, M. Dulligan, C. Wittig, J. Phys. Chem. 99 Ž1995.
7
446.
w18x B. Krakow, R.C. Lord, G.O. Neely, J. Mol. Spectrosc. 27
Ž1968. 148.
w19x M.N.R. Ashfold, J.E. Baggott ŽEds.., Molecular Photodisso-
ciation Dynamics, Royal Society of Chemistry, London,
Acknowledgements
1
987.
w20x R.E. Continetti, D.R. Cyr, D.L. Osborn, D.L. Leahy, D.M.
We thank Liming Wang and Steve Chambreau for
their assistance in this work and Nader Ari and Prof.
Xuechu Li for their help in the early stage of setting
up the instruments. We thank Prof. Curt Wittig for
donating an excimer laser and a vacuum chamber
used in this work. This work is supported partially
by a UC Regents Faculty Fellowships and Faculty
Development Award and partially by a Camille and
Henry Dreyfus New Faculty Award.
Neumark, J. Chem. Phys. 99 Ž1993. 2616.
w21x D.W. Rogers, F.J. McLafferty, J. Chem. Phys. 103 Ž1995.
8
302.
w22x R.N. Zare, Mol. Photochem. 4 Ž1972. 1.
w23x U. Meier, Dissertation, Ruhr-Universit ¨a t Bochum, 1988.
w24x A.E. Douglas, W.J. Jones, Can. J. Phys. 43 Ž1965. 2216.
w25x R.A. Beaman, T. Nelson, D.S. Richards, D.W. Setser, J.
Phys. Chem. 91 Ž1987. 6090.
w26x M.E. Jacox, J. Phys. Chem. Ref. Data 19 Ž1990. 1415, and
references therein.
w27x G. Chambaud, P. Rosmus, J. Chem. Phys. 96 Ž1991. 77.
w28x C.R. Brazier, P.F. Bernath, J.B. Burkholder, C.J. Howard, J.
Chem. Phys. 89 Ž1988. 1762.
w29x A.D.J. Walsh, J. Chem. Soc. 1953, 2266.
w30x J.J. Klossika, H. Fl o¨ thmann, C. Beck, R. Schinke, K. Ya-
mashita, Chem. Phys. Lett. 276 Ž1997. 325.
References
w1x H. Okabe, Photochemistry of Small Molecules, Wiley, New
w31x
J.E. Stevens, Q. Cui, K. Morokuma, J. Chem. Phys. 108
York, 1978, pp. 287–289, and the references therein.
Ž1998. 1452.