Velocity-map ion-imaging of the NO fragment from the UV-photodissociation of nitrosobenzene

Article abstract of DOI:10.1039/b302319b

The velocity and angular distribution of NO fragments produaed by UV photodissociation of nitrosobenzene have been determined by velocity-map ion-imaging. Excitation of the S₂-state by irradiation into the peak of the first UV absorption band at 290.5 nm leads to a completely isotropic velocity distribution with Gaussian shape. The average kinetic energy in both fragments correlates with the rotational energy of the NO fragment and increases from 6% of the excess energy for j = 6.5 to 11% for j = 29.5. A similar isotropic distribution albeit with larger average velocity is observed when the ionization laser at 226 nm is also used for photodissociation, corresponding to excitation into a higher electronic state S_n (n ≥ 3) of nitrosobenzene. It is concluded that photodissociation occurs on a timescale much slower than rotation of the parent molecule, and after redistribution of the excess energy into the vibrational degrees of freedom.

Full text of DOI:10.1039/b302319b

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Velocity-map ion-imaging of the NO fragment from the
UV-photodissociation of nitrosobenzene
Thorsten J. Obernhuber, Uwe Kensy and Bernhard Dick*
Institut fu¨r Physikalische und Theoretische Chemie, Universita¨t Regensburg, D 93040
Regensburg, Germany. E-mail: Bernhard.Dick@chemie.uni-regensburg.de
Received 27th February 2003, Accepted 2nd May 2003
First published as an Advance Article on the web 27th May 2003
The velocity and angular distribution of NO fragments produced by UV photodissociation of nitrosobenzene
have been determined by velocity-map ion-imaging. Excitation of the S2-state by irradiation into the peak of the
first UV absorption band at 290.5 nm leads to a completely isotropic velocity distribution with Gaussian shape.
The average kinetic energy in both fragments correlates with the rotational energy of the NO fragment and
increases from 6% of the excess energy for j ¼ 6.5 to 11% for j ¼ 29.5. A similar isotropic distribution albeit
with larger average velocity is observed when the ionization laser at 226 nm is also used for photodissociation,
corresponding to excitation into a higher electronic state Sn (n ꢀ 3) of nitrosobenzene. It is concluded that
photodissociation occurs on a timescale much slower than rotation of the parent molecule, and after
redistribution of the excess energy into the vibrational degrees of freedom.
time-of-flight distributions of both fragments were measured.28
1. Introduction
This study came to the conclusion that the velocity distribution
is anisotropic with an anisotropy parameter b ¼ ꢁ0.64. A
negative anisotropy parameter indicates fast dissociation and
preferred perpendicular arrangement of the transition dipole
and the recoil velocities. It was rationalized by the assumption
that the initially excited state has np*-character.
Since our investigations lead us to a different conclusion,
namely that photodissociation is slow compared to rotation
of the parent molecule, we decided to apply the technique
of velocity-map ion-imaging31,32 to the photodissociation of
nitrosobenzene. This method measures a 2D projection of
the 3D fragment velocity distribution. If this distribution is
cylindrically symmetric around an axis parallel to the image
plane, the 3D distribution can be reconstructed by Abel inver-
sion. During the ten years since the development of this
method it has been applied to the study of many fragment
atoms and some diatomic fragment molecules. The NO frag-
ment from the photodissociation of NO2 has been monitored
in an early study,33 but larger nitroso compounds have not
been studied by this technique until very recently.
The photophysical and photochemical processes exploited in
our experiment are indicated in the level diagram in Fig. 1.
Nitrosobenzene is excited into one of its UV bands leading
to population of the states S2 and above. These states are well
above the dissociation threshold of ED ¼ 18 955 cmꢁ1 34 and
the fragment radicals NO and phenyl will be created with high
efficiency. A second laser ionizes the NO fragment via 1 + 1
REMPI resonant with the A 2S+ (v0 ¼ 0) X 2P1/2,3/2
(v00 ¼ 0) transition. These ions are accelerated towards a posi-
tion-sensitive detector (multichannel plate, MCP) by a static
electric field. The ions impinging on the detector create a pro-
jected image of the velocity distribution which is monitored
by a CCD camera interfaced to a computer. Analysis of these
data allows the reconstruction of the complete three-dimen-
sional velocity distribution since it is axially symmetric around
the polarization axis of the photolysis laser.
Many nitroso compounds are prone to efficient photodissocia-
tion leading to the comparatively stable radical NO. The
mechanism of this photodissociation has been studied exten-
sively for the classes of aliphatic C-nitroso compounds R-
NO and nitrites RO-NO.1–23 Detailed investigations of the
product state distributions of the rotational and vibrational
states of NO by LIF or REMPI as well as measurements of
the alignment and the Doppler profiles have been performed.
The results indicate that the dissociation follows either of
two mechanisms. The statistical mechanism, which is observed
for most aliphatic nitroso compounds, leads to a quasi thermal
population and velocity distribution with little or no aniso-
tropy. The other mechanism involves very fast dissociation
on a repulsive potential surface and is frequently observed with
the aliphatic nitrites. In all these aliphatic compounds the
photodissociation is initiated by excitation into the first excited
singlet state which is of np*-character.
Nitrosobenzene is so far the only aromatic nitroso com-
pound whose photodissociation has been studied in a similar
fashion.24–29 In this case the excitation energy of the first
np*-transition is not sufficient to break the C–NO bond. The
energy gap between S1 and S2 is very large, but excitation to
S2 does not lead to fluorescence but to efficient fragmentation.
The origin of the S0 ! S2 transition could be resolved in an
argon matrix30 and a supersonic jet,25 and the homogeneous
linewidth of this transition indicates a lifetime of 60–90 fs for
the excited state.
Although this fact might point to a very fast dissociation
reaction, LIF measurements showed a quasi thermal popula-
tion of the rotational and vibrational levels of NO.26,27 The
Gaussian shape of the Doppler profiles is consistent with an
isotropic and thermal velocity distribution. However, a devia-
tion from an isotropic distribution can be observed in the Dop-
pler profiles only for specific arrangements of the beam and
polarization directions. The critical dip in the lineshape could
easily be overlooked or be washed out when the distribution
of velocities is very broad. Such a broad distribution was
indeed proposed by a study in which the angularly resolved
We have built an apparatus for the measurement of these
ion images and report here first results. In order to assess the
performance of the new machine we give data for the well
DOI: 10.1039/b302319b
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Fig. 2 Schematic setup of the ion imaging apparatus.
electrodeposition of copper onto a stainless steel mold as
described in 25. The shape of the skimmer was originally
designed as light baffle for optimized stray light rejection.35
For the beam skimmer we used a radius of 30 mm and an
opening of 1 mm diameter, resulting in an asymptotic aperture
angle of 25ꢂ.
2.3. Ion imaging apparatus
The ion imaging apparatus was constructed in our laboratory
in analogy to a similar setup published by Wrede et al.36 in
2001 and is described here for the first time. The molecular
beam enters the photolysis and ionization chamber through
a hole in the repeller electrode. This chamber is pumped by a
turbomolecular pump (EXT250H, BOC Edwards) to a pres-
sure of 10ꢁ7 mbar. Halfway between repeller and extractor
electrode the molecular beam is intersected perpendicularly
by two counterpropagating, linearly polarized dye laser beams.
One laser photolyses the sample molecules, the second state-
selectively ionizes the NO fragments by 1 + 1 REMPI. Both
lasers are focused into the region of overlap with the molecular
beam by quartz lenses (f ¼ 400 mm, Linos Photonics). The
ions are accelerated by a static electric field between repeller
and lens electrode towards the field free drift region of 500
mm length, which is pumped by an additional turbomolecular
pump (EXT70, BOC Edwards). After passing the drift region
the ions impinge upon the position sensitive detector which
consists of two multichannel plates (Proxitronic), coupled to
a phosphor screen (P43, 40 mm active diameter). These images
are recorded by a CCD camera (Imager 3, 1280 ꢃ 1024 pixel,
LaVision) through an objective (Schneider-Kreuznach XENON
25/0,95) and processed on a PC with the DAVIS 6.0 Software.
In a typical experiment about 5000 laser shots were accumu-
lated. If the ionization laser also dissociates the parent mole-
cules a background image caused by the ionization laser
alone was accumulated for the same number of shots and sub-
tracted from the data image. If the probability of ionization of
the fragments is independent from the relative orientation of
the fragment velocity and the polarization of the ionizing laser
beam, the recorded ion-images are projections of the three
dimensional velocity distribution of the neutral fragments.
In addition to the ion imaging measurements, 1 + 1 REMPI
spectra of the NO fragments can be recorded with the same
apparatus. For this purpose the CCD camera was replaced
by a photomultiplier (Thorn Emi RFIB289) and the probe laser
was scanned through the rotational transitions of interest of
the NO fragments while the photolysis wavelength was fixed.
The integral emission of the phosphor screen was detected
by the photomultiplier and processed on a gated boxcar inte-
grator (SRS 245, Stanford Research Systems) and a PC.
The optimum voltage ratios for velocity mapping were
determined by simulations with the Simion 6.0 program37
Fig. 1 Energy level scheme for the photodissociation of nitrosoben-
zene and the ionization of NO by 1 + 1 REMPI. ED ¼ 18 955 cmꢁ1
marks the dissociation energy of the C–N-bond of nitrosobenzene.
D0 and D1 indicate the two lowest doublet states of the phenyl radical.
known photodissociation of NO2 for comparison. Application
of the method to the photodissociation of nitrosobenzene from
S2 shows convincingly that the velocity distribution is comple-
tely isotropic and has a Gaussian shape.
2. Experimental
2.1. Chemicals
Nitrosobenzene (98%) was obtained from Fluka and purified
by vacuum sublimation at 65 ꢂC. Nitrogen monoxide and
nitrogen dioxide (99.5 + %) were obtained from Aldrich and
used without further purification.
2.2. Vacuum chambers and molecular beam
The vacuum apparatus consists of a sequence of three cham-
bers which serve for the generation of the cold supersonic
beam of educt molecules, the photofragmentation and ioniza-
tion of the fragments, and the field free flight region for the
ions, respectively. The main components of the setup are
shown schematically in Fig. 2 (not to scale). The supersonic
beam was created by expansion of helium through a pulsed
nozzle (General valve No. 9, 0.5 mm orifice) into a cylindrical
stainless steel chamber of 28 cm diameter pumped by the com-
bination of an oil diffusion pump (DN 250 ISO-K, 3000 l sꢁ1
Leybold), a roots pump (RUVAC WAU 251, 253 m3 hꢁ1, Ley-
bold) and a rotary pump (TRIVAC D40B, 40 m3 ꢁ1, Ley-
,
h
bold) to a final pressure of 10ꢁ6 mbar. Nitrosobenzene was
seeded into the beam by heating it to 60 ꢂC in an oven adjacent
to the nozzle. Beams seeded with NO or NO2 were produced
by expanding through the nozzle a mixture with helium (5 :
95) prepared in a steel cylinder. The molecular beam is colli-
mated by a skimmer with an opening of 1 mm diameter before
it enters the ionization chamber. The skimmer was made by
2800
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and the geometry of the electrodes as input. The best focussing
was found for the ratio of 1 : 2 : 2.45 of the voltages at the lens
electrode (Vl), the extractor electrode (Ve), and the repeller
electrode (Vr), with the electrode at the beginning of the drift
region at ground potential. The total voltage was adjusted so
that the image filled a large fraction of the MCP detector.
For NO2 a voltage of Vr ¼ 1200 V was used, for nitrosoben-
zene a voltage of Vr ¼ 2000 V. Calibration was based on simu-
lations of trajectories of NO fragments with different recoil
energies and different voltages Vr . Plots of the simulated image
diameter versus the initial recoil velocities are well fitted by
straight lines for fixed repeller voltages. This fact was used to
translate pixel positions into velocities. The velocity distribu-
tion for the photodissociation of NO2 obtained in this way is
in very good agreement with that published previously33 (see
results section). We also observe the expected linear relation-
ship when the experimentally determined image diameter is
plotted versus the inverse of the square root of the acceleration
voltage.
found it convenient to transform to another distribution func-
tion defined by
qffiffiffiffiffiffiffiffiffiffiffiffi
Cðv; zÞ ¼ 2pv2Qðv 1 ꢁ z2; vzÞ
pffiffiffiffiffiffiffiffiffiffiffiffiffiffi
ð2Þ
In this expression, v ¼ r2 þ z2 is the absolute value of the
velocity, and z ¼ z/v ¼ cos y measures the angle between the
velocity vector and the z-axis. The velocity distribution G(v)
and the angular distribution H(z) are obtained by the integra-
tions
Z
1
GðvÞ ¼
HðzÞ ¼
dz Cðv; zÞ
dv Cðv; zÞ
ð3Þ
ð4Þ
ꢁ1
1
Z
0
which correspond to summing over all rows or all columns,
respectively, in the matrix C(v,z). A photodissociation initiated
by a one-photon absorption is usually described well by a dis-
tribution function of the form:40–42
2.4. Laser and data acquisition
1
Pðv; yÞ ¼ pðvÞ½1 þ bP2ðcos yÞꢅ;
ð5Þ
2
The two dye lasers (Lambda Physik LPD 3002 and FL 3002)
used for the photolysis of nitrosobenzene/NO2 and the ioniza-
tion of the NO fragments were pumped by a single XeCl exci-
mer laser (Lambda Physik Lextra 200). The probe laser was
optically delayed by about 10 ns. The typical laser pulse energy
was about 20–40 mJ for both lasers. The polarisation direction
of the photolysis laser was rotated parallel to the vertical axis
of the detector surface by a double-Fresnel rhomb (Halle). The
ionization laser was polarized parallel to the photolysis laser
and operated in the spectral region of the A 2S+ (v0 ¼ 0) X
2P1/2,3/2 (v00 ¼ 0) transition of NO near 226 nm by frequency
doubling the output of the coumarin 120 dye with a BBO I
crystal. The fundamental wavelength was removed by a set
of 4 Pellin-Broca prisms or a filter (Schott UG 5). For the
photolysis of NO2 the second laser was tuned to 360 nm.
Nitrosobenzene was studied at the photolysis wavelengths
290.5 nm and 226 nm. In the first case the photolysis laser
was operated with the dye rhodamine 6G and frequency
doubled with a KDP crystal. In the second case, a single laser
was used for photolysis and ionization of the fragments.
The nozzle, lasers and CCD camera were synchronized with
suitable delays by a home built trigger unit. The repetition rate
was kept below ꢄ2 Hz to avoid an increase of the pressure in
the ion drift chamber above 10ꢁ6 mbar. Typically 5000 laser
shots were taken for each image, and the CCD camera read
out after every shot.
where P2 is the second Legendre polynomial. This distribution
function is normalized according toy
Z
Z
p
1
1 ¼
sin y dy
dv Pðv; yÞ
ð6Þ
0
0
Analysis of an ion image resulting from this distribution
with the method outlined above yields
GðvÞ ¼ pðvÞ
HðzÞ ¼ 1 þ bP2ðzÞ
ð7Þ
ð8Þ
This separates the velocity distribution from the angular dis-
tribution. A fit of a parabola to H(z) yields the anisotropy
parameter b.
3. Results and analysis
3.1. NO from the photodissociation of NO2
The photodissociation of NO2 has been studied by a number
of groups with various experimental methods. The angular dis-
tribution of the NO and O fragments and the partitioning of
the excess energy onto the states of the NO products are
known for many different photolysis wavelengths.42–48 Early
ion imaging experiments by Houston et al. show a strongly
anisotropic distribution of the NO fragments (anisotropy
parameter b ¼ 1.46 ꢆ 0.20) formed in the photodissociation
at 355 nm.33,49 Because of this high anisotropy and the large
number of former measurements, nitrogen dioxide is well sui-
ted for testing our newly built ion imaging apparatus.
2.5. Data analysis
The image produced on the CCD camera is a two dimensional
data array B(x,z), where the axis z is parallel to the polariza-
tion of the photolysis laser, and x is perpendicular to it. This
image is the projection of a three dimensional distribution
function F(x,y,z) onto the (x,z)-plane. If this distribution func-
Fig. 3a shows the velocity-map ion-image of NO formed in
2
the P1/2 (v ¼ 0, j ¼ 13.5) state by photodissociation of NO2
at 360 nm. This corresponds to an excess energy of ꢄ2650
cmꢁ1 above the threshold for dissociation.50 The fragments
were ionized by 1 + 1 REMPI resonant with the P21 ,Q11 tran-
sition. The image size is 360 ꢃ 360 pixel. Most of the intensity
lies along the vertical axis parallel to the polarization direction
of the photolysis laser. This indicates a positive correlation
between the transition dipole moment ~m of the parent molecule
tion is cylindrically symmetric, i.e. if it depends only on the dis-
tance r ¼ x2 þ y2 from the z-axis, it can be regained from the
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
38
projection by the inverse Abel transform
Z
1
1 @B
@x
dx
pffiffiffiffiffiffiffiffiffiffiffiffiffiffi
Qðr; zÞ ¼ Fðr; 0; zÞ ¼ ꢁ
ð1Þ
x2 ꢁ r2
p
r
Since this transform does not depend on the value of the
z-coordinate, it can be performed for each line in B(x,z)
separately. Several algorithms have been proposed for the
numerical implementation of this integral transform.39 We
have chosen the matrix method as described in 39 and apply
the transform to each line of the image separately.
~
and the velocity vector v of the NO fragments. Most of the
fragments recoil along ~m, i.e. the dissociation process has pre-
dominantly parallel character. A large anisotropy of this kind can
only be observed if the time constant for photodissociation is
y Note that in contrast to the usual convention for integration in sphe-
rical coordinates the factor v2 is already included in the distribution
function.
For the analysis of the Abel inverted image Q(r,z) in terms
of a velocity distribution and an angular distribution, we
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Fig. 4 Velocity distribution p(v) and angular distribution H(cos y)
for the NO fragment from the photolysis of NO2 , calculated from
the Abel inverted image of Fig. 3.
than the value of ꢄ230 m sꢁ1 that appears in the work by
Houston et al..
Summation of all rows of C(v,z) in Fig. 3c in the velocity
range from v ꢄ 650–850 m sꢁ1 results in the angular distribution
shown in the lower part of Fig. 4. The latter is well fitted by a
parabola with the anisotropy parameter b ¼ 1.35 ꢆ 0.05. The
values for the mean velocity, the width of the distribution, and
the anisotropy measured with our new apparatus are in good
agreement with values reported from earlier experiments.33,42,44
Fig. 3 (a) Ion velocity image of NO fragments from the photolysis
of NO2 at 360 nm. A frequency doubled dye laser tuned to the
P21 ,Q11(13.5) line of the A 2S+(v0 ¼ 0) X 2P1/2 (v00 ¼ 0) transition
of NO at 226 nm was used as probe beam. (b) Abel inverted image
of (a). (c) Transformation of the Abel inverted image to the distribu-
tion function C(v,z) in the orthogonal coordinates v and z ¼ cos y.
For details see text.
3.2. Ion imaging of NO from the photolysis of nitrosobenzene
Cooling of molecules seeded into a supersonic beam can lead
to the formation of Van der Waals dimers or higher aggre-
gates. The tendency for dimer formation increases with the
polarity and polarizability of the molecules and their partial
pressure in the expansion. For example, with t-butylnitrite a
sharp onset of cluster formation has been reported when the
fraction of this compound in the gas mixture exceeds 5%.8
The rotational distribution of the NO fragments produced
from photolysis of these clusters is quite different from that
of the monomers. Drucker and Flade51 have reported the
vapor pressure of solid nitrosobenzene in the temperature
range 25–66 ꢂC. We have measured the absorbance of the
absorption band at 212 nm in this temperature range in vapor
cells with 1 cm and 0.2 cm length, with some solid nitrosoben-
zene at the bottom. For this absorption band, which peaks at
218 nm in mixtures of ethanol and n-heptane, an extinction
coefficient of e ¼ 6480 l Mꢁ1 cmꢁ1 and a constant shape has
been reported independent of the solvent composition.52 In
contrast to this, the first two UV absorption bands show a
strong solvent dependence. Assuming the same extinction
coefficient for the vapor phase, we obtain vapor pressures of
10 mbar at 50 ꢂC, 24 mbar at 60 ꢂC, and 49 mbar at 69 ꢂC,
in good agreement with the early measurements of
Drucker and Flade.51 In our ion-imaging measurements we
short compared to the rotational period of the parent molecule.
Based on their data of alignment and anisotropy from TOF
measurements, Mons and Dimicoli have previously estimated
an upper limit of 250 fs for the lifetime of the excited 2B2 state.45
The velocity distribution Q(r,z) obtained by Abel inversion
of these data is shown in Fig. 3b. This image corresponds to
a cut through the three-dimensional velocity distribution along
the polarization axis of the photolysis laser. Transformation of
these data according to eqn. (2) leads to the representation
shown in Fig. 3c, in which the velocity and the angular coord-
inates are orthogonal. Summation of all columns yields the
velocity distribution integrated over all angles, as shown in
the upper part of Fig. 4. This velocity distribution peaks at
740 m sꢁ1 corresponding to a kinetic energy of 687 cmꢁ1 of
the NO fragments. This peak velocity is close to the value of
729 m sꢁ1 found by Houston et al.33 by ion imaging of NO
in the state 2P1/2(v ¼ 0, j ¼ 25.5) formed by photodissociation
of NO2 at 355 nm. The full width at half maximum of the velo-
city distribution in Fig. 4 is ꢄ125 m sꢁ1, which is narrower
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Fig. 5 1 + 1 REMPI spectrum of NO fragments from the 290.5 nm
photolysis of nitrosobenzene. The spectrum was measured in the ion
imaging apparatus by integration of the whole image area while tuning
the probe laser.
used a stagnation pressure of 2500 mbar and observed no
change in the ion images in the temperature range of 50–
60 ꢂC. Hence we believe that our data are not affected by clus-
ter formation.
In our previous measurements of Doppler profiles of the NO
fragments from the photodissociation of nitrosobenzene we
used the photolysis wavelengths 290, 266, and 255 nm. The
two latter wavelengths excite into parts of the absorption spec-
trum with higher absorption cross section and hence lead to
larger signals. However, these wavelengths fall already into
the second UV absorption band, i.e. the initially prepared
excited state is S3 or even higher. Since we are mainly inter-
ested in the mechanism of photodissociation following excita-
tion into the S2-state, we have concentrated on the photolysis
wavelength of 290 nm in the present study.
Fig. 5 shows part of the 1 + 1 REMPI spectrum of the NO
fragments produced by 290.5 nm photodissociation of jet-
cooled nitrosobenzene. It was measured by integration of the
whole ion signal on the phosphor screen by a photomultiplier
while the ionization laser was scanned. After assignment of the
transitions several well isolated lines with reasonable intensity
in the range of j ¼ 6.5–29.5 were chosen for the ion imaging
experiments.
Fig. 6 (a) Ion velocity image of NO fragments from the photolysis of
nitrosobenzene at 226 nm. A frequency doubled dye laser tuned to the
P21 ,Q11(14.5) line of the A 2S+(v0 ¼ 0) X 2P1/2 (v00 ¼ 0) transition
was used simultaneously as photolysis and probe beam. (b) Ion velo-
city image of NO fragments from the photolysis of nitrosobenzene at
290.5 nm. The probe laser was tuned to the same transition as in (a).
486 ꢆ 12 m sꢁ1. (The small difference in the widths for the ana-
lysis of the columns and the rows can partly be accounted for
by the fact that the ionization volume extends more into the
horizontal direction, i.e. the propagation direction of the laser
beams.) The corresponding analysis for six images of the
photolysis at 226 nm yields the average width of 594 ꢆ 20 m
The wavelength of the ionization laser near 226 nm falls
close to a minimum of the absorption spectrum between the
second and the third UV band. Nevertheless we observed that
this laser produced a considerable amount of fragments by
itself. The experiment with the photolysis laser tuned to
290.5 nm resonant with the maximum of the first UV band
hence yields an ion image which has contributions from photo-
dissociation with photons of 226 and 290.5 nm. These two
components were separated by subtraction of an ion image
measured with the same number of laser shots but only the
ionization laser at 226 nm operating. In all experiments with
290.5 nm photolysis the intensity of the ionization laser was
attenuated to such an extent that the contribution by this laser
alone was small compared to the signal due to both lasers. Two
ion images, representing the velocity distributions of NO in the
s
ꢁ1. It was checked that not only the total projection but also
individual horizontal lines of each image were fitted by a Gaus-
sian with the same width. In this special situation the image is
proportional to its own Abel inverse, as shown in the appen-
dix. Making use of this fact we have integrated the images
directly and obtained the velocity and angular distributions
shown in Fig. 8. This avoids artifacts, which are produced
by the matrix method (and other methods too) of Abel inver-
sion due to amplified noise, in particular near the symmetry
axis of the velocity distribution. As a consequence of the Gaus-
sian nature of the projection onto one coordinate, the total
velocity distribution is a Maxwell Boltzmann distribution with
vrms ¼ 840 m sꢁ1 for photolysis at 290.5 nm and vrms ¼ 1030
m sꢁ1 for photolysis at 226 nm. The angular distributions
found for both photolysis wavelengths are almost horizontal
lines. The anisotropy parameters obtained from the fits are
close to zero. The simulated shape for an angular distribution
with b ¼ ꢁ0.64 is shown for comparison.
2
state P1/2(v ¼ 0, j ¼ 14.5) produced by photodissociation at
226 and 290.5 nm, are shown in the upper and lower part of
Fig. 6.
Both images appear to be completely isotropic, with the
highest intensity in the center. The projections of these images
onto the horizontal and vertical axes, corresponding to the
summation of all rows or all columns in the data array, are
very well fitted by Gaussians. Fig. 7 shows these projections
and the Gaussian fits for a photolysis experiment at 290.5
nm. Six of these measurements yielded the average width of
Measurements of ion images have been performed for sev-
2
eral rotational states of the NO fragment in the P1/2(v ¼ 0)
state. In all cases an isotropic and Gaussian velocity distribu-
tion was found. The mean velocities, average kinetic energies
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Table 1 Analysis of ion imaging data for nitrosobenzene. Mean velo-
cities vrms of NO fragments obtained by analyzing the rows and the
columns of the data matrix were used to calculate the average transla-
tional energies (Etr) of the NO and the phenyl fragments. The last two
columns present the fraction of the excess energy converted to kinetic
energy and the anisotropy parameter b. Errors are the standard devia-
tions from six measurements
lph/
nm
vrms(NO)/ Etr(NO)/ Etr(phenyl)/
m sꢁ1 cmꢁ1 cmꢁ1
J
Etr/Eexc
b
225.96 14.5 1029 ꢆ 35 1328 ꢆ 90 517 ꢆ 35
0.073
0.064
0.086
0.080
0.100
0.112
+0.05
+0.00
+0.01
ꢁ0.04
ꢁ0.05
+0.03
290.50 6.5 753 ꢆ 20
290.50 12.5 873 ꢆ 24
290.50 14.5 842 ꢆ 21
711 ꢆ 37 277 ꢆ 15
956 ꢆ 52 372 ꢆ 20
889 ꢆ 44 346 ꢆ 17
290.50 22.5 943 ꢆ 27 1114 ꢆ 64 433 ꢆ 25
290.50 29.5 995 ꢆ 32 1243 ꢆ 80 483 ꢆ 31
4. Discussion
4.1. NO2
For an instantaneous photodissociation a value of the aniso-
tropy parameter of b ¼ 2P2(cos a) is expected, where a is the
angle between the transition dipole moment and the vector
for the recoil velocity in the center of mass frame of the mole-
cule. The extremal values are b ¼ 2 and b ¼ ꢁ1 if these
vectors are parallel or perpendicular. With increasing duration
of the photodissociation process the anisotropy will decay
Fig. 7 Integration of Fig. 6b along all columns (a), and along all
rows (b). This corresponds to the distribution of the velocity compo-
nent perpendicular (a) or parallel (b) to the polarization vector of
the photolysis light. Both curves are well fitted by Gaussians.
2
towards b ¼ 0. The transition dipole ~m for the 2A1 ! B2 tran-
sition lies along the line connecting the two oxygen atoms.
Values of 134ꢂ and 102ꢂ have been reported for the ONO bond
angle in the ground state 2A1 and the excited state 2B2 , respec-
tively.53 Hradil and coworkers have chosen the geometry of the
ground state and the velocity vector along the breaking NO
bond.49 The result a ¼ 23ꢂ and b ¼ 1.54 have subsequently
been used by others.48 With the geometry of the excited state
the same choice for the velocity vector results in a ¼ 39ꢂ and
b ¼ 0.81 which is smaller than the experimental value and
hence not reasonable. Alternatively one could assign the velo-
city vector pointing from the dissociating O atom towards
the center of mass of the NO fragment. This choice assigns
all the angular momentum to the rotation of NO and none
to the relative motion of the two fragments. In this case
a ¼ 7.4ꢂ or 13.8ꢂ and b ¼ 1.83ꢂ or 1.95ꢂ for the geometry of
the ground state or the excited state.
of the fragments, the fraction of excess energy converted to
kinetic energy, and the anisotropy parameters are collected
in Table 1. Apparently the average kinetic energy of the NO
fragment increases with the rotational energy.
Due to the finite lifetime tD of the excited state, the initial
anisotropy will decay by the rotation of the parent molecule after
excitation and before dissociation. We can model the instant-
aneous velocity distribution at time t after the photolysis by
expðꢁt=tDÞ
Pðv; y; tÞ ¼
pðvÞ½1 þ bðtÞP2ðcos yÞꢅ;
ð9Þ
2tD
Assuming an exponential decay of the anisotropy with a
characteristic rotational time constant tR ,
bðtÞ ¼ b0 expðꢁt=tRÞ;
ð10Þ
the time-integrated velocity distribution will have the form of
eqn. 5 with
ꢀ
ꢁꢁ1
tD
tR
b ¼ b0 1 þ
ð11Þ
The ratio of the observed value of b ¼ 1.35 and the initial
anisotropy b0 can thus be used to calculate the ratio tD/tR
of the dissociation and rotation times. The result depends on
the choices made for the initial geometry of the dissociating
molecule and the direction of the velocity vector. Above we
have discussed three choices, namely (i) ground state geometry
and velocity along the NO bond, (ii) ground state geometry
and velocity along the line connecting the O atom and the
Fig. 8 (a) Normalized velocity distribution p(v) and (b) angular dis-
tribution H(cos y) of NO fragments in the state j ¼ 14.5 from the
photolysis of nitrosobenzene at 290.5 nm (left scale) and 226 nm (right
scale). The simulated angular distribution for an anisotropy parameter
b ¼ ꢁ0.64 is also shown (left scale).
2804
Phys. Chem. Chem. Phys., 2003, 5, 2799–2806

View Article Online
center of mass of the NO fragment, and (iii) excited state geo-
metry and velocity as in (ii). For these three cases the ratio of
the time constants tD/tR is 0.14, 0.36, and 0.44.
bending mode or the CN stretch have a large contribution in
the reaction coordinate, they will be highly excited in the
moment of dissociation. This could lead to a correlation of v
and J. This is not a contradiction to the absence of alignment
(i.e. a correlation of J and the transition moment m) found in
ref. 29.
4.2. Nitrosobenzene
Nitrosobenzene exhibits three strong absorption bands in the
UV with peaks at 34 500, 37 000, and 46 500 cm in the gas
phase.26 The corresponding excited states can be phenomeno-
logically assigned to S2 , S3 , and S4 . However, many more
electronic states are expected for an organic molecule of this
size in this energy range. Indeed, semiempirical calculations
indicated a number of states with a large contribution of dou-
bly excited configurations.30 These will carry little oscillator
strength but might gain intensity via vibronic coupling to
higher electronic states. The fact that the intensity of the first
UV-band strongly increases with the polarity and the polariz-
ability of the solvent could be due to such a state mixing.
Although we can not exclude the presence of more than one
electronic state in the range of 31 000–36 000 cmꢁ1, the maxi-
mum-entropy deconvoluted spectrum in argon matrix30 and
the spectrum in the supersonic jet25 measured indirectly by
monitoring the product yield of NO could both be analyzed
into progressions of five vibrations built on a single electronic
origin. The homogeneous line width of this origin band led us
to conclude a lifetime of 60–90 fs for the S2-state.
In our previous investigations on the photodissociation of
jet-cooled nitrosobenzene we have studied Doppler profiles
and product state populations of the NO fragment produced
by photodissociation with photons of 320, 308, 290, 266, and
250 nm wavelength.26,29 All these data were consistent with
a statistical mechanism of photodissociation. The primarily
populated S2-state decays through internal conversion within
60–90 fs into the S1 or S0 state, and dissociation occurs on
the potential surface of the lower state on a much longer time
scale.
Such a mechanism leads to Gaussian lineshapes for the
Doppler profiles of individual rotational lines of the NO frag-
ment. On the other side, a very fast dissociation would lead to
an anisotropic velocity distribution which could be detected by
the observation of dips in Doppler profiles measured with spe-
cific polarization conditions. However, such a dip might be
obscured even in the presence of a considerable anisotropy if
the spread of velocities is large.29 Quantitative evaluation of
Doppler profiles requires deconvolution of these lineshapes
since the laser linewidth is of the same order of magnitude as
the Doppler width. In contrast to Doppler profile measure-
ments, ion imaging obtains the complete velocity distribution
and no deconvolution is required.
The increase of the average velocity with the rotational
quantum number of the NO fragments indicates that these
two degrees of freedom are not statistically independent. A
similar tendency was already observed in our previous study
of Doppler widths.29 A small line broadening could, however,
also be explained by the increase of the L-splitting with
increasing rotational quantum number. A rigorous mechanis-
tic analysis of this correlation would require quantum dynami-
cal wavepacket calculations on the potential energy surface
involved, a task which is far beyond the scope of the present
work. In a simplified classical picture such a correlation could
be the result of an impulsive torque at the N end of NO. If the
center of mass of the NO fragment remains in the plane of the
phenyl radical, the dissociation coordinate will be a linear
combination of the C–N stretch, the CNO and CCN bending
modes, and the CCNO torsional mode. These four coordinates
define the relative position of the two fragments and will,
after dissociation, describe four degrees of freedom of relative
motion. The latter are the two components of the velocity vec-
tor (constrained to the plane of the phenyl radical) and the two
rotational degrees of freedom of the NO radical. If the CNO
A negative anisotropy of b ¼ ꢁ0.64 has recently been
reported as the result of a study of 266 nm photodissociation
by angularly resolved time-of-flight measurements of the neu-
tral fragments.28 These data are at variance with our findings,
and possible reasons for this should be discussed. From the
photolysis of nitrites it is known that NO fragments formed in
the dissociation of clusters have different rotational and vibra-
tional populations and lower anisotropy than the fragments
ejected from monomers.54 At the temperature of 60 ꢂC the
vapor pressure of nitrosobenzene is ca. 24 mbar. With a stagna-
tion pressure of 2.5 bar the concentration of nitrosobenzene is
far below the critical value of 5% observed for cluster formation
with nitrites. The higher anisotropy has been reported for the
experiment with the higher concentration of nitrosobenzene.
Hence it is unlikely that the difference was caused by the pre-
sence of clusters in one of the two experiments.
In ref. 28 a focused Nd:YAG laser beam with pulse energies
of 40 mJ at 226 nm was used for photodissociation of nitroso-
benzene. Assuming a cross section at the focus of one square
millimeter and a typical pulse duration of 4 ns, the average
intensity is I ꢄ 109 W cmꢁ2. It has been shown that intense
laser fields can trap and align neutral molecules in the gas
phase. Efficient alignment of CS2 followed by multiphoton dis-
sociation and ionization has been achieved with 35 ps light
pulses at 532 nm and intensities in the range 1012–1014
cmꢁ2 55,56
Larsen and coworkers succeeded in aligning 3,4-
dibromothiophene along all three axes by elliptically polarized
light at 1064 nm and intensities of 1011–1012 W cmꢁ2 57
The
W
.
.
alignment is proportional to the anisotropy of the polarizabil-
ity. The latter is strongly enhanced when the frequency of the
electric field is resonant with an electronic transition. If this
resonance enhancement is of the order of 102–103, the light
pulses used by Huang and coworkers might be strong enough
to align nitrosobenzene.
Since light at 266 nm is resonant with a strong absorption
band of nitrosobenzene, multiphoton excitation should also
be considered. The electronic state initially populated by UV
excitation decays very quickly (ꢄ60 fs) by internal con-
version to either the S1 or the S0 state. The isotropic distri-
bution of the fragment velocities indicates that dissociation
occurs on a much slower timescale, probably in the range of
10–100 ps. During this time the system can absorb a second
photon. If internal conversion leads to the S1 state, absorption
of a second photon will populate electronic states at ꢄ6.1 eV
above the ground state. The potential energy surfaces of these
states could be repulsive, leading to an anisotropic velocity dis-
tribution of the fragments generated via this two-photon disso-
ciation. The yield of this process relative to that of one-photon
dissociation depends on the ratio of the rate constant k of dis-
sociation of the hot S1 state and the rate sF of the transition
S1 ! Sn excited by the second photon. The intensity of
I ¼ 109
W
cmꢁ2 corresponds to
a
photon flux of
F ¼ 1.3 ꢃ 1027 photon cmꢁ2
s
ꢁ1. With an estimated cross
section of s ¼ 10ꢁ17 cm2 for excited-state absorption the exci-
tation rate will be sF ¼ 1.3 ꢃ 1010
s
ꢁ1. Since k ꢄ 1010 sꢁ1 is of
similar magnitude, a substantial fraction of the molecules
could be pumped into this two-photon dissociation channel.
Rotational relaxation will be essentially complete before the
second photon is absorbed. The anisotropy of the fragment
velocities will hence reflect the orientation of the transition
dipole of the excited-state absorption. If the latter is perpendi-
cular to the molecular plane, as is expected for a transition
from a np* state to a pp* state, it will be perpendicular to
the velocity vector, and negative anisotropy will be observed.
Phys. Chem. Chem. Phys., 2003, 5, 2799–2806
2805

View Article Online
12 D. Schwartz-Lavi, I. Bar and S. Rosenwaks, Chem. Phys. Lett.,
1986, 128, 123.
5. Conclusions
13 C. S. Effenhauser, P. Felder and J. R. Huber, J. Phys. Chem.,
1990, 94, 296.
Applying the technique of velocity-map ion-imaging we have
determined the three-dimensional velocity distribution of NO
fragments from the photodissociation of jet-cooled nitroso-
benzene. Photolysis was performed at 290.5 nm and 226 nm,
corresponding to excitation of S2 and a higher singlet state,
respectively. The ion images presented here are completely iso-
tropic and can be modelled very well by a Maxwell–Boltzmann
distribution. The negative anisotropy of b ¼ ꢁ0.64 recently
reported from a study of 266 nm photodissociation might be
an artifact produced by the high laser intensity applied. A
strong light field could either align and trap the nitrosobenzene
molecule, or lead via two-photon excitation to a different very
fast dissociation channel.
14 E. Kades, M. Ro¨sslein, U. Bruhlmann and J. R. Huber, J. Phys.
¨
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6. Appendix: Invariance of the Gaussian with
respect to Abel transformation
Application of the Abel inversion to a Gaussian with mean 0,
standard deviation s, and area A,
ꢀ
ꢁ
ꢁ
A
x2
2s2
pffiffiffiffiffi
f ðxÞ ¼
exp ꢁ
;
ð12Þ
s
2p
27 Ya-Min Li, Ju-Long Sun, Ke-Li Han and Guo-Zhong He, Chem.
Phys. Lett., 2001, 338, 297.
yields the integral expression
Z
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Guo-Hong He, J. Phys. Chem., 2000, 104, 10 079.
29 R. Seiler and B. Dick, Chem. Phys., 2003, 288, 43.
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33 T. Suzuki, V. P. Hradil, S. A. Hewitt, P. L. Houston and B. J.
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34 J. Park, I. V. Dyakov, A. M. Mebel and M. C. Lin, J. Phys. Chem.
A, 1997, 101, 6043.
ꢀ
1
A
x2
2s2
x
pffiffiffiffiffiffiffi
pffiffiffiffiffiffiffiffiffiffiffiffiffiffi
x2 ꢁ r2
FðrÞ ¼
dx exp ꢁ
:
ð13Þ
s3 2p3
r
pffiffiffiffiffiffiffiffiffiffiffiffiffiffi
This can be solved with the substitution y ¼ x2 ꢁ r2, lead-
ing to
ꢀ
ꢁ
Z
1
A
r2 þ y2
pffiffiffiffiffiffiffi
FðrÞ ¼
dy exp ꢁ
ð14Þ
ð15Þ
2s2
s3 2p3
0
ꢀ
ꢁ
A
¼
r2
2s2
exp ꢁ
:
s2p
35 W. R. Gentry and C. F. Giese, Rev. Sci. Instrum., 1975, 46, 104.
36 E. Wrede, S. Laubach, S. Schulenberg, A. Brown, E. R. Wouters,
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2629.
37 D. A. Dahl, SIMION 3D 6.0, Ion Source Software, Idaho
Falls, ID.
Hence the Abel inverse F(r) of f(x) is a Gaussian with mean
0 and the original standard deviation, but with the scaled area
rffiffiffi
A
2
~
A ¼
:
ð16Þ
s
p
38 R. N. Bracewell, The Fourier Transform and its Applications,
McGraw-Hill, New York, 1986.
Acknowledgements
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TJO gratefully acknowledges a fellowship from the Fonds der
Chemischen Industrie.
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