Spatial discrimination of Rydberg tagged molecular photofragments in an inhomogeneous electric field

Article abstract of DOI:10.1063/1.1408286

A new approach for obtaining spatial and energetic information is described. It is shown how the field configuration and other experimental parameters can be optimized so as to obtain the best spatial resolution. For demonstration purposes, an electrostatic octople, comprising eight cylindrical electrodes of alternating polarity equally spaced around a circle is considered.

Full text of DOI:10.1063/1.1408286

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Spatial discrimination of Rydberg tagged molecular photofragments in an
inhomogeneous electric field
O. L. A. Monti, H. A. Cruse, T. P. Softley, and S. R. Mackenzie
Citation: The Journal of Chemical Physics 115, 7924 (2001); doi: 10.1063/1.1408286
View online: http://dx.doi.org/10.1063/1.1408286
View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/115/17?ver=pdfcov
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JOURNAL OF CHEMICAL PHYSICS
VOLUME 115, NUMBER 17
1 NOVEMBER 2001
Spatial discrimination of Rydberg tagged molecular photofragments
in an inhomogeneous electric field
O. L. A. Monti, H. A. Cruse, and T. P. Softleya)
Physical and Theoretical Chemistry Laboratory, University of Oxford, South Parks Road,
Oxford OX1 3QZ, United Kingdom
S. R. Mackenzie
Department of Chemistry, University of Warwick, Coventry CV4 7AL, United Kingdom
͑Received 11 June 2001; accepted 13 August 2001͒
A new approach to detecting the spatial and energetic distribution of photodissociation products is
demonstrated, in which an inhomogeneous electric field is used for spatially selective field
ionization of Rydberg molecules. State-selected NO fragments from NO2 photodissociation in a
supersonic beam are excited to high-n Rydberg states and allowed to expand through the
inhomogeneous field provided by an octopole. The field in the octopole results in the field ionization
of those fragments that have moved to large radial distances (ϳ5 mm͒ from the beam axis, and
therefore this device transmits only those Rydberg molecules with a low velocity component in the
direction perpendicular to the beam. A detailed characterization of the properties of this
‘‘steradiancy analyzer’’ is carried out and factors limiting the energy resolution ͑currently
approximately 100 cmϪ1 at an excess energy of 250 cmϪ1), along with possible improvements, are
discussed in detail. © 2001 American Institute of Physics. ͓DOI: 10.1063/1.1408286͔
I. INTRODUCTION
experimental parameters might be optimized so as to obtain
the best spatial resolution in such an experiment.
A. Spatially selective field ionization
The first system investigated and reported here uses an
electrostatic octopole, comprising eight cylindrical elec-
trodes of alternating polarity equally spaced around a circle,
to provide the inhomogeneous field. The photodissociation of
NO2 in a supersonic beam is the simple reaction under study.
The experimental apparatus is illustrated schematically in
Fig. 1. After photodissociation of NO2 in Region I, the NO
photofragments are excited to high-n Rydberg states (n
ϳ120) by the probe lasers and the fragment cloud is allowed
to expand as the beam passes centrally through the octopole
in Region II. A pulsed voltage is applied to the octopole
when the fragment cloud is well inside this device and the
resulting field rises sharply with radial distance from the
beam axis. Fragments produced at energies very close to a
dissociation threshold have essentially zero kinetic energy in
the center-of-mass frame and remain in the molecular beam.
Fragments produced at energies significantly above the rel-
evant dissociation threshold may have high velocities per-
pendicular to the molecular beam axis. These fragments
travel to a region where the field strength is large when the
pulsed voltage is applied, and are field ionized. The ions
produced in the octopole are discarded, while the Rydberg
states that survive the octopolar field are transmitted into
Region III, where they are ionized and accelerated onto a
detector.
The application of ion-imaging techniques to measure
the spatial anisotropy and the energy distribution of product
molecules from simple gas-phase chemical processes has
been a rapidly developing field in recent years.1,2 The mea-
surements provide fundamental tests of the quality of ab
initio potential energy surfaces and of the dynamics on those
surfaces. In our own laboratory we have developed an inter-
est in the near-threshold dynamics,3 as these may be sensitive
to more subtle features of the potential energy surface and a
wider range of nuclear configurations. In this paper a new
approach for obtaining spatial and energetic information is
described, which is potentially of value in these near-
threshold studies and in a wider sphere. Product molecules
from a photodissociation event are excited into high-n Ryd-
berg states, and are then subjected to a pulsed inhomoge-
neous electric field. The probability of field ionization de-
pends on the spatial position of the fragments at the time the
field is applied. Thus by detecting either the Rydberg-tagged
molecules surviving the applied pulsed field, as in the
present experiments, or by detecting the ions produced by
field ionization in the inhomogeneous field, spatial discrimi-
nation is achievable. Furthermore, it should be possible to
establish a complete picture of the spatial distribution by
varying the field magnitude and its time of application. In
this paper we discuss how the field configuration and other
The octopole provides a mechanism for discriminating
against fragments produced with nonzero kinetic energies,
thus representing a first step in the direction of a tunable
kinetic energy aperture. The effective size of the aperture is
changed by varying either the principal quantum number of
Rydberg states excited ͑i.e., the excitation wavelength͒ or the
a͒
Address correspondence to Dr. T. P. Softley, Physical and
Theoretical Chemistry Laboratory, University of Oxford, South
Parks Road, Oxford OX1 3QZ, United Kingdom. Electronic mail:
tim.softley@chemistry.ox.ac.uk; fax: ϩϩ44 1865 275410
0021-9606/2001/115(17)/7924/11/$18.00
7924
© 2001 American Institute of Physics
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J. Chem. Phys., Vol. 115, No. 17, 1 November 2001
Rydberg tagged molecular photofragments
7925
FIG. 1. Schematic of the experimental setup.
applied octopole voltage. The device has been fully charac-
terized and its behavior is analyzed in detail using a combi-
nation of ion imaging and Mass Analyzed Threshold Ioniza-
tion ͑MATI͒ spectroscopy. The octopole constitutes an
example of a more general class of experiments employing
inhomogeneous electric fields and Rydberg excitation to
manipulate4,5 or analyze spatial distributions of neutral atoms
or molecules.
FIG. 2. MATI probing scheme for NO photofragments ( ϭ0, J ϭ17/2)
vЉ
as used in the PHOMPFI ͑photofragment multiphoton pulsed-field ioniza-
tion͒ and imaging experiments ͑see Sec. III͒.
Љ
II. EXPERIMENT
A. Excitation and detection
The experimental set-up, illustrated in Fig. 1, is a modi-
fication of the apparatus described in Ref. 3. The pulsed su-
personic beam of NO2, seeded in He at 0.5% molar fraction,
is introduced into the experimental chamber ͑maintained at
less than 10Ϫ7 mbar͒ via a 0.5 mm diameter skimmer. The
molecular beam is intersected at right angles by the photoly-
sis laser beam and by two additional laser beams required to
excite the NO fragments to the high-n Rydberg states. The
probing scheme is illustrated in Fig. 2. The nascent NO frag-
B. NO2 photodissociation
The dissociation process under study may be written as
3
NO ϩh␯→NO ,J ͒ϩO P ͒.
͑1͒
͑v
͑
Љ Љ
2
J
The photodissociation channels are defined by the possible
vibration–rotation states of the NO fragments ( ,J ) and
v
Љ Љ
the three possible spin–orbit states (J) of the oxygen atom
3
3
3P2 , P1, and P0. Energy conservation requires that
ment molecules in the Љ ϭ0, JЉ ϭ17/2 state are probed in
vNO
NO
h␯ϭD ϩE ,J ͒ϩE J͒ϩKE,
͑2͒
͑v
͑
Љ Љ
a (1ϩ1 ) mass analyzed threshold ionization ͑MATI͒
Ј
0
˜ ˜
scheme via the Q11 /P21(17/2) transitions in the A X(0,0)
neglecting the initial energy of the parent NO2 molecule,
where KE is the total kinetic energy of both products, and
D0 is the dissociation energy. The NO fragments are detected
band of NO. The detection of the J ϭ17/2 channels for the
Љ
NO product was chosen because the population of this level
in the background NO impurities contributed negligibly to
the NOϩ ion yield at the rotational temperature of 5–8 K in
the supersonic expansion. Also, the chosen rotational transi-
tion from J ϭ17/2 corresponds to an isolated line in the
(1ϩ1 ) REMPI ͑resonance-enhanced multiphoton ioniza-
state-selectively in the present experiments, fixing E( ,J ),
v
Љ Љ
and thus the quantity Eavl is defined here as the available
excess energy for distribution among the kinetic energy of
the products and the spin–orbit energy of the O atom,
Љ
Ј
EavlϭKEϩE J͒ϭh␯ϪD ϪE
,J ͒.
͑3͒
͑
͑v
Љ Љ
Љ Љ
netic energy of the products, KE, will be equal to Eavl
0
˜
tion͒ spectrum via the A state and, as shown below, the broad
For any detected quantum state ( ,J ) of NO the total ki-
MATI peaks converging to the various accessible ionic
thresholds of NO are widely separated. Therefore, high-n
Rydberg states with a single core angular momentum and
narrow range of n could be tagged when probing the frag-
ments.
v
,
E
avlϪ158 cmϪ1 and
E
avlϪ226 cmϪ1 for the NO
(
,J )ϩO(3P ), Jϭ2,1,0 channels, respectively.
v
Љ Љ
NO2 photodissociation was chosen as the system to test
J
the octopolar device, building on the previous measurements
of angular distributions for near-threshold photodissociation
of this molecule in our laboratory. The state-selected Ryd-
berg tagging of NO was demonstrated in previous
experiments,3 guaranteeing the long lifetimes necessary for a
successful application of the octopole. Furthermore the pho-
todissociation anisotropy parameters, of importance in the
present work, were investigated in detail in Ref. 3 for the
channels studied here.
The linearly polarized output of a frequency-doubled,
Nd:YAG-pumped optical parametric oscillator ͑Quanta Ray
GCR 190/MOPO 730, 10 Hz, 8 ns pulse, ϳ2 mJ per pulse͒,
acts as the tunable photolysis source in the near-threshold
region at ␭ϳ390 nm. The photolysis laser is weakly focused
and has a linewidth ⌫photϷ0.3 cmϪ1. The frequency-tripled
output of a Nd:YAG-pumped dye laser ͑Spectron 4000G/
SL800, Pyridine-1/DCM dye mixture, ϳ226 nm, 300 ␮J
per pulse͒ is used to excite the desired NO fragment mol-
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7926 J. Chem. Phys., Vol. 115, No. 17, 1 November 2001
Monti et al.
ecules to the A state. A second, frequency-doubled Nd:YAG-
pumped dye laser ͑Spectron 4000G/SL800, DCM dye, ϳ326
nm, 4 mJ per pulse͒, is used to excite from the A state to the
high-n Rydberg states. The probing laser beams are intro-
duced coaxially and are perpendicular to both the molecular
beam axis and the photolysis laser beam. They are gently
focused by a cylindrical fused-silica lens (fϭ15 cm͒. The
laser providing excitation to the high-n Rydberg states is
constantly monitored by a pulsed wavemeter ͑Burleigh WA
4500͒ so as to correct for any wavelength drifts in the course
of the experiments. This ensures that, throughout the data
acquisition, only states within the selected range of principal
quantum number n are excited.
The photodissociation and fragment excitation take place
in Region I between the skimmer plate and a fine-mesh elec-
trode, both kept at ground potential. The fragment cloud
travels into Region II at the beam velocity, where it passes
centrally through the octopole. A positive voltage pulse,
ϩU8p , of 4 ␮s duration and 50 ns rise time is applied to
every second octopole rod, while a carefully matched nega-
tive voltage pulse, ϪU8p , is applied simultaneously to the
remaining rods. A small constant repelling field is applied
across Region II to suppress any signal due to prompt ions or
NO Rydberg states field ionized in the octopole. The frag-
ments that are not ionized by the discrimination or octopole
fields in Region II, enter Region III, where a 200 V/cm elec-
tric field pulse is applied. The Rydberg-tagged NO molecules
are field-ionized by the pulse and this field serves also to
accelerate the fragment cloud towards the detector. The elec-
tric field pulse is carefully timed so as to ionize the whole
fragment sphere.
The ionized fragment cloud is accelerated further by a
stack of dynodes and crushed onto a microchannel plate de-
tector ͑Gallileo, 40 mm diameter͒ coupled to a phosphor
screen. The total ion signal can be detected by a photomul-
tiplier tube, amplified, averaged over typically ten shots in a
boxcar integrator, and fed via an analog–digital converter to
a computer. In the imaging mode, the phosphor screen output
is recorded by a fiber-optically coupled CCD camera ͑Prox-
itronic HR0, 768ϫ576 pixels͒. The captured frames are sent
to a frame grabber ͑Coreco Ultra II͒ and the acquisition is
controlled from a PC. Images are averaged over 20 shots
using standard image acquisition and analysis software ͑Visi-
log 5, Noesis S.A.͒ and accumulated over typically 300
cycles. Fewer than 100 ions are obtained per laser shot. Ow-
ing to the low kinetic energy of the fragments, the probing
lasers do not need scanning over the Doppler profile during
image acquisition. The acquired images are centered, sym-
metrized and filtered prior to Abel inversion,6 which gener-
ates the original three-dimensional fragment distribution.
FIG. 3. Overview MATI spectrum of nascent NO (JЉNOϭ17/2) produced by
near-threshold photodissociation of NO2 ͑excitation via N ϭ8 as shown in
Ј
Fig. 2͒.
V/cm is employed to reject prompt ions and no voltage is
applied to the octopole. The spectrum shows a group of
broad features associated with the field ionization of high-n
Rydberg series converging to the various ionic thresholds,
Nϩϭ5 to 10, accessible from N ϭ8.3,7 The Rydberg series
Ј
converging to the Nϩϭ10 threshold corresponds to the high-
est series accessible with significant oscillator strength. The
sharp peaks within the Nϩϭ5 to 9 broad features, for ex-
ample, the peak at 74710 cmϪ1 or the series in the range
74730–74760 cmϪ1, are due to channel interactions between
the Rydberg states converging on a higher threshold ͑higher
Nϩ, lower n) and the detected high-n pseudo-continuum.8
The intensity associated with the Nϩϭ9 threshold is derived
almost entirely from such interactions. The Nϩϭ10 feature
is the only one not to show obvious perturbations due to
these channel interactions, and it is therefore ideally suited
for characterizing the octopole. However, in some of the ex-
periments shown below it was necessary to use the Nϩϭ8
MATI peak in order to gain sufficient transition intensity.
The substructure in the main Nϩϭ8 feature reveals strong
final-state interactions with the series converging to the Nϩ
ϭ9 and Nϩϭ10 thresholds. The Rydberg series, simulated
¯
with an average quantum defect of ␦pϭ0.7, are indicated in
Fig. 4.
C. Potential and field of an octopole
An ideal multipole with 2n poles at potential ⌽0, is
defined as one for which the electric potential ⌽(r,␸) and
field magnitude F are given in a plane perpendicular to the
axis by9
B. MATI spectrum of NO
Figure 3 shows an overview of the (1ϩ1 ) MATI spec-
Ј
trum of NO produced by photolysis of NO2 just a few cmϪ1
above the dissociation threshold for the channel detected.
ˆn
⌽ r,␸͒ϭ⌽ r cos n␸,
͑4͒
͑
0
The NO (J ϭ17/2) fragments are excited via the A ( Ј
vNO
Љ
⌽
0
ជ
ˆnϪ1
nr ,
ϭ0, NЈ ϭ8) intermediate with the J ϭ17/2 and 15/2 lev-
Ј
NO
Fϭ
͉
Ϫٌ⌽ r,␸͒
͉
ϭ
͑5͒
͑
r0
els unresolved ͑see Fig. 2͒. A discrimination field of 1.8
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