A REMPI study of styrene(Ar) n , n=4 -12 clusters

Article abstract of DOI:10.1016/S0009-2614(00)00803-4

New experimental spectra for styrene(Ar)_n, n=4-12 clusters have been recorded using the technique of resonant one-color two-photon ionization and time-of-flight mass spectrometry. Transition energies and spectral shifts relative to styrene have been determined and are reported. These spectra are discussed in terms of progressive solvation dynamics.

Full text of DOI:10.1016/S0009-2614(00)00803-4

25 August 2000
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Chemical Physics Letters 326 2000 389–394
www.elsevier.nlrlocatercplett
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A REMPI study of styrene Ar , ns4–12 clusters
n
T.A. Zensen, P. Jarski, J.G. Eaton)
Center for Main Group Chemistry, Department of Chemistry, North Dakota State UniÕersity, Fargo, ND, 58105, USA
Received 25 May 2000; in final form 13 June 2000
Abstract
Ž
.
New experimental spectra for styrene Ar , ns4–12 clusters have been recorded using the technique of resonant
n
one-color two-photon ionization and time-of-flight mass spectrometry. Transition energies and spectral shifts relative to
styrene have been determined and are reported. These spectra are discussed in terms of progressive solvation dynamics.
q 2000 Published by Elsevier Science B.V.
1. Introduction
solvent molecules, in competition with the aromatic
ring. However it does not substantially increase the
polarity of the aromatic compound, as do the amine
or chlorine substituents in aniline and chlorobenzene.
Resonance enhanced multiphoton ionization spec-
An understanding of microsolvation at the molec-
ular level has been a long-term goal of cluster sci-
ence. An important area of this research has been the
study of the interaction of aromatic molecules with
small solvent molecules 1,2 . There are two aspects
to these studies. One is a detailed understanding of
the positions and interactions of the first few solvent
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.
troscopy REMPI and time-of-flight mass spectrom-
w
x
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.
etry TOF-MS have contributed significantly in these
studies. The REMPI technique allows energetic in-
formation about the interaction to be extracted.
Time-of-flight mass spectrometry provides a means
of relating this energetic information to a particular
cluster size, and permits the variation in the energet-
ics with varying cluster size to be studied. Generally,
in these techniques, clusters of the species under
study are produced using either a pulsed or cw
supersonic expansion nozzle source. The clusters are
resonantly excited with pulsed laser light and then
ionized using either the same pulsed laser light source
w
x
molecules 3–5 . Another aspect, however, is the
stepwise evolution of this interaction over the addi-
tion of a sufficiently large number of solvent
w
x
molecules 6–9 . This will give insight into the
variation of interactions as the system evolves to the
solvated state.
Styrene and substituted styrenes have been useful
in several of these studies 10–17 . The unsaturated
side chain provides a site for p interaction with
w
x
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.
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.
one color , or a second pulsed laser two color . We
have recorded one color REMPI spectra of
)
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styrene Ar , ns4–12 clusters using a recently con-
Corresponding author. Fax: q1-701-231-8831; e-mail:
n
structed REMPI-TOF-MS apparatus.
joeaton@badlands.nodak.edu
0009-2614r00r$ - see front matter q 2000 Published by Elsevier Science B.V.
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PII: S0009-2614 00 00803-4

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T.A. Zensen et al.rChemical Physics Letters 326 2000 389–394
2. Experimental
A schematic diagram of this recently constructed
beam apparatus is shown in Fig. 1. In this apparatus,
a cluster beam is produced by expanding a gas
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.
mixture through a pulsed valve General Valve a9
with a 0.8 mm nozzle. Clusters are formed in the
cold, condensation prone region of the expansion on
the high vacuum side of the nozzle. In these experi-
ments, the valve was operated with 1 to 3 atmo-
spheres of argon gas seeded with 1 to 10 torr of
styrene vapor. The pulsed nozzle is operated at 10
Hz, with the valve actually open for about 140 ms to
Ž
400 ms per pulse. This corresponds to an open
duration setting of 290 ms to 550 ms on our general
valve controller. Upon this apparatus becoming oper-
Fig. 1. Schematic diagram of the REMPI apparatus.
Ž .
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.
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Fig. 2. Mass spectra of styrene-argon clusters recorded using varying source conditions: a room temperature 288C styrene, a nozzle open
Ž .
.
time of 200 ms, and a 1.945 ms delay between the pulsed nozzle opening and the laser Q-switch firing, b cold 108C styrene, a nozzle
Ž .
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open time of 380 ms and a 1.475 ms delay between the pulsed nozzle opening and the laser Q-switch firing c cold 108C styrene, a nozzle
open time of 380 ms and a 1.945 ms delay between the pulsed nozzle opening and the laser Q-switch firing. All spectra were recorded with
25 psia of argon as a carrier gas and an ionization wavelength of 266 nm.

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T.A. Zensen et al.rChemical Physics Letters 326 2000 389–394
391
ational, we noted that ;150 ms of the open duration
setting seemed to be needed before the valve actually
opened. This was confirmed by varying the nozzle
open to ionization laser Q-switch delay while moni-
toring styrene ion signal as a diagnostic of the actual
matching angle of the doubling crystal was then set
to maximize the UV output power at that wave-
length. The energy of the tunable UV pulse was
typically 200 mJ with a 5 ns pulse length and a
spectral width of -1 cmy1. The mass spectra in
Fig. 2 were generated using the Nd:YAG 4th har-
.
time length of the expansion pulse. The expansion
chamber is pumped with a Varian 10XX VHS diffu-
sion pump equipped with a water cooled baffle
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.
monic 266nm .
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.
2500 Lrs and achieves a working pressure of
;2=10y5 torr under the above expansion condi-
tions.
3. Results and discussion
The expanded pulse passes through a 1 mm diam-
Mass spectra of styrene clusters produced under a
variety of source conditions are shown in Fig. 2.
When the liquid styrene is held at room temperature
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eter skimmer Beam dynamics 25 mm downstream
of the nozzle and passes into a differentially pumped
spectroscopy chamber. The neutral cluster beam
travels 32 cm in the spectroscopy chamber and en-
ters the extraction region of a 2 stage, Wiley–Mc-
Laren type ion source of a linear time of flight mass
spectrometer. Here, a tunable laser pulse resonantly
excites and ionizes the neutral clusters. The ionized
clusters are extracted perpendicularly into the TOF-
MS, travel 40 cm up the flight tube and impact a 40
Ž
;25 8C, corresponding to an equilibrium vapor
.
pressure of 8 torr and expanded with 1 atmosphere
of argon, styrene clusters predominate Fig. 2a .
Cooling the liquid styrene with cold tap water
Ž
.
Ž
;28C, corresponding to an equilibrium vapor pres-
.
sure of 1–2 torr the styrene clusters are suppressed,
and styrene Ar clusters are produced Fig. 2b . The
styrene Ar cluster distribution can be shifted to
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.
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.
n
Ž
.
n
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.
mm dual microchannel plate MCP detector. The
detector signal is time resolved, digitized and inte-
grated by a LeCroy 9350 digital oscilloscope. Result-
ing mass spectra were transferred to a personal com-
larger cluster sizes by increasing the argon gas pres-
sure or increasing the open time of the pulsed nozzle.
Photoionizing later in the gas pulse by increasing the
delay between the nozzle open and laser Q-switch
also shifts the cluster size distribution to larger clus-
ter sizes, as shown in Fig. 2c.
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puter Macintosh 7200 via GPIB where the ion
intensity data were normalized to the laser pulse
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.
energy and compiled versus wavelength. The spec-
The REMPI spectra of styrene and styrene Ar ,
n
XX
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.
troscopy chamber and TOF-MS is pumped by a 6
ns1–3, near the origin of the S1 S0 electronic
transition, have been recorded and are shown in
Fig. 3. These spectra are similar to previously re-
ported spectra 4,13,14 and were helpful in tuning
and calibrating our new instrument. The styrene Ar
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.
Turbo pump Leybold TMP 361, 350 Lrs which
maintains a pressure below 2=10y6 torr under
operating conditions. The TOF-MS is a Wiley–Mc-
Laren type with a 2 stage acceleration source. Typi-
cal extraction and acceleration voltages were 240 V
and 3760 V, respectively. These conditions produce
mass resolution of mrDm;150.
w
x
Ž
.
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.
and styrene Ar origin transitions are red shifted 32
2
cmy1 and 63 cmy1, respectively, relative to styrene.
Essentially, red or blue shift indicates the differences
between the amount the upper, excited state is stabi-
lized by cluster formation relative to the amount the
ground state is stabilized. If the upper excited state is
stabilized by clustering more than the ground state,
The tunable UV excitation and ionization pulse
used for the REMPI spectra was generated by a
Ž
frequency doubled dye laser Dakota Technologies
.
Inc. pumped by the 2nd harmonic of a Q-switched
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.
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.
Nd:YAG laser Quantel Brilliant B operating at a
pulse repetition rate of 10 Hz. The visible output of
the dye laser was passed through a frequency-dou-
bling crystal mounted on a computer controlled rota-
tional stage. For each wavelength, the angle of the
rotation stage was scanned while the power of the
frequency doubled output was monitored. The phase
the spectrum is red shifted. Thus, in styrene Ar , the
excited state is stabilized by cluster formation 32
cmy1 more than the ground state, while the excited
state of styrene Ar is stabilized 63 cmy1 more
Ž
.
2
than the ground state. These first two argon clusters
of styrene obey the additivity rule 1,14 : that is the
spectral shift varies linearly with the number of
w
x

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T.A. Zensen et al.rChemical Physics Letters 326 2000 389–394
some of this energy must be carried away by the
ionized electron, and not all will remain within the
ionized cluster. However, excess energy could cause
cluster fragmentation, and cluster fragmentation on a
sub-microsecond time scale can lead to spectral con-
tamination of clusters smaller than the fragmenting
parent cluster ion. It is unlikely that fragmentation is
making a significant contribution to the spectral fea-
tures in these spectra. While some features in the
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.
spectra of styrene Ar ns0–3 suggest some frag-
n
mentation, these features are very small and do not
obscure or complicate the spectral assignment.
Fig. 3. The mass resolved resonant 1 color, 2 photon, ionization
Ž
.
,
n
spectra of styrene and styrene Ar
ns1, 2, 3. The zero
y1
Ž
.
wavenumber cm
point corresponds to the styrene origin tran-
sition at 34,764.1 cmy1
.
adatoms. This is reasonable in that it is likely they
occupy equivalent sites which will prevent them
from interacting strongly with each other. Consalvo,
w
x
et al. 14 explained these shifts as argon atoms
adding first above, and then below the plane of the
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styrene aromatic ring. The spectrum of styrene Ar ,
3
w
x
also previously published by Consalvo, et al. 14 , is
more complicated, containing both sharp and diffuse
features. This is likely caused by the presence of
more than one isomer.
These spectra are scanned over the range 0 to
y100 cmy1 relative to the styrene S1 S0 origin
transition at 34,764.1 cmy1. The ionization potential
of styrene is 68,267 cmy1 and styrene Ar is 68,151
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.
cmy1 10 . Since a one color technique is used, two
excitation photons are 1100 to 1400 cmy1 greater
than the ionization potential of the cluster, and up to
this amount of excess energy can be deposited in the
cluster upon photoionization. It should be noted that
Fig. 4. The mass resolved resonant one color, two photon ioniza-
w
x
Ž
.
,
n
tion spectra of Styrene Ar
ns4–12. For the spectra of
Ž
.
Ž
Styrene Ar ns4–8, the upper trace is a higher resolution 1-2
n
y1
cmy1 scan of the main peak. The zero wavenumber cm
.
Ž
.
point corresponds to the styrene origin transition at 34,764.1
cmy1
.

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T.A. Zensen et al.rChemical Physics Letters 326 2000 389–394
393
Ž
.
The REMPI spectra of larger styrene-argon clus-
REMPI spectra of fluorostyrene Ar clusters exhibit
a small blue shift for n(15.
n
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.
ters, styrene Ar , ns4–12, have also been recorded
n
and are presented in Fig. 4. Energies for the origin
transitions observed for these species are listed in
Table 1. These spectra are dominated by a single
band, red shifted from the styrene origin transition
In solvatochromic theory, solvation shifts with
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.
nonpolar ms s0 solvents are related to the differ-
ence between the dipole moments of the solute
ground and excited states, the polarizability of the
solute, and the dielectric constant and refractive in-
by 24–37 cmy1 and with a FWHM of ;50 cmy1
.
w x
Higher resolution scans of several of the clusters
have been included in Fig. 4. The spectra of
dex of the solvent 18,19 . Styrene in the ground
w
x
state has a negligible dipole moment 20 , while the
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.
styrene Ar , ns4–8 clearly contain partially re-
change in dipole moment upon excitation is "
n
w
x
Ž
.
solved features. Shifts and transition energies for
these spectra were measured from the two most
prominent features near the peak of the band.
While all of the bands in the spectra of
0.13 D 21 . A red bathochromic solvation shift is
generally indicative of an increase in the dipole
moment upon excitation. It is reasonable to think that
the dipole moment would increase upon excitation,
due to the increased participation of the ethylenic p
Ž
.
styrene Ar , ns4–12, are red shifted relative to
n
the styrene origin transition, this red shift is essen-
tially constant with increasing cluster size over this
range. The larger argon clusters no longer exhibit
spectral shift additivity. This is similar to spectral
shifts observed in the REMPI spectra of phenylacety-
p system. Effects of the
orbitals with the ring
solvent dielectric constant and refractive index are
often best analyzed by comparison to solvents with
similar values for these parameters and known solva-
tion shifts. Pentane is a solvent with appropriate,
although slightly larger, constants; however the ef-
fect of the difference in molecular geometry between
argon and pentane is more difficult to estimate, and
is not addressed in solvatochromic theory. The solva-
tion shift of styrene in pentane was measured by
UV–Vis absorption to be 335 cmy1. The solvation
shift, adjusted for the slightly smaller dielectric con-
Ž
.
Ž
.
lene Ar and paraxylene Ar clusters by Dao and
n
w
x
nŽ
.
Ž
.
Castleman 6,7 . From styrene Ar to styrene Ar ,
there is a small, gradual shift of several wavenum-
4
8
Ž
.
bers to the red, while styrene Ar
through
9
Ž
.
styrene Ar have shifted back to the blue by about
12
the same amount. This is comparable to the progres-
w x
sion observed by Faltin, et al.
5 in which the
stant of argon, is approximately 250 cmy1
.
Although the spectral shift is relatively constant
over the ns4 to ns12 range observed, it is still an
order of magnitude less than expected on the basis of
solvatochromic theory. Perhaps these cluster beam
spectra are representative of an ensemble of one or
several rigid isomeric structures, which are a poor
comparison with the fluid environment of solutions.
Another possible explanation is temperature. The
internal temperature of the clusters is likely to be
Table 1
Transition Energies and spectral shifts relative to styrene of the
clusters, styrene Ar n, ns4–12. The red and blue indicates the
two small features at the top of the peak of several of the spectra
Ž
.
y1
Ž
Shift cm
Ž
.
Transition
y1
Ž
.
.
energy cm
reltive to styrene
Ž
.
.
.
.
.
Sty Ar
red
blue
red
blue
red
blue
red
blue
red
blue
34732
34743
34728
34742
34728
34743
34728
34739
34725
y32
y21
y36
y22
y36
y21
y36
y25
y38
y27
y27
y27
y27
y27
4
5
6
7
8
w
x
Ž
very low; Consalvo et al. 14 estimate 10 to 15 K
for their comparable spectra. These very cold species
might not be directly comparable to room tempera-
Sty Ar
Ž
Sty Ar
w
x
ture solutions 18 . A more appropriate comparison
for these spectra might be with rare gas matrix
isolated aromatic chromophores.
Ž
Sty Ar
Ž
Sty Ar
Ž
.
.
.
.
Sty Ar
34737
34727
34727
34727
9
Ž
4. Conclusion
Sty Ar
10
11
12
Ž
Sty Ar
We have reported new experimental spectra for
Ž
Sty Ar
Ž
.
styrene Argon , ns4–12 clusters. These spectra
n

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T.A. Zensen et al.rChemical Physics Letters 326 2000 389–394
w x
5
C.A. Haynam, D.V. Brumbaugh, D.H. Levy, J. Chem. Phys.
were recorded using a newly constructed cluster
beam time-of-flight mass spectrometer. Transition
energies and spectral shifts relative to styrene have
been determined from these spectra and are reported.
Essentially, clusters in this size range have origin
transitions red shifted 36–27 cmy1 relative to styrene
monomer.
Ž
.
80 1984 2256.
w x
6
P.D. Dao, S. Morgan, A.W. Castleman Jr., Chem. Phys. Let.
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.
111 1984 38.
w x
7
P.D. Dao, S. Morgan, A.W. Castleman Jr., Chem. Phys. Let.
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113 1985 219.
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M.S. El-Shall, R.L. Whetten, Chem. Phys. Let. 163 1989
41.
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239 1988 83.
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10 J.M. Dyke, H. Ozeki, M. Takahashi, M.C.R. Cockett, K.
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Bowen, Dr. John R. Lombardi, Dr. Kenton Rodgers,
and Dr. John Hershberger for helpful discussion
during the course of this work. Financial support of
the ND-EPSCoR program and North Dakota State
University is gratefully acknowledged.
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