The effect of carbonyl complexation on highly exothermic vanadium oxidation reactions

Article abstract of DOI:10.1016/S0301-0104(00)00259-7

The effects of CO complexation on highly exothermic vanadium oxidation reactions is evaluated. We study the chemiluminescent (CL) reaction products formed when vanadium vapor entrained in Ar or CO is oxidized by O₃ or NO₂. The multiple collision V + Ar + O₃ → VO*(C⁴Σ^-, ⁴Φ, ²X) + Ar + O₂ reactive encounter yields two previously unreported VO excited states, whereas the V + Ar + NO₂ → VO* + Ar + NO reactive encounter populates states up to and including VO* C⁴Σ^-. The multiple collision V + nCO + O₃ reactive encounter would appear to form a VOCO excited state complex, emitting in the region 420-560 nm, via the formation and oxidation of V(CO)₂ viz. V(CO)₂ + O₃ → VOCO* + CO + O₂ and a relaxed VO excited state emitter via V + nCO + O₃ → VO* + nCO + O₂ where the VO excited state excitation is mediated by V-CO complexation. In complement, the much less exothermic V-NO₂ encounter displays an emission which, in concert with previous studies of CO complexation, suggests the formation of a VO(CO)₂ excited state complex viz. V(CO)₂ + NO₂ → VO(CO)₂/* + NO. The experiments characterizing CL are complemented by comparative laser-induced fluorescence studies of the VO X⁴Σ^-CO and Ar interactions and their influence on the VO C⁴Σ^-X⁴Σ^- laser-induced excitation spectrum. These studies, in conjunction with further attempts to excite LIF in the 420-560 nm region, suggest that the observed complex emissions result primarily from VO excited state interactions. Complementary time-of-flight mass spectroscopy of vanadium and vanadium-oxide-carbonyl complex formation demonstrates the formation of V(CO), V(CO)₂, V₂(CO), and VOCO, the latter three of which demonstrate clear metastable-ion dissociation peaks for the processes VOCO⁺ → V⁺ + CO₂, V(CO)₂/⁺ → V⁺ + 2CO, and V₂(CO)⁺ → V₂/⁺ + CO, suggesting that these vanadium complexes when formed in a reaction-based environment may be photodissociated with light in the visible and ultraviolet regions. (C) 2000 Elsevier Science B.V.

Full text of DOI:10.1016/S0301-0104(00)00259-7

Chemical Physics 260 (2000) 367±382
www.elsevier.nl/locate/chemphys
The e€ect of carbonyl complexation on highly exothermic
vanadium oxidation reactions
M.J. McQuaid, James L. Gole *
School of Physics, Georgia Institute of Technology, Atlanta, GA 30332-0430, USA
Received 10 July 2000
Abstract
The e€ects of CO complexation on highly exothermic vanadium oxidation reactions is evaluated. We study the
chemiluminescent (CL) reaction products formed when vanadium vapor entrained in Ar or CO is oxidized by O3 or
Ã
4
2
NO2. The multiple collision V ‡ Ar ‡ O3 ! VO ꢀC4R , U, X† ‡ Ar ‡ O2 reactive encounter yields two previously
unreported VO excited states, whereas the V ‡ Ar ‡ NO2 ! VOà ‡ Ar ‡ NO reactive encounter populates states up to
and including VOà C4R . The multiple collision V ‡ nCO ‡ O3 reactive encounter would appear to form a VOCO
excited state complex, emitting in the region 420±560 nm, via the formation and oxidation of VꢀCO† viz.
2
VꢀCO† ‡ O3 ! VOCOà ‡ CO ‡ O2 and a relaxed VO excited state emitter via V ‡ nCO ‡ O3 ! VOà ‡ nCO ‡ O2
2
where the VO excited state excitation is mediated by V±CO complexation. In complement, the much less exothermic V±
NO2 encounter displays an emission which, in concert with previous studies of CO complexation, suggests the for-
Ã
mation of a VOꢀCO† excited state complex viz. VꢀCO† ‡ NO2 ! VOꢀCO† ‡ NO. The experiments characterizing
2
2
2
CL are complemented by comparative laser-induced ¯uorescence studies of the VO X4R ±CO and Ar interactions and
their in¯uence on the VO C4R ±X4R laser-induced excitation spectrum. These studies, in conjunction with further
attempts to excite LIF in the 420±560 nm region, suggest that the observed complex emissions result primarily from VO
excited state interactions. Complementary time-of-¯ight mass spectroscopy of vanadium and vanadium-oxide±carbonyl
complex formation demonstrates the formation of V(CO), V(CO)2, V2(CO), and VOCO, the latter three of which
‡
‡
‡
‡
demonstrate clear metastable-ion dissociation peaks for the processes VOCO ! V ‡ CO2, VꢀCO† ! V ‡ 2CO,
2
‡
‡
and V2ꢀCO† ! V2 ‡ CO, suggesting that these vanadium complexes when formed in a reaction-based environment
may be photodissociated with light in the visible and ultraviolet regions. Ó 2000 Elsevier Science B.V. All rights re-
served.
1. Introduction
processes [1]. However, while there has been con-
siderable research on the volatile multiply ligated
metal carbonyls, relatively little experimental work
has focused on the basic metal monocarbonyl, M±
CO, binding energies and MCO spectroscopy is
sparse [2,3]. The M(CO)x systems also play an
important role in organometallic chemistry where
again one ®nds the strongest focus on heteroge-
neous catalysis [1±3] and the in¯uence of the
M±CO bond strength. In fact, the most celebrated
Transition metal complexes have been the focus
of a number of studies primarily because of the
important role of metal±CO bonding in catalytic
* Corresponding author. Tel.: +1-404-894-5201; fax: +1-404-
894-9958.
E-mail address: jim.gole@physics.gatech.edu (J.L. Gole).
0301-0104/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved.
PII: S0301-0104(00)00259-7

368
M.J. McQuaid, J.L. Gole / Chemical Physics 260 (2000) 367±382
and studied gas±surface interaction continues to be
that of carbon monoxide on metal surfaces [4,5].
The metal surface±CO interaction has been the
subject of innumerable studies employing a wide
variety of experimental [2] and theoretical methods
[3±5]. These studies have been, in large part, a re-
sponse to the industrial versatility of methods
running the gamut from methanol formation [6] to
information and impetus for expanding these
studies.
The present study contributes to this database
as we assess the e€ects of CO complexation on
highly exothermic vanadium oxidation reactions.
We study the chemiluminescent (CL) reaction
products formed when vanadium vapor entrained
in Ar or CO is then oxidized by O3 or NO2. For
entrainment in argon, all observed CL emissions
are ascribed to vanadium monoxide, as features
involving two previously unreported VO states are
characterized. For entrainment in CO, the CL
emissions from the vanadium±ozone system dis-
play a series of distinctly di€erent features which
appear to be associated with emission from a VO±
CO excited state complex. Similar pronounced
changes are also noted for the much less exother-
mic V±NO2 system; however, it is suggested that
the observed features indicate the formation of a
VO(CO)2 excited state complex. The experiments
characterizing CL are complimented by compara-
tive LIF studies of the VO X4R ±CO and Ar in-
teraction which suggest the signi®cant role which
VOñCO interactions play in the excited complex
emission process. Finally, time-of-¯ight (TOF)
mass spectroscopy is used to characterize meta-
stable state formation and the nature of the vana-
dium and vanadium-oxide±carbonyl complexation
processes.

the Fisher±Tropsch synthesis [7,8], to the selective
transition metal oxide promotion of catalytic sites
for CO hydrogenation [5]. Although a large num-
ber of theoretical papers concerning the electronic
and bonding properties of these species now can be
found in the literature [2,3,5], data on metal and
metal oxide±carbonyl interactions in the gas
phase, to which this paper contributes, is still
sparse [2,9±11]. The available experimental results
come primarily from matrix isolation spectros-
copy. Here, matrix infrared absorption data has
been obtained for the ®rst row transition metal,
M±CO interaction [12], and for V(CO)x [13],
x ˆ 1±5. ESR studies [14] have also been presented
for V(CO)x, x ˆ 1±3. The Ni±CO [15], Cu±CO [16],
and Pd±CO [17] systems have been studied more
or less systematically [18].
Considerable theoretical and some experimental
work has been directed to structural determina-
tion, the qualitative evaluation of binding energies,
and the determination of relative reaction proba-
bilities for CO adsorbed on small metal clusters
[19]. The envisioned [19] simplicity of small metal
clusters, compared with the complexities inherent
in treating a metal surface, has been one of the
prime motivating factors for this theoretical work,
nickel being the focus of much attention. Rich-
ardson and Bradshaw [20] have determined the
vibrational frequencies of the Ni5CO cluster as a
model for on-top surface site interaction and
the Ni6CO complex as a means of modeling the
bridge surface site interaction. Parra and Micha
[21] have recently used nine and 14 metal atom
clusters to carry out similar calculations. Perhaps
the most detailed studies to date have been those
of Barnes and coworkers [3,22] who have studied
ScꢀCO†xˆ1;2, TiꢀCO†xˆ1;2, VꢀCO†xˆ1;2, and NiCO
[22] complexes to evaluate the MCO and M(CO)2
ligand bonding. Data on metal and metal oxide
carbonyl interactions can provide the necessary
2. Experimental
The purpose of these experiments has been to
gain information on vanadium carbonyl complex
formation and its e€ect on highly exothermic va-
nadium oxidation processes. We establish a com-
parison between the CL emission resulting from
these highly exothermic vanadium atom oxidation
processes and that resulting from notably less
exothermic VꢀCO† complex oxidations. We con-
x
sider the vanadium oxide related CL reactions
observed when argon- or CO-entrained vanadium
atoms are oxidized viz.
V ‡ NO2 ‡ Ar ! VOà ‡ NO ‡ Ar
V ‡ O3 ‡ Ar ! VOà ‡ O2 ‡ Ar
ꢀ1†
ꢀ2†

M.J. McQuaid, J.L. Gole / Chemical Physics 260 (2000) 367±382
369
2200 K. This produced near-e€usive ¯ux condi-
tions. These experiments were run primarily using
high-density graphite crucibles (micromechanisms
T6). For the vanadium oxidation studies, the sys-
tem was brought to speci®ed temperature for at
least 30 min to aid in the removal of a vanadium
oxide outer coating from the sample being va-
porized. This procedure also served to remove any
additional lower temperature vaporizing contami-
nants. Source temperatures were monitored with a
disappearing ®lament optical pyrometer (Leeds
and Northrup 8636C).
V ‡ NO2 ‡ nCO
! VꢀCO† ‡ NO2 ‡ ꢀn x†CO
ꢀ3a†
ꢀ3b†
ꢀ3c†
x
VꢀCO† ‡ NO2 ‡ ꢀn x†CO
x
! VOà ‡ NO ‡ nCO
VꢀCO† ‡ NO2 ‡ ꢀn x†CO
x
! VOÃꢀCO† ‡ NO ‡ ꢀn x†CO
x
V ‡ O3 ‡ nCO ! VꢀCO† ‡ O3 ‡ ꢀn x†CO ꢀ4a†
x
The CL appeared as a well-de®ned, conical,
¯ame which was ꢁ1 cm in diameter at its base and
from 1 to 5 cm in height. When established, the CL
¯ame for the vanadium±argon±ozone or NO2
systems ranged from whitish blue to yellow±green.
The CL, monitored at total background pressures
ranging from 1.5 to 8 Torr, was observed at 90°
and recorded photoelectrically through a viewing
port capped by a fused quartz or pyrex window.
The viewing port was equipped with light ba‚es to
minimize the amount of blackbody radiation
reaching the detection system. The CL was focused
onto the entrance slit of a 1 m scanning mono-
chromator (Spex 1704), operated in ®rst order with
a Bausch and Lomb 1200 groove/mm grating
blazed at 500 nm. For these experiments, a dry-ice-
cooled EMI9808 photomultiplier tube (PMT),
operated at 1500 V, was used. The PMT out-
put signal was recorded by a fast picoammeter
(Keithley) and sent to an appropriate recording
VꢀCO† ‡ O3 ‡ ꢀn x†CO
x
! VOà ‡ O2 ‡ nCO
ꢀ4b†
VꢀCO† ‡ O3 ‡ ꢀn x†CO
x
! VOÃꢀCO† ‡ O2 ‡ ꢀn x†CO
ꢀ4c†
x
In complement, we have also studied the e€ect
of argon and CO entrainment on the laser-induced
¯uorescence (LIF) spectrum of the VO C4R ±
X4R band system and we have interpreted the
formation of V(CO), and VꢀCO†2, V2ꢀCO†, and
VOCO metastable complexes formed in an en-
trainment ¯ow supersonic expansion and charac-
terized in a TOF mass spectrometer.
The experimental con®guration used for the CL
studies has been described in detail elsewhere
[2,23,24]. Brie¯y, all experiments were carried out
using a single vacuum chamber-oven con®gura-
tion. This con®guration has been developed to
provide intense, steady, ¯uxes of metal atoms and
molecules [2,23,24]. The entire system was typi-
cally evacuated by a two-stage, rotary vacuum
pump (Welch model 1397), facilitating operation
in a multiple collision pressure regime. For these
experiments, the typical operating pressures ran-
ged from 0.5 to 8 Torr with a baseline (no gas
load) pressure of the order of 20 mTorr. The
pressure was monitored by a Wallace and Tierman
gauge, an MKS Baratron, and/or a thermocouple
gauge.
ꢀ
device. Typical scan rates were 2 A/s. Wavelength
calibration was accomplished with low-pressure
sodium and mercury lamps and the entire system
was calibrated for relative spectral response using
a standard lamp.
Argon entrainment gas (Holox, 99.999%) was
used without further puri®cation. CO (Matheson,
99.5%) was passed over iodine crystals, then
through an activated charcoal trap in order to
extract metal carbonyls which form in its gas cyl-
inder. Ozone was generated utilizing a commercial
ozonizer (Welsbach T-408). The ozone was col-
lected through adsorption onto a silica gel trap
immersed in an ethyl alcohol/dry ice slush bath.
Prior to an experimental run, the trap was evacu-
ated for approximately 15 min at the baseline
A metallic ¯ux was obtained by vaporizing va-
nadium from a specially designed crucible, ®t to a
resistively heated tungsten basket heater (R.D.
Mathis), at temperatures ranging from 1800 to

370
M.J. McQuaid, J.L. Gole / Chemical Physics 260 (2000) 367±382
pressure of the vacuum system to remove residual
oxygen resulting from ozone decomposition.
A schematic of the LIF excitation system
[26,27] is indicated in Fig. 1. The 488 nm output of
ucts of laser vaporization from a vanadium rod,
whose surface was oxide covered, were entrained
in a high-pressure helium ¯ow seeded with CO to
form VꢀCO†x, V2ꢀCO†x, and VOꢀCO† complexes.
x
‡
an Ar laser (Spectra Physics model 171) was used
These complexes, which are formed when a high-
pressure burst of the helium carrier gas is sent
down an entrainment channel undergoing super-
sonic expansion to vacuum, are sent via a molec-
ular beam to the TOF mass spectrometer depicted
schematically in Fig. 2. Here the complex con-
taining molecular beam was ionized using either a
single excimer laser (ArF at 193 nm or F2 at 157
nm) or through a mass-resolved two photon res-
onantly enhanced ionization. The acceleration and
focusing region of the mass spectrometer [28], de-
picted in Fig. 2, consists of three ®eld acceleration
regions created by a repeller (A1), drawout grid
(A2), and ®eld plate (A3). The drawout grid was
maintained at a slightly lower voltage than the
repeller plate, typically 2100±2600 V. The ®eld
to pump a Coumarin 540 dye laser whose output
was tuned over the range of 520±575 nm. A con-
stant dye laser output power was maintained
throughout the scanned wavelength region utiliz-
ing a Spectra Physics model 373 light stabilizer.
The tunable dye laser beam entered the vacuum
chamber depicted in Fig. 1 (containing the vana-
dium oven) crossing the ¯ow from the oven source
at a right angle. The sample was not treated to
remove the oxide and, in fact, was exposed to
oxidant at lower temperature to enhance the ox-
ide concentration. The resulting LIF from the
VO C4R state was monitored by a blank photo-
multiplier tube (Hamamatsu R446) mounted on
the chamber as shown. Typical dye laser powers
were between 50 and 100 mW over the entire range
of the scan. To maximize the collection of the LIF,
a 2.5 in. focal length lens was used to focus the
image of the ¯uorescence zone directly onto a slit
in front of the phototube.
V±CO complexes were also prepared using a
laser vaporization cluster source which has been
described in detail elsewhere [25±28]. The prod-
Fig. 1. Experimental con®guration used for LIF studies of
oven produced molecules. A dye laser is used to pump mole-
cules produced by the oven source into excited electronic states.
The resulting ¯uorescence is imaged onto a slit and detected by
a photomultiplier tube to obtain an excitation spectrum.
Fig. 2. TOF spectrometer ion acceleration and focusing region
used to characterize vanadium atom and dimer complexation
with carbon monoxide.

M.J. McQuaid, J.L. Gole / Chemical Physics 260 (2000) 367±382
371
plate was grounded. In this manner, the ®rst ac-
celeration ®eld (labeled E1 in Fig. 2) was held at a
signi®cantly lower value than the second acceler-
ation ®eld (labeled E2). The grounded mesh A3
was used to prevent vertical acceleration electric
®elds from entering the ¯ight tube. Following
photoionization, created ionic clusters experience
a force F ˆ qE, where q is the charge on the cluster
(typically one electronic charge) and E is the
electric ®eld strength. Formed ionic clusters are
accelerated intact into the ¯ight tube or, in the case
of weakly bound species, are found to dissociate in
acceleration yielding the signature of a metastable
mass peak.
3. Results and analysis
Fig. 3. Comparison of CL emission spectra for V ‡ CO ‡ O3,
V ‡ Ar ‡ NO2, and V ‡ Ar ‡ O3 reaction systems.
3.1. Chemiluminescent reactions
Ar entrainment and the emission recorded again to
ensure reliable comparisons. In this way, we were
able to reliably assess di€erences induced in the CL
by CO interactions and complexation with vana-
dium and vanadium oxide.
With a vanadium oven temperature in the range
1800±2200 K, the CL intensity from the vanadium
reactions of interest was maximized by adjusting
entrainment gas and oxidant ¯ow rates to obtain
the best S=N ratio.
Fig. 3 presents a comparison of the CL emis-
sions for the V ‡ O3 ‡ Ar, V ‡ NO2 ‡ Ar, and
V ‡ O3 ‡ CO systems. The V ‡ Ar ‡ O3 system
clearly demonstrates a UV band system which is
not present in the much less exothermic V ‡
NO2 ‡ Ar or V ‡ O3 ‡ CO systems. The longer
wavelength emission from the ozone system cor-
responds to the VO C4R ±X4R transitions easily
recognizable by comparison with the data ob-
tained for the V ‡ Ar ‡ NO2 system; however, the
e€ects of the enhanced reaction exoergicity are
clearly apparent. The UV features observed in the
ꢀV ‡ O3 ‡ Ar† system are considered to result
from VO and correspond to two new band sys-
tems. The population of the VO C4R state in the
ozone-based systems is clearly a€ected by the pres-
ence of CO entrainment, interaction, and com-
plexation.
In order to develop comparisons between the
optical signatures for the V ‡ Ar ‡ OX and V ‡
CO ‡ OX systems, experimental conditions (oven
temperature, entrainment and oxidant gas ¯ow
rates, etc.) were ®rst established with Ar entrain-
ment and the CL emission spectrum recorded.
CO was then gradually introduced as the entrain-
ment gas while reducing the Ar ¯ow. This was
done to maintain the total background pressure
and oven temperature. For those experiments in-
volving CO entrainment, the temperature of the
entrainment gas was found to play an important
role in the modi®cation of the optical signatures
associated with the argon-based systems under
consideration. This was observed visually as a
change in the ¯ame color from whitish blue to
deep purple. The most dramatic changes were
obtained by cooling the CO with liquid nitrogen
via both a brass jacket containing the CO inlet and
a double-pipe heat exchanger connected to the
boundary of the vacuum system. After recording
the emission using pure CO entrainment, the ex-
perimental con®guration was converted back to
3.1.1. V ‡ Ar ‡ NO2 ! VOà (C 4R )‡NO ‡ Ar
Fig. 4 depicts the CL emission spectrum ob-
served when vanadium vapor is entrained in Ar
and subsequently oxidized by NO2. The emission

372
M.J. McQuaid, J.L. Gole / Chemical Physics 260 (2000) 367±382
oxidation of vanadium vapor entrained in argon.
In conjunction with the supplementary informa-
tion we have obtained from LIF studies of VO
under the same entrainment conditions, the dif-
fering CL spectra provide information on the na-
ture of CO complexation with vanadium atoms
and vanadium oxide molecules to form V(CO)x
and VO(CO)x adducts.
Fig. 3 compares the CL signatures for (1) va-
nadium vapor entrained in argon and oxidized by
O3 or NO2, and (2) vanadium vapor entrained in
CO and oxidized by O3. The analysis presented in
Fig. 4 suggests that the resolved features in the
argon-entrained systems are associated with
emission from VO. However, the spectra obtained
when vanadium vapor is entrained in CO and
oxidized by O3 is signi®cantly di€erent from either
of the argon-entrained systems. With CO en-
trainment, the UV features observed in the
V ‡ O3 ‡ Ar system are almost completely quen-
ched, the VO C4R vibrational excitation, as in-
dicated by the starred features in Fig. 5, is clearly
diminished, and a new vibrational progression
appears in the range from 420 to 560 nm. As Fig. 5
clearly demonstrates through a comparison of the
V ‡ O3 ‡ CO and V ‡ NO2 ‡ Ar systems, this
progression would appear to correspond to some
of the features of the VO C4R ±X4R transitions
(marked with a star) which result from the con-
siderably diminished VO vibrational excitation
corresponding to Dv ˆ ‡1, v0 ˆ 1, Dv ˆ ‡2, v0 ˆ 2
and Dv ˆ ‡3, v0 ˆ 3 sequence bands in the range
from 520 to 560 nm. However, in the range from
420 to 520 nm, the features deviate signi®cantly
from the VO C±X band system. This deviation is
clearly shown through comparison of the spectrum
for the V ‡ CO ‡ O3 system with the spectrum for
the V ‡ Ar ‡ NO2 system.
Fig. 4. CL emission spectrum for the V ‡ Ar ‡ NO2 system.
This portion of the spectrum is dominated by emission from the
Dv ˆ ‡1 to ‡4 transitions of the VO C 4R±X 4R band system
with Dv ˆ v0 v00.
features are associated with the VO C4R ±X4R
transitions which have been previously deter-
mined.
The reaction leading to the observed CL spec-
trum is presumably
4
2
Vꢀ F† ‡ NO2ꢀ B1† ‡ Ar
2
! VOà ‡ NOꢀ P† ‡ Ar
ꢀ5†
and the energy available to populate VO excited
electronic states via this reaction is approximately
EintꢀVO† ˆ D00ꢀV±O† D00ꢀON±O†
ˆ 6:44 eV 3:16 eV27
ˆ 3:29 eVꢀ26460 cm †:
1
ꢀ6†
Using the spectroscopic constants for the
VO C4R ±X4R transition, we determine that the
populated v0 ˆ 11 level of the C4R state is 26 030
The diminished vibrational excitation for the
VO C4R excited state which results from the
V ‡ O3 ‡ CO system might result from a signi®-
cant CO collisional deactivation of the VO C4R
state. However, both the radiative lifetime of the
C4R state and the results of experiments, which
we will consider, to characterize the VO C±X LIF
spectrum suggest that this is an unlikely possi-
bility. Rather, the decreased vibrational excitation
most likely results from a decreased reaction exo-
1
cm
above the X4R ground state, in close
agreement with the reaction exothermicity (26 460
cm 1) calculated using Eq. (6).
3.1.2. V ‡ CO ‡ O3; NO2
Not surprisingly, the CL emission observed
when vanadium vapor, entrained in CO, is oxi-
dized with O3 and NO2 is found to be signi®cantly
di€erent from the signature associated with the

M.J. McQuaid, J.L. Gole / Chemical Physics 260 (2000) 367±382
373
Table 1
Observed band centers for spectrum correlated with VOCO
1
Measured band centers
D (cm )
1
(nm)
(cm )
547.7
525.7
505.5
486.5
468.0
450.8
435.2
420.3
18 253
19 016
19 776
20 549
21 361
22 176
22 971
23 785
763
760
773
812
815
795
814
Table 2
Low-lying electronic states of vanadium monoxidea (all values
1
in cm
)
b
T0
xe
xeve
22Dr
D4Dr
b ‡ 9595:8
19 148.1
a ‡ 8127:0
a ‡ 7208:1
17 420.1
b
12 605.6
ꢁ12 430
a
±
±
[835]
±
±
2 2P3=2
1 2Pr
C 4R
±
[927]
863.4
±
±
5.35
±
1 2Ur
B 4Pr
910.9
[ꢁ1000]
[1020]c
[884]
[936]
1011.3
5.0
±
‡
1 2R
1 2Dr
±
A
0 4Pr
0 4Ur
9498.9
7255.0
0
±
A
±
4.86
X 4R
a Adapted from Refs. [29,31].
b Bracketed values are DG1=2
.
Fig. 5. Comparison of CL emission spectra for V ‡ CO ‡ O3,
and V ‡ Ar ‡ NO2 systems.
c Uncertain.
thermicity due to the formation and reaction of a
VCO complex.
transitions associated with the V ‡ Ar ‡ NO2 or
O3 systems is notable since the vibronic widths of
the VO-based features are expected to be narrower
with CO entrainment due to enhanced rotational
relaxation. This is certainly true for those VO
features indicated in Fig. 5 (520±560 nm). Energy
transfer processes between rotational manifolds
[30] (R±R) are typically faster than energy transfer
processes between translational and rotational
manifolds (R±T). As CO has rotational degrees of
freedom while Ar does not, CO should more ef-
fectively relax the rotational manifold of emitting
reaction products, producing narrower vibronic
bands. The lack of structure associated with these
features would be consistent with the formation of
a complex most reasonably attributed to VOCO.
The band centers for the prominent progression
associated with the V ‡ CO ‡ O3 system from 420
to 520 nm are tabulated in Table 1 along with the
energy di€erence between adjacent bands. The
di€erence in energy between adjacent bands is
similar to, but notably lower than any of the
DGꢀ0:5† ꢂ x0 values for the ground and excited
VO states tabulated in Table 2. This observation
was taken to suggest that the emission features
might be associated with a VO±CO complex.
Attempts to more clearly resolve the newly
observed features in the range from 420 to 560 nm
in the V ‡ CO ‡ O3 system were unsuccessful. The
broadness of these features versus the VO C±X

374
M.J. McQuaid, J.L. Gole / Chemical Physics 260 (2000) 367±382
3.2. Laser-induced ¯uorescence of the C 4R ±X 4R
band system
The complexation of CO to VO would increase the
density of states and produce broader bands than
obtained with a a ``simple'' VO emitter.
In order to assess the possibility that CO com-
plexes with VO chemically or quenches electronic
emission via energy transfer, LIF excitation spec-
tra were recorded for the excitation of VO va-
porized directly from a sample and entrained in Ar
or CO at the same pressures and ¯ow rates em-
ployed in the CL experiments. The results of these
experiments are depicted in Fig. 7. The excitation
spectrum corresponds to the ¯uorescence intensity
detected by a PMT/short wavelength pass ®lter
combination while scanning a Coumarin dye laser
through the designated spectral region. The PMT/
short pass ®lter combination is sensitive to radia-
tion in the range from 200 to 560 nm and thus
¯uorescence is detected ``on-resonance''. The fea-
tures at 18 113.3, 18 272.9, 18 944.6, and 19 114.9
In addition to the prominent bands listed in
Table 1, weaker and broader bands at 520 nm,
near 545 and 570 nm are also observed. It is not
certain if these features are associated with the
carrier of the more prominent bands and their
further analysis was not pursued.
3.1.3. V‡CO ‡ NO2
In an attempt to obtain some further insight
into the processes leading to the features observed
in the V ‡ CO ‡ O3 system, two other experiments
were performed. In the ®rst, a comparison was
made of the optical signatures associated with the
V ‡ CO ‡ NO2 and V ‡ Ar ‡ NO2 systems. The
results of this experiment are depicted in Fig. 6.
The VO C4R ±X4R vibronic transitions ob-
served for the V ‡ Ar ‡ NO2 system are appar-
ently quenched and broadened for the V ‡ CO ‡
NO2 system; however, an e€ective sequence posi-
tioning is clearly maintained. The most likely ex-
planation for this observation is the complexation
of vanadium with CO prior to oxidation with a
minimal loss of reaction exothermicity as a VO
excited state based product is formed. In other
words, the results in Fig. 6 are consistent with the
formation and oxidation of a vanadium±carbonyl
complex at the vanadium center with minimal loss
of reaction exoergicity vs. VOÃ formation in the
V ‡ Ar ‡ NO2 system.
1
cm in Fig. 7 correspond to the observed LIF
intensity subsequent to the excitation of the des-
ignated VO C4R ±X4R transitions. The assign-
ments are based on the spectroscopic constants of
Table 2. It can be immediately inferred from the
spectra, that any di€erence between the eciency
of CO and Ar with respect to VO C4R electronic
state energy transfer or quenching processes does
not play a signi®cant role at the pressures em-
ployed for the LIF experiments. Since the CL ex-
periments were carried out at the same pressure
and ¯ow rates as the LIF experiments, these con-
Fig. 6. Comparisons of CL emission spectra for the V ‡
CO ‡ NO2 and V ‡ Ar ‡ NO2 systems.
Fig. 7. Comparison of VO LIF excitation spectra obtained
with CO and Ar entrainment of the vanadium oxide.

M.J. McQuaid, J.L. Gole / Chemical Physics 260 (2000) 367±382
375
sidered relaxation e€ects cannot account for the
changes observed in Fig. 6.
process. This, of course, is consistent with the
observed LIF spectrum obtained for CO-entrained
VO (Fig. 7). The results also suggest an alternative
possibility which we have investigated.
3.3. Time-of-¯ight spectroscopy of vanadium-based
carbonyl complexes
In the course of our TOF study of the V±CO
system, we have observed the signatures of meta-
stable-ion mass peaks, signaling the occurrence of
parent-ion ®ssion processes during the acceleration
which follows laser-induced ionization above the
TOF repeller grid (Fig. 2). The TOF mass spectra
depicted in Figs. 8 and 9, covering the range from
5 to 150 amu, have been analyzed to suggest that
V(CO), V(CO)2, V2(CO), and VOCO metastables
are formed from a supersonic expansion in which a
combination of vanadium and vanadium oxide are
laser vaporized from an oxide-coated vanadium
rod and subsequently entrained in a CO-seeded
helium expansion.
In order to analyze the TOF spectra in Figs. 8
and 9, we have assumed that the laser-induced
ionization of vanadium-based carbonyl complexes
can be followed by their ®ssion, either directly
after ionization and before acceleration, or during
an intermediate phase over the time scale to full
acceleration at the ®eld plate (Fig. 2). The ioniza-
tion region and the maximum acceleration point
Fig. 7 demonstrates that a Coumarin 540 dye
laser operative from 520 to 570 nm excites only the
VO C±X band system when probing a CO-
entrained VO mixture. Attempts to excite an LIF
spectrum corresponding to the features catalogued
in Table 1 and tentatively assigned to VOCO were
also unsuccessful. We have also attempted to as-
sess this tentative VOCO assignment by applying
the mass-resolved resonance-enhanced multipho-
ton ionization technique [25±28]. Using the TOF
system described in Section 2, we attempted to
assign this optical signature using two photon
resonant ionization by monitoring the mass
channel associated with VOCO (95 amu) and
scanning a pulsed dye laser in the range from 420
to 520 nm, providing a second ionizing photon
with an ArF excimer laser. These experiments also
were unsuccessful, suggesting that complexes of
vanadium or vanadium oxide with CO might be
dicult to excite as they are lost in a dissociative
Fig. 8. TOF mass spectrum in the range 50±150 amu for vanadium/vanadium oxide±carbonyl complexes. See text for discussion.

376
M.J. McQuaid, J.L. Gole / Chemical Physics 260 (2000) 367±382
Fig. 9. TOF mass spectrum (a) in the range from 5 to 55 amu for vanadium/vanadium oxide±carbonyl complexes and (b) closeup of
24±36 amu region. See text for discussion.
‡
de®ne a range of metastability for various possible
complexes. This range is de®ned for several com-
plexes and their ®ssion processes in Table 3. For
subsequent ®ssion to V ‡ CO2 should result in a
‡
metastable peak corresponding with V if ®ssion
immediately follows ionization and to a metastable
peak at 27 amu if ®ssion occurs after full acceler-
‡
example, the formation of the VOCO ion and its

M.J. McQuaid, J.L. Gole / Chemical Physics 260 (2000) 367±382
377
Table 3
Metastable ®ssion map
Initially
formed ion
Fission process
Location of mass peak
Fission directly after ionizationa
(amu)
Fission directly after full accelerationb
(amu)
‡
‡
‡
‡
‡
VCO
VOCO
VOCO
VCO ! V ‡ CO
V (51)
V (51)
32.9
27.4c
47.3
79.8
58.3
24.3
95.2
36.5
73.4
65.7
37.1
41.8
‡
‡
‡
‡
VOCO ! V ‡ CO2
‡
‡
‡
VOCO ! VO ‡ CO
VO (67)
‡
‡
‡
‡
V2CO
V(CO)2
V2CO ! V2 ‡ CO
V2 (101.8)
‡
‡
‡
‡
‡
VꢀCO† ! VCO ‡ CO
VCO (79)
‡
2
‡
‡
V(CO)2
V2OCO
VO(CO)2
VꢀCO† ! V ‡ 2CO
V (51)
‡
2
‡
‡
‡
V2OꢀCO† ! V2O ‡ CO
V2O (117.8)
‡
‡
‡
‡
‡
VOꢀCO† ! VO ‡ 2CO
VO (67)
‡
2
‡
‡
VO(CO)2
V2(CO)2
VOꢀCO† ! VOCO ‡ CO
VOCO (95)
‡
2
‡
‡
‡
‡
‡
V2ꢀCO† ! V2 ‡ 2CO
V2 (101.8)
‡
2
‡
‡
‡
V(CO)2
VOCO
VꢀCO† ! VC ‡ CO2
VC (63)
‡
2
‡
VOCO ! VC ‡ O2
VC (63)
a
Initial ionization point, acceleration potential zero, mà ˆ m ˆ mass of ®ssion ion.
‡
b Full acceleration point, mà ˆ m2=m0 with m ˆ mass of ®ssion ion, m0 ˆ initial ion mass.
c
‡
Al contribution considered as laser vaporization and expansion channel constructed from aluminum (see Fig. 9). Laser vaporization
‡
‡
‡
from an aluminum target in a carbonyl-seeded expansion yields only minor evidence for Al(CO) and Al(CO)2 despite an intense Al
peak.
ation in the vicinity of the ®eld plate. Clearly, there
is an intermediate range which suggests that the
features ranging from 27 to 34 amu in Fig. 9 cor-
‡
respond to the ®ssion of VOCO to V ‡ CO2, in
various stages of acceleration of the parent ion and
that the process is dominated by ®ssion at almost
‡
full acceleration. Similarly, the ®ssion of V2CO to
‡
V2 ‡ CO should produce the metastable peak ex-
tending from 79.8 to 101.8 amu depicted in Fig. 8.
The peak at 24.6 amu in Fig. 9 is likely caused by
‡
‡
the ®ssion of VꢀCO† to V ‡ 2CO. The mass
2
spectra in Figs. 8 and 9 would also suggest that
‡
VCO is a reasonably stable ion with only a small
Fig. 10. CL emission spectrum for the V ‡ Ar ‡ O3 system.
probability of ®ssion after ionization. However,
this does not preclude the dissociative removal of
the CO ligand via a highly exothermic oxidation
process.
The sequence structure of previously unobserved UV bands.
observed bands are associated with a suciently
small change in the V±O bond length so as to re-
sult in sequence structure. To second order, the
separation between successive bands in a sequence
is given by [29],
3.4. V ‡ O3 ‡ Ar ! VOà ‡ O2 ‡ Ar ± the UV
band systems
0
00
0
~
00
00
omꢀv ‡ 1; v ‡ 1† mꢀv ; v † ˆ ꢀx0 x00
~
‡ 2x0 Dv† ꢀx0v00 x0v000†ꢀ2v00 ‡ 1†;
ꢀ7†
We have characterized two previously unre-
ported VO band systems not reported in Table 2
[31]. Fig. 10 depicts the CL emission from these
UV systems which appear to be in the range from
318 to 374 nm and from 285 to 318 nm. These
~
where m is the di€erence in energy between the
upper and lower levels of the electronic transition,
and x0 and x0v0 are vibrational constants.

378
M.J. McQuaid, J.L. Gole / Chemical Physics 260 (2000) 367±382
The bands at 330.4, 344.2, and 354.5 nm are
suggestive of Dv ˆ ‡1, Dv ˆ 0, and Dv ˆ 1 se-
vanadium carbonyls, VꢀCO†x, x ˆ 1±5, is relatively
limited.
quences, respectively, where x000 < x00 . Using these
The combination of the results obtained from
the V ‡ CO ‡ NO2 vs. V ‡ Ar ‡ NO2 experiments
and the observed LIF excitation spectra for VO
entrainment suggest that vanadium complexes
with CO prior to oxidation by O3 and NO2 but
that these complexes can be dissociated with visi-
ble or UV radiation. A mechanism for these com-
plexation processes may be written:
~
~
~
peaks as estimates for mꢀ1; 0†, mꢀ0; 0† and mꢀ0; 1†
respectively, estimates of D0Gꢀ0:5† and D00Gꢀ0:5†
can be obtained as
0
DG0ꢀ0:5† ˆ G0ꢀ1† G ꢀ0† ˆ mꢀ1; 0† mꢀ0; 0†
~
~
1
ˆ 1168 cm
ꢀ8†
V ‡ xCO ! VꢀCO†
ꢀ10†
and
x
00
DG00ꢀ0:5† ˆ G00ꢀ1† G ꢀ0† ˆ mꢀ0; 0† mꢀ0; 1†
VꢀCO† ‡ OX ! VO Á ꢀCO† ‡ ꢀx y†CO ‡ X
~
~
x
y
1
ꢀ11†
ˆ 947 cm
:
ꢀ9†
The suggested assignment of this transition to a
4U±4U system terminating on the A Ur state, as
Ishikawa et al. [43] have used the time-resolved
infrared kinetic absorption technique to study CO
recombination reactions involving V(CO)x (x ˆ
3±5, and possibly 2). These researchers found that
the rates of CO recombination with V(CO)x (x ˆ
3±5) for the reactions
0
4
discussed in detail by McQuaid [32], is quite con-
sistent with the exothermicity of the V ‡ O3 reac-
tion.
Like the 318±374 nm system, the violet de-
graded band at 302.4 nm appears to represent the
Dv ˆ 0 vibrational sequence for the 285±318 nm
system in which x000 < x00. The band peaking at
312.5 nm may then correspond to a Dv ˆ 1 se-
quence. Using the band peaks at 302.4 and 312.5
1
‡
VꢀCO† ‡ COꢀ R † ! VꢀCO†
ꢀ12a†
ꢀ12b†
ꢀ12c†
5
6
5
4
1
‡
VꢀCO† ‡ COꢀ R † ! VꢀCO†
4
1
‡
VꢀCO† ‡ COꢀ R † ! VꢀCO†
~
~
nm as estimates for mꢀ0; 0† and mꢀ0; 1†, respectively,
3
and substituting into Eq. (9), we ®nd DG00ꢀ0:5† ˆ
all approach gas kinetic values. Their results sug-
gest that the V(CO)3±5 complexes have low-spin
doublet ground states like the well-studied stable
free-radical V(CO)6. The stepwise recombination
of CO with V and V(CO) has not been observed,
and the results for the reaction
1069 cm 1. This value is 68 cm greater than
1
DGꢀ0:5† for the VO X4R state and more than
1
130 cm greater than any other low-lying VO
quartet state suggesting that this system may in-
volve previously unobserved doublet transitions of
VO. Further discussion is given by McQuaid [32].
1
‡
VꢀCO† ‡ COꢀ R † ! VꢀCO†
ꢀ12d†
2
3
are considered tentative, although an absorption
feature ascribed to V(CO)2 suggests that reaction
(12d) is a slow, spin forbidden process.
4. Discussion
4.1. Rates of complex formation
If the stepwise reaction of CO with V, V(CO),
and V(CO)2 proceeds through ground state (or
low-lying) species as also observed for other metal
carbonyl±CO reactions [44,45], then the following
sequence would be expected
The saturated form of vanadium carbonyl is the
well-known, stable free-radical V(CO)6. This mol-
ecule has been thoroughly studied using infrared
[33,34], MCD [35], ultraviolet (UV) [36], electron
and X-ray di€raction [37,38], ESR [39±41], and
mass [42] spectroscopies. In contrast, experimental
or theoretical work characterizing the unsaturated
4
1
‡
VꢀCO†2ꢀ R; 6R; 4Pg† ‡ COꢀ R †
2
! VꢀCO†3ꢀ A†
ꢀ13†

M.J. McQuaid, J.L. Gole / Chemical Physics 260 (2000) 367±382
379
‡
6
1
4
VCOꢀ R† ‡ COꢀ R † ! VꢀCO† ꢀ R; 6R; 4P†
at the surface of a heated vanadium sample. Based
on these considerations, we postulate that the
dominant species reacting with O3 and NO2 to
yield the emission features observed in Figs. 3 and
6 for the vanadium±carbon monoxide system is
V(CO)2. This species can be rapidly formed and a
bottleneck to higher ligation appears to exist. This
result is also consistent with our observation of
metastable structure in the TOF mass spectrum
(Fig. 9) which we associate with the ®ssion of
2
ꢀ14†
ꢀ15†
‡
Vꢀ F; 6D† ‡ COꢀ R † ! VCOꢀ R†
4
1
6
where the symmetry designations are based on the
studies of Ishikawa et al. [43] and discussions by
Seder and coworkers [44,45] on the iron carbonyls.
The symmetries assigned to VCO and V(CO)3 are
well corroborated. A R ground state for V(CO)
was determined in the ESR study by van Zee et al.
[46] and predicted in the ab initio calculations of
Barnes and Bauschlicher [3]. Van Zee et al. [46],
on the basis of ESR measurements, determined a
low-spin doublet ground state for V(CO)3, as sug-
gested in the transient infrared absorption study
by Ishikawa et al. [43].
6
‡
‡
V(CO)2 .
4.2. Complexation manifest in the V ‡ CO ‡ O3
and V ‡ CO ‡ NO2 systems
Barnes and Bauschlicher [3] have evaluated the
bonding in VCO and V(CO)2 in an extensive ab
initio calculation. These researchers predict (1)
For V(CO)2, general agreement has not been
reached. Barnes and Bauschlicher [3] have pre-
sented convincing arguments that V(CO)2 will
2
‡
6
a R ꢀr1d p2† ground state for VCO with
‡
D00 V±ꢀCO† ˆ ꢀ0:90 Æ 0:30† eV, and (2) a Rg
6
‡
6
have a R ground state with a low-lying excited
4Pg state approximately 0.1 eV higher in energy.
Their calculations predict only a small energy
di€erence between the linear conformation and
\CVC angles down to 140±150°. This ¯at bending
potential may explain the observation of three
forms of V(CO)2 in the matrix IR experiments
reported by Hanlan et al. [13]. The 4Pg state is also
found to have an extremely ¯at bending potential.
Thus, it is considered possible that matrix e€ects
could produce small bends, lifting the degeneracy
ground state for V(CO)2 with D00 V±ꢀCO† ˆ
2
ꢀ1:35 Æ 0:35† eV. The spin multiplicity predicted
for VCO agrees with the ESR results of Van Zee
et al. [46], while the multiplicity predicted for
V(CO)2 does not. Barnes and Bauschlicher present
the argument we have outlined previously which
perhaps reconciles their results with both the ESR
and IR results for V(CO)2.
With the conclusion that vanadium complexes
with CO, prior to oxidation, to form V(CO)2, the
reaction
4
of the Pg state and allowing observation of this
VꢀCO† ‡ O3 ! VO Á COà ‡ CO ‡ O2
ꢀ16†
state in the ESR experiment [46]. Despite some
uncertainty in the assignment of the ground state
of V(CO)2, the dicarbonyl appears to have a higher
multiplicity than V(CO)3, suggesting that reaction
(13) should be spin forbidden and relatively slow.
This suggests a bottleneck to the conversion to
highly ligated V(CO)x (x P 3) species which are not
expected to chemiluminesce.
2
is indicated as a possible source of the unique
emission features which we catalog in Table 1 for
the V ‡ CO ‡ O3 system. In view of this conclu-
sion, it is pertinent to examine the possibility that
the spectrum associated with the V ‡ CO ‡ NO2
system (Fig. 6) corresponds to a vanadium-based
carbonyl complex oxidative emission where the
complex is formed in a manner which parallels
reaction (16).
Within the present framework, we must explain
why the V ‡ CO ‡ O3 system gives rise to the de-
creased excitation of the VO C 4R excited state
and the structured spectrum pictured in Fig. 5 and
catalogued in Table 1, whereas the V ‡ CO ‡ NO2
The data obtained for V(CO)2 suggest that both
6
‡
4
the R and Pg states will be thermodynamically
important. Thus, reaction (14) should be fast. The
signi®cant decrease in the intensity of the UV
transitions observed in Fig. 3 and the signi®cant
quenching of the C±X transitions in Fig. 6 for the
V ‡ CO ‡ NO2 system suggest that reaction (15) is
also fast. These reactions may indeed be initiated

380
M.J. McQuaid, J.L. Gole / Chemical Physics 260 (2000) 367±382
spectrum (Fig. 6) appears to be a partially quen-
ched, manifestly broadened, although very similar
distribution to that of the VO spectrum obtained
for V ‡ Ar ‡ NO2. We suggest that the features
which characterize the V ‡ CO ‡ NO2 system in-
deed correspond to an emitter with a substantial
density of states relative to VO but with a clearly
de®ning VO base. We further suggest that an ex-
planation for the di€erences in the V ‡ CO ‡ O3
and V ‡ CO ‡ NO2 emission features is, in fact,
related to the considerable di€erence in the O3
(1.04 eV [4]) and NO2 (3.16 eV [4]) bond energies.
The V ‡ O3 reactive encounter is of the order of 2
eV more exothermic than the V ‡ NO2 reaction.
For this reason, as V(CO)2 reacts with NO2 to
form an oxide bond, the vanadium carbonyl
structure should be better able to support the en-
ergy provided to the reacting complex as this bond
is formed. In other words, we suggest that the NO2
oxidation proceeds via the reaction sequence
gests that CO and O2 must carry away a
considerable portion of the exothermicity associ-
ated with reaction (16).
4.3. Tentative assignment of the VOCO band
system
The possibility that the band system catalogued
in Table 1 might be attributed to VO2, V2, or V2O
can be readily eliminated and has been discussed
by McQuaid [32]. The proposed VOCO assign-
ment to the bands in Table 1 is consistent with
observations in similar systems. For example, the
complexation of CO with MFn molecules, where
M is a metal atom, has been previously studied in
argon matrices [47]. In these studies, it was ob-
served that the stretching frequency associated
with M±F in an MFn á CO complex was lower than
that for the ``free'' MFn species. For the transition
metal halides: CrF2 (m1 ˆ 654:5 cm 1), MnF2
(m1 ˆ 700:1 cm 1), and NiF2 (m1 ˆ 779:6 cm 1),
complexation with CO reduced the ground state
frequencies by 16.6, 23.5, and 65.7 cm 1, respec-
tively. The stretching frequency associated with
CO, on the other hand, was observed to be higher
in the complex than that corresponding to the free
ligand.
These shifts were understood as a r donation
from the CO to the metal ion without concomitant
back donation into the antibonding p orbital on
CO. This is in contrast to M(CO)x complexes,
where back donation weakens the CO bond, re-
ducing its stretching frequency. The CO ! MF r
donation reduces the oxidative state of the metal
ion and thus the strength of the M±F bond.
Analogous studies on MOÁCO complexes are not
found in the literature. However, the mechanism
proposed to explain MFn ÁCO complexes might
well apply to MOÁCO complexes as well. The
stretching frequency of CO adsorbed on metal
oxide surfaces is observed to be higher than its gas-
phase value [48]. Thus, by analogy, the stretching
frequency of an MO molecule should decrease
upon complexation with CO.
Ã
VꢀCO† ‡ NO2 ! VOꢀCO† ‡ NO
ꢀ17†
2
2
thus leading to a VO-like emission corresponding
to the polyatomic complex VO(CO)2 with little
loss of reaction exoergicity relative to V ‡ NO2 ‡
Ar. This is, of course, in contrast to the proposed
O3 oxidation which reaction (16) suggests ruptures
one of the V±CO ligand bonds dissociatively pro-
ducing a CO in addition to O2. Clearly, the density
of states for a weakly bound dicarbonyl complex
with VO is expected to greatly exceed that for
VOCO. This also represents a consistent explana-
tion for the broadening observed in the V ‡ CO ‡
NO2 vs. V ‡ Ar ‡ NO2 system. We recall that CO,
acting not as a ligand but as a collision partner
should be more e€ective than Ar in relaxing the
rotational manifold of VO, producing sharper
emission features. This is not consistent with the
observation. Thus, the broad features in the V ‡
CO ‡ NO2 spectrum would appear to be due to an
increased density of states associated with V(CO)2
complex formation.
It is also worth noting that the V ‡ CO ‡ O3
system displays clear VO C 4R vibrational struc-
ture consistent with a considerable lowering of the
vibrational excitation in the VO C 4R state rela-
tive even to the V ‡ NO2 ‡ Ar system. This sug-
Finally, we suggest that the complex-based
emission which we associate with the oxidation of
V(CO)2 to form VOCO likely takes place on an
excited state potential and may involve a relatively

M.J. McQuaid, J.L. Gole / Chemical Physics 260 (2000) 367±382
381
[4] J.N. Allison, W.A. Goddard III, Surf. Sci. 115 (1982) 553.

short-lived intermediate observed in consequence
of a short excited state radiative lifetime. The data
in Figs. 8 and 9 and Table 3 suggest that a VOCO
intermediate, once formed, may well rearrange and
dissociate to atomic vanadium and CO2. The ki-
netics of the CO ligation of vanadium would
suggest that both V(CO) and V(CO)2 are readily
formed, consistent also with the data in Figs. 8 and
9 and Table 3. The radiative lifetime of the VO
C 4R state is 415 ns [49]. This means that VOÃ will
undergo between one and ten collisions before
radiating. Our ability to observe the emission
system catalogued in Table 1 may result, as the
excited state lifetime associated with this complex
optical signature is of the order of that for the VO
C 4R state and corresponds to a carbonyl weakly
ligated to the VO C 4R excited state.
[5] T. Hartmann, H. Knozinger, Z. Phys. Chem. 197 (1996)
113 and references therein.
[6] K. Kilar, Adv. Catal. 31 (1982) 243.

[7] F. Fischer, H. Tropsch, Brennst. Chem. 7 (1926) 97.
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5. Conclusion
Evidence has been presented which suggests a
signi®cant di€erence in the emission products of
highly exothermic vanadium oxidation reactions in
the presence of argon and CO. This di€erence is
attributed to the interaction±complexation of va-
nadium and vanadium oxide with CO. Evidence
for V(CO)2 complex oxidation with O3 (to produce
VOCOÃ) and NO2 (to produce VO(CO)Ã2) is out-
lined. This work contributes to our understanding
of V(CO)x (x 6 2) complex formation and interac-
tion where highly exothermic reaction processes
are operative.
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