Photodissociation
measurements
Tiz, Vi, Co,‘, and Co;
of bond dissociation
energies:
Larry M. Russon, Scott A. Heidecke, Michelle K. Birke, J. Conceicao, Michael D. Morse,
and P. B. Armentrout
Department of Chemistry, University of Utah, Salt Lake City, Utah 84112
(Received 26 October 1993; accepted14 December 1993)
The bond dissociation energiesof Ti:, V: , Co:, and Co: have been measuredfrom the sudden
onset of predissociation in the photodissociationspectra of these molecules, yielding values of
Di(Tii)=2.435+0.002
eV, D~(V~)=3.140-+0.002 eV, D~(Co~)=2.7651!~0.001 eV, and
D~(Co;)=2.086+0.002 eV. Thesevalues are in good agreementwith values previously determined
from collision-induced dissociation experiments. General criteria for the interpretation of
predissociationthresholdsas bond dissociationenergiesand periodic trendsin the bonding of the 3d
transition metal diatomic neutrals and monocationsare discussed.
I. INTRODUCTION
Investigationsinto the bonding of small transition metal
clusters have been carried out for many years now. Part of
the interest in this field arises from the complex interplay of
opposing forces which are at work in these difficult electronic systems. On one hand, d-orbital contributions to the
bonding can strengthenthe bond. This effect lessensas one
moves left to right across the periodic table becausethe radial size of the d orbitals decreasessignificantly relative to
the radial size of the s orbitals. On the other hand, there is
often an energetic price that must be paid to promote the
constituent atoms to an electron configuration that diabatitally correlates to ground state molecules. This promotion
energy may weaken the adiabatic bond energy relative to the
diabatic bond energy.
Significant studies of the electronic spectraof many neutral transition metal dimers have been performed by using
resonant two-photon ionization (R2PI) spectroscopy.‘-5
From such studies molecular term symbols, vibrational and
rotational constants,and bond lengths have been determined.
Bond strengthshave beenmeasuredby the observationof the
suddenonset of predissociationin R2PI spectra.5-7Knudsen
cell mass spectrometry8p9
has also provided bond strength
information for many such molecules. Together, all of this
information contributes to a better understanding of the
bonding in these complicated systems.
With the advent of resonance-enhanced
photodissociation (REPD) spectroscopy,10-‘similar
2
studies of transition
metal clusters with nonzerochargeare now possible.As yet,
relatively little rotationally resolved spectroscopyhas been
performed, severely limiting the types of information that
can be obtained.Nevertheless,the bond strengthsof charged
specieshave been measuredby the observationof predissociation thresholds.‘0*““3Such thermodynamicinformation is
also accessibleby collision-induced dissociation (CID)‘4-21
studies,although thesetwo methodsdo not always agree.For
example, Lessen and co-workers” recorded a REPD spectrum of Cri and observeda predissociationthresholdat 2.13
eV, while Su et al. measured the bond strength as
Di(Cr;)=1.30+0.06 eV by CID of Crz with Xe.14It is apparent from this difference that a predissociationthresholdis
not always an accurate representation of the true bond
strength,a fact also demonstratedby the observationthat not
all transition metal diatomics exhibit a sharp predissociation
threshold.4
We report here the bond strengthsof Tit, Vl , Co:, and
Co: measuredfrom the sudden onset of predissociationin
REPD spectra.Thesevalues are in good agreementwith values obtainedby Armentrout and co-workers,‘5-18again using
CID with Xe. From these studies and those performed on
neutral species using R2P1, criteria for the assignment of
bond strengthsto predissociationthresholdshave been determined.
The experimental apparatusused in this study is described in detail in Sec. II, and results of the studies of Ti,f ,
V,‘, Co:, and Co: are presentedin Sec. III. In Sec. IV, we
discuss criteria for the interpretation of a predissociation
threshold as an accuratemeasureof the bond strength, and
periodic trends in the bond strengths of homonuclear diatomic moleculesand monocationsin the 3d transition metal
series are also examined.A brief summary is presentedin
Sec. V.
II. EXPERIMENT
This study was performed on a recently completed,jetcooled ion photodissociation apparatus (Fig. 1). The ion
source is a laser vaporization, supersonicexpansionsimilar
in design to that used in the R2PI apparatusof the Morse
laboratories.22A metal target disk is rotated and translated
against a stainless steel vaporization block23mounted on a
magnetically actuated, double solenoid valve.24 Helium at
-10 psig, seeded with a small amount of argon (cl%),
passes through a 3A molecular sieve trap maintained at
-78 “C (dry ice/isopropanolbath), and is used as the carrier
gas.At the approximatepeak of the gas pulse, 248 nm radiation (KrF) from an excimer laser (LambdaPhysik, EMG 101
MSC, 20-40 mJ/pulse) is focused onto the metal disk by a
47 cm focal length lens. The resulting metal plasma is swept
through a clustering region 2 mm in diameter and approximately 3 cm in length. Ion formation in this source is sufficient to preclude the necessity of any secondaryionization.
The vapor then expandssupersonicallythrough a diverging
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Russon et al.: Bond strengths of Ti;, Vi, Co:,
Q
F
A
B
C
nu’ ?iYs%%E
10” Din.
4
pump
J=b
1
6” Difhtak
F
FIG. 1. Schematic of the ion beam photodissociation spectrometer used in
this study. Note that the two flight tubes are shortened to more easily illustrate the details. (A) gate valve, (B) 248 nm excimer laser radiation, (C)
pulsed gas valve, (D) two-dimensional turning quadrupole, (E) WileyMcLaren acceleration assembly, (F) einzel lenses, (G) deflector plates, (H)
dual microchannel plate detectors, (I) reflectron assembly, (J) tunable dye
laser radiation.
nozzle into a low pressureregion (~10~~ Ton; Varian VHS10, 10 in. diffusion pump). The resulting cooling of the internal degrees of freedom is sufficient to produce cobalt
atomic ions with several helium atoms attached(Fig. 2).
The metal vapor is collimated by a 6.5 mm conical skimmer as it enters a differentially pumped extraction chamber
(-2X10m6 Ton; 6 in. diffusion pump). Positive ions are extracted using a two-dimensional turning quadrupole.=In this
device, four stainless steel, quarter-circle rods (7.5 cm long,
3.8 cm radius) are placed at the comers of a squarestainless
steel box (11 cm square,7.5 cm high). Electrodesin opposite
FIG. 2. Mass spectrum of laser-vaporized cobalt entrained in helium carrier
gas. For this spectrum, no argon was seeded into the helium. Iron and water
are incidental impurities.
and Co;
comers are held at the same electrostatic potential while
those in adjacent comers have opposite applied potentials,
referenced to the potential of the surrounding box. Shim
electrodes,positioned betweenthe box and the circular electrodes, help to generatethe requisite hyperbolic equipotential
lines and carry potentials that are intermediatebetween that
of the nearestrod electrode and the surrounding box.
As the various species in the source beam have been
acceleratedto nearly the full supersonicvelocity of the carrier gas, they will be traveling with nearly the same velocity.
Because the species have different masses, however, each
will have a characteristic translational energy. The quadrupole, being an electrostatic device, affects ions of different
energy differently, making it a mass-sensitivedevice in this
application. To overcomethis effect, the box surroundingthe
rods carries a negativevoltage (- - 180 V), thus accelerating
the positive ions as they approach. Because the ions start
with only a few electron volts of energy,the addition of 180
eV greatly narrows the relative energy difference between
ions of different mass. Defining the potential on the box as
V,, the potentials on the circular electrodesare (l.O+-0.8)Vo
and those on the shim electrodes are ( l.0?0.4)Vo. In this
configuration, negative ions are repelled by the potential on
the box, neutral speciespass through largely unaffected,and
positive ions are turned 90”, pass through a single-element,
electrostatic lens (maintained at -- 1350 V), and enter a
Wiley-McLaren time-of-flight (TOF) accelerationregion.26
The TOF source consists of two steel tubes 10 cm in
diameterand 5 cm long. They have thin metal plateswith 1.5
cm holes in the center attachedto one end and are assembled
with their open ends facing each other, separatedby 1 mm.
This assemblyconstitutesthe repeller and draw-out plates in
the Wiley-McLaren scheme.A third piece of 10 cm diam
tube, 3.8 cm long, is left open on both ends and is mounted
1 mm away from the second tube. This shield electrode is
held at ground potential. There is no ground plate per se but
the first element of an einzel lens, which is kept at ground
potential, is about 2.5 cm away from the final element of the
TOF assembly.The repeller and draw-out plates are kept at
ground potential until the ions fill the intervening region, and
are then pulsed to 900 and 750 V, respectively,using a homebuilt circuit with rise times of 800 and 670 ns, respectively.
This voltage is maintainedfor about 8 ms, which is sufficient
for all of the ions to be acceleratedout of this region.As the
experiment is operatedat 10 Hz, there is no problem in returning the plates to ground potential before the next experimental cycle. Following acceleration, the ion beam passes
through two einzel lenses2.5 and 30.0 cm downstreamfrom
the TOF source. In each lens, the outer elementsare 0.8 cm
thick and the center element is 1.6 cm thick. The outer elements are separatedfrom the inner element by 0.8 cm. The
inside diameter of the outer elements is 3.2 cm and that of
the inner element is 4.8 cm. The central element of both
lenses is held at approximately - 1100 V.
The combination of the Wiley-McLaren sourceand einzel lensesallows the ion beam to be brought to a longitudinal
and radial focus in the spectroscopychamber, 1.73 m downstream from the center of the quadrupole.This chamber is
pumpedby a 6 in. diffusion pump (Edwards 160M diffstak),
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Russon et al.: Bond strengths of Ti:, V,+, I&!,
and CO;
I lcol
1
which maintains a pressureof -9X 10m7Torr. Here the ions
Ti;
*Ti’
+ Ti
are irradiated with the output of a tunable dye laser (Lumon9Xits, HD-500) pumped by 308 nm excimer laser radiation
(Lumonics, EX-700 running on XeCl). Typically the laser
radiation counterpropagatesalong the axis of the ion beam.
For higher resolution work it is necessaryto overcome the
Doppler broadening inherent in the system. This is accomplished by directing the laser radiation across the ion beam
path at right angles.In this arrangementthe residual Doppler
width is below the laser linewidth of 0.03 cm-‘.
As the ions exit the spectroscopychamber, they pass
19400
19800
through another einzel lens and horizontal and vertical deEnergy (cm-‘)
flecting plates before entering a reflectron flight tube. The
reflectron is mounted 2.9 m downstreamfrom the center of
the quadrupole.The einzel lens is identical in design to those
FIG. 3. The predissociation threshold of Ti: detected in a photodissociation
previously describedand is also maintainedat about -1100
action spectrum. This spectrum was obtained by scanning the dye laser
V. The stainlesssteel deflection plates are 2.5X3.2X0.6 cm
using coumarin 500. The arrow marks where a congested spectrum rises
with matching pairs mounted 6.3 cm apart. The voltage on
abruptly out of background noise at 19 640 cm-‘, giving a bond strength of
Db(Ti:)=2.435?0.002
eV. The line across the top of the arrow indicates the
each set of plates is centered around 0 V. The first set of
uncertainty.
deflector plates is maintained at about ?20 V in order to
changethe horizontal flight path of the ions for proper flight
through a gridless reflectron assembly.The reflectron is censcanned.In each case, a congestedspectrum arose out of
tered in the secondflight tube, offset from the first flight tube
background
noise apparently caused by collision-induced
by 2.86 cm, so that the deflection angle is slight (1.6”). The
dissociationof the metal cluster ions with residual helium or
secondset of deflector plates changesthe vertical flight path
pump oil molecules.The spectrawere calibrated by collectto adjust for any vertical m isalignmentof the apparatusand
ing an I, absorptionspectrum,while simultaneouslycollectis usually operatedin the range of -C5 V.
ing the experimentaldata, and comparing it to the I, atlas of
The reflectron consists of a stack of 19 stainless steel
Gerstenkomand Luc.~~Where the laser radiation was to the
plates with an outside diameter of 13.3 cm, an inside diamblue of the Z2 atlas, a high-pressureH, cell was used to
eter of 8.9 cm, and a separationof 5.0 m m . The first plate is
Ramanshift the light back into the range of the atlas.At 500
6.35 m m thick and is shorted to ground. The 2nd through
psig, H2 gives a precisely known Q(1) Raman shift of
17th plates are 0.5 m m thick and are connectedin series by
100 kfl resistors. The 17th through 19th plates are shorted 4155.162 cm-1.28As a final correction, the energy scalesof
togetherwith a mesh on the 17th plate to prevent penetration the spectra were shifted to correct for the Doppler shift experiencedby the ions as they approachedthe source of the
of ground potential into the reflectron. A potential of apradiation. This correction rangedfrom 1.68 cm-’ for Co; to
proximately 400 V is applied to the second plate and ap3.33
cm-’ for V:.
proximately 950 V is applied to plate 17 for reflection of the
full ion beam. In order to separateundissociatedparent ions
from fragment ions, the voltages in the reflectron, on the
111.RESULTS
third einzel lens and the deflector plates are reducedby the
ratio of the mass of the fragment to the mass of the parent A. The bond strength of Ti:
(e.g., in the case of a homonuclear dimer, l/2). Fragment
Figure 3 shows a strong, abrupt increasein the Ti+ fragions are then detected at the same time of flight as unfragment signal at 19 640 cm-‘. This is assignedas the predismented parents would be at full voltage on the reflectron. sociation thresholdwith the uncertainty of 2 15 cm-’ arising
Reflected ions are detectedby a dual m icrochannelplate defrom the difficulty of precisely determining the threshold.
tector (Galileo, 3025). Any ions that are not reflectedmay be
Weak, broad features observed below the threshold are the
detected by another dual m icrochannel plate detector result of periodic fluctuations in the parent ion signal that are
(Hamamatsu,F2221-21) mounted directly behind the reflecassociatedwith the rotational period of the sample rotary
tron. The signal is amplified (Pacific Instruments, model
drive mechanism.These source fluctuations contribute to an
2A50 video amplifier, Xl00 gain, 150 MHz) and digitized
uncertainty in the measured threshold. The threshold of
by a 40 MHz digital oscilloscope (Markenrich Corp.,
19 6402 15 cm-’ (2.435+0.002 eV) is in good agreement
WAAGII) mounted in a 386-basedpersonalcomputer (&OS,
with the CID bond strength measurement of
386-20/8). The data are then summed and stored for later
Di(Til)=2.37+0.07
eV,t5 and we therefore assign this
analysis.
threshold as the bond strength of this molecule.
Predissociationthresholdswere obtained by monitoring
In a spectroscopicinvestigation of Ti2,t Doverstil et al.
fragment ion signal intensity as a function of dye laser frereport a lower lim it to the Ti, bond strength as
quency.The purposeof this study was to determinethe bond
Di(Ti,)S1.349 eV. By using the thermochemicalcycle,
strengthsof the moleculesinvestigated,so only thoseregions
D;(M,) + IE(M) =D;(M;) + IE(M,),
(1)
of the spectrum where a threshold was expected were
J. Chem. Phys., Vol. 100, No. 7, 1 April 1994
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4750
Russon et al.: Bond strengths of Ti;, Vi, Co;,
FIG. 4. The predissociation threshold of V; detected in a photodissociation
action spectrum. These data were collected while scanning the dye laser
using exalite 398. The arrow indicates where the predissociation threshold
arises at 25 326 cm-‘, giving a bond strength of 3.140-t-0.002 eV. The line
across the top of the arrow indicates the uncertainty.
this lower limit on the dititanium bond strength,in combination with our measuredvalue of the Ti: bond dissociation
energy and the ionization energy of titanium, IE(Ti)
=6.828 12t0.000 04 eV,29provides a lower limit on the ionization energyof Ti, as IE(Ti.Jb5.742 eV This may be combined with the upper limit on the ionization energy of Ti2
measuredby Doverstil et al., IE(TiJ~6.125 eV, to provide
IE(Ti2)=5.93t0.19 eV. The same thermochemical cycle
then provides D”,(Ti&=1.54+0.19 eV.
B. The bond strength
of Vz
The photodissociationspectrumof Vz shown in Fig. 4 is
relatively weak, making it difficult to pinpoint the predissociation threshold. Nevertheless,an abrupt increasein the V+
fragment signal is evident at 25 326+15 cm-‘, providing
D”,(Vl)=3.140+0.002 eV. This value is in excellent agreement with the CID measurementof Di(V,f)=3.13%0.12
eV.i6 Given the bond strengthof V, measuredfrom a predissociation
threshold
’
R2PI
spectrum,
Di(V&=2.753?0.001 eV,6%d I~(V)=6.740~0.002 eV3’
Equation (1) provides an ionization energyfor divanadium of
IE(V2)=6.353?0.003 eV. This compares very favorably
with a recent direct measurementof IE(V,)=6.356?0.001
eV,31providing strong evidencethat predissociationoccurs at
the thermochemical threshold in V2 and Vl. Employing a
more recent,highly precise value for the ionization energyof
atomic vanadium, IE(V)=54411.7+-0.1 cm-’ (6.746 19
+-0.00001 eV),32the ionization energy predicted for V2 using Eq. (1) and the bond energies of V2 and Vz is
IE(V2)= 6.359? 0.002 eV, which remains within experimental error of the directly measuredvalue.
C. The bond strength
of Co:
In the predissociation threshold of Co;,” there is an
abrupt rise in the Co+ fragment ion signal at 22 300 cm-‘,
and above this energy the spectrum is very congested, as
shown elsewhere.”Although most of the featuresabove this
energy are reproducible, no assignmentwas attempted be-
and Co;
FIG. 5. A photodissociation spectrum of Co; cohected by monitoring the
Co; signal intensity while scanning the dye laser using a 7:3 mix of
rhodamine 590 and rhodamine 610. The arrow marks the predissociation
threshold at 16 82.5 cm-‘, giving a bond strength of 2.086ZO.002 eV. The
line across the top of the arrow indicates the uncertainty.
causeof the complexity of the spectrum.The predissociation
threshold was determinedto lie at 22 30025 cm-‘, providing Di(Col) =2.765 +O.OOl eV The lower uncertainty limit
in this example reflects a sharper, more precisely defined
threshold. This bond energy again compares very favorably
with the CID value of Di(Coz)=2.75t0.10 eV.17
D. The bond strength
of Co;
To illustrate the extension of this technique to larger
clusters, the dissociation energy of Co: was also investigated. Figure 5 displays the predissociation threshold for
Co: dissociating to Co:+Co. The fragment Co: ion signal
measuredin this experiment is slightly weaker than that for
Vz becauseof the lower Co: parent ion signal. A second
difficulty in recording this predissociation threshold results
becauseit occurs in a spectral region where no single dye
lases efficiently. Figure 5 was obtained by scanning light
output from a 7:3 mixture of rhodamine 590 and rhodamine
610 laser dyes, and as a result only covers a limited range.
The threshold and bond strength are assignedas 16825t 15
cm-‘=2.086+0.002 eV. This again agreeswell with the CID
value of Di(Col-Co)=2.04?0.13 eV.‘*
IV. DISCUSSION
A. Assignment
strengths
of predissociation
thresholds
as bond
In addition to the molecules presentedhere, the observation of predissociation thresholdsin R2PI experimentshave
yielded the bond strengthsof TiV,6 TiCo,6 V2,6 VNi,6 Ni2,7
NiPt,2 Pt2,3 and AlNi.33 Other transition metal systems,
NiPd4 and PdPt4for example, do not exhibit sharppredissociation thresholds.From this information, Spain and Morse6
proposedthat in order to interpret a predissociationthreshold
as a measureof the true bond strength certain factors must
hold. First, the molecule must have a large density of electronic states near the ground separatedatom limit, and second, this separated atom limit must generate repulsive
curves. Table I lists the number of relativistic, adiabatic
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100, No.to7,AIP
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RUSSOn et al.: Bond strengths of Tii, V,‘, Co,‘, and Co;
TABLE I. Number of adiabatic Hund’s case (c) potential energy curves evolving from separated atom limits
6 10 000 cm-’ above the ground separated atom limit and bond strengths of selected diatomic transition metal
molecules.
Number of
curves
Di, eV
Molecule
Number of
curves
Dh, eV
Ti;
TiV
TiCo
5114
2186
1388
2.43Sa
2.068b
2.401b
co;
Ni,
NiPd
3458
960
291
2.76Sa
2.042d
...
v;
v2
4772
2711
3.140a
2.753b
NiPt
PdPt
630
159
2.798e
...
VNi
Cr:
1516
222
2.100b
1.30c
pt2
AlNi
392
123
3.14'
2.29g
Molecule
‘This work.
bReference 6.
‘Reference 14.
dReference 44.
[Hund’s case (c)] potential energy curves that arise within
10000 cm-’ of ground state atoms for selected transition
metal diatomic neutrals and cations. Clearly the ions that are
the subject of this report have a very large number of lowlying electronic states, thus fulfilling the first requirement
stated above.
From the ground states of Ti (4~~3d’,~F) and Ti+
(4~3d*,~F),~~ 588 curves are generated.Of those, 441 correlate diabatically to s~$(~Ads(~F)di(~F) attractive curves
while the remaining 147 curves correlate to
s~~sa~d:(“F)d~(~F) repulsive curves. Thus, Tii strictly
follows both of the guidelines outlined above. In the case of
V,’, however, all of the 350 curves evolving from the lowest
separatedatom limit of V (4.~~3d~,~F)-i-V+(3d~,~D),~’correlate diabatically to s~~d~(~F)d~(~D) attractive curves.
This diabatic correlation for both Tii and Vl is based on
guidelines set forth by Armentrout and Simons35which consider the 4s bonding alone. It neglects any contributions of
the d orbitals to the bond and is therefore most useful for
predicting long range attractive or repulsive behavior. The
more efficient mixing of wave functions among crossed
curves as compared to nestedcurves might explain why the
predissociationof Vl does not show as sharp a threshold as
does Ti: . However, Co: has a nearly identical pattern of
diabatic correlations as Vl. Ground state Co (4s23d7,4F)
and Co+ ( 3d8,3F)3” combine to generate588 curves, all of
which correlate to scridi(4F)dff(3F)
attractive surfaces.
Within the first 10 000 cm-’ above the ground state separated atom limit, there are fewer curves generatedthan for
Vi (see Table I), yet the Co: threshold is the strongest of
those presentedin this paper. Figure 6 displays a qualitative
depiction of the predicted pattern of potential energy curve%
evolving from the first several separatedatom limits of Co
and Cot to form Co:. The number of curves is calculated
for the various combinations of separatedatom terms; the
curves themselves are drawn to approach the lowest spinorbit level of each separatedatom limit. The bands drawn
representthe rich density of states formed from these atoms.
These are drawn assuming no d-orbital interaction, as is appropriate for transition metal atoms from the right side of the
3d transition series.
The second rule developed by Spain and Morse6 con-
eReference 2.
‘Reference 3.
gReference 33.
cerned repulsive curves, and was introduced to account for
the lack of strong predissociationthresholdsin the spectraof
NiPd4 and PdPt.4Like Vz and Co:, theseare molecules that
generateonly attractive curves from their lowest separated
atom limits. As shown in Table I, NiPd and PdPt have a
much lower density of states than Co: or V:. This occurs
becausethe high stability of the ‘So(d”) ground term of
palladium greatly limits the density of electronic states in
NiPd and PdPt. The handicap of having no repulsive curves
evolving from the ground separatedatom limit may apparently be overcome in the casesof Co: and V: becausethere
is a sufficient density of electronic states. One might ask
whether repulsive curves are required at all becausethe NiPd
and PdPt molecules may have lacked a sufficient density of
states to fulfill the first requirement.However, another molecule studied by R2P1, AlNi,33 has fewer electronic states
than either NiPd or PdPt (see Table I), but neverthelessdisplays a sharp predissociation threshold, presumably because
of repulsive curves evolving from the Al (s~~~,~P”)
o’Yd’d’ - 294 curves
a’o’d’d’ - 980 CUW~S
=y\
u’dd’d’
s “/
co
- 588 curves
t
Co’
‘F(s’d’) + ‘D(sOd*)
‘F($d’)
+ ‘F(s’d’)
‘F(s’d*) t ‘F(sOd*)
-
‘F(s’d’)
----w--‘F(s’d’)
t”F(s’d’)
+‘F(s’d’)
*F(s’d’) + ‘F(r’d’)
-------
‘F(s’d’)
+ ‘F(sOd’)
$d’
‘ d’ - 588 curves
FIG. 6. Qualitative potential energy curves for Co: illustrating the enormous density of states. The shaded bands represent the great number of
potential curves deriving from each separated atom asymptote. The different
shading patterns are to distinguish states with an even number of s electrons
from states with an odd number of s electrons. The 4F(~1d8) +3F(sods) and
the 4F(s2d7) +‘F(.s’d’) states are nearly, but not exactly, degenerate.
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J. Chem. Phys., Vol. 100, No. 7, 1 April 1994
4752
Russon et a/.: Bond strengths of Ti:, V;, Co;,
+Ni(s1d9,3D) limit. Repulsive curves evolving from the
ground state asymptotecan allow efficient predissociationin
those systemswhere the density of statesis too low for predissociation among nestedcurves to occur to an appreciable
degree.
In the caseof Cr: ,I0 the predissociationthreshold is not
as abrupt as those presentedhere and there are distinct features in the signal above the thresholdthat suggestthere may
be Franck-Condon difficulties in either the excitation or the
predissociation step. Additionally, assuming a “2,’ ground
state with an sold” configuration evolving from ground
state Cr(4s13d5,g7S)+Cr+(3d5,6S), the first strongly allowed optical transition in Cri is to a state that correlatesto
the Cr(4s’3d5,‘S) +Cr+(3d5,6S) excited separated atom
limit, 0.94 eV37 above the ground separatedatom limit.
When the photodissociation threshold is corrected by this
energy,it is in quantitative agreementwith the CID result.14
With these ideas in mind we revise and extend the
Spain-Morse rules as follows. A predissociation threshold
may be inferred to correspondto the bond dissociation energy if (1) The threshold is sharp and well defined, without
evidenceof Franck-Condon difficulties in either the excitation or predissociationstep; (2) Dissociation can occur to the
ground separatedatom limit while preservingthe good quantum numbers,such as a, g/u, and +; (3a) Either the ground
separatedatom limit generatesa suitable number of molecular potential energy surfaces,some of which are repulsive; or
(3b) The ground separatedatom limit generatesa sufficiently
large number of attractive molecular potential energy surfaces to allow weaker predissociationprocessesto dominate.
B. Electronic
configurations
7. 77:
Ti, is known by R2PI spectroscopy’to have a 3Ag,r
ground state indicating that the dug orbital is mixed with the
sag orbital to the extent that it lies higher in energy than the
d rr,, orbitals, resulting in an s uid rrzd aid 8: configuration.
The removal of one electron in order to form the ion leads to
either a 22+(sa~dv~du~)
or a 2Ag(su~dn~dS~) ground
state. Knig&’ has performed electron spin resonancespectroscopy on a sample believed to contain Til but no spectrum was observed,making “2,’ an unlikely assignmentfor
the ground state. This is consistent with the idea that in the
transition metal cations the d orbitals are stabilized relative
to the s orbital, enhancing the interaction between the su
and du molecular orbitals to the point that the dag orbital
lies abovethe d Sg orbital in energy.Assuming a ‘ha,, ground
state for Ti: , the ground level will possessfi=3/2. Allowed
electric dipole transitions can then access ungerude states
with R=1/2, 312, or 512, all of which may be produced by
combining a ground state titanium atom (3Fz) with a ground
state titanium ion (4F3n).As a result, Til can dissociate to
ground state atoms while preserving its value of fl and g/u
symmetry. The 3F(4s23d2) and 4F(4s3d2),34 ground states
of Ti and Ti+, respectively,combine diabatically to form Til
with a su~su~d~(3F)d~(3F)
configuration. On the other
hand, combination of a ground state titanium atom,
3F(4s23d2), with the 4F(3d3), excited state of Ti+, which
and Co;
lies only 907.96 cm-’ higher than the ground state of Tic,
should diabatically form Til without an su: antibonding
electron, resulting in a stronger bond with an
suidi(3F)di(4F)
configuration and more d-orbital electrons available for additional bonding. The ground state of
titanium dimer cation is thereforeexpectedto arise from this
excited separatedatom limit.
2. v-l;
From R2PI data’the ground state of V, is known to be
‘2, arising from an sazdrr;fd4dc$
configuration. Because
of su-da hybridization it is likely that the dug orbital lies
betweenthe drr, and the d6, orbitals in energy.Removal of
one electron to form the cation will most likely result in a
“c, state, regardlessof the relative energiesof the do, and
d Sg orbitals, as long as the dug orbital lies above the dr,
orbitals, which is very probable.Van Zee and Weltner39have
determinedthe ground state of the isoelectronic neutral molecule, TiV, to be 4c-, further supporting the 4Xg ground
state assignmentfor Vl . From this state !J= l/2, and 3/2,
are generated,which can be excited to s1~5/2, states under
dipole selection rules. The ground separatedatom limit of
V(4s23d3,4F3,2)+V+(3d4,5Do)30 yields Hund’s case (c)
states of n=1/2, and 312,) allowing predissociation of
a= l/2, and 312, stateswhile preservingthe values of R and
g/u. The 0=5/2, states, which may be excited from the
4c,(3/2g) would be expectedto provide a secondpredissociation threshold in this experiment.Our inability to observe
a second threshold may indicate complete cooling to a
ground, 42;( l/2,) level. The ground state of vanadium
atom is 4F(4s23 d3) and that for the vanadium atomic ion is
5D( 3d4).%
These may combine to form an
s$gd~(4F)d~(5D)
bond, plus contributions from the
d-orbital electrons.This is consistentwith the postulateof an
suidrriduid
6, “C, ground state for Vz .
3. co,’
The ground state of Co: has been determinedfrom ESR
spectroscopy@to be “2. Possible R values are l/2, 312, or
5/2. An electric dipole transition from the ground state can
then accessstateswith fi=1/2, 312,512,or 712.All of these
n values are generatedby the combination of a ground state
cobalt atom (4s23d7,4F9,2) with a ground state cobalt ion
(3d8,3F4) so there is no a-based restriction to prevent the
states reachedby the optical excitation of ground state Co:
from predissociating to ground state atoms. Becausecobalt
occurs later in the transition metal series where the 3d orbitals are quite contracted, the 3d orbital contributions to the
bonding in Co; should be greatly reduced as compared to
Til and Vz . This effect may be mitigated to some extent by
the contraction of the s orbital, which provides greater
s-electron density between the nuclei for bonding. The
4F(4s23d7) and 3F(3d8)36 ground states of Co and Co+,
res ectively, are perfectly set up for the formation of an
s J gdi(4F)di(3F)
bond by the donation of the two s electrons of Co into the empty s orbital of Co+.
J. Chem.
Phys., Vol. subject
100, No. to7, AIP
1 April
1994
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see http://ojps.aip.org/jcpo/jcpcr.jsp
Russon et al.: Bond strengths of Ti;, Vl, Co:,
C. Promotion
TABLE Ii. Bond strengths and promotion energies of diatomic transition
metal ions and neutrals. All values in eV.
Measured 0; ’
Molecule
2.435(2)
1.54(19)
3.140(2)
2.753(l)’
1.30(6)h
l&3(56)’
31.4
0.3(3)k
2.74(10)’
1.14(l)”
2.765(l)’
0.7-1.4O
2.08(7)q
2.042(2)’
1.84(8)’
2.03(2)”
Tif
-I$
V;
V?
C$
Cr2
Mni
Mn2
Fe;
Fez
coy
co2
Nif
Ni,
cu;
cu2
4x
energies
In order to understand the bonding among transition
metal dimers, it is important to consider the expectedextent
of d-orbital interactions and the magnitudeof the promotion
energiesinvolved. In many cases,the ground state configuration of the molecule does not correlate diabatically to
ground state atoms. Such promotion effects often occur in
the neutral specieswhere the atomic ground state configuration is usually s2dn. Notable exceptionsalong the 3d transition metal series are Cr(s’d’) and Cu(s’d”). Atoms with
sldn+ 1 configurationsare expectedto form a strongly bound
said nt Id”+’ molecular configuration. In the case of the
monocations, the usual ground atomic configuration is
sod”+’ which will combine readily with the usual ground
state neutral atom (s’d”) to form an suid”+‘dn
molecular
configuration. Scandium, titanium, manganese,and iron are
the exceptions among the 3d atomic cations, having sldn
ground state configurations. In each of these metals, it is
energeticallymore favorable to promote the ion to an s’d”+l
configuration than to promote the neutral to an sldn+l configuration. The homonuclear dimer cation with an
su2dn+l d n configuration is then formed from
M[s’d”)+Mt(sod”“).
For this discussion, we define the
promotion energy as the energy required to promote each
atom from the lowest J level of the ground state to the lowest
J level of the lowest lying excited state having the appropriate electron configuration for formation of an suidn”dnfl
molecular configuration for neutrals, or an suidn+*dn
molecular configuration for ions. The diabatic bond strength
will accordingly be defined as the measured,adiabatic bond
Epavb
0.1136
2X0.813d
2XO.2628
Diabatic 0; a2b
2.548(2)
3.17(19)
3.140(2)
3.277(l)
l&3(56)
0.232m
2X0.859m
2XO.432p
2X0.025’
2.97(10)
2.86(l)
2.765(l)
1.6-2.3
2.08(7)
2.092(2)
2.03(2)
4753
and Co;
3
1
0 I
FIG. 7.
strengths
transition
definition
I
I
I
I
I
I
l-i
V
Cr
Mn
Fe
Co
I
Ni
I
I
Cl!
Measured (open symbols) and diabatic (filled symbols) bond
of neutral (squares) and cationic (triangles) homonuclear diatomic
metal molecules from the data listed in Table II. See the text for
of diabatic bond strength.
strength plus the promotion energy.These concepts are discussedfurther in a review of CID data by Armentrout et ~1.~~
Table II lists the adiabatic and diabatic bond strengths
and promotion energies,E, , of all the homonucleardiatomic
neutrals and monocations in the 3d transition metal series
from Ti to Cu. These data are also representedgraphically in
Fig. 7. The diabatic bond strengthsof Crl, Mnzf , Mn2, and
Cul are not representedbecausethey are not expected to
have SU$YU: ground state configurations. Instead, so--u:
and scr~su~ground configurationsare expectedfor Mnz and
Mn2, respectively, and an suj ground configuration is expected for Crl14 and Cul . While this configuration for Crg
has not been proven conclusively, it does correctly predict
the observed predissociation threshold for Cri reported by
Lessenet al.,” as previously discussed.14
D. Periodic trends
Reviewing Table II and Fig. 7 there are several observations concerningthe bonding that can be made. (A discussion
including larger clusters in the 3d series,and Nb and Ta can
be found in the review by Armentrout er a1.)41The diabatic
bond strengths of Til and Ti, are lower than those for V$
and V,, suggestingan increasein the bonding among the d
orbitals in the vanadium dimers, which place more d electrons in bonding orbitals than do the titanium dimers. The
lower diabatic bond strength of titanium and vanadium cationic dimers compared to their neutral counterparts is the
result of the removal of an electron from a bonding d molecular orbital. This phenomenonis reversedfor the iron and
cobalt dimers, where the electron is removed from an antibonding d molecular orbital.
The diabatic bond strength of Co2 plotted on Fig. 7,
taken as the midpoint of the rangeof values listed in Table II,
seemsanomalously low. The lower limit of this range is the
lower limit of a bond strength for Co2 calculated by Shim
‘Uncertainties in the final digits are
given in parentheses.
‘See the text for definition.
This work.
dReference 34.
‘See the text.
‘Reference 6.
aReference 30.
‘Reference 14.
kReference 47.
‘Reference 19.
“Reference 48.
“Reference 49.
‘See the text.
aReference 36.
4Reference 20.
‘Reference 44.
‘Reference 50.
‘Rcfcrcnce
45.
‘Reference
51.
and Gingerich.42The upper limit is derived from an upper
‘Reference 46.
“Reference 52.
limit to the ionization energy of Co2, IE(Cods6.42 eV
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J. Chem. Redistribution
Phys., Vol. 100, subject
No. 7, 1toApril
4754
Russonet al.:
Bond strengths of Ti2+, Vi, Co;,
which was establishedby noting that CoZ is readily ionized
with 193 nm ArF laser radiation.43The Co, bond strength is
thus easily calculated through Eq. (l), using IE(Co)=7.864
eV.36Based on a comparison to similar molecules as shown
in Fig. 7, it appearsthat the true bond strength of Co, is
probably closer to the upper limit and that the true ionization
energy of CoZ is probably close to the 6.42 eV upper limit.
The diabatic bond strengths of the Fez, Fez, and Co:
molecules are higher than those of Ni2, Nizf , and Cu,. Copper dimer neutral, having a full 3d subshell, is representative
of a transition metal diatomic molecule with no d-orbital
contributions to the bonding. As the nickel dimer species
have similar bond strengths, they also appear to have no
d-orbital bonding. The higher diabatic bond strengths for
Fez, Fez, and Cot seem to indicate that d-orbital interactions may be occurring in these species.The diabatic bond
strengthsof Fe, and Fe; are slightly higher than thoseof Co2
and Co;, consistent with both greater d-orbital interactions
and higher d orbital bond orders in the iron species.
V. SUMMARY
The sudden onset of predissociation in the resonanceenhancedphotodissociation spectra of Til, Vz, Co:, and
Cot has been observed and is interpreted as providing the
thermochemicalbond strengthof thesemolecules.Thesevalues, Di(Tiz)=2.435?0.002 eV, D~(V~)=3.140+0.002 eV,
Di(Co,‘)=2.765+0.001 eV, and D~(Co~)=2.086+0.002 eV,
are in good agreementwith values obtained from collisioninduced dissociation experiments.The presentmeasurements
are the most precise currently available for these molecules.
Combined with auxiliary data for Ti, and VZ, these results
give D~(TiJ=1.54+0.19 eV, IE(Ti2)=5.93t0.19 eV, and
IE(V,)=6.359?0.002 eV. This last result is in excellent
agreement with a recent direct measurement of
IE(V,)=6.356+0.001 eV.31Togetherwith the bond strengths
of several neutral diatomic speciesmeasuredfrom predissociation thresholds in resonant two-photon ionization spectroscopy,the data presentedhere illustrate the general applicability of this technique to transition metal cluster cations
that meet certain criteria which are discussedin detail. The
addition of the triatomic metal cluster cation, Co;, demonstrates the extension of this technique to larger clusters.
Comparisonsof diabatic bond strengthsfor homonucleardiatomic molecules and monocationsin the 3d transition metal
series from Ti to Cu are made. Periodic trends in the metal
dimer bond energies show that d-orbital interactions play
major roles in the bonding of the early transition metal diatomics, with the importance of these roles decreasinglater
in the series.
ACKNOWLEDGMENTS
This research is funded by the Department of Energy,
Office of Basic Energy Sciences(P.B.A.); the National Science Foundation under Grant No. CHE-9215193 (M.D.M.);
and partial support is provided by the Donors of the Petroleum ResearchFund, administeredby the American Chemical Society (M.D.M.). Funds used to purchasethe excimer-
and Co;
pumped dye laser system employed in these experiments
were provided by the National Science Foundation under
Grant No. CHE-8917980 (P.B.A./M.D.M.).
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.I Chem
Phvn
Vnl
IlXl
Nn
7 1 Aoril
1994