J. Phys. Chem. 1995,99, 15633-15641
15633
Photoionization Dynamics and Charge Separation Reactions of Iron Penta-, Ennea-, and Dodecacarbonyl Complexes Induced by Photoabsorption in the 20-90 eV Energy Range A. J. R. Heck? and T. Drewello Hahn-Meitner-Institut Berlin, Bereich Physikalische Chemie, Glienickerstrasse 100, 0-14109 Berlin, Germany
M. Fieber-Erdmann, R. Weckwerth, and A. Ding* Optisches Institut, Technische Universitat Berlin, Strasse des 17, Juni 135, 0-10623 Berlin, Germany Received: March 16, 1995; In Final Form: August 23, 1995@
The results of a study of the photoionization dynamics of iron penta-, ennea-, and dodecacarbonyl complexes are presented. In these experiments the gaseous iron carbonyl complexes are ionized via absorption of photons in the 20-90 eV energy range. Ionic products are analyzed using time-of-flight mass spectrometry. Cation pairs formed via double ionization of the iron carbonyl complexes are analyzed using photoion-photoion coincidence techniques. Photoion pairs corresponding to a direct two-body charge separation of high-mass iron carbonyl dications, such as CO+/Fe(CO),+, with n L 3, could not be detected. However, several ion pairs, such as C+/Fe(CO),+, with n = 0, 1, and CO+/Fe(CO),+, with n = 0, 1, have been observed generated by charge separation reactions originating from doubly charged iron carbonyl fragments. The total photoionization efficiency decreases gradually in the studied energy range between 20 and 90 eV, although a broad relative maximum in the photoionization efficiency is observed for iron pentacarbonyl around 60 eV photon energy. This relative maximum is attributed to an increased ionization efficiency caused by the iron 3p inner shell resonant excitation.
1. Introduction Charge separation and charge transfer processes are being investigated not only to obtain static properties such as energy levels, bond energies, or equilibrium distances but also to elucidate the dynamics of such collisions. Ionic systems usually feature many interacting potential energy surfaces, whose knowledge is essential to predict the reaction and collision rates of molecular ions. The modeling of many ionic collision processes such as those for reactions in the outer atmosphere and in space rely heavily on detailed knowledge about such systems. One outstanding pioneer in this field is Zdenek Herman, who has been developing the necessary tools for such investigations for the last three decades. He not only devised the first ion beam experiment to obtain angular and energy-resolved cross sections of ion molecule reactions' but also published many important contributions on the dynamics of charge separation reactionse2 The spectroscopy and ionization dynamics of mononuclear transition metal carbonyls have received considerable attention over many years, since these compounds not only are important in homogeneous catalysis but may also be considered as model compounds for the bonding of carbon monoxide molecules to metal surfaces. Moreover, these species might serve as a model for heterogeneous clusters consisting of an inner core (the metal center) and an outer sphere (the CO ligands). The excitation and ionization dynamics of the iron carbonyl complexes have been studied previously using electron3 and photoionization4 mass spectrometry, photoelectron spectroscopy~photoelectronphotoion coincidence and inner shell electron energy loss s p e c t r o s c ~ p y . ~ ~ ' ~ 1.1. Mass Spectrometry. Mass spectrometric characterization of the mononuclear transition metal carbonyls was already
* Author to whom correspondence
should be addressed. Present address: Department of Chemistry, Mudd Bldg., Stanford University, CA 94305-5080. Abstract published in Aduunce ACS Abstrucrs, October 1, 1995. @
attempted in the 1930~."-'~ With the availability of more advanced mass spectrometers in the 1960s more comprehensive studies on the metal carbonyls appeared.',2q'4 Upon electron impact ionization of iron pentacarbonyl all possible ions of type Fe(CO),+ are formed together with the analogous doubly charged species. Furthermore, a number of ions of type FeC(CO),+ are observed. From metastable ion decomposition studies and photoionization experiments it has been proposed that metal carbonyl ions decompose via successive elimination of neutral carbon monoxide2,6%s (eqs 1-3): Fe(CO),+ FeC(CO),+ Fe(C0),2+
-
Fe(CO),-,+ FeC(CO),-,+ Fe(C0),-,2'
+ CO
+ CO + CO
(1) (2) (3)
The appearance energies of many of the decomposition reactions 1-3 have been determined with high a c ~ u r a c y ! ~ ~ ~A~few ~'~ transition metal carbonyl complexes, in particular iron pentacarbonyl and tungsten hexacarbonyl, have been used extensively to determine energy deposition in these systems upon activation via electron impact ionization, charge exchange reactions,I6 low-" and high-energyI8 collisional activation, and surfaceinduced decompo~ition.'~ For many transition metal carbonyls multiphoton ionization, using photons of typically 3.5-6.5 eV, yields different results than electron impact ionization. Multiphoton ionization of transition metal carbonyls mainly generates bare metal ions.20-23 It has been shown that the observed fragmentation pattern depends strongly on the laser power. Some of these multiphoton ionization results have been rationalized by invoking mechanisms in which dissociation takes place prior to i ~ n i z a t i o n . ~ ~ For the iron e ~ e a c a r b o n y l ~and ~ -iron ~ ~ dode~acarbonyl~~ complexes a few electron impact ionization studies have been reported.
0022-365419512099-15633$09.00/0 0 1995 American Chemical Society
15634 J. Phys. Chem., Vol. 99, No. 42, 1995
Heck et al.
A study on photoelectron-photoion-photoion coincidence (PEPIPICO) of Fe(CO)z(NO)Z has been performed by Simonz8 to investigate the fragmentation processes at photon energies above 400 eV. 1.2. Charge Separations and Photoion-Photoion Coincidences. Doubly charged ions are often found to be metastable. Generally two different modes of fragmentation are observed: firstly, “covalent” neutral loss channels, mz+ mI2+ m2, which are usually endoergic and thus occur at higher appearance energies; secondly, charge separation channels, m2+ ml+ m2+, which are driven by Coulombic repulsion, but are often hindered by high potential barrier^.^^,^^ Dissociative doubleionization processes via charge separation channels are efficient in small systems.31 In contrast, covalent channels are expected to be more efficient in larger systems, as the Coulombic repulsion will diminish due to the possible larger distance between the two charges. Coincidence techniques, such as photoion-photoion coincidence (PIPICO), which is based on the measurement of correlated fragment ion pairs, are especially suited for the investigation of charge separation reactions of dications. Most previous photoionization studies on the charge separation reactions of dications have been carried out on relatively small molecules, although recently a few examples have been reported on larger systems, such as hexafluorobenzene,32 1, 1,l-trimethyl-2,2,2-tri~hlorosilane,3~ various organometallic systems such as m e t a l l o c e n e ~and ~ ~alkylated metals,35 and noble gas cluster^.^^,^^ For a recent example on related alkylidyne tricobalt nonacarbonyl complexes see ref 38. Furthermore, zero kinetic energy electron spectroscopy with a triplecoincidence technique was applied to study correlated pair fragmentation of (C0& clusters.39 “Covalent” neutral loss channels (cf. eq 3) have been observed for long-lived metastable, doubly charged iron pentacarbonyl dications (Fe(CO)s2+)generated by electron bombardment. No evidence, however, was found for charge separation reactions (eq 4),although specific searches have been ~ndertaken?~.~’
-
Fe(CO),’+
-
Fe(CO),-lf
+ CO+
-
+
+
(4)
On the basis of thennodynamic arguments, it can be shown that for instance for Fe(C0)z2+ the “covalent” neutral loss channel (3) is a few electronvolts more endoergic than the charge separation channel (4).42943Still, the neutral loss channel is observed exclusively in the metastable decomposition of this doubly charged ion, indicating that the barrier for the alternative charge separation channel is quite high. 1.3. Core-Level Photoexcitation and Photoionization. Dynamic processes following core-level excitation leading to ionization is a topic of much In contrast to valence electrons, which can be delocalized over the molecule, the corelevel electrons are normally localized near the atoms they belonged to originally. Core-level excitation may lead to selective bond breaking in polyatomic molecules. For instance, it has been demonstrated that characteristic differences in the fragmentation pattern can be observed when the core hole is located in different atoms or in different core-excited states. This is even true when the core hole is located in similar atoms such as the two carbon atoms in CF3CH346and the two nitrogen atoms in N20.47 The electronic configuration of atomic iron is 3p63d64s2. The iron 3d atomic orbitals of the iron pentacarbonyl complexes are split by the field of the carbonyl groups. These atomic orbitals mix with the carbonyl molecular orbitals. Therefore, most valence orbitals in iron pentacarbonyl have contributions from both the Fe atom and the carbonyl ligand. Valence
molecular orbital diagrams have been reported for iron pentacarb~nyl.~~.~~ Information about low-lying core orbitals has been obtained through X-rayso and synchrotron a b s o r p t i ~ n ,photoelectron ~~.~~ ~pectroscopy,5~-~~ electron energy loss s p e c t r o ~ c o p y ? ~ ~and ~-~‘ ab initio calculations.49~6z~63 Electron energy loss spectra have been recorded for iron pentacarbonyl and iron enneacarbonyl; the maxima around 60 eV are attributed to 3p e x ~ i t a t i o n . ~ * ~ ’ This is slightly lower than the 3p ionization energy of the gaseous atomic iron, which is around 66 eV.@ Here we report the results of a study of the photoionization dynamics of iron penta-, ennea-, and dodecacarbonyl complexes induced by photoabsorption in the 20-90 eV energy range. Special attention is paid to the charge separation dynamics following double-ionization processes, using photoion-photoion coincidence techniques.
2. Experimental Section A detailed description of the experimental setup has been given p r e v i o u ~ l y . ~Briefly, ~ , ~ ~ synchrotron radiation from the BESSY storage ring in Berlin is dispersed by a toroidal grating monochromator (TGM7) in the 15-120 eV photon energy range, providing an energy resolution EIAE = 500. The photon flux of typically 10” photons*s-’ makes multiple photon ionization events rather unlikely. The experiments are operated in the following modes: (I) Mass spectra at a given photon energy are obtained by use of a linear time-of-flight (TOF) mass spectrometer. A periodically pulsed extraction field ( E = 108 Vlcm) is used to extract the ions, formed within a certain time interval, into the flight tube. The linear TOF mass spectrometer has a massresolving power greater than mlAm = 500. In our experiments this resolution is sufficient to detect all possible ions with unit mass resolution. However, isobaric ions, e.g. CO+ and Fe2+, cannot be resolved. (11) Photoion yield curves of selected cations are measured with pulsed cation extraction ( E = 108 Vlcm). The ions are extracted into the time-of-flight tube by applying a repetitive extraction field in the ionization region. The detected ions with mass-to-charge ratios (mlz) in a preset range are monitored by use of the TOF mass spectrometer as a function of the photon wavelength. These photoion yield curves are corrected for time dependent variations of the photon flux and for the variation of sensitivity as a function of photon energy. (111) Charge separation mass spectra are monitored using photoion-photoion coincidence (PIPICO) techniques ( E = 216 Vlcm). The principle of the PIPICO method is to measure the time-of-flight difference of the two singly charged fragment ions generated by the decay of a doubly charged parent ion. The undissociated doubly charged ions or doubly charged fragment ions, formed via loss of a neutral molecule, cannot be observed with this technique. The lighter ions of the fragment ion pairs provide the start signal, and the heavier ones, the stop signals for a time-to-amplitude converter. The peaks in the PIPICO spectra are normally significantly broadened due to the high amounts of kinetic energy released by the Coulombic repulsion. The kinetic energy released in these charge separation processes can be determined from the peak broadening of the PIPICO spectra. The inlet system was operated at room temperature (ca. 300 K) because the iron carbonyl complexes have a sufficient vapor pressure. Typical iron metal carbonyl pressures in the ionization chamber were Pa. The background pressure in the chamber is normally Pa.
Iron Penta-, Ennea-, and Dodecacarbonyl Complexes
J. Phys. Chem., Vol. 99, No. 42, 1995 15635
In systems such as iron pentacarbonyl ion assigriinent is difficult. This is due to the fact that the mass of the atomic iron is-within the experimental mass resolution-double that of the CO ligand. Furthermore, there are other mass coincidences between singly and doubly charged fragments (for example, ions of formula FeZ(CO)z,+ overlap with ions of formula Fe(C0),2+).
3. Results and Discussion 3.1. Mass Spectra. The 70 eV photoionization (PI) timeof-flight mass spectra of the iron penta-, ennea-, and dodecacarbonyl complexes are displayed in Figure 1. These spectra are compared with electron impact (EI) data obtained at the same electron energy (Table I). For this comparison one has to keep in mind that in contrast to photoionization only a fraction of the impact energy is transferred to the molecule in the case of electron bombardment ionization. This results in a lower amount of fragmentation. Fe(C0)s. Table l a compares the 70 eV photoionization and electron impact mass spectra of iron pentacarbonyl. The electron impact data are taken from the literat~re.~As mentioned before, the mass resolution of the time-of-flight mass spectrometer is not sufficient to distinguish isobaric ions such as Fez+ and CO+. Therefore, different ions may contribute to the same mlz peak. In particular, an overlap is expected for the signals corresponding to mlz = 28 (Fe2+and CO+), mlz = 56 (Fe+ and Fe(C0)z2+), and mlz = 84 (FeCO+ and Fe(c0)4'+). This makes the unequivocal assignment of the ion signals difficult. The relative abundance of the larger singly charged ions, i.e. Fe(CO),+ (n = 2-5), is clearly reduced; that of the doubly charged ions FeC02+ and FeC(CO)2+ is increased in the 70 eV photoionization mass spectrum, compared with electron bombardment. Furthermore, the 70 eV photoionization mass spectra of iron pentacarbonyl shows larger fragmentation. FeZ(C0)g. Table l b compares the 70 eV photoionizationand electron impact mass spectra of iron enneacarbonyl. The electron impact data are taken from the l i t e r a t ~ r e ? ~ The - ~ ~most obvious difference between the photoionization and electron impact mass spectra is the relative high abundance of dimetallic ions Fez(CO),+ as compared to the monometallic ions Fe(CO),+ in the photoionization mass spectrum. However, the abundance of dimetallic ions decreases dramatically when the sample is heated. Therefore, it seems that the observed differences in the reported mass spectra (Table lb) are not exclusively caused by the different ionization methods, but could be partly due to differences in experimental conditions (internal excitation of the sample). In the experiments described here the sample was maintained at room temperature. The electron impact mass spectra were recorded at source temperatures of typically 250 0C.24-26Thermal decomposition of iron enneacarbonyl is expected to occur above 50 "C, generating atomic iron, carbon monoxide, some iron pentacarbonyl,and iron dode~acarbonyl.~' These compounds are, however, not observed in our experiments. Fe3(CO)12. Table IC compares the 70 eV photoionization and electron impact mass spectraz6of iron dodecacarbonyl. As in the case of iron pentacarbonyl,the ion assignment is difficult. For instance, ions of formula Fe2(CO),-Z+ overlap with ions of formula Fe(CO),+. The 70 eV photoionization mass spectrum is very similar to the 70 eV electron impact mass spectrum. The relative abundance of Fe3(C0)12+ and F ~ ~ ( C O ) IisI +higher in the photoionization mass spectrum. Furthermore, the signals of mlz = 280 and 112 are more abundant in the photoionization
2.0
0.0
6.0
4.0
8.0
time-of-flight (p) i
R(CO),'
B
7 0
I
2
3
4
5
7
0
~
~
IC
"
~
'
1
"
"
'
~
"
~
'
~
1
~
"
5
4
~
"
~
6
"
1
"
8
~
"
Y
~
"
"
PC(CO),,,. I
2
I 0
2.0
I
I
4.0
3
4
:
I
1
1
4
5
6
6.0
7
8
1 Fc&O):
9
1
8.0
time-of-flight (ps)
Figure 1. Time-of-flight mass spectra of (A) iron pentacarbonyl, (B) iron enneacarbonyl, and (C) iron dodecacarbonyl taken at a photon energy of 70 eV. The applied extraction and acceleration voltages are different for the three spectra, leading to different flight times of the ions in the three spectra.
mass spectrum. The assignment of these signals is again difficult. For instance the ions of mlz = 112 may have contributions from Fe(CO)2+, Fe2(CO)dZ+,and Fe3(C0)z2+.
"
"
~
~
'
15636 J. Phys. Chem., Vol. 99, No. 42, 1995
Heck et al.
TABLE 1: Comparison of 70 eV Photoionization (PI) and Electron Impact (EI) Mass Spectra m/z
PI (%)
28 42 48 56 68 70 84 112 140 168 196
28 5 100 6 3 51 6 1 5 4
Fe(C0)s E1 (%)".b.1.2 2 3 90 11 4 100 30 16 30 25
a
assignment
(84/6) (98/2)
Fe2(COh m/z
42 56 68 70 84 112 140 168 196 224 252 280 308 336 364
PI (%) 14 100 5 8 84 26 14 35 30 18 2 2 9 4 6
E1 (%y'b,26 E1 (%)o,b.24,25 80
75
80 82 55
100 26 15 30 -
-
-
78 100 2 1 0.5 1
-
assignment FeC02+ Fe+lFe(C0)Z2+ FeC+ Fe(CO)3z+ FeCO+, Fe(C0)d2+ Fez+, Fe(C0)2+ FezCO+, Fe(CO)3+ Fez(C0)2+, Fe(C0)4+ Fez(CO)3+, Fe(C0)5+ FedC0)4+ Fez(CO)s+ Fez(C0)6+ FeACOh+ FedCO)s+ Fe2(COh+
FedC0)12 m/z
PI(%)
42 56 68 84 112 140 168 196 224 252 280 308 336 364 392 420 448 476 504
14 96 5 56 100 15 43 31 35 33 61 31 42 41 6 1 17 18 24
a
EI(%)",b*27
80 9 100 48 14 43 43 22 26 30 28 35 27 4 1 16 6 9
assignment FeC02+ Fe+, Fe(C0)z2+ FeC+ FeCO+ Fez+, Fe(C0)2+ FeZCO+, Fe(CO)3+ Fe3+, Fe2(C0)2+, Fe(C0)4+ Fe3CO+, Fez(CO)3+, Fe(C0)5+ FedCO)z+, FedC0)4+ FedCO)3+, Fez(COh+ FedCOh', Fe2(C0)6+ FedCO)s+, Fez(C0)7+ FedC0)6+, Fez(C0)8+ FedC0)7+, FedC0)9+ FedCO)s+ FedC0)9+ FedCO)lo+ FedCOh I + FedCO) 12+
The mass resolution is not sufficient to distinguish Fe2+ and CO+.
-, no ions observed.
The mass spectra of the iron carbonyls vary only little with photon energy in the range between 40 and 100 eV (cf. Figure IC). Below 40 eV, however, the mass spectra change dramatically. The photoionizati'on mass spectrum, however, shows a much lower abundance of the atomic iron ions; that is, at lower photon energies fewer fragment ions are formed. This behavior is normally also observed in electron impact mass spectra of transition metal carbonyl^.^ Figure 2 compares the mass spectra of iron dodecacarbonyl taken at photon energies of 20 and 45 eV. 3.2. Charge Separation Reactions. Fe(CO)5. Figure 3 shows the photoion-photoion coincidence spectra of iron pentacarbonyl recorded at photon energies of 45 eV (upper) and 90 eV (lower). The PIPICO signals are significantly
broadened as compared to the corresponding signals in the normal time-of-flight mass spectra. The broadening is caused by the large kinetic energy release (KER) accompanying the charge separation reaction. Broad rectangular peak shapes are observed in the PIPICO spectra if the angular distribution of the fragment ions is isotropic.68@ In the experimental setup used here, ions are formed in the ionization region via charge separation reactions of doubly charged ions. Ions with velocity components mainly in the direction of the time-offlight mass spectrometer are detected more efficiently than those ions moving perpendicular to the direction of the flight tube. Therefore, the observed PIPICO signals in our experimental setup show a double maximum corresponding to ions scattered into the forward and backward direction. The present setup did not allow the detection of ion-ion coincidence signals with a time-of-flight difference of less than 0.8 ps, like C+/O+. At low photon energies (45 eV) three charge separation signals can be identified. By comparing the observed time-offlight difference in the PIPICO spectrum with the time-of-flight for singly charged ions in the normal TOF mass spectrum, recorded under identical conditions, the charge separation signals can be attributed to the following processes a, b, and c:
-
FeC02' Fe(C0):' Fe(C0),2'
+ CO' FeCO' + COS Fe'
Fe(C0)2f
+ CO+
(c)
Ion pairs indicating a two-body charge separation reaction of Fe(C0)42+ and/or Fe(C0)S2+ are not observed. This clearly indicates that a charge separation reaction which involves the latter ions would be connected to a "covalent" neutral loss channel preceding and/or following the actual Coulombic explosion. At higher photon energies (above 55 eV) three additional channels are observed which are attributed to reactions d, e, and f FeCC02+
FeC2'
-
+ CO' FeCO' + CS Fe' + C' FeC'
(4
Due to the specific extraction and acceleration voltages used, identification of processes b and f is obscured by the fact that these signals overlap in the PIPICO spectrum at higher energies. Figure 4 shows the PIPICO signals of the processes b and f i n more detail. At photoionization energies of 45 eV only one charge separation process is observed, which corresponds to the FeCO+/ CO+ ion pair. The shape of the observed signal changes dramatically if the photon energy is increased. By subtraction of the normalized PIPICO spectrum at 45 eV from the normalized PIPICO spectrum at 90 eV, a second charge separation process can be positively identified, which is centered at a slightly higher time-of-flight difference corresponding to the Fe+/C+ ion pair. The width of the PIPICO signals is a measure of the kinetic energy release (KER) accompanying the charge separation processes. Table 2 displays the signal widths of the observed processes a-f. The KERs are estimated assuming that all charge separation processes are two-body processes (see below). We estimate the error of these values to be approximately 10%. With this assumption the KER for the process
J. Phys. Chem., Vol. 99,No. 42, 1995 15637
Iron Penta-, Ennea-, and Dodecacarbonyl Complexes
-
m1m22+ ml+
+ m:
(5)
t
is given by68
t
0
1
1
3
4
5
6
7
6
9
j
ICO'
in which At is the signal width in nanoseconds, Eext is the extraction field in volts/centimeter, and ml and m2 are the masses (in daltons) of the two ions involved in the charge separation reaction. The intercharge separation distance R can be determined from the kinetic energy release in a point charge model which
(7)
R[A] = 14.1/E,~[eV]
More complex charge separation mechanisms other than the suggested two-body dissociations can however not be excluded. For instance, the Fe+/CO+ ion pair can also be formed via an alternative mechanism. Fe(C0);'
-
FeCO'
+ CO'
-
Fe' m2
+ co + CO' m3
(8)
ml
This alternative mechanism leads to an alternative value for the kinetic energy release EKER given by
It is assumed that the loss of neutral CO preceding the Coulombic explosion does not carry any significant amount of kinetic energy. Formula 9 can be derived by considering the conservation of momentum,
4.0
2.0
6.0 time-of-flight (p)
8.0
Figure 2. Time-of-flight mass spectra of iron dodecacarbonyl recorded at photon energies of 20 eV (upper spectrum! and 45 eV (lower spectrum).
and the conservation of energy,
and u2 are the velocities of ml and m2, respectively. The change of the time-of-flight At1 for a mass ml in a static electric field is given by VI
dence techniques such as PEPIPIC071-75would be needed to elucidate the real mechanism of the charge separation process in greater detail. Further alternatives for the two-body dissociations are listed in the following, together with the corresponding intercharge distances (the final product ions are indicated in bold letters). Fe(C0)32'
-
CO'
+ Fe(C0)2+ -CO' + FeCO' + CO (13)
Atl = ulml/eEext
(12)
The KER determined from the observed Fe+/CO' ion pair (Table 2 ) assuming this alternative mechanism (eq 8) with m3(CO) = 28 u amounts to EKER= 3.4 eV, which corresponds to R = 4.2 A. With a mass m3(4CO) = 112 u the KER was determined to EKER = 4.6 eV (corresponding to R = 3.1 A). TJsing a simple picture for the dissociating Fe(C0L2' dications with the two charges at a maximum separation from each other and assuming the Fe-CO bond length of neutral Fe(CO)5 to be 2.7 yields lower KER values than obtained experimentally. All intercharge distances evaluated with the method described above suggest that transition structures and the evaporation of neutral CO molecules before and after the charge separation event play a dominant role for the resulting KER values. Unambiguous evidence for sequential dissociations, i.e. the evaporation of neutrals, can be deduced from our PIPICO experiments in a very rudimentary fashion. Multiple coinci-
(EmR = 2.3 eV, R = 6.0 A) Fe(C0):'
-
CO'
+ Fe(CO),'
-
CO+
+ FeCO' + 2CO (14)
(EmR = 2.7 eV, R = 5.2 A) Fe(C0);'
-
CO'
+ Fe(CO),+ CO'
-
+ Fe(CO),+ i2CO (15)
(EmR= 2.2 eV, R = 6.5 A) It is to be noted that the processes a-c exclusively involve the loss of intact CO molecules, whereas the ion pairs d-f obviously include the more energy-demanding break of the CO bond. The
15638 J. Phys. Chem., Vol. 99, No. 42, 1995
Heck et al.
Fe' I CO'
1
1I 1
1
FeCO'ICO'
Fe' I CO'
pFZKV-1 C' I Fe'
FeCO'?'
I\
1 1
Fe(C0); / CO'
\
C'IFeCO'
I
I 1500
I 1000
I 2000
I
I
I
2500
3000
3500
1450
40
time-of-flightdifference (ns)
Figure 3. Photoion-photoion coincidence spectra of iron pentacarbony1 recorded at photon energies of 45 eV (upper spectrum) and 90 eV (lower spectrum). following reactions show two possible sequences:
-
-
I
1750
time-of-flight difference (ns) Figure 4. Detailed view of the photoion-photoion coincidence spectra of iron pentacarbonyl in the region between 1450 and 1750 ns timeof-flight difference. The lower spectrum has been observed at a photon energy of 45 eV (dashed line) and 90 eV (solid line). The upper spectrum shows the difference between the two spectra taken at 45 and 90 eV photon energy.
(EKER = 2.8 eV, R = 5.0 A)
TABLE 2: Experimentally Determined Kinetic Energy Releases for Simple Coulombic Fragmentation According to Eqs 5 and 6 for a Photon Energy of 90 eV (The Intercharge Distances Have Been Calculated Using Eq 7) process assumed ion pair At (ns) EKER(eV) R (A) a Fe+/CO+ 188 2.7 5.3 b FeCO+/CO+ 170 1.9 1.3 165 1.7 8.2 C Fe(CO)2+/CO+ d FeC+/CO+ 185 2.4 5.9 e FeCO+/C+ 160 3.4 4.1 f Fe+/C+ 163 3.8 3.1
In line with the expectations, the ion pairs d-f are observed efficiently only for higher photon energies. Interestingly, the FeCC02+ dication shows two competitive charge separation reactions, resulting in the ion pairs FeC+/CO+ and FeCO'/C+ being formed. The heat of formation of the product ions are known: AH,(FeC+) = 15.52 eV,16 AH,(CO+) = 12.88 eV$2 AHr(FeC0') = 9.84 eV,43 and AH&?) = 18.69 eV.42 From these values it is estimated that both charge separation processes are equally endothermic. However, these two ion pairs might also be formed via altemative mechanisms and not exclusively via the FeCC02+ doubly charged ion. Figure 5 shows the time-of-flight difference mass spectra in the region of the Fe+/CO+ ion pair signal. For a low photoionization energy (45 eV) the width of the ion pair signal is smaller than for a high photoionization energy (90 eV). Also the kinetic energy released in this charge separation process increases from 2.67 eV at 45 eV photon energy to 2.74 eV at 90 eV photoionization energy. This behavior might be caused
by ionization processes occumng differently at low and high energies. For photon energies above 60 eV iron 3p inner shell excitation and ionization processes might play an important role in the photoionization dynamics. A general trend observed is the increase in kinetic energy release with decreasing doubly charged ion mass. A straightforward explanation is the size of the ions. It might be expected that the charge separation distance increases with the size of the dication. This will lead to less repulsion between the two charges and therefore to a lower kinetic energy release in the charge separatiod process. The same argument can also be used to explain the exclusive observation of the "covalent" neutral loss channels for the "larger" high-mass doubly charged ions; that is, the intercharge repulsion will be small in these systems, and therefore it may be that the Coulombic repulsion driving the charge separation reaction is too weak. The statistics of the experimental method are not sufficient to determine accurate appearance energies of the observed
FeC(C0)22+
CO'
+ FeCCO'
CO'
+ FeC' + C O (16)
= 3.0eV, R = 4.7 A) Fe(C0);'
-
CO+
+ FeCO'
-
CO+
+ FeC' + 0
(17)
Iron Penta-, Ennea-, and Dodecacarbonyl Complexes
J. Phys. Chem., Vol. 99, No. 42, 1995 15639
“.,5.
C
.s 8
-\,
I
i2
’c
50.0
S5.0
65.0
60.0
Photon energy (eV)
”-A.
9.-
--e..
36
I
~
I
40
44
48
~
52
I
~
56
I
~
I
~
68 72 Photon energy (eV)
60
64
I
~
I
76
80
84
’
I
88
-*.
~
I
92
Figure 6. Total photoion yield as a function of the photon energy. The inset shows the region around 60 eV, for which a linear background was subtracted to reveal the relative maximum in the photoion yield more clearly.
1050
850
time-of-flight difference (ns) Figure 5. Photoion-photoion coincidence spectra of iron pentacarbony1 in the region between 830 and 1050 ns time-of-flight difference. The figure shows the signal, corresponding to the Fe+/CO+ ion pair, recorded at a photon energy of 45 eV (dashed line) and 90 eV (solid line).
doubly charged ions. However, the appearance energies of the singly charged ions are known quite The appearance energies of the Fe(CO),+ ions are generally a few electronvolts lower than the appearance energies of the corresponding FeC(CO),+ ions. For instance, the appearance energies of Fe+, FeC+, and FeCO+ are 14.4, 23.6, and 13.4 eV, r e ~ p e c t i v e l y . ~As ~ ’a~rule of thumb, the ionization potential to form a doubly charged ion is expected to be 2.8 times higher than the energy to generate the corresponding singly charged ion.77 Therefore, a rough estimate of the appearance energies of the doubly charged ions, obtained by applying this rule, is in agreement with the experimental findings that the charge separation reactions corresponding to the FeC2+ and FeCC02+ ions occur only at higher photon energies, whereas the charge separation reactions corresponding to the Fe(C0),2+ ions occur also at low photon energies (see Figure 3). Fez(C0)9 and FedC0)12. The observed PIPICO spectra of iron enneacarbonyl are very similar to the PIPICO spectra of iron pentacarbonyl. For low photon energies (45 eV) the processes a, b, and c are observed; for higher energies the additional processes d, e, and fare also observed. It is difficult to establish whether this behavior is due to charge separation reactions of iron enneacarbonyl or iron pentacarbonyl, which is thought to be formed via thermal decomposition of iron enneacarbonyl. If the observed charge separation channels result from the iron enneacarbonyl complexes, this would further underline the above statement on “covalent” neutral loss channels being more dominant for the “larger” doubly charged ions, whereas charge separation channels are preferably observed for the “smaller” doubly charged systems. The PIPICO spectra of iron dodecacarbonyl gave only poor counting statistics. Only one coincidence signal has been observed, which is attributed to the Fe+/COf ion pair. In general it can be concluded that, in contrast to the metastable dissociation, the prompt decay of doubly ionized iron carbonyl systems shows extensive reactivity toward charge separation. The explanation for this observed behavior may be that metastable doubly charged ions (with lifetimes of a few microseconds) do not have sufficient intemal energy to decompose via charge separation channels. This means that the barrier
for charge separation reactions is higher than the barrier for the “covalent” neutral loss channels at least for the Fe(C0),2+ (n = 1 , 2 , 3 ) ions. The observed charge separation dynamics will therefore, most probably, occur on a much shorter time scale. 3.3. Photoion Yield and Inner Shell Resonances. The total photoionization cross section of iron pentacarbonyl in the 30100 eV energy range is shown in Figure 6. It can be seen that the photoionization cross section decreases gradually in the studied energy range. However, a broad relative enhancement in the photoionization cross section is observed with the maximum centered at 60 eV. This maximum lies about 6 eV below the inner shell 3p ionization limit.@ The photoabsorption and photoelectron spectra of free gaseous atomic iron have been studied in detail.5’$52 Gaseous atomic iron shows strong absorption resonances at 53 and 56 eV attributed to 3p 3d excitations. Electron energy loss spectroscopy of iron pentacarbonyl shows a maximum oscillator strength around 60 eV,9S6’ in perfect agreement with our data. Therefore, it seems that the relative maximum observed in the photoionization cross section may be attributed to processes in which ionization is preceded by resonant inner shell 3p excitations to vacant valence shell orbitals. These inner shell excited states may decay via autoionization or Auger channels, leading to an enhancement of the photoionization cross section. No selective fragmentation pattems were observed at a photon energy equal to the observed resonance energy, in contrast to observations made for chromium, molybdenum, and tungsten hexacarbonyl c ~ m p l e x e s . ~ ~ This may have an experimental reason, namely, the overlap in the mass spectrum of many isobaric ions in the case of the iron carbonyl complexes, which complicates the ion mass assignment in the case of the iron carbonyl complexes, as mentioned above.
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4. Conclusions The ionization of iron penta-, ennea-, and dodecacarbonyl using single photons of 20-90 eV was studied using time-offlight mass spectrometry. The findings were compared to earlier investigations on the electron impact behavior of these species. The broad relative enhancement of the gradually decreasing photoionization cross section revealed an inner shell 3p excitation, for which no selective fragmentation behavior could be observed with the present experimental setup. In spite of the limited information obtained using the PIPCO method, some interesting conclusions on the probable charge separation mechanisms can be extracted from the experimental data. Using the models described in eqs 5 and 8 which assumes the evaporation of neutral CO ligands before and after the actual Coulombic explosion event, and taking into account the experimentally determined time differences for the different ion
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pairs (cf. Table 2), some of the fragmentation channels may be ruled out. In particular, it is rather unlikely that the relatively large time differences measured in the PIPICO experiment would correspond to low KERs, as predicted by the simple Coulombic explosion model (eqs 5 , 6). The assumption has therefore to be made that a significant amount of evaporation of neutral CO molecules takes place after the Coulombic explosion event. In the case of the Fe+/CO+ pair (process a) even the evaporation of four CO molecules correlates to a charge separation which is larger than the equilibrium distance of the neutral Fe-CO radical. In the case of the FeCO+/CO+ pair (process b) the fragmentation is more likely to proceed according to eq 14; a charge separation process is described by eq 13 would require unphysical charge separations larger than the diameter of the molecule. However, it has to be kept in mind that there exist models for large charge separations in which the actual charge exchange takes place well after the separation of the particle (in our case CO). The neutral fragment then captures an electron when it has reached a certain distance from the ionic core, and Coulombic repulsion forces separate the two particles. This process is fairly unlikely. Processes requiring the breaking of the neutral CO bond exhibit larger KERs and therefore larger equilibrium distances. This is in line with the prediction that these fragmentation events take place prior to the Coulombic explosion.
Acknowledgment. The research was partially funded by the German Research Council (SFB 337). One of us (A.J.R.H.) gratefully acknowledges the support of the Hahn-Meitner-Institut Berlin and in particular Prof. K.-Dieter Asmus for making this collaboration possible. For making high-quality samples of the compounds studied here available to us we are deeply indebted to Dr. A. Willnow (HMI Berlin), M. Kempf (TU Berlin), and Prof. J. Muller (TU Berlin). References and Notes
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