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J. Phys. Chem. 1996, 100, 5660-5667
Chemiluminescent Emission from the Reaction of Manganese Vapor (Mnx) and Halogen Molecules (Cl2, F2) T. C. Devore† and J. L. Gole* School of Physics, Georgia Institute of Technology, Atlanta, Georgia 30332 ReceiVed: December 4, 1995; In Final Form: January 24, 1996X
The chemiluminescent reactions between an agglomerated flux of entrained manganese vapor (atoms and small manganese molecules) and halogen vapor (chlorine or fluorine) have been investigated under multiple collision conditions. Both reaction groups produce emission features that are readily correlated with manganese atom transitions, manganese(I) halide molecular emission, and broad features centered at 464, 530, and 720 nm which might be attributed to the manganese cluster halides or to manganese dimer. Chemical reactions involving manganese atoms and small manganese clusters are required to explain the observed emission features. The energetics of several possible metal atom, dimer, and trimer reactions are considered and correlated with the observed emission features. The energetics of some of these reactions are used to estimate the a5Σ-X7Σ energy separation in MnF. This value, 2500 ( 500 cm-1, is consistent with the value of 1743 cm-1 determined for MnH and a value of 3000 ( 500 cm-1 estimated previously for MnF. Vibrationally resolved emission features are observed for five previously investigated MnF transitions. While limited information was obtained for the system a (350 nm), c (504 nm), or d (690 nm) transitions, revised vibrational analyses for the b (495 nm) and the e (832 nm) band systems of MnF are presented. The b system bands are found to be blue degraded, with a vibrational analysis suggesting that the observed transition terminates in the X7Σ ground state. Twenty bands belonging to the origin sequence of the e system have been identified and fit using a nonlinear least squares analysis. Energetic arguments dictate that the lower state of this transition must be either the ground state or an electronic state differing little in energy from the ground state. The location of the observed broad emission features common to both the chlorine and fluorine systems are in reasonable agreement with absorption bands reported previously for Mn2 isolated in rare gas matrices. While their correlation with Mn2 is a viable possibility, the features in the 464 nm region would seem to be more reasonably associated, at least in part, with Mn2F* (Mn2Cl*) emission products. Emission profiles suggest that there are significant differences in the bond lengths for those states involved in the transitions giving rise to the broad emission features.
Introduction To date, there have been relatively few studies of chemiluminescent transition metal oxidation reactions, a minority of these involving the reactions of d block metals with the halogens. This has resulted in part because of the complexity of those spectra which characterize the transition metal halogenation processes and in part because of the relatively smaller bond energies of the transition metal halides, leading to a much smaller energy release for the population of excited electronic states. Thus, the only published data involving transition metal atom halogenations appear to be those for the early transition metals and primarily the reactions of scandium (Sc), yttrium (Y), and lanthanum (La).1-4 The chemiluminescent spectra for the Sc and Y reactions with F2, Cl2, Br2, and I2 are complex and dominated by a selectively formed excited state emitter whose formation kinetics is clearly second order in the halogen,1-4 however, the origin of this second order behavior is still the subject of controversy.5,6 While the halogenation reactions of the transition metal atoms may not be sufficiently exothermic to populate the excited electronic states of the transition metal halides emitting in the visible and ultraviolet regions, the multicentered reactions of transition metal molecules can provide a considerable increase † Permanent address: Department of Chemistry, James Madison University. X Abstract published in AdVance ACS Abstracts, March 15, 1996.
0022-3654/96/20100-5660$12.00/0
in the energy release from the oxidation process. This increased energy release in the simultaneous formation of two metal halides can be partitioned not only to these product halides but also to molecular and atomic fragments, Mn-1 and Mn-2 of the oxidized metal molecules, Mn. Thus, the reactions of chromium dimers with molecular fluorine have been shown7 to populate several higher lying states of the product metal fluoride, and the multicentered reactions of nickel molecules have been used8 to establish a dissociation energy and fundamental frequency for the ground state of NiF. The information extracted from these processes has come, in no small part, from their study across a wide pressure range encompassing both single collision9 reactive encounters and the controlled extension to multiple collision entrainment flow conditions. Here, we investigate the kinetics of the formation of excited electronic states and their relaxation. In this study, we extend our characterization of transition metal systems to the manganese molecule-halogenation reactions. The unique electrical and magnetic properties of small manganese clusters have been investigated using a variety of theoretical10-16 and experimental methods.17-33 Although clusters with up to five manganese atoms24,25 have been identified, the majority of these investigations have concentrated on the manganese dimer, Mn2. To date, all of the spectroscopy reported for Mn2 has been done in rare gas matrices. The visible absorption spectrum,17-23 the laser Raman spectrum,20-23 the ESR spectrum,24,25 and the MCD spectrum19 of Mn2 have been © 1996 American Chemical Society
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reported. Coupled with theoretical calculations,10-16 this research has led to a reasonable understanding of the low-lying states of Mn2. Since Mn2 has a long chemical bond and a very small dissociation energy,26,27 it is considered to be a van der Waal’s molecule. All data indicate that while the d electrons are spinpaired giving a 1Σg+ ground state, the d orbitals are not extensively involved in the bonding. Such states as the highspin 11Σg+ state are only a few wavenumbers higher in energy. The ground state of Mn2+ (12Σ) has been established from matrix isolation ESR28 and from theoretical calculations.16 The dissociation energy of Mn2+ exceeds that of Mn2, but there is disagreement over its exact value.29-32 Bier and co-workers have extended their studies to larger manganese molecules, investigating the resonance Raman spectrum of Mn3 21,22 and determining that Mn3 has a slightly distorted trigonal planar structure. The chemical reactions of manganese molecules (clusters) are also in the early stages of study. Armentrout and coworkers34,35 found that Mn2+ reacts much faster with O2 than does Mn+, and Larsen et al.36 reached a similar conclusion by studying the reactions between N-donor bases and Mn2+. The reasons for this enhanced reactivity are not yet understood. The oxidation of small neutral manganese clusters with ozone has been explored in this laboratory.37,38 These initial studies have revealed the first optical signature for Mn2O and suggested that unique reaction products and dynamical processes characterize cluster reactions. Here, we present the results of an initial study of the oxidation of manganese atoms and small manganese clusters with fluorine and chlorine oxidizers. The chemiluminescence which characterizes these systems not only is associated with manganese halide and manganese atom transitions but also may be associated, in part, with the excited states of Mn2 and the manganese cluster halides, MnxX. The latter tentatively correlated features may then represent the first gas phase emission spectra obtained for these species.
Figure 1. Chemiluminescent spectrum resulting from the interaction of helium entrained manganese vapor and molecular chlorine. Features assigned to MnCl emission systems, Mn atomic transitions, and possible “Mn2” or “Mn2Cl” emissions are indicated in the figure. Spectral resolution is 12 Å. See text for discussion.
The metal flux issuing from the crucible was entrained in helium (Selox, 99.999%) at temperatures ranging from 300 K (room temperature) to 96 K (liquid nitrogen cooling). At a suitable point above the flow, F2 (Air Products, 98.0%) intersected the entrained metallic flux, entering from a concentric ring injector inlet. As the vacuum chamber was evacuated by a mechanical pump (Welch 1397), the effluent was passed over 8-14 mesh activated alumina to scrub F2 and prevent fouling of the pump oil. In this configuration, the oxidant pressure ranged from 10 to 40 mTorr, and the total pressure in the system, including entrainment gas, was 2-3 Torr. The pressure was measured with a capacitance manometer (MKS Instruments). The chemiluminescent emission was focused onto the entrance slit of a 1-m Spex 1704 scanning monochromator operated in first order with a Bausch and Lomb 1200 groove/ mm grating blazed at 5000 Å. A dry ice cooled EMI 9808 photomultiplier tube was used to detect the dispersed fluorescence. The signal from this phototube was fed to a Keithley 417 fast picoammeter. The output from the picoammeter drove a Leeds and Northrup strip chart recorder onto which the spectra were recorded or was fed to a computer for data storage and subsequent analysis.
Experimental Section In order to study several processes involving the formation of metal atom and metal cluster complexes, we have developed a number of “variable flux” continuous metal flow entrainment sources. These sources are discussed in detail elsewhere.37,39 In this study, manganese metal was evaporated from a graphite crucible (Micromechanisms, Billerica, MA) placed in a resistively heated tungsten basket heater. The basket heater was heavily insulated with zirconia cloth (Zircar, Florida, NY), which allowed ready operation of the heater at temperatures near the upper limit of its performance specifications. The manganese was heated over a range from 1350 to approximately 1500 K, producing a vapor pressure in the range 10-1 to 2 Torr.
Results and Discussion Mnx + Cl2. The chemiluminescent spectrum observed when a moderate and agglomerated flux of entrained manganese vapor reacts with chlorine molecules is depicted in Figure 1. The strongest features are the z8P7/2-a6S5/2, the z8P5/2-a6S5/2, and the z6P7/2-a6S5/2 manganese atom transitions.40 The A7Π-X7Σ transition41 and the system II42 and III43 bands of MnCl are easily identified (Table 1). Two broad structureless features at 464 and 720 nm and at least three broad blended features in the 530 nm region are also apparent. These features may result from a small manganese molecule or from a manganese cluster
TABLE 1: Observed Transitions of MnF and MnCl molecule
transition
Te (cm-1)
ωe′ (cm-1)
ωe′′ (cm-1)
ωexe′ (cm-1)
ωexe′′ (cm-1)
comments
MnF MnF MnF MnF MnF MnCl MnCl MnCl MnCl
7Π-X7Σ
28 526 20 206 19 807 14 493.6 12 194 27 004.6 20 115.4 19 900 11 420.3
648.0 640.8 640.0 597.4 625.9 407.2 378.0 385 385.4
624.2 624.2 645.92 645.92 645.92 380.6 385.5 410 397.9
1.6 3.9(4) 3.6 3.15 3.18
3.2 3.2 3.22 3.22 3.22
ref 49 reassign. of ref 42 ref 44 ref 47 this work + refs 44 and 49b
A ?? -X7Σa d5Π-a5Σ c5Σ+-a5Σ b5Π-a5Σ(?) A7Π1-7Σc Π-Σd Π-Σd Π-Σ
ref 42smay need revision ref 42 ref 43
a Old values ν0 ∼ 20298.2, ωe′ ) 637.1, ωe′′ ) 649.1. See text for discussion. b The vibrational constants were determined by assuming that the state assignments given in refs 44 and 49 are correct. They were determined from a nonlinear least squares fit of the band heads observed for the origin bands of this transition. c Hayes, W.; Nevin, T. E. Proc. R. Ir. Acad. 1955, 57, 15. d Hayes, W. Proc. Phys. Soc. 1955, A68, 1097.
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Figure 2. Chemiluminescent spectrum resulting from the interaction of helium entrained manganese vapor and molecular fluorine. Features assigned to MnF emission systems, Mn atomic transitions, and possible “Mn2” or “Mn2F” emissions are indicated in the figure. Spectral resolution is 10 Å. See text for discussion.
halide. Bier et al.21,22 have reported structureless absorption bands for Mn2 isolated in argon matrices centered at 458, 491, 500, 531, 535, 587, 625, 650, and 749 nm. Allowing for possible matrix shifts and the red shift in band maxima expected for emission versus absorption, there is sufficient agreement between these spectra to suggest that some of the broad bands presented in Figure 1 could result from Mn2* emission. The features identified with the three MnCl band systems in Figure 1 are broad and of such an intensity as to render further spectroscopic analysis inappropriate. The additional broad features observed in the spectrum are of roughly Gaussian shape, suggesting that the equilibrium bond length is significantly different for the two states involved in the transitions. The observed band shapes are consistent with those expected for the visible transitions of Mn2, involving modestly d-orbital bound excited states and a van der Waal’s bound ground state. However we will suggest that the feature at 464 nm might best be associated with emission from a manganese cluster halide. Mnx + F2. Five previously reported MnF transitions could be identified in the spectrum of the manganese-fluorine system. The overall spectrum is depicted in Figure 2. e System (System I). The complex multiheaded system centered at 830 nm (Figure 3) is the e system first reported by Hayes and Nevin43 in their study of MnX2 vapors dissociated in a hollow cathode discharge. Twenty bands (Table 2) with a nearly constant spacing of approximately 20 cm-1 were observed. Even at the resolution of the current experiment, each band appears to contain several heads. The resolution used here does not permit the position of these heads to be accurately determined, however, the band centers observed correlate well with those reported by Hayes and Nevin.43 The features observed are fit to near the experimental error with a linear least squares fit, however, a nonlinear fit provides a slightly better result. The expression
ν ) 12194.1 - 20.05(V + 1/2) - 0.038(V + 1/2)2
(1)
fits the observed band centers to within the experimental error. The molecular constants determined differ from those calculated using a nonlinear least squares fit of the band heads reported by Hayes and Nevin43 (∆ωe ) -15.7 ( .3 cm-1 and ∆ωexe ) -0.424 ( 0.017 cm-1). However, the equation derived from Hayes and Nevin’s data does not fit the high-V bands. Since these bands are sufficiently well-resolved in the present study, a revised fit is presented. Hayes and Nevin43 have concluded that the e system corresponds to a Π-Σ transition. Launila and Simard44 have referred to, but apparently not yet published, high-resolution
Figure 3. Chemiluminescent emission corresponding to the MnF e band system formed from the reaction Mn (a6S) + He + F2 f MnF* + He + F under multiple collision conditions. Spectral resolution is 1.5 Å. See text for discussion.
TABLE 2: Observed Bands for the ∆W ) 0 Sequence of the e System of MnFa V′′
obsd freq (cm-1)
calcd freq (cm-1)
diff (cm-1)
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
12 183 12 166 12 146 12 125 12 105 12 079 12 060 12 046 12 015 12 000 11 980 11 958 11 936 11 915 11 899 11 877 11 857 11 835 11 811 11 784 11 765
12 184 12 164 12 144 12 123 12 103 12 082 12 062 12 042 12 021 12 000 11 979 11 958 11 937 11 916 11 895 11 874 11 853 11 832 11 810 11 789 11 767
-1 2 2 2 2 -3 -2 4 -6 0 1 0 -1 -1 4 3 4 3 1 -5 -2
a
Calculated using ν ) 12194.1 - 20.05(ν + 1/2) - 0.038(ν + 1/2)2.
spectra for this transition. They conclude that this transition correlates with the lowest energy transition for the analogous MnH molecule,45 and they have assigned it as the b5Π-a5Σ band system of MnF.46 If this is correct, then the vibrational constants for the upper state would be approximately ωe ) 625.97 cm-1 and ωexe ) 3.20 cm-1. However, reaction energetics (see below) suggest that the lower state of this transition could lie ∼2000 cm-1 below the lower state of the d band system. Since the d band lower state is known to be the lowest a5Σ+ state,46,47 these energetics suggest that the lower state of the e system transition could in fact be the X7Σ+ ground state. Additional information will be needed to identify the states involved in this band system. d System (System II). The system d bands reported by Hayes and Nevin46 and rotationally analyzed by Launila and Simard47 are observed and well-resolved in this study (Figure 4). Transitions from levels up to and including V′ ) 11 have been identified and fit with the molecular constants reported previously (Table 1). The band spacings are regular, and there is no indication of vibrational perturbations at the current resolution. Both states in this band system have now been wellcharacterized, and the spectrum is associated with the MnF c5Σ+-a5Σ+ transition by Launila and Simard.47
Reaction of Manganese Vapor and Halogen Molecules
Figure 4. Chemiluminescent emission corresponding to the MnF d band system formed from the reaction Mn (a6S) + He + F2 f MnF* + He + F under multiple collision conditions. The spectrum is dominated by a strong ∆V ) 0 diagonal sequence accompanied by weaker ∆V ) (1, +2 groupings. V′ denotes the upper state level from which emission is observed. Spectral resolution is 1 Å. See also Figure 2 and text for discussion.
Figure 5. Chemiluminescent emission corresponding to the MnF c and b band systems formed from the reaction Mn (a6S) + He + F2 f MnF* + He + F under multiple collision conditions. Additional * contributions may also result from the Mn2 (1Σ+ g ) + He + F f MnF + Mn reaction. The b system is dominated by a strong ∆V ) 0 sequence structure and likely terminates in the ground electronic state of MnF. Spectral resolution is 2.5 Å. See text for discussion.
b(c) Systems (Systems III, IV). The MnF blue-green b and c band systems first reported by Hayes42 are depicted in Figure 5. The complex red degraded system c bands are broad and poorly resolved. Hayes42 has discerned that the c system corresponds to a series of Q heads separated by approximately 75 cm-1, and Launila and Simard44 have now established that these heads are the origin subbands of the d5Π-a5Σ+ transition. These authors45 also found some indication of hot bands in their jet cooled spectrum, with the heads to these bands identified with a quintet-quintet system. Because the system is badly blended and poorly resolved, little additional information can be obtained. Bands arising from the ∆V ) +1, 0, -1 sequences of the b system are apparent in the spectrum of Figure 5. Although sharper than the c system bands, the b system bands for the ∆V ) (1 sequences are weak and cannot be well-resolved. Band heads identified with the stronger ∆V ) 0 sequence suggest that the vibrational structure is blue degraded. Additional evidence for this assignment comes from the pressure dependence observed for this band system. As the pressure in the reaction zone is increased, the excited state vibrational excitation should be relaxed (quenched) with the (0,0) band emerging as the dominant feature. The changes observed in the ∆V ) 0 sequence structure as the pressure in the reaction zone is increased to promote vibrational relaxation are depicted in Figure 6. Because the dominant quenching occurs in the blue region
J. Phys. Chem., Vol. 100, No. 14, 1996 5663
Figure 6. Closeup of chemiluminescent spectrum corresponding to the ∆V ) 0 diagonal sequence of the MnF b band system showing the effect of increasing pressure and the excited state vibrational relaxation accompanying this increase. Spectra A-C correspond to increasing entrainment gas pressure in a given experimental run. Spectral resolution is 1.5 Å. See text for discussion.
of the sequence structure, the strong feature at 20 212 cm-1 (494.6 nm) and not the feature at 20 298 cm-1 (492.5 nm) is assigned as the origin band of this system, counter to previous suggestion.42 A new vibrational analysis for the b system is needed. Because the ∆V ) +1 sequence is not well-resolved and the ∆V ) -1 sequence is blended with the c system in the spectra observed here, an analysis based only on this data can only be approximate. However, an attempt to supplement this data using the band heads reported by Hayes42 offers some improvement. Some of the features reported by Hayes42 are not apparent in the present emission spectra, whereas some of the features observed in the present study are not catalogued in the tables in Hayes’ papers.42 The Deslandre’s scheme developed from the combined data is given in Table 3. The chemiluminescent spectra suggest that x in Table 3 is either 1 or 2. For x ) 1, a nonlinear least squares fit of the combined bands gives
ν ) 20206 + 640.8(1.5)(V′ + 1/2) - 3.9(.4)(V′ + 1/2)2 623.7(1.4)(V′′ + 1/2) + 3.4(.3)(V′′ + 1/2)2 (2) The lower state frequency is within experimental error of the value reported by Launila et al.49 for the X7Σ state and is in reasonable agreement with the value of 618.8 cm-1 reported previously by Rao et al.48 This suggests that the lower state of this transition is the ground state. Launila et al.49 have suggested that the known A-X transition of MnF may not correspond with the lowest energy septet excited state. Launila et al.49 and Hayes42 both have concluded that the b bands involve pentet states. Further, since the upper state vibrational constants agree within experimental error with those reported for the c system,44 it is possible that both transitions have a common upper state. This would suggest that the separation of the a5Σ and X7Σ states could be as low as 400 cm-1. However, neither Launila et al.49 nor Hayes have indicated that the same upper state is involved in these transitions. Further, the 400 cm-1 a-X separation is much less than that estimated by Launila and Simard,47 also casting doubt on this conclusion. Better data will be needed to
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TABLE 3: Portion of the Deslandre’s Table Developed for the b System of MnF (cm-1) Using the Spectra Observed in the Present Study and the Features Reported by Hayes V′′/V′ x x+1 x+2
x
x+1
20231.6 [631.1] [616.8] 19614.8 [632.5]
20862.7 [615.4] 20247.3 [612.1] 19635.2 [624.8]
x+3 x+4
x+2
x+3
x+4
20273.8 [607.4]
20881.1 [595.3] 20285.9
-
20260.0 [603.4] 19656.6 [617.2]
-
TABLE 4: Spectral Assignments for the Bands Tentatively Assigned to Mn2F assigmnta,b 0
01 1 03 2 3 0
obsd freq calcd freq obsd freq calcd freq (cm-1) (cm-1)a assigmnta,b (cm-1) (cm-1)a 22 293 22 400 22 500
22 293 22 400 22 499
3
03
1
01
1
1 01 03(?)
22 590 22 698 22 830
22 591 22 698 22 810
a Calculated from ν ) 22293 + 405ν1 + 110ν3 - 3.5ν32. b Indicated vibrational numbering is arbitrary.
Figure 7. Expanded view of the chemiluminescent emission spectra for the Mnx + (F2,Cl2) reaction systems in the region close to 450 nm. Features in this region might be attributed to manganese molecules or the manganese cluster halides. The observed spectra appear to display a background continuum-like emission on which, for the fluorine system, is superimposed a discrete spectrum which might reasonably be attributed to the Mn2F molecule formed in an Mnx>2 + He + F2 f Mn2F* + He + Mnx-2 reactive encounter. Trace A is from the manganese vapor-Cl2 system. Trace B is from the manganese vaporF2 system. Trace C is an expanded view of trace B. The features in the spectrum are tentatively associated with a short progression in an Mn-F stretch (ν1) and the Mn-Mn stretch, ν3. However, the latter features might also correspond to the bending mode, ν2, which is expected to be similar in magnitude to ν3. Spectral resolution is 8 Å. See text for discussion.
resolve this discrepancy and to produce more accurate upper state vibrational constants. A-X System. The A7Π-X7Σ+ transition of MnF48,49 has also been observed in this study. The observed features correlate with the ∆V ) 0, (1, (2 sequences, although individual band structure is not resolved. Since this transition is now wellcharacterized, further analysis was not attempted. Possible MnxF and Mn2 Emission Features. A broad feature close to 450 nm is observed in both the chemiluminescence spectra produced from the reactions between manganese clusters and chlorine or fluorine. The observed emission, shown in expanded view for the fluorine reaction in Figure 7, appears to display evidence for a structured spectrum superimposed on a background continuum. The structured features appear much more apparent for the fluorine system than they do for chlorine. The blue degraded features resulting from the fluorine reaction might tentatively be correlated with an Mn2 electronic transition as the peak-to-peak separation for the six features indicated in Figure 7 is ∼100 ( 5 cm-1, a reasonable value for an excited state vibrational frequency of Mn2. By comparison, the upper state vibrational frequency of the 650 nm system of Mn2 is ∼111 cm-1.17 However, the assignment is problematic for it is
unlikely that a vibrationally resolved emission spectrum for Mn2 would show no obvious features that can be correlated with the Mn2 ground state frequency of 76 cm-1 and demonstrate only an excited state frequency. Further, the observed intensity pattern would require an unusual excited state vibrational distribution or surprising Franck-Condon factors. Alternatively, the structured features depicted in the fluorine based spectrum of Figure 7 and catalogued in Table 4 might be correlated with an MnxF emitter where x is likely 2. As indicated in the figure, the spectrum might correspond to the combination of a short MnF stretch, ν1, on which is superimposed short progressions in the polyatomic bending mode, ν2 or the Mn-Mn stretch, ν3. This assignment not only is consistent with the observed intensity pattern expected for an emission dominated by progressions in the lower state frequency separations but also with the expected frequencies for the ground state of an Mn2+F- molecule, ν1 ∼ 400-450 cm-1 and ν2 and ν3 of the order of 100 cm-1. While it is not surprising that the corresponding emission in the chlorine system is red shifted from that of the fluorine based emitter, attempts to resolve a well-defined structure superimposed on a background broad emission feature have thusfar been unsuccessful. It is tempting to ascribe the remaining observed unresolved emission features depicted in Figure 2 to Mn2.. We note that a similar, albeit much more pronounced and enhanced, emission has been observed for certain excited state vibrational levels of Na2 produced from the reaction of Na3 and halogen atoms.50 The band maxima for the remaining broad emission features are similar for the chlorine or fluorine reaction systems and agree reasonably with the absorption maxima reported for Mn2 by Bier et al.21-23 It is tempting to suggest that these features might result from Mn2, although additional experiments will be needed to establish an assignment. Mechanistic and Energetic Considerations. Based upon the previously determined dissociation energy for MnF, 4.35 ( 0.15 eV,51 the reaction
Mn + F2 f MnF + F
∆H ≈ -2.75 eV
(3)
is approximately 2.75 eV exothermic. This exothermicity is not sufficient to populate the manganese z6P5/2 (3.07 eV) excited atomic state in an energy transfer step or the A7Π state of MnF (3.53 eV) in a single reaction step. Therefore, the energy needed to populate the A7Π or the z6P5/2 states must result from a
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Figure 8. Chemiluminescent spectra for the interaction of helium entrained manganese vapor and molecular fluorine. Traces A-C depict the changes in the chemiluminescent spectrum as the vapor pressure and hence concentration of the entrained manganese vapor is increased to promote clustering. Trace A corresponds to the lowest manganese metal flux, whereas trace C corresponds to the highest. Spectral resolution is 8 Å. See text for discussion.
multicentered metal molecule reaction, from the reaction of a low-lying atomic manganese metastable state, or from a multiple collision energy transfer up-conversion. There is little evidence for the latter two processes in the data of the present experiments. Thus, multicentered manganese molecule reactions are favored as the source of the most energetic atomic and molecular emissions. The multicentered reactions of manganese dimer, Mn2, via
Mn2 + F2 f MnF* + MnF
∆H ≈ -7.0 eV29-32,51
Mn2 + F(secondary) f MnF + Mn* ∆H2 ) -4.1 eV (4) are sufficiently exothermic to populate the Mn z6P5/2 or the MnF A7Π states if the energy is unequally partitioned among the reaction products. The multicentered reactions of manganese clusters, viz.,
Mn3 + F f MnF + Mn2* Mn4 + F2 ∼ Mn2* + 2MnF
∆H2 ≈ -4 eV ∆H ≈ -7 eV
(5)
can also provide the energy needed to populate the electronically excited states of Mn2. Energetic arguments also suggest that all of the features observed in the manganese-chlorine reaction system are produced via the multicentered reaction of manganese molecules. The reaction of ground state manganese atoms
Mn + Cl2 f MnCl + Cl
∆H ≈ -1.2 eV
(6)
is exothermic by ∆E ∼ D00 (MnCl)52 - D00(Cl2)53 ∼ 1.2 eV. Clearly this energy is insufficient to populate any of the observed excited state emitters in a one step process. However, the dimer reactions
Mn2 + Cl2 f MnCl* + MnCl Mn2 + Cl f MnCl* + Mn
∆H ≈ -4.7 eV ∆H ≈ -3.5 eV
(7)
and the trimer reactions
Mn3 + Cl2 f 2MnCl* + Mn
∆H ≈ -4.5 eV
Mn3 + Cl f MnCl* + Mn2
∆H ≈ -3.5 eV
(8)
are sufficiently exothermic to produce the monitored chemiluminescent emitters. Metal concentration experiments, the results of which are exemplified in Figure 8, and constant flux cooling experiments, the results of which are exemplified in Figure 9, were performed
Figure 9. Chemiluminescent spectra for the interaction of helium entrained manganese vapor and molecular fluorine showing the effects of entrainment gas cooling. Traces A and B were obtained using the same modest metal flux. In trace A, water cooling was used. In trace B, the helium + manganese vapor combination was cooled with liquid nitrogen. The key change with increased cooling involves a strengthening of the MnF A-X band system emission. Spectral resolution is 8 Å. See text for discussion.
to determine if manganese molecule reactions could be responsible for a portion of the observed chemiluminescence. At low metal fluxes, virtually no chemiluminescence is observed for the manganese flow reacting with molecular chlorine. However, the fluorine molecule reactions are characterized by a base emission at even the lowest metal fluxes. The failure to observe chemiluminescence from the chlorine reactions indicates that few excited state metastable manganese atoms are produced using the present source configuration. The reaction between ground state manganese atoms and molecular chlorine must produce only ground state products. As the vaporization temperature of the manganese source is increased, chemiluminescence is observed for both the fluorine and chlorine reactions. As the vaporization temperature increases and the concentration of metal atoms in the cooled metal vapor flow increases, the intensity of the chemiluminescence produced from reaction 3 will increase linearly with the metal atom concentration. However, as the seed for agglomeration is created, the relative concentration of manganese molecules in the flow is expected to increase at a rate proportional to the metal concentration raised to the appropriate power.37,38 The chemiluminescence produced from manganese molecule reactions increases more rapidly than the increase in intensity associated with reactions involving atoms, while the increase in intensity due to metal atom reactions is somewhat muted by their agglomeration. Therefore, features which grow in intensity relative to the remaining spectrum can be reasonably associated with manganese molecule reactions. A further cooling of the entrained metal flux to temperatures between those of dry ice and liquid nitrogen also enhances clustering. The enthalpy change for these molecule forming reactions is negative since chemical bonds are formed. The entropy change for the agglomeration reactions is also negative. Thus, lowering the temperature in the region where reaction occurs causes the equilibrium constant for manganese molecule reactions to increase, generating larger relative molecule concentrations in the flow. Changes in relative intensity with entrainment gas cooling thus aid in identifying the contribution of manganese atom and molecule reactions as they produce the observed chemiluminescence. The interdependence of the species in the flame does make it difficult to unequivocally determine the precise reaction sequence and branching producing a given emission feature. For example, the reaction between ground state manganese atoms and halogen molecules produces halogen atoms (reactions 3 and 6). Thus, an increase in the metal atom concentration will produce a corresponding increase in the halogen atom concentration in the reaction zone for subsequent reaction with
5666 J. Phys. Chem., Vol. 100, No. 14, 1996 manganese molecules. This interdependence makes the isolation of all contributing reactions under multiple collision conditions tenuous as certain excited states can be produced from more than one reaction. For example, the A7Π state of MnF or MnCl can be populated by four different manganese molecule reactions. The available data indicate that the emission from the e and the d bands of MnF is largely produced as a result of the direct atom reaction 3. The intensity of both systems mirrors the concentration of the metal atoms in the flow and shows no dramatic changes as conditions for clustering are created. This agrees with the recent findings of Huges et al.,54 which indicate that ground state manganese atoms will populate the e, d, and b(c) MnF band systems. These results suggest that the upper states of the b(c) systems may also be partially populated by the reaction of a ground state manganese atom. However, as demonstrated in Figure 9, the intensity of these systems increases relative to the d and e systems as the flow is cooled, indicating that at least a portion of the b and c state emitters are produced by manganese molecule reactions. The lower state for the e system has not been established with certainty; however, reaction energetics suggest that it is either the X7Σ ground state or the a5Σ state which lies very near the ground state in energy. It is apparent that 20 excited state vibrational levels are populated in the reaction. This requires a minimum of 23 450 cm-1 for the heat of reaction 3, indicating that the sum of the MnF dissociation energy and reactant relative and excess internal energies is greater than or equal to 4.53 ( 0.1 eV; this value is slightly larger than, but within the measured uncertainty of, the reported mass spectrometric determination of the bond energy. The agreement suggests that if the lower level of this transition is not the ground X7Σ state but the a5Σ state, the a5Σ and X7Σ states are separated by 400 cm-1 as measured from the b-c system separation. This value must represent a lower bound for the a5Σ+-X7Σ energy separation. The d bands are known to terminate in the a5Σ state, not the X7Σ ground state. The energy spacing between the a and X states is not known but can be estimated from reaction energetics. Since the V′ ) 11 level of the c5Σ state is populated via reaction 3, 20 958 cm-1 of reaction exoergicity is accounted for from the highest excitation characteristic of the emission features. If the lower state for the MnF e system is the ground electronic state, the difference in the d band energy register and the reaction exoergicity monitored from the e system, 2500 ( 500 cm-1, provides an upper limit estimate for the energy separation between the low-lying a5Σ and the ground X7Σ states. This value compares favorably to the recently determined separation of 1745 cm-1 in MnH55 and is in agreement with the value of 3000 ( 1000 cm-1 estimated by Launila and Simard.48 The value calculated using the mass spectrometric dissociation energy lies between 1000 and 2000 cm-1. Huges et al.54 have found that the emission from the b system is also produced from the reaction between ground state manganese atoms and fluorine. If the lower state of this transition is the ground state, there is sufficient energy produced from reaction 3 to populate the V′′ ) 4 or 5 levels of the b state. Emission is observed from at least these levels in the present study; however, the intensity changes observed with increased clustering suggest that the observed features result, at least in part, from manganese molecule reactions. The MnF A7Π state must be populated by multicentered metal cluster reactions which include reaction 4. As well, the energy released from a cluster reaction process is needed to populate the higher lying Mn atom states in an energy transfer process.
Devore and Gole The dimer reaction with fluorine atoms (reaction 4) is a strong candidate to produce the z6P atomic emission. There is a close match between the energy released in reaction 4 and the energy needed to excite this state, which suggests that the process should have a favorable reaction cross section. Further, there are no close-lying MnF states to readily promote possible energy transfer reactions. Preliminary investigations exploring the reaction between metal clusters and fluorine atoms, generated from an electrical discharge through SF6, do produce emission from the z6P state, which would seem to support this conclusion.56 In contrast, several cluster reactions and/or energy transfer processes are possible candidates to produce emission from the z8P state of Mn. The kinetics for the manganese-chlorine reactions are less established. The reactions between manganese molecules and chlorine molecules (reactions 7 and 8) probably produce the majority of the MnCl emission. The Mn z6P atomic emission could be produced from the manganese dimer/Cl atom reaction (reaction 7). If so, the energetics for this reaction indicate that the dissociation energy of Mn2 must be less than 0.5 eV consistent with previous determinations.57 The reactions between manganese trimer and a halogen atom are the most likely reactions to produce the excited state emitters that can be tentatively correlated with Mn2. These reactions, of course, also can produce emission from Mn2X via
Mn3 + X f Mn + Mn2X*
∆H ∼ 4 eV
(9)
The reactions of larger clusters could also produce these features, but this seems less likely in the present experiments. Conclusions and Quandries Emission features that have been attributed to the manganese halides, manganese atoms, the manganese cluster halides, and possibly manganese dimer have been observed when a cooled, entrained flux of manganese vapor reacts with chlorine or fluorine molecules. A coupling of energetic and kinetic considerations suggests that manganese atoms, Mn2, and Mn3 contribute to the observed emission features. There is some evidence that halogen atoms produced in the reaction sequence might also react to produce a portion of the observed emission features. A revised vibrational analysis of the b system of MnF suggests that this transition terminates in the ground state. Launila et al.49 find an anomalous spin orbit parameter for the A7Π state which they compare to that for the supposed isoelectronic d5Πi state. They suggest that this could indicate that the A state is not the lowest excited septet state. If the b system also terminates in the ground state, then the upper states of the b and c system bands may both result from an electron promoted from the ground state ...8σ23π4[1δ24π29σ1]10σ1 electron configuration to the ...8σ23π4[1δ24π9σ1]5π2 excited electron configuration in analogy to the corresponding transitions in MnH. The A7Π-X7Σ+ transition would then result from an alternate transition. One possibility might correspond to the transition between states described by the ...8σ23π4[1δ24π29σ1]10σ1 and the ...8σ23π3[1δ24π39σ1]10σ1 electron configurations. However, it is hard to rationalize how this promotion would produce a more stable excited state configuration than that ascribed to the A state. Energetics suggest that the lower state of the e system bands could lie 2500 cm-1 lower than the lower state of the d system bands. This requires that the energy exhibited through population of the vibrational levels of the e system be released via the reaction between manganese atoms and molecular fluorine. This
Reaction of Manganese Vapor and Halogen Molecules also suggests that the e system terminates in the ground state. However, no obvious transition exists that can explain these bands, and no analogous transition exists for MnH, thus making this explanation tenuous. A second possible reason for the current observations could be that the c5Σ+ state is predissociated. It is also possible that there is an activation barrier to forming this state. The situation is complex, and the solution to this problem must await accurate assignment of the states which contribute to the e band system. The energy of reaction can provide a method for estimating the energy difference between the MnF X7Σ+ and a5Σ+ states. The difference between the observed energy monitored from the c5Σ+-a5Σ+ transition and the heat of reaction expected from the mass spectrometric determination of the MnF dissociation energy, coupled with reasonable estimates for the translational energy of the reactants, suggests a separation of ∼2500 ( 500 cm-1. This agrees well with the value estimated by Launila and Simard45 on the basis of Boltzmann arguments. References and Notes (1) Chalek, C. L.; Gole, J. L. Proceedings of the Symposium on High Temperature Metal Halide Chemistry, 1977. Proc. Electrochem. Soc. 1978, 78-1, 278. (2) Preuss, D. R.; Chalek, C. L.; Gole, J. L. J. Chem. Phys. 1977, 66, 548. (3) Preuss, D. R.; Gole, J. L. J. Chem. Phys. 1977, 67, 850. (4) Brayman, H. C.; Fischell, D. R.; Cool, T. A. J. Chem. Phys. 1980, 73, 4246. (5) Rosano, W. J.; Parson, J. M. J. Chem. Phys. 1986, 84, 6250. (6) Parson, J. M. J. Phys. Chem. 1986, 90, 1811. (7) Devore, T. C.; McQuaid, M.; Gole, J. L. High Temp. Sci. 1990, 29, 1. (8) Devore, T. C.; McQuaid, M.; Gole, J. L. High Temp. Sci. 1991, 30, 83. (9) Gole, J. L. Probing Ultrafast Energy Transfer Excited Small High Temperature Molecules. In Gas Phase Chemiluminescence and Chemiionization; Fontijn, A., Ed.; Elsevier Science Publishers: New York, 1985; p 253. (10) Nesbet, R. K. Phys. ReV. 1964, A135, 460. (11) Harris, J.; Jones, R. O. J. Chem. Phys. 1979, 70, 830. (12) Wolf, A.; Schmedthe, H. H. Int. J. Quantum Chem. 1980, 18, 1187. (13) Salahub, D. R.; Baykaun, N. A. Surf. Sci. 1985, 156, 605. (14) Gutsev, G. L.; Lutatskaya, V. D.; Klyagina, A. P.; Levin, A. A. Zh. Strukt. Khim. 1987, 28, 22. (15) Klyagina, A. P.; Fuvsoua, V. D. Zh. Strukt. Khim. 1987, 28, 39. (16) Bauschlicher, C. W., Jr. Chem. Phys. Lett. 1989, 156, 95. (17) Devore, T. C.; Ewing, A.; Franzen, H. F.; Calder, V. Chem. Phys. Lett. 1975, 35, 78. (18) Klotzbucher, W. E.; Ozin, G. A. Inorg. Chem. 1980, 19, 3776. (19) Rivoal, J. C.; Shanks Emampour, J.; Zeranjue, K. J.; Vala, M. Chem. Phys. Lett. 1982, 92, 313. (20) Moskovits, M.; Dilella, D. P.; Limm, W. J. Chem. Phys. 1984, 80, 626. (21) Bier, K. D.; Haslett, J. L.; Kirkwood, A. D.; Moskovits, M. J. Chem. Phys. 1988, 89, 6. (22) Bier, K. D.; Haslett, T. L.; Kirkwood, A. D.; Moskovits, M. Faraday Discuss. Chem. Soc. 1988, 86, 181.
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