Energetics, molecular electronic structure, and spectroscopy of forming Group IIA dihalide complexes

Chemical Physics 241 Ž1999. 221–238

Energetics, molecular electronic structure, and spectroscopy of forming Group IIA dihalide complexes T.C. Devore 1, J.L. Gole

)

School of Physics, Georgia Institute of Technology, Atlanta, GA 30332-0430, USA Received 7 October 1998

Abstract Multiple-collision relaxed Žhelium. chemiluminescence and laser-induced fluorescent spectroscopy have been used to demonstrate the highly efficient collisional stabilization of electronically excited Group IIA dihalide collision complexes formed in M ŽCa,Sr. q X 2 ŽXY. ŽCl 2 , Br2 , ICl, IBr, I 2 . reactive encounters. The first discrete emission spectra for the CaCl 2 , CaBr2 , SrCl 2 , SrBr2 , and SrICl dihalides are observed and evaluated; however, the low-pressure ‘continuous’ chemiluminescent emission observed for forming barium dihalide ŽBaX 2 . complexes is quenched under these experimental conditions. The reactions of the Group IIA metals with molecular fluorine do not readily produce the corresponding dihalide. While the lowest-lying observed dihalide visible transition is, as predicted, found to result in an extended progression in a dihalide complex bending mode ŽSrCl 2 ., the observed progression suggests the presence of a residual halogen ŽCl–Cl. bond. Two higher-lying transitions are dominated by a vibrational mode structure corresponding to progressions in the symmetric stretching mode or, for nominally forbidden electronic transitions, odd quanta of the asymmetric stretching mode. Some evidence for sequence structure associated with the dihalide bending mode is also obtained. These observations are consistent with complex formation as it is coupled with a modified valence electron structure Žcorrelation diagram. associated with the highly ionic nature of the dihalides. The bonding in the Group IIA dihalides Žand their complexes., whose atomization energies are more than twice the metal monohalide bond energy, strongly influences the evaluation of energetics and the determination of monohalide bond energies from chemiluminescent processes. Discrepancies between those bond strengths determined by mass spectrometry and chemiluminescence are discussed with a focus on energy partitioning in dihalide complex formation and its influence on chemical vapor deposition. q 1999 Published by Elsevier Science B.V. All rights reserved.

1. Introduction Metals, whether as additives or impurities, are present and can play a significant role in combustion-based systems w1x. Therefore, the behavior of these metals, both beneficial and deleterious, must be )

Corresponding author. Permanent address: Department of Chemistry, James Madison University, Harrisonburg, VA, USA. 1

understood in order to evaluate their influence in enhancing or controlling the process of interest. Recently, these metallic interactions have gained further import as combustion synthesis has become a method of choice for the production of nanoscale refractory solids of controlled narrow size distribution and high purity w1,2x. Within this framework, it is also becoming increasingly clear w3x that the ability of metals to form complexes in the gas-phase oxidation or reduction process can have an impact on the nature of

0301-0104r99r$ - see front matter q 1999 Published by Elsevier Science B.V. All rights reserved. PII: S 0 3 0 1 - 0 1 0 4 Ž 9 8 . 0 0 4 0 1 - 7

222

T.C. DeÕore, J.L. Gole r Chemical Physics 241 (1999) 221–238

these synthesis processes as well as on chemical vapor deposition in general ŽMOCVD. w3,4x. The reactions of alkaline earth atoms with halogen molecules form the Group IIA dihalides, MX 2) , in a termolecular process, second order in the halogen concentration at micro-Torr pressures w5–7x. Recently, a mechanism consistent with the halogen pressure dependence has been established w8–10x which demonstrates the collisional stabilization of a highly vibrational excited MX 2) † q intermediate viz. M q X 2 ™ MX 2) †

Ž 1a .

MX 2) † q X 2 ™ MX 2) q X 2

Ž 1b .

MX 2) ™ MX 2 q hn

Ž 1c .

where the stabilization process, a radiative three-body recombination ŽR3BR., overcomes the propensity for a ‘rapid’ dissociation of the highly excited MX 2) † complex. Through a series of controlled multiple-collision chemiluminescent studies w10x, we have confirmed the long-range collisional stabilization of high-temperature Group IIA dihalide complexes of some considerable spatial extent. Controlled relaxation also reveals the first vibrationally resolved electronic emission for the calcium and strontium dihalides. The vibronic structure of the observed emission spectra correlates well with expectations based on the molecular electronic structure of the ground and low-lying electronic states of the dihalides while also suggesting the formation of a long-lived complex. In this paper, we will consider this vibronic structure and its assignment in more detail, evaluating its implications for long-range collisional stabilization accompanied by complex formation Žand subsequent emission.. The Group IIA dihalides have been studied extensively by Hildenbrand w11x who has used an astute combination of mass spectrometric data and a variety of structural models to evaluate the validity with which ionic models can describe these systems. As Table 1 demonstrates, the IIA chlorides, bromides and iodides are unusual in that the strength of the second halogen bond is considerably greater than that of the first. In other words, the atomization energy, especially for the heavier halogen dihalides, is at least twice the dissociation energy of the metal monohalide. This bonding characteristic not only has implications for the evaluation of energy partitioning

Table 1 Experimentally determined bond energies of Group IIA monoand dihalides Do8 kcalrmola F Mg Ca Sr Ba

109 126 266Ž1408. 129 260Ž1208. 139 271Ž1008.

Cl

Br

I

75 94 214Ž1808. 96 211Ž1408. 104 220Ž1208.

Ž58. 73 186Ž1808. 79 189Ž1808. 85 197Ž1508.

Ž45. 62 155Ž1808. 64 155Ž1808. 72 166Ž1808.

a

For each entry, the upper value corresponds to the metal monohalide bond energy and the lower value corresponds to that for the metal dihalide.

but also suggests that the Group IIA–halogen molecule reactions might present ideal systems in which to study few body complexation processes and hence the efficient collisional stabilization of intermediate complexes. Structural w12,13x and spectroscopic data obtained on the dihalides, in combination with a firm thermochemical base, can also furnish an important link to further refine the extent and validity of the ionic models which Hildenbrand w11x and others w14x have developed to aid our understanding of higher pressure systems. In this paper, in addition to modeling the vibronic structure observed for the CaCl 2 , CaBr2 , SrCl 2 , SrBr2 , and SrICl molecules, we evaluate dihalide complex formation as it influences energy partitioning and the formation of metal monohalide excited states. This analysis may account for the discrepancies between those monohalide bond strengths determined by mass spectrometry and ‘single-collision’ chemiluminescent techniques.

2. Experimental 2.1. Sources Multiple-collision chemiluminescent and laser-induced fluorescence ŽLIF. experiments were carried out in an entrainment device similar to that used previously w15–20x. The Group IIA metals calcium, strontium, and barium were vaporized from specially designed and capped c.s. grade graphite crucibles at temperatures between 900 and 1200 K producing a vapor pressure between 10y3 and 10 1 Torr for most

T.C. DeÕore, J.L. Gole r Chemical Physics 241 (1999) 221–238

of the experiments considered in the present study. The crucibles were machined to fit inside a commercial tungsten basket heater ŽR.D. Mathis, Long Beach, CA. which was lightly wrapped with zirconia cloth ŽZircar Products, Florida, NY.. The Group IIA metal flux was entrained in a helium ŽHolox 99.998%. or argon ŽHolox 99.99%. buffer gas and transported to the reaction zone. At a suitable point above the flow, the halogen molecule oxidants intersected the entrained Group IIA metals, entering from a concentric ring injector inlet. Typical operating pressures ranged from 10 to 80 mTorr oxidant and between 1 and 2 Torr of helium buffer gas. 2.2. Detection systems and LIF studies Both chemiluminescent and LIF techniques w21,22x were used to monitor Group IIA mono- and dihalide reaction products in the multiple-collision pressure regime. The chemiluminescence from the dihalides of interest was monitored at right angles to the metal flow. Spectral emissions were dispersed with a 1-m Spex scanning monochromator operated in first order with a Bausch and Lomb 1200 groovermm grating ˚ Either cooled RCA 1P28 or 4840 blazed at 5000 A. or a dry ice-cooled EMI 9808 photomultiplier tube were used to detect the dispersed fluorescence and provide a signal for a Keithley 417 autoranging picoammeter. The output signal from the picoammeter was then sent to a personal computer for storage and subsequent analysis. All spectra were wavelength calibrated with a mercury arc lamp Žsee e.g., Refs. w23,24x. or with respect to Group IIA metal atomic emissions Žsee e.g., Refs. w23,24x.. In order to carry out LIF experiments, the second harmonic of a Quanta-ray Nd:YAG laser Ž0.53 m . was used to pump a Spectra-Physics PDL-3 pulsed tunable dye laser system operated with DCM or LDS698 dye. The output of the pulsed dye laser Žwith a linewidth of 0.07 cmy1 and a pulsewidth of 9 ns. was then either mixed with the fundamental output of the YAG laser or frequency doubled in a frequency mixer ŽQuanta-Ray WEX-1. to produce UV Žin the range 310–410 nm. coherent radiation. The laser beam was introduced to the reaction chamber in a direction perpendicular to both the reactant flow and detector. The YAG laser was triggered by a digital pulse generator ŽSRS DG535. with a repetition rate of 15 Hz. The Q-switching signal of the

223

YAG oscillator was used to trigger a boxcar integrator ŽSRS SR250. for better synchronization. The fluorescence induced by the UV laser pulse was collected with an RCA 1P28 photomultiplier Ž2.2 ns rise time. and, through a fast preamplifier ŽCLC 100 Video Amplifier, 500 MHZ., sent to the gated integrator to record the spectrum as a function of the laser frequency. A fast digital oscilloscope ŽHP 54111D, 0.7 ns rise time. was used to real-time monitor and record the fluorescence decay. The integration gate was set to a proper width in the range from 20 to 300 ns, dictated by the nature of the monitored fluorescence decay, with a delay timed such that the gate opened just after the short laser scattering pulse, thus reducing background noise. A personal computer ŽPC. drives the dye laser stepper motor, scanning the dye laser frequency and acquiring the averaged output data from the boxcar synchronously. In order to achieve a linear scan in wavelength when the dye laser frequency was mixed with the infrared, the scan step size of the dye laser was calculated in real time using the PC. The output frequency of the WEX-1 was calibrated using aluminum atom lines w23,24x.

3. Periodicity of Group IIA dihalide electronic states In order to evaluate the dihalide emission spectra which characterize the Group IIA metal–halogen molecule reactions, the nature of the ground and low-lying electronic states of these highly ionic 16 valence electron molecules must be considered. Klemperer et al. w25–28x used electric quadrupole deflection of molecular beams to establish the grid of dihalide ground state geometries outlined in Table 2. Table 2 Geometry of Group IIA dihalides Metal

Be Mg Ca Sr Ba

Halide F

Cl

Br

I

l l

l l l

l l l l

l l l l

b

b

b b b

l , linear; b, bent.

b b

T.C. DeÕore, J.L. Gole r Chemical Physics 241 (1999) 221–238

224

Table 3 Group IIA dihalides – low-lying electronic states, electronic transitions in D`h ŽAbsorption. or C 2v ŽEmission. symmetry Electron configurationa ŽAbsorption.

State designation ŽAbsorption.

Electron configurationa ŽEmission.

State designation ŽEmission.

. . . Ž x, y,pg . 4c linear . . . Ž x, y,pg . 3 Ž l sg . c linear

X 1 Sq g ground state 1,3 P g A–X region

. . . Žxa 2 . 2 Žyb 2 . 2 . . . Žxa 2 .1 Žyb 2 . 2 Ž l a 1 .1

X 1A 1 ground state 1,3 A 2 A–X regione

. . . Ž x, y,pg . 3 Žmpu . c linear

1,3

S u B–X region

. . . Žxa 2 . 2 Žyb 2 .1 Ž l a 1 .1 . . . Žxa 2 .1 Žyb 2 . 2 Žmb1 .1

1,3

1,3

P u C–X region

1,3

. . . Žxa 2 . 2 Žyb 2 .1 Žmb1 .1 . . . Žxa 2 .1 Žyb 2 . 2 Žna 1 .1

1,3

. . . Žxa 2 . 2 Žyb 2 .1 Žna 1 .1

1,3

B2 B 2 B–X regionf,g

A 2 C–X region g A 2 ŽC–X region. g

1,3

Commentsb

A–X region transitions1 will produce large change bond angle B–X region transitions1 will produce significant change in bond length C–X region transitions1 will produce significant change in bond length

B2

a

Highest occupied orbitals Ž x, y . and lowest promotion orbitals Ž l ,m,n.. B 2 –X 1A 1 transition is electric dipole allowed, 1A 2 –X 1A 1 transition is vibronically allowed through coupling with the ground or excited state asymmetric stretch of b 2 symmetry. The corresponding triplet–singlet transitions will be weaker. X c For MXY ŽSrICl, . . . . we have Ž x, y,p . 4 , Ž x, y,p . 3 Ž l s ., and Ž x, y,p . 3 Žmp .. Y X X Y X X X Y d For MXY ŽSrICl, . . . . we have Žfor example. Žxa .Žya . 2 Ž l a .1 , 1,3A ; Žxa . 2 Žya .Ž l a ., 1,3A in the A–X region. e For the A–X region see Figs. 2 and 3. f For the B–X region see Figs. 4–8 and 10. g For the C–X region see Fig. 9. b 1

Linear geometries are favored by the light metal– heavy halogen combinations whereas bent structures are favored by the heavy metal–light halogen combinations. These geometric trends can be explained w29–32x on the basis of a modification of Walsh’s correlation diagrams for AB 2-type molecules to take into account the influence of normally unfilled dorbitals on the metal atom. The Group IIA dihalides are predicted to be linear when only the s- and p-orbitals on the metal are involved in the bonding. However, as the d-orbitals increase in importance for the heavier alkaline metals and their singly charged ions w29–32x, the non-linear structure becomes more favorable. This, and the strongly ionic character of the Group II dihalides lead to a modification of the valence correlation diagram for these systems w8,9,33x. The makeup of the valence molecular orbitals for the linear 16 valence electron BeF2 molecule and its correlation diagram has been discussed w8,9,33x and comparisons to the valence molecular orbitals for CO 2 have been made. The lowest energy excited states in BeF2 are predicted to result from transitions from the highest occupied pg Žb 2 . and pg Ža 2 . orbitals

of the ground state to the lowest unoccupied sg Ža 1 . molecular orbital. This is in contrast to CO 2 where experiments have confirmed the prediction that the lowest energy transitions are from the pg Žb 2 . and pg Ža 2 . to the puŽa 1 . molecular orbital. This predicted change in the ordering of the puŽa 1 . and the

Fig. 1. MNDO–PM 3 electronic structure calculations showing the predicted changes in ground electronic state vibrational frequencies of CaCl 2 as the molecule is bent. This calculation indicates that n 1 Žsymmetric stretch. and n 2 Žbend. increase and n 3 Žasymmetric stretch. decreases.

T.C. DeÕore, J.L. Gole r Chemical Physics 241 (1999) 221–238

sg Ža 1 . orbitals suggests that the lowest energy transitions for linear BeF2 and several of the remaining Group II dihalides will be quite weak since a g-to-g transition is electric dipole forbidden. In fact, this lowest energy transition has not been observed in absorption even for the highly bent BaF2 molecule w34x where the inversion selection rule no longer applies and the transition probability is predicted to be stronger. The electronic transitions predicted for the excited states of the dihalides are summarized in Table 3. At the lowest energies, it should be possible to observe a reasonably intense emission from the low-lying 1 B 2 excited state. This state is strongly bent for all of the Group IIA dihalides. The emission spectrum of this state is expected to involve long progressions in the bending mode since there are substantial changes in the bond angle between the strongly bent excited state and the linear or near-linear ground state. Transitions from the 1A 2 state are expected to be weak based upon the electronic selection rules for a C 2v molecule. Any observed transition must involve odd quanta of the asymmetric stretch if it is to be observed. Spin forbidden transitions from the corresponding triplet state are expected to be even weaker. Transitions involving excited states which result from a pg to pu valence molecular orbital excitation will be found at higher energy. Transitions among the singlet states are fully dipole allowed and predicted to involve excited states of nearly the same bond angle as the ground state. The transition into the anti-bonding pu orbital should result in a fairy large increase in the bond length and produce at least a moderate progression in the symmetric stretching mode w8,9,33x. Because the emission spectra which we observe will be determined by Ž1. the dynamics of excited state formation and Ž2. the nature of vertical transitions at the excited state geometries, influenced also by the reactive formation of the dihalide, it is appropriate that we evaluate changes in the ground state vibrational mode frequencies as a function of bond angle. Molecular electronic structure calculations 2

225

Table 4 Ground state vibrational frequencies for select Group IIA dihalides Dihalide Žvibrational mode. CaCl 2

CaBr2

SrCl 2

SrBr2

Žn 1 . Žn 2 . Žn 3 . Žn 1 . Žn 2 . Žn 3 . Žn 1 . Žn 2 . Žn 3 . Žn 1 . Žn 2 . Žn 3 .

Vibrational frequencies Žcmy1 . 265a , 275 b , 217 c , 238–243 d 50 a , 48 b , 71.5e , 64 d 442 a , 420 b , 395e , 402 d 171a , 170 b , 148c 33 a , 44 b , 67 e 382 a , 350 b , 335e 251a , 256 b , 270 c 19 a , 20 b , 44 e 337 a , 307 b , 300 c 160 a , 152 b , 157 f 13 a , 11b , 37 f 267 a , 242 b , 263 f , 223–269 d

a

Ref. w12x. Ref. w39x. c See JANAF thermochemical tables in Ref. w38x. d IR gas-phase and matrix studies as quoted by Ref. w12x. See Refs. w40–43x. e Ref. w40x. f Estimated by Ref. w43x. b

on the ground electronic states of CaCl 2 and CaBr2 , have been combined with force field calculations to produce the data exemplified for CaCl 2 in Fig. 1. Vibrational frequencies were calculated from potential curves minimized as a function of bond angle. For force field calculations on CaCl 2 , CaBr2 , SrCl 2 , and SrBr2 , the Wilson FG-matrix method was applied w37x. The G-matrix was calculated using the ground state geometry given in the JANAF Tables w38x. This matrix and the experimental vibrational frequencies ŽTable 4. were used to determine the F-matrix. This F-matrix was then used to calculate frequencies as changes in the structure of the molecule generated a modified G-matrix. Both calculations indicated that the n 2 Žbending. and n 1 Žsymmetric stretch. mode frequencies increase as the dihalide molecule is bent while the n 3 Žasymmetric stretch. mode frequency decreases.

4. Observed emission spectra

2

The calculations performed used MNDO–PM3 to evaluate relative potentials and establish trends Žsee Refs. w35,36x..

The Group IIA dihalide molecules for which we have obtained vibronically resolved emission spectra include CaCl 2 , CaBr2 , SrCl 2 , SrBr2 , and SrICl. The

226

T.C. DeÕore, J.L. Gole r Chemical Physics 241 (1999) 221–238

Fig. 2. SrBr2 qSrBr multiple collision chemiluminescent spec˚ Formation of the di- and trum taken at a resolution of 10 A. He

monobromide results from the reactive process SrqBr2 ™SrBr2) q hn 1 or SrBrqBrq hn 2 at a background helium pressure of 0.8–1.2 Torr. The dihalide A–X, B–X, and C–X emission regions Žsee Table 3. are indicated in the figure. See text for discussion.

overview spectrum we have observed for the reaction of strontium with molecular bromine Žthis reaction mixture flowing in helium. depicted in Fig. 2 is exemplary. Features arising from strontium atomic transitions, electronic emissions from the strontium monohalides, and three additional band systems labeled as A–X, B–X, and C–X are observed. Similar, albeit wavelength shifted, spectra were observed for the Sr–Cl 2 and Sr–ICl systems while only the A–X and B–X systems characterize the calcium reactions. The emission intensity for the A–X, B–X, and C–X systems in excess helium was approximately second order in the halogen pressure signaling there association with the dihalide w8–10x. In the SrBr2 overview spectrum Žand for similar spectra obtained for the Ca–Cl 2 , Ca–Br2 , and Sr–ICl reactive systems. the A–X band system Žfeature., which extends from the fringes of the ultraviolet through the visible, appears to be dominated by a long progression in the bending mode of an SrX 2 complex. This is more apparent in the SrCl 2 emission spectrum depicted in Fig. 3. The observed features, catalogued in Table 5, can be associated with the analog of the 1 B 2 ŽŽ1a 2 . 2 Ž4b 2 .Ž6a 1 .. ™

X 1A 1ŽŽ1a 2 . 2 Ž4b 2 . 2 . transition in BeF2 which correlates in linear configuration to a 1 P g ™1 Sq g transition involving the sg and pg molecular orbitals. For all of the systems studied we have found that both Group IIA metal and halogen molecule collisions very effectively quench this feature, a fact which is not surprising in view of the anticipated longer lifetime Žtradiative 0 10y5 s. associated with the emitting excited state w8–10x. The higher energy, B–X band system region depicted in Fig. 2 is displayed in greater detail for the Ca q Cl 2 , Ca q Br2 , Sr q Cl 2 , Sr q Br2 , and Sr q ICl reactive encounters in Figs. 4–8, and catalogued in Tables 6 and 7. The weaker, higher energy, C–X emission region which appears energetically accessible only to the Sr q Cl 2 , Br2 and ICl reactions is displayed in greater detail in Fig. 9 and catalogued in Table 8. The B–X emission region would appear to correspond to the electric dipole allowed 1 B 2 Ž . . . Ž1a 2 .Ž4b 2 . 2 Ž2b 1 .. and ŽŽ1a 2 . 2 Ž4b 2 .Ž3a 1 ... –X 1A 1 transitions in BeF2 ŽTable 3. which correlate in linear configuration to the allowed 1 P u – 1 Sq g transition involving the pu and pg molecular orbitals.

Fig. 3. Multiple collision chemiluminescent spectrum for SrCl 2 in the dihalide A–X emission region Žsee Table 3. taken at a ˚ The emission corresponds to a long progresresolution of 10 A. sion in the dihalide bending mode. The dihalide emitter is formed He

via a collisionally stabilized SrqCl 2 ™SrCl )2 reactive encounter. The helium gas background pressure ranges from 0.8 to 1.2 Torr. The labeled emission features are catalogued in Table 5 and discussed in the text.

T.C. DeÕore, J.L. Gole r Chemical Physics 241 (1999) 221–238

227

Table 5 Vibronic bands observed for SrCl 2 in the dihalide ‘A–X’ transition regiona Wavelength ˚. ŽA

Frequency b Žvacuum-cmy1 .

1

4387

22788.2

2

4411

22664.2

3

4434

22546.7

4

4456

22435.4

5

4480

22315.2

6

4500

22216.0

7

4524

22098.2

8

4548

21981.6

Band number Žsee Fig. 3.

DŽfrequency. Žcmy1 . 124 118 111 120 99 118 117

He

115 9

4572

21866.2

10

4596

21752.0

11

4620

21639.0

12

4646

21517.9

13

4672

21398.2

14

4700

21270.7

15

4726

21153.7

16

4754

21029.1

17

4782

20905.9

18

4806

20801.5

19

4836

20672.5

20

4862

20561.9

21

4891

20440.0

Fig. 4. Multiple collision chemiluminescent spectrum for CaCl 2 in the dihalide B–X emission region Žsee Table 3. taken at a ˚ The dihalide emitter is formed via a collisionresolution of 10 A.

114 113 121 120 128 117

ally stabilized CaqCl 2™CaCl )2 reactive encounter. The helium gas background pressure ranges from 0.8 to 1.2 Torr. The spectrum appears to be dominated by dihalide stretching modes. The labeled emission features are catalogued in Table 6 and discussed in the text.

MX 2 or MXY stretching modes. The frequency separations between the observed features are consistent with moderate progressions in the dihalide stretching modes. The emission features ŽTable 7. associated with the C–X region are considerably weaker than those associated with the A–X and B–X regions. This may

125 123 104 119 112 122 a

See Table 3 and Fig. 3. Frequencies are "5 cmy1 . Additional poorly resolved features to shorter and longer wavelength.

b

This transition which, for the MX 2 ŽMXY. molecules in absorption, involves primarily a change from M–X nonbonding to M–X antibonding character, should be dominated in emission by progressions in the

Fig. 5. Multiple collision chemiluminescent spectrum for CaBr2 in the dihalide B–X emission region Žsee Table 3. taken at a ˚ The dihalide emitter is formed via a collisionresolution of 10 A. He

ally stabilized CaqBr2™CaBr2) reactive encounter. The helium gas background pressure ranges from 0.8 to 1.2 Torr.

T.C. DeÕore, J.L. Gole r Chemical Physics 241 (1999) 221–238

228

Fig. 6. Multiple collision chemiluminescent spectrum for SrCl 2 in the dihalide B–X emission region Žsee Table 3. taken at a ˚ The dihalide emitter is formed via a collisionresolution of 10 A. He

ally stabilized SrqCl 2™SrCl )2 reactive encounter. The helium gas background pressure ranges from 0.8 to 1.2 Torr. The spectrum appears to be dominated by dihalide stretching modes. The labeled emission features are catalogued in Table 7 and discussed in the text.

result from a lack of reaction exoergicity to populate these states Žwhich will lie at somewhat higher energy for the calcium dihalide.. Alternatively the

Fig. 7. Multiple collision chemiluminescent spectrum for SrBr2 in the dihalide B–X emission region Žsee Table 3. taken at a ˚ The dihalide emitter is formed via a collisionresolution of 10 A.

Fig. 8. Multiple collision chemiluminescent spectrum for SrICl in the dihalide B–X emission region Žsee Table 3. taken at a ˚ The dominant dihalide emitter is formed via a resolution of 10 A. He

collisionally stabilized SrqICl 2™SrICl ) reactive encounter Žsome SrCl )2 contamination is also evident due to the SrClqICl ™SrCl )2 qI reaction. The helium gas background pressure ranges from 0.8 to 1.2 Torr. The spectrum appears to be dominated by dihalide stretching modes. The labeled emission features are catalogued in Table 7 and discussed in the text.

emission may correlate with a vibronically allowed transition Žsee following.. The assignments which we suggest for the A–X, B–X, and C–X emission band systems are all consistent with the expected lowest energy transitions of the dihalides ŽTable 3.. The close proximity of the calcium and strontium dichloride and dibromide emission systems ŽTable 9. may, at first, seem surprising. However, the small energy increment is reasonable since both the Group IIA metal neutral and positive ion states ŽTable 10., which are the atomic dissociation products to which the dihalide excited states must correlate, also lie close in energy. There have been several studies w12,39–43x of the vibrational spectroscopy of the Group IIA dihalide ground states, both experimental and theoretical, which suggest the ground state vibrational frequency ranges summarized in Table 4 for the CaCl 2 , CaBr2 , SrCl 2 , and SrBr2 molecules.

He

ally stabilized SrqBr2 SrqBr2 ™SrBr2) reactive encounter. The helium gas background pressure ranges from 0.8 to 1.2 Torr. The spectrum appears to be dominated by dihalide stretching modes. The labeled emission features are catalogued in Table 8 and discussed in the text.

4.1. Details of obserÕed emission spectra The resolved features for the SrCl 2 A–X band system ŽFig. 3. extending from ; 420 nm well into

T.C. DeÕore, J.L. Gole r Chemical Physics 241 (1999) 221–238 Table 6 Vibronic bands observed for CaCl 2 in the dihalide ‘B–X’ transition regiona Moleculer Wavelength Frequency b ˚. ŽA Žvacuum-cmy1 . band number 1

3500

28563.3

2

3530

28320.6

3

3555

28121.4

4

3590

27847.2

5

3625

27578.3

6

3655

27352.0

7

3700

27019.4

8

3745

26694.7

9

3785

26412.6

10

3845

26000.4

11

Ž3890.

Ž25699.7.

12

3915

25535.6

13

3985

25087.0

14

4045

24714.9

15

4070

24563.1

16

Ž4090.

Ž24443.0.

17

4130

24206.2

18

4180

23916.7

DŽfrequency. Žcmy1 .

243 199 274 269 226 333 325 282 412 Ž301. Ž164. 449 372 152 272 cmy1 120

229

vations suggest that the chemiluminescent emission from long-lived excited states emanates from the lowest vibrational levels of the upper state w17,18x. This in turn suggests that the vibrational progression corresponds to a ground state frequency. The average separation Ž117 " 2 cmy1 ., however, is considerably larger than the Õ 2 frequency observed for SrCl 2 in rare-gas matrices w40x Ž44 cmy1 . or predicted from theoretical calculations w12,39x. The discrepancy between the observed frequency separations and the expected vibrational frequency increments might be attributed to a transition from a highly bent upper state Žas predicted ŽTable 3... A vertical transition from a highly bent excited state might result in a n 2 frequency approaching 120 cmy1 for a transition terminating in highly excited bending levels ŽFig. 1. of the ground state. However, it is surprising that the observed emission does not appear to demonstrate obvious variations as manifest in an decrease in the frequency separation with increase or decrease in wavelength. As an alternate more feasible explanation for the considerably larger frequency separations Žvs. the expected dihalide bending mode frequency. we consider the reactive formation of the dihalide and suggest that the observed emission reflects the remnants of bonding between the two chlorine atoms. In other words, we suggest that the observed SrCl 2 A–X band system is consistent with emission from an intermediate complex formed via the approach of strontium to Cl 2 viz.

238 290 a

See Table 3 and Figs. 4 and 5. Frequencies are averages of two or more spectra values to "20 cmy1 . c See Fig. 5. b

the visible Ž) 520 nm., are consistent with a long progression in the bending mode of the SrCl 2 emitter. This feature and the corresponding features in the CaCl 2 , CaBr2 , SrBr2 , and SrICl systems are readily quenched by either increasing the metal atom flux or the halogen pressure. The ease of quenching of this band system is consistent with the transition having a relatively long lifetime w44x. Previous obser-

and that the emitting complex correlates with a local minimum on the SrCl 2 potential surface which has not yet completely rearranged to the final product dihalide. The corresponding states would preserve

T.C. DeÕore, J.L. Gole r Chemical Physics 241 (1999) 221–238

230

Table 7 Vibronic bands observed for SrX 2 , SrXY ŽX sCl, Br; Ys I. in the dihalide ‘B–X’ transition regiona Moleculer band number SrCl 2 ŽFig. 6. 1

Wavelength ˚. ŽA

Frequency b Žvacuum-cmy1 .

DŽfrequency. Žcmy1 .

Table 7 Žcontinued. Wavelength Frequency b ˚. Žvacuum-cmy1 . band number ŽA

Moleculer

SrBr2 ŽFig. 7. 5 3959

DŽfrequency. Žcmy1 .

25251.8 140

3687

27114.6

2

3715

26910.2

3

3750

26659.1

4

3782

26433.5

5

3818

26184.3

6

Ž3831.

7

3852

25953.2

8

3900

25633.8

9

3923

25483.5

10

3940

25373.5

11

3964

25219.9

12

3994

25030.5

13

4015

24899.6

14

4038

24757.7

15

4076

24526.9

16

4117

24282.7

17

4156

24054.8

6

3981

25112.2

7

4017

24887.2

8

4049

24690.5

9

4071

24557.1

10

4099

24389.3

11

4109

24330.0

12

4141

24141.9

13

4161

24025.9

14

4176

23939.6 172 cmy1

15

4191

23853.9

16

4239

23583.8

17

4273

23396.2

18

Ž4295.

Ž23276.3. 195 cmy1

19

4309

23200.7

20

4341

23029.7

21 4373 SrICl ŽFig. 8. 1 4347

22861.2

225

204

197

251

133

226

168

249

59

Ž85. Ž26099.4. 231

188

Ž146.

116

319

86

150

86

110

270

154

188

189

Ž120.

131

Ž76.

142

171

231

169

244 228

22997.9 178

229 2

4381

22819.5

3

4417

22633.5

4

4451

22460.5

23152.4

5

4483 281

22300.3

3873

25812.5

6

4505

22191.4 256

2

3898

25646.9

7

4535 273

22044.6

3

Ž3905.

Ž25600.9.

8

4561

21918.9

4

3928

25451.0

18

4196

23825.5

19

4233

23617.2

20

4275

23385.2

21 SrBr2 ŽFig. 7. 1

4318

186

208

173

232

160

233

109 147

166

126

Ž46. Ž149. 199

a b

See Table 3 and Figs. 6–8. Frequencies are "10 cmy1 .

T.C. DeÕore, J.L. Gole r Chemical Physics 241 (1999) 221–238

231

Table 8 Vibronic bands observed for SrX 2 , SrXY ŽX sCl, Br; Ys I. in the dihalide ‘C–X’ transition regiona Moleculer band number

Wavelength ˚. ŽA

Frequency b Žvacuum-cmy1 .

SrBr2 ŽFig. 9. 1 3211

31134.0

2

3233

30922.1

3

3253

30732.1

4

3277

30507.0

5

3287

30414.2

6

3297

30322.0

7

3321

30102.9

8

3365

29709.3

9 3409 SrICl ŽFig. 9. 1 3277

29325.8

2

3297

30322.0

3

3317

30139.2

4

3337

29958.5

5

3347

29869.0

6

3357

29780.0

7

Ž3382.

Ž29559.9.

DŽfrequency. Žcmy1 .

212 190 225 93 92 219 394 384

30507.1 185 183 181 90 89 Ž220.

a b

Fig. 9. Collage of multiple collision chemiluminescent emission spectra for SrCl 2 , SrBr2 , and SrICl in the dihalide C–X emission ˚ The dihalide region Žsee Table 3. taken at resolutions of 10 A. emitters were formed via the collisionally stabilized SrqCl 2 , Br2 , He

ICl™SrCl )2 , SrBr2) , SrICl reactive encounters. The helium gas background pressure ranges from 0.8 to 1.2 Torr. The spectra appears to be dominated by dihalide stretching modes. The labeled emission features are catalogued in Table 8 and discussed in the text.

See Table 3 and Fig. 9. Frequencies are "20 cmy1 .

remnants of the Cl–Cl bond. Vertical transitions from these highly bent states would be expected to terminate in highly bent excited bending vibrational levels of a ground state complex. It is apparent that the bands in the B–X and C–X emission regions are dominated by progressions in the stretching modes of the dihalide. The B–X region is complex and several features are blended. Further, one must be cognizant of the C 2 P–X 2 Sq metal monohalide transitions in this region. In part, for this reason we have obtained LIF spectra for the Sr q Cl 2 and Sr q Br2 reaction systems which we

T.C. DeÕore, J.L. Gole r Chemical Physics 241 (1999) 221–238

232

Table 9 Summary of Group IIA dihalide relaxed spectral emission regions

Table 10 Lowest-lying Group IIA metal neutral and ionic states

Group IIA dihalide

‘A–X’ region ˚. ŽA

‘B–X’ region ˚. ŽA

Atom or ion

Electron configurationa

Energy Žcmy1 .

CaCl 2 CaBr2 SrCl 2 SrBr2 SrICl

4200–5200 4500–5700 4220–5400 4600–6100 4800–6100

3500–4200 3600–4300 3700–4360 3850–4400 4360–4580

Ca Sr Ca Sr Ca Sr Caq Srq Caq Srq Caq Srq

. . . Ž3p. 6 Ž4s. 2 1 S . . . Ž4p. 6 Ž5s. 2 1 S . . . Ž3p. 6 Ž4s.1 Ž3d.1 . . . Ž4p. 6 Ž5s.1 Ž4d.1 . . . Ž3p. 6 Ž4s.1 Ž4p.1 . . . Ž4p. 6 Ž5s.1 Ž5p.1 . . . Ž3p. 6 Ž4s. 2 S . . . Ž4p. 6 Ž5s. 2 S . . . Ž3p. 6 Ž3d. 2 D . . . Ž4p. 6 Ž4d. 2 D . . . Ž3p. 6 Ž4p. 2 P . . . Ž4p. 6 Ž5p. 2 P

0 0 21849 20149 23652 21698 0 0 13650, 13710 14556, 14836 25192, 25414 23715, 24514

compare with the B–X emission region in Fig. 10. From this comparison, we find that there is a very poor overlap of the SrBr LIF spectrum with the observed B–X emission region and further that there is virtually no B–X emission intensity in the SrBr origin regions. This suggests that the SrBr C–X bands make no contribution to the observed chemiluminescence spectrum for the Sr–Br2 reactive encounter. The evaluation of the B–X region is more complicated for the Sr–Cl 2 system where there is potentially some overlap with the SrCl C 2 P–X 2 Sq bands. Again, however, the assignment of these bands to the metal monohalide requires that the intensity of the origin sequence be much less than expected. While there is sequence structure which is quite apparent in those bands which lie close to the regions of the SrCl C–X system, the pattern of the chemiluminescence simply does not fit that expected for the SrCl C 2 P–X 2 Sq system. The Sr–Cl 2 reaction is not sufficiently exothermic to populate the C 2 P state of SrCl; however, it could be argued that a selective excitation of one C 2 P component over another, through an energy pooling process, might produce a contribution to the chemiluminescence resulting from the interaction of highly vibrationally excited ground state SiCl viz. SrCl Ž X 2 Sq, Õ . q SrCl Ž X 2 Sq, ÕY . ™ SrCl Ž C 2 P , ÕX . . However, a simulated SrCl vibrational and rotational temperature close to 450 K w10x for the SrCl C 2 P–

a

1

D D 1 P 1 P 1

Highest coupled atomic orbitals are denoted.

X 2 Sq LIF spectrum does not seem consistent with the required ground state vibrational energies. Fi˚ nally, we note that those bands to the red of 4080 A are far too intense to be reasonably assigned as SrCl transitions. The C–X band system of SrCl, at best, makes only a small contribution to the CL spectrum observed in the dihalide B–X region. The apparent sequence structure associated with each of the SrCl 2 B–X features in Fig. 10a may be reasonably assigned to the dihalide bending mode. The distribution of this structure Žresulting from a small change in bond angle. within each of the major bands is very much analogous to that observed for similar transitions in SiF2 w45x. The dominant long wavelength features in Fig. 6 Žlabeled features 14–21. are readily assigned to SrCl 2 . Their energy separation, ; 232 cmy1 , is slightly lower than the value observed for the IR symmetric stretch frequency in rare-gas matrices w38x. The appearance of the band system and the apparent sequence structure suggests that the bond angle changes little during this transition. The ab-initio calculations for BeF2 w33x suggest that this transition results from the second 1 B 2 excited state listed in Table 3 resulting in a transition

Fig. 10. Ža. Comparison of LIF spectrum for SrCl obtained by pumping ground state SrCl formed in the Sr–Cl 2 reaction to the SrCl C 2 P state and a portion of the SrCl 2 chemiluminescent emission in the B–X dihalide region. Sequence structure in the chemiluminescent spectrum is also noted and associated with the dihalide bending mode. Žb. Comparison of LIF spectrum for the SrBr obtained by pumping ground state SrBr formed in the Sr–Br2 reaction to the SrBr C 2 P state and a portion of the SrBr2 chemiluminescent emission in the B–X dihalide region. See text for discussion.

T.C. DeÕore, J.L. Gole r Chemical Physics 241 (1999) 221–238

233

234

T.C. DeÕore, J.L. Gole r Chemical Physics 241 (1999) 221–238

with which is associated a progression in n 1. At shorter wavelength, the spectrum becomes considerably more complicated possibly due to Fermi resonances between the bend and symmetric stretch modes of the ground state or as a result of some contribution from the ; 300–310 cmy1 asymmetric stretch mode Žsee also following.; however, the longest wavelength features Žbands 7, 1–5. again clearly display frequency separations indicative of the symmetric stretching Ž n 1 . mode. The spectrum observed for SrBr2 ŽFig. 7. is similar but more complicated than the spectrum observed for SrCl 2 . Two spacings Ž; 190 and 170 cmy1 . are observed in the long wavelength region. Either separation could be assigned to n 1. While, the 170 cmy1 value is in reasonable agreement with the data in Table 4, the 190 cmy1 frequency could, at a slightly higher frequency, reflect the effect of a residual Br–Br interaction. The difference between these frequencies Ž; 20 cmy1 . could also reasonably be associated with sequence structure in the bending mode, the combination of progression and sequence band structure producing the observed separations. The frequencies associated with the symmetric stretch are larger than those normally ascribed to the dihalide ŽTable 4.. Based again upon the mode frequency variations detailed in Fig. 1, we suggest that this may result from a smaller excited state bond angle associated with an SrBr2 complex formed in the Sr–Br2 reactive encounter. Here, in analogy to the Sr–Cl 2 system, the remnants of the Br–Br bond may be manifest. Indeed, several of the observed band spacings are indicative of a short progression in a bending mode frequency of order 75–90 cmy1 , much larger than that for the nominal ground state of the dibromide. The spectral emission features observed for SiICl are not subject to those symmetry constraints which apply to the SrX 2 dihalides. The B–X and C–X emission region frequency separations display a number of similarities. The bands 1–4 in Fig. 8 ŽB–X region. are reasonably fit by a frequency spacing of 180 cmy1 which we associate primarily with a coupled and Sr–I dominated stretch. A second frequency of ; 270 cmy1 , which links the remaining bands of the system, is reasonably assigned to an Sr–Cl stretch. In both cases, these frequencies would appear to be somewhat higher than nominally associ-

ated with the normal dihalide, again suggestive of residual halogen molecule bonds. The CaCl 2 and CaBr2 emission features in the B–X region are weaker and less clearly resolved than those for the strontium systems. Based upon the similar atomization energies for the chlorides and bromides given in Table 1 and the shift to higher energy of the CaX 2 B–X emission bands, this is not surprising. The emission feature for CaCl 2 is somewhat weaker than that for CaBr2 and the spectral features indicated in Fig. 4 and Table 6 are certainly blended; however, several of the separations for the long wavelength bands appear to reflect a symmetric stretch frequency separation. At the resolution attainable, it was not feasible to evaluate the corresponding features for the CaBr2 B–X system. The C–X transition region is accessible only to the strontium–halogen reactions, suggesting that its weak nature relative to the A–X and B–X band systems may result, at least in part, from reaction exoergicity. The observed bands for the SrCl 2 system are weak and their resolution precludes a detailed analysis of the spectrum pictured in Fig. 9. The C–X bands for SrBr2 are considerably sharper. All of the features in this system could arise from one progression with an average spacing of ; 200 cmy1 . However, there is a clear systematic variation in the spacing of the features given for this assignment which is larger than the uncertainty in the measurements. We find that assigning the bands labeled 2, 4, 7, 8, and 9 ŽFig. 9. to one progression and the bands labeled 1, 3, and 6 to a second progression eliminates this variation ŽTable 8.. While the average spacing within these progressions ; 410 cmy1 is too large to arise from the ground state of SrBr2 , the frequency increment is in reasonable agreement with estimates for two quanta of the ground state asymmetric stretching frequency. This assignment requires that every other line in the proposed n 3 progression be missing. The data in Table 3 suggests the presence of a 1A 2 state in this region. The assignment of the observed system to a 1A 2 – 1A 1 transition of the strontium dihalide would be reasonable since the coupling of an odd quanta of the n 3 stretch is required for this transition to be observable. Thus the weak nature of the C–X bands could indicate that they correspond to an electronically forbidden transition.

T.C. DeÕore, J.L. Gole r Chemical Physics 241 (1999) 221–238

The C–X transition region emission intensity for SrICl exceeds that for SrCl 2 and SrBr2 . This may result in part from reaction exoergicity; however, a more likely possibility is that it results from the lifting of the C 2v selection rules for the asymmetric molecule. The bands 1–4 follow a clear progression with a spacing of ; 180 cmy1 , well fit by the expression

n Ž cmy1 . s 30507y 187 n q 1.5 n 2 .

Ž 2.

Those features which are not fit directly using Eq. Ž2. would appear to be connected to these bands through the presence of a 270 cmy1 frequency. The correlation with the observed features in the B–X region is quite evident and suggests a remnant I–Cl interaction.

5. Evaluation of metal monohalide bond energies from single-collision chemiluminescence In figure 8 of Ref. w10x we depict a collage of dihalide and monohalide emission features produced as atomic strontium reacts with varying mixtures of Br2 and Cl 2 . These spectra correspond in part to sums of emission features resulting from the dichloride and dibromide with no eÕidence for the mixed halogen ŽSrBrCl. emitter. This result provides a strong confirmation of the highly efficient R3BR collisional stabilization mechanism ŽEqs. Ž1a., Ž1b. and Ž1c.. w8–10x. We also observe that a sum of both SrCl and SrBr emission features characterize the monohalide emissions for all of the chlorine–bromine mixtures. Although these observations imply that both electronically excited SrCl and SrBr are formed in this reactive environment, the data in Table 1 suggests that the fast bimolecular Sr q X 2 ™ SrX q X reactions are not sufficiently exothermic to populate the electronically excited states of SrCl or SrBr. Likewise, a collision-induced energy pooling from highly vibrationally excited ground state SrCl or SrBr to form the excited state emitters can be eliminated as a possibility. We have found that the careful and controlled extension of chemiluminescent studies from single- to multiple-collision conditions Žsee Ref. w16x. preserves the maximum internal excitation observed under single-collision conditions with no exceptions. While we observe rotational followed by

235

vibrational relaxation and thermalization at higher pressure, no enhanced excitation appears to be manifest due to energy transfer pooling conditions in the multiple-collision environment of the present study. This is apparent on energetic grounds and also on the basis of the observed thermalization of the ground state rovibrational distribution, which is evident from Fig. 10. The LIF spectrum for SrCl Žsimilar results for SrBr. depicted in Fig. 10 corresponds to a thermalized distribution ŽTvib ; 450 K, Trot ; 450 K. among the lowest vibrational levels of the ground electronic state of the monochloride w10x. The LIF spectra of Fig. 10, taken in a wavelength region characterized by strong dihalide emission, indicate that either the radiative lifetime of the dihalide excited states considerably exceeds that of the monohalide Žtrad typically ; 10y8 s. or the population of the lowest ground state dihalide levels is not significant. While the population of the electronically excited states of SrCl and SrBr via the corresponding Sr–X 2 reactions at first seems puzzling, there is a strong synergism between the development of the dihalide emission features and the subsequent increase in intensity of the monohalide emission features. We can also realize that the more energetic reactive process forming the dihalide provides sufficient energy to produce the excited states of the monohalide via a dissociative process. For example, under multiple-collision conditions, the exothermicity of the reaction Sr q Cl 2 ™ SrCl q Cl

Ž 3.

given closely by the difference in the SrCl and the Cl 2 molecule 3 bond energies, is 39.8 kcalrmol Ž; 13920 cmy1 .. This is not a sufficient energy to produce monochloride excited state emission. However, the energy available through the R3BR process and dihalide complex formation viz. Sr q Cl 2 ™ SrCl )q ™ SrCl ) q Cl 2

Ž 4.

can correspond closely to an increment of order 49.3 kcalrmol Ž; 17243 cmy1 s 1r2 AEŽSrCl 2 . y

3 Do8ŽCl 2 . s 2.479367 eV Ž57.2 kcalrmol. from Ref. w46x; Do8ŽBr2 . s1.971 eV Ž45.5 kcalrmol. from Ref. w47x; Do8ŽICl. s 2.152 eV Ž49.6 kcalrmol. from Ref. w48x.

236

T.C. DeÕore, J.L. Gole r Chemical Physics 241 (1999) 221–238

Do8ŽCl 2 . w43x. where AEŽSrX 2 . is the dihalide atomization energy taken from Table 1. This significant difference in energy, which can be manifest to pump excited electronic monohalide states results from a substantially larger dihalide bond energy. For the Sr–Br2 system, the energetics compiled from Table 1 yield an available energy of 33.5 kcalrmol Ž; 11717 cmy1 . for the direct process Ž3. and suggest an energy of order 49 kcalrmol Ž; 22600 cmy1 . for a process which maximizes internal energy for the monohalide product of dihalide complex dissociation. 5.1. Bond energy determination The formation of a long-lived dihalide complex via the reaction of the Group IIA metals with chlorine, bromine, and iodine may also have implications for the determination of metal monohalide bond energies using single-collision chemiluminescent techniques w49x. Because the Group IIA metal–halogen molecule reactions involve the interaction of metals with reasonably low ionization potentials w23,24x and halogen molecules of high electron affinity Žsee for example Refs. w50–53x., they are expected to proceed via an electron jump process whereby the Group IIA atom harpoons the halogen molecule to form an Mq Xy 2 complex. Here there are key periodic differences in the observed ‘low-pressure’ emission from the combinations of Group IIA metal–homonuclear halogen molecule reactions w8,9x. In contrast to the calcium, strontium, and barium reactions with chlorine, bromine, and iodine which yield a spectrum dominated by Group IIA dihalide emission, the reactions involving molecular fluorine are strongly if not completely dominated by emission from the monofluorides. This dichotomy clearly represents an important periodic trend in the formation of the dihalides which bears on mechanistic considerations w8–10x. The observed dichotomy of mono- and dihalide emission features may result, at least in part, because the fluorine molecule negative ions produced in the electron jump process dissociate much more rapidly y y Ž than do the corresponding Cly 2 , Br 2 , or I 2 ions see y y w x w x for F2 , Refs. 54,55 ; for ClF , Ref. 56 ; for Cly 2, y w x Refs. w54,57,58x; Bry 2 , Refs. 58,59 ; for I 2 , Ref. w60x.. This results not only from the considerably

smaller bond energy of the fluorine molecule but also from the nature of the vertical electron attachment process which creates the fluorine negative ion high on the repulsive wall of its ground state potential w54,55x. In contrast, the process for the more tightly bound heavier halogens, whose size better accomodates electron attachment, can lead to the formation of a considerably longer-lived negative ion. While it is difficult to create a long-lived Mq Xy 2 complex in those reactions with molecular fluorine, forcing a direct forward scattered reaction, complex formation can be accomodated for the heavier halogens. Based upon these observations, it is relevant that we consider the bond energies determined by Menzinger w49x, under ‘single-collision’ conditions, for the Group IIA monohalides. The energetics is summarized in Table 11. The lower bound bond energies determined for the bromides and chlorides considerably exceed those values given in Table 1 whereas those for the fluorides are in good agreement. This difference may be understood if we consider the implications of complex formation and the nature of the mass spectrometric equilibrium determinations outlined in Table 1 as they apply to the determination of the energetics of the Group IIA–halogen molecule reactions summarized in Table 11. For example, according to Table 1, the formation of CaBr from the direct Ca q Br2 reaction corresponds to a process exothermic by approximately 27.5 kcalrmol Ž Do8ŽCaBr. y Do8ŽBr2 . w43x.. In contrast, the formation of CaBr from the dissociation of a freshly formed CaBr2 complex can potentially supply an energy of order 47.5 kcalrmol Ž1r2 AEŽCaBr2 . s 93.0 kcalrmol-Do8ŽBr2 . Žsee footnote 3.. These energy differences can, if fully exploited, lead to excited state formation and account for the

Table 11 Dissociation energies of the alkaline earth monohalides MX ŽM s Ca, Sr, Ba; X s F, Cl, Br. determined from ‘single-collision’ chemiluminescencea

Ca Sr Ba a

F

Cl

Br

128.4"2 127.3"1.5 141.4"5

115.0"1.5 123.9"3 119.7"1.5

97.4"1.5 93.5"2 104.6"2

From Ref. w49x.

T.C. DeÕore, J.L. Gole r Chemical Physics 241 (1999) 221–238

prediction of bond energies from chemiluminescence significantly higher than those given in Table 1 as one evaluates the chlorides, bromides, and iodides. The bond energies determined for the calcium, strontium, and barium fluorides, formed in direct fluorine molecule reactions, believed not to proceed through a long-lived complex w8–10x, are thus in good agreement with the values obtained from mass spectroscopy ŽTable 1. 4 .

6. Conclusions Complex formation in the Group IIA metal– halogen molecule reactive systems can play an important role in the partitioning of energy, the nature of reaction products, and their available energy content. With the increased evidence for complexation processes in reactive environments and the clear evidence for the role which the chemistry of these processes may play in chemical vapor deposition, the evaluation of complex formation can be of import to device fabrication. Therefore, the presence of these complexes, alternate routes to product formation, and their implications must be carefully assessed.

References w1x A. Fontijn, Pure Appl. Chem. 70 Ž1998. 469. w2x K. Brezinsky, in 26th Symposium ŽInternational. on Combustion, The Combustion Institute, Pittsburgh, PA, 1996, p. 1805. w3x A. Fontijn, private communication. w4x H. Komiyama, private comminication. w5x D.J. Wren, M. Menzinger, Chem. Phys. Lett. 27 Ž1974. 572. w6x M. Menzinger, Adv. Chem. Phys. 42 Ž1980. 1. w7x M. Menzinger, Acta Phys. Pol. A 73 Ž1988. 85. w8x J.L. Gole, D.R. Grantier, High Temp. Sci. 33 Ž1995. 126. w9x J.L. Gole, J. Chem. Phys. 102 Ž1995. 7425. w10x J.L. Gole, H. Wang, J.S. Joiner, D.E. Dawson, J. Chem. Phys. 102 Ž1995. 7437.

4

In a study of the dynamics of the electronically chemiluminescent CaqX 2 ŽX 2 s F2 , Cl 2 , Br2 . reactions w61x, a note in proof suggest that the MqF2 ™ MF)qF reactions are elementary and bimolecular, while the MX) ŽX sCl,Br. products seem to be formed, contrary to the assumption of the referenced paper, mainly in a two step mechanism probably involving MX) as the intermediate.

237

w11x D.L. Hildenbrand, in Proceedings of the Symposium on High Temperature Metal Halide Chemistry, vol. 78–1, The Electrochemical Society, p. 248. w12x L. Seijo, Z. Barandiaran, S. Huzinaga, J. Chem. Phys. 94 Ž1991. 3762. w13x M. Hargittai, M. Kolonits, D. Knausz, I. Hargittai, J. Chem. Phys. 96 Ž1992. 8980. w14x M.C. Drake, G.M. Rosenblatt, in Proceedings of the Symposium on High Temperature Metal Halide Chemistry, vol. 78-1, The Electrochemical Society, p. 234. w15x M. McQuaid, K. Morris, J.L. Gole, J. Am. Chem. Soc. 110 Ž1988. 5280. w16x J.L. Gole, in: A. Fontijn ŽEd.., Gas Phase Chemiluminescence and Chemiionization, Elsevier, 1985, p. 253. w17x G.J. Green, J.L. Gole, Chem. Phys. 100 Ž1985. 133. w18x R.W. Woodward, J.S. Hayden, J.L. Gole, Chem. Phys. 100 Ž1985. 153. w19x J.R. Woodward, S.H. Cobb, K.K. Shen, J.L. Gole, JQE 26 Ž1990. 1574. w20x K.K. Shen, H. Wang, J.L. Gole, JQE 29 Ž1993. 2346. w21x H. Wang, J.L. Gole, J. Chem. Phys. 98 Ž1993. 9311. w22x H. Wang, J.L. Gole, J. Mol. Spectrosc. 161 Ž1993. 28. w23x NBS Special Publication 505, Bibliography on Atomic Transition Probabilities Ž1914 through October 1977. and Supplement I ŽNovember 1977 through March 1980., US Department of CommercerNational Bureau of Standards. w24x C.E. Moore, ‘Atomic Energy Levels’, National Bureau of Standards, 1949. w25x L. Wharton, R.A. Berg, W. Klemperer, J. Chem. Phys. 39 Ž1966. 2023. w26x A. Buchler, J.L. Stauffer, W. Klemperer, L. Wharton, J. Chem. Phys. 39 Ž1963. 2299. w27x A. Buchler, J.L. Stauffer, W. Klemperer, J. Chem. Phys. 40 Ž1964. 3471. w28x A Buchler, J.L. Stauffer, W. Klemperer, J. Am. Chem. Soc. 86 Ž1964. 4544. w29x E.F. Hayes, J. Phys. Chem. 70 Ž1966. 3740. w30x D.R. Yarkony, W.J. Hunt, H.F. Schaefer, Mol. Phys. 26 Ž1973. 941. w31x R.L. DeKock, M.A. Peterson, L.K. Timmer, E.J. Baerendo, P. Vernooijs, Polyhedron 9 Ž1990. 1919. w32x J.L. Gole, A.K.Q. Siu, E.F. Hayes, J. Chem. Phys. 58 Ž1973. 857. w33x J.L. Gole, J. Chem. Phys. 58 Ž1973. 869. w34x R.H. Hauge, private communication. w35x J.J.P. Stewart, J. Comput. Chem. 10 Ž1989. 209. w36x J.J.P. Stewart J. Comput. Chem. 10 Ž1989. 221. w37x E.B. Wilson, Jr., J.C. Decius, P.C. Cross, Molecular Vibrations: The Theory of Infrared and Raman Vibrational Spectra, McGraw-Hill, 1955. w38x M.W. Chase Jr., C.A. Davies, J.R. Downey Jr., D.J. Frurip, R.A. McDonald, A.N. Syverud, J. Phys, Chem. Ref. Data 14 Ž1985. Supplement 11-2. w39x M. Kaupp, Pv.R. Schleyer, H. Stoll, H. Preuss, J. Am. Chem. Soc. 113 Ž1991. 6012. w40x F. Ramondo, L. Bencivenni, C. Nunziante, K. Hilpert, J. Mol. Struct. 192 Ž1989. 83.

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w41x D. White, G.V. Calder, S. Hemple, D.E. Mann, J. Chem. Phys. 59 Ž1973. 6645. w42x V. Calder, D.E. Mann, K.S. Seshadri, M. Allevena, D. White, J. Chem. Phys. 51 Ž1969. 2093. w43x D.E. Mann, G.V. Calder, K.S. Seshadri, D. White, M.J. Linevsky, J. Chem. Phys. 46 Ž1967. 1138. w44x R.B. Bernstein, R.D. Levine, Molecular Reaction Dynamics, Oxford University Press, New York, 1974. w45x J.L. Gole, R.H. Hauge, J.W. Hastie, J.L. Margrave, J. Mol. Spectrosc. 43 Ž1972. 441. w46x R.J. Leroy, Molecular Spectroscopy, vol. 1, The Chemical Society, Washington, DC, 1973, p. 113. w47x J.A. Horsley, R.F. Barrow, Trans. Faraday Soc. 63 Ž1967. 32. w48x W.E. Eberhardt, W.C. Cheng, H. Renner, J. Mol. Spectrosc. 3 Ž1959. 664. w49x M. Menzinger, Can. J. Chem. 52 Ž1974. 1688. w50x R.S. Berry, C.W. Reimann, J. Chem. Phys. 38 Ž1963. 1540.

w51x R.S. Berry, J. Chem. Phys. 27 Ž1957. 1288. w52x W.S. Struve, J.R. Krenos, D.L. McFadden, D.R. Herschbach, J. Chem. Phys. 62 Ž1975. 404. w53x R.C. Oldenborg, J.L. Gole, R.N. Zare, J. Chem. Phys. 60 Ž1974. 4032. w54x J.A. Ayala, W.E. Wentworth, E.C.M. Chen, J. Phys. Chem. 85 Ž1981. 768. w55x W.A. Chupka, J. Berkowitz, D. Gutman, J. Chem. Phys. 55 Ž1971. 2724. w56x H. Dispert, K. Lacmann, Int. J. Mass. Spectrom. Ion Phys. 28 Ž1978. 49. w57x H. Dispert, K. Lacmann, Chem. Phys. Lett. 45 Ž1977. 311. w58x A.P.M. Baeda, Physica 59 Ž1972. 541. w59x D.L. Baulch, R.A. Cox, P.J. Crutzen, R.F. Hampson Jr., J.A. Kerr, J. Troe, R.T. Watson, J. Phys. Chem. Ref. Data 11 Ž1982. 327. w60x A.P.M. Baeda, J. Averbach, D.J. Los, Physica 64 Ž1973. 134. w61x M. Menzinger, Chem. Phys. 5 Ž1974. 350.