Populations of BaO states in the Ba+N2O chemiluminescent flame using the BaO C 1Σ+state as a probeAfranio TorresFilho and J. Gary Pruett Citation: The Journal of Chemical Physics 70, 1427 (1979); doi: 10.1063/1.437580 View online: http://dx.doi.org/10.1063/1.437580 View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/70/3?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Chemiluminescence of argon bromide. II. Potential curves of ArBr and population distributions in the B (1/2) andC (3/2) electronic states J. Chem. Phys. 72, 442 (1980); 10.1063/1.438871 Al+O3 chemiluminescence: Perturbations and vibrational population anomalies in the B 2Σ+ state of AlO J. Chem. Phys. 66, 3886 (1977); 10.1063/1.434464 HTFFR kinetic studies of the fate of excited BaO formed in the Ba/N2O chemiluminescent reaction J. Chem. Phys. 66, 3256 (1977); 10.1063/1.434302 Product state analysis of BaO from the reactions Ba + CO2 and Ba + O2 J. Chem. Phys. 61, 4450 (1974); 10.1063/1.1681763 Excited State Microwave Spectroscopy on the A 1Σ State of BaO J. Chem. Phys. 57, 2209 (1972); 10.1063/1.1678554

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Populations of BaO states in the Ba + N20 chemiluminescent flame using the BaO C 1~ + state as a probe

Afranio Torres-Filho and J. Gary Pruett

Department of Chemistry. University of Pennsylvania, Philadelphia, Pennsylvania 19104 (Received 18 May 1978)

The reaction Ba+N20-->BaO+N2 in the presence of argon buffer gas (25011-) has been studied using laser induced fluorescence to probe the nonchemiluminescent states present. We have observed ground electronic state (X'I+) vibrational levels from v' = 7 to v' = 32 by exciting them using a N2 laser pumped tunable dye laser to various vibrational levels of a highly excited electronic state (C'I+) and observing the resulting ultraviolet fluorescence to low ground state vibrational levels (v' < 12). Low ground state vibrational levels (v· = 0-9) have also been observed by exciting the molecules to the A 'I+ state and observing vibrationally resolved visible fluorescence. We find the C'I+ state to be highly perturbed, but still useful as a population probe for the X 'I + state. We measure the C 'I + radiative life time to be 25± 10 nsec. The X'I+ vibrational population from v' = 5 out to v' = 32 has a characteristic temperature of about 3600oK. We find evidence of nonthermal population of v' = 13-16. We also observe population in the A' '1T and the a 31T states with v' = 0-4. The implications of these results on the identity of the nonchemiluminescent precursor states in the Ba+ N20 reaction are discussed.

INTRODUCTION

Chemiluminescent reactions of metal atom vapors have been investigated for several years. In particular, the reaction between Ba atoms and N20 was subjected to early studiesl - 3 because of its high light output. Studies of this genre focused on the identity and populations of the chemiluminescent molecular states, although other nonemitting states were known to be chemically acces­sible. 4 Subsequent studies revealed that the light output is dependent on the total reaction pressure (mostly in­ert buffer gas), and that light production efficiency in­creases with pressure from about 3% at pressures below 10-3 Torr to a maximum of about 25% at pressures around 10 Torr, before decreasing as a result of colli­sional quenching. 5-8

The clear implication is that subsequent collisions, following initial reaction, aid in transferring energy from a predominantly produced nonchemiluminescent precursor state or states to molecular levels which radi­ate within the residence time in the reaction observation zone. The identity of these precursor states has not been experimentally established, although various can­didates have been proposed. The two main candidates for precursor states are either the high vibrational lev­els of the ground electronic state, or metastable elec­tronic states such as the sublevels of the a 3n state. There are really two questions involved, however. The first is the identity of the initially formed products of the reaction and the second is the relative roles of all nonemitting states in the energy exchange and pooling processes resulting from subsequent collisions. Eck­strom, Edelstein, Huestis, Perry, and Benson have proposed that the reaction products are initially formed in the ground electronic state. Indeed, they were able to reproduce the pressure dependent chemiluminescence efficiency using that basis, together with reasonable es­timates of collisional energy transfer rates. 9 Field, Jones, and Broida, however, have suggested that the initial product is the a 3n state, and were also able to explain the pressure dependent chemiluminescent phe-

nomenon. 10 Since both were able to satisfy known ex­perimental data it may be that knowledge of the exact identity of the initial product is unnecessary to account for the bulk behavior at higher pressures. However, the identity of the precursor is necessary to obtain a working understanding of the chemiluminescence pro­duction mechanisms in this and, by analogy, in other systems.

If indeed one of the possible classes of precursor states dominates the initial reaction products, it may be possible to determine the usefulness of electronic cor­relation diagrams in predicting the important products in reactions of this family.

Re velli , Wicke, and Harris have recently made in­roads into understanding the populations of the dark states in the Ba + N20 + Ar flame at medium pressures (0.3-10 Torr) by observing cw laser induced photolumi­nescence in the A 1~+ _Xl~+ system of BaO. 11 They were able to obtain relative populations of the low vibrational levels of the ground state from v" =0 to v" =4, which seemed to indicate a thermal population with T near 500 OK. Because of the low energies of these levels, little could be said concerning their connection with any precursor state for the chemiluml.nescence. They were unable to observe higher vibrational levels. In order to avoid interference from the chemiluminescence, they observed the fluorescence through a monochromator tuned to a single vibrational fluorescence pathway in the BaO (A -X) system. They were thus limited to excita­tion transitions within their dye ranges which resulted in strong Franck-Condon allowed fluorescence within their photomultiplier range.

We have removed these problems by utilizing a here­tofore incompletely characterized excited electronic state of BaO, the Cl~+ state. This state was first ob­served by Parkinson12 and labeled Bin. Field, Capelle, and Revelli observed some of the same bands as Parkin­son13 and in more recent work Gottscho, Koffend, Field,

J. Chern. Phys. 70(03), 1 Feb. 1979 0021-9606/79/031427-10$01.00 © 1979 American Institute of Physics 1427

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1428 A. Torres-Filho and J. G. Pruett: Ba + N20 chemiluminescent flame

~ ~=--

PM c2:::=:::::::!::,PHOTO DIODE

FIG. 1. Schematic diagram of flame apparatus and detection system. Not shown are the filters or the monochromator, one of which is normally inserted between the flame apparatus and the photomultiplier tube.

and Lombardi have established that its predominant character is 1'6+. 14

By using this state as our fluorescing state excited by a pulsed laser, we have characterized many new bands in the BaO (C-X) system and determined relative vi­brational population of the v" = 7 -32 levels of the X state at a reaction pressure of 250 J.L.

We have also determined the X state v" =0-9 relative populations using techniques similar to those of Revelli et al. 11 Also we have directly observed populations in the A' 1n , A 1'6+, and a 3n sta tes by exciting these mole­cules to the C 1'6+ state and observing the C 1'6+ -X 1'6+

emission.

APPARATUS

The reaction Ba + NzO- BaO + Nz was run in the gas phase at low pressure in the presence of 250 J.L of Ar buffer gas using a standard flow system for such reac­tions shown in Fig. 1. Barium atoms are produced by evaporation from a resistively heated crucible filled with barium cut from commercially available rods (Alfa-Ventron). Argon carrier gas entrains the barium and carries it out of the oven region into the reaction zone. An opposing flow of NzO gas is introduced from a long delivery tube to obtain a compact flame located at the center of the observation window. Reactants, prod­ucts, and buffer gas are pumped from the reaction chamber by two 150 l/min mechanical vacuum pumps. Mass flow rates are not determined. Argon buffer gas is metered into the oven region to obtain a steady state pressure of 250 J.L (uncorrected thermocouple gauge) at the reaction chamber side wall. The Ba oven is heated to around 1000 OK and the NzO flow adjusted to give a bright compact flame. The resulting flame is steady and typically lasts 8-10 h.

The light from a pulsed dye laser (Molectron DL -300) passes through the brightest portion of the flame to probe the molecular states present. In order to maximize the overlap between the laser beam and the flame, the Ba +Ar flow is concentrated at the laser overlap region by a conical shroud topped by a modified Bunsen burner

wing cap whose opening slit runs parallel to and just be­low the laser beam. The resulting flame has a horizon­tal length of 3 cm, a width of 0.5 cm, and a height of 1 cm. The laser beam enters and exits through care­fully aligned baffle arms to reduced scattered light.

Laser induced fluorescence is observed at right angles to the laser beam through anf-1. 5, 4 in. diam quartz lens placed at its focal length from the reaction zone. A 2 in. diam quartz window, S-20 response photomulti­plier tube (EMI-9816QA) is used to detect the pulsed fluorescence. The photomultiplier tube is placed in one of two positions depending on the particular experiment. At all times the tube must be protected from the bright chemiluminescence with A >400 nm. The tube is either protected by two Corning 7 -54 glass filters (transmitting A< 400 nm) or it views the flame through a t m Jarrell­Ash grating monochromator. When viewing through the monochromator, the flame is focused onto the entrance slits with a secondf-1.5, 4 in. diam quartz lens.

The pulsed fluorescence induced by the laser and ob­served by the photomultiplier tube is detected by a PAR Model 162 gated integrator with a Model 164 plugin. The boxcar gate is triggered by a signal from a photo­diode which observes the fluorescence of the dye in the dye laser cell. The boxcar gate is then set for optimum delay and duration for separation of the exponentially decaying fluorescence from the scattered laser light and chemiluminescent background. The output of the boxcar integrator is recorded on a Fisher Recordall 5000 strip­chart recorder.

Beam measurements

In preliminary investigations the flame was observed under molecular beam conditions. For those experi­ments, the same detection techniques were used, but the apparatus was modified to use diffusion pumping in the flame zone and differential pumping of the metal atom beam source.

SPECTROSCOPY OF THE C lL+ STATE

Experimental

The previously known bands of the Parkinson system all involve transitions to X1'6+ v" ~ 11 and appear at wave­lengths below 400 nm, most of them beyond the range of currently available dyes. Since we wanted to use this state as a population probe, it was necessary to first ob­serve and analyze the visible extension of this system. Since the visible extension results from transitions be­tween high vibrational levels of the ground state and medi­um vibrational levels of the C state, fluorescence back to low vibrational levels of the ground state occurs in the ultraviolet. In order to shield the photomultiplier tube from chemiluminescence, it was necessary to re­strict observation to the ultraviolet region.

For bands with A> 400 nm, the two Corning 7 -54 fil­ters eliminated chemiluminescence and scattered laser light from the photomultiplier tube. For bands with 400 nm > A> 350 nm the monochromator was used with no slits (~A = 50 nm) and centered at A = 325 nm.

J. Chem. Phys., Vol. 70, No.3, 1 February 1979

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A. Torres-Filho and J. G. Pruett: Ba + N20 chemiluminescent flame 1429

The combination of a 7-54 filter and an S-20 photo­cathode allows light between 680 and 850 nm to be de­tected. This resulted in the observation of some bands of the A-X system when fluorescence occurred in that region. These we eliminated by using a less red sensi­tive photocathode (when necessary). By observing only uv light, no interference was obtained from chemilumi­nescence or scattered laser light. The boxcar gate was thus optimized using no delay between the laser pulse and the gate opening and a gate width comparable to twice the C state lifetime (see Results section). Wavelength cali­bration of the dye laser was obtained at several points throughout the visible by observing atomic lines of Ba, and of impurities of Ca, Sr, and Mg. Ba and Sr reso­nance lines could be seen even through the 7 -54 filters, while other lines required removal of the filters and extinguishing the flame by interrupting the N20 flow. Wavelength calibration for dyes within which there were no atomic lines was obtained by secondary calibration on common band heads of adjacent calibrated dyes. Bands below ;\=400 were calibrated using accurate emis­sion wavelengths of these bands observed by Gottscho, Koffend, Field, and Lombardi. 14 Overall calibration thus obtained is ± O. 03 nm.

Spectroscopic results

We have observed 103 new bands of the Cl~+_Xl~+ system of BaO as well as some of the bands of this sys­tem previously seen by Parkinson. 12 All bands of suffi­cient intensity show sharp band heads and are degraded to the red. Figure 2 shows a fast scan using Pilot 386 as the laser dye. Figure 3 shows a slow scan of the (0,12) band showing more clearly the rotational struc­ture in the band. The bands appear very much like the A -X bands of BaO because both transitions result in a large change in the equilibrium internuclear separation.

Vibrational assignments were made explicitly for some of the bands by observing the Franck-Condon envelope of the fluorescence emission. Figure 4 shows such enve­lopes for VI = 0-2 and 6. The nodes are well developed and unambiguous. Bands with identical emission FC

0,12) I<J (J

Z I<J (J

(1,13) U)

(0,11) 0,11)

I<J II:: 0 ~ ....I ...

400 395 390 385

LASER ).nm

FIG. 2. Excitation spectrum using Pilot 386 as the lasing dye. Observation of uv fluorescence here is through a i-meter monochrometer with no slits tuned to pass 325 ± 25 nm. The strong line near (3,13) is an atomic Ba line. Note the double headed appearance of all bands with Vi = 1.

LU (J

z UJ (J

U)

LU II:: o ;:)

...J ...

401 400

LASE R ). nm

399

FIG. 3. Slow scan through the (0,12) band of the C-X system. P-R doublets are not resolved even at higher values of J.

envelopes were assigned to the same upper vibrational level.

Lower level assignments were unambiguously made by measuring accurate spacings between band heads of tran­sitions to a common upper level and matching those dis­tances to known ground state vibrational spacings. Ground state numberings were confirmed by assigning the deepest ultraviolet emission in fluorescence to vI/ = 0 and counting back to the fluorescence band at the laser wavelength.

Bands whose upper state assignment was not made by counting nodes in the fluorescence FC envelope were as­signed by finding a series of bands separated by ground state spacings which were translated from a previously assigned series by an amount equal to an upper state spacing around 430 cm-1

• The upper state spacings thus observed were quite irregular, indicating complex per­turbations of the estate. The irregular spaCing is quite

"'A~ 4'00 ' 3'00 500 400 300

FIG. 4. Vibrationally resolved fluorescence spectra from the BaO C 1~. state Vi = 0,1,2, and 6 showing countable nodes in the Franck-Condon envelopes: used to assign upper state vibra­tional number. Abnormally intense lines in Vi = 1 and Vi = 2 emission are due to scattered laser light.

J. Chern. Phys., Vol. 70, No.3, 1 February 1979

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1430 A. Torres-Filho and J. G. Pruett: Ba + N20 chemiluminescent flame

TABLE 1. C-X transition of BaO.

(v',v") V .. e (em-I) Aur (nm) (v',v")

(0,7) (0, 8)J a

(0,9) J (0,10) (0,11) (0, 12)J (0,13) (O,14)J (0,15)

(1,8) (1,10)J (l,l1)J (1,12) (1,13)J (1,14)J (1,15) (1,16) (1,17) (1,18)

(lE,9)b

(lE,10) (lE,l1) (lE,12) (lE,13) (lE,14) (lE,15) (lE,16) (lE,17)

(2,9) (2,10) (2,11) (2,12) (2,13) (2,14) (2,15)J (2,16)J (2,17) (2,18) (2,19) (2,20) (2,21)

(3,10) (3,11)J (3,12) (3,13)

(3,15) (3,16) (3,17) (3,18) (3,19)J (3,20) (3,21)

(4,13) (4,14) (4,15) (4,17)J (4,18)J (4,19) (4,20)J (4,21) (4,22) (4,23) (4,24)

28178.4 27540.7 26908.2 26279.8 25656.2 25039.9 24425.4 23815.3 23206.9

28007.8 26735.9 26113.1 25492.1 24879.7 24270.6 23663.9 23065.0 22462.6 21872.8

27328.0 26697.6 26076.5 25456.5 24845.1 24233.8 23628.1 23027.0 22427.3

27783.3 27154.8 26527.3 25911.1 25295.8 24689.5 24083.2 23480.5 22883.9 22292.3 21706.6 21124.1 20545.0

27593.5 26969.0 26346.3 25733.8

24~~.0. 9 23919.6 23321. 1 22729.1 22139.2 21558.2 20980.5

26148.0 25538.2 24936.2 23737.3 23143.8 22558.1 21975.0 21398.1 20826.2 20257.0 19696.2

354.78 362.99 371. 52 380.41 389.65 399.24 409.29 419.78 430.78

356.94 373.92 382.84 392.16 401. 82 411 90 422.46 433.43 445.05 457.06

365. 82 374.45 383.37 392.71 402.38 412.53 423.10 434.15 445.76

359.82 368.15 376.86 385.82 395.21 404.91 415.11 425.76 436.86 448.45 460.56 473.26 486.60

362.30 370.69 379.45 388.48

407.70 417.94 428.67 439.82 451. 56 463.73 476.49

382.32 391. 45 400.91 421. 15 431. 95 443.17 454.93 467.20 480.03 493.51 507.57

(5,11) (5,13) (5,18) (5,20) (5,218 (5,22) (5,23) (5,24)

(6.14) (6.15) (6.17) (6,18) (6,22)] (6,23)J (6,24) (6,25) (6,26) (6,27) (6,28)

(7,17) (7,20) (7,23) (7,24)J (7,25)] (7,26)] (7,27) (7,28) (7,29)

(8,14) (8,15) (8,25) (8,26) (8, 271 (8,28) (8, 29~ (8,30)

(9,17) (9,20) (9,23) (9,26) (9,27)

(9,28)J (9,29) (9,30) (9,31)

(10,19) (10,21) (10,22) (10,26) (10,29) (10,30)] (10, 31)J (10.32)

(11,30) (11,31) (11,32) (11,33)

27828.3 26581. 9 23586.9 22417.0 21838.0 21265.0 20697.3 20135.4

26440.4 25831. 3 24644.8 24051.9 21728.3 21160.1 20597.5 20040.5 19487.0 18939.5 18404.3

25066.3 23304.3 21589.4 21025.9 20466.0 19913.2 19366.2 18826.2 18285.5

27274.8 26671. 3 20871. 0 20315.4 19770.2 19228.7 18690.8 18157.6

24167.6 22443.4 20772.6 20225.8 19682.7 19145.9 18613.9 18087.3

25148.2 23986.7 23415.4 21174.5 19550.0 19017.9 18491. 3 17986.3

19420.7 18897.0 18373.9 17862.4

359.24 276.08 423.84 445.96 457.78 470.12 483.01 496.49

378.10 387.01 405.65 415.65 460.10 472.45 485.36 498.85 513.01 527.85 543.20

398.82 428.98 463.06 475.47 488.47 502.03 516.21 531. 02 546.72

366.53 374.82 478.99 492.10 505.67 519.91 534.87 550.58

413.66 445.44 481. 26 494.28 507.91 522.15 537.08 552.72

397.53 416.78 426.94 472.13 511. 36 525.67 540.64 556.38

514.77 529.03 544.09 559.67

"Bracket indicates pairs of bands used for relative population measurements.

bIE is used here to designate the extra head seen in the bands of v' =1.

worrisome, but the assignment is validated by the fact that irregular spacings are also apparent for the bands whose assignments are based on the FC nodes of the fluorescence spectrum. In addition, all observed bands from 350 to 570 nm fit into the C -X band system with intensities consistent with Franck-Condon factor trends.

Table I lists all the bands observed and their assign­ments grouped in sets of common upper vibrational lev­el. Table II gives the observed !loo and lists average ground state spacings and average upper state spacings calculated using the band head positions. Ground state spacings are compared with very accurately known val­ues. 13,15

The perturbed nature of the bands is best illustrated by the bands with VI = 1. Figure 2 shows several bands with VI = 1. The double headed appearance is due to a strong perturbation observed recently in VI = 0 by Gottscho, Koffend, Field, and Lombardi14 using optical­optical double resonance (OODR) techniques. The other bands are apparently not suffiCiently perturbed near the head to develop the perturbing band as in the case of the VI = 1 bands. Until the perturbations are understood we do not feel compelled to try to extract "deperturbed" vibrational spacings through least squares fitting of the observed spaCings to standard formulas. We did, how­ever, assume a regular spaCing close to the observed spaCings when calculating potential curves and Franck­Condon factors for the C -X system as explained later.

Lifetimes

As additional information on the extent of the pertur­bations, the lifetimes of several of the bands were mea­sured. Lifetimes were recorded by scanning the delay time of a 5 nsec boxcar gate through the pulsed fluores­cence signal and recording the boxcar output on the re­corder. Time base was taken from a simultaneous os­cilloscope display (Tektronix 465). We were unable to obtain quantitatively different lifetimes for different vi­brational states because of the short lifetime of all states and the time uncertainty introduced by the laser pulse width and the detector gate width. Thus we give only a rough lifetime measurement of 25 ± 10 nsec.

Interfering bands

Several bands we have observed at long wavelengths do not fit into the C-X system. Some show band heads while others show no head and abnormal intensities of particular rotational lines. Figure 5 illustrates this ef­fect. These bands are apparently arising from visible transitions in the C-A system, followed by C-X emis­sion in the ultraviolet. The C-A bands observed are listed in Table III. The steady-state concentration of molecules in the A state (which produces much of the chemiluminescence) is apparently sufficient to produce these bands. The abnormal rotational intensities are due to accidental optical-optical double resonances (OODR) between the X and A states and the A and C states. The initial absorption from the X to the A state results in an abnormally high population of a particular rotational level of the A as evidenced by the abnormal inten-

J. Chern. Phys., Vol. 70, No.3, 1 February 1979

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A. Torres-Filho and J. G. Pruett: Ba + N20 chemiluminescent flame 1431

TABLE II. Vibrational spacings of X and C states of BaO.

X'l:+ C 'l:+ voo = 32 752 em-'

v" AGv+ 112 (em-')a AG v+ 112 (em-')b v' AG v +'1l (em-')b

0 665.7 0 455.5 1 661. 6 1 418.1 2 657.5 2 437.0 3 653.3 3 416.2 4 649.1 4 440.5 5 645.1 5 463.3 6 640.8 6 425.6 7 636.4 637.3 7 403.8 8 632.2 632.5 8 455.6 9 627.9 629.1 9 403.5

10 623.6 623.9 10 404.7 11 619.1 619.2 12 614.7 613.2 13 610.2 609.3 14 605.6 606.0 15 601.1 601.0 16 596.5 599.3 17 591. 9 591.8 18 587.2 587.3 19 582.4 582.2 20 577.6 578.2 21 572.7 572.3 22 567.8 568.4 23 562.9 562.2 24 557.8 558.5 25 552.7 554.0 26 547.6 546.6 27 542.4 540.0 28 537.1 538.5 29 531. 7 532.4 30 526.4 525.6 31 520.8 523.1 32 515.4

aValues from Ref. 13. ~his work.

sity of its C -.A transition. The OODR effect is con­firmed by noticing that when the laser is fixed at one of the abnormal features, the C-X fluorescence is greater than first order in laser intensity. 16

1&1 U Z 1&1 U fII 1&1 a:: o ::;)

oJ ...

15115

(C-A,A-X) OOOR

1510 15615 1560

LASER ~ nm

FIG. 5. OODR enhanced rotational lines due to simultaneous C-A, A-X (6,2), (2,0) excitation by the laser. No bandhead is visible on this scale.

1&1 U Z 1&1 U fII 1&1 a:: o ::;)

oJ ...

655

L~SER Anm

C-a (0,0)

650 645

FIG. 6. C-A' (0,0) and C-a (0,0) bands. Additional features more than 4 nm from bands are not part of these systems. The band at 644 nm is the C-A (0,1) band, others are unassigned. Other weak Av = 0 bands may account for some of the intensity around the (0,0) bands.

Excited states

There are yet other bands of unusually high intensity which are not associated with the C-.A or A-X, C-.A OODR system. These bands we have assigned to transi­tions in the C l~+ -A I III and C 1~+ -a 3IIl systems by match­ing their transition energies with the band pOSitions ob­served in emission following OODR in BaO by Gottscho, Koffend, Field, and Lombardi14 and calculated band posi­tion obtained by using our observed C state vibrational spacings. Figure 6 shows the C-A'(O,O) and the C-a

. (0,0) bands. Some interference is undoubtedly present from weaker but overlapping Av = 0 transitions. The assignment of these bands is confirmed by observing vibrationally resolved emission from the C l~+ v' =0 state when the laser is fixed at the P branch peak. Only the a 3IIl sublevel (predominant n = 1 character) is ob­served due to the large partial III character of that one sublevel. 17 No new spectroscopic information is obtained in this observation, other than the general appearance of the bands. Since the internuclear separations for the states involved are nearly equal and since the transi­tions are predominantly ~ -II tranSitions, the appearance of a sharp band head in the P branch together with close­ly spaced (and unresolved) Q and R branches is expected (and observed).

The spectra observed at A> 600 nm are quite com-

TABLE III. Observed bands of BaO C-A sys­tem.

(v' ,v,,)a "VIC (em-') (v' ,V,,)b vvac (em-')

(0,1) 15533.8 (0,0) 16030.2 (1,1) 15989.1 (3,2) 16350.4 (2,1) 14407.7 (6,2) 17670.4 (3,1) 16843.5 (4,1) 17259.6

"Band heads observed. bOb served in OODR.

J. Chem. Phys., Vol. 70, No.3, 1 February 1979

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1432 A. Torres-Filho and J. G. Pruett: 8a + N20 chemiluminescent flame

ILl

0 Z ILl

0 I/)

ILl

0:

0 ::;)

..J

U.

C-A (0,1)

C-A't::.va+1 , i

640 635

LASER ).nm

C-a

630

FIG. 7. Groups of unresolved bands probably due to Av =+ I, C-A' and C-a systems. Again the C-A (0,1) band at 644 nm is just visible.

plicated and we have not been able to assign all the tran­sitions. In fact, very few of the transitions can be given specific energies because they do not show sharp band heads or other definitely identifiable features. Since the upper states excited are probably all perturbed to some extent, we could conceivably be exciting transi­tions in a large number of systems such as the B In or highe r lying triplet states above a 3n . Some of the more intense groupings of overlapping bands, however, ap­pear to be explained by C-a and C-A' bands with Av =± 1, ± 2, ± 3. One such group is illustrated in Fig. 7.

POPULATIONS OF BaO STATES

Data reduction

Excitation line intensities, normalized to constant laser power, are related to the individual rotationless18

vibrational level populations by

J(v', v")/p=N{v") (1/1'1 iJ.11/I"/ Dv' , (1)

where p is the laser intensity, assumed A dependent; N{v"}is the population of the lower level; (1/1' I iJ.11/I'')z is the total dipole transition moment squared =Rz; and Dv'

is the total detector sensitivity including molecular emission patterns and geometric factors. 19

In the Born-Oppenheimer approximation the transition moment is usually divided into a vibrational and an as­sumed constant electronic part, giving Franck-Condon factors and electronic transition strength, respectively. If, however, one of the states is not described adequate­ly by Born-Oppenheimer wave functions but rather by two Born-Oppenheimer functions (<p~ and 1/1~) weighted by coefficients determined through a first order perturba­tion calculation including the neglected interaction, then the proper form of the transition moment becomes

(2)

where a=[1_(W/AE)z]1/z; j3=W/AE; and where W= (1/1; I Wlzi %) = perturbation matrix element; AE is the zero order eigenvalue difference.

The transition moment is then evaluated for each up­per zero order state:

each one of which reduces to the familiar form

(3)

(4a)

(4b)

where q is the Franck-Condon factor. The total transi­tion moment is then

R = aR1(ql)1/2 + {3R z(qz)l/Z ,

which when squared gives

RZ=azRiql +2aj3R 1 Rz(ql)l/Z(qZ)l/Z + {3zR~qz

(5)

(6)

If now we assume that the interacting level has a small electronic transition moment in zero order, the second and third terms of the above equation becomes small compared to the first, and the transition moment can again be divided into an electronic part (reduced by a) and the vibrational part of the zero order dipole in­teraction:

(7)

Equation (1) now becomes (putting in the explicit upper and lower vibrational level dependence)

(8)

This amounts to ignoring the interference term due to products of vibrational overlaps which give rise to in­tensity anomalies seen by Gottscho et al. 14 These terms must be small, however, since we see no evidence of other strong band systems in this region. Also, we can reproduce vibrationally resolved emission intensities within 5% using only the q(v', v") of the C state.

In Eq. (8) the mixing coefficient a will of course strongly depend on the level being excited, but the zero order electronic transmission moment, I R1 1z, will only have a very weak and negligible vibrational level depen­dence. The detector sensitivity Dv' will also depend strongly on v' due to the different methods of detection used. It is most useful, therefore, to observe popula­tion ratios for pairs of lower state levels by measuring relative intensities for bands with a common upper state level. For such pairs

J(v' ,a)/p N(a) q(v' ,a) J(v',b)/p = N(b) q(v',b) ,

where a and b specify particular lower levels.

(9)

The a(v') and Dv' have cancelled exactly and we have neglected the I R1 1

a v" dependence. Calculation of popu­lations now only requires Franck-Condon factors and band intensities.

Franck-Condon Factors

Spectroscopic constants for the ground electronic state were taken from the OODR work by Field, Capelle, and Revelli 13 and electric resonance work by Wharton and

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A. Torres-Filho and J. G. Pruett: Ba + N20 chemiluminescent flame 1433

Klemperer . zo These constants were used to construct the ground state X 1~ + potential curve using the Rydberg­Klein-Reese method21 in a program obtained from R. N. Zare. The potential curve obtained was within O. 00001 A for the inner and outer tUrning points at each energy listed by Field et ai. 13 The C 1~+ state constants used were obtained from the present spectroscopic work. Be­cause of the irregular nature of the vibrational levels, vibrational constants were chosen to give a reasonable fit to the observed levels. A least-squares procedure was not used since the source of error is due to molecular perturbations and is therefore inherently nonrandom. Instead, constants were chosen which represented the average spacing of the levels. The resulting vibrational constants were harmonic with we=429 cm-I

, wexe=O.O cm-1 • Because of the absence of rotational analysis of the bands, the rotational constants were obtained in a somewhat unorthodox manner. A reasonably good value for Bo was obtained from the work of Gottscho et ai. 14

starting with this value, appropriate values of Be and Cie

were chosen and a potential curve for the C state con­structed. Another computer program was then used to calculate Franek-Condon factors between the X and the C states and the results of those calculations were com­pared with observed Franck-Condon factors in emission (data from Fig. 4 properly adjusted for detector response and frequency factors). This procedure was repeated until values of Be and Cie were found which gave the fixed Bo and which accurately reproduced the Franck-Condon envelope observed for C I~ -X 1~ v' = 6, v" = 0-26.

The values of Be and Ci e obtained were Be = 0.23931, Ci e = 0.0017. The potential curve thus obtained was used to calculate aU the C-X Franck-Condon factors needed for the population measurements. Since the fractional error in the resulting q(v', v") is dependent on the ab­solute size of the q's only bands with strong observed and calculated q's were used to obtain population ratios. Computed and observed q's were within 5% of each other.

Band intensities

Band intensities used in Eq. (1) were taken as band head heights above background from interfering bands. Because of the densities of bands, it was not possible to integrate individual band areas, as this would introduce more uncertainty than using band height. A few well­separated bands were integrated and the integrated ratio compared with the band height ratio. The two ratios were usually within 10% of each other. We do not expect that the two ratios would be much different since the line density at the head is essentially the same for all bands of the same system as long as the difference in the rota­tional constants is not changing. The range of aBv values for the bands observed is 0.063 cm-1 to 0.047 cm-1 which represents a variation of ± 30%. This error, however, represents the cumulative error over a change in v" of 25 levels and only slightly affects the apparent tempera­ture. Intensities of specific bands were obtained in one of three methods outlined below. In some cases, during long experimental runs, individual bands were observed a second time in order to correct for flame intensitydeg­radation.

c-x bands with A> 400 nm

Relative intensities of the C -X bands in the visible were obtained by observing fluorescence directly through a 7 -54 filter and normalizing in the following manner. Immediately before or after each scan through a full dye lasing region in which good band intensities were ob­served, the dye laser intensity was recorded as a func­tion of wavelength using a wavelength independent power meter (pyroelectric). The observed bands for the dye were then normalized to constant laser intensity. The effects of saturation were cheeked and the laser operated such that deviations from nonlinearity were less than 10%.

C-X bands with A < 400 nm

In order to observe the Parkinson bands arising from v" = 7 to v" = 13 (some bands of v" = 12 and 13 were also observed as above) a i-m monochromator was used as a filter as described in the experimental section.

The laser was scanned and the fluorescence intensity within the monochromator slit function was recorded. When the laser overlapped a transition between v" =7-13 and any v' whiCh was being monitored by the monochro­mator, a strong signal was obtained. Because of the wide slit function of the monochromator, emission from any v' was monitored, although with differing detection efficiencies.

A-X bands with v" = 0-9

Since ~1l C -X transitions for v" < 7 are beyond the range of currently available N2 laser pumped dyes, popu­lation information on v" from 0 to 9 was obtained using the A -X visible transitions of these vibrational levels. The technique used was essentially the same as that em­ployed by Revelli, Wicke, and Harris, 11 with the differ­ence being the use of a pulsed laser and gated detection. The technique is similar in nature to the one described above for observation of the ultraviolet C-X bands. The monochromator was used with 2 mm slits, however, to observe fluorescence only from a single A-X vibra­tional band outside of the dye laser scanning range. Ta­ble IV lists the dyes used, the bands excited, the mono­chromator settings and bands used to observe the result­ing fluorescence.

Cross normalization

The preceding methods provided groups of bands, each group consisting of bands with a common laser dye, a common upper level, and a common detection technique. Within any such group, Eq. (8) may be applied directly to obtain population ratios between lower levels in those groups. This of course results in a large duplication of information, since any lower level pair exhibits common absorption to several upper levels. Many times, how-· ever, one of the bands of that pair suffered from the ex­perimental error associated with low laser power, small Franck -Condon factor, over lap of band heads, or some combination of these. Therefore, band pairs not exhibit­ing any of these qualities were chosen to calculate the population ratios. When two or more consecutive ratios

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1434 A. Torres-Filho and J. G. Pruett: Ba + N20 chemiluminescent flame

TABLE IV. Detection scheme for XI~+ v" = 0-9 levels.

Band excited (A-X) Band monitored (A-X) Dye and wavelengths (nm) and wavelength (nm)

Rh6G (1,0) 580.5 (1,3) 656.3 Rh6G (1,1) 603.9

RhB (1,1) 603.9 (1,3) 656.3 RhB (1,2) 629.1 (1,3) 656.3

Rh6G+ Pilot (5,2) 560.2 (5,0) 521.4 495 a (5,3) 581.7 (5,0) 521. 4

Rh6G (5,3) 581. 7 (5,0) 521.4 (5,4) 604.6 (5,0) 521.4

RhB (5,4) 604.6 (5,0) 521. 4 (5,5) 629.3 (5,0) 521.4

Rh6G+ (9,5) 562.1 (9,1) 489.9 Pilot 495 a (9,6) 583.2 (9,1) 489.9

(9,6) 583.2 (9,1) 489.9 (9,7) 605.8 (9,1) 489.9

RhB (9,7) 605.8 (9,1) 489.9 (9,8) 630.1 (9,1) 489.9

RhB+ (9,8) 630.1 (9,1) 489.9 cresyl violet (9,9) 656.3 (9,1) 489.9

"New England Nuclear.

come from one particular detection group, those ratios are the most reliable. When two adjacent ratios are taken from two different detection groups, a random type error is introduced due to the uncertainty in the calibration factors between those groups. This error could be cumulative, but is more likely to cancel after several such connective operations. By assigning the most populated level a value of 1, all other populations are then calculated from the previous population by multi­plying it by the best ratio, or average of best ratios, between it and the next level.

RESULTS The relative populations for the X IL+ levels from

v" = 0-32 are displayed in Fig. 8. The bands primarily used to compute the ratios are noted in Table I. In es­sential agreement with Revelli, Wicke, and Harris ll we find that the low vibrational levels of the ground state are described by a temperature of around 600 oK. How­ever, at levels just beyond their observation and neglect­ing local variations, we observed a rather sharp break in the population to a new "temperature" of 3600 oK. This temperature is maintained out to the limits of our observation. We estimate that the major sources of error in the general trend of the populations are due to time irregularities in the flame and nonlinear absorp­tion. We did not normally monitor the flame intensity, but on one occasion (while observing v" = 0-9) when we did, we noticed a 4-h drop of intensity of about a factor of 2.

Scans through individual dye laser ranges, however, only required about 20 min so that the variation in flame intensity during that time is more likely to be around 10%. Constructing the population ratios as described above thus automatically compensates for flame inten-

sity variations between different dye scans. Although we did periodically check for absorption saturation ef­fects, we did not do this for every dye range used. Since each dye could deviate somewhat in bandwidth and collimation, and therefore, power density, it is pos­sible that some apparent populations were reduced near the peak of the dye range. We expect, however, only a few « 10%) percent error from this source based on the extent of saturation evidenced when we did check for that effect using neutral density filters.

The resulting error in individual population ratios is therefore approximately 20%. Most of the fluctuations from thermal behavior are within this range. The in­creased population around v" = 15, however, seems to be beyond the error range, indicating a real sustained deviation from thermal behavior in those levels. Since these local 20% error fluctuations can cumulatively af­fect the overall temperature observed, we must estimate that cumulative effect to set error bounds on the general population trend. A standard differential propagation of errors treatment predicts an overall temperature uncer­tainty of ± 600 ° K for the 3600 ° K observed.

Excited electronic states

Even though we have seen transitions between the elL state and the excited electronic states a 3n, A' In, and A IL+, we cannot at this time report relative popula­tions in these states because of the lack of information on relative transition strengths between these states. Within anyone such state, e.g., the A,ln state, we can only qualitatively estimate the levels which are popu­lated. The spectrum in the region of these bands is highly congested, such that many such bands overlap caUSing considerable difficulty in interpreting relative

0.1

0.01

0.001

0.0001

..

:ab.: • 3~

~K 10K I~K 20K ENERG Y em-I

FIG. 8. Relative populations of the first 33 vibrational levels of the BaO X 1~+ state. Error bars shown at v" = 14 are repre­sentative of the fractional error at each vibrational level. Representative temperatures have a similar 20% uncertainty.

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A. Torres-Filho and J. G. Pruett: Ba+ N20 chemiluminescent flame 1435

TABLE V. Reactions considered in kinetic model.

Production

Ba + N20 - BaO (X state, v":5 27) + N2• Ba+Np-BaO (reservoir) +N2 (either directly or via a

nascent precursor) Ba + N20 - BaO (A or A' states) + N2

Emission:

BaO (A and A') - BaO (X state, v":5 27) +hv

Quenching and transfer:

BaO (reservoir) + M - BaO (A or A' states) + M BaO (reservoir) + M - BaO (X state, v":5 27) + M

Diffusion:

BaO (any state) - out of flame

intensities. Until we are able to synthesize the spectra using good molecular constants and laser bandwidth in­formation in a convolution program, we can only state that we observe population in at least v" = 0-4 of the A' in and a 3n states, with v" =0 having the highest popu­lation. We also see strong transitions from v' = 1 of the A 1:0+ state which will eventually relate the population of the nonemitting states reported here to the A 1:0 + state populations observed in chemiluminescence. 5

DISCUSSION

The laser induced fluorescence spectrum from 360 to 750 nm is dominated by the C-X band system. The in­tensities in the observed (C-A') and (C-a) systems quali­tatively only accounts for about 0.1% of the total signal integrated over all wavelengths. Although we do not have relative transition strengths for these bands, we feel that the majority of the population will be in the ground state. It is not surpriSing that in a flame, the ground state population would be large because any excited state or reservoir state with a loss mechanism faster than the rate of diffusion out of the flame would be diluted with respect to molecules whose only loss mechanism is dif­fusion, (as is the case for low vibrational levels of the ground state). Using a simple kinetic model similar in nature to others already published, 9 one can evaluate this effect for the flame taken as a whole. We have done this assuming there to be three types of states present in the flame: (1) ground states below v" = 27, (2) reser­voir states, and (3) radiating states (see Table V). Us­ing the beam condition photon yields of Dickson et al. 8

and pressure dependent photon yields from a composite of several labs, 2.3.5-9 the rate of net collisional transfer from the reservoir state to radiating states can be esti­mated at our pressure to be 0.15 - O. 03 = 0.12 relative to the total BaO production rate. A similar rate is as­sumed for the quenching of reservoir states to the ground state. These relations result from the model in which the rate of transfer into any radiation state is measured by the rate of extra photon emission from that state. We make the assumption then tlrat the original branching fractions for reservoir states (or precursors to a reser­voir state) and v":5 27 of the ground state are essentially equal and given by (1. 0 - 0.03)/2 - 0.5. At worst these

fractions are limited by the maximum photon yield to be 0.25 for the reservoir and 0.75 for v:5 27 of the ground state. Using a steady state analysis we find that from 5% to 2~ of the average population in the flame should be in the energy pooling reservoir state. We have used v" = 27 as the cutoff point for "relaxed" ground state molecules because the energy of levels below v" = 27 is less than the energy of the v = 0 level of the lowest lying radiative state of BaO, so ground state levels below that level cannot contribute to a reservoir for collisionally pumped chemiluminescence. Our present results show that the observed population above v" = 27 represent less than 0.2% of the total observed ground state population. We estimate that the population that we do see represents from 80% to 95% of the total flame BaO population (allow­ing for 20% to 5% additional in the reservoir state).

Unless the ground state vibrational population makes a dramatic turn to a much higher vibrational tempera­ture or becomes inverted above v" = 32, it is unlikely that the unobserved ground state vibrational states are acting as a reservoir for the chemiluminescence. We must emphasize however, that this says nothing about the initial product branching into high vibrational levels, since such population may be drastically altered by col­lisions after reaction..

We are not able to say at this time that the observed population of the fl 3n state can account for the unassigned 5%-20% of the population, because we do not yet have ac­curate relative transition strengths for the (C-X) and (C-a) systems.

The apparent increase in population above a thermal value in v" = 13-15 presents an intriguing bit of datum for speculation. If the area under that portion of the curve which represents above-thermal population is in­tegrated and compared with the total area under the en­tire distribution, we find that about 2% of the molecules are in excess of the thermally expected populations.

The exact mechanism of production of this apparent excess population cannot be determined by the present experiments. There are two possibilities which seem plausible. The first possibility is that this population represents radiatively relaxed vibronic states whose Franck-Condon factors favor radiation to the X 1:0+ v" = 13-16 levels. In fact, the very low vibrational levels of the known radiating state A' in radi~te preferentially to vibrational levels in this area. At our experimental pressures, this A'-X emission could represent a rea­sonable fraction of the chemiluminescent emission. 5

There is still a significant amount of A 1:0+ emission, however, which would tend to wash out any local in­crease in ground state vibrational populations. In fact, using the A state populations given by Jones and Broida5

and by dividing the resulting radiatively produced popu­lation among the various ground state levels according to (A-X) Franck-Condon factors, we calculated that the radiatively populated ground state molecules would ex­hibit a smooth vibrational population decrease roughly characterized by a temperature of - 4000 0 K with no sharp nonthermal features. This radiatively produced population is certainly occurring, but we cannot say that

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1436 A. Torres-Filho and J. G. Pruett: Ba + N20 chemiluminescent flame

it is responsible for the majority of the population that we see.

The second possible explanation for an excess popula­tion near v" = 15 is that there is yet another electronic state with !loo near 10 000 cm -1 which acts as a localized population reservoir, transferring energy to the X 1~+ state collisionally around X (v" = 15). A low-lying 3~+ state should exist, but its presence has not yet been spectroscopically observed. No perturbations of the X state are known in this region, but an electronic state with 3 ~+ character, if present, would not interact with the 1~ + ground state. 22 Gottscho and Field have ob­served emission to an apparent 3~+ state following OODR, but it appears to be at considerably higher energy than 10000 cm-1 • 23 The fact that a3~+ and 1~+ states are non­interacting would reduce considerably the energy trans­fer rate between them, but energy transfer rates com­parable to electronic quenching rates of other nonper­turbed BaO states should still be possible. 24 There is also a 3~ - state which arises from ground state atoms and which is allowed to perturb the 1~ + state, but this state is expected to be high lying or repulsive. 25017

We cannot at this time assign the source of nonthermal population definitely to either of these sources, but when more complete population maps are available which in­clude excited electronic state populations, more quanti­tative comparison between radiative and collisional re­laxation can be made.

During the early phases of this investigation, attempts were made to observe nascent reaction products under molecular beam conditions. These attempts were suc­cessful but we were unable to identify the transitions ob­served due to the very high apparent rotational excitation (no band heads visible). Work in this direction is con­tinuing, taking advantage of the spectroscopic results of the present work.

ACKNOWLEDGMENTS

We gratefully acknowledge helpful discussions with Robert W. Field and Richard A. Gottscho, for sharing their results with us prior to publication and for critical reading of the manuscript. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the ACS, for partial support of this work. One of us (A.T.F.) would like to acknowledge C.A.P.E.S. (Brazil­ian Govt. Agency) and the "Instituto de Quimica," Uni­versity of Bahia, Brazil, for support. Partial support for this work was also provided by the National Science Foundation under grant CHE76-11468.

lCh. ottinger and R. N. Zare, Chern. Phys. Lett. 5, 243 (1970).

2R. H. Obenauf, C. J. Hsu, and H. B. Palmer, J. Chern. Phys. 57, 5607 (1972).

3C. R. Jones and H. P. Broida, J. Chern. Phys. 59, 6677 (1973).

4The reaction exothermicity of 94 kcal (32500 em-I) is suffi­cient to populate the A 11;+ (voo = 16 722. 3 cm-I, a 3" (voo = 17 337.00 cm-l) and A,l" (voo = 17 573.00 cm-l) states.

5C. R. Jones and H. P. Broida, J. Chern. Phys. 60, 4369 (1974).

6D. J. Eckstrom, S. A. Edelstein, D. L. Huestis, B. E. Perry, and S. W. Benson, J. Chern. Phys. 63, 3828 (1975).

IC. J. Hsu, W. D. Krugh, and H. B. Palmer, J. Chern. Phys. 60, 5118 (1974).

BC. R. Dickson, S. M. George, and R. N. Zare, J. Chern. Phys. 67, 1024 (1977).

9D• J. Eckstrom, S. A. Edelstein, D. L. Huestis, and B. E. Perry, and S. W. Benson, Semiannual Tech. Rep. #1, Con­tract DAA HOl-74-C-0524, S. R. I., August, 1974. Also Rep. #3, September, 1975.

lOR. W. Field, C. R. Jones, and H. P. Broida, J. Chern. Phys. 60, 4377 (1974).

11M. A. Revelli, B. G. Wicke, and D. O. Harris, J. Chern. Phys. 66, 732 (1977).

12W. H. Parkinson, Proc. Phys. Soc. London 78, 705 (1961). 13R. W. Field, G. A. Capelle, and M. A. Rivelli, J. Chern.

Phys. 63, 3228 (1975). 14R• A. Gottscho, B. Koffend, R. W. Field, and J. Lombardi,

J. Chern. Phys. 68, 4110 (1978). 15T • Wentink, Jr. and R. J. Spindler, J. Quant. Spectrosc.

Radiat. Trans. 12, 129 (1972). 16Second order behavior is expected with no steady state popu­

lation of the A state. With steady state population the order is less than second and greater than first.

IIR . W. Field, J. Chern. Phys. 60, 2400 (1974). lBFor simplicity of notation the rotationless states are treated.

Rotational line strength and degeneracies must be included for J'" O. If the same rotational branch line(s) are observed for two different bands these factors will also cancel in Eq. (9).

19Dv ' can be written more explicity as: Dv' =

=kL,v,.qv"v"v~"v,J)v',v'.Fv"v'" where k is a wavelength inde­pendent geometrical factor; qv',v" is the Franck-Condon factor for each fluorescence pathway; vV',v" is the frequency at which the transition occurs; Dv' ,v" is the PMT response (expressed as quantum efficiency) at Av',v"; and F v',v" is either the 7-54 filter transmission function or the slit func­tion (when the monochromator is being used).

2oL. Wharton and W. Klemperer, J. Chern. Phys. 38, 2705 (1963).

21R . J. Spindler, Jr., J. Quant. Spectrosc. Radiat. Trans. 9, 598 (1969).

22G. Herzberg, Spectra of Diatomic Molecules (Van Nostrand Reinhold, New York, 1950).

23R• A. Gottscho, R. W. Field, and R. Bacis, Symp. Mol. Spectrosc., 33rd, Paper RF9, Ohio State University, 1978.

24S. E. Johnson, J. Chern. Phys. 56, 149 (1971). 2iiA. D. McLean and M. Yoshimine, "Tables of Linear Mole­

cules Wave Functions," I. B. M. Corp., 1967.

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