Electronic excitation of the d 3Δi and a′ 3Σ+ states of carbon monoxide by metastable kr(3p2) atoms

Volume 143, number 5

CHEMICAL PHYSICS LETTERS

29 January 1988

ELECTRONIC EXCITATION OF THE d “Ai AND 8’ ‘Z+ STATES OF CARBON MONOXIDE BY METASTABLE Ih(3P,) ATOMS

Masaharu TSUJI, Kazuo YAMAGUCHI, Sumio YAMAGUCHI and Yukio NISHIMURA Institute of Advanced Material Study and Department of Molecular Science andTechnology, Graduate School ofEngineering Sciences, Kyushu University,Kasuga-shi, Fukuoka 816, Japan Received 17 August 1987; in final form 23 October 1987

Molecular CO emission, resulting from the Kr( ‘PI) + CO reaction in the flowing afterglow, indicates that a few near-resonant vibrational levels of the d ‘A, and a’ ‘Z+ states are preferentially populated. The mechanism of excitation transfer is discussed in terms of energy resonance and Franck-Condon criteria.

1. Introduction

The electronic energy transfer from resonance and metastable krypton atoms to carbon monoxide has captured our attention, because product state distributions depend on the spin-orbit state. According to a photosensitized fluorescence study by Vikis [ 1,2], the reaction of Kr( ‘PI) with CO gives very specific vibrational-rotational levels of CO(A ‘II), especially for certain isotopic combinations of carbon and oxygen. On the other hand, Stedman and Setser [ 31 found that the reaction of Kr(‘PZ) with CO gives a broad distribution of vibrational levels in the CO(A ‘FI, a 311r, a’ ‘C +) states. Among the observed product states, the a’ 3Z+ state was dominant. In this work, CO molecular emission due to the Kr( 3P2)+ CO reaction has been re-examined in the flowing afterglow under better optical resolution than the experiment of Stedman and Setser [ 31. In contrast to their earlier work, the present spectral analysis demonstrates that CO molecules are primarily excited into a few near-resonant vibrational levels of the d ‘Ai and a’ 3C+ states.

2. Experimental

quartz discharge tube (12 mm in diameter) and a stainless steel main flow tube (60 mm in diameter). The earlier experiment of Stedman and Setser [ 31 has been performed at buffer gas pressures between 0.3 and 10 Torr, usually 0.5 Torr (1 Torr= 133 Pa). In order to achieve lower pressure and maintain an adequate concentration of metastable atoms, a high capacity (10000 II min- ’ ) booster pump was employed. Krt3P,) atoms were produced by adding a small flow of Kr to the Ar buffer gas either before or after a microwave discharge; the density of Kr( 3P2) generated by the two methods was found to be similar from the observed fluorescence intensity. The reagent CO gas was injected about 23 cm downstream from the center of the discharge. Partial pressures,of Ar, Kr, and CO in the reaction zone were 50-80, 20-30, and lo-30 mTorr, respectively. The absence of Ar( 3Po,z) atoms in the reaction zone was confirmed by the absence of N2( C ‘l-I,-B ‘II& emission from the Ar( 3Po,z)+N2 reaction. Fluorescence spectra in the 200-600 nm region were analyzed by a 1 m monochromator equipped with a cooled Hamamatsu R376 photomultiplier tube. A mercury pen lamp was employed for the wavelength calibration. The relative response of the optical detection system was calibrated using a halogen lamp.

The flowing afterglow apparatus used for the present optical spectroscopic study has been described in detail previously [ 4,5]. The reactor consisted of a 482

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possible (v’, u”) transitions are indicated in fig. 1. As is seen from the figure, the u’ =23-26 levels are probably involved in the emission, though a higher resolution spectral analysis is necessary to make an unambiguous assignment. The present assignment is inconsistent with the earlier flowing-afterglow result of Stedman and Setser [ 31. They found that the emission spectrum from the Kr( ‘PJ + CO reaction consisted almost entirely of the CO( a’ ‘IZ‘-a ‘IT,) Asundi system for v’ = 13-22 with an apparent maximum population around V’= 20,

3. Results and discussion The emission from Kr* t CO is strongest in the 330-520 nm region (fig. 1). The predominant emmission system is a triple-headed, red-degraded band in the 330-410 nm region. Partially superimposed upon this system, another broad red-degraded emission system appears in the 410-520 nm region. The measured bandhead positions of the former triplet system are arranged in the Deslandres table given in table 1, where triple bandheads, with a separation of 70-90 cm-‘, are included. By reference to the reported spectroscopic constants of CO [ 6,7], it was definitely assigned to the d ‘Ai-a ‘IIr triplet system for u’ ~20 and 21 with V; = l-4. Such a straightforward assignment was difficult for the latter broad band, because clear bandhead structure is absent probably due to a heavy overlap of some vibronic transitions for each band. However, a careful comparison of the observed position and the intensity distribution with predicted ones from the reported constants and RKR Franck-Condon (FC) factors for the a’ 3C+-a ‘IIr transition led us to conclude that it is due to the a’-a Asundi system ofC0 from high vibrational levels. Predicted bandhead positions of

Kr(3P,)+CO+CO(a’

‘Z+:u’=13-22)+Kr,

M~=-10482to-2573cm-*.

(1)

Although the observed spectrum was not shown in their article, a direct comparison of the present spectrum with one recorded recently at Kansas State University [ 81 indicated that both spectra were essentially identical. However, the resolution of the Kansas spectrum was insuffrcient to resolve the triplet structure. We suppose, therefore, that the spectral resolution of the earlier Stedman and Setser spec-

v”C5 b

CO(d3Ai-a3N,)

v’=25

= 2

350

3

4

400

450

500

nm

Fig. 1. Emission spectrum resulting from the Kr( 3P2)+CO reaction in flowing afterglow. Predicted bandhead positions of the (2 I, 5) and (20, 5) bands of the CO(d ‘Ai-a 311,) system are pointed out by dotted lines.

483

Volume 143, number 5 Table 1 Deslandres

CHEMICAL

PHYSICS LETTERS

table ai for the CO( d ‘A,-a ‘II ,) system observed from the Kr( lP2) -I CO reaction 0” = 1

V’

20

21

v”Z3

0” =4

a=1

29817

(1689)

28188

(1656)

26532

(1624)

24908

52=2 Q=3

29188 29711

(1682) (1683)

28106 28028

(1658) (1657)

26448 2637 1

(1626) (1622)

24822 24749

n=1 a=2 Q=3

a’ Uncertainty

1988

’)

(in cm-

u” = 2

(764) (760) (764)

(772) (776) (774) 30649 30564 30485

(1697) (1698) (1693)

(762) (770)

(759) (757) (764)

(770)

28952 28866 28792

(1658) (1648) (1651)

27294 27218 27141

(1627) (1639) (1628)

25667 25579 25513

+ 5 cm-‘.

trum was similarly too poor to make a correct assignment. The intensity distribution of each band was essentially independent of the partial pressures of Ar, Kr, and CO, indicating that vibrational relaxation was negligible within the present experimental conditions. RKR FC factors and r-centroids calculated for the observed v’ = 20 and 21 U” progressions of CO(d-a) are listed in table 2. The relative vibrational populations of u’ = 20 and 2 1 levels in CO(d) were estimated from the corrected emission intensity I,.,. at frequency yU.uSG using the relation

(2) where qv.“..is the FC factor and R,( Tn.,,,)is the electronic transition moment in the r-centroid approximation. The R,( fU,,.,) function was estimated by plotting (Zu~v~.lqv~u~~v~~v~~)“2 versus fUS,,*. In fig. 2, the relative R,( Fuf,,,) dependence for the 0’ = 20 and 2 1 v” progressions are resealed by using Fraser’s method Table 2 RKR Franck-Condon

[9]. From the second-order fit of the data, the relation R,( TV’“,,) =canst.( 1 - 1.8208~~,,.+0.8366&) l.O464
VI

0” = 1

20

0.1504(-2) 1.0589 h’

21

0.8308( -3) 1.0464

”

is derived. The vibrational population ratio u’= 2 11 v’ = 20 thus obtained was 0.77 &0.07. In fig. 3 are shown the energy level diagram of metastable and resonance states of Kr and the observed vibrational levels of CO(d, a’). The Kr( ‘P,) and Kr( ‘P, ) resonance states can be removed from possible candidates for the excitation source of CO( d, a’) on the basis of Vikis’s photosensitized fluorescence data [ 1,2]. The higher metastable Kr ( ‘PO) state would not be the excitation source, because it relaxes before reaching the reaction zone [lo] and emissions are not detected from the 22< v’ < 27 (842921 cm-‘) levels of CO(d) and the 27~~~~33 (84897 cm-‘) levels of CO(a’). Thus it was con-

v” =2

v” =3

0.91 lO( -2) 1.0799

0.2793( 1.0891

0.5774( -2) 1.0688

0.2065( - 1) I .0784

the entry.

,

(3)

factors for the CO(d ‘A,-a ‘II,) system observed from the Kr( ‘Pz) +CO reaction

:: !.,,.: (n) is power of 10 multiplying r,*,.. in A.

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29 January

1)

v”=4

v” = 5

0.4429( - 1) 1.1181

0.2995( - 1) 1.1354

0.4008( 1.1080

1)

0.3819(-l) 1.1257

CHEMICAL PHYSICS LETTERS

Volume 143, number 5

eluded that the CO(d, a’) states are excited by the Kr( 3Pz) + CO reaction:

. V’=ZO A v’= 21

.

29 January 1988

AH! z 10 cm- ’ for the 51~ 2 component,

(4)

Kr(3P,)+CO+CO(d3A,:v’=21)+Kr, AH! =:780 cm-’ for the L?=2 component.

Fig. 2. Plot of the relative electronic transition moment versus rcentroid for the vibrational bands of the CQ(d-a) system observed in the present study.

It should be noted that both processes are endoergic. Taking account of the relative kinetic energy of colliding particles (fig. 3) and the rotational energy of CO at thermal energy, process (4) is expected to take place easily. On the other hand, high relative kinetic energy and/or rotational energy of CO must be SUPplied for process (5): fractions of kinetic and rotational energies higher than 780 cm-’ are evaluated to be 5.8 and 2.196, respectively, assuming a Boltzmann distribution at 300 K. Such a large endoergic process has been found for the Xe( ‘Pz) + Nz reaction [3,11-141:

AH! =377 cm-’ .

-3 pO

3pl 3p2

CO(d3Ai)

CO(aJX+)

v’

v’

-

21 20

-26 -25

7.

Fig. 3. Energy-level diagram of krypton atoms in the metastable and resonance states and vibrational levels of CO( d ‘A,, a’ 32 ‘) detected in the present study. A 300 K Boltzmann translational profile is also displayed.

(5)

(6)

The formation of N, (B : v’ = 5) has been explained not only by the contribution of relative kinetic energy [ 111 but also by the effect of vibrationally excitedN,(X ‘C+:v”=l) molecules [12]. ForCO, the relative population of N,,,= ,lNVVS =. at thermal energy is very small ( x 3X 10S5). Although the sum of the energies of Kr( ‘Pz) and the v” = 1 level (2143 cm- ’ ) can excite the d ‘Aistate up to v’ = 23, the triplet band system from v’ = 22 and 23 was absent. On the basis of these facts, the contribution of CO( X 3Z+ : II” = 1) will be unimportant. Energy transfer reactions between metastable and diatomic molecules have been discussed in terms of FC factors and energy resonance criteria. In table 3 are listed RKR FC factors for the X( v” = 0) +d( u’) and X( v” = 0) +a’( v’) vertical excitation calculated using reported molecular constants [ 6,7]. In table 3 are also included relative vibrational populations predicted from the golden rule model which has often been employed to explain vibrational distributions observed in energy transfer reactions between rare gas metastable atoms and diatomic molecules [ 13,151: ,485

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CHEMICAL PHYSICS LETTERS

Table 3 RKR Franck-Condon factors and golden rule distributions for transitions from CO(X ‘Z+:u”=O) to CO(d ‘Ai:v’) and CO(a’ %+:u’) XII:+-d’Ai

0

0.75( -4)

1 2

0.52( -3) 0.19( -2)

0.003 b’ 0.021 0.072

3 4 5

0.51( -2) 0.11(-l) 0.19(-l)

0.171 0.320 0.504

6 I

0.29( 0.40( 0.51(-l) 0.61(-l) 0.69( 0.74(-I)

0.693 0.855 0.961 1.000 0.971 0.885

8 9 10 II 12 13 14 15 16 17 18 19 20 21

*)

I) I)

I)

0.22( -3) 0.14( -2) 0.46( -2) 0.11(-l) 0.21(-l) 0.33( - 1) 0.47( - 1) 0.60( - 1) 0.71(-l)

0.006 0.035 0.109 0.239 0.417 0.615

0.78( - 1) 0.81(-l) 0.80( - I )

0.797 0.930 1.ooo 0.992 0.929 0.825

0.76( 0.75( 0.73( -

0.762 0.619 0.474

0.77( - 1) 0.71(-l) 0.63(-l)

0.699 0.567 0.443

0.55( - I)

0.341 0.228 0.139

0.55(-l) 0.47(-l) 0.40( -

0.334 0.244 0.172

0.073 0.030 0.006

O-33( - 1) 0.27( - 1) 0.22( - 1) 0.17(-l)

I) I) I) 0.68( - I ) 0.62( - I ) 0.48( - I) 0.41(-l) 0.34( - 1) 0.28( - 1)

I)

0.117 0.077 0.048

22 23 24

0.14(-l) 0.11(-l) 0.85( -2)

0.029 0.016 0.008 0.004

25 26

0.66( -2) OSl(-2)

0.001 0.000

a) qtmaa: (n) is power of IO multiplying the entry. b, qt,&-E,.~)“2.

N,, aq~v~(E-Ev~)3’2 ,

(7)

where E is the available energy including the electronic energy of Kr( 3Pz), the translational energy of the system and the rotational energy of the CO(X) molecule; and E,, the internal energy of the CO( d, a’) states. Favorable vibrational levels based on the vertical FC and the golden rule models are the V’= 8-12 levels. These levels are much lower than the experimental observation which shows the preferential formation of a few near-resonant vibrational levels in CO (d, a’). On the basis of this fact the energy resonance requirement takes precedence over the FC factor for the present reaction system. The discrepancy between the observed and calculated distribu486

29 January 1988

tions is especially large for the golden rule model, indicating that the excitation-transfer process is not controlled by the density of the final state. The total quenching cross section of Kr(3P2) by CO has been measured as 10.5 A2 [ 161. When the quenching cross section was estimated for the chargetransfer mechanism through a Kr+ + CO- ionic curve by using an electron affinity reported by Balamuta and Golde [ 171, a .(TCT value of 20 A’ was observed with a cross point (R,) at 2.5 A. This value is larger than the experimental value. Since repulsive forces are operative at 6 5 A for the metastable rare gas atoms [ 161, the charge-transfer mechanism is probably unimportant for the present system. It is, therefore, reasonable to assume that the formation of the near resonant CO(d, a’) levels proceeds through a direct curve crossing between covalent surfaces. In the measurement of Stedman and Setser [ 31, the following less intense band systems have also been identified: the Cameron (a ‘II,- X ‘): + ) system for V’< 3 and the fourth positive (A ‘II -+X ‘Z ’ ) system for V’< 12 with the highest population at o’ = 6. We could also identify weak band systems of a-X for v’ < 3 and the fourth positive system of A-X for v’ = 6. The a 311rstate is probably produced from the d-a and al-a radiative cascade. Meanwhile, the formation of A ‘II, which requires some breakdown of the spin-conservation rule, may involve secondary collision processes as proposed by Stedman and Setser [ 3 1. We are planning to carry out a low pressure experiment to identify the CO states directly excited. In summary, the emission spectrum from the Kr( 3P2)+ CO reaction has been studied in the flowing afterglow. In contrast to the earlier result of Setser and Stedman [ 31, near-resonant vibrational levels of the d ‘Ai and a’ 3E + states were found to be excited preferentially. This shows that the energyresonance requirement takes precedence over the FC factors for excitation in the present system.

Acknowledgement We wish to thank Professor D.W. Setser for sending an unpublished spectrum due to the Kr(3~2) +CO reaction, Dr. H. Obase for his helpful discus-

Volume 143, number 5

CHEMICAL PHYSICS LETTERS

sion, and Drs. J.T. Hougen, T. Kasuya and T. Munakata for providing us programs of RKR FC factors developed by Dr. D.L. Albritton.

References [ I] AC. Vikis, Chem. Phys. Letters 57 (1978) 522. [ 21 AC. Vikis, J. Photochem. 9 (1978) 154. [ 31 D.H. Stedman and D.W. Setser, J. Chem. Phys. 52 (1970) 3957. [4] M. Tsuji, T. Susuki, M. Endoh and Y. Nishimura, Chem. Phys. Letters 86 (1982) 411. [S] M. Tsuji, T. Susuki, K. Mizukami and Y. Nishimura, J. Chem. Phys. 83 (1985) 1677. [ 61 P.H. Krupenie, NSRDS Nat]. Bur. Stand. 5 (1966). [ 71 S.G. Tilford and J.D. Simmons, J. Phys. Chem. Ref. Data 1 (1972) 147.

29 January 1988

[ 81 D.W. Setser, private communication. [9] P.A. Fraser, Can. J.Phys. 32 (1954) 515. [IO] J.H. Kolts and D.W. Setser, in: Reactive intermediates in the gas phase, ed. D.W. Seter (Academic Press, New York, 1979) p. 152. [ 111 ES. Fishbume, G.L. Siebert and S.S. Lazdinis, J. Chem. Phys. 48 (1968) 1424. [ 121 T.D. Nguyen, N. Sadeghi and J.C. Pebay-Peyroula, Chem. Phys. Letters 29 (1974) 242. [ 131J. Be1Bruno and J. Krenos, Chem. Phys. Letters 74 (1980) 430. [ 141 N. Sadeghi and D.W. Setser, Chem. Phys. Letters 82 (198 1) 44. [ 151T.D. Nguyen and N. Sadeghi, Chem. Phys. 79 (1983) 41. [ 161J.E. Velazco, J.H. Kolts and D.W. Setser, J. Chem. Phys. 69 (1978) 4357. [ 171J. Balamutaand M.F. Golde, J. Chem. Phys. 76 (1982) 2430.

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