Electronic states of the CeO molecule: Absorption, emission, and laser spectroscopy

JOURNAL OF MOLECULAR SPECTROSCOPY

102, 441-497 (1983)

Electronic States of the CeO Molecule: Absorption, and Laser Spectroscopy

Emission,

C. LINTON Physics Department, University of New Brunswick, Fredericton, New Brunswick E3B 5A3, Canada

M. DULICK AND R. W. FIELD Department

ofChemistry,Massachusetts

Institute of Technology, Cambridge, Massachusetts 02139

P. CARETTE UER de Physique Fondamentale, UniversitC des Sciences et Techniques, 59655 Villeneuve dilscq Cedex, France

AND P. C. LEYLAND AND

R. F. BARROW

Physical Chemistry Laboratory, Oxford University, South Parks Road, Oxford OX1 3Q.Z. England

The electronicspectrumof the CeO molecule is characterizedby the existence of many O-O bands resulting from transitionsbetween various Q components of excited states and the 16 lower tl states which arise from the lowest configuration. . (4 f )(6s). Classical studies of rotational structure of absorption and emission spectra have been extended, and argon-ion and tunable dye (coumarin 460, rhodamine 6G, rhodamine 101) lasers have been used to excite known transitions in bands which had previously been rotationally analyzed. The resulting fluorescence spectra have been used to establish the relative energies of the lower states. By tuning the lasers to excite analyzed transitions from different known electronic states it has been possible to determine the energies of 16 low-lying states, to assign quantum numbers to 14 with certainty, and to suggestassignmentsfor the other 2. The resultingenergylevel diagram of lower states is discussed and shown to correlate well with the 4f 6s configuration of the Ce*+ ion. From the energies of the low-lying states, those of the higher excited states are calculated and in some cases new values of vibrational and rotational constants are derived. I. INTRODUCTION

The spectroscopy of the cerium oxide molecule has become of increasing interest following the discovery of CeO bands in the absorption spectra of S-type stars (Z3). Cerium oxide also provides a good example in which to begin the study of the electronic structure and coupling in molecules with partially filled f orbitals. At the start of the present work, information about the low-lying states was fragmentary; it had been derived as follows. An extensive series of high resolution (Doppler-limited) absorption and emission experiments (4, 5) identified many bands which involve transitions to one of four low-lying states, X&I = 2), X,(Q = 3), X,(Q = 4), and X&I 441

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442

LINTON ET AL. TABLE I Constants for the Low-Lying States of CeO (cm-‘)

state u3

o+

“2

1

TI

O-

“4

1

“3

2

w4 2 W3 3 X4 3 x3

4

Ul

o+

“2

l

VI

o-

w2 2

3

x1

2

*

Levels

a.

b.

c.

3

l/2

94

457.7

(30)

4

712

*4

133

(5)

4

7/Z

3

l/2

4

II2

3 462.6

(25)

0.355

3

712

2 771.7

(15)

0.35999

4

l/2

2 617.3

(21)

3

712

2 140.6

(15)

0.35658

(2)

2.72

(2)

824.1

4

II2

2 039.8

(21)

0.35327

(1)

2.24

(1)

822.1

3

5/Z

ll

931.8

(30)

0.377

‘1

0

Bo

3 821.5 *3

3

512 5/z

3

642

820.7 (1)

2.21

823.4

(1)

-0.356

824.7

869.7

(30)

0.343

(25)

0.35788

(1)

2.52

512

912.2

(15)

0.36139

(6)

7.3

(4)

823 .O

0.35692

(1)

2.71

(1)

822.8

0.35454

(1)

2.46

(1)

824.3

512

811.6

(25)

512

80.3

(15)

2

5/Z

0

the

Numbers

in

tion

the

in the -

0.375

1 679.4

of

correct

a.

parentheses last

exceptions

Note

(5)

2

With

AGl12

0.367

(30)

3

are

107Do

Jc

2

w1 l x2

T

Ja

-0.361

closest

to

represent

two

the

predicted

estimates

positions.

of

the

standard

devia-

digits. of

the

X1 and X2

states,

uncertainties

in

AGIl

2 cm-l.

These

states

J -25

:

have

their

each

B values

d.

Originally

labelled

v(2)

e.

Originally

labelled

r(3)

f.

1030. = 1.4

5.

103a

h.

Originally

labelled

u(2)

i.

Originally

labellod

y(l)

j.

1030

= 1.27

= 1.9

been are

observed not

known.

only

once,

in

fluorescence,

at

ELECTRONIC

443

STATES OF CeO

= 3).’ Rotational analyses showed that there are least four other low-lying states. Rotational constants of the eight low-lying states and of the many upper states connected (via observed transitions) to them were determined. The only additional information pertaining to the electronic level diagram provided by these experiments (4, 5) is that (i) the X,3 state is 2060 cm-’ above X23, and (ii) for X23, AGi,z = 822.76 cm-‘. Observations of laser induced fluorescence were used subsequently (i) to determine the vibration frequencies (6) of X,2 and X34, and (ii) to derive a scheme of electronic energy levels (7) in which the relative energies of seven low-lying states, namely, X,2, X23, X34, X,3, and three others, then labeled u(2), u(2), and w(3), were determined. The aims of the work described in the present paper were to use laser induced fluorescence combined with classical absorption and emission spectroscopy at high resolution (- 300 000) to derive the properties of as many states as possible and thus to build up a comprehensive and coherent scheme of energy levels which might then be interpreted to elucidate the nature of the bonding in CeO. Experiments were done in four laboratories. Ring furnace, absorption spectra were taken in Oxford of selected wavelength regions to supplement the extensive observations in absorption by Clements. Some spectra were also photographed in emission using a Bacis-type hollow-cathode discharge. High resolution emission spectra were also photographed at Lille, where pulsed laser induced fluorescence experiments were also done. Laser induced fluorescence and excitation spectra were recorded both at MIT and at the University of New Brunswick. Many experiments were complementary: the analyses of laser induced fluorescence spectra led to the successful search for new bands, while the detailed rotational analyses indicated good wavelengths for laser induced fluorescence and helped in the assignment of laser transitions. The observation and characterization of most of the 16 states expected from the lowest electron configuration enable us to introduce a rational system of nomenclature for the lower states: it remains necessary to use an empirical classification for the excited states. It will be shown that the lower states can be labeled by their values of ’ Henceforward, these states will be labeled X,2, X23. . . .

k.

Three m is are tively been

lower the the

states lower

lower

states

assigned observed

have

state

as in

I!

not

yet

of

three

of

single

= 2 -

absorption,

been

related

to

bands.

m2,

m3 and

bands, 1

and

k in

2

k2 -

3.

-

k Tix

emission.

B0

and

listed

‘4

E2

9. and

Constants

1OWS:

state

those

107Do

m (?2)

0.35214

(1)

2.45

(1)

i’ (?3)

0.35065

(2)

2.30

(4)

k (?l)

0.36035

(6)

0.32

(14)

above.

m,

and

-

9..

k and tenta-

n bands are

as

have fol-

C

444

LINTON

ET AL.

J, (and of course by their experimentally determined value of a). The X1 . . . X4 states are states for which Q = J,, where J,, for . . . 4f6s is3F, takes the values 2, 3, and 4. Thus we have X,2, X23, X34, and X,3, in that order. The four states Wi are the states for which Q = J, - 1, the Vi states have 52 = J, - 2; there are three Vi states with Q = J, - 3, and a single T state corresponding to Q = 0 (J, - 4). This system of nomenclature is used throughout the present paper: the correspondence with some earlier state labels is given in notes to Table I. For the upper states, whose characterization is less complete, apart from the Q values, we decided to use the empirical notation, taken from atomic spectroscopy, [energy, T&l. Values of To, with respect to D = 0 in X12, are given in thousands of cm-‘, so that, for example, [22.0]4 is the state with Q = 4 and TO - 22 000 cm-‘. The correspondence with earlier labels is given in Table II. II. EXPERIMENTAL

METHODS

Absorption spectra were photographed using cerium oxide, CeOl, in molybdenum boats heated in a King furnace to temperatures in the range 2200-2500 K, in the presence of about 300 Torr of argon. The carbon tubes were 35 cm long, but the effective path was shorter than this, perhaps 15-20 cm. For emission experiments, Bacis-type hollow cathodes (8) were prepared by pressing a mixture of Ag + Ce02 in a 2:l mole ratio, and in typical experiments spectra were excited with a current of 75 mA at 150 V in presence of about 0.3 Torr of argon. In the experiments at Oxford, the spectra were photographed on a 3.4-m Jarrell-Ash spectrograph using a plane grating blazed at 59” with 57 000 lines. In the Lille experiments, a similar composite wall hollow-cathode lamp was used but cooled with liquid nitrogen and photographed on a 4-m Littrow mounted grating spectrograph providing a theoretical resolution of 500 000 in second order. The dye laser experiments at Lille employed a pulsed nitrogen-laser-pumped tunable dye laser (Molectron UV 300, DL 400) which delivered light pulses of 5-nsec duration, power lo-40 kW, 0.01~nm linewidth (FWHM), 15-Hz repetition rate. Coumarin 460 dye was used in the 456~nm wavelength region. The laser beam was directed vertically into the fluorescence cell. Spectra were recorded photoelectrically using a Hamamatsu RS 928 photomultiplier after dispersion through a THR 500 Jobin-Yvon monochromator. A second photomultiplier detected directly the total emitted fluorescence through a cutoff interference filter to block the scattered laser light. Both signals were fed into a two-channel PAR 162-163 boxcar averager using Tektronix S-2 sampling heads (75 psec rise time), triggered by a photodiode set in the dye laser and running in the “A/B mode” to obviate the effects of intensity fluctuations. All of the cw laser experiments used a Broida oven (9, 6) in which CeO was formed by (i) reacting Ce vapor with 02, (ii) heating a mixture of Ce and Ce02, or (iii) heating CeOz. In each case, the resulting CeO vapor was entrained in argon and carried into the observation region at a total pressure of - l-2 Torr. All three methods were effective in producing CeO and each required a temperature in the range 18002000 K. In experiments at MIT and at UNB, fluorescence was excited using an argon ion laser (single mode and multimode) and tunable single mode cw dye lasers (Spectra

445

ELECTRONIC STATES OF CeO TABLE II Constants for the Excited States of CeO (cm-‘)

state

T

a

128.611 L26.714

E3

126.213

%

L26.112

Fl

C25.313

G2

125.012

%

1O’D

Bo

0

(eJ0.36122 (fI0.36079

0

AG1/2

[28.611tVl

O-

795.5

[26.714.X3

4

732.54

[26.213+X2

3

[26.112+X1

2

3.04

28

596.1

26

715.2

0.35294

4.9

26

200.0

0.3514

2.2

Transitionsb

26

067.5

0.3522

3.75

25

292.3

0.3502

3.03

733.90

[25.313+X2

3

25

011.5

0.34863

2.81

744.3

[25.012
2

[23.710-

23

674.3

0.35348

0.48

[23.710-wV1

[22.710-

22

722.8

0.35407

2.63

[22.710-TV1

22

556.0

0.35069

2.61

l22.615

D3

122.511

22

505.8

t22.513

22

498.4

h2

786.0

6.2

(fI0.3576

792.3

5.0

0.3568

122.014

A4

4

[22.511+‘1

O-

t22.513++W2

121.411

“1

C21.112

g2

3,

F2

120.313

cl

L19.911

012.3

0.35443

2.81

[22.014-X4

3

0.35353

2.14

121 .719’X2

3

21

379.2

0.35241 0.35203

2.75 2.59

[21.411+X1

2

21

061.9

0.35662

3.98

[21.112+X2

20

914.5

0.35294

2.97

20

273.8

0.34988

2.99

19

926

783.6

c3

19 287.5

0.3485

0.4

18 386.2

0.34176

2.3

750.7

[20.313-X1

2

119.911+x1

2

2, 2,

e2

17

0.35037

169.5

2.44

765.0

Earlier

b.

F.

designations

indicates

of

a transition

states

are

observed

given in

in

the

fluorescence.

Of,

[18.414+X3

4

4,

-X2

[17.213-X2

3

-w2

8.

-Ill

3

F.-XI

second

2, 2.

-x2

-w4

column.

3,

-W4 2.

[19.31&X4

2

-W2 2,

-W4 2

121.112+W2

-X4 3

F.-X3

r 17.213

3:

3,

[20.91kX2

-V3

18.414

3,

713.2

F.-XI

119.313

-x4

21

F.-X2 I20.914

2

22

768.0

O-

[22.615+X3

F.-X2

r21.714

O-

-W2 2,

-VI -U3

0-. O+.

2 -W4

2

-V2 -Tl

1. O-

3,

-X4

3.

-W3 3

3,

-X3

4,

-x4

3.

446

LINTON

ET AL.

TABLE II-Continued

state

To

116.711

107Do

Bo

4.28 3.45

(ej0.35616 (fJ0.35643

16 714.8

Transitions

A%/2

[16.711+Vl F.-Xl

116.512

16

Bl

2. o-.

-v2

1,

-U3

O+.

-T1

O-

116.512+X1 F.-Xl -w2

L16.514

5.1

0.35363

16 495.4

E2

2, 2,

-w3

B3

114.713 L14.213

A3

[14.210L13.914

B2

C13.211 L12.812

A2

Ll2.613

c.

%

Values

of

[16.013+X2

3

813.8

[15.814+X2

3

15 489.4

0.34534

2.45

[15*515+X3

4

14

0.35255

2.51

[14.713+W2

2,

14 201.8

0.36024

4.48

[14.213+X3

4

14

197.0

0.35216

7.8

[14.210-+V1

13

884.3

0.34711

2.86

0.35974 0.35921

4.97 5.51

[13.211ttX1

2

12 768.0

0.3535

4.5

[12.812ttX2

3

12

0.34672

2.90

[12.613++Xl

2

To

201.6

595.8

have

yet

to

be

state

d.

Values

The

The in

0.34933

2.65

m4-m

19 727.0

0.34857

2.82

m3 -

m

?l

11

0.3572

1.2

In2 -

q

e,

?2

11 787.0

0.35844

5.74

e2 -e

k2

?2

15

0.35924

1.75

k2 -

as

2-2

128.98

-X4

3,

-W2 2,

[14.713*W4

2

[13.914+X4

3

O3,

An=+1 AQ = +l An = -1 An=-1 k

AI-I = +l

follows: 0.0018 0.00212

t14.713

assigned

963.4

-Xl

Transition

20 792.29

in

the

107Do

1,

3,

states:

?3

Of a are

band

following

?l

transitions

fact

the

m4

respectively, f.

fcr

B0

“0

113.914+x2

q3 q2

126.213 122.014

e.

determined

o+.

3

3.03

13

‘1

-X4

4.

5.3

747.39

-W4 2. -ul

3 -X3

0.35216

701.6

2,

-w4 2

3,

0.35953

15

c2

[15.515

3,

039.2

16

D2

[15.814

-w3

3,

-W2 2. -v3

2 -X2

[16.514+X2

753.9

F.-X2

116.013

1,

-Wl

-vl

720.7

0.3436

524.2

O-

122.615 118.414

x- W2 2

and

122.513 L13.914

0.00192 0.00138

[16.711

+

Vl

O-

0.00172 0.00177

were

denoted

cm -1

systems

n

and

o,

cm -’

is

(4). in band

(4) of

as

the

r13.914

O-O band -X,

3.

of

a transition

b3

-

X3 at

13

652.07

ELECTRONIC

STATES

OF CeO

447

Physics 580A, Coherent Radiation 599-21) using coumarin 460 (450-480 nm), rhodamine 6G (580-620 nm), and rhodamine 101 (620-670 nm) dyes. Spectra were recorded photoelectrically using either a Jarrell-Ash 0.5-m Ebert or Spex 1-m Czerny Turner monochromator with appropriate photomultipliers and, in some cases, photon counting detection systems. Mercury, argon, and neon lamps were used for calibration. The relative accuracy of wavelength measurements varied upwards from 0.025 nm depending on the slitwidths required to record the spectra. Some high resolution laser excitation experiments were performed in order to identify and measure the first few lines in the Q branch of certain bands. In these experiments the CR 599-21 dye laser was scanned, 1 cm-’ at a time, across the wavelength region of interest. The laser linewidth was - 1 MHz and the resolution was limited by the Doppler width (- 1 GHz). III. RESULTS

AND ANALYSIS

A. General Features and Data Reduction Methods In this section, before detailed spectra are discussed, general features common to nearly all the spectra are outlined and the standard methods used to analyze the data are described briefly. (i) General. The fluorescence spectra obtained typically consist of doublets (Rp) or triplets (RQP) arising from transitions from a common upper state to various lowlying electronic and vibrational states. Very often, especially when the laser was not used in single mode operation, several transitions were excited simultaneously and the spectra were quite complex. All spectra were attributed to the most abundant isotopic species, ‘40Ce’60 (88.5%) although there is an appreciable population of ‘42Ce’60 ( 11.1%). However, the ratio of the reduced masses, 142Ce’60/‘40Ce’60, is only 14.378/14.357 = 1.00145, so that possible errors caused by excitation of the less common isotope would generally be less than 1 cm-‘, and well within the error of measurement. (ii) Rotation. The J value of the upper state was determined in the usual manner from the measured separation of the P and R lines in a doublet or triplet. To calculate J, a value of B” = 0.35 cm-’ was assumed. This value is typical of all the known low-lying states of CeO. J could generally be determined to an accuracy of +l. In many cases, it was possible to assign the lines uniquely by comparing their frequencies with the high resolution measurements (Appendix I). (iii) Vibration. Vibrational frequencies of low-lying states of CeO are typically -820 cm-‘. Doublets or triplets with the same RP separation and separated by -820 cm-’ were thus assigned as belonging to a vibrational progression. The strongest member of the progression was assigned as a Au = 0 transition and the vibrational numbering assigned either by comparison with previously analyzed bands (6, 7) or by counting the number of anti-Stokes components in the progression. This latter method can lead to error in the event that very weak members of the progression are not detected.

448

LINTON

ET AL.

(iv) Electronic. As the rotational constants of all the known low lying states of CeO are very similar, the PR separations of all fluorescence transitions from a common upper state are nearly identical. It was thus possible, even in the most complex spectra when several transitions were excited simultaneously, to determine which RQPgroups involved a common upper state and hence calculate the separation of the low-lying states. The relative D values of the low-lying states could be determined from the relative intensities of the R, Q, and P lines. In the absence of perturbations, the presence of an RP doublet, with maybe a weak Q line, indicates that An(=Q’ - V) = 0. If Zo > ZR> ZP, AQ = +I, and if Zo > ZP > ZR, Afl = - 1. However, in the CeO molecule, with many low-lying states, one may expect that heterogeneous perturbations will cause many intensity anomalies in the observed spectra. The matrix element mixing states whose Szvalues differ by one is J dependent and vanishes at very low J. To determine AQ correctly, it was therefore necessary to examine the intensities of transitions at low J. For all transitions excited with dye lasers, the laser was tuned to several different J’s and the variation of intensity with J was examined. In most cases, fluorescence was examined in a known transition for which the value of D for both states had previously been established and it was therefore possible to assign Q for all the other states in the fluorescence spectrum. Observation of fluorescence spectra over a range of values of J is valuable also in that it enables differences in B value for two lower states to be determined. For, considering fluorescence from a single upper level to two lower states, I and II, Av(J) = RI(J) - R,,(J) = ATo + (B,I - BMJ + 1). Thus observations over a range of J enable, for example, BI, to be determined if BI is known, and lead to the determination of TO,provided of course that the effects of centrifugal distortion and of perturbations can be ignored. Rotational structure in the high resolution spectra (Appendix I) was measured against Th or Fe-Ne lines from hollow-cathode lamps and reduced to wavenumbers in the usual way.

B. Detailed Results General The results may be considered under two headings: (i) the determination of the relative energies of the low-lying states by laser induced fluorescence, and (ii) the analyses of the high resolution emission and absorption spectra which led to the determination of rotational constants and of the energies of the high-lying states. The experiments which led to the construction of the energy-level scheme were performed in four stages: (i) Construction of a manifold of low-lying states connected to X23. This emerged by examining the most intense features in fluorescence spectra excited at 4764.9 and 4632.0 A, and by several transitions in the [16.514-X23 O-O and l-l bands.

ELECTRONIC STATES OF CeO

449

(ii) Determination ofthe separationoftheX12andX23statesby excitingtransitions withinthe [16.512-X12 and [ 17.213~X23 O-Obandsand examiningthe resultingfluorescence. (iii) Determination of the separation of the X34 and X43 states through excitation of transitions in the O-O bands of the [17.213-X23, [16.514-X23 and [18.414-X34 systems. (iv) Detection and determination of the energies of a series of a = 0 and 1 states and one further Sz = 2 state by exciting the upper state of the O-O band of the system “o”, [16.7]1-V,O- analyzed by Barrow et al. (4). The transitions are set out in Fig. 1. In most cases, the assignments of laser transitions were confirmed, and, in some cases, the accuracy of measurement of energy separations improved, by rotational analyses of new emission and absorption bands. The fluorescence measurements, given below in Table III, refer to spectra taken with the laser operating single mode, with the exception of transitions following [2 1.7]3- IV22excitation. Where laser induced fluorescence spectra were taken at several different values of 1, only identified transitions at one J are listed. Details (i) The X23 statemanifold. (a) [21.112-X23. In a summary ofabsorption and emission bands of CeO (4), a red-degraded head is listed at 4764.76 A. This is the Q head of a weak and overlapped n2-Xz3 band which has now been analyzed both from absorption (Oxford) and from emission plates (Lille). The two sets of measurements are in good agreement, apart from a small constant shift of about 0.08 cm-’ and, following the identification of R(3 l), Q(32), and P(33) from the fluorescence work, it was possible to assign R and P lines between J = 3 and J = 52, and closely spaced Q lines at somewhat higher J. Within this range, the upper state seems to be unperturbed. The 4765-A line of the argon laser excited the most intense fluorescence so far observed in CeO. The laser, even when operated single mode, excites two Q-branch transitions which seem to be in the same band. The relevant portions of the spectrum are shown in Fig. 2 and indicate that the transition is Aa = -1. Other intense transitions with AQ = 0, - 1, 0 appear -830, 2060, and 2964 cm-‘, respectively, from the laser transition. The 2060-cm-’ separation indicates that these two states are X23 and X,3, so that the laser excites into the Q branch of the [21.112-X23 transition and fluorescence occurs to two fl = 2 states labeled IV22 and IV,2 and the X43 state. The most intense observed lines are listed in Table IIIA and the assignments of the [21.112-X23 lines are shown to be consistent with the high resolution measurements. There are many weaker lines which will be discussed elsewhere and are not listed in Table IIIA. Having completed the analysis of the [2 1. 112-X23 band a search was made for the transitions to the other states detected in fluorescence. The [2 1. 112-X43 and [21.1]2IV42 bands could not be found, but a very weak, overlapped, two-branch band was found in absorption, at the position calculated for the [2 1.1]2-IV22 transition. Agreement of upper state combination differences indicated that the upper state was [2 1.112.

450

LINTONET AL.

a 24000-

20000cm-’

%OO 16000-

116,512

i 1lolo I:,,I_ fll!

5000cm-’-

2

O-

2 lo

2 1

O

0

0

Xl

1 0

x*

WI

w2

x4

w, %

FIG. 1. Laserinduced fluorescence transitions in CeO. (i) Excitation: a [17.313-X23 (Table IIIC); b [2 1.1]2X23 (Table MA); c, d [22.5]3-IV,2 (Table IIIB); (ii) [16.512-X,2 (Table IIID); (iii) a, b [16.514-X23 (Table IIIE); d [18.414-X34 (Table IIIF); (iv) a (16.7]1-W22 (Table IIIG); c [19.911-X,2 (A? 5017 A: see p. 28).

The B value of the lower state was found to agree closely with that of the lower state of system n (4), which is thus identified as [ 14.7]3-W22. From the band origin separations in [21.1]2-W22, -X23, the IV22state is found to lie at an energy of 831.89(5) cm-’ above X23. This is in satisfactory agreement with the value 833.9(28) cm-’ given by Linton et al. (7). It has been mentioned (4) that v = 1 of the X23 state is perturbed. This is consistent with our assignment of u = 0 of the IV22state which is within 10 cm-’ of the expected position of X23, u = 1, and is most probably responsible for the perturbation. The rotational constants obtained from high resolution analysis of the R and P branches in the [21.112-X23 band (9) show that the Q branch forms a single reddegraded head. The excitation of two transitions at nonconsecutive J in the same

451

ELECTRONIC STATES OF CeO

@

19000- [El*414

L16.514

-

C)

bAl

16000- a

1

II

I

v=i v=o

5000cm-'

-

O-

@ 20000-

17000-

119.911

‘.“1

5000cm-' -

FIG. 1-Continued.

branch of the same band, even when the laser is operated single mode, therefore indicates that there is probably Cldoubling in one of the states. None of the analyses of the many known bands involving X23 have indicated any measurable doubling in this state so that the doubling must occur in [21.1]2. The band is red-degraded, and therefore, for both the e andfcomponents, v[Q(15)] > v[Q(30)]. In order that v[Q(15)] = v[Q(30)], the laser must be exciting the lower component at J = 15 and the upper component at J = 30. When the laser etalon was tuned across the Doppler profile of the laser line, different Q-branch lines were excited but there were always two different J transitions. The J value of the lower J lines varied more rapidly with laser frequency than did that of the higher J lines which is to be expected as the lines should be closer together at low J. (6) [22.5]3- W22. The rotational analysis of the O-O and l-l bands of a system assigned as [22.5]3-IV22 was carried out at Lille from plates taken in emission. The O-O band has a red-degraded Q head at 4631.3 A. Dye laser radiation (coumarin

LINTON ET AL.

452

TABLE III Observed Laser Induced Fluorescence Transitions in CeO A. [2 I. 1]2-X23 Excitation Wavenumber -1 (cm )

Assignment Y',""

lower state

21 OOl.Z(Z.7) 20 980.9 L 20 958.1t9.2)

R(29) 0.0 Q(30) P(31)

X2 3

20 165.8C7.3) 20 122.OC3.4)

R(29) 0,O P(31)

w2 2

18 920.5 942.0

Q(30) R(29) 0.0

X4 3

18 898.7

P(31)

18 307.9 18 264.4

R(29) 0.0 P(31)

w4 2

20 991.1t2.1) 20 980.9 L 20 969.1(0.1)

R(14) 0.0 Q(15) P(16)

X2 3

20 155.3 20 133.7

R(14) 0.0 P(16)

w2 2

18 931.6 18 920.5 18 908.3

R(14) 0.0 Q(15) P(16)

X4 3

18 298.2 18 275.1

R(14) 0.0 P(16)

W4 2

460) at 4633.5 A excited Q(49) in the O-O band of [22.5]3-Wz2, and fluorescence to the same four states, X23, X43, W22, and W,2, as is excited by Ar+ 4765 A in the O-O band of [2 1.112-X23, is observed (Table IIIB). At the same time, other transitions are seen, particularly to a lower state designated V,2. The spectrum is somewhat simplified when the laser is operated single mode (Fig. 3), but this has been examined only in the X,3, W22, X43, and W42 regions. (c) [16.514-X23. Barrow et al. (4) have rotationally analyzed a band with heads at 6080.28 (R branch) and 6083.90 A (Q branch) and assigned it as the O-O band of the [ 16.514-X23 transition. On the basis of the known 2060 cm-’ separation between the X23 and X43 states, they assigned a bandhead at 6964.33 A as the Q head of the [16.514-X43 O-O band, When the dye laser, with rhodamine 6G, was tuned to this band, several fluorescence features were observed. The three most intense features are shown in Fig. 4. The two strongest features are separated by 2060 cm-’ and can be assigned as [16.514-X23 = +l andthereis and [16.514-X43. The third transition at -72OOAalsohasACl thus another fi = 3 state -475 cm-’ above X,3. This state has been assigned as W33. When the laser was tuned in the 6 110-A region, spectra similar to that at the bottom of Fig. 4 were obtained. The similarity to the [16.514-X23 O-O fluorescence is striking and shows that the laser was exciting a [16.514-X23 transition, probably the l-l band. Very close to this transition, there are three very weak lines which appear to form an RQP triplet with the same RP separation. The Q line is the most

453

ELECTRONICSTATESOF CeO TABLEIII-Continued B. [22.5]3-IV22 Excitation Assignment

Wavenumber (cm-1)

v',v"

10Vier state

22 448.8 22 379.5

R(48) 0.0 PC501

X2 3

21 610.8(0.8) 21 575.8(5.7)L 21 539.9

R(48) 0.0 PC491 P(50)

w2 2

21 630.4 21 561.8

R(48) 0.1 P(50)

X2 3

20 388.2 20 319.0

R(48) 0.0 P(50)

X4 3

19 752.2 19 717.5 19 681.0

R(48) 0.0 Q(49) P(50)

w4 2

19 074.9 19 039.3 19 004.6

R(48) 0.0 Q(49) P(50)

"3 2

18 930.5 18 896.5 18 861.2

R(48) 0.1 Q(49) P(50)

W4 2

21 399.0 22 368.0 22.338.0

R(401 1.0 Q(41) P(42)

W2 2

22 412.6 22 355.0

R(40) 1.1 P(42)

X2 3

21 575.8(6.O)L 21 547.OC6.7) 21 516.4

R(40) 1.1 Q(41) P(42)

W2 2

20 350.9 20 288.5

R(40) 1.1 P(42)

X4 3

19 717.5 19 687.2 19 658.2

R(40) 1.1 Q(41) P(42)

19 039.3 19 010.7 18 981.9

R(401 1.1 Q(41) P(42)

22 424.2 22 406.1

R(11) 0.0 P(13)

21 593.5C4.0) 21 585.1C5.3) 21 575.8(5.9)L

R(11) 0.0 Q(12) P(13)

w2 2

21 602.8 21 585.1

R(11) 0.0 PC131

X2 3

20 463.3 20 457.0 20 447.4

R(11) 0.0 Q(lZf P(13)

X3 4

19 734.6 19 725.1 19 717.5

R(11) 0.0 Q(12) P(13)

w4 2

w4 z

"3 2

x2 3

454

LINTON ET AL. TABLE III-Continued C. [ 17.213~X22Excitation WaVeIllldler (cm-1)

Assignment V',"')

lOW.YI state Xl 2

17 192.1 17 165.3 17 139.7

R(34) 0.0 Q(35) P(36)

17 105.1(5.3)L 17 079.6

R(34) 0,O Q(35)

17 054.6f4.7)

P(36)

16 362.4 16 337.5 16 312.7

R(34) 0.1 Q(35) P(36)

x1 2

16 280.0 16 255.1 16 229.5

R(34) 0.1 Q(35) P(36)

X2 3

267.0 16 242.2

Q(35) R(34) 0.0

W2 2

16 215.6

P(36)

15 543.7 15 518.1

R(34) 0.2 Q(35)

Xl z

15 463.4 15 439.3 15 413.6

R(34) 0.2 Q(35) P(36)

X2 3

15 446.0 15 420.9 15 397.2

R(34) 0.1 Q(35) P(36)

W2 2

15 146.6 15 121.2 15 095.6

R(34) 0.0 Q(35) P(36)

X3 4

15 042.0 15 017.4 14 991.7

R(34) 0.0 PC351 P(36)

X4 3

14 627.8 14 602.4 14 577.1

R(34) 0.2 Q(35) P(36)

W2 2

14 409.6 14 384.5 14 358.5

R(34) 0.0 Q(35) P(36)

W4 2

13 590.0 13 565.3 13 539.1

R(34) 0.1 Q(35) P(36)

W4 2

X2 3

intense and can just be seen in Fig. 4 (bottom) at 6 120 A. The lines have the appearance of a very weak AR = +1 transition and their frequencies are those calculated for the 1-O band of [ 16.5]4- W22. Even though this AR = 2 transition is nominally forbidden, there is sufficient mixing between u = 1 of X23 and u = 0 of IV22 for the transition to be observed. Several other features were observed in [16.514-X23 fluorescence spectra. These will be discussed in a later section. (d) [22. 7&X23. The O-O band of a transition [2 1.714-X23 lies in a very crowded region of the CeO spectrum and is overlapped by other bands of comparable intensity.

455

ELECTRONICSTATESOF CeO TABLEIII-Continued D. [16.512-X,2Excitation Assignment

Wavenumber (cm -1)

Y’,V’I

lower state

16 531.2(1.9)L 16 520.5 16 510.4(1.4)

R(13) 0.0 Q(14) PC151

x1 2

16 448.0 16 438.5 16 421.7

R(13) 0.0 Q(14) P(15)

X2 3

15 715.0 15 694.2

R(13) 0,O P(15)

WI l?

15 704.8 15 684.2

R(13) 0.1 P(15)

x1 2

15 621.4 15 611.7 15 600.9

R(13) 0.1 Q(14) PC151

x2 3

15 613.4 15 603.9 15 593.4

R(13) 0.0 Q(14) P(15)

w2 27

14 884.6 14 864.5

R(13) 0.2 P(15)

xl 2

14 793.6 14 771.6

R(13) 0.1 PC151

w2 2

14 387.4 14 377.7 14 367.2

R(13) 0.0 Q(14) P(15)

x4 3

13 989.8 13 980.2 13 969.5

R(13) 0.3 Q(14) P(15)

X2 3

13 974.7 13 964.8 13 954.3

R(13) 0.2 Q(14) P(lS)

W2 2

13 911.3 13 901.3 13 890.5

R(13) 0.0 Q(14) P(15)

w3 3

13 758.6 13 738.0

R(13) 0,O P(15)

w4 2

R, Q, and P lines were assigned in the rotational analysis and the Sl value of the upper state is taken to be 4 from the relative intensities of the branches, Q > R > P. It was not possible to follow the structure to low values of J. Pulsed laser excitation at 4633.5 A excited P(63) of the O-O band of [2 1.714-X23. Weak emission lines were assigned as [2 1.7]4-W33 and [2 1.7]4-W,2: the latter indicates that the upper state has some S2= 3 character. The four lower states that have been connected with X23 should give characteristic fluorescence patterns which, as will be shown, can be used to confirm the identification of the X23 state in further studies. (ii) Separation of the Xl2 and X23 states: The [17.213-X23 and [16.512-X,2 systems. The O-O band of the [17.213-X,3 system is a red-degraded band with a head at 5843.7 1

456

LINTON ET AL. TABLE III-Continued E. [16.514-X23 Excitation Assignment

Wavenumber

loner

-1 (cm )

v’,v”

state

16 421.7(4.5)L 16 412.3t4.5) 16 401.5

R(13) 0.0 Q(l4) P(15)

X2 3

15 598.7 15 589.5 15 579.3

R(13) 0.1 Q(14) P(15)

x2 3

14 465.9 14 455.9 14 445.8

R(13) 0.0 Q(14) PC151

X3 4

14 364.5 14 355.4 14 344.9

R(13) 0.0 Q(14) P(15)

X4 3

13 887.2 13 877.9 13 867.5

R(13) 0.0 Q(14) P(15)

13 538.7 13 529.9 13 519.5

R(13) 0.1 Q(14) P(15)

13 062.5 13 053.1 13 044.3

R(13) 0.1 Q(14) P(15)

w3 3

R(14) 1.1 Q(15) P(16)

X2 3

16 345.5 16 334.8

R(14) 1.0 Q(15)

W2 2

15 117.6 15 106.6 15 095.2

R(14) 1.0 Q(15) P(16)

x4 3

14 642.6 14 632.3 14 621.0

R(14) 1.0 Q(15) PC161

w3 3

14 395.7 14 384.9 14 373.8

R(14) 1,l Q(15) P(16)

X3 4

14 294.4 14 283.6 14 272.0

R(14) 1.1 Q(15) P(16)

X4 3

13 818.1 13 807.4 13 795.6

R(14) 1.1 Q(15) P(16)

W3 3

13 414.3 13 463.9 13 452.5

R(14) 1.2 Q(15) P(16)

X4 3

12 996.6 12 985.8 12 973.8

R(14) 1.2 Q(15) P(16)

W3 3

16 353.8 16 342.8 16 331.3

L

w3 3

A. This R head has been observed in stellar spectra (I). Recent rotational analysis (9) has established the lower state as X23. No Q lines were detected in the absorption spectrum, but laser excitation of the Q(4) to Q( 12) lines has been recorded. From the absence of an extensive strong Q branch An is taken to be 0. Further evidence

ELECTRONIC STATES OF CeO

457

TABLE III-Conhzued F. [ 18.414-X34 Excitation Assignment

lhvanumber (cl-l)

v',v"

1OWCr state

16 671.9 16 651.2 16 630.5

R(Z8) 0.2 Q(29) P(30)

X2 3

16 356.2 L 16 314.3C5.2)

R(28) 0.0 PC301

x3 4

16 253.0 16 231.9 16 210.9

R(28) 0.0 Q(29) P(30)

X4 3

15 114.2 15 153.4

R(28) 0,O Q(29)

w3 3

15 531.1 15 488.3

R(Z8) 0.1 P(30)

x3 4

15 426.9 15 405.7 15 384.6

R(28) 0.1 Q(29) P(30)

x4 3

14 950.7 14 929.1

R(28) 0.1 Q(29)

W3 3

14 609.2 14 588.1 14 567.5

R(28) 0,Z Q(29) P(30)

x4 3

14 133.0 14 111.6 14 089.6

R(28) 0,2 Q(29) P(30)

w3 3

that t-l = 3 in the upper state comes from the appearance of Q doubling with a J dependence of Au = 3.30 (f0.06) X lo-‘* J3 (J + 1)3. The [ 17.213 state is extensively perturbed by states of smaller B value. The [16.512-X,2 O-O band is red-degraded, has two branches, a head at 6045.77 8, and has been rotationally analyzed (4, 5). To determine the separation between the X,2 and X23 states, the dye laser, with rhodamine 6G, was tuned to excite transitions in each of the above bands. Fluorescence spectra from both transitions showed many features. Of particular interest were features in the vicinity of the laser line and these are shown in Fig. 5. When the [ 17.213-X23 transition is excited at 5844.6 A, another transition appears at -85 cm-’ to the blue. The relative intensities of the lines indicate that, for this transition, AR = + 1 or - 1, and spectra taken at lower J show that A0 = + 1. The lower state therefore has Q = 2 and could be X12. The presence of a transition to the X43 state, at 6646.2 A, -2060 cm-’ to the red of the laser transition, confirms that the lower state of the excited transition is X23. Several vibrational levels of the IV22 and IV42 states were also identified. A complete list of identified transitions is given in Table IIIC. When the [16.512-X,2 transition was excited at 6047.5 A, a transition was observed at 6078.1 A, -83 cm-’ to the red of [16.512-X12. This transition clearly had AQ = - 1; thus, the lower state had a = 3. That this was the X23 state was confirmed

458

LINTON ET AL. TABLE III-Continued G. [ 16.7]1-W22 Excitation Wavenumber

Assignment lower state

V’,Y”

(cm -1) 16 16

721.1 713.0

R(11) Q(12)

16

704.4

P(13)

15 15

910.3 891.3

15 15

808.4 800.7

15

791.2

P(13)

15 15 15

042.9t4.6) 034.3 025.3C6.7)

R(11) Q(12) P(13)

0.0

Vl

14 14 14

851.2 843.7 835.6

R(11) Q(12) PC131

0.0

V2 l

14 14

792.1 782.7

R(11) Q(12)

0.0

14

772.4

P(13)

13 13 13

950.9 942.2 932.9

R(11) Q(12) P(13)

0.0

w4 2

13 13 13

259.5 251.4 241.7

R(11) Q( 12) P(l3)

0.0

“3

12 12 12

899.0 890.5 880.6

R(11) Q( 12) P(13)

0.0

Tl

O-

12

255.3 263.7

Q(12) R(11)

0.0

u3

o+

12

245.8

P(13)

L

I,

Laser

8.

Rclstive

b.

x1

R(11) P(13)

0.0

w1 l

R(11) Q(l2)

0.0

W2 2

u1

o-

o+

2

transition.

Vslues

accuracy in

measurements resolution For

2

0,O

example,

-

parentheses in

cm

-1 .

are

the

absorption

(typically in

:

the

first

or 250

last

0001,

entry.

two

emission

digits at

rounded (2.7)

of

higher off

stands

to for

0.1 21

cm -1 002.7

. cm-l.

by observation of a weak, AQ = -1, transition [16.512-X43, at 6948.6 A, 2060 cm-’ to the red of [16.512-X23. Weaker features in the spectrum were identified with the W,2, W33, and W42 states. Observed transitions are listed in Table IIID. The intensities in Fig. 5 are slightly anomalous. In [17.213-X,2, where ZRshould be greater than ZP, the two lines are of equal intensity. In [16.512-X23, the ratio ZP/ZRis slightly larger than expected. These anomalies are not large and indicate slight heterogeneous mixing of the X12 and X23 states. The intensity patterns become more normal at low J. To obtain a more accurate value for the separation of the Xi2 and X23 states, spectra were obtained for the [16.512-X,2 transitions at about 12 different J’s ranging from J = 4 to J - 50. A significant increase in separation was observed as Jincreased. Analysis of the data using the method outlined earlier shows that X23 is 80.4 f. 0.7 cm-’ above X,2 and B(X&B(X,) = 0.0025 + 0.0004 cm-’ (high resolution value (4) is 0.0024 cm-‘).

ELECTRONIC

STATES OF CeO

459

[214]2

-X23

(O,O) I ISER

m

I

;

5470 FIG.

I

_L_L I ; 5280 4970

i

4940

L_L_i,

4770

4740 ii

2. Fluorescence following excitation in [2 1.1]2-X,3.

22 513-x23 [22 533-

(;

I

/

/

4470

I

jJ_j&

I

4460

w,z

4450

---AT-122 5]3-WqZ :-----~~

[22513-x43 -__~

I

1

I I

, I 4920

J 1

I 4910 FIG.

I

/

4900

I

5060

I 5c70

3. Fluorescence following excitation in [22.5]3-W,2.

I

I 5060

B

460

LINTON ET AL.

O-Q J’.l4

[ 7210

7‘20(

I-I J’=l5

.1_:,:‘,,,;

-.

7230

7250

FIG.

7010

6990

6120

6110

8

4. Fluorescence following excitation in [ 16.514~X23.

[17-213-X23

J’=36

[17-213-X,2

r-1

I

[I7 213-x43

I

I

I

L ,-‘L :k

I

6670

J’=

I 6650’

:i ’

‘c

x IO

:

I 5860 [I6 512-X23

I

I

5040

I

I 5020

8

[I6 512-X,2

14

FIG. 5. Fluorescence following excitation in [ 17.213~X23and [16.512-X,2.

ELECTRONIC

461

STATES OF CeO

To check that the laser was in fact exciting the [17.213-X23 and [16.512-X12 transitions, the frequencies of the laser lines and the accompanying R or P lines were compared with those obtained from the high resolution measurements (4, 5, 9). For each J at which fluorescence was excited, agreement was within experimental accuracy. (iii) Separation of the X34 and X,3 states: The [27.213-X23, [16.514-X23, and [18.44-X4 systems. To determine the position of the X34 state relative to X12 and X,3, we tuned the laser to excite the [16.5]4-X23, [17.213-X23, and [18.4]4-X34 systems. High resolution spectra of [16.514-X23 and [17.213-X23 have already been mentioned. The [18.414-X34 O-O band is red-degraded, has a head at 6 111.93 A, and has been analyzed by Barrow et al. (4). These three transitions were chosen as they are all convenient for excitation in the rhodamine 6G region. Fluorescence spectra obtained from excitation of [ 16.514-X23 and [17.213-X23 were discussed in the previous section, but only in relation to the X23 manifold and the X,3-X,2 separation. In Fig. 6 we show portions of the same spectra in the vicinity of the [16.514-X43 l-l band, J’ = 15 (top), and the [ 17.213-X43 O-O band, J’ = 36 (middle). In each case another transition is observed - 100 cm-’ to the blue, indicating a state 100 cm-’ below X43. The bottom portion of Fig. 6 shows part of the spectrum obtained when the laser excited the [18.414-X34 O-O band at J’ = 29, close to the band head. The splitting in the P-branch line indicates that two rotational transitions were excited simultaneously. Another transition is observed - 100 cm-’ to the red of [ 18.414-X34.

[18,4]4

-X4

[I8 414

3

J’= 29

o-2 I

I

I 6170

I

-X34

I

I 6150

O-l I

I

I 6130

FIG. 6. Fluorescence following excitation in [16.5)4-x23,

1

I [17.213-X,3,

I 6110

ii

and [18.414-X,4.

462

LINTON ET AL.

The immediate deduction from the spectra in Fig. 6 is that X,3 is - 100 cm-’ above X34. However, this requires further investigation as the intensities of the lines can only be consistent with this conclusion if there is significant heterogeneous mixing between the X43 and X34 states. In the top spectrum, the relative intensities of the [16.514-X34 lines are not typical of an 52 = 4 - 4 transition. The Q line should be undetectable and the R and P lines should have almost equal intensities. In the middle spectrum the intensities of the [17.213-X34 transition are more typical of D = 4 3 transitions, The anomalously large value of ZR/ZPin this transition is matched by an anomalously small value in [17.213-X43. In the bottom spectrum, [18.414-X43 has the appearance of a Afi = - 1 transition. ZP/ZRis anomalously large for this transition and anomalously small for [18.414-X34. The anomalies are typical of heterogeneous perturbations. These observations confirm the correctness of the assignments and show that the states are strongly mixed. To obtain a more certain assignment of the values of R and hence establish that the states are X34 and X,3, [16.514-X23 and [18.414-X34 spectra were obtained at many different J’s. If the above interpretation is correct, then the mixing should decrease at lower J and intensity distributions should become normal. Figure 7 shows the [ 18.414-X34 and [18.414-X,3 transitions at J’ = 7 and 29. There is a striking difference between the two spectra. The S = 29 transitions are clearly anomalous, as described above, whereas at J’ = 7, the intensity distributions of [18.414-X34 and [18.414-X43 are typical of 4 - 4 and 4 - 3 transitions (at low J, A0 = 0 transitions have a strong Q branch, especially at high n, and for ASl = + 1, the R branch is considerably stronger than the P branch). The same situation was observed with [16.514-X23 O-O, l-l, and [17.213-X23 excitation. In all cases the intensities approached theoretical values at low J and the assignments are thus self-consistent. Further evidence for the correctness of the assignment is shown in Fig. 8 which shows vibrational progressions in [ 18.414-X34 and [ 18.414-X43 at J’ = 29. The intensity anomalies in [ 18.414-X43 are caused by a “borrowing” of transition strength from m J’=7

m /

d.__

I

6170

I

I

6150

I

I

6130

I

I

6110

8

FIG. 7. Lines of the [18.414-X,4and [18.414-X43 O-O bands at J’ = 7 and 29.

463

ELECTRONIC STATES OF CeO

[I6

514-X,3

[I6

514-X34

I

I

I

7010 J’=3~j [W2]3-X,

I

I

I

I

I

6970

6990

I

6950

a

[I7 213-X94

3

r- 7

ri--1

T+++_l& 6600

6620

6640

6660

%

I

_JI

I

6170 FIG.

1

I

6150

I

I

6130

I

I

6110 H

8. Lines of the [18.414-Xx4and [18.414-X,3progressions at J’ = 29.

[ 18.414-X34. The intensities therefore depend on the degree of mixing between the two states and the transition strength of [ 18.414-X34. In the O-O transitions Z([18.4]4X,)/Z([ 18.414-X4) > 10, the transition strength of [18.414-X34 is much larger than that of [18.414-X,3 and the anomaly is large (Zp > ZR). For the 0- 1 transitions the intensity ratio is -4, the transition strength is thus smaller, and the anomaly is not so great (ZP- ZR).The transition strength of the O-2 band of [ 18.414-X34 is negligible and this band is not observed. The [18.414-X43 O-2 band is very weak but its intensity distribution is normal (bottom of Fig. 8). In an earlier paper, which concentrated on establishing vibrational frequencies, weak lines were observed - 100 cm-’ to the red of the laser transition when the [22.615-X34 transition at J’ = 36 was excited by Ar+ 4880 A (Ref. (6), Fig. 1). At the time, they could not be assigned but were assumed to arise from a AQ = +l transition. At J = 36, there is considerable mixing between X34 and X43 and the X,3 state will have some Q = 4 character. The observed lines thus belong to the “forbidden” [22.615-X43 transition which, because of mixing, is weakly allowed, and has the intensity characteristics of a An = 5 - 4 transition. The above examples show that it is dangerous to assign Q values on the basis of intensity distribution alone. Complete listings of assigned fluorescence transitions from excitation of [ 17.2]3X23, [ 16.514~X23,and [18.414-X34 are given in Tables IIIC, IIIE, and IIIF, respectively. Several other states, apart from those mentioned in the previous discussion, were

464

LINTON

ET AL.

observed in the spectra and very often appeared to be perturbed. Whenever high resolution data were available (O-O bands of all three transitions) these were compared with measurements of the fluorescence lines in the laser band as a check that we were exciting the assigned transitions. (iv) The V,O- state manifold. All of the states described in previous sections have 52> 2. In none of the fluorescence spectra were any low-lying states observed with n = 0 or 1. In their high resolution absorption spectra, Barrow et al. (4) observed the O-O and 1- 1 bands and analyzed the O-O band of an fi = 1-Q = 0 system which they labeled system o, now described as [ 16.71l-V,O-. The assignment was made on the basis of observed combination defects in the differences A+‘(J), and the Q doubling in the [16.7]1 state was determined. The R head of the O-O band is at 6628.53 A, and the Q head at 6649.14 A. This was the first band system identified in absorption as having Q < 2. In later work, four further transitions from V,O- were found on the absorption plates. The upper states are [ 14.2]0-, [22.7]0-, [23.7]0-, and [28.6]1. (The symmetries of the a = 0 states are discussed below.) The [ 14.2]0--ViO- band is weak and overlapped. It seems not to be perturbed in the short range of J for which it has been observed, J = 15-38. The [ 16.7]1-V,Otransition is the strongest of those involving the V,O- lower state. The determination of the correct numbering of the Q branch in this band is not straightforward: trial and error fits indicate that the most probable numbering is two units of J lower than originally given by Clements (5). The head of the [22.7]0--ViO- band lies at 4743.2 A. It seems to be perturbed at J - 34, and the constants given are for the levels with J > 37. The analysis of possible extra lines suggests that the B value of the perturbing state is about 0.351 cm-‘. The [23.7]0- u = 0 level seems not to be perturbed in the rather limited range of J in which lines were assigned, J = 37 to 70: the value of D is anomalously small, but not very well determined. Finally, the [28.6]1-V,O- O-O band has a violet-degraded Q head at 37 14.1 A. Both components of the upper state are perturbed and there is measurable 52doubling. It was decided to use the [ 16.71l-I/o- band as a basis for a search for low-lying 52= 0 and 1 states by laser excited fluorescence. The resulting spectrum showed some fairly intense features (apparently with Afl = -1) in the 6320-A region. As the dye laser is more intense in this wavelength region, we attempted to excite the [ 16.711 state through this latter transition and look for fluorescence at -6630 A to confirm the upper state assignment. The fluorescence spectrum obtained at a laser wavelength of 6327.1 a is shown in Fig. 9. Nine fluorescence transitions are clearly discernible in the figure and a weak transition at -5980 A is not shown. Slower scans of each transition showed the intensity distributions more clearly and enabled us to determine that J’ - 12. Four transitions at -6650,6760,7760, and 8 160 A appeared to have AhR= + 1, indicating that the lower states were Q = 0 states. The lines in the 6650-A region were shown to belong to the O-O band of the o system by comparing their frequencies with the high resolution measurements (5). The laser transition appeared to be AQ = - 1 (Q 1 - Q2) as did three others, at -5980, 7 170, and 7540 A. There was one AL? = 0 transition at -6290 A, the intensity distribution of which was anomalous and for which AQ could not be assigned. There are thus transitions to four Q = 0, one Q = 1, four Q = 2 states, and one state of uncertain Q.

465

ELECTRONIC STATES OF CeO

w22 WI’

I

I

I

I

I

I

I

I

TI O-

[16.7]I -+O+

I

I

I

I

1

H

6500

7000

1

8000 FIG. 9. Fiuomnce

I

“32

I

I

I 7500

I

I

A

following excitation in [l&7)1-W22.

The transitions to the 51 - 2 states were then examined to see if they could be identified with any known states, X,2, Wz2, W,2. The separation of the transitions in the 6320- (laser region) and 7 170-A regions is 1858 cm-‘, the same as that between W22 and W,2. The separation of the weak AQ = -1 transition at -5980 18,and the laser transition is 913 cm-‘, the same as that between X12 and W22. It thus appears that the laser excited the [ 16.71l- W22 transition and the features at 5980 and 7 170 A can be assigned as [ 16.7] l-X,2 and [ 16.7]1- W,2. Having established that the laser was exciting molecules from the W22state, the lower states of alI the other fluorescence transitions can be placed on the same energy scale. In particular, this spectrum establishes that VIOL is 766 cm-’ above W22. The An = -1 feature at -7540 A shows that there is an 62= 2 state (labeled V32) -2550 cm-’ above W,2. A transition to this state was also observed very faintly in the fluorescence obtained from [21.1]2-

466

LINTON ET AL.

X,3 excitation at 4764.9 A. The transition at -6280 A shows that there is an Q = 1 state (labeled W, 1) -100 cm-’ below W22. This is just - 10 cm-’ below X,2, 2) = 1, and may account for very weak features - 10 cm-’ to the blue of the [ 16.5]2Xi2 O-l fluorescence observed when the O-O band is excited. This may also account for the perturbed features at -4940 %, in Fig. 2. From fluorescence spectra, it is possible to make definite identification of Ql-QO transitions without relying on intensity distributions. It is also possible to determine which of the 52= 0 states are of the same symmetry (+ or -) though it is not possible to establish the actual symmetry of the state. Because of D doubling, each J’-J” transition in an fil-n2 band is a close doublet. Whether or not the splitting can be detected depends mainly on the size of the doubling in the Q = 1 state. However, as the doublet separation increases with J, it should be possible, at high enough J, to tune a single mode dye laser to either component of the doublet and thus selectively pump either the e or theflevel in the Q = 1 state. If the laser populated the e level of the D = 1 state then, because of the selection rule (+ * -), fluorescence to O+ states would consist of R and P lines and no Q line while $2l-90- fluorescence would consist of a single Q line. If the laser is then tuned to populate theflevel the pattern would reverse. Of the transitions to states labeled Vi, VZ, U1 , Tl , Us in Fig. 9, four appear to be R l-fit0 and the other, to V2, is uncertain. The laser was tuned to J = 20 and these five transitions were examined in detail. The upper part of Fig. 10 shows the transitions when the laser frequency was adjusted until the intensity of the R line of transition [ 16.71l-V,O- was a maximum. Transitions to Vi and T, have strong R and P and weak Q lines, whereas I’,, Ui , and lJ, have strong Q and weak R and P lines. The lower part of Fig. 10 shows the spectrum obtained when the laser was tuned to maximize the Q line of transition [ 16.71l-ViO-. The intensities were reversed in all five transitions. This strongly suggests that all five transitions are nl-no, and as I’, is a O- state, then Ti will be O- and VZ, UI,and U, will be O+. The initial choice of symmetry and the labeling of the fl = 0 states is discussed in Section IV. The patterns were not exactly the singlets and doublets expected at high J as, at J = 20, excitations to the e andfcomponents in [ 16.711 overlapped. When the laser was tuned to the peak of one component, it also was exciting in the wing of the other. There were thus weak Q lines accompanying strong P and R lines and vice versa. A high resolution excitation scan of the J = 20 region showed a partially resolved doublet with a separation of 0.030 cm-’ which is in good agreement with the separation of 0.029 cm-’ calculated using the constants listed by Barrow et al. (4). The intensity distributions of all the other observed features in Fig. 9 were unaffected by tuning from one component to the other, confirming that they did not involve n = 0 states. Spectra were obtained at several different J’s, ranging from 8 to 63. At the higher J’s, transitions to the five lower states Vi to 17, were either single lines or doublets grouped as above (Vi, T, : VZ, UI, U3) indicating that the excitations to e and to f levels were well resolved. It was also observed that, as J increased, the intensity of the transition [ 16.71l-v2 increased significantly relative to the others. At J = 8 it was hardly detectable. It has already been noted that the intensity distribution in this transition was anomalous. Whenever R and P lines were observed, IP > I,, whereas for the other four transitions ZR > IP. These features can be explained if the lower state were not Q = 0, but an !J = 1 state which appears in the spectrum only because

ELECTRONIC

b t=

STATES OF CeO

467

468

LINTON ET AL.

it is mixed with the nearby O+ state. This mixing would only affect the e levels of the fl = 1 state with the result that in transitions from another Q = 1 state it behaved like a O+ state. The mixing is J dependent, thus the transition would be very weak at low J. The transition is thus assigned [ 16.71l-V,l. The other four states are all Q = 0 states. A complete list of transitions observed at J’ = 12 is given in Table IIIG. It was also observed that, as J was varied, the separations of some of the transitions changed considerably, indicating that the lower states had different B values. Plots of the separations against J(J + 1) were linear and were used to derive values of ATo, the state separations at J = 0, and of AB. As the B value for Vi is known (4), that of the other states could be determined. All the information is listed in Table IV. The low B value of V,I and the high B value of U,O+ are consistent with the suggestion, mentioned above, that the two states are mutually mixed. One effect of the mixing will be to depress the levels of the lower state V,, and elevate those of the upper state U1 , by an amount which is proportional to J2. This would manifest itself in the spectrum as a lowering of B for VI and a raising of B for U, in agreement with the observations. When we average the two B values, we would expect the result to be close to the deperturbed values and typical of that of most of the other low lying states. The average value is -0.360 cm-‘, which satisfies the above conditions. It is possible that perturbations may also be responsible for the larger than normal B values of T, and US but it is not possible to establish this at present. Variation of separation with J was also observed for the states other than the Q = 0 and 1 states above. The separation, V,3-W22, decreased by 26 cm-’ between J = 12 and 63. Thus B is lower for V32than W22. The separation W22- WI 1 appeared to increase by 2 cm-’ between J = 12 and 46, but this is within experimental uncertainty. It can be concluded that the B values of the two states are very similar. Identification of the states assigned above on the basis of fluorescence from the 116.711 state was reinforced by observation of fluorescence excited by the 5017.2-A Ar+ laser line. The spectra showed that the laser excited an Q = 1-X12 transition and resulting fluorescence features were identified with transitions to all the five lower states V, to U3 as well as to the W42 and V22 states. The assignment of a transition [22.513-F/32 (Table IIIB), with a V32- W$? separation of 2536 cm-’ at J = 49, is consistent with the data given in Table IV. Similarly, the

TABLE IV Separations and Differences in Rotational Co+ants ~ofStates Observed in Transitions from [ 16.71I AT

Reference state

I

Vl

o-

Vl

state

II

V2 l

(*II

Ul

o+

Tl

O-

Vl

u3 o+

2718.3

v3

2530.4

2

2

(BII -

252.4

o-

x2

TI)

190.3

VI oo-

AR

0

-

2142.

-

Bfl

0.015 0.019

1

0.017 0.0089 -

0.0066

469

ELECTRONIC STATES OF CeO

estimate of the VrO--W, 1 separation is confirmed by the results of experiment in which R(34) of the O-O band of [22.710--W, 1 at 4559.4 At J = 34, the separation was found to be 863.2 cm-‘, as compared 867.8 cm-’ at J = 0, given in Table I. Constants of the 14 low-lying states discussed in this and in the sections are listed in Table I.

a pulsed laser A was excited. with the value previous three

Identification of the State W22 A decrease of - 10 cm-’ was observed in the separation of the VrO- and W22 states between J = 12 and 63. This is too small to get anything more than a very crude estimate that B for W22 is 0.003 cm-’ greater than that of V,O-, thus B( W22) - 0.361 cm-‘. To see if Wz2 could be identified with the lower state of any of the systems, 1, m, or n observed by Barrow et al., a more accurate B value was determined. The laser was tuned to a low J line in the Q branch of the [16.7]1-Wz2 at -6327 A and the spectrometer was set to detect the Q branch of the [ 16.7]1- VIO- band at -6650 &The laser, operating single mode, was then scanned through 1 cm-’ and the excitation spectrum recorded is shown in Fig. 11. As the spectrometer was detecting fluorescence in the Q branch of a fi = 1-a = 2 transition, only one component of the 52doublets would show up in the excitation spectrum and the features in Fig. 11 are thus single lines. The scan covered the first nine lines of the Q branch and continued into the zero gap. The first two lines were not resolved but all the others were. The spectrum was calibrated using an etalon with a free spectral range of 750 MHz. This enabled the separations of the lines to be measured. No attempt was made to determine the absolute frequencies of the lines as only line separations were of interest. Measurements led to a value of AB, B(W22) - B([16.7]1), equal to 0.0053 + 0.0005 cm-‘, so that with B([16.7]1) = 0.3564 cm-‘, for thefcomponent, B(W22) = 0.3617 f 0.0005 cm-‘. As the lower state n” of the system n(4), with B = 0.3614 cm-‘, was thought to have fi = 2, it seemed likely that n” was to be identified with W,2. This was confirmed by the observation and analysis of the O-O band of [2 1.1]2- Wz2 (Section III B(i)a).

::p&~&p 9

I 0.6 FIG.

I 0.5

I

I

I

I

0.4

03

0.2

0.1

I 0 cm-’

11. An excitation spectrum, showing the Q branch of the O-O band of [16.711-W22.

470

LINTON ET AL.

The above assignment of states was confirmed by the rotational red-degraded three-branch band with an origin at 11 929.98 cm-‘. the O-O band of [ 14.7]3-W42, giving the W’.,-Wz separation as compared with the value 1859.0 + 2.2 cm-’ derived from cence data.

analysis of a weak This proved to be as 1859.46 cm-‘, 11 sets of fluores-

Other Low-Lying Electronic States Assignments and energies of all states discussed in previous sections have been confirmed by independent identification in fluorescence spectra. In many spectra, excited with both the dye and ion lasers, features were observed which did not fit into any regular pattern and which have not been identified. In several cases, some pattern could be discerned which indicated the existence of further low-lying states. However, each of these states was only observed in one spectrum, and was usually perturbed. Two such states observed when the [21.4] lX,2 O-O transition was excited by the Kr’ 4680-A line have been assigned as 52 = 1 states, possibly belonging to the ground state configuration, and have been listed in Table I as I’, 1 and & 1. Until corroborated by further observations, these assignments must be regarded as tentative. Fluorescence observed when the [2 1.113-X23 transition was excited by the Ar+ 4765-A line showed features indicating that there may be states at about 4140, 4400, and 4594 cm-‘. It was not possible to assign s2 values to these states. Vibrational Frequencies Transitions to u = 0, 1, and, in some cases, u = 2, were observed for many of the states in Table I. As mentioned in a previous paper (7), all vibrational frequencies for these states are near 820 cm-‘, but scatter in the data prevented determination of accurate vibrational constants. Very little can be added to the data already presented (7) but, for completeness, the vibrational frequencies are listed in Table I. RotationalAnalyses and Assignment of Bandheads Since the earlier publication (4), a number of bands have been the subject of rotational analyses at Lille and at Oxford. Some of the results of the new analyses have already been mentioned: constants have been given in Tables I and II and the line lists are given in the Appendix. The results have enabled some new identifications to be made in the tables of bandheads given in Ref. (4) and have also led to some corrections to those tables. The new assignments are summarized in Table V. IV. DISCUSSION

These experiments have enabled us to assign values of Q and to determine the absolute energies of 14 low-lying electronic states of CeO. Several other states of low energy have been observed and tentative values of Q determined. The classical high

471

ELECTRONICSTATES OF ‘30 TABLEV NewAssignmentsof Bandheadsin the Spectrum of CeO Read-forming I.R

Degradation

“I

System

Branch

-

“03

8 381.06

L14.71

2 -

w4 2

o-o

8 368.14

114.71

2 -

w4 2

o-o

7 971.96

L14.21

0-

7 316.06

l13.91

4 -

-

6 608.1

k2(?2)

-

6 115.90

f16.51

4 -

6 083.90

116.~51

5 863.5

L17.21

5 843.71

C17.21

VI x2

0 3

k(?l)

o-o 2-2 o-o

X2 3

l-l

2 -

X1

2

l-l

3 -

X2

3

l-l

3 -

X2

3

o-o

x4

3

o-o

5 830.59

119.31

3 -

5 067.87

m3(?3)

-

m(?Z)

o-o

5 059.86

m3(?3)

-

q(?Z)

o-o

4

940.83

C20.31

3 -

Xl

2

l-l

4

934.25

C20.31

3 -

Xl

2

l-l

4 815.05

L20.91

4 -

x2

3

l-l

4

808.19

(?l)m4

-

4

800.28

[22.51

1 -

Vl

o-

o-o

4 764.76

c21.11

2 -

x2

3

o-o

4 631.26

[22.51

3 -

w2 2

o-o

4 625.20

t22.51

3 -

w2 2

o-o

(?Z)m

o-o

4 621.35

Q

121.71

4 -

X2

3

o-o

3 714.07

Q

L28.61

1 -

Vl

O-

o-o

resolution spectra were crucial for establishing rotational assignments and accurate rotational constants. The laser spectra enabled observation of additional low-lying states, determination of energy linkages among groups of states, assignment of absolute Q values and, from measurements of A B, determination of B values for levels observed only in laser fluorescence experiments. Figure 12 presents the 16 lowest energy levels of CeO (including two tentatively identified Q = 1 levels, Uzl and vdl), arranged to emphasize the similarity between the free-ion Ce2’(4f6s) levels, plotted as dotted lines, and the CeO molecular states. All of the low-lying states listed by Barrow et al. (4) except the 1 and m states are identified with levels displayed in Fig. 12.

472

LINTON ET AL.

3 3w42 4-3 W3

J;&! 2

4-4 x3

J 3......

110

4......4f6s(z,$ UI "2 13) '3t-_~_---"+ "I

!

3...... 2.. . . . .4f64y

I FIG.

1

n= J.

I

Jo-1

I

Jo-2

lower electronic states of 0,

1

Jo-3

1

Jo-4

arranged according to energy, J., and s2.

CeO is unique among the lanthanide monoxides in its number of well-characterized low-lying states. It is reasonable to expect that Fig. 12 contains sufficient information to identify the electronic configuration of the lowest states of CeO. The evidence will now be summarized which supports assignment of all 16 known low-lying states of CeO as the 16 states expected from the Ce*+ (4f6s) free-ion configuration split by the axially symmetric electric field of the O*- ligand. This ligand field theory Ce*+O*(4f6s) superconfiguration picture will be shown to be more appropriate and of greater interpretive power than the traditional molecular orbital picture involving &J, &J, rru, and uu configurations.* Of course, in the real molecule, the overall separation of charge is likely to be smaller than +2, -2. It has already been suggested (4, II) that the electronic structure of La0 is that of a single electron outside a closed shell. The electronic states correspond to a series of La-centered orbitals derived from 6s, 6p, and 5d atomic orbitals. On going from La0 to CeO it is reasonable to add an electron to the extremely compact inner-shell 4forbital. This expectation is supported by the presence of at least one 4felectron in all of the lowest configurations of free Ce, Ce+, and Ce*+. What is known so far about the molecular constants for the states of CeO gives ’ The distinction between crystal field and ligand field has been drawn differently by different authors. Our choice of the term ligand field theory is dictated by two considerations: (i) the parameters describing the field of the ligand are fitted to observed energy levels and are not calculated for a point charge model as originally done by Bethe; (ii) the use of the term crystal with reference to diatomic molecules in the gas phase is not helpful.

ELECTRONIC STATES OF CeO

473

little help in assigning configurations, but those values of AGil for the lower states which have been determined are remarkably similar, close to 822 cm-’ (see Table I), and suggest a common basic configuration. The repeated energy and B pattern of groups of four CeO states is strikingly similar to the energy and J pattern of Ce*+ (4f6s) states. In addition, whether one adopts a ligand field or a molecular orbital picture, one predicts that the 4f6s configuration will give rise to 16 Hund’s case (c) molecular states: one St = 4, three Q = 3, four fl = 2, four 52 = 1, two Q = O+, and two s2 = O-. Figure 12 shows all 16 predicted Q levels. The free Ce*+ ion 4f6s configuration exhibits&j rather than L-S coupling because the spin-orbit coupling strength associated with the 4f orbital is much larger than the energy separation of the ‘Fj and 3FJ L-S basis functions. The singlet-triplet splitting arises from the exchange integral

(Aft 1)6s(2)le2/r,214f(2)6s( 1,) which is small because of the large difference in size between 4fand 6s orbitals. j-i coupling in the free Ce*+ (4f6s) ion suggests Hund’s case (c) rather than case (a) coupling in the CeO (4f6s) molecular superconfiguration. Lanthanide monoxide superconfigurations involving several 4f electrons should exhibit a form of case (c) coupling in which several atomic quantum numbers remain well defined. For (4f)N6s, the interaction between the N electrons in forbitals will be more important than the interactions of individualfelectrons with the O*- ligand. Thus (4f)N couples to give L,, SC, J, inner quantum numbers. For CeO, N = 1 and L, = 3, S, = l/2, J, = 5/2 and 7/2. The (4f)N core then J,, j, couples with the single ~6s orbital to give J, = J, f l/2. The resultant CeO( J,, Ja) levels are (5/2, 2), (5/2, 3), (7/2, 3), and (7/2, 4). Finally, each state of total atomic angular momentum J, splits in the field of the ligand into IMJ.I = Q projections of Ja onto the internuclear axis. For N = 1-3 and 8-10 and a negatively charged ligand, the order of energies of the R components arising from a given J, is Q = J, lowest, then Q = J, - 1, and so on. This is the reason for the organization of Fig. 12 into stacks with constant J, - Sz. It is also the basis for the names of the molecular states: X, W, V, U, T, representing Q = J,, J, - 1, J, - 2, J, - 3, J, - 4. For example, in the name V21, V implies 0 = J, - 2, the subscript identifies it as the second from lowest of the V states, and 1 is the value of Q. The lowest state with Q = 0 is that with J, = 2. The symmetry of a single 0 state, O+ or O-, is determined by the value of JI + JZ + Zl, + 212 (JO). In the present case, J1 = 2, J2 = 0, 21, = 3, and 212 = 0, and the sum is odd. The lowest 0 state, V,, is therefore a O- state. The single 0 state with J, = 4, T,, is also O-, while the two 0 states with J, = 3, U, and U,, are then both O+. The case (a), case (b) states 3110+ and ‘Z+, and 3110-and 3Z+, of course also give two O+ and two O- states. The highest state would be expected to be ‘Z+, and this is in accord with the case (c) designation of U3 as O+. The symmetries of the excited 0 states follow from the selection rules 0’ c, 0’. There is one objection to the interpretation of the low-lying states of CeO as belonging to the Ce*+O*- (4f6s) superconfigurations namely, that the lowest free Ce*+ ion configuration is 4f*, not 4f6s. However, quantitative ligand field calculations

474

LINTON ET AL.

using Bums’ rule estimates (II) of 4f and 6s orbital sizes predict a reordering of free-ion configurations: 4f6s44f2<4f5d<4f6p. Shielding effects cause compact metal-centered orbitah (4f; 5d) to be significantly destabilized relative to more diffuse orbit& (6s, 6~). In fact, for all of the lanthanide monoxides except EuO and YbO, the lowest-lying molecular states should arise from the (4 f )N6s superconfiguration rather than (4f )N+‘. Eleven of the energy levels shown in Fig. 12 have been fitted to a ligand field model (12). Five parameters are required by the model but only three are varied to fit the observed molecular levels: 5‘4/,G3(4f - a), Bg(4f ), Bd(4f ), and B8(4f ). C4~is a spin-orbit parameter which appears to be within 5% of its free-ion Ce2+ 4 f 6s value. G3 is a 4 f - u exchange integral which is reduced from its free-ion 4f - 6s value by a factor of three. B& B& and B$ express the strength of the ligand field and may be expressed as explicit functions of the effective metal nuclear charge seen by the 4f electron, ZM, the effective charge on the ligand, ZL, and the internuclear distance. Bi and B?, may be estimated from the fitted value of Ba. Detailed discussions of the quantitative ligand field treatment of the (4 f 6s) and (4 f 26s) superconfigurations of CeO and PrO are presented elsewhere (12, 13). One of the yet unrealized goals of a ligand field treatment of the CeO energy levels is a global electronic structural model which explains all of the electronic states, not simply those which belong to the lowest superconfiguration. One use for the inner quantum numbers (L,, S,, J,, Ja) discussed above may be to explain the relative intensities of transitions between superconfigurations. The remaining unidentified fluorescence features, as well as several which have been tentatively assigned, suggest that there are many other states below 8000 cm-‘. There are also numerous higher-lying states (upper states of observed transitions) which can all be placed in energy relative to the ground state. These are listed in Table II and illustrated in Fig. 13. So far, these states have not been fitted into an orbital scheme and it is not certain that a simple relationship between Ce2+ and CeO levels will be recognizable for the higher energy configurations. It should be expected that there will be considerable configurational mixing and that correlation between atomic and molecular orbitals and observed states will be more difficult to unravel (Fig. 13). V. CONCLUSIONS

The experiments described above have led to a determination of vibrational frequencies for many electronic states and the assignment of several previously uncharacterized or incorrectly analyzed bands. The main result has been the establishment of the quantum numbers and energies of a large number of low-lying states, leading to the conclusion that the ground state manifold results from a (4f 6s) superconfiguration. The quantitative agreement between the CeO and Ce2+ 4 f 6s energy spacings indicates that the ligand field model is a good first approximation to the bonding in the low-lying states. However, in this limiting model, all the binding energy is ionic: a more sophisticated model would doubtless include contributions from back donation

475

ELECTRONIC STATES OF CeO

XT cm“

-

25000353

352 --

351

- 349

350

358

xr

-CT

_554357

XT20000-

354

-z-_

%F 353

Jso J49 350 _

356

-

344

15000352

I n = o-

-

359

I 1

- 354 1 2

-

342 354

360

-

353

TE-

360 347

347

I 3

1 4

345

-

1 5

FIG. 13. Excited states of CeO, arranged according to energy and s1. The numbers given by each level are values of 10’ BO.

of charge from the oxygen anion. To treat this effect properly, ligand field calculations will be necessary and ligand induced configuration interaction with higher-lying atomic configurations may become significant. The present work represents the first step in an attempt to gain some insight into the details of the electronic redistribution within rare-earth atoms on formation of diatomic molecules. Further work remains to be done on the states of lowest energy. The lower states of systems, k, 1, and m have yet to be identified with states found by laser fluorescence and placed on the molecular energy diagram. Several states, so far observed only in laser fluorescence, are not well characterized and rotational analysis of bands involving these states will be necessary to give precise energies and molecular constants. Many electronic states remain to be discovered or characterized and it may be estimated that not more than 10% of the lines in the absorption spectrum in the region 10 000 to 30 000 cm-’ have yet been assigned. The configurations 4f6s, 4f6p, 4 f 6d, and 4f2 of Ce*+ give 200 molecular states of CeO. In addition to the 16 lower states discussed above, some 40 excited states have been characterized directly, while others have been revealed by perturbations. However, the perturbations are for the most part rather weak, so that few extra lines have been assigned, and the interactions have not been analyzed in detail. It is not known then whether these perturbations result from interactions with new states or with higher vibrational levels of states already identified. Further study of some of these interactions, for example, by laser excitation or fluorescence spectroscopy, may well give interesting results. While there are obviously many problems still to be solved, these experiments have demonstrated that single mode laser induced fluorescence, especially when combined with high quality, high resolution absorption and emission data, provides a very powerful tool in our attempt to understand what seems to be the exceedingly complicated electronic structure of molecules containing rare-earth atoms.

LINTON ET AL.

476

E x . 2

x ‘0 i

ELECTRONIC

i

STATES OF CeO

477

478

ELECTRONIC STATES OF CeO

479

480

LINTON ET AL.

ELECTRONIC STATES OF CeO

481

482

LINTON

ET AL.

ELECTRONIC STATES OF CeO

483

484

LINTON ET AL.

ELECTRONIC

STATES OF CeO

485

486

LINTON ET AL.

ELECTRONIC STATES OF CeO

487

488

LINTON ET AL.

ELECTRONIC

STATES OF CeO

489

490

LINTON ET AL.

ELECTRONIC

STATES OF CeO

491

492

LINTON ET AL.

493

494

LINTON

ET AL.

ELECTRONIC STATES OF CeO

495

ELECTRONIC

STATES OF CeO

497

ACKNOWLEDGMENTS The beginnings of the present work were based on the detailed analyses of Dr. R. M. Clements. Many of the measurements listed in the Appendixes are taken directly from his thesis. Experiments at the University of New Brunswick have been supported by grants from the Natural Sciences and Engineering Research Council of Canada. Several summer students, Linda Ireland, Maureen Mayhew, and Terry Sturtevant assisted with these experiments. Experiments at MIT have been supported by grants from the Air Force Geophysics Laboratory (F19628-77-C-0061) and the National Science Foundation (NSF PHY-81-12546). We thank Exeter College, Oxford, for a grant to one of us (P.C.L.). RECEIVED:

July 25, 1983 REFERENCES

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.

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