Observation and analysis of the 21Σu+ “double minimum” state of Li2

JOURNAL OFMOLECULAR SPECTROSCOPY

137,235-241 (1989)

Observation and Analysis of the 2’Zl State of Liz

“Double Minimum”

C. LINTON,* F. MARTIN,? R. BACIS,~ AND J. VERG&$ *PhysicsDepartment, University of New Brunswick, Fredericton. New Brunswick, Canada E3B SA3; TLaboratorie de Spectrometrie Ionique et Moleculaire, Universite’ Claude-Bernard Lyon I, 43 boulevard du I1 Novembre 1918, 69622 Villeurbanne Cedex, France; and $.Laboratoire Aim6 Cotton, Centre National de la Recherche Scientifique II. Blitiment 505, 91405 Orsay Cedex, France Fluorescence observed after excitation of ‘Liz and “Liz with the ultraviolet lines of argon and krypton ion lasers has been examined at high resolution with a Fourier transform spectrometer. Spectra in the 3 000-12 000 cm-’ region have beenassigned to the 2’I;:-2’2:, 2’2:-1 ‘I&, C’Il,-2’Z:, and C’II.-1 ‘II, transitions. The 2’2:-2’ZIj transitions have been analyzed and the molecular constants for the 2’2: state are found to satisfy the isotope relations and are in very good agreement with ab initio predictions. Q 1989AcademicFWSS,hc. INTRODUCTION

Over the past few years, using collisionally induced fluorescence (CIF) excited by visible lines of an argon ion laser, we have been able to access the previously unobserved

2 ‘2 g’and 1 32 g’ states of both 6Liz and 7Li2 ( l-3). Examination of the transitions to the A ‘Z : and 13Z: states at high resolution using a Fourier transform spectrometer led to a complete analysis of the vibrational and rotational structure of these states and construction of accurate potential curves. It is also known (4-6) that the UV lines of argon and krypton ion lasers excite various rovibrational levels in the C’II, state resulting in CIF from higher lying triplet states. However, although the 2 ‘2: “double-minimum” state is predicted ( 7, 8) to be in the same region as C’II, and should be accessible to UV excitation from the ground state, it has so far escaped detection. In this paper, we discuss our recent experiments in which fluorescence excited by Ar and Kr UV laser lines has been examined, at very high resolution, with a Fourier transform spectrometer in the 3 000-20 000 cm-’ region. Several electronic transitions have been observed and we shall concentrate on those involving the 2 ‘2 i state. The heat pipe Liz source and experimental arrangement have been discussed in previous publications ( l-3). Both 6Li2 and 7Liz were examined using argon and krypton lasers operating with all-lines (undispersed) UV output. RESULTS

Assignment of Infrared Transitions Because the lasers were operated “all lines,” all transitions were excited simultaneously. In the infrared region (3 000-12 000 cm-‘), many lines were observed and 235

OO22-2852189$3.00 CopyrightCQ1989 by Academic Press, Inc. All rightsof reproductionin any form -ed.

236

LINTON

ET AL.

the spectrum appeared quite complex. However, it was obvious that there was little or no rotational relaxation and that we were observing direct fluorescence from one or more excited states. In the visible region, the spectra were relaxed and probably resulted from CIF. Examination of the potential diagrams show that only two states, C’II, and 2 ‘2: ( 7,8), are accessible to the UV laser lines through allowed transitions from the ground X ‘2: state. Further examination shows that the only singlet gerade states accessible through subsequent fluorescence in the 3 000- 12 000 cm-’ region are 2 ’ Zg’ and 1 ‘I& Thus, the only possibilities for infrared fluorescence are Cl&- 1 ‘I&, Cl&-2 ‘I;:, 2 ‘Z:-1 ‘IQ,, 2 ‘Z z-2 ‘2:. The spectra should therefore consist of vibrational progressions in the 1 ‘I’&and 2 ‘Zi states. The 2 ‘Zl state has vibrational frequencies in the region of - 128 cm-’ for 7Li2 and - 142 cm-’ for 6Li2 and an unusual distribution in which the vibrational separations increase up to 2, = 8 and then decrease (1, 2). The 1 ‘I& state of 7Liz has recently been observed by Miller, Bemheim, and Gold (9) and has a vibrational frequency of - 90 cm -l. By searching for features with separations of - 130 and 90 cm-’ we were able to pick out 14 progressions in 7Liz and 14 in 6Li2 and assign the lower states. Transitions were observed to n = 0- 15 in the 2 ‘2: state and u = O-33 in 1 ‘I&. The known rotational constants were used to assign the J values from the RP-branch separations. For the 1 ‘I&, state, the 6Li2 constants were calculated using the 7Li2 values and the isotope relations. The assignment of the upper states was more difficult. Energies of the upper states were calculated by adding the frequencies of the fluorescence transitions to the energies of the lower state, which were known for 2 ‘2: (10) and calculated from the constants (9) for 1 ‘I&. As the J values of the upper states and the constants for C’II, (4) were all known, it was relatively easy to calculate the energies of different vibrational levels of C’II,, compare with the observed levels, and assign all the transitions originating in the C’II, state (4 for 7Liz and 7 for 6Liz). In order to determine whether the other transitions originated in the 2 ‘L:: state, the expected energies of various vibrational states at the appropriate J were calculated for 7Li2 from the ab initio constants ( 7). These were compared with the experimental energies and it was found that, for each observed state, there was a 2 ‘z1: level that was calculated to be -20-30 cm-’ below it. The T, value was then increased by 25 cm-’ and the isotope relations were used to calculate the 6Li2 levels. It was then possible to assign every observed level to a 2 ‘Z: state that was calculated to be within 10 cm-’ of the measured value. Analysis of 2’2 :-2’2,’

Transitions

All the observed fluorescence transitions are listed in Tables I and II. There were nine 2 ‘Z i-2 ‘2: transitions assigned for 7Li2 and seven for 6Li2. These were fitted to a Dunham expansion for each isotope in order to determine the constants of the 2 ’ 2: state. However, because only five vibrational levels were observed and, for most of these, there were only one or two rotational levels, the number of constants that could be determined for this state was severely limited (6 for 7Li2, 7 for 6Liz). There was very little relaxation to provide more rotational information. The constants, which are listed in Table III, are statistically very well determined and fit the observed lines to an RMS deviation of 0.005 1 cm -’ for ‘Lig ( 127 lines) and 0.0018 cm-’ for 6Li2

237

THE 2 ‘Z: STATE OF Liz TABLE I Transitions Observed in ‘Liz after UV Excitation Exciting Transitiona

Transition

v'

3'

v"

LSSW

X(m)

21c;-2'2+ g

0

19

o- a

Kr

337.5

P(20) O+l

0

36

l- 7

Ar

351.1

P(37) 0+4

1

5

O-10

Ar

333.6

R(4)

1~1

1

17

o-11

Ar

334.4

P(la)

it1

1

38

o-12

Ar

335.8

R(37)

l+lb

1

2

o- 9

Ar

333.6

R(1)

1+-l

1

7

5- a

AK

351.1

P(a)

1~5

2

30

o-13

Ar

351.1

R(29)

2+6

3

40

o-15

Ar

334.4

R(39)

3~2

0

19

5-14

Kr

337.5

P(20) O+l

3

34

o- 5

Kr

337.5

Q(34)

3~4

4

54

o- 5

Kr

324.0

R(53)

4+0

4

54

o- 2

Kr

324.0

R(53)

4cO

12

21

4- 6

Kr

324.0

Q(21) 12+6

2yl'ng

mu-l'll

g

C'll-21I' " g

+he exciting transition is 'Au + X'E; where the upper state is either 212; or ClIf".

%entative assignments. Other excitation transitions also fit the data within the accuracy of the calculations. Direct fluorescence in the laser region is required for a definitive assignment.

( 12 1 lines). However, because they are very highly correlated, the addition of more constants, which would be required with a more complete data set, would probably change some of the values in Table III by amounts that are greater than the quoted uncertainties. Also listed in Table III are 7Li2 constants that were calculated, using the isotope relations, from the 6Li2 values. The agreement between the two sets of 7Li2 constants shows that they appear to be isotopically consistent. The fact that the difference between the isotope constants is outside the quoted accuracy, especially for the higher order constants, is a result of the very limited data set and the fact that one more parameter was required to fit 6Liz than 7Liz. A more realistic assessment of the isotope effect requires a more complete data set with each isotope fitted to the same constants. Comparison of the constants in Table III with the ab-initio values ( 7, 8) attests to the high quality of the calculations, especially those of Schmidt-Mink et al. ( 7),

LINTON

238

ET AL.

TABLE II Transitions Observed in 6Li2 after UV Excitation

Exciting Transition

v*

J'

vm

2'2'-212' u 8

0

32

l- 8

0

43

1

c'n -2l2' u 8 C'lIu-l'll 8

Laser

Urn)

Transitiona

Ar

334.5

R(31)

o- 9

Kr

337.5

P(44) oto

14

o-11

Ar

334.5

P(15)

ltlb

2

25

o-14

Ar

335.8

R(24)

2~2

3

31

5-15

Ar

333.6

R(30)

3+2

3

7

9-15

Kr

324.0

R(6)

3+Ob

5

49

o- 1

Kr

324.0

R(48)

SC0

10

15

3- 5

Kr

356.4

Q(l5) lot14

O+Ob

0

31

1-29

Kr

337.5

R(30)

0

35

6-25

Ar

351.1

P(36) O+Sb

1

5

o-33

Kr

337.5

P(6)

1~3

1

29

o-11

KK

350.7

R(28)

1+6

2

12

o-22

Ar

351.1

Q(12)

2-7

4

7

0-

AK

333.6

P(8)

4+4

6

0+2

%e exciting transition is ‘Au f X'Z' where the upper state is either K 21X' or C'II u u* b Tentative assignments. Other excitation transitions also fit the data within the accuracy of the calculations. Direct fluorescence in the laser region is required for a definitive assignment.

DISCUSSION

We have presented the results of the first experimental observation of the 2 ‘2: state of Liz and have shown that the state is 24 cm-’ higher than predicted ( 7) and that the rotational and vibrational constants, even though limited by the small number of data, are in very good agreement with predictions. The state is predicted to have a double minimum and we have observed only the inner limb of the potential curve. To observe the outer limb would probably require two-photon excitation to a higher state with a larger re, followed by fluorescence to the outer limb of the potential curve. It would be interesting to try to excite higher vibrational levels and observe states near the maximum, which is predicted to be 1892 cm-’ above the minimum ( 7). The ab initio potential curves were used to compute Franck-Condon factors for the transitions of interest and these were an invaluable aid in assigning the lines. There were gaps in most of the observed progressions corresponding to transitions with very low Franck-Condon factors. The calculations enabled us to predict where the tran-

239

THE 2’8+” STATE OF Liz TABLE III Equilibrium Molecular Constants’ for the 2 ‘I: : State of ‘Liz and 6Li2 Thf%Ol?Y

Constant

Te w e Wexe

'Li,

'Li,

'Li, from 'Li*

30100.26(50)

Ref(7) 30077

259.900(42)b

279.857(10)

259.128(10)

259.20

273.5

2.2252(73)

2.4528(22)

2.1029(20)

1.566

3.42

0.502054(90)

0.584014(34)

0.500701(30)

0.5036

10Qxe

6.625(48)

7.1964(56)

5.7128(46)

6.0

1O6D e

7.262(68)

9.133(14)

6.713(10)

Be

10'8 e r= e

Ref(8)

9.948(27) 3.0937(5)

3.0978(2)

3.089

3.055

aAll constants are in cm-'. b Numbers in parentheses represent the two standard deviation uncertainty. 'Calculated from Be.

sitions should be observable again after the gaps and we were able to pick up the progressions again and continue the assignments. As the spectra resulted from simultaneous excitation by all the UV lines and no spectra were taken in the vicinity of the laser lines, it was not immediately obvious which laser lines were responsible for exciting each sate and what rovibrational level of the ground state was involved in the excitation process. For the 2 ‘2 i-2 ‘Z:d fluorescence, this is not of great importance as the infrared fluorescence provided enough information to permit assignment of the 2 ‘2: levels. However, for the Cl&-1 ‘I$ transitions, this is of critical importance. In order to account for the A-doubling, it is essential to know whether the upper state is excited by a Q-branch transition which excites the f levels or by P- and R-branch transitions which excite the e levels. For C’II,-2 ‘2: fluorescence, we can determine this information, as fluorescence from an f level will result in a progression of single Q lines whereas the e levels will result in PR doublets. However, only three of these transitions have been observed. In order to determine the exciting transitions it is necessary to subtract the frequencies of all the Ar or Kr laser lines from the energy of the excited state to give a set of possible ground-state energies. The ground-state constants must then be used to search for a coincidence between one of these energies and the energy of a vibrational level of the ground state at the correct J. Tentative assignments of the exciting transitions are listed in Table III. In several cases, there are other possibilities and these can only be sorted out by observing fluorescence in the laser region or dispersing the UV lines before they enter the lithium source. Several of the exciting transitions for the C’II, state have been confirmed by the recent work of Bahns, Stwalley, and Pichler (6)) who observed fluorescence progressions in the laser regions.

240

LINTON ET AL.

It appears as if Bahns et al. observed direct fluorescence only from the C’II, state and not any direct 2 ‘Z i-X ’ 2: fluorescence. This raises the question of why this was not observed when the 2 ‘Z s-2 ‘Zg’ fluorescence was so intense. The ab initio calculations of the electronic transition dipole moment functions ( 7) show that, at internuclear separations corresponding to the inner limb of the 2 ‘Zg potential curve, the dipole moment of the transition to 2 ‘2: is about 17-20 times greater than that to the ground state, corresponding to an intensity ratio of 300-400. The theory is therefore consistent with the lack of observation of transitions to the ground state. It is of interest to note that the dipole moment for transitions from the outer limb to 2 ‘2: is twice as large as that for the inner limb. However, the Franck-Condon factors are not so favorable. Nevertheless, if it were possible to excite states in the outer limb of 2 ‘B: , one would expect to see transitions to the higher vibrational levels of 2 ‘Zz, possibly enabling us to examine the predicted potential hump in this state. In only one instance ( 7Li2, u’ = 0, J’ = 19 ) was 2 ‘Z i-2 ’ Z l fluorescence accompanied by fluorescence to 1 ‘II,. The dipole moment for 2 ‘2: is predicted to be about double that for 1 ‘IYIp.In addition, the Franck-Condon factors are greater for 2 ‘2: than for 1 ‘II, transitions and the v4 factor in the intensity expression also strongly favors the 2 ‘2: state. A combination of all these factors would suggest that 2 ‘2:-2 ‘2; transitions can be expected to be up to 100 times more intense than 2 ‘Xi- 1 ‘I&, which supports the experimental observations. Analysis of the C’II,-1 ‘III, transitions is in progress. Because of the lack of data for ‘Liz it is unlikely that we will be able, at present, to extend the previous analysis of the 1 ‘I& state ( 9). For 6Li2, for which there are no previous data on 1 I&, there are just enough data to determine low-order constants. A strong perturbation was observed in 6Li2 between 1 ‘I$, v = 5, J = 30 and 2 ‘Xg’, 2, = 15, J = 30 and another between 1 ‘I&, 2)= 4, J = 4 and 2 ‘Zg’, u= 15, J = 4. Using the isotopically corrected constants for 1 ‘I’&, it was determined that 1 ‘I$, u = 5, J = 30 is 1.653 cm-’ above 2’Zi, u = 15, J = 30 and they are connected by a matrix element of 3.732 cm-‘. It was also determined that 1 ‘I&, v = 4, J = 4 is 0.235 cm-’ below 2’2:, o = 15, J = 4 and the matrix element is 0.184 cm- ‘. These figures are strongly dependent on the accuracy of the 1 ‘I& constants and may change if new data require an adjustment to the constants. The o = 15 level of 2 ‘Zg has not previously been observed for 6Li2. A complete analysis of the 1 ‘I&, state and the perturbations will be presented in a future publication, In the visible region, rotationally selective but relaxed spectra were obtained using both lasers. Many transitions in 6Li2 have been assigned to the 13A&311n transition and the predissociation in the b311, state is very clearly observed. Analysis is in progress and the results will be published shortly. CONCLUSIONS

Excitation by the UV Ar and Kr laser lines has proved to be a fertile source of new transitions in both isotopomers of Liz. Because the 2 ‘2: state is the upper state of the transitions, the constants had to be determined from a very small number of rovibrational levels. Nevertheless, these preliminary constants were shown to be isotopically consistent and have demonstrated, once again, that the ab initio calculations for Liz

THE 2’2:

STATE OF Liz

241

are very reliable. The intensities are also consistent with the ab initio dipole moment functions. The analysis of the spectra is continuing and we shall soon be reporting an extension to the previous data (9) on the 1 ‘I& state and an extensive high resolution analysis of the 1‘Ag-b311, transition. ACKNOWLEDGEMENTS This work has been funded by grantsfrom NATO and the National Sciencesand EngineeringResearch Council of Canada.The authorsthankDr. A. J. Ross for providingand helpingwiththe computerprograms and ProfessorR. A. Bemheim for providingthe constantsfor the 1‘II, state. RECEIVED:

April 26, 1989 REFERENCES

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J. Mol. Spectros. 119, 38-50 ( 1986). 3. F. MARTIN,R. BACIS,J. VERGES,C. LINTON,G. BUJIN,C. H., CHENG,ANDE. STAD, Spectrochim. Acta A 44, 1369-1376 (1988). 4. G. ENNEN,CH. OTTINGER,K. K. VERMA,AND W. C. STWALLEY,J. Mol. Spectrosc. 89, 413-420 (1981). 5. F. ENGELKEANDH. HAGE,Chem. Phys. Left. 103,98-102 ( 1982). 6. J. T. BAHNS,W. C. STWALLEY,ANDG. PICHLER, J. Chem. Phys. 90,2841-2847 ( 1989). 7. I. SCHMIDT-MINK,W. MULLER,AND W. MEYER,Chem. Phys. 92,263-285 (1985). 8. D. D. KONOWALOWANDJ. L. FISH,Chem. Phys. 84,463-477 ( 1984). 9. D. A. MILLER,R. A. BERNHEIM, AND L. P. GOLD, in “42nd Symposium on Molecular Spectroscopy, Columbus, Ohio,” PaperFC8. 10. F. CARROT,Th&scde doctoratde 32me cycle, Universitt? Claude Bernard,Lyon I, 1986.