High resolution UV laser spectroscopy of CaF: Rotational analysis of the C2Π-X2Σ+ (0, 0) system

JOURNAL

OF MOLECULAR

SPECTROSCOPY

153,81-90 ( 1992)

High Resolution UV Laser Spectroscopy of CaF: Rotational Analysis of the C*II-X *Z+ (0, 0) System WOLFGANG Department

E. ERNST

ofPhysics, The Pennsylvania State University. 104 Davey Laboratory, University Park, Pennsylvania 16802

AND JORN KANDLER ’ AND OLAF KNUPPEL Institut fiir Molekiilphysik, Freie Universitiit Berlin, Arnimallee 14. W-1000 Berlin 33, Germany

The CaF C*II-X22+ (0,O) system has been investigated by molecular beam spectroscopy with a frequency-doubled dye ring laser. The X-doubling components in the C*II state were assigned by Stark-effect labeling. Spectroscopic constants of the C state were determined from a leastsquares fit. The main parametersare T o. = 30215.956( 16) cm-‘, A0 = 29.243(30) cm-‘, B0 = 0.32342(7) cm-‘,p = -4.89(27) X lo-‘cm-‘, and q = 1.05(2) X 10dcn-‘. Local perturbations were observed in both spin-orbit components of C*II and attributed to an interaction with a *Z+ state approximately 50 cm-’ below the C state. This state could be the lowest vibrational state of D*2+, previously placed 600 cm-’ above C’II, U’ = 0. o 1992Academic hess, IIIC. 1. INTRODUCTION As ionic radicals with a relatively simple electronic structure, the alkaline earth monohalides (MX) have attracted interest from experimental and theoretical spectroscopists, as well as from researchers studying reaction dynamics. Much of the experimental work has concentrated on the investigation of the ground and first excited states. To a first approximation, the electronic structure in these states is described by an unpaired electron and a molecular ion core consisting of two closed shell ions M*+ and X- . In the X2Z +, A *II, and B*Z + states, the unpaired electron is mainly metal centered and strongly hybridized in the field of X -, in an orbital given by a mixture of M+ atomic wavefunctions. Different ionic bonding models have been developed which describe the molecules either in terms of the electrostatic interaction of the polarizable partner ions (I, 2) or the electric field effect of the negatively charged Xligand on the M+ ion (3). Structure of different electronic states is derived in the first case by assuming different polarizabilities of the M+ ion in the different states (2), while in the ligand field approach by Rice, Martin, and Field, the mixing of different free M+ ion orbitals in the field of X- has been calculated (3). Both models made predictions about a metastable *A state in close neighbourhood to the A*II and B*Z+ states. In CaF, the *A state was recently confirmed by d’Incan et al. experimentally (4). Furthermore, a modified version of our polarization model was published by Mestdagh and Visticot this year and successfully applied to alkaline earth hydroxides (5)) which show strong similarities to the monohalides. ’ Present address: VDI Technologiezentrum,

Postfach 1139,W-4000 Dusseldorf 1, Germany. 81

0022-2852192 $5.00 Copyright 0

1992 by Academic Press, Inc.

All rights of reproduction in any form reserved.

82

ERNST, KANDLER, I

I 10 5

I 10 5

8’5 7.5

I 6.5

ICN~PPEL I

I 12.5

11.5

35

AND

I 11 5 I 5.5

13 5

I 12 5 I A.5

I 1L 5

Q21(J")

R2

(J")

I

13.5 I 35

601.8MHz

FIG. 1. A section of the C2113,Z-X2B* (0,O) band near the Qz, /Rz bandhead ( wavenumbers increasing to the left).

While in the first excited states of MXthe M+-centered electron is strongly polarized away from X- (I-.?), the situation changes for the C*II state, which requires about twice as much excitation energy. The influence of the different polarization of the unpaired electron on the C’II state hyperfme structure was observed for BaI (6). On the other hand, ab-initio calculations can be performed more easily for lighter molecules, and we continued our C-state studies rather for CaF. By measuring the Stark effect of the A 211and C*II states of CaF, we directly observed the effect of differently polarized electron orbitals on the electric dipole moments of these states ( 7). While these electric field measurements were performed for low-dlines in the C2113,&Y2X+ (0,O) system which we could easily assign, it took a much larger effort to analyze the complete C*II-X22+ (0,O) system. In the following, we present a rotational analysis from sub-Doppler resolution UV spectra. Two local perturbations were found, and lines observed in addition to the investigated transition suggest that another electronic state lies in close nei~borho~ to the C*II state. By comparison with unpublished results (8, 9) and recent abinitio calculations by Biindgen, Engels, and Peyerimhoff (IO), we conclude that this new state may be 21’= 0 of the D*Z+ state so far assumed to lie at higher energy ( f J ) . II. EXPERIMENTAL

DETAILS

An effusive CaF molecular beam was produced from a high-temperature reaction of Ca and CaF2 at about 1250 K and collimated to a ratio of 1: 100. A Coherent 6992 1 ring dye laser operated with DCM and an intracavity frequency doubler provided about 1 to 2 mW of tunable single-mode UV light around 33 1 nm at 2 MHz linewidth. Absolute wavenumbers of the visible fundamental output through a high-reflectance output coupler could be detrains with a home-made traveling Michelson wavemeter ( 12) or by simultaneous recording of an iodine spectrum ( 13). The laser beam crossed the molecular beam under a right angle while laser-induced fluorescence was monitored by a photomultiplier. In this way, UV spectra could be recorded at about 20 MHz linewidth. The laser was scanned continuously over 60

83

CaF @II-X *8 +

Q/2 ?/2

s/2 312

I

I

I

30229.091cm-'

1

e

l/2

J”

Parity

FIG. 2. (a) Portion of a Stark-effect spectrum showing electric fieId-induct satellite lines next to the R2 and Q* lines. The energy separation AVextrapolated to zero field corresponds to the upper state A-splitting labeled AVin (b) the energy level diagram for a 2J13,2-2Z+ transition.

GHz without problems. However, long-range scans were accompanied by the problem of optimization of many intracavity elements. Absolute wavelength calibration represented another problem, since the low intensity of the fundament~ dye laser output did not allow for high-precision operation of our wavemeter, and even the iodine spectrum was partly weak. Although line spacings within a 60”GHz UV scan could be determined precisely due to the recording of markers from a stabilized Fabry-Perot interferometer (12), the absolute UV wavenumber accuracy was only 0.01 to 0.03 cm-‘, depending on the laser power. We recorded and assigned about 220 lines of the CaF C’II-X2X+ (0, 0) system and determined line positions of about another 100 lines which do not belong to the C-Xsystem but lie in the investigated region between 30 191 cm-’ and 30 232 cm-‘. As our apparatus also allowed for Stark-effect measurements ( 7, 12), we recorded a part of the spectrum in the presence of an electric field of up to 1 kV/cm. In this way, we could observe S~rk-effe~-induced satellite lines, i.e., transitions which are not allowed at zero field but can help to determine A-type splittings in the 211 state ( 12).

84

ERNST, ICANDLER, AND

23 5* I 205

21 5

v-'

'

I

I

I

225*

KN~~PPEL

2L5+

255*

I 19.5

I

'

'

30212.1586cm-'

’

Rl

I 18 5

601.8MHz r--Y I

I

”

I 17.5

I

I,

II

FIG.3.A section of the C’II, ,2-X%+ (0,O) band near the R, bandhead (perturbed lines labeled with an *).

III. RESULTS

Figure 1 shows a portion of the C’II s,z-X2Zt spectrum near the Q2,/R2 bandhead. As the ground state spin-rotation interaction is responsible for the spacings between corresponding Q2, and R2 lines and the spin-rotation interaction parameter y is known from the work of Childs, Goodman, and Goodman (14), the quantum number assignment was easy. In the C211 state, the sign of the A-doubling parameters p and q was unknown as was the relative position of e and fparity levels for a particular J’. By applying an external electric field, we observed zero field forbidden satellite lines ( 12). For example, next to a strong R2 line, a second line appeared corresponding to

815

7.5 6.5

9:5 I L.5

I 7.5

6.5 5.5

R12

a!5

I L.5

I 3.5

P2,132.5)

a2 131.5)

I

I

v-

Ql

I 5.5

J ’

I

I

I

I

601.8MHz

I

I

I

II

I

I

I

30202.681Lcm-' FIG. 4. A section of the CaF C-X (0,O) band with R,,/Q, and a strong line of another system ( next to P2 (20.5 ) )

bandhead, high-/lines

of *II3,2-‘2+ branches,

CaF C211-X22+

a transition into the other A-doubling component. In Fig. 2a, the spectrum in the neighborhood of the R2( 22.5 ) and Q2, (23.5 ) lines is shown for different electric field strengths. With the help of the corresponding energy level diagram in Fig. 2b, it can be seen that the distance Av between the RZ and Q2, lines and their respective satellites, extrapolated to zero field, is equal to the spacing of the A-doublet for J’ = 23.5 in C211J12,o’ = 0; in this case Au = 0.0105 cm-‘. As the energy in the spectrum of Fig. 2a increases to the left, the experiment also showed us that in the C*II3,2,2r’ = 0 state the e-parity level of a rotational state J’ lies under thef-parity level. While the assignment of C211jj2- X*8+ (0, 0) branches around 30 230 cm-’ was relatively easy, the C*II, ,*- X*I: + (0,O) spectra around 30 200 cm-’ gave us more of

86

ERNST, KXNDLER,

J”++

AND &.MEL

CaF

L23831302622-

ialLlo621 I”

30230

18

I

)

30220

1

n s I 30210

t

I’1

1

L

30200

Wavenum ber km-’

G II

30190

I

FIG. 5. Fortrat diagram of the CaF C*II-X *Z+ (0, 0) system. The solid parts of the curves represent assigned unperturbed lines.

a puzzle. A local ~~urbation was observed in the R, branch near the bandhead (Fig. 3). Furthermore, a considerable number of lines in the spectrum could not be assigned to any C-Xbranch, but obviously belong to another electronic transition. An example is shown in Fig. 4, where the rather strong line next to Pz( 20.5 ) is part of an unassigned sequence. This result is consistent with the unpublished observations of Bernath (8) who monitored fluorescence from the high-lying E and E’ states of CaF and saw an unknown *E+ state approximately 30 100 cm-’ above the ground state, which he called C’ 2Z+ + He also cites Pouilly and Schamps in Lille (9) for unpublished measurements of an unknown ‘zl’ state of CaF in the UV spectral region. Lines assigned to the C211-X*Z+ (0, 0) transition are listed in Table I. Between 30 2 16 and 30 225 cm-’ and above 30 232 cm-‘, no spectra were recorded due to laser problems. A Fortrat diagram of the assigned branches is given in Fig. 5. Table II contains lines which could not be identified and do not belong to the C-X system. They are grouped according to their hyperfine splitting and may be of use for further investigations. Molecular constants were derived by applying a nonlinear least-squares fit to the measured unperturbed line positions (Table I) using standard matrix elements for the 211 and 2Z + stat es ( 15 ) . The ground state constants were kept fixed in the fit to the following values: The spin-rotation constant y. of X’Z: *, o” = 0 was taken from the work of Childs, Goodman, and Goodman ( 14) as y O”+ f /2y lo, which is precise enough for this purpose, since the known centrifugal distortion parameters y O’,yo2, etc. (14), only affect the sixth digit. The rotational constant BO of the vibrational ground state had been determined by Weiler ( 16)’ by microwave spectroscopy in the 100-GHz region. Within the standard deviation, his results are the same as those of Dulick, Bernath, and Field ( Z7) from their B2Z+-X2X + analysis. The parameter Do

87

CaF C’II-X22+ TABLE II

Measured Transition Wavenumbers (cm-‘) of Unassigned Sequences, Grouped according to the Size of Hyperfine Splitting: (a) Unresolved Hyperfine Structure, (b) Hyperfme Splittings on Order of 80 MHz, (c) Hypefine Splittings on Order of 170 MHz a 30191.998 30192.834 30193.048 30193.454 30193.828 30194.186 30194.571 30194.838 30194.866 30195.361 30195.632 30196.391 30196.647 30198.295 30198.383 30199.533 30199.903 30200.524 30200.624 30201.650 30203.916 30207.820 30211.296 30212.072 30214.039 30215.817

c

b 30191.791 30192.106 30192.550 30192.694 30193.724 30194.438 30194.228 30196.136 30196.512 30198.479 30198.485 30198.494 30200.123 30200.172 30200.422 30200.561 30202.392 30202.484 30203.543 30203.547 30203.551 30204.258 30204.641 30204.721 30205.245 30205.254 30206.870 30209.335

30192.155 30192.189 30192.490 30193.488 30194.221 30194.911 30194.947 30195.303 30195.710 30197.233 30198.429 30198.456 30199.306 30199.855 30200.267 30201.180 30201.558 30201.664 30201.753 30201.801 30202.018 30202.523 30202.557 30202.806 30203.220 30203.502 30203.616 30203.842

30204.294 30204.629

30204.918 30204.972 30205.104 30205.710 30206.794 30206.867 30209.015 30209.025 30209.090 30209.192 30209.953 30213.607 30213.653 30215.952 30215.990

was also taken from Weiler. Table III lists the results for C*II, I,+= 0 together with the X*I;+, 2)”= 0 input parameters. The accuracy of all ground state constants is at least 100 times greater than of the determined excited state parameters. Therefore, the standard deviation of the C state constants in Table III is relevant. As mentioned above, a local perturbation was observed in the R, branch for J’ N 30, revealing energy shifts of the e-parity levels in the *III,* state. Line shifts in the perturbed region are shown in Fig. 6. Unfortunately, we did not find the R, sequence at high quantum numbers. Therefore, the exact J’ value for the avoided curve crossing cannot be given here. Another perturbation was found in the R2 and Q2, branches near J’ = 36.5. This means that thef-parity levels of the 2II3,2 state are affected by the interaction with another electronic state. Since the wavenumber range between 30 216 and 30 225 cm-’ was not scanned, it is well possible that there are also perturbations off-parity levels in C’II,,, and e-parity levels in C211X12.As discussed in the next chapter, we believe that the perturbations are due to a *X+ state which would interact with both D components of 211 via the L-uncoupling operator and with the 211,,2 state, in addition, through spin-orbit and spin-electronic interaction ( 18). IV. DISCUSSION

With an R, value of about 2.0 I A and w, = 48 1.7 cm-’ ( 11)) the C’II state has a larger internuclear distance and a less steep potential than the other known electronic states of CaF. The spin-orbit coupling constant Ao = 29.243 cm-’ also indicates a strong change if compared with the A*II state with A = 7 1.475 cm-’ ( 19). The values are consistent with the picture of the unpaired Ca+-centered electron being polarized toward F- in the C*II state (2, 3, 7).

88

ERNST, KANDLER,

AND KN~~PPEL

TABLE III Spectroscopic Constants for u = 0 of the X2Z+ and @II States of CaF (All in cm-‘, Standard Error in Parentheses ) X2X+

‘

V’=O

iO215.956(16)

Too

n,

29.243(30) 6.21(17)

x lo5

AD

c%, V’+J

Bo

0.3424882(1)a

0.32342(7)

Do x lo7

4.688(10)'

5.71(8)

l)* x 103

1.317514(1)b

p. x 103

-4.89(27)

p, x 105

-1.39(7)

q.

x

104

1.05(2)

a From

Ref.

(J&)

b From

Ref.

(J,&)

Thus far, known C*II states of other alkaline earth monohalides like CaCl (20), BaCl (21)) and BaI (22) exhibit a similar behavior of their spectroscopic constants. However, a difference appears in the A-doubling. In CaCl C211 both parameters p and q are positive (20), in BaCl both have a negative sign (21), in BaI p is positive and q

FIG. 6. Pe~ur~tion

in the R,branch: deviation of the rotational lines from their “unperturbed” positions.

CaF C211-X 22 +

89

negative (22), and in CaF we find a negative p and a positive q. In CaF C2113,2, o’ = 0 the f-parity levels clearly lie above the e levels for a particular J’ (Hund’s case a), whereas in C2111,2, o’ = 0 the situation is reversed, with e abovef. While all alkaline earth monohalides have A 211and B2Z + states forming fairly well a unique perturber pair with the A state below the B state and both states having similar potential curves, the situation appears quite differently for the C state. For CaF, the Huber and He&erg tables ( II) list a D22 + state approximately 500 cm-’ above the C state. Like other high-lying states, it has a vibrational constant of about 650 cm-‘, and we assume that it has a rotational constant comparable to the E2Z+ and Ef211 states (23), i.e., around 0.36 cm-‘. A unique perturber pair cannot simply be assumed in this case. Moreover, we observed lines of another electronic transition near 30 200 cm-’ and perturbations shifting the flevels of C21’13,2,o’ = 0 around S = 36.5 and the e levels of C’II,,,, V’ = 0 around J’ = 30.5. After excitation of the E*Z+ and E’*ll states, Bernath (8) found cascade fluorescence from a previously unobserved C’2Z+ state to the A*Il state. From the observation of a few lines, he was able to derive for the C22+ state Too = 30 157 (2) cm-‘, B. = 0.367( 10) cm-‘, and a spin-rotation coupling constant y = 0.01 cm-’ (8). Th e rotational constant corresponds to an R, value similar to the E and E’ states, presumably also similar to the D2Z+ state. Bernath cites unpublished measurements of the group at Lille (9) of a violet degraded band at 331.55 nm, in agreement with a Cf2Z+-X2Z:+ system. Assuming a 2Z+ state with B. = 0.367 cm-’ to perturb the C211 state with avoided crossings for the rotational quantum numbers above would place this unknown 2Z+ state approximately 50 cm-’ below the C211 state, yielding 7’& 2ZZ+)N 30 165 cm-‘. This estimate is only based on the magnitude of the rotational constants. The D22 + state was observed by Fowler in 194 1(24) and is listed with T, = 30 77 1.9 cm-’ , w, = 650.7 cm-’ and w,x, = 2.89 cm-‘. If the lowest observed vibrational state was not V’ = 0, but the first excited state 2)’= 1, the D state would be lower by one vibrational quant. In order to be consistent by giving TOOvalues, we calculate a new Too(D22’) = 30 158 cm-’ by using ground state vibrational constants from Ref. ( 17). This would be in perfect agreement with the values above for the unknown *2 + state. Recently Btindgen, Engels, and Peyerimhoff (10) calculated dipole moments and various spectroscopic constants of several electronic states of CaF. Their general picture of the electronic structure agrees remarkably well with the known experimental results, although the term values of excited states come out too high in energy by 1000 to 2500 cm-‘. However, on a relative scale the states including their equilibrium internuclear distances are placed well. The only 22:+ state in close neighborhood to the C211 state, has been calculated to lie 24 cm-’ below the C state, with w, and B, values in reasonable agreement with those given above for D221+ and C’2Z:+, respectively. In conclusion, we think that the C’ and D states are identical, with a recommended Too value of 30 158 cm-‘. Murphy et al. (25) studied Rydberg states of CaF which they assigned to several series. The D state belongs to the ‘$2 series and the lowering of the term value by about 650 cm-’ would increase the quantum defect from p = 2.4 (25) to 2.45. The new value would fit even better into the smooth plot of quantum defects versus principal quantum number in their work. ACKNOWLEDGMENTS The experimental work was performed at the Freie UniversitSt Berlin and supported by the Deutsche Forschungsgemeinschaft. W.E.E. thanks Robert W. Field for helpful discussions.

90

ERNST, KANDLER, AND KN~PPEL

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