The spectra of YF+ and YF molecules in the 3400–3700 Å region

The spectra of YF+ and YF molecules in the 3400–3700 Å region

JOURNAL OF MOLECULAR SPECTROSCOPY 63, 23-32 (1976) The Spectra of YF+ and YF Molecules E. A. SHENYAVSKAYA Deportment of Chemical Thermodynamics...

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JOURNAL

OF MOLECULAR

SPECTROSCOPY

63, 23-32

(1976)

The Spectra of YF+ and YF Molecules E. A. SHENYAVSKAYA Deportment

of Chemical

Thermodynamics,

in the 3400-3700

AND B.

A Region

S. RYABOV

Institute for High Temperatures,

Moscow,

1I.S.S.R.

The violet-degraded band system (3400-3700 d) has been found in the spectrum of a discharge through helium with a trace of YF, vapor. The emitter of this system is the hitherto unknown YF+ ion. Vibrational analysis of the band system has been carried out. Rotational analysis has shown that the bands are due to the 211-2A transition. Molecular orbital configurations which give rise to both participating states are discussed. The ‘2-X% (O-O) band of YE’ (X3.572 A) obtained in absorption has been reanalyzed.

INTRODUCTION

According to present-day concepts (I), the ems and 6(n - 1)d orbitals are the lowest-energy molecular orbitals in diatomic molecules containing a transition metal. These concepts were proven experimentally for the isoelectronic series of molecules, such as monoxides of the IVth Group and monohalides of the IIIrd Group. The 2A[6(n - l)d] states in the scandium subgroup monoxides were unknown for quite some time. A theoretical calculation of the ScO molecule (2) has shown that the “A(63d) state should be the first excited molecular state. Quite recently, the 2A(6.5d) state with the excitation energy of about 7800 cm-’ has been found in the spectrum of La0 (3). This leads one to assume that the zA[6(n - l)d] states are low lying for the isoelectronic series of molecules such as monoxides and ions of monohalides of the scandium subgroup. Data on the 2A states are required both for understanding the electronic structure of molecules and for calculating the thermodynamic functions of corresponding gases. In the present paper we report an investigation of the band system of YF+ which involves the 2A(64d) state. EXPERIMENTAL

DETAILS

Weak violet-degraded bands in the 3470-3580 A region ascribed here to the molecule-ion YF+ were first revealed during investigation of the emission spectrum of the YF molecule (4). A description of the discharge tube and the method used to excite spectra has been given in Ref. (5). The emission spectrum of YF (4) was excited in an uncondensed discharge using powdery YF, in the tube with Ar Upon replacing argon with the Ar + He mixture and, further, with carrier-gas. helium, the intensity of the violet-degraded bands increased considerably, this being a common occurrence for spectra of the molecular ions. However, the external heating of the capillary had to be increased at the same time, which had an unfavorable 23 Copyright All rights

0

lY76

by

of reproduction

Academic Press, Inc. in any

form

reserved.

24

SHENYAVSKAYA

AND

RYABOV

FIG. 1. Discharge tube. (1) Graphite capillary. (2) Graphite disks. (3) Heat shields. (4) Quartz (5) Auxiliary quartz tube. (6) Electrodes. (7) Current leadouts. (8) Inlet and outlet of circulating gas. (9) Water-cooled iacket.

rings. inert

effect upon the discharge tube-a breakdown of the quartz wall of the capillary occurred after several hours of operation. Therefore, a new discharge tube was provided, based on the original idea of Pearse and Gaydon (6). The high current density produced by constricting the discharge through a capillary of suitable bore generated the heat to volatilize compounds placed along the capillary. The use of such tubes is described in Refs. (7, 8). The discharge tube described here operated at currents up to 10 A and appears to be quite robust: while investigating the tube has run without breakdown. The new light source (Fig. 1) consisted of a wide quartz tube 700 mm long, 60 mm in internal diameter, which had a water-cooled jacket, two side tubes for the inlet and outlet of the circulating inert gas and two wide side tubes designed to accommodate the current leadouts. The capillary (100 mm long, with an inner diameter of 5 mm) and disks (the length of contact with quartz, about 25 mm) were made of graphite. For convenience in assembly, the capillary and supporting disks, as well as the heat shields were mounted in a short quartz tube, the disks being closely fitted to the tube walls and secured at the edges with ground quartz rings. As a rule, the presence of those rings was sufficient to prevent the bypassing of the disks by the discharge. The thus assembled short tube was inserted in the center of the discharge tube, and the gaps between the walls of the two tubes were packed with kaolin wool. Cylindrical stainless steel electrodes, 100 mm long and 50 mm in diameter, were secured on the current leadouts after mounting the capillary. The current leadouts, molybdenum rods 4 mm in diameter, were sealed in water-cooled molybdenum glass plugs which were made to fit onto the current leadout pipes. An intense spectrum of the YF+ molecule was obtained under the following conditions: carrier gas, helium (S-10 mm Hg); voltage across the tube electrode, 1.5 to 2 kV; discharge current, 5 to 7 A. The spectrum was photographed in a DFS-13 spectrograph equipped with a grating

THE SPECTRA

15 1.

OF YF+ AND YF

having 1200 lines/mm and a blaze angle corresponding to a wavelength of 12 000 A. In the 3rd order, the dispersion was about 0.5 A/mm. In order to make sure that the bands belong to the ion, absorption spectra were taken vapor over yttrium trifluoride at 2000-23OO’C. A tantalum-heater vacuum furnace used for the purpose has been described in Ref. (9). No violet-degraded bands were observed under such conditions. In place of the most intense violetdegraded bands, there was observed a previously described (4) ‘S-X% (O-O) band of the YF molecule with a well-developed rotational structure. Because of strong overlapping in the emission spectrum, it appeared advisable to reanalyze this band. The band was photographed in a PGS-2 spectrograph with a dispersion of 0.6 A/mm. The spectra were measured on an IZA-2 comparator. The corresponding wave numbers in vacuum were calculated on the basis of Edlen’s equation for the indes of refraction of standard air using a computer program (10). VIBRATIONAL

ANALYSIS

AND ELECTRONIC

ASSIGNMENT

All the violet-degraded bands between 3400 and 3700 A have been arranged into one system. The system consists of two subsystems, separated by about 100 cm-l. Every band has two marked heads P and Q. Moreover, a closer examination shows that the P-heads of the “red” subbands are doubled. The analysis reported here demonstrated that the two P branches of the “red” subbands arise from a large A-doubling. This doubling indicates that both states involved in the transition have A # 0, and the observation of strong Q branches implies AA = f 1. The A-type doubling in the “violet” subbands is not resolved. So the “red” subbands have been assigned as %+* 2Ar and the “violet” subbands as %t+ 2Ar transition. Both states are in Hund’s coupling case a. The experimental data do not unambiguously indicate which is the upper and TABLE

I

Deslandres Table for the Band Heads of the 211-2A System of YF+ iin cm-l) u”

0

1

2

3

4

5

6

7

8

9

\4 0

27981.3 28079.7

27321.9 27419.5

1

28691.1 28791.5

28032.2 28131.7

27377.5 27476.1

28737.1 28838.7

28083.0 28183.2

27432.6 27532.5

28782.8 ?88.85.5

2813?.9 28232.5

27487.8 27588.2

28828.4 28932.3

2PlS3.2 28286.2

27542.3 27644.1

28874.0 28978.9

28232.5 28335.9

2

3

1

5

0

27596.8 27700.1 21651.0 27758.1

28919.0 29"25.2 28963.8 29071.3

27706.7 27810.5 27758.1

8 5

29053.1 29162.7

26

SHENYAVSKAYA

AND

TABLE

28012.05 13.16 14.40* 15.451 16.75 I',.;:" .

31.5 32.5 33.5 34.5 35.5 36.5 37.5 38.5 39.5 40.5

20.41 21.66 23.08* 24.22* 25.46*x 26.88 2a.30* 29.61s 30.90* 32.43

20.70 21.91* 23.08* 24.53 25.88* 27.141 20.63 30.05 31.40 32.81

41.5 42.5 43.5 44.5 45.5 46.5 41.5 48.5 49.5 50.5

33.89 35.44 36.84 3a.42* 39.84s 41.29 42.77+* 44.58 46.32+ 47.06+

51.5 ::*: 5415 55.5 56.5 51.5 58.5 59.5 60.5 61.5 62.5 63.5 64.5 65.5 66.5 67.5 68.5 69.5 70.5

overlapped

osloulatlon

P

Qb

a

96:61 97.07 97.62 98.13 98.78 99.28 99.93

27995.81 96.25 96.77 97.31 97.79 98.37 98.96 99.53 28000.21

'8000.53 01.25 01.85 02.52 03.28 03.99 04.80 05.59 06.40 07.15

00.91 01.41 02.14 02.82 03.66 04.33 05.04 05.87 06.67 07.55

27982.42 82.59

34.40 35.7-i* 37.30 38.77 40.47* 41..93* 43.37* 45.09 46.16 48.32

08.02 08.85 09.74 10.67 11.61 12.52 13.43 14.40 15.45 16.51

08.35 09.19 10.09 10.96 11.97 12.91 13.86 14.86 15.81 16.90

82.80 83.04 83.30 83.65 83.92 84.26 84.60 84.96 85.34 85.73

e3.04* a3.30* 83.65+ 83.9H 84.26s 84.60s 84.96* 85.34'c a5.73* 86.20*

49.43+ 51.26 52.83 54.64 56.24 58.01 59.86 61.64 63.31 65.21

49.96 51.58 53.31 55.08 56.68 58.52 60.16* 61.89+ 63.91 65.79

17.59 18.56 19.66 20.79 21.91 23.08 24.22 25.46 26.61 21.85

17.95 19.04 20.15 21.20 22.38 23.47 24.72 25.88 27.14 28.30

86.20 86.65 87.12 07.56 88.07 88.66 89.19 89.78 90.42 91.15

86.65* 87.12s 87.56s aa.o7* 88.66*

67.17 68.9%* 70.91* 72.661 74.73

67.69 69.45 71.47 73.31 75.26 77.12 79.01 a1.11* 83.26* 85.10s

29.08 30.38 31.69 33.00 34.40 35.71 37.06 38.42 39.04 41.29

29.61 30.90 32.17 33.54 34.78 36.23 37.58 39.02 40.47 41.83

91.76 92.02*, 93.19 93.92 94.50 95.31 96.25 96.77* 97.62* 98.61*

92.om 92.88 93.61 94.40 95.10 95.01* 96.61* 97.31* 96.37s

87.21+ 89.38 91.40 93.46

42.77 44.28 45.76 47.24 48.05

43.37 44.80 46.32 47.86 49.43

:E: 01:25* 02.14* 03.09

28012.32 13.43* 14.86x* 15.81X 16.90s 18.22 19.32

78.62 80.64s 82.72

71.5 72.5 73.5 74.5 75.5 *

Q,

%

22.5 23.5 24.5 25.5 26.5 27.5 28.5 29.5 30.5

II

Lines of ‘W h2A) (C-O) Subband

Rotational J

RYABOV

by atomia of

or

rotational

YF+

line

oonstants

27999x62

+* h -

the

lines

evsrlappsd

MI%

27981.33h 81.33h 81.33h 81.33h 81.33h 81.33h 81.33h

not

by band

27981.58h 81.58h B1.58h 81.58h 81.58h 81.58h 81.58h

27902.42* 82.59* 82.80*

xx go:33 90.94 91.59

99.93s 28000.91* 01.85* 02.82 03.661

takeninto aooountin head

which is the lower of the two states. The reasons why we consider 2A as the lower state are given below. The intensities and spacing of the bands enable the GO subbands to be easily recognized. The clear sequences facilitate the construction of the Deslandres scheme in Table I. Only the PI, and PZ heads are listed since in many cases the Q and Plb heads could not be measured accurately enough. The accuracy of band head measurements is somewhat less than that of rotational lines. Measurements are reproducible to better than 0.05 cm-’ but each band head is blended with rotational lines of other branches or with atomic lines. Assuming that the accuracy of the band head

THE

SPECTRA

measurements is better than ~~0.5 cm-’ the following culated using a computer program (II) : VE =

, we = 714.8 f ?, we = 663.7 f

27

955.7 f

= 664.7 f

constants

0.06

0.4

LL~~“x~” = 2.29 f

0.06

28 053.7 f

0.5

~efxef = 2.38 f

0.4

o;‘xg”

ROTATIONAL

were cal-

0.5

~e’xe’ = 2.41 f

ye =

We

molecular

0.4

f we = 716.6 f 0.4 I,

35

OF YF+ AND YF

0.07

= 2.30 f

0.05

ANALYSIS

The wavenumbers of rotational lines in the O-O (211-2Ag and 211~-2A~) and 1-O (*II~-zA~) bands are given in Tables II-IV. The 211+-2A~subbands consists of six branches (R,, Rt,, Qa, Qt,, P,, and Pb). The 211i-2As subband consists of single R, Q, and P branches. The numbering has been established by finding an agreement between the combination differences AlF: R(J) - Q(J) = Q(J + 1) - P(J + 1) for upper state and R(J) - Q(J + 1) = Q(J) - P(J + 1) for lower state. As it would be expected from the vibrational analysis the combination differences ArF” and AzF” of the 14 and 0-O subbands (“&-“A$ agree within the accuracy of measurements. In good agreement with our assignment, the splitting in the 211+-2A; subbands can TABLE

III

Rotational Lines of 211+-aAi (O-O) Subband J

Q

A

19.5 20.5 21;5 22.5 23.5 24.5 25.5 26.5 27.5

28105.28* 06.12+ 07;27 08.35 09.46 10.50 11.73* 12.80 14.06

28.5 29.5 30.5 31.5 32.5 33.5 34.5 35.5 36.5 37.5

15.27 16.50 17.73 19.10 20.42+ 21 .66* 23.15 24 -3-l 25.80 27.20

38.5 39.5 40.5 41.5 42;s 43.5 44.5 45.5 46.5 47.5

28.67 30.08 31.78*1( 33.04 34.6lS-S 36.02* 37.55* 39.26 40.77 42.33

*

overlapped

calouletion

28092.61 93 03 93.46 ?3.88 94.40

. ..__

97.23 97.83 98.47 99.19 99.85 28100.55 01.32 02.04 02.80 03.63

by atomio

of

P

04.44 05.28 06.12 07.02 07.94 08.81 09.80 10.73 11.73 12.80 OF YF+

79.83h 79.83h 79.83h 79.83h 79.8331

J

R

Q

P

48.5 49.5 50.5 51.5 52.5 53.5 54.5 55.5 56.5

28144.02 45.75 47;32 49.06 50.84 52.52 54.28 55.99 58.02

28113.81 14;80 15.89 16.95 18.11 19.30 20.42 21.66 22.83

28084.17 84.61 85.10 85.57 86.08 86.60 87.21 87.71 88.36

80.64

57.5 58.5 59.5 60.5 61.5 62.5 63.5 64.5 65.5 66.5

59.82 61.64 63.48 65.23 67.11 68.9O+r 71.12 73.11 75.10 77.18

24.00 25.24 26.51 27.87 29.11 30.46 31.78 33.14 34.61 36.02

80.92 81.11 81.41 81 .62 81 .YO 82.23 82.61 82.99 83.26 83.74

67.5 68.5 69.5 70;5 71.5 72.5 73.5 74.5 75.5 16.5

79.27

37.55 38.97 40.42 41.95 43.49 44.89 46.53 48.78 49.72 51.42

60.25

lins

*

tha

rotbticnal ocnstants h -

llnss

overlapped

weme not

taken

by band head

into

88.92 89.60 90.16 90.95 91.8P-S 92.40 93.03 93.88 94.68 95.51 96.37 97.23 98.08 99.01 99.05* 28100.85 01.78 02.80 03.91

aooount

in

28

SHENYAVSRAYA

AND RYABOV

TABLE

IV

Rotational Lines of 211-2At (1-O) Subband .I

%

Qa

Qb

25.5 2b.5 27.5 28.5 29.5 30.5

28723.43 24.55 25.C2 26.68 27.6% 29.04*

28723.61 24.8H 25.79* 26.96 28.04* 29.25

28707.41 07.87 08.28 08.83 09.29 09.81

28707.59 08.12 08.55 09.00 09.55 10.10

31.5 32.5 33.5 34.5 35.5 3C.5 37.5 38.5 39.5 40.5

30.07* 31.34 32.42 33.C3* 34.75* 36.OC 37.41 38.56s 40.00 41.13

30.35 31.50* 32.64* 33.85 35.2W 36.35* 37.02 38.99 40.34* 41.49

10.47 10.92 11.47 12.11 12.72 13.34 13.99 14.6C 15.31 lb.00

10.59 11.25 11.82 12.35 13.02 13.67 14.24 14.93 15.C5 lb.58

41.5 42.5 43.5 44.5 45.5 45.5 47.5 48.5 49.5 50.5

42.C8t 43.92s 45.13s 4C.5C 48.01 49.40* so.75 52.09* 53.10 55.154

42.84 44.09 45.54 4C.05* 40.23+

lb.71 17.44 18.21 18.99 19.80 20.63 21.38 22.23 23.03 23.97

17.07 11.79 18.59 19.37 20.11 20.93 21.78 22.Cl 23.43 24.38

51.5 52.5 53.5 54.5 55.5 56.5 57.5 58.5 59.5 CO.5

56.70

57.03

24.82 25.79 25.68 27.69 28.58 29.53 30.52 31.50 32.64 33.63

25.27 2C.19 27.20 28.04 29.04 30.07 31.04 32.05 33.08 34.13

93.72 94.09 94.33 94.61 95.03 95.41 95.82 96.17 9C.68 91.12

Cl.5 62.5 C3.5 C4.5 (5.5 6C.5 61.5 (8.5 69.5 70.5

34.75 35.88 3C.93 38.10 39.24 40.34 41.49 42.U 43.92 45.13

35.22 36.3C 31.41 38.56 39.18 40.94 42.10 43.19 44.53 45.G

97.54 91.98 98.53 99.04 99.58 28700.13 OO.CB 01.29 01.91 02.59

71.5 12.5 73.5 74.5 15.5 -lb.5 77.5 10.5 79.5 * overlapped

4C.24 47.58 48.94 50.20 51.58 52.88 54.25 55.74 57.03 * the

oaloulatlon

52.54 53.95 55.74s

by stomio

or Wt

of rotsticnal

line

oonstnnts

h-

28691.41h 91.41h x::: ;;::;; 91:4lh 91.4lh

28C19.32 92.27 92.40 K 93:11 93.44

03.12 03.17 04.54 05.18 05.89 06.72 07.59 oe.za* 09.29

46.85 40.23 49.40 50.75 52.09 53.50 5!.83 56.21 57.60 lires were

not

taken

into

2802.40% 92.CCX 92.83* 93.11* 93.11* ;::::: 94.09* ::*::I 95:03* ::*::I 9c:17* 9C.C8* 97.12* 97.54* 97.90* ;;*:3: 99:50* 287oo.l3* Oo.C8* 01.29+ 01.91* 02.59* 03.12* 03.-n* 04.54* 05.1s* 05.85 06.59 07.27 OB.lz* 08.83* 09.8.1 moount

in

warlapped by hand head

be represented by the formula:

Qd-0 - Qa(J> = Rb(J - 1) - R&7 -

1)

= PO

1)

All(J) =

= lPl(Jf The

+

1) -

P,(J

(1)

3).

values of p, given in Table VI, were calculated

using only the lines of the Q-branch.

+

by the least-squares

techniques

THE SPECTRA

OF YF+ AND YF

29

In accordance with the fact that A-doubling in *&, *An, and 2A; is negligibly we have used the following expressions for the rotational terms (F,(J)

+ 1)’ -

I p 1(J + $)/2,

= B&7(J + 1) - D&q_7 + 11’ +

/ pj (J + 4)/2,

= &J(J

2n*\F&q

+ 1) - D;P(J

small

(2)

2rI( F(J)

= B$J(J + 1) - QP(J

+ l)?,

(3)

*A; F(J)

= &J(J

+ 1) -

@P(J

+ 1)2,

(4)

2A5 F(J)

= B$J(J

+ 1) - QJ”(J

+ l)*,

(3

where p is the A-doubling The effective rotational

coefficient. constants,

B;,Bt,... of the F1 and F2 sublevels are related

TABLE

V

Rotational Lines of %X12 J

20 21 22 23 24 25 26 27 28 29

A 2798b.3oh 8b.3oh 8b.30h

85.11 84.90

P

J

84.55 04.24 83.88 83.42 83.02 82.bO 82.07 81.53 80.92 80.32

50.91 49.47 47.99 46.56 44.99 43.48 41.74 40.24 38.55 36.54

40 41 42 43 44 45 46 47 48 49

79.14 79.05 76.38 77.bb 76.89 7b.15 75.38 74.53 73.85 72.81

35.29' 33.48 31.72 29.90 28.11 26.20 24.35 22.43 20.57 18.bb

50 51 52 53 54 55 5b 57 58 59

71.88 71.02 69.99 69.03 (8.00 66.92 65.89 64.73 (3.56 62.41

lb.69 14.57* 12.54* 10.40* 08.36 06.18 04.08 01.83 27899.65 97.30

:: 65 it overlnpped

61.22 60.01 58.72 57.32 5b.07 54.61 by YF

R

P

27953.25 51.89 50.37 48.89 47.45 45.88 44.20 42.49 41.08 39.37

27880.67 78.10 75.75 73.14 70.53 b7.93 65.40 62.52 59.91 57.14

37.58 35.86 34.16 32.28 30.44 28.56 2b.73 24.83 22.90 20.77

54.35 51.59 48.75 45.94 43.04 40.10 37.14 34.07 31.21 28.13

18.83 16.87 14.57 12.54 10.40 08.21 06.06 03.81 01.39 27899.b5

25.12 21.92 18.73 15.64 12.41 09.09 05.99 02.50 27799.23 96.05

96.77 94.35 91.96 89.59 87.07 84.51 82.00 79.32 7b.72 74.15

92.43 89.39 85.63 82.21 78.55 75.03 71.39 67.81 64.30 60.65

71.50 68.79 66.04 63.22 60.46

56.75 53.05 49.30 45.33 41.60 37.72

27963.56+ 62.41+ 61.41 (0.25 59.03 57.b3 56.47 55.11 53.73 52.39

3": 32 33 34 35 36 37 38 39

:: 62

(0-O) Band of YF

95.00 92.67 90.32 88.05 85.62 83.17 line

h -

:: 78 79 :: 82 83 ::

96 97 ;i 100 101 102 103 104 105 106 107 108 109 110 111 overlapped

by band

head

30

SHENYAVSKAYA AND RYABOV TABLE

VI

Rotational Constants of ‘Qp2Ar Transition of YF+ (cm-l) 2nt 0.3055

BO

2f11

2A3

0.3065

0.2953

0.3040 2.2 2.1 0.0079 0.0080

Do x 107 n, x 107 P(v = 0) P(v = 1) r&Q*

Error

0.2960

f0.0002

0.2956

0.3060

Bl

2A%

2.2

2.0

2.2

f0.0002 *0.3 50.3 10.0002 10.0002

1.909

1.876

*/.LA= 15.653

to each other : &ff(Fd

=

B(1

-

WAN,

Beff(F2)

=

BU

+B/AN.

(6’

Equations (6) were used to determine whether the states are regular or inverted. The rotational constants, B and D, and the band origins were calculated by the least-squares method using the following relations : A#(J) R(J -

1) + P(J) Q(J)

= 2B(J + 1) -

4D(.7 + l)“,

= 2~0 + 2(B’ -

B”)J2 - 2(D’ -

= y0 + (B’-

B”)J+

(B’-

(7) D”)J2(J2 + l),

B”)J2.

(8) (9)

Constants for 211: were obtained by averaging over the two A-type doubling components. The origins of the O-O subbands were calculated most accurately from Eq. (8), but for the 1-O band (211+- “A;) Eq. (9) had to be used. The molecular constants of YF+ derived from the rotational analysis are reported in Tables VI and VII. The relationship of the B values of the subbands [see Eq. (6)] indicates both the 211 and 2A are regular states. Table V contains wave numbers of lines of the ‘Z - X’I: (O-O) band of YF. The molecular constants of YF derived from the analysis are given in Table VIII. The band origin was calculated using Eq. (8), the rotational constants using the relation A$

= 4B(J +$) TABLE

-

SD(J + $)“.

(10)

VII

Band Origins of the YF+ Bands (cm-l)

zn t-*A+

*n +-2A$ CbO

1-o

27 990.34 f 28 701.87 f

0.10 0.05

28 088.37 f

0.06

THE SPECTRA

OF YF+ AND YF

TABLE VIII Molecular Constants of Yttrium Monofluoride (cm-l) TO 12

X’Z

BO

27 980.81 zt 0.01 0

0.27536 f 0.00009 0.28970 f 0.00009

n, x 107 2.76 f 0.06 2.43 f 0.06

DISCUSSION

It is known that molecules with the same number of electrons, and not differing too much in the charges on their nuclei, have similar spectra. Such similarity of the isoelectronic molecules ScF and TiO was used (12, 13) for predicting the yet unobserved electronic states of these molecules. One should expect a close resemblance between the electronic states of monoxides and ions of monofluorides of the scandium subgroup. Experimental studies, as well as theoretical calculations, show that the ground state of the monoxides is XZ2 +. The first excited state A’2A has been predicted for ScO (2) and found experimentally for La0 (3). The excitation energies of the unobserved A’2A states of ScO and YO were predicted by Green in Ref. (14). Green’s method is based on the accumulated theoretical and experimental evidence that the unpaired electron of the low-lying electronic states is in a molecular orbital which is very atom-like. The ground state of ScO derives from the configuration . . .9a, the A211 state is from the configuration . . -Jr, and the unobserved Af2A from the configuration . . .16, where 9~ is mainly 4s (SC), 4?r is essentially 4~ (SC), and 16 is 3d (SC). Green suggested the evaluation of the molecular orbital energies from the atomic orbital energies. The values ea and ~6 are assumed to be identical with the atomic excitation energies Q, - es and cd - es. The latter were calculated by Green from the energies of the 2D atomic states of s2d, sd2, and sdp configurations. The use of this method for evaluating the excitation energies of the 2A(64d) and 211(r.5p) states of YF+ shows that 2A(64d) is a very low-lying state (all the states of Yf of the 4~15s configuration are below 3300 cm-‘), while 211(?r5p) h as a rather high excitation energy’ (the states of the 5s5p configuration are above 23 400 cm-r). It should be noted that according to the calculation given in Ref. (Z), the 2A states corresponding to other configurations are highly- excited ones and have not been observed experimentally. From the foregoing we assume that 2A(64d) is the lower state of the analyzed transition of YF+. The upper state is probably ‘II( The calculation of EJ and tr from the “center of gravity” for the configuraticns 4d5s (3D and ID) and 5sSp (“P and ‘P) gives the values of about 1720 and 29 300 cm-l, respectively. Hence, for the 211-2A transition, we obtain ve = 27 600 cm-‘, which is in good agreement with the experimental values of V~= 27 955.7 and ve = 28 053.7 cm-‘. As in the case of Lao, the internuclear distance in 211(np) is slightly less than in ‘A(6d). The comparison of the internuclear distances of YF in the 3A(6~) state [r, = 1.968 w (15)] and of YFf in the ‘A(6) state (ro = 1.909 A) provides evidence of the nonbonding character of the &a-orbital.

SHENYAVSKAYA

32

AND RYABOV

ACKNOWLEDGMENT The authors wish to thank Prof. F. M. Gerasimov for the gratings made in his laboratory and Prof. L. V. Gurvich for his keen interest in this work. RECEIVED:

February

6, 1976 REFERENCES

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