Electronic states of diatomic lanthanide and actinide halides

0 (Degree)

5. Electronic states of diatomic lanthanide and actinide halides

In this section, we summarize the results of recent experimental and theoretical studies on diatomic lanthanide and actinide halides. It is pointed out that there exists a large set of spectroscopic data on these species. These are already summarized in H u b e r and Herzberg (1979). The present section is focused primarily on those diatomic molecules for which extensive studies have been made recently. The reader is referred to the excellent b o o k of H u b e r and Herzberg for previous studies. A recent paper by Gotkis (1991) provides theoretical insight and systematic comparison of bonding in lanthanide halides.

5.1. L a F

The electronic states of L a F are qualitatively similar to LaH, although L a F is significantly more ionic c o m p a r e d to LaH. The ground state of L a F is also of l y + symmetry and is very ionic. Among others, Barrow et al. (1967) have studied the diatomic L a F molecule. They have analyzed the A ~ X, B*--X, C*--X, D ~ X, and E ~ X systems with %o values of 11662, 16184, 20960, 22485 and 2 2 5 7 4 c m -1 respectively. The A, B, C, D and E states of L a F are assigned to A 1Z+, B 1H, C 1H, D 1E+, and E lE+ states, of course, the X ground state is of 1E+ symmetry.

Schall et al. (1983) have studied L a F using resolved fluorescence laser spectroscopy.

Tunable dye lasers were used to excite known systems of LaF. However, the resolved fluorescence spectra subsequent to excitation of B 1H X 12+ and C 1 H - X 1E+ bands also showed transitions to X 1E+ and a 3A states. They found yet another ,(2= 2 state which was tentatively assigned to 1A 2. These high-resolution spectral bands facilitated accurate determination of the spectroscopic constants of these states. Schall et al.

(1983) also used the ligand-field theory to rationalize their assignments.

Table 48 shows the spectroscopic constants of LaF. The values for the X, a 1, az, 1A2, 0 +, B lI-I state are improved and from Schall et al.'s work (1983). Other constants are from H u b e r and Herzberg (1979). Figure 31 summarizes the electronic states of L a F observed up to now.

There are several similarities between the electronic states of L a H studied by Das and B a l a s u b r a m a n i a n (1990) theoretically and L a F (compare tables 13 and 48). The ground state of L a H is also X 1E+ with R e = 2.08 A and co e = 1433 c m - 1. The shorter L a F bond length is primarily due to large ionicity of the La F bond. The 3A1, 3A 3 and 3A 2 states of L a H are computed at 2580, 2588, 2789 cm 1, respectively, while the 3A a and 3A 2 states of L a F are at 1432 and 1808 c m - 1 , respectively. The 1A 2 s t a t e s of L a H has a Te of 6567 cm 1 while the corresponding state of L a F has a T v of 5478 c m - 1

The A 1E + (II) state of L a H has a T e of 13 025 c m - 1 while the corresponding state of L a F has a T~ of 16638cm 1. The B 1II state of L a H is also close to the A state. The

102 K. B A L A S U B R A M A N I A N TABLE 48

Spectroscopic constants of LaF. Prepared from Schall et al.

(1987) and H u b e r and Herzberg (1979).

State R~ (,~) o 4 ( c m - 1) T 2 ( c m - 1)

X i Z + 2.0265

a 1 3A I 2.05994

a 2 3 A 2 2,0543

IA2

AO+(1E +) 2.0929

B 11-1 2.0971

C 11-1 [2.0671]

D 1E + [2.04]

E IE+ [2.10]

c 1 aA 3

c 2 3A 2 2.0548

c 3 3A 1 d 3~

ea~p

570 0

537.14 1432(3)

537.65 1808

528 5478

[489] 16637.96

475.91 16184

[549] 20959

22485

[421] 22574

25 F

A I 5

/ E ' Z +

~ . D ' Z*

CIFi

5 e 3 ~ 2

. . . 4 3 ~3~ t 2

~ 11

-F- -~

/c~A2

-- b~ 2 c,3A,

.O~A3

-/J~:21o~a21 X' E "+

Fig. 31. Energy levels of L a F known up to now, The broken lines are uncertain in an absolute sense but are placed cor- rectly relative to other states. The solid vertical lines stand for laser-induced transitions studied by Schall etal.

(1983) while the broken vertical lines are observed fluorescence transitions.

Reproduced from Schall et al. (1983).

C l rI state of LaH has a T e of 20 170 cm - 1 while the corresponding state of LaF has a Te of 20 959 c m - 1. Consequently, there are m a n y similarities between LaH and LaF. This similarity appears to reveal that these excited states arise primarily from the excitations of the La electrons to upper orbitals c o m p o s e d predominantly of La.

5.2. YF and YCI

The diatomic yttrium halides have been the topic of both ab initio and experimental studies. Fischell et al. (1980) have studied the excitation spectra of the YC1 diatomic molecule using the laser-induced fluorescence (LIF) method. More recently, Xin et al.

(1991) have studied the B I I I - X 1E ÷ system of YC1 in high resolution. The rotational analysis of the observed bands has yielded very accurate molecular constants for the X and B states of YC1. Shirley et al. (1990) have studied the molecular-beam optical Stark spectrum of the B lII(v = 0)-X 1E+(v = 0) band system of YF. The permanent dipole moment p and the magnetic hyperfine parameter a for the B 117 state have been determined as 2.96(4)D and 146.8(3)MHz, respectively. The dipole moment of the X lie + state was determined as 1.82(8)D. More recently, Shirley et al. (1991) have employed the molecular-beam millimeter-wave optical pump-probe spectroscopy to study pure rotational transitions of the YF ~ ÷ ground state. This study has yielded improved ground-state rotational constants as B = 8683.65(1) MHz and D = 0.0079(2)- MHz, respectively.

TABLE 49

Spectroscopic constants for the excited states of YC1. Reproduced from Langhoff et al. (1988). R e is reported in a o and T~ and co e are given in c m - 1

M R C I M R C I + Q Exp."

S t a t e R e coo T~ R e coo To R e oJ e T~

X IE ÷ 4.592 365 0 4.590 364 0

A IA 4.733 335 9594 4.729 335 9557

B ~H 4.775 315 12 417 4.770 315 12 284

C 1 Z + 4.744 314 15 260 4.741 316 15 161

D IFI 4.723 391 23 147 4.704 337 22 765

E 1A 4.723 326 23 916 4.715 327 23 792

F 1 F 4.760 312 25841 4.743 308 25460

G 12+ 4.729 325 25985 4.723 341 25425

H 1~ 4.749 342 26 624 4.729 322 26 023

I1A 4.801 313 28 599 4.775 314 27 722

J 1 H 4.758 315 28 770 4.727 318 27 747

a3A 4.712 339 6524 4.707 339 6673

b 3 ~ 4.722 331 8753 4.716 332 8788

c a Z ÷ 4.732 316 10154 4.724 323 10255

d 3~ 4.771 309 19 165 4.765 310 19 305

e 3 ~ 4.718 325 20076 4.701 339 20001

g 3 A b 4.786 306 20 332 4.779 307 20 459

h 3H 4.756 347 21 027 4.763 340 21 018

4.547 381 0

4.667 330.6 12 128

4.694 325 14908

4.654 345 22 787

4.668 335 27116

a H u b e r and Herzberg (1979), except the B 1FI state whose constants are from Xin et al. (1991).

bThe l e t t e r f i s reserved for the lowest 3 Z - state, which is expected to lie in this region - s e e text.

104 K. BALASUBRAMANIAN

Langhoff et at. (1988) have computed the spectroscopic constants of scandium and yttrium halides using ab initio method. In particular, the YC1 diatomic molecule has been studied using CASSCF followed by MRCI and MRCI + Q methods. These authors have computed the spectroscopic constants of 18 electronic states of YC1 and reassigned the observed spectra of Fischell et al. (1980). We discuss this in this section.

Table 49 shows the MRCI and MRCI + Q results of Langhoff et al. (1988) for YC1 together with the experimental data from Huber and Herzberg (1979). Fischell et al.

have observed the excitation spectra of YC1 using LIF but the two observed bands were not assigned. Figures 32 and 33 show the energy-level diagrams for the singlet and triplet states of YC1, together with all dipole transitions.

Langhoffet al. (1988) assigned to the excitation spectra of YC1 at 27 116 cm- 1 to the J l r I - x 1E + transition. They noted that their computed lifetime (13 ns) is lower than the experimental value of 21ns. They found that this system contained significant component of Y+ 5s --* 5p atomic excitation.

The spectra observed by Fischell et al. with T e ~ 22787 c m - t were assigned by Langhoffet al. (1988) to the D arI-X tZ + system. The computed lifetime of 23 ns was found to be in excellent agreement with the experimental value of 28 ns supporting further the D aFI-X 1~'~+ agreement.

25000--

20000--

'7 E 15000

w u J

10000--

5000-- ~.

j l l l

~

^11,~

HI~ I + d | F ["

,~ EI~.

--DIll

i C1Z +

Blll

A I ~

x l ~ + Fig. 32. Energy-level diagrams and the dipole-allowed transition for the singlet states of YCI. Reproduced from Langhoffet al. (1988).

20000

o ~ 7 . ,

o o h311 d

~, e 3 l l d3,1 ,

c1¢

LU Z

I.U

1 5 0 0 0 - -

5 0 0 0 - -

. +

~I b311 Fig. 33. Energy-level diagrams

o I and the dipole-allowed transi-

tions for the triplet states of

a3tk YC1. Reproduced from Langhoff et al. (1988).

As seen from table 49, A and B states were assigned to 1A and 1H, respectively, by Langhoff et al. The B - X system was also recently analyzed by Xin et al. (1991). The lifetime of the B 1H state was computed as ~ 291 ns by Langhoffet al. (1988). Xin et al.

(1991) confirmed Langhoff et al.'s computed properties of the B 1FI state. The experimental T~ for the B state obtained by Xin et al. (12 128 c m - 1) is in excellent agreement with the computed value of Langhoff et al. (12 284 c m - 1).

As seen from table 49 and figs. 32 and 33, there are numerous allowed dipole transitions predicted for YC1 many of which are yet to be observed. Hence, there is further scope for experimental studies on YCI.

5.3. UF and UF

Krauss and Stevens (1983a) have studied the diatomic U F and its ions. The bonding in U F was found to be similar to alkaline-earth fluorides. These authors studied the 6A state of UF. The SCF configuration used near R e by these authors, is

1~ 2(F 1 s) 2~ 2(U6s) 3~ 2(F2s) 4~ 2(U6p) 5~ 2(F2p) 6~ (U7s) l~4(U6p) 2~t4(F2p) 3~(U5f) 16(U5f)26(U6d)l~(U5f).

The U F molecule was found to be ionic with a U 7s electron transferring to the F 2p at long distance. The spectroscopic constants of the 6A ground state of U F were computed as R e = 4.12 bohr, ~e = 511 c m - 1, ~o~x e = 2.0 c m - 1 and D r = 4.92 eV relative to the neutral separated atoms.

Figure 34 shows the amplitude contour plots of the 56 and 6~ orbitals of U F for the 6A state at 4.5 bohr. Figure 35 shows the amplitude contour plots of the atomic valence orbitals (4~, 3~, 18, 14) of 6A state of U F at the same distance. The 5~ orbital is predominantly F2p~ although the lobe towards U is compressed by overlap with uranium orbitals. On the other hand, 6~ orbital is mainly composed of U 7s.

106 K. B A L A S U B R A M A N I A N

• ,. , - . : ' . - . ' , . N , , ~ , , \ , , , ,

, , . : - - . . ' . e , , , , ' , ' , ' , ; , , , , ', h h h h , , l l l l , I i I I

,.r " . " - ; ' ; / ; ; ; ' : : • : ; : :

i " . " o t , ~ t t # t t , t o e ~ . ,

• . ' i " . " . : ' , , , • ,

•

, ' - " z'-'-'.C,, o ' , ' l ,' ,' ; "

~ . y . . , ~ ".,-.-...-.'...'.. ,, ,, •

I I I I I I

Fig. 34. The amplitude contour plots of the 5~ and 6~ orbitals of U F 6A state at R = 4 - 5 bohr (see fig. 28 for definition). Reproduced from Krauss and Stevens (1983a).

Krauss and Stevens (1983a) considered the 5A state of U F - . The spectroscopic constants of this state at the SCF level were R e = 4 . 3 0 b o h r , ~oe= 429cm -1,

~oex e = 1.7cm -1 and D e = 3.1 eV. The larger D e and smaller R e for U F - compared to U H - was rationalized by Krauss and Stevens based on the smaller size of F - compared to H - .

5.4. G r o u n d s t a t e s o f L a F - L u F

Dolg and Stoll (1989) have computed the ground-state properties of L a F - L u F using different ECPs and SCF/SDCI + Q levels of theory. We focus mainly on their computed R& and Des. The reader is referred to their original paper for the vibrational frequencies and dipole moments.

Tables 50 and 51 show the Res and Des computed by Dolg and Stoll for L a F - L u F . We omitted the SCF result from their original tables since the SCF results are inferior as they do not include electron-correlation effects at all. The computed Res of

I I I I 1 •

(b) .::".-."--X--.., .,, ,.,,;-, -2- ;.-~

,,",,',,'.;'.;.-~

i ~ I I ea e I ' . o

- - ' ~ , , ~ , , , ~

• . . - a #l ! i ~ - - . J s

, , , , , , , . . . . . . . . , . , , . , ;

a ' L ~ " - " " o " - • •

" . . . . . - °

I l I l l l

I I 1 1 1 1

1 J i I I I

Fig. 35. The amplitude c o n t o u r plots of the atomic valence (a) 40, (b) 3~, (c) 18, and (e) 14) orbitals of U F , 6 A a t

4-5 bohr (see caption for fig. 28). Reproduced from Krauss and Stevens (1983a).

108 K. BALASUBRAMANIAN TABLE 50

Bond lengths of the rare-earth monofluorides (A,) from SCF and CI (SD) calculations [including Davidson's correction ( + Q)] and experiment. The core charges Q = 11 and Q = 10 denote the pseudopotentials for a (4f") o 2 and a (4f" + 1)ol superconfiguration, respectively. Reproduced from Dolg

and Stoll (1989).

Q = 1 1 Q = 1 0

LnF CI + Q CI + Q

LaF a, d 2.178 2.181

b, d 2.179 2.183

c, d 2.169 2.172

b,e 2.115 2.116

c, e 2.070 2.072

b,f 2.116 2.118

c, f 2.068 2.070

CeF a, d 2.156 2.159

PrF a, d 2.142 2.145

NdF a, d 2.127 2.130

PmF a, d 2.112 2.115

SmF a, d 2.099 2.103

EuF a, d 2.087 2.090

b, d 2.084 2.088

b, e 2.020 2.022

b, f 2.017 2.018

GdF a, d 2.076 2.079

TbF a, d 2.065 2.067

DyF a, d 2.055 2.059

HoF a, d 2.045 2.049

ErF a, d 2.037 2.041

TmF a, d 2.029 2.033

YbF a, d 2.021 2.025

b, d 2.013 2.017

b, e 1.953 1.955

b, f 1.944 1.946

LuF a, d 2.014 2.018

b, d 2.002 2.007

b, e 1.944 1.946

b, f 1.934 1.936

2.249 2.252

2.240 2.242

2.226 2.229

2.214 2.216

2.203 2.206

2.204 2.208

2.144 2.145

2.138 2.138

2.146 2.148

2.143 2.146

2.136 2.140

2.079 2.080

2.066 2.067

(a) MEFIT, HF pseudo-potential (b) MEFIT, WB pseudo-potential

(e) Ln (7s6p5dlf)/[5s4p3dlf], F (9s6pld)/[4s3pld]

(f) Ln (7s6p5d2f)/[5s4p3d2f], F(9s6p2d)/[4s3p2d]

L a F - L u F a l s o s h o w t h e l a n t h a n i d e c o n t r a c t i o n t r e n d . T h e e x p e r i m e n t a l Res f o r L a F , T b F , H o F , Y b F a n d L u F a r e 2.027, 1 . 9 6 , 1 . 9 4 , 2 . 0 1 6 a n d 1 . 9 1 7 A r e s p e c t i v e l y . T h e g r o u n d - s t a t e r e s u l t s o f D o l g a n d S t o l l a r e t h u s i n g o o d a g r e e m e n t w i t h e x p e r i m e n t . A t p r e s e n t , t h e r e is n o t m u c h k n o w n o n t h e e x c i t e d e l e c t r o n i c s t a t e s o f l a n t h a n i d e h a l i d e s . T h e S D C I + Q m e t h o d b a s e d o n a s i n g l e - r e f e r e n c e c o n f i g u r a t i o n u n d e r e s t i m a t e s t h e d i s s o c i a t i o n e n e r g i e s u n i f o r m l y a s s e e n f r o m t a b l e 51. T h i s is e x p e c t e d i n v i e w o f a

TABLE 51

Dissociation energies of the rare-earth monofluorides (eV) [AC denotes the CI (SD) + Q results after the energy correction to the experimentally observed atomic ground state of the rare-earth atom was applied]. Reproduced from Dolg and Stoll (1989). See table 47 for a - f footnotes. Most of the experimental constants from Huber and Herzberg (1979). Readers are referred to Dolg and Stoll (1989) for further details.

Q = l l Q = 1 0

LnF CI + Q + AC CI + Q Exp.

LaF a,d 5.14 5.45 5.45

b, d 5.46 5.77 5,77

c,d 5.50 5.81 5.81

b,e 5.55 5.98 5.98

c,e 5.68 6.11 6,11

b, f 5.56 6.02 6,02

c,f 5.68 6.15 6.15

CeF a, d 5.17 5.48 5.48

PrF a,d 5.19 5.50 4.95

NdF a, d 5.22 5.54 4.70

PmF a, d 5.26 5.58

SmF a,d 5.31 5.63 3.39

4.84 4.99 4.83 4.98 4.79 4.94 4.77 4.91

EuF a,d 5.36 5.68 2.35 4.75 4.89

b, d 5.77 6.09 2.76 4.38 4.51

b,e 5.88 6.32 2.98 4.58 4.87

b, f 5.90 6.37 3.04 4.68 5.02

GdF a,d 5.41 5.73 5.73

TbF a, d ~ 5.46 5.78 5.75

DyF a,d 5.53 5.85 4.91

HoF a, d 5.60 5.92 4.87

ErF a, d 5.66 5.98 5.09

TmF a, d 5.72 6.04 4.41 4,57 4.72

YbF a,d 5.80 6.12 3.24 4.52 4.67

b, d 6.38 6.69 3.81 4.03 4.20

b,e 6.47 6.91 4.03 4.25 4.54

b, f 6.50 6.97 4.09 4.36 4.70

LuF a,d 5.87 6.18 6.18

b, d 6.45 6.76 6.76

b, e 6.56 6.99 6.99

b, f 6.57 7.04 7.04

6.20

5.87 5.46 5.81 5.42 5.60

6.08 5.46 5.57 5.83 5.25 4.80 5.00

single-reference and SCF treatment for the MOs. Yet, one could obtain a meaningful relative trend as seen from table 51.

Gotkis (1991) has recently developed a simple electron-structure model to investi- gate bonding in lanthanide halides. He has investigated the ionization energies for various lanthanides and has obtained systematic trends. Gotkis (1991) has shown that deviations from monotonic trend are due to a field-stimulated restructuring of the lanthanide cation core, which involves p r o m o t i o n of an electron from 4f to an out-of-core extended ~6~ orbital. He has grouped lanthanide fluorides into two groups;

one which has f" core (PrF, NdF, SmF, EuF and YbF) and the other with an f " - 1 core (LaF, G a F , TbF, HoF, ErF, LuF). The former group was found to be similar to

110 K. BALASUBRAMANIAN

alkaline-earth monohalides while the latter group consisted of Ln +3 (4f"-1) and F - and a lone pair ((~6s) 2 localized on the lanthanide atom. Gotkis (1991) found that the energies of the two configurations are close for D y F and TmH. He has predicted the R~

and co e values for the ground state and excited states of these halides. He has discussed the Johnson rule for the variation of properties of lanthanide halides. For the actual tables of molecular constants that Gotkis predicts, the reader is referred to his paper.

Dolg etal. (1991b) has recently used the C A S S C F / M R C I / C P F method in con- junction with quasi-relativistic pseudo-potentials to study both YbF and YbH including s p i n - o r b i t coupling. For YbF, they computed the molecular constants as R~ = 2.045 ,~, D O = 4.87 eV and (o e = 492 c m - 1, in excellent agreement with the experimental values of R~ = 2.016 A, D o = 4.80 eV, or > 5.36 eV, ~o e = 502 cm - 1. The ground state of YbF was found to be a f2=½ state arising from the 4flg(32(sz'g.4(~ 2 (252+) configuration. An excited A 252 + state of YbF was also found by Dolg et al. (1991b) with T e = 0.44 1.51 eV depending on the level of treatment. The predicted ground state of YbF was in accord with the ESR studies of Van Zee et al. (1977).

Dalam dokumen Edited by: Karl A. Gschneidner, Jr., LeRoy Eyring, G.H. Lander and G.R. Choppin ISBN: 978-0-444-81724-2 (Halaman 113-122)