19
Photoionization of
Alkali
and
Halide
Photoelectron
Monomers,
Spectroscopy
Dimers,
and
Trimers
J. BERKOWITZ, C. H. BATSON, and G. L. GOODMAN
Downloaded by CORNELL UNIV on May 31, 2017 | http://pubs.acs.org Publication Date: March 8, 1982 | doi: 10.1021/bk-1982-0179.ch019
Argonne National Laboratory, Argonne,IL60439
The ultraviolet photoelectron spectra of diatomic alkali halide molecules are reviewed and inter preted. Data for lithium halide dimers, Li X , are presented and it is shown that the dimers have significantly larger ionization thresholds than the corresponding monomers. Some histori cal controversies regarding the presence of dimers and their ionization energies are clari fied. Photoionization mass spectrometry is used to determine the adiabatic ionization potential of lithium chloride trimer, in order to probe the trend of I.P. with cluster size. The pre dictions of Hartree-Fock, Χα and ionic model calculations on this point are presented. Finally, the very weak stability of M X + and M X + and the high stability of M X+ and M X + are discussed, and conjectures are made regarding the geometric structure of these entities. 2
2
3
3
2
2
2
3
2
It is now a well-recognized fact that the saturated vapors of alkali halides contain associated species (dimers, trimers, etc.) as well as the diatomic molecules. The first evidence of this behavior was an electron-impact mass spectrometric study by Ionov (1). He reported the observation of K2l and Na2l , which he attributed to the process +
M
I
22
+
5
M
2
I +
+
1
+
2 i >
w h e r e
M
=
K
>
+
Na
+
He also indicated that similar data were obtained for K2C1 , Na Cl and L i C l . Subsequently, Friedman (2) reported a similar, but more extensive study of lithium iodide vapor, which indicated that V50% of the ion current was from a dimer species. A brief debate occurred in the middle 1950 s, when Miller and Kusch (3) interpreted the velocity profile of alkali halide molecules effusing from an oven in terms of heavier species as +
2
+
2
f
0097-6156/82/0179-0275$06.25/0 © 1982 American Chemical Society Gole and Stwalley,; Metal Bonding and Interactions in High Temperature Systems ACS Symposium Series; American Chemical Society: Washington, DC, 1982.
19
Photoionization of
Alkali
and
Halide
Photoelectron
Monomers,
Spectroscopy
Dimers,
and
Trimers
J. BERKOWITZ, C. H. BATSON, and G. L. GOODMAN
Downloaded by CORNELL UNIV on May 31, 2017 | http://pubs.acs.org Publication Date: March 8, 1982 | doi: 10.1021/bk-1982-0179.ch019
Argonne National Laboratory, Argonne,IL60439
The ultraviolet photoelectron spectra of diatomic alkali halide molecules are reviewed and inter preted. Data for lithium halide dimers, Li X , are presented and it is shown that the dimers have significantly larger ionization thresholds than the corresponding monomers. Some histori cal controversies regarding the presence of dimers and their ionization energies are clari fied. Photoionization mass spectrometry is used to determine the adiabatic ionization potential of lithium chloride trimer, in order to probe the trend of I.P. with cluster size. The pre dictions of Hartree-Fock, Χα and ionic model calculations on this point are presented. Finally, the very weak stability of M X + and M X + and the high stability of M X+ and M X + are discussed, and conjectures are made regarding the geometric structure of these entities. 2
2
3
3
2
2
2
3
2
It is now a well-recognized fact that the saturated vapors of alkali halides contain associated species (dimers, trimers, etc.) as well as the diatomic molecules. The first evidence of this behavior was an electron-impact mass spectrometric study by Ionov (1). He reported the observation of K2l and Na2l , which he attributed to the process +
M
I
22
+
5
M
2
I +
+
1
+
2 i >
w h e r e
M
=
K
>
+
Na
+
He also indicated that similar data were obtained for K2C1 , Na Cl and L i C l . Subsequently, Friedman (2) reported a similar, but more extensive study of lithium iodide vapor, which indicated that V50% of the ion current was from a dimer species. A brief debate occurred in the middle 1950 s, when Miller and Kusch (3) interpreted the velocity profile of alkali halide molecules effusing from an oven in terms of heavier species as +
2
+
2
f
0097-6156/82/0179-0275$06.25/0 © 1982 American Chemical Society Gole and Stwalley,; Metal Bonding and Interactions in High Temperature Systems ACS Symposium Series; American Chemical Society: Washington, DC, 1982.
Downloaded by CORNELL UNIV on May 31, 2017 | http://pubs.acs.org Publication Date: March 8, 1982 | doi: 10.1021/bk-1982-0179.ch019
276
METAL BONDING AND INTERACTIONS
as w e l l as diatomic molecules, while Klemperer and Rice (4, 5) were unable to detect the i n f r a r e d s p e c t r a of the presumed heavier species and concluded that they c o n t r i b u t e d l e s s than 15% to the vapor composition. In 1958, Berkowitz and Chupka (6) examined 13 a l k a l i h a l i d e s with a more s e n s i t i v e mass s p e c t r o meter, and r u l e d out c o l l i s i o n a l processes (such as ion-molecule r e a c t i o n s ) as p o s s i b l e mechanisms f o r the production of heavier ions. They were a l s o a b l e to observe i o n i c species a t t r i b u t a b l e to t r i m e r s i n a l l but one case, and to tetramers i n four cases. T h e i r r e s u l t s are summarized i n Table I. I t can be seen that the degree of a s s o c i a t i o n i s strongest f o r the l i t h i u m h a l i d e s and diminishes monotonically toward the cesium h a l i d e s . The p r e c i s e r e l a t i v e abundance of n e u t r a l monomers, dimers, t r i m e r s , e t c . i s d i f f i c u l t to deduce from mass spectrometric measurements, s i n c e the r e l e v a n t i o n i n t e n s i t i e s are i n f l u e n c e d by i o n i z a t i o n cross s e c t i o n s and fragmentation processes. Several schemes have been a p p l i e d to overcome these problems. However, s u b l i m a t i o n energies can be determined by measuring the temperature dependence of i o n i n t e n s i t i e s , l a r g e l y u n a f f e c t e d by the aforementioned problems. These measurements enable one to compute heats of formation, heats of d i m e r i z a t i o n , e t c . f o r the n e u t r a l s p e c i e s . In a d d i t i o n , m a t r i x - i s o l a t i o n techniques have y i e l d e d i n f r a r e d a c t i v e v i b r a t i o n a l frequencies of dimers (and p o s s i b l y one trimer) and geometrical s t r u c t u r e s of monomers and some dimers have been obtained from a combination of microwave s p e c t r a and e l e c t r o n d i f f r a c t i o n . The a c q u i s i t i o n of such an extensive set of p r o p e r t i e s f o r t h i s c l a s s of molecules has prompted a number of authors to explore the use of i o n i c models f o r s y s t e m a t i c a l l y c h a r a c t e r i z i n g these p r o p e r t i e s . We have r e c e n t l y reviewed some of the successes and f a i l u r e s of these models,(7) i n c l u d i n g r e f e r e n c e s to the v a r i o u s authors. The newer technologies of p h o t o e l e c t r o n spectroscopy and p h o t o i o n i z a t i o n mass spectrometry, when adapted to high temperature s t u d i e s , have re-opened t h i s t o p i c f o r f u r t h e r i n v e s t i g a tion. In p a r t i c u l a r , p h o t o e l e c t r o n spectroscopy has provided i n f o r m a t i o n about the e l e c t r o n i c s t r u c t u r e of these s p e c i e s , a property l a r g e l y ignored i n the e a r l i e r i n v e s t i g a t i o n s . Photoelectron S p e c t r o s c o p y — I t s Vapors
A p p l i c a t i o n to the A l k a l i H a l i d e
Photoelectron spectroscopy i s the study of the e l e c t r o n k i n e t i c energy spectrum produced upon p h o t o i o n i z a t i o n of molecules with monochromatic r a d i a t i o n . We may w r i t e the b a s i c p h o t o e l e c t r i c equation as hv + AB -> AB If AB
+
+
+ e
i s produced i n i t s ground s t a t e ( e l e c t r o n i c , v i b r a t i o n a l
Gole and Stwalley,; Metal Bonding and Interactions in High Temperature Systems ACS Symposium Series; American Chemical Society: Washington, DC, 1982.
19.
BERKOWITZ ET AL.
Photoionization
and Photoelectron Spectroscopy
277
TABLE I Relative
Ion I n t e n s i t i e s o f A l k a l i
Vapors, Produced by E l e c t r o n
Downloaded by CORNELL UNIV on May 31, 2017 | http://pubs.acs.org Publication Date: March 8, 1982 | doi: 10.1021/bk-1982-0179.ch019
M
+
MX
+
M X
+
2
Halide
Impact
M X 3
a
+ 2
M
X
4 3
+
LiF
51.7
11.5
100.
7.2
0.038
LiCl
18.2
21.8
100.
4.3
0.14
LiBr
21.8
22.0
100.
3.9
0.11
29.8
0.58
0.0058
NaF
100.
NaCl
100.
59.0
71.3
0.59
-
NaBr
100.
70.7
64.8
1.01
-
53.3
0.33
-
17.5
0.12
-
Nal
85.6
2.60
100.
KF
100.
KC1
100.
16.8
21.4
0.058
-
KBr
100.
28.6
22.3
0.086
-
KI
100.
55.7
16.7
0.040
-
RbCl
100.
7.03
13.6
0.0061
-
CsCl
100.
1.16
0.18
7.15
