- Email: [email protected]
Absorbed ion emission from laser-irradiated tungsten
Volume 19, number 8
PHYSICS LETTERS
data a r e c o n s i s t e n t l y 3-4 eV higher than X - r a y data r e p o r t e d by M e r r i l l and Du Mond [8], and these in t u r n a r e about 2 eV higher than those r e p o r t e d by Cauchois et al. [7]. This i n d i c a t e s that the r e f e r e n c e e n e r g y m a y not have been the s a m e in the different m e a s u r e m e n t s . D i s c r e p a n cies a r e as l a r g e as 10 eV b e t w e e n the X - r a y e m i s s i o n and a b s o r p t i o n data r e p o r t e d in r e f s . 6-9. The e l e c t r o n line P t NHI (CuK~I) could be obtained f r o m the p l a t i n u m b a c k i n g of the plutoniu m s a m p l e . The NIH e n e r g y in p l a t i n u m d e t e r m i n e d f r o m this line was i n a c c o r d a n c e with Xr a y data for p l a t i n u m [10] to within 0.5 eV. At the s a m e t i m e , if the line was u s e d for an a l t e r native c a l i b r a t i o n of the plutonium s p e c t r u m , the r e s u l t s w e r e in a c c o r d a n c e with the e l e c t r o n s p e c t r o s c o p i c data l i s t e d in table 1 to within 0.5 eV. We plan to extend the m e a s u r e m e n t s to other s h e l l s and s u b s h e l l s and also to different v a l e n c e s t a t e s of plutonium and other a c t i n i d e s with the a i m to find the c h e m i c a l shifts. F u r t h e r m o r e it is our i n t e n t i o n to study the p l u t o n i u m m e t a l . where it might be p o s s i b l e to throw s o m e light on the type of c h e m i c a l bonds i n the d i f f e r e n t p h a s e s , if the d i f f e r e n c e in c h e m i c a l shifts is l a r g e enough. We m a y then also study the m e c h -
1 January 1966
a n i s m of the phase changes. Another i n t e r e s t i n g p o s s i b i l i t y i s the study of the v e r y i n i t i a l s t e p s of the c o r r o s i o n p r o c e s s of plutonium m e t a l as oxide l a y e r s of only a few tens of ,Fmgstrt}ms can be e a s i l y m e a s u r e d .
References 1. A.Fahlman, S.Hagstrb2n, K. Hamrin, R.Nordberg, C. Nordling and K. Siegbahn, to be published in Arkiv Fysik. 2. C.Nordling and S.HagstrS'rn, Arkiv Fysik 15 (1959) 431. 3. C.Nordling and S. Hagstrb2n, Z.Physik 178 (1964) 418. 4. S.Hagstrthn, C.Nordling and K. Siegbahn, Z.Physik 178 (1964) 439. 5. S. Hagstrb'm, C .Nordling and K. Siegbahn, Physics Letters 9 (1964) 235. 6. Y. Cauchois, C. Bonnelle and L.De Bersuder, Compt rend. 256 (1963) 112. 7. Y. Cauchois, C. Bonnelle and L.De Bersuder, Compt. rend. 257 (1963) 2980. 8. J. Merrill and J.Du Mond, Ann. Phys. 14 (1961) 166. 9. J. -L. Bobin and J.Depres, Compt. rend. 252 (1961) 1302. 10. J.A. Bearden, "X-Ray Wavelengths", Publ. by the U. S. Atomic Energy Commission, Division of Technical Information Extension, Oak Ridge, Tennessee (1964).
ABSORBED ION EMISSION FROM L A S E R - I R R A D I A T E D TUNGSTEN *
E. B E R N A L , J. F. R E A D Y and L. P. LEVINE Honeywell Inc., Corporate Research Center Hopkins, Minnesota, U.S.A. Received 27 November 1965 A t i m e of flight s p e c t r o m e t e r with an e n e r g y a n a l y z e r has been used to study the m a s s e s and e n e r g i e s of ions of a d s o r b e d s p e c i e s e m i t t e d by a t u n g s t e n s u r f a c e upon i r r a d i a t i o n with a Qswitched r u b y l a s e r . The m e a s u r e m e n t s w e r e c a r r i e d out at light fluxes for which p r e v i o u s work in this l a b o r a t o r y [1] indicated that the e l e c t r o n e m i s s i o n could be explained on a t h e r m a l b a s i s . We find that at the s a m e power l e v e l s a l a r g e f r a c t i o n of the ions e m i t t e d have e n e r g i e s above 100 eV. The f r a c t i o n found with such l a r g e e n e r g i e s is f a r too l a r g e to be a t t r i b u t a b l e to t h e r m a l effects. High e n e r g y ions have been r e p o r t e d p r e v i o u s ly for light fluxes two o r d e r s of m a g n i t u d e higher * Work supported by Ballistic Research Laboratories, Aberdeen Proving Grounds, Maryland.
than those used in our e x p e r i m e n t s [3-6]. F o r the higher fluxes extensive v a p o r i z a t i o n of the t a r g e t o c c u r s and m o r e complex i n t e r a c t i o n s can be expected to take place in the high d e n s i t y p l a s m a that is emitted. B e c a u s e we a r e p r i m a r i l y i n t e r e s t e d in studying the s p e c i e s s o r b e d on the s u r f a c e , we have p u r p o s e l y worked at power levels at which t h e r e is no v i s i b l e c r a t e r i n g of the target. A Q-switched r u b y l a s e r with an output of ~ 0.1 joule was used in all the e x p e r i m e n t s . The p u l s e shape with a c r y p t o c y a n i n e Q-switch i s v e r y n e a r l y t r i a n g u l a r with a full width of 30 n a n o s e c o n d s at the base. The peak power was m e a s u r e d with a c a l i b r a t e d Philco 4501 photodiode. M a x i m u m peak power was 6 MW. The peak flux at the t a r g e t s u r f a c e was 20 M W / c m 2 for the unfocused b e a m . A lens was used in some i n 645
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s t a n c e s to i n c r e a s e the flux to about 80 M W / c m 2. At t h e s e p o w e r l e v e l s t h e r e i s no c r a t e r i n g of the target surface, nor is there a visible plume e m a nating f r o m the s u r f a c e . F o r our e x p e r i m e n t a l conditions we find that m o s t of the ions o b s e r v e d have m a s s - t o - c h a r g e r a t i o s l e s s than s i x t y a . m . u . / e . F u r t h e r m o r e , we find the high e n e r g y ions to be m a i n l y a l k a l i m e t a l ions. The t e m p e r a t u r e r i s e of the m e t a l s u r f a c e f o r the f l u x e s u s e d in t h e s e e x p e r i m e n t s has been c a l c u l a t e d using c l a s s i c a l t h e r m o d y n a m i c s [1] and found to be of the o r d e r of 2000OK. HIGH VOLTAGE IN
d~
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a c c u r a t e d e t e r m i n a t i o n of m a s s e s can be m a d e f r o m the t i m e - o f - f l i g h t alone. T h i s is n o r m a l l y not t r u e in a s i m p l e diode s y s t e m with a s h o r t f l i g h t - p a t h . A m o r e d e t a i l e d d e s c r i p t i o n of the s p e c t r o m e t e r i s published e l s e w h e r e [7].
ID
Fig. 1. Schematic of time-of-flight spectrometer. The t i m e - o f - f l i g h t s p e c t r o m e t e r is shown in fig. 1. It c o n s i s t s of an a c c e l e r a t i n g r e g i o n A, followed by a one m e t e r long d r if t tube B, held at ground potential. On the opposite end of the d r i f t tube t h e r e a r e two g r i d s , c and d, that a r e grounded and at an a r b i t r a r y p o t e n t i a l V ' , r e s p e c t i v e l y . The m e t a l to be studied i s used a s one of the e l e c t r o d e s in the a c c e l e r a t i n g s t r u c t u r e . It i s held 1.5 cm away f r o m the end of the d r if t tube by a l u m i n a s p a c e r s , and a p o s i t i v e dc p o t e n t i a l V i s applied to it. Th e l a s e r is a i m e d at the m e t a l s u r f a c e through a s a p p h i r e window in the g l a s s e n v e l o p e s u r r o u n d ing the s p e c t r o m e t e r . Ions e m i t t e d by the s u r f a c e upon i r r a d i a t i o n a r e a c c e l e r a t e d a c r o s s A and a l lowed to d r i f t t h r o u g h B. A f t e r B they a r e a c c e l e r a t e d into the cathode of an e l e c t r o n m u l t i p l i e r tube by the n e g a t i v e p o t e n t i a l applied to the l a t t e r . The m u l t i p l i e r c u r r e n t i s m e a s u r e d a c r o s s a 220 ~2 r e s i s t o r and d i s p l a y e d on the s c r e e n of a T e k t r o n i x 555 o s c i l l o s c o p e . The t i m e constant of the d e t e c t i o n c i r c u i t i s < 10 -7 sec. Ion e n e r g y m e a s u r e m e n t s that a r e independent of ion m a s s a r e p e r f o r m e d by s i m p l y m e a s u r i n g the p o t e n t i a l that m u s t be applied to g r i d d in o r d e r to e x t i n guish a given m a s s peak. Once the ion e n e r g y f o r a given peak has been d e t e r m i n e d , the m a s s c o r r e s p o n d i n g to that p e a k m a y be c a l c u l a t e d f r o m the t i m e of flight f r o m t a r g e t to d e t e c t o r . It i s w o r t h w h i l e to note, h o w e v e r , that the s e p a r a t i o n b et ween m a s s p e a k s in t h i s s y s t e m i s l a r g e c o m p a r e d to the width of the p e a k s , so that a f a i r l y
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Fig. 2. Typical spectrum obtained from time-of-flight spectrometer. Lower trace shows laser pulse with sweep of 0.1 p s / c m . Upper trace is mass spectrum with sweep of 2 ~ s / c m a) V = V ' = I000 V, vertical sensitivity of upper trace (L2 V/cm b) V = 1000 V, V' = 1170 V, vertical sensitivity of upper trace 0.05 V/cm. Fig. 2 shows s o m e t y p i c a l data obtained in the s p e c t r o m e t e r with an a c c e l e r a t i n g p o t e n t i a l V = 1000 V applied to a tungsten t a r g e t . No s p e c i a l c a r e w as taken in c l e a n i n g the s u r f a c e , since it w as d e s i r e d to o b s e r v e the a d s o r b e d s p e c i e s as w e l l as the b a s e m e t a l . It can be s e e n that the p e a k s a r e s e p a r a t e d f a r enough f r o m e a c h other to allow identification of the m a s s e s . Fig. 2a is taken with no d e c e l e r a t i n g p o t e n t i a l (V' = V) and shows that m a n y ionic s p e c i e s a r e p r e s e n t . F r o m it and s i m i l a r p h o t o g r a p h s , the f o l l o w i n g s p e c i e s have been identified: H +, C +, H2 O+, Na +, Co +, + + K +, CO 2, W . The m a j o r p e a k s c o r r e s p o n d to Na + and K +. Fig. 2b i s taken with a d e c e l e r a t i n g p o t en t i al of 170 V (V' =1170 V) applied to g r i d d. Since the ions had only been given 1000 eV of e n e r g y in A, the p r e s e n c e of the two m a j o r m a s s p eak s is e v i d e n c e that ions with e n e r g i e s in the e x c e s s of 170 eV a r e p r e s e n t in the e m i s s i o n f r o m the tungsten s u r f a c e . F u r t h e r i n c r e a s e of the de c e l e r a t i n g p o t e n t i a l to 1200 V e l i m i n a t e s a l l p e a k s , indicating that t h e s e a r e indeed c h a r g e d p a r t i c l e s and not s p u r i o u s signals. Many p o s s i b l e
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s o u r c e s of s p u r i o u s s i g n a l s have been c o n s i d e r e d and the c o n c l u s i o n r e a c h e d is that the s i g n a l s obs e r v e d can only be due to ions of a d s o r b e d s p e c i e s f r o m the t a r g e t , some of which p o s s e s s kinetic e n e r g i e s in e x c e s s of 170 eV. M e a s u r e m e n t s of the a r e a s u n d e r the m a s s peaks for data t a k e n up to m a s s 200 show that the n u m b e r of t u n g s t e n ions r e a c h i n g the d e t e c tor i s only ~ 25% of the n u m b e r of p o t a s s i u m ions. T h i s i s to be expected s i n c e we a r e not c a u s i n g much v a p o r i z a t i o n of the s u r f a c e . Calc u l a t i o n s of the s p e c t r o m e t e r a p e r t u r e show that t h e r e a r e ~ 108 K+ ions e m i t t e d p e r l a s e r pulse. While it m a y s e e m s u r p r i s i n g that the m a j o r p a r t of the e m i s s i o n should c o n s i s t of alkali m e t a l s , r a t h e r than such g a s e s a s CO, one m u s t r e m e m b e r that only charged p a r t i c l e s a r e b e i n g d e t e c t e d , and it i s well known that the alkali m e t a l s will boil off t u n g s t e n m a i n l y a s ions. In fact, independent m e a s u r e m e n t s of n e u t r a l p a r t i c l e e m i s s i o n m a d e in a quadrupole m a s s s p e c t r o m e t e r [8] show that the total n u m b e r of p a r t i c l e s e m i t t e d f r o m the s u r f a c e i s o r d e r s of magnitude g r e a t e r than the n u m b e r of a l k a l i ions m e a s u r e d in these e x p e r i m e n t s .
ON T H E
OPTICAL
1January1966
In c o n c l u s i o n , the m a s s and the e n e r g y of the ions of a d s o r b e d s p e c i e s e m i t t e d by a t u n g s t e n s u r f a c e upon i r r a d i a t i o n with a Q-switched l a s e r have been m e a s u r e d . It is found that some of the a d s o r b e d a l k a l i m e t a l ions e m i t t e d have e n e r g i e s of over 170 eV, f a r too high to be explained on a t h e r m a l b a s i s . C a l c u l a t i o n s of ion e n e r g i e s att a i n a b l e by i n v e r s e B r e m s t r a h l u n g [9-11] for exp e r i m e n t a l conditions d e s c r i b e d above also fail to explain the e n e r g i e s o b s e r v e d . 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.
SPECTRA
J.F.Ready, Phys.Rev. 137 (1965) A620. W.I. Linlor, Appl.Phys. Letters 3 (1963) 210. W.I. Linlor, Phys.Rev. Letters 12 (1964) 383. A.W. Ehler, Am.Phys. Soc.Bull. 9 (1964) 536. H.Opower and E.Burlefinger, Physics Letters 16 (1965) 37. N.R. Isenor, Appl. Phys. Letters 4 (1964) 152. E.Bernal, L.P.Levine and J.F.Ready, Rev. Sci.Inst., in p r e s s . J.F.Ready, L.P.Levine, E.Bernal, Bull.Am. Phys.Soc. 10 (1965) 596. J.M.Dawson, Phys. Fluids 7 (1964) 981. B.A.Tozer, P.R.Smy, J.K. Wright, Proc. Phys. Soc. (London), 86 (1965) 45. J. F. Ready, L.P. Levine and E. Bernal, Eighteenth Gaseous Electronics Conf. (1965).
OF
G d 3+ IN C a F 2 *
J. MAKOVSKY Department of Physics, The Hebrew University, Jerusalem Received 25 November 1965 The absorption and emission spectra of Gd3+ : CaF 2 are studied and the crystalline energy level schemes of Gd3+ ions in eight different sites are established. We have studied the a b s o r p t i o n and e m i s s i o n ** s p e c t r a of Gd3+: CaF2 and e s t a b l i s h e d the c r y s t a l line e n e r g y level s c h e m e s (6p, 61 and 6D s t a t e s ) [4] of Gd3+ ions in eight different s i t e s . T h i s study, we b e l i e v e , i s the f i r s t in which a " m u l t i ple" optical s p e c t r u m of a t r i v a l e n t r a r e - e a r t h (RE 3+) in C a F 2 has been disentangled. In this note we d e s c r i b e some of the b a s i c f e a t u r e s of t h e s e s p e c t r a - d e m o n s t r a t e d by r e sults r e l a t e d to the 6 P ! and 6p~ l e v e l s . The optical spectra2of RE 3+ in CaF 2 a r e a m o n g the m o s t c o m p l i c a t e d RE 3+ s p e c t r a . Despite n u m e r o u s i n v e s t i g a t i o n s , the c r y s t a l l i n e e n e r g y l e v e l s of t h e s e s y s t e m s r e m a i n e d e s s e n t i a l l y u n * Supported by the National Bureau of Standards, Washington, D.C. ** Excitation with X-rays of optical emissions, which h a s m a n y a d v a n t a g e s , h a s b e e n extensively used
[1-3].
known. The m o s t c o n s p i c u o u s difficulty in the i n t e r p r e t a t i o n of t h e s e s p e c t r a is the e x c e s s i v e n u m b e r of lines. Though m a n y l i n e s of ambiguous o r i g i n a p p e a r only in s p e c t r a of c r y s t a l s with high i m p u r i t y c o n c e n t r a t i o n s , s u p e r n u m e r a r y l i n e s a p p e a r even at v e r y low c o n c e n t r a t i o n s (e.g. 10 . 4 mole%). Fig. 1 (a)-(d) d e m o n s t r a t e the 6p~ --. ~S~ e m i s s i o n t r a n s i t i o n s obtained at room t e m~p e r a t u"r e f r o m v a r i o u s s a m p l e s of G d 3 + : C a F 2. The d i v e r s i t y and v a r i a b i l i t y of the s p e c t r a l s t r u c t u r e , and the e x c e s s i v e n u m b e r of l i n e s w h e r e at m o s t four e l e c t r o n i c t r a n s i t i o n s a r e exp e c t e d , can be noticed at a glance. Stepanov and Feofilov [5] d e m o n s t r a t e d the i n f l u e n c e of the r e d u c t i o n - o x i d a t i o n conditions u n d e r which the c r y s t a l s a r e grown on t h e i r optical s p e c t r a . A c c o r d i n g to t h e m , at l e a s t two types of s p e c t r a - which they d e s i g n a t e d I and H - ex647
PHYSICS LETTERS
data a r e c o n s i s t e n t l y 3-4 eV higher than X - r a y data r e p o r t e d by M e r r i l l and Du Mond [8], and these in t u r n a r e about 2 eV higher than those r e p o r t e d by Cauchois et al. [7]. This i n d i c a t e s that the r e f e r e n c e e n e r g y m a y not have been the s a m e in the different m e a s u r e m e n t s . D i s c r e p a n cies a r e as l a r g e as 10 eV b e t w e e n the X - r a y e m i s s i o n and a b s o r p t i o n data r e p o r t e d in r e f s . 6-9. The e l e c t r o n line P t NHI (CuK~I) could be obtained f r o m the p l a t i n u m b a c k i n g of the plutoniu m s a m p l e . The NIH e n e r g y in p l a t i n u m d e t e r m i n e d f r o m this line was i n a c c o r d a n c e with Xr a y data for p l a t i n u m [10] to within 0.5 eV. At the s a m e t i m e , if the line was u s e d for an a l t e r native c a l i b r a t i o n of the plutonium s p e c t r u m , the r e s u l t s w e r e in a c c o r d a n c e with the e l e c t r o n s p e c t r o s c o p i c data l i s t e d in table 1 to within 0.5 eV. We plan to extend the m e a s u r e m e n t s to other s h e l l s and s u b s h e l l s and also to different v a l e n c e s t a t e s of plutonium and other a c t i n i d e s with the a i m to find the c h e m i c a l shifts. F u r t h e r m o r e it is our i n t e n t i o n to study the p l u t o n i u m m e t a l . where it might be p o s s i b l e to throw s o m e light on the type of c h e m i c a l bonds i n the d i f f e r e n t p h a s e s , if the d i f f e r e n c e in c h e m i c a l shifts is l a r g e enough. We m a y then also study the m e c h -
1 January 1966
a n i s m of the phase changes. Another i n t e r e s t i n g p o s s i b i l i t y i s the study of the v e r y i n i t i a l s t e p s of the c o r r o s i o n p r o c e s s of plutonium m e t a l as oxide l a y e r s of only a few tens of ,Fmgstrt}ms can be e a s i l y m e a s u r e d .
References 1. A.Fahlman, S.Hagstrb2n, K. Hamrin, R.Nordberg, C. Nordling and K. Siegbahn, to be published in Arkiv Fysik. 2. C.Nordling and S.HagstrS'rn, Arkiv Fysik 15 (1959) 431. 3. C.Nordling and S. Hagstrb2n, Z.Physik 178 (1964) 418. 4. S.Hagstrthn, C.Nordling and K. Siegbahn, Z.Physik 178 (1964) 439. 5. S. Hagstrb'm, C .Nordling and K. Siegbahn, Physics Letters 9 (1964) 235. 6. Y. Cauchois, C. Bonnelle and L.De Bersuder, Compt rend. 256 (1963) 112. 7. Y. Cauchois, C. Bonnelle and L.De Bersuder, Compt. rend. 257 (1963) 2980. 8. J. Merrill and J.Du Mond, Ann. Phys. 14 (1961) 166. 9. J. -L. Bobin and J.Depres, Compt. rend. 252 (1961) 1302. 10. J.A. Bearden, "X-Ray Wavelengths", Publ. by the U. S. Atomic Energy Commission, Division of Technical Information Extension, Oak Ridge, Tennessee (1964).
ABSORBED ION EMISSION FROM L A S E R - I R R A D I A T E D TUNGSTEN *
E. B E R N A L , J. F. R E A D Y and L. P. LEVINE Honeywell Inc., Corporate Research Center Hopkins, Minnesota, U.S.A. Received 27 November 1965 A t i m e of flight s p e c t r o m e t e r with an e n e r g y a n a l y z e r has been used to study the m a s s e s and e n e r g i e s of ions of a d s o r b e d s p e c i e s e m i t t e d by a t u n g s t e n s u r f a c e upon i r r a d i a t i o n with a Qswitched r u b y l a s e r . The m e a s u r e m e n t s w e r e c a r r i e d out at light fluxes for which p r e v i o u s work in this l a b o r a t o r y [1] indicated that the e l e c t r o n e m i s s i o n could be explained on a t h e r m a l b a s i s . We find that at the s a m e power l e v e l s a l a r g e f r a c t i o n of the ions e m i t t e d have e n e r g i e s above 100 eV. The f r a c t i o n found with such l a r g e e n e r g i e s is f a r too l a r g e to be a t t r i b u t a b l e to t h e r m a l effects. High e n e r g y ions have been r e p o r t e d p r e v i o u s ly for light fluxes two o r d e r s of m a g n i t u d e higher * Work supported by Ballistic Research Laboratories, Aberdeen Proving Grounds, Maryland.
than those used in our e x p e r i m e n t s [3-6]. F o r the higher fluxes extensive v a p o r i z a t i o n of the t a r g e t o c c u r s and m o r e complex i n t e r a c t i o n s can be expected to take place in the high d e n s i t y p l a s m a that is emitted. B e c a u s e we a r e p r i m a r i l y i n t e r e s t e d in studying the s p e c i e s s o r b e d on the s u r f a c e , we have p u r p o s e l y worked at power levels at which t h e r e is no v i s i b l e c r a t e r i n g of the target. A Q-switched r u b y l a s e r with an output of ~ 0.1 joule was used in all the e x p e r i m e n t s . The p u l s e shape with a c r y p t o c y a n i n e Q-switch i s v e r y n e a r l y t r i a n g u l a r with a full width of 30 n a n o s e c o n d s at the base. The peak power was m e a s u r e d with a c a l i b r a t e d Philco 4501 photodiode. M a x i m u m peak power was 6 MW. The peak flux at the t a r g e t s u r f a c e was 20 M W / c m 2 for the unfocused b e a m . A lens was used in some i n 645
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s t a n c e s to i n c r e a s e the flux to about 80 M W / c m 2. At t h e s e p o w e r l e v e l s t h e r e i s no c r a t e r i n g of the target surface, nor is there a visible plume e m a nating f r o m the s u r f a c e . F o r our e x p e r i m e n t a l conditions we find that m o s t of the ions o b s e r v e d have m a s s - t o - c h a r g e r a t i o s l e s s than s i x t y a . m . u . / e . F u r t h e r m o r e , we find the high e n e r g y ions to be m a i n l y a l k a l i m e t a l ions. The t e m p e r a t u r e r i s e of the m e t a l s u r f a c e f o r the f l u x e s u s e d in t h e s e e x p e r i m e n t s has been c a l c u l a t e d using c l a s s i c a l t h e r m o d y n a m i c s [1] and found to be of the o r d e r of 2000OK. HIGH VOLTAGE IN
d~
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1January 1966
a c c u r a t e d e t e r m i n a t i o n of m a s s e s can be m a d e f r o m the t i m e - o f - f l i g h t alone. T h i s is n o r m a l l y not t r u e in a s i m p l e diode s y s t e m with a s h o r t f l i g h t - p a t h . A m o r e d e t a i l e d d e s c r i p t i o n of the s p e c t r o m e t e r i s published e l s e w h e r e [7].
ID
Fig. 1. Schematic of time-of-flight spectrometer. The t i m e - o f - f l i g h t s p e c t r o m e t e r is shown in fig. 1. It c o n s i s t s of an a c c e l e r a t i n g r e g i o n A, followed by a one m e t e r long d r if t tube B, held at ground potential. On the opposite end of the d r i f t tube t h e r e a r e two g r i d s , c and d, that a r e grounded and at an a r b i t r a r y p o t e n t i a l V ' , r e s p e c t i v e l y . The m e t a l to be studied i s used a s one of the e l e c t r o d e s in the a c c e l e r a t i n g s t r u c t u r e . It i s held 1.5 cm away f r o m the end of the d r if t tube by a l u m i n a s p a c e r s , and a p o s i t i v e dc p o t e n t i a l V i s applied to it. Th e l a s e r is a i m e d at the m e t a l s u r f a c e through a s a p p h i r e window in the g l a s s e n v e l o p e s u r r o u n d ing the s p e c t r o m e t e r . Ions e m i t t e d by the s u r f a c e upon i r r a d i a t i o n a r e a c c e l e r a t e d a c r o s s A and a l lowed to d r i f t t h r o u g h B. A f t e r B they a r e a c c e l e r a t e d into the cathode of an e l e c t r o n m u l t i p l i e r tube by the n e g a t i v e p o t e n t i a l applied to the l a t t e r . The m u l t i p l i e r c u r r e n t i s m e a s u r e d a c r o s s a 220 ~2 r e s i s t o r and d i s p l a y e d on the s c r e e n of a T e k t r o n i x 555 o s c i l l o s c o p e . The t i m e constant of the d e t e c t i o n c i r c u i t i s < 10 -7 sec. Ion e n e r g y m e a s u r e m e n t s that a r e independent of ion m a s s a r e p e r f o r m e d by s i m p l y m e a s u r i n g the p o t e n t i a l that m u s t be applied to g r i d d in o r d e r to e x t i n guish a given m a s s peak. Once the ion e n e r g y f o r a given peak has been d e t e r m i n e d , the m a s s c o r r e s p o n d i n g to that p e a k m a y be c a l c u l a t e d f r o m the t i m e of flight f r o m t a r g e t to d e t e c t o r . It i s w o r t h w h i l e to note, h o w e v e r , that the s e p a r a t i o n b et ween m a s s p e a k s in t h i s s y s t e m i s l a r g e c o m p a r e d to the width of the p e a k s , so that a f a i r l y
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Fig. 2. Typical spectrum obtained from time-of-flight spectrometer. Lower trace shows laser pulse with sweep of 0.1 p s / c m . Upper trace is mass spectrum with sweep of 2 ~ s / c m a) V = V ' = I000 V, vertical sensitivity of upper trace (L2 V/cm b) V = 1000 V, V' = 1170 V, vertical sensitivity of upper trace 0.05 V/cm. Fig. 2 shows s o m e t y p i c a l data obtained in the s p e c t r o m e t e r with an a c c e l e r a t i n g p o t e n t i a l V = 1000 V applied to a tungsten t a r g e t . No s p e c i a l c a r e w as taken in c l e a n i n g the s u r f a c e , since it w as d e s i r e d to o b s e r v e the a d s o r b e d s p e c i e s as w e l l as the b a s e m e t a l . It can be s e e n that the p e a k s a r e s e p a r a t e d f a r enough f r o m e a c h other to allow identification of the m a s s e s . Fig. 2a is taken with no d e c e l e r a t i n g p o t e n t i a l (V' = V) and shows that m a n y ionic s p e c i e s a r e p r e s e n t . F r o m it and s i m i l a r p h o t o g r a p h s , the f o l l o w i n g s p e c i e s have been identified: H +, C +, H2 O+, Na +, Co +, + + K +, CO 2, W . The m a j o r p e a k s c o r r e s p o n d to Na + and K +. Fig. 2b i s taken with a d e c e l e r a t i n g p o t en t i al of 170 V (V' =1170 V) applied to g r i d d. Since the ions had only been given 1000 eV of e n e r g y in A, the p r e s e n c e of the two m a j o r m a s s p eak s is e v i d e n c e that ions with e n e r g i e s in the e x c e s s of 170 eV a r e p r e s e n t in the e m i s s i o n f r o m the tungsten s u r f a c e . F u r t h e r i n c r e a s e of the de c e l e r a t i n g p o t e n t i a l to 1200 V e l i m i n a t e s a l l p e a k s , indicating that t h e s e a r e indeed c h a r g e d p a r t i c l e s and not s p u r i o u s signals. Many p o s s i b l e
Volume 19, number 8
PHYSICS LETTERS
s o u r c e s of s p u r i o u s s i g n a l s have been c o n s i d e r e d and the c o n c l u s i o n r e a c h e d is that the s i g n a l s obs e r v e d can only be due to ions of a d s o r b e d s p e c i e s f r o m the t a r g e t , some of which p o s s e s s kinetic e n e r g i e s in e x c e s s of 170 eV. M e a s u r e m e n t s of the a r e a s u n d e r the m a s s peaks for data t a k e n up to m a s s 200 show that the n u m b e r of t u n g s t e n ions r e a c h i n g the d e t e c tor i s only ~ 25% of the n u m b e r of p o t a s s i u m ions. T h i s i s to be expected s i n c e we a r e not c a u s i n g much v a p o r i z a t i o n of the s u r f a c e . Calc u l a t i o n s of the s p e c t r o m e t e r a p e r t u r e show that t h e r e a r e ~ 108 K+ ions e m i t t e d p e r l a s e r pulse. While it m a y s e e m s u r p r i s i n g that the m a j o r p a r t of the e m i s s i o n should c o n s i s t of alkali m e t a l s , r a t h e r than such g a s e s a s CO, one m u s t r e m e m b e r that only charged p a r t i c l e s a r e b e i n g d e t e c t e d , and it i s well known that the alkali m e t a l s will boil off t u n g s t e n m a i n l y a s ions. In fact, independent m e a s u r e m e n t s of n e u t r a l p a r t i c l e e m i s s i o n m a d e in a quadrupole m a s s s p e c t r o m e t e r [8] show that the total n u m b e r of p a r t i c l e s e m i t t e d f r o m the s u r f a c e i s o r d e r s of magnitude g r e a t e r than the n u m b e r of a l k a l i ions m e a s u r e d in these e x p e r i m e n t s .
ON T H E
OPTICAL
1January1966
In c o n c l u s i o n , the m a s s and the e n e r g y of the ions of a d s o r b e d s p e c i e s e m i t t e d by a t u n g s t e n s u r f a c e upon i r r a d i a t i o n with a Q-switched l a s e r have been m e a s u r e d . It is found that some of the a d s o r b e d a l k a l i m e t a l ions e m i t t e d have e n e r g i e s of over 170 eV, f a r too high to be explained on a t h e r m a l b a s i s . C a l c u l a t i o n s of ion e n e r g i e s att a i n a b l e by i n v e r s e B r e m s t r a h l u n g [9-11] for exp e r i m e n t a l conditions d e s c r i b e d above also fail to explain the e n e r g i e s o b s e r v e d . 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.
SPECTRA
J.F.Ready, Phys.Rev. 137 (1965) A620. W.I. Linlor, Appl.Phys. Letters 3 (1963) 210. W.I. Linlor, Phys.Rev. Letters 12 (1964) 383. A.W. Ehler, Am.Phys. Soc.Bull. 9 (1964) 536. H.Opower and E.Burlefinger, Physics Letters 16 (1965) 37. N.R. Isenor, Appl. Phys. Letters 4 (1964) 152. E.Bernal, L.P.Levine and J.F.Ready, Rev. Sci.Inst., in p r e s s . J.F.Ready, L.P.Levine, E.Bernal, Bull.Am. Phys.Soc. 10 (1965) 596. J.M.Dawson, Phys. Fluids 7 (1964) 981. B.A.Tozer, P.R.Smy, J.K. Wright, Proc. Phys. Soc. (London), 86 (1965) 45. J. F. Ready, L.P. Levine and E. Bernal, Eighteenth Gaseous Electronics Conf. (1965).
OF
G d 3+ IN C a F 2 *
J. MAKOVSKY Department of Physics, The Hebrew University, Jerusalem Received 25 November 1965 The absorption and emission spectra of Gd3+ : CaF 2 are studied and the crystalline energy level schemes of Gd3+ ions in eight different sites are established. We have studied the a b s o r p t i o n and e m i s s i o n ** s p e c t r a of Gd3+: CaF2 and e s t a b l i s h e d the c r y s t a l line e n e r g y level s c h e m e s (6p, 61 and 6D s t a t e s ) [4] of Gd3+ ions in eight different s i t e s . T h i s study, we b e l i e v e , i s the f i r s t in which a " m u l t i ple" optical s p e c t r u m of a t r i v a l e n t r a r e - e a r t h (RE 3+) in C a F 2 has been disentangled. In this note we d e s c r i b e some of the b a s i c f e a t u r e s of t h e s e s p e c t r a - d e m o n s t r a t e d by r e sults r e l a t e d to the 6 P ! and 6p~ l e v e l s . The optical spectra2of RE 3+ in CaF 2 a r e a m o n g the m o s t c o m p l i c a t e d RE 3+ s p e c t r a . Despite n u m e r o u s i n v e s t i g a t i o n s , the c r y s t a l l i n e e n e r g y l e v e l s of t h e s e s y s t e m s r e m a i n e d e s s e n t i a l l y u n * Supported by the National Bureau of Standards, Washington, D.C. ** Excitation with X-rays of optical emissions, which h a s m a n y a d v a n t a g e s , h a s b e e n extensively used
[1-3].
known. The m o s t c o n s p i c u o u s difficulty in the i n t e r p r e t a t i o n of t h e s e s p e c t r a is the e x c e s s i v e n u m b e r of lines. Though m a n y l i n e s of ambiguous o r i g i n a p p e a r only in s p e c t r a of c r y s t a l s with high i m p u r i t y c o n c e n t r a t i o n s , s u p e r n u m e r a r y l i n e s a p p e a r even at v e r y low c o n c e n t r a t i o n s (e.g. 10 . 4 mole%). Fig. 1 (a)-(d) d e m o n s t r a t e the 6p~ --. ~S~ e m i s s i o n t r a n s i t i o n s obtained at room t e m~p e r a t u"r e f r o m v a r i o u s s a m p l e s of G d 3 + : C a F 2. The d i v e r s i t y and v a r i a b i l i t y of the s p e c t r a l s t r u c t u r e , and the e x c e s s i v e n u m b e r of l i n e s w h e r e at m o s t four e l e c t r o n i c t r a n s i t i o n s a r e exp e c t e d , can be noticed at a glance. Stepanov and Feofilov [5] d e m o n s t r a t e d the i n f l u e n c e of the r e d u c t i o n - o x i d a t i o n conditions u n d e r which the c r y s t a l s a r e grown on t h e i r optical s p e c t r a . A c c o r d i n g to t h e m , at l e a s t two types of s p e c t r a - which they d e s i g n a t e d I and H - ex647