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Dependence of non-classical electron emission from metals on the direction of polarisation of laser beams
Volume 24A. number 2
16 January1967
PHYSICS LETTERS
DEPENDENCE OF NON-CLASSICAL ELECTRON METALS O N T.HE D I R E C T I O N OF POLARISATION
EMISSION FROM OF LASER BEAMS
Gy. FARKAS, Zs. N.~RAY and P. VARGA
Central Research Institute for Physics, Budapest, Hungary Received 5 December 1966
Electron emission experiments on silver irradiated by a high power laser beam, performed under conditions with negligible Richardson effect, are reported. Electron emission is observed which depends on the direction of polarisation relative to the surface. The effect may be attributed to multiphoton processes.
When high p o w e r l a s e r b e a m s a r e a b s o r b e d in g a s e s the f r e e e l e c t r o n s a r e p r o d u c e d by multiphoton ionization, as p r e d i c t e d by the t h e o r y [1,2] and c o n f i r m e d e x p e r i m e n t a l l y [3-6]. On the o t h e r hand, the multiphoton photoeffect (or f i e l d e m i s sion) s i m i l a r l y p r e d i c t e d f o r the a b s o r p t i o n of high p o w e r l a s e r b e a m s by s o l i d s [7] could not be e x p e r i m e n t a l l y c o n f i r m e d yet b e c a u s e of the p r e domin an ce of the R i c h a r d s o n effect induced by c l a s s i c a l a b s o r p t i o n in the e x p e r i m e n t s r e p o r t e d so f a r [8,9]. To get e x p e r i m e n t a l e v i d e n c e on the m u lt ip h o ton photoeffect the e l e c t r o n e m i s s i o n of m e t a l s was m e a s u r e d within our e x p e r i m e n t a l p r o g r a m m e [10] un d er e x p e r i m e n t a l conditions with the R i c h a r d s o n effect kept negligible and a s t r o n g dependence of e l e c t r o n e m i s s i o n on the d i r e c t i o n of p o l a r i s a t i o n of the incoming light b e a m was observed. A high p o w e r light pulse of T = 30 nsec duration was g e n e r a t e d by a ruby l a s e r with a p a s s i v e Qswitch. This light p u ls e p a s s e d through a G la n T h o m p s o n p r i s m , s t r i k i n g at angle of in c i d e n c e 0 = 87 ° the m a t e r i a l to be i n v e s t i g a t e d . The light b e a m r e a c h e d the t a r g e t by p a s s i n g through a s l i t which p r e v e n t e d the e d g e s of the t a r g e t to be il l u m i n a t e d . The d i r e c t i o n of p o l a r i s a t i o n of the incident light, i . e . the angle ~ of the plane of p o l a r i s a t i o n of the e l e c t r i c v e c t o r to the plane of i n c i d e n c e , can be v a r i e d by r o t a t i o n of the G la n T h o m p s o n p r i s m . The v a r i a t i o n of the p a r a l l e l I El, i and p e r p e n d i c u l a r 1E±I components of the e l e c t r i c field v e r s u s (p is shown in fig. 1. The l a s e r b e a m was not f o c u s e d on the t a r g e t and the peak p o w e r density of the light p u ls e on the s a m ple was only 0.5 M W / c m 2. In the p r e s e n t i n v e s tigation as t a r g e t s i l v e r was c h o s e n in o r d e r to e l i m i n a t e the l i n e a r photoeffect. (The w o r k func-
•
U
~x
E~
5
20
~
60
~o
~°
Fig. 1, Values of the peak electron c u r r e m / m a x and of
the parallel Ehl and perpendicular E± components of the electric field plotted versus angle ~ . q~ is the angle between the plane of polarisation of the electric vector and the plane of incidence. lion of s i l v e r is 4.7 eV which e x c e e d s the 1.8 eY quantum e n e r g y of single photons). The c a l c u l a t e d i n c r e a s e in t a r g e t t e m p e r a t u r e due to the light p u l s e n e v e r e x c e e d e d AT = 10oc, owing to the high r e f l e c t a n c e of s i l v e r at the angle of i n ci d en ce 0 = 87 ° and to the r e l a t i v e l y s m a l l p o w e r density used. The t h e r m i o n i c e m i s s i o n c a u s e d by the R i c h a r d s o n effect was thus n e g l i gible, in c o m p a r i s o n with the signal c u r r e n t observed. The vacuum in the bulb containing the t a r g e t
Volume 24A, number 2
PHYSICS LETTERS
was 10 -8 T o r r . To c o l l e c t the e l e c t r o n s e m i t t e d f r o m the t a r g e t by the l a s e r p u l s e , a p o t e n t i a l of U = 1000 V was applied to the m e a s u r i n g c e l l . Intensity m e a s u r e m e n t s with both p h o t o m u l ° t i p l i e r and c a l o r i m e t e r showed the i n t e n s i t i e s of the b e a m f al l i n g on the t a r g e t to be equal f o r the d i r e c t i o n s of p o l a r i s a t i o n with ~ = 0 ° and q~ = 90 ° The a m p l i t u d e ?max of the s i g n a l p r o d u c e d by the e m i t t e d e l e c t r o n s was m e a s u r e d by o s c i l l o sc ope at d i f f e r e n t d i r e c t i o n s of p o l a r i s a t i o n (see fig. 1). The m a x i m u m e l e c t r o n e m i s s i o n was obs e r v e d at q~ ~ 20 o. This c o i n c i d e s with the m a x i m u m value of the p a r a l l e l c o m p o n e n t E H (i. e. the component of the e l e c t r i c f i e l d p a r a l l e l to the plane of i n c i d e n c e ) . The duration of the s i g n a l s was o b s e r v e d to be s m a l l e r than that of the light pulse, As a p p a r e n t f r o m fig. 1 a s t r o n g dependence of the e l e c t r o n e m i s s i o n on the d i r e c t i o n of p o l a r i s a t i o n was o b s e r v e d . At the bulb p r e s s u r e s u s e d the p r e s e n c e of a m o n a t o m i c l a y e r on the s a m p l e may have s o m e influence on the e m i s s i o n p r o c e s s . The shape of the s i g n a l was found to depend slightly on p o l a r i sa ti on. The e l e c t r o n e m i s s i o n o b s e r v e d may be e x p l a i n e d by a l t e r n a t i v e o r c o m b i n e d multiphoton photoeffect, fi el d e m i s s i o n or i n v e r s e B r e m s strahlung.
MEASUREMENTS IN AN A R G O N
OF THE SINGLE
16 January 1967
I n v e s t i g a t i o n on the dependence of e l e c t r o n y i e l d on the m a t e r i a l used on the p o w e r density, as well as on the r o l e of the m o n a t o m i c l a y e r and the d i f f e r e n c e in the e l e c t r o n s i g n a l shape a r e in progress. Thanks a r e due to M r . J . Bakos, Mr . E. F a z e k a s , M r . I. Czig~iny, M r . L. I m r e and M r s . Zs. SzUts.
References 1. A.Gold and B. Bebb, Phys.Rev. Letters 14 (1965) 60. 2. L.V. Keldish, Zh.Eksp. i Teor. Fiz.47 (1964)1945. 3. G.S. Voronov, G.A. Delone and H. B. Delone, JETP Letters 3 (1966) 480. 4. A.I. Peressini, Physics on quantum electronics, Puerto Rico, 1965 (McGraw-Hill, New York, 1966) p. 499. 5. Che Jen Chen, Phys.Rev. Letters 16 (1966). 833. 6. R.Rompe, E. Hantzsehe and H.Wolf, Communication on the Conference Laser und ihre Anwendungen, 1966, Berlin. 7. F.V. Bunkin and M.V. Fedorov, Zh.Eksp. i Teor. Fiz. 48 (1965) 1341. 8. J.F.Ready, Phys°Rev°137A (1965) 620. 9. M.Januzzi and R.Williamson, Nuovo Cim. 36 (1965) 1130. 10. Gy. Farkas, Zs. N~ray and P. Varga, KFKI KSzl. 14 (1966) 129.
LONGITUDINAL SOUND VELOCITY CRYSTAL AT 76.8OK AND 4.2°K
M. GS~NGER, H. E GG ER and E. LOSCHER
Physics Department, Technische Hochscule M~nchen, Germany Received 3 December 1966 The longitudinal sound velocities in the [llO]-direction of an argon single crystal have been determined to be 1446 * 7 m / s e c at 4.2°K and 1291 * 6 m / s e c at 76.8°K.
M e a s u r e m e n t s of the sound v e l o c i t i e s in s o li d a r g o n a r e of i m p o r t a n c e b e c a u s e they p r o v i d e i n f o r m a t i o n about the i n t e r a t o m i c f o r c e s in r a r e gas s o l i d s . S e v e r a l d e t e r m i n a t i o n s of the l o n g i tudinal v e l o c i t y of sound in s o l id a r g o n above 50°K have p r e v i o u s l y been r e p o r t e d by v a r i o u s a u t h o r s [1,2]. We wish to r e p o r t our m e a s u r e m e n t s of the longitudinal sound v e l o c i t y in s o li d
a r g o n which, to our knowledge, a r e the f i r s t to be p e r f o r m e d on a l a r g e s i n g l e c r y s t a l , whose quality and o r i e n t a t i o n have been d i r e c t l y d e t e r m i n e d by neutron s c a t t e r i n g m e t h o d s . The a r g o n c r y s t a l s a m p l e was p r e p a r e d f r o m c o m m e r c i a l l y obtained argon, whose i m p u r i t y content was l e s s than 4 p . p . m . *. * Supplied by the Edelgas GmbH, Diisseldorf. 135
16 January1967
PHYSICS LETTERS
DEPENDENCE OF NON-CLASSICAL ELECTRON METALS O N T.HE D I R E C T I O N OF POLARISATION
EMISSION FROM OF LASER BEAMS
Gy. FARKAS, Zs. N.~RAY and P. VARGA
Central Research Institute for Physics, Budapest, Hungary Received 5 December 1966
Electron emission experiments on silver irradiated by a high power laser beam, performed under conditions with negligible Richardson effect, are reported. Electron emission is observed which depends on the direction of polarisation relative to the surface. The effect may be attributed to multiphoton processes.
When high p o w e r l a s e r b e a m s a r e a b s o r b e d in g a s e s the f r e e e l e c t r o n s a r e p r o d u c e d by multiphoton ionization, as p r e d i c t e d by the t h e o r y [1,2] and c o n f i r m e d e x p e r i m e n t a l l y [3-6]. On the o t h e r hand, the multiphoton photoeffect (or f i e l d e m i s sion) s i m i l a r l y p r e d i c t e d f o r the a b s o r p t i o n of high p o w e r l a s e r b e a m s by s o l i d s [7] could not be e x p e r i m e n t a l l y c o n f i r m e d yet b e c a u s e of the p r e domin an ce of the R i c h a r d s o n effect induced by c l a s s i c a l a b s o r p t i o n in the e x p e r i m e n t s r e p o r t e d so f a r [8,9]. To get e x p e r i m e n t a l e v i d e n c e on the m u lt ip h o ton photoeffect the e l e c t r o n e m i s s i o n of m e t a l s was m e a s u r e d within our e x p e r i m e n t a l p r o g r a m m e [10] un d er e x p e r i m e n t a l conditions with the R i c h a r d s o n effect kept negligible and a s t r o n g dependence of e l e c t r o n e m i s s i o n on the d i r e c t i o n of p o l a r i s a t i o n of the incoming light b e a m was observed. A high p o w e r light pulse of T = 30 nsec duration was g e n e r a t e d by a ruby l a s e r with a p a s s i v e Qswitch. This light p u ls e p a s s e d through a G la n T h o m p s o n p r i s m , s t r i k i n g at angle of in c i d e n c e 0 = 87 ° the m a t e r i a l to be i n v e s t i g a t e d . The light b e a m r e a c h e d the t a r g e t by p a s s i n g through a s l i t which p r e v e n t e d the e d g e s of the t a r g e t to be il l u m i n a t e d . The d i r e c t i o n of p o l a r i s a t i o n of the incident light, i . e . the angle ~ of the plane of p o l a r i s a t i o n of the e l e c t r i c v e c t o r to the plane of i n c i d e n c e , can be v a r i e d by r o t a t i o n of the G la n T h o m p s o n p r i s m . The v a r i a t i o n of the p a r a l l e l I El, i and p e r p e n d i c u l a r 1E±I components of the e l e c t r i c field v e r s u s (p is shown in fig. 1. The l a s e r b e a m was not f o c u s e d on the t a r g e t and the peak p o w e r density of the light p u ls e on the s a m ple was only 0.5 M W / c m 2. In the p r e s e n t i n v e s tigation as t a r g e t s i l v e r was c h o s e n in o r d e r to e l i m i n a t e the l i n e a r photoeffect. (The w o r k func-
•
U
~x
E~
5
20
~
60
~o
~°
Fig. 1, Values of the peak electron c u r r e m / m a x and of
the parallel Ehl and perpendicular E± components of the electric field plotted versus angle ~ . q~ is the angle between the plane of polarisation of the electric vector and the plane of incidence. lion of s i l v e r is 4.7 eV which e x c e e d s the 1.8 eY quantum e n e r g y of single photons). The c a l c u l a t e d i n c r e a s e in t a r g e t t e m p e r a t u r e due to the light p u l s e n e v e r e x c e e d e d AT = 10oc, owing to the high r e f l e c t a n c e of s i l v e r at the angle of i n ci d en ce 0 = 87 ° and to the r e l a t i v e l y s m a l l p o w e r density used. The t h e r m i o n i c e m i s s i o n c a u s e d by the R i c h a r d s o n effect was thus n e g l i gible, in c o m p a r i s o n with the signal c u r r e n t observed. The vacuum in the bulb containing the t a r g e t
Volume 24A, number 2
PHYSICS LETTERS
was 10 -8 T o r r . To c o l l e c t the e l e c t r o n s e m i t t e d f r o m the t a r g e t by the l a s e r p u l s e , a p o t e n t i a l of U = 1000 V was applied to the m e a s u r i n g c e l l . Intensity m e a s u r e m e n t s with both p h o t o m u l ° t i p l i e r and c a l o r i m e t e r showed the i n t e n s i t i e s of the b e a m f al l i n g on the t a r g e t to be equal f o r the d i r e c t i o n s of p o l a r i s a t i o n with ~ = 0 ° and q~ = 90 ° The a m p l i t u d e ?max of the s i g n a l p r o d u c e d by the e m i t t e d e l e c t r o n s was m e a s u r e d by o s c i l l o sc ope at d i f f e r e n t d i r e c t i o n s of p o l a r i s a t i o n (see fig. 1). The m a x i m u m e l e c t r o n e m i s s i o n was obs e r v e d at q~ ~ 20 o. This c o i n c i d e s with the m a x i m u m value of the p a r a l l e l c o m p o n e n t E H (i. e. the component of the e l e c t r i c f i e l d p a r a l l e l to the plane of i n c i d e n c e ) . The duration of the s i g n a l s was o b s e r v e d to be s m a l l e r than that of the light pulse, As a p p a r e n t f r o m fig. 1 a s t r o n g dependence of the e l e c t r o n e m i s s i o n on the d i r e c t i o n of p o l a r i s a t i o n was o b s e r v e d . At the bulb p r e s s u r e s u s e d the p r e s e n c e of a m o n a t o m i c l a y e r on the s a m p l e may have s o m e influence on the e m i s s i o n p r o c e s s . The shape of the s i g n a l was found to depend slightly on p o l a r i sa ti on. The e l e c t r o n e m i s s i o n o b s e r v e d may be e x p l a i n e d by a l t e r n a t i v e o r c o m b i n e d multiphoton photoeffect, fi el d e m i s s i o n or i n v e r s e B r e m s strahlung.
MEASUREMENTS IN AN A R G O N
OF THE SINGLE
16 January 1967
I n v e s t i g a t i o n on the dependence of e l e c t r o n y i e l d on the m a t e r i a l used on the p o w e r density, as well as on the r o l e of the m o n a t o m i c l a y e r and the d i f f e r e n c e in the e l e c t r o n s i g n a l shape a r e in progress. Thanks a r e due to M r . J . Bakos, Mr . E. F a z e k a s , M r . I. Czig~iny, M r . L. I m r e and M r s . Zs. SzUts.
References 1. A.Gold and B. Bebb, Phys.Rev. Letters 14 (1965) 60. 2. L.V. Keldish, Zh.Eksp. i Teor. Fiz.47 (1964)1945. 3. G.S. Voronov, G.A. Delone and H. B. Delone, JETP Letters 3 (1966) 480. 4. A.I. Peressini, Physics on quantum electronics, Puerto Rico, 1965 (McGraw-Hill, New York, 1966) p. 499. 5. Che Jen Chen, Phys.Rev. Letters 16 (1966). 833. 6. R.Rompe, E. Hantzsehe and H.Wolf, Communication on the Conference Laser und ihre Anwendungen, 1966, Berlin. 7. F.V. Bunkin and M.V. Fedorov, Zh.Eksp. i Teor. Fiz. 48 (1965) 1341. 8. J.F.Ready, Phys°Rev°137A (1965) 620. 9. M.Januzzi and R.Williamson, Nuovo Cim. 36 (1965) 1130. 10. Gy. Farkas, Zs. N~ray and P. Varga, KFKI KSzl. 14 (1966) 129.
LONGITUDINAL SOUND VELOCITY CRYSTAL AT 76.8OK AND 4.2°K
M. GS~NGER, H. E GG ER and E. LOSCHER
Physics Department, Technische Hochscule M~nchen, Germany Received 3 December 1966 The longitudinal sound velocities in the [llO]-direction of an argon single crystal have been determined to be 1446 * 7 m / s e c at 4.2°K and 1291 * 6 m / s e c at 76.8°K.
M e a s u r e m e n t s of the sound v e l o c i t i e s in s o li d a r g o n a r e of i m p o r t a n c e b e c a u s e they p r o v i d e i n f o r m a t i o n about the i n t e r a t o m i c f o r c e s in r a r e gas s o l i d s . S e v e r a l d e t e r m i n a t i o n s of the l o n g i tudinal v e l o c i t y of sound in s o l id a r g o n above 50°K have p r e v i o u s l y been r e p o r t e d by v a r i o u s a u t h o r s [1,2]. We wish to r e p o r t our m e a s u r e m e n t s of the longitudinal sound v e l o c i t y in s o li d
a r g o n which, to our knowledge, a r e the f i r s t to be p e r f o r m e d on a l a r g e s i n g l e c r y s t a l , whose quality and o r i e n t a t i o n have been d i r e c t l y d e t e r m i n e d by neutron s c a t t e r i n g m e t h o d s . The a r g o n c r y s t a l s a m p l e was p r e p a r e d f r o m c o m m e r c i a l l y obtained argon, whose i m p u r i t y content was l e s s than 4 p . p . m . *. * Supplied by the Edelgas GmbH, Diisseldorf. 135