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Submitted on 1 Jan 1979
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SIMPLE TECHNIQUE FOR OBSERVING QUANTUM BEATS
G. Michael Lanham, John Stoner, Jr
To cite this version:
G. Michael Lanham, John Stoner, Jr. SIMPLE TECHNIQUE FOR OBSERVING QUANTUM BEATS. Journal de Physique Colloques, 1979, 40 (C1), pp.C1-340-C1-341. �10.1051/jphyscol:1979173�.
�jpa-00218453�
JOURNAL DE PHYSIQUE Colloque C1, suppldment au n O 2, Tome 40, fdvrier 1979, page C1-340
SrMPLE TECHNIQUE FOR OBSERVING QUANTUM BEATS*
6. Michael Lanham
-
John 0. Stoner, J r .Department of Physics, The University of Arizona, Tucson, Arizona 85721 Abstract. I f a luminous p a r t i c l e beam i s imaged onto the plane of the entrance s l i t of a spectrometer, s p a t i a l structure in the beam due t o quantum beats, decay, e t c . , appears in each image of the beam i n the e x i t - s l i t plane. By using a wide entrance s l i t and a narrow e x i t s l i t , and scanning an isolated spectral l i n e , one can observe quantum beats, short l i f e t i m e s , e t c . , without moving t h e f o i l , spectrometer, o r coupling optics. We have demonstrated t h i s technique using Stark-induced quantum beats i n Hg.
Quantum beats in l i g h t emitted from f o i l - excited ion beams have been used t o study f i n e s t r u c t u r e , hyperfine s t r u c t u r e , Stark e f f e c t s , Zee- man e f f e c t , and coherence produced i n t h e e x c i t a t i - on process. ( I ) We report a simple method f o r ob- serving such beats.
A conventional grating spectrometer pro- duces an image of t h e entrance s l i t in t h e plane of the e x i t s l i t f o r each wavelength present i n t h e illuminating radiation. Variations in i n t e n s i t y across t h e entrance s l i t w i l l appear i n every mono- chromatic image i n the e x i t - s l i t plane. I f t h e in- cident radiation, consists of well-separated spec- t r a l l i n e s , the image of each of these w i l l show i t s p a r t i c u l a r s p a t i a l structure. Rotation of t h e grating moves these images across the e x i t s l i t . S e t t i n g t h e e x i t s l i t much narrower than the en- trance s l i t permits the study of t h e s p a t i a l struc- t u r e of each image as the "wavelength" s e t t i n g of the spnctrometer i s varied. I f an excited ionbeam i s imaged onto and perpendicular t o the entrance s l i t , s t r u c t u r e due t o quantum beats o r ordinary
removed by refocusfng.(2) The relationship between position along t h e beam and wavelength s e t t i n g varies w i t h wavelength (although c a l i b r a t i o n i s simp1 e )
.
Me t e s t e d t h i s method using Stark beats i n the HB l i n e of atomic hydrogen. H: ions a t 280 keV were passed through pairs of 6 pg/cm2 carbon f o i l s . The excited beam was imaged (Fig. 1)- onto the entrance s l i t of a 1-meter Czerny-Turner spectrometer via a spherical mirror and a cylindri- cal lens, providing a demagnification o f 1:5. W i t h the entrance s l i t w i d t h a t 2.8 mm we could view a length of 14 mm a t t h e ion beam. This corresponded t o varying the wavelength s e t t i n g of t h e spectrom- e t e r by about 20
a.
decay can be displayed without trans1 a t i n g t h e
f o i l , spectrometer, or coupling o p t i c s . The scan- Fig. 1
-
Schematic representation of t h e apparatus.ning of this s t r u c t u r e can be as reproducible a s The grounded f o i l wheel (F) holds pairs of permitted by t h e wavelength d r i v e of t h e spectrom- carbon f o i l s i n s e r i e s . The optical sys-
tem produces a demagnified image of the e t e r .
This method has several obvious l i m i t a t i - beam on the entrance s l i t . E = screen elec- ons. A spectral l i n e of i n t e r e s t m u s t be isolated trode f o r applying e l e c t r i c f i e l d t o beam;
from nearby l i n e s when a wide entrance s l i t i s I = ion beam, C = Faraday cup, M1 = plane de- used. The e n t i r e s p a t i a l s t r u c t u r e of i n t e r e s t f l ecting mirror,
M2
= concave imaging m5.r-Tor, L = cylindrical l e n s , S =entrance s l i t (beats, decay curve) must be imaged onto t h e en-
t o spectrometer G . trance s l i t ; a demagnifying'optical coupling sys-
tem between ion beam and spectrometer may therefore
be necessary. Line broadening l i m i t s the a t t a i n - In order t o minimize noise due t o beam- able s p a t i a l resolution; t h i s may only in part be current fluctuations we square-wave modulated the
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979173
e l e c t r i c f i e l d applied t o the excited atoms between 0 and 45 V/cm a t 1500 Hz, and used lock-in detec- tion a t t h a t frequency. Fig. 2 shows typical ex- amples of t h e d.c. photomultiplier signal and t h e ( a . c . ) lock-in amplifier signal vs wavelength s e t - t i n g . Beats a r e c l e a r l y present i n the a .c.signal w i t h s ~ a c i n g consistent w i t h orevious observati- ons. ( 3 )
A.C.
1 ~i h
SIGNAL
iMB1TRdRY UNITS)
D.C.
Fig. 2
-
Top trace: Stark beats Pn the H (48611) s p e c t r a l l i n e . Theton
beam was B 15 H A o f HH+ ions a t 280 keV; the beam was subject- ed t o a square-wave e l e c t r i c f i e l d (0 and 45 vol ts/cm) of 1500 Hz frequency. The en- trance s l i t was s e t a t 2.8 mm and the e x f t s l i t was s e t a t .25 mm.Bottom trace: H (4861A) l i n e (d.c. signal):
The beam was the same as in a ) except a 0
constant f i e l d of 100 volts/cm was applied t o the emitting atoms. [With t h i s detec- tion method, no beats could be resolved a t any f i e l d strength because of beam-current fluctuations
.]
BIBLIOGRAPHY
[l] S.Bashkin, ed., BEAM-FOIL SPECTROSCOPY, Spring- Lett,
2,
477-479 (1971); Optica Acta XI-, 435-er-Verlag; Berlin (1976). 448 (1973).
H.J.Andrf, Ph>s.Scr.
9,
257-80 (1974).[3] S. Bashkin, W.S. Bickel, D. Fink, and R.K.
H.G.Berry, Rep.Prog. Phys.
9,
155-217 (1977).Wangness
,
Phys.
Rev. Letters15,
284 (1965).[2] J.O. Stoner, J r . , and J.A. Leavitt, Appl.Phys.
*This work was supported i n part by ONR and NSF.
