Exploiting the unique time-structure of synchrotron radiation at SSRL

NUCLEAR

INSTRUMENTS

AND METIIODS

152 ( 1 9 7 8 )

255-259

, ©

NORTH-HOLLAND

PUBLISHING

CO

EXPLOITING THE UNIQUE TIME-STRUCTURE OF SYNCHROTRON RADIATION AT SSRL ÷ K M M O N A H A N ' and V

REHN ~'

Michelson Labotatopy. NWC. Ch;na Lake. C.4 93555. U S A

Synchrotron radiation arnves at SSRL in subnanosecond pulses w~th an mterpulse period of 780 ns The unique combination of short pulses and a relative]y long mterpulse period makes SSRL a highly attracuve source for h m m g experiments The discussion covers time-resolved photoluminescence emission and exc~tauon spectroscopy as well as nolse-reducuon techmqucs and hfetJmc measurements In the measurement of hfct~mes less than ~ I ns, problems related to variations in pulse duration, amplitude and period are d~scusscd along w~th potential soluuons

1. Introduction: source characteristics

Synchrotron radiation (SR) at the Stanford Synchrotron Radiation Laboratory (SSRL) is characterized by an intense, continuous spectral distribution from the IR to X-ray regions, a highly collimated and polarized beam, and a pulsed time-structure governed by the orbital period of electron bunches m the storage ring l) The advantages of this last characteristic have attracted the interest of research groups in Orsay (R Lopez-Delgado et al.), Berkeley (D A Shirley et al ), China Lake (K. M. Monahan and V Rehn) and Hamburg (G Zimmerer et ai ) All of these, with the exception of the Hamburg group, have at one time used the 8° VUV line at SSRL for time-resolved measurements The SR time-structure at SSRL is uniquely suited for this purpose by virtue of its very sharp pulses and relatively long lnterpulse period. SPEAR, the positron-electron storage ring which Is the source of synchrotron radiation at SSRL, operates m a single bunch, colliding beam mode The rf accelerating field operates at 358 MHz, the 280th harmonic or the 128 MHz orbital frequency Thus, one may visualize a single bunch of electrons traveling around the ring in only one of 280 electromagnettc buckets producing a pulse of SR every 780 ns Circulating m the opposite direction is a single bunch of positrons

which effectively sweeps any stray electrons from the beam. Although, at relativistic velocities, both electrons and positrons emit synchrotron radiation, the SR ports at SSRL are set up to intercept only radiation emanating from the electron bunch in a direction tangent to the electron orbit. The duration, amplitude and period of SR pulses are precisely correlated with the longitudinal spread, population and orbital period of the electron bunch. The pulse duration is known to be a strong function of beam current (bunch population) and can range from 0 06 to 0 4 ns (see fig 1) Both the pulse duration and amphtude are affected by synchrotron oscillations in the bunch2). The amplitude can fluctuate from one half to twice the time - - averaged amplitude at frequencies of 25 kHz In addition, random dephaslng of the 1.55 GeV Do~a

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Resolution 120 ps FWHM

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200ps

~ Work supported by the NWC Independent Research Fund

and NSF Grant No DMR73-07692 in cooperation with SLAC and ERDA + NRC Resident Research Associate Madmg address Stanford Synchrotron Radmt~on Laboratory, Bin 69, SLAC, P O Box 4349, Stanford, CA 94305 ++ Until June 1978, Visiting Scientist, Lawrence Berkeley Laboratory, Matenals and Molecular Research D~v~sion, Bldg 70A, Berkeley, CA 94720, and Visiting Senior Research Associate, Stanford Synchrotron Radiation Laboratory

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Bosei,~eJ Fill 1 SR pulse from SPEAR at low (lop) and high (bottom) beam current Deconvolvinll the instrument resolution

( ~ 120 ps) shows those pulse widths to be about 80 and 400 ps, respectively (ref 2)

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256

K

M

MONAIIAN

electron bunch w~th respect to the rf field produces a 200ps jitter m the mterpulse period so that reference t~mes based on the rf clock can be considered accurate only to one part m five thousand. All of these changes m pulse duration, amphtude and period are normally e~ther neghgtble or easily compensated for experimentally.

2. Instrumentation The instrumentation used by the China Lake Group for time-resolved spectroscopy (TRS) at SSRL ts shown schematically m fig. 2 Synchrotron radiation from SPEAR, dBpersed by the McPherson 231-M3 Seya-Namloka exotatlon monochromator (EMC), exotes a fluorescent sample m an ultra-high vacuum, sample chamber. A collecting mirror focuses the fluorescence onto the entrance sht of the McPherson 218 analyzing monochromator (AMC). Pulses from the photomult~pher (PM) mounted at the exit sht are amplified (AMP) and fed through a discriminator (DIS) into the START input of a t~me-to-amphtude converter (TAC). The 1.28 MHz reference signal from the SPEAR rf clock ts fed through a discriminator, an adjustable delay, and a pulse generator into the STOP input of the TAC The adjustable delay ts necessary to ensure that the reference s~gnal coincides with the SR hght pulse from the detector The TAC output ~s used to perform the three separate TRS experiments dlustrated m fig 3 Time-resolved fluorescence decay (TRFD) curves are obtained by putting the TAC output into a

AND

V

REHN

multi-channel analyzer (MCA) operating m the pulse-height analys~s mode The fluorescence decay data may then be d~splayed on the oscdloscope (OSC) or dumped into the memory of the central processing umt (CPU). Alternatively, t~me-resolved excitatxon (TREX) and time-resolved emission (TREM) spectra are obtained by putting the TAC output through a single channel analyzer (SCA) and using the CPU coupled with an on-hne counter as a multi-channel scaler. Data m the CPU memory can then be dumped on magnetic tape (TAPE) or plotted on chart paper (XY). The time-resoluuon of the system described above is entirely adequate for the measurement of lifetimes greater than a few nanoseconds. The electromc response function coupled with that of a typical fast photomult~pher, such as the RCA31034A (GaAs) or the E M R 5 1 0 G (solar bhnd), broadens the cumulatwe dlummatlon function, ln(t'), to about 1.5 ns fwhm, as shown m Eex, Eem

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2

S c h e m a t , c o f i n s t r u m e n t a t i o n u s e d for t , m e - r e s o l v e d

spectroscopy ExcJtauon monochromator (EMC), analyzing monochromator (AMC), photomultlpher (PM), amphfier (AMP), discriminator (DIS), time-to-amplitude converter (TAC), multichannel analyzer (MCA), single channel analyzer (SCA), central processing unit (CPU), oscilloscope (OSC), chart recorder (XY) and magnetic tape drive (TAPE)

, m e n t s At t h e top, fluorescence i n t e n s , t y is plotted as a function o f t i m e after t h e excitation pulse T h e excitation and e m i s s i o n energies, Eex a n d E e m , are set on t h e E M C and AMC, respectively In the m~ddle, t h e t i m e acceptance-gate Ter n , is set on the S C A , Ecru ts held c o n s t a n t as beforc, and the fluorescence i n t e n s i t y is plotted as a f u n c t i o n excitatior energy At t h e b o t t o m , Tern a n d £ex are held c o n s t a n t , and t h e fluorescence i n t e n s i t y ,s plotted as a f u n c t i o n o f e m i s s i o r energy

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turn beats) in the TRFD with a frequency proportional to the energy separation TREX spectr#) of

fig 4 N

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(1)

tl=|

/

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quasi-Gausslan For critical a p p l i c a t i o n s , as in the m e a s u r e m e n t

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of very short lifetimes (~ 1 ns), it is necessary to abandon the SPEAR rf clock as a reference source and take the reference signal directly From a stripline antenna located inside the storage ring For ultra-short hfeumes ( ~ 0 1 n s ) , t t n s essential to deconvolve the fluorescence decay function, /'(t-t'), From the tllummation function, lo~(t') Disregarding the PM and electronic response functlons, we may write a simple expression for the cumulative fluorescence intensity, l(t), observed on the MCA display after N mterpulse periods

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Fig 4 Cumulatnvc )llummatnon functuon (top) measured w~th an R C A 31034A p h o t o m u l t t p h c r The same data plotted on a log scale (bottom) to show weak structure m the PM response function 105

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where the mtegrand is assumed to be well-behaved and the hmits, _+6, are intended to indicate the time interval ovcr which the excitation pulse IS effective. The significance of the result in eq (4) is that only the cumulative illumination function need be measured in order to deconvolve the fluorescence decay function from the experimentally observed, cumulative fluorescence intensity Ideally, I(¢) and lo(t') should be obta;ned when the SPEAR operating parameters (l e beam current and electron energy) are nearly constant, as is the usual case during steady, stable operating periods

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Examples of time-resolved spectroscopy on the 8~ VUV line at SSRL are shown m figs 5-7. The TRFD curve 3) of fig. 5 illustrates the advantage of a very sharp SR excitation pulse for simultaneous excitation of two closely spaced, Zeeman-spht, atomic levels in Kr vapor. Interference between the two emittmg levels produces modulation (quanVII

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Fig 5 Fluoresccnt decay of the 5s (3/2) level of atomic Kr (top) Quantum beats m the fluorescent decay of the Zccmanspilt 5s (3/2) level of atomtc Kr (bottom) The detector us an EMR 510G solar blind photomulttpher (ref 3)

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F~g 7 T ~ m e - r e s o l v e d emission s p e c t r a (at ~ 3 5 K ) o f K r O c x o m e r l u m i n e s c e n c e m sohd krypton doped with 1% N 3 0 T h e rime acceptance-gate ~s from D to D + A after the exotat~on pulse

the intrinsic luminescence m solid Xe, shown m fig 6, demonstrate the advantage of the long ~nterpulse period at SSRL by their striking contrast The 0 - 5 0 n s spectrum, dominated by scattered photons from the "prompt" excitation pulse, is essentially a reflecnon spectrum, the 5-30 ns spectrum resembles conventional excitation spectra; but the 150-470 ns spectrum reveals fascinating new information about the long-lived components of the solid Xe luminescence L~kew~se, the long period makes possible the two TREM spectra ~) shown m fig. 7 Comparing these, one can immediately note that the 2 09 and 2 20 eV features of the KrO e x o m e r emission in solid Kr have long but roughly comparable hfetlmes at ~ 3 5 K The SR trine-structure at SSRL is thus uniquely suited for purposes requiring either very sharp pulses or a relatively long lnterpulse period The sharp pulses permit measurement of quantum beats and nanosecond hfeumes6), while the long lnterpulse pertod makes possible TREX and TREM spectra such as those m fig 6c and fig. 7 Benmst d'Azy et al 7), have described a method of

analyzing "fluorescence pale-up" to obtain lifetime measurements at storage rings with shorter interpulse periods (73 ns at ACO), but, m most cases, this method breaks down if an emission has more than one lifetime component. The same method can be used at SSRL to measure single component decay times of several microseconds. As an added dividend, the 780 ns period at SSRL has been used to facilitate determining photo-electron energies by time-of-flight s) and to improve signal-to-noise rauos m the measurement of prompt phenomena such as reflectance, transmission, or non-specular scattering The latter resembles a TREX measurement where the time gate is set to accept only prompt photons A 50-fold reducnon in dark count was obtained with a sodium salicylate-coated PM, permitting the measurement of angle-resolved scattering from extremely smooth surfaces in the VUV for the first nine 9) One of us~°), has suggested another extension of TREX technique to the measurement of X-ray Induced UV luminescence m alkali hahdes.

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References I) I Ltndau and H Wmtck, Proc 4th Conf on Sctenttlqc and industrial apphcattons o~ small accelerators. SSRL Report No 76/12 2) p B Wilson, R Servranckx, A P Sabersky, J Gareyte, G E Fischer and A W Chao. Proc 1977 Particle Accelerator C o n f , Chicago, Illinois, March 16-18, 1977, and A P Sabcrsky (private commumcatlon) 3) E Matthms, M G White, R A Rosenberg, E D Pohakoff. S T Lce and D A Shlrely (unpublished data) 4) K M Monahan, V Rchn, E Matthlas and E D Pohakoff

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(unpublished data) s) K M Monahan and V Rehn (unpublished data) 6) R Lopez-Dclgado, J A Mlehe and B Sipp, SSRL Report No 76/04 7) O Benolst d'Azy. R Lopez-Delgado and A Trainer. Chem Phys 9 (1975) 327 8) R Z Bachrach, M Sklbowskl and F C Brown, Phys Rev Lett 37 (1976) 40 9) A D Baer and V Rehn. Angle-resolved scattering b) .%mooth surlaces m the ultraviolet ()n preparation) 10) A Blancom, D Jackson and K M Monahan (submitted to Phys Rcv B)

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