LIDARS FOR STUDYING THE CHARACTERISTICS

OF ATMOSPHERIC AEROSOLS

A. P. Ivanov, A. P. Chaikovskii, K. N. Dyatlov, and I. S. Khutko

UDC 551.508

With the appearance of pulsed l a s e r s fundamental ly new poss ib i l i t i es for studying the s t ruc tu re Of the a t m o s p h e r e and the phys ica l p r o c e s s e s occu r r ing in the a tmosphe re have a r i sen . L ida r s ( laser rangef inders ) , with which m a c r o - and mic ro inhomogene i t i e s in the a tmosphere can be probed r emo te ly a re used in this work. A l idar cons is t s of a l a s e r t r a n s m i t t e r , an optical s y s t e m r e c e i v e r , an optical de tec to r , and a r e c o r d e r . I ts opera t ing pr inc ip le is based on in te rpre t ing the echo signal produced by sending a shor t light pulse into a m e - dium. Since at any given m om en t the radia t ion incident on the r e c e i v e r comes f r o m a l imi ted por t ion of the med ium, a t ime scan of the detected signal contains informat ion about the spat ia l s t ruc tu re of the medium along the probe path. The main advantages of l idar s y s t e m s a re the i r abili ty to opera te at a d i s tance , the i r ease of opera t ion , the amount of informat ion they can provide , and their high spat ia l and t empora l resolut ion. In addition, as opposed to devices employing contact methods , l a s e r rangef inders do not d is turb the volume being observed .

The echo signal f r o m the med ium may be due to luminescence or to ae roso l , combination (i.e., Raman) , r e sonance , or mo lecu l a r sca t te r ing . Depending on the purpose , l ida rs can be used to study the ae roso l and gas composi t ion , the par t i c le concentra t ion and size d is t r ibut ion, the speed of winds and f lows, the exis tence of objects in the med ium, the t e m p e r a t u r e , p r e s s u r e , and humidity of the a t m o s p h e r e , and so on. The t r a n s - m i t t e r and r e c e i v e r components can be combined so that a single lens is used for sending a signal into the m e - dium and for detect ing the s ca t t e r ed radiat ion. Such s y s t e m s make it poss ib le to study the c h a r a c t e r i s t i c s of the env i ronment next to the rangef inder and a r e used, e .g . , in probing s eawa te r and fog. However , dur ing o p e r a - tion o v e r extended paths the range of var ia t ion of the energy of the detected signal is e x t r e m e l y l a rge and the dynamic range of the photodetec tors and detect ion s y s t e m s is g rea t ly inc reased .

In o rde r to avoid sca t t e r ed s ignals f r o m the near zone fall ing on the de tec to r , l idars with sepa ra t e t r a n s - m i t t e r s and r e c e i v e r s a re often used. In this case , a "dead zone," of length de te rmined by the dis tance be - tween the de tec tor and the source and by the i r ape r tu re angles , ex is t s along the probing path.

Work on l ida rs and l a s e r probing of the a tmosphe re was begun at the Insti tute of Phys ics of the Be lo- ru s s i an Academy of Sciences in 1965 by A. P. Ivanov, I. D. Sherbaf , and A. L. Skrelin. In this a r t i c le we desc r ibe s e v e r a l of these l idars which m e a s u r e ae roso l and molecu la r sca t t e r ing of light.

L-1 Fixed L idar for A tmosphe r i c Aeroso l Studies. The L-1 l idar is used for m e a s u r i n g the optical c h a r a c t e r i s t i c s of a t m o s p h e r e s at wavelengths of 0.53 and 1.06 ~m along hor izonta l and inclined paths. One fea tu re of this appara tus is the poss ib i l i ty of changing the base line z (distance between the source and r ece ive r ) of the l idar . This , combined with a fa i r ly powerful l ight source , o f fe rs a wide range of poss ib i l i t i es for probing clean and turbid a t m o s p h e r e s ove r extended paths . A block d i ag ram of the L - 1 is shown in Fig. 1. The t r a n s - m i t t e r 1 is fixed mounted on an optical bench 8 and sends a shor t l ight pulse at two wavelengths with a d i v e r - gence angle of 27tran into the medium to be studied. The r e c e i v e r 2 is located a dis tance z f r o m the source and picks up light backsca t t e r ed by the a tmosphe re into an angle 2Yrec. A specia l optical device , a beam sp l i t - t e r mounted behind the col lect ing lens , divides the rece ived flux of sca t t e red l ight into two spec t r a l components and sends these to appropr ia t e photodetec tors along sepa ra t e channels. The e l ec t r i ca l s ignals f r o m the photo- mul t ip l i e r s a r e fed into the r ecord ing s y s t e m through two coaxial cables .

The energy of the light sent into the a tmosphe re is moni tored by a v a c u u m photodiode. An e lec t ron beam osc i l lo sc rope is used as a r ecord ing s y s t e m in the l idar . The t r ace on the c a t h o d e - r a y t u b e is photographed on an R F - 3 photographic plate. A "Silu6t" device is used to r ead out the informat ion on the photograph and the digit ized data a r e ex t rac ted on punched tape.

Inst i tute of Phys ic s , Academy of Sciences of the Be lo russ i an SSR. Trans l a t ed f r o m Zhurnal Pr ikladnoi Spektroskopi i , Vol. 29, No. 6, pp. 1044-1052, D e c e m b e r , 1978. Original a r t i c l e submit ted July 10, 1978.

0021-9037/ '78/2912-1479507.50 �9 Plenum Publishing Corpora t ion 1479

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Fig. 1. A block diagram of the L-1 two-wavelength t r amsmi t t e r : 2) r e - ceiver sys tem; 3) l idar control unit; b) power supply unit; 5) cooling sys - tem; 6, 7) osci l loscopes; 8) optical bench; DL 1 and DL 2 delay lines.

A mechanical ly Q-switched lase r is used as a source of short light pulses in the lidar. The optical l ay- out of the laser is shown in Fig. 2. A neodymium doped glass rod of d iameter 10 and length 130 mm is used as the active element of the osci l la tor 1. An IFP-1200 flashlamp is used for pumping. The l a se r cavity is made up of a pile of plane-paral le l plates 5 and a total internal reflection p r i sm 4. The p r i sm 4 is attached to the shaft of a DID-2-TA motor which has a speed of up to 30,000-35,000 rpm. The output power of the light pulse at 1.06 ~m is 20-25 MW and the pulse length is 30 nsec. The osci l la tor output goes into the amplifier II. The active element of the amplif ier is a neodymium glass rod 6 whose ends are cut at the Brewste r angle. The amplif ier rod is pumped by two IFP-5000 flashlamps. The amplified power of the pulse is 70-90 MW. The l a se r beam passes through a nonlinear frequency conver ter 8 made of a c rys ta l of KDP. At the output of this c rys ta l there is light at two wavelengths: unconverted with k = 1.06 pro, and converted with ?, =0.53 pro. Par t of the light energy is deflected by two t ransparen t plane-paral le l plates 9 into Fl~K-11 and FI~K-09 vacuum photodiodes 11 which a re used to monitor the pa ramete r s of the initial probe pulses. The appropriate wave- length is separated f rom the total flux by a f i l ter 10 mounted immediately in front of the input window of the photocathode.

Figure 3 is an optical d iagram of the rece iver sys tem of the L-1. The main element is a standard "Uran- 16" ser ia l photographic lens with an input pupil d iameter of 210 mm and a focal length of 750 ram. The view- ing angle 2~frec is selected with the aid of a set of interchangeable diaphragms located in the focal plane of the lens. The diaphragms are made in the form of calibrated aper tu res of different d iameters dril led along a c i r - cle on the disk 2 ,whichcan be rotated about its center in the focal plane of the lens. The diaphragms are shifted remote ly f rom the control panel by means of an e lec t r ic motor mounted in the rece iver . Behind the disk with the diaphragms 2 there is an identical disk 3 with a set of neutral density f i l ters with optical dens i - t ies of 0-5. The f i l ters are changed by an e lec t r ic motor upon receipt of a command f rom the control panel. The attenuated flux enters the beam spl i t ter , a plane-paral le l wavelength-select ive plate 4 mounted at 45 ~ to the direct ion of the radiation. The plate t ransmi t s light with a wavelength of 1.06 ~m and ref lects that with k = 0.53 ~m. The detectors are Ft~U-83 and Fl~U-36 photomultipl iers. In front of each photomultiplier there is a narrow-band interference f i l ter .

St ructural ly , the t r ansmi t t e r and r ece ive r of the L-1 l idar are made in the form of separate compact assemblies . Both assembl ies are mounted on special plates , equipped with runners , and placed on the optical bench. The base line between the t r ansmi t t e r and rece iver can be changed f rom 0.45 to 3 m. The angle of inclination of the probing path is changed automatical ly on command f rom the control panel according to a previously chosen program.

The power supply and control sys tem are constructed as a single unit and serve to control the operat ion of all components of the lidar. On the front panel of the unit there are knobs for controll ing the operat ing r e - gime of the l a se r and for changing the diaphragms and neutral density f i l ters in the rece iver . The e lect ronics make it possible to operate in two t r ansmi t t e r r eg imes : single pulses and repeated pulses at a low repetit ion rate (0.02-0.04 Hz).

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/ I , I . 4

Fig. 2. The transmitter of the L-I lidar: I) oscillator; 2) IFP- 1200 flashlamp; 3) DID-2TA motor; 4) Q-switch prism; 5) stacks of plane-parallel plates; 6) neodymium glass rod; 7) IFP-5000 flashlamp; 8) K-DP frequency converter crystal; 9) beamsplitter pla tes ; 10) na r row-band light f i l t e r s ; 11) F]~K-11 vacuum photo- diode; 12) Ft~K-09; I) act ive e lement is a neodymium glass rod of length 130 and d i ame te r 10 m m ; II) ampl i f ie r .

5 6 7

Fig. 3. The optical layout of the r e c e i v e r s y s t e m of the L-1 l idar : 1) "Uran-16" ae r i a l photographic lens; 2) d isk with a se t of in- te rchangeable d iaphragms; 3) d isk with a se t of n e u t r a l - d e n s i t y f i l t e r s ; 4) p l ane -pa ra l l e l wave length se lec t ive plate; 5) m i r r o r plate; 6) in t e r fe rence f i l t e r s ; 7, 8) FI~:K- 36 and FI~U -83 photode tec tors , r espec t ive ly .

The L-1 has been used to develop l a s e r rangefinding techniques, to study the optical cha rac t e r i s t i c s of c l ea r and turbid a t m o s p h e r e s , and to study the s t ruc tu re of optical hazes produced by l a s e r s in a cloudless a tmosphe re [1-3].

L-1M Ai rborne Lidar . A number of p rob lems in me te ro logy and envi ronmenta l p ro tec t ion involve in- ves t iga t ions of the optical c h a r a c t e r i s t i c s of the a t m o s p h e r e over wide regions and in different c l imat ic zones of our planet. The spec i f ics of this p rob l em entai l the construct ion of a specia l c lass of optical appara tus , the a i rborne l idar . While re ta in ing the same bas ic design as fixed l ida r s , a i rborne l ida rs have a number of s t r u c - tura l f ea tu res which p r i m a r i l y take into account the m o r e s t r ingent r equ i r emen t s of re l iabi l i ty and l imited s i ze , weight, and power consumption. Since the main fac tor which pe rmi t s studies to be done over la rge r e - gions is the mobi l i ty of an a i rbo rne l a b o r a t o r y , the ranging dis tance of the l idar i t se l f is usual ly smal l .

At the Inst i tute of Phys ic s of the Be lo rus s i an Academy of Sciences the a i rborne L-1M l idar (Fig. 4) has been built for studying the optical c h a r a c t e r i s t i c s of the a tmosphe re at two wavelengths (0.53 and 1.06 pm) s imultaneously.

During opera t ion the l idar t r a n s m i t t e r sends a pulse of light at two wavelengths into the a tmosphe re through a side por t on the a i rp lane . The l idar r e c e i v e r detects the radia t ion sca t t e r ed by the a tmosphere and conver ts it into an e l ec t r i ca l signal which is r eco rded on the s c r een of a dua l -beam m e m o r y osci l loscope. The t r a ce s a r e then photographed on a plate. T h e informat ion is read off the plate in the manner descr ibed above.

The t r a n s m i t t e r in the L-1M is one of th ree in terchangeable specia l ly developed solid s tate l a s e r s with a single type of mounting and power supply connection. The L-1M includes the following sources in the equip- ment: a neodymium glass l a s e r with an ampl i f i e r and with f requency doubling; a neodymium glass l a s e r without an ampl i f i e r with f requency doubling; and a ruby l a s e r . The f i r s t two t r a n s m i t t e r s a re intended to opera te s imul taneous ly at two wavelengths (0.53 and 1.06 ~m) and the third ope ra t e s at a wavelength of 0.694 pm. The

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Fig. 4. Overal l view of the L-1M l idar .

connector design makes it poss ib le to rapidly change one source for another during an exper iment .

The neodymium glass l a s e r o s c i l l a t o r - a m p l i f i e r has the s ame design as in the L-1 . This source is mainly used for probing along long paths. For s h o r t - r a n g e probing the l a s e r without an ampl i f i e r is used. I ts power in the IR is 15-20 MW.

The light source with a wavelength of 694 nm is a "Groin" type ruby l a s e r [4] developed at the Insti tute of Phys ics of the Beloruss ian Academy of Sciences. The r e c e i v e r s y s t e m detects radiat ion at the wavelengths mentioned above. I ts design is s i m i l a r to that of the L-1.

An L-1M l idar was instal led on board an I1-18 a i rborne l abo ra to ry and used to study the var iab i l i ty in the optical c h a r a c t e r i s t i c s of the a tmosphere over extended hor izonta l paths [5, 6]. The expe r imen t s were done over var ious pa r t s of the Soviet Union at al t i tudes of up to about 10 kin. The cha rac t e r i s t i c sca les and d i spers ion of the dis tr ibut ion of optical c h a r a c t e r i s t i c s of the a tmosphe re were m e a s u r e d , along with their co r re la t ion with me te ro log ica l p a r a m e t e r s .

L-2 L idar for Spectra l and Pola r iza t ion Studies in the Atmosphere . R e s e a r c h on the fo rmat ion and t r a n s p o r t of a e r o s o l s , the solution of a number of c l imatological p r o b l e m s , the moni tor ing of a tmosphe r i c pollution, and other applied p rob l ems p resen t ly requi re e x t r e m e l y detai led informat ion on the m i c r o s t r u c - ture of the ae roso l component of the a tmosphere . It has been shown theore t ica l ly and exper imenta l ly that a l a rge p a r t of this information can be obtained f r o m spec t r a l m e a s u r e m e n t s ove r a wide wavelength range. This has become poss ib le with the development of new dye l a s e r s which can be tuned over a wide spec t ra l range.

Using this type of l a s e r the exper imen ta l L-2 device has been developed and const ructed to probe the a t m o s p h e r e in the v is ib le and nea r inf rared .

The L-2 consis ts of a t r a n s m i t t e r and r e c e i v e r . The cha rac t e r i s t i c s of the t r a n s m i t t e r have been d i s - cussed in [7]. The r e c e i v e r includes a m i r r o r object ive , a s y s t e m for separa t ing the sca t t e red radia t ion into two perpendicu la r ly po la r ized components , and a photodetector circuit . A te lescopic s y s t e m for the t r a n s - mi t t e r and a s y s t e m of ro ta table m i r r o r s for ensur ing coupling of the t r a n s m i t t e r and r e c e i v e r optics a r e mounted on the base of the r e c e i v e r (Fig. 5).

Pe rpend icu la r to the plane of the f igure and in the upper pa r t of the te lescope a s semb ly there a re two vacuum photodiodes (types F]~K-09 and Ft~K-11) for r ecord ing a r e f e r ence signal and synthesizing the detec- tor sys t em.

During opera t ion the r e c e i v e r and t r a n s m i t t e r a r e located re la t ive to one another so that the l a s e r beam exact ly coincides with the output ape r tu r e of the te lescopic s y s t e m through a s y s t e m of m i r r o r s 19 mounted on one of the a r m s of the U-shaped f r a m e . When the angle of inclination of the probing path is changed the moveable m i r r o r 19 ro ta tes by half the angle. In this way synchronous rotat ion of the optical axes of the t r a n s m i t t e r and r e c e i v e r is achieved.

When the wavelength dependence of the intensi ty of the sca t t e r ed light is being m e a s u r e d the signal r e - f lec ted by the a tmosphe re is detected by the of the photomul t ip l ie rs (FEU-84 or F]~U-83). When the degree of polar iza t ion of the sca t t e r ed l ight is being m e a s u r e d the signal is s imul taneously taken f r o m two identical photomul t ip l ie rs .

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5 2

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Fig. 5

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Fig. 5. The optical setup of the L-2 l idar : 1) m i r r o r object ive; 2) r e f l e c - tor ; 3) p ro tec t ive quar tz window; 4) disk with a se t of d iaphragms; 5) disk with a set of neutra l densi ty f i l t e r s ; 6, 7) knobs for changing d iaphragms and f i l t e r s , r e spec t ive ly ; 8, 9) photodetectors ; 10) r emovab le a s sembly ; 11, 13) axle pinions with ro ta to r mechan i sm; 12) knob for changing angle of e l e - vation; 14) t r a n s m i t t e r te lescope; 15) te lescope m i r r o r ; 16) p ro tec t ive quar tz window; 17) t r a n s m i t t e r ; 18) t r a n s m i t t e r control and power supply console; 19) movable m i r r o r s for aligning the t r a n s m i t t e r beam with the output a p e r - ture of the te lescope; 20) d iaphragm.

Fig. 6. The f ield ve r s ion of the "Glor iya" mul t i f requency l idar : 1) light source ; 2) s y s t e m for detect ing and moni tor ing the initial light; 3) cooling s y s - t em; 4) r e c e i v e r object ive; 5) blind; 6) beamsp l i t t e r a s s e m b l y and photodetec- t o r s ; 7) s y s t e m for m e a s u r i n g the polar iza t ion p a r a m e t e r s of the sca t t e red light; 8) U-shaped support f r a m e ; 9) stand.

The equipment is supplied f r o m the 220-V, 50-Hz ac power grid. The appara tus weights 400 kg.

In the fall of 1977 the L-2 l idar was used to inves t igate the m i c r o s t r u c t u r e of the a tmosphe r i c ae roso l in the city of Minsk [8]. Spectra l s tudies were made of the r e c i p r o c a l and total sca t t e r ing coefficients of the a tmosphe re ove r the range 0.475-1 ~m and f r o m these studies informat ion was der ived on the s ize dis tr ibut ion of the ae roso l pa r t i c l e s .

"Glor iya" Mult ifrequency Lidar . The mul t i f requency "Glor iya" l idar is an improved appara tus for spec - t r a l s tudies of the a tmosphe re . The main goal of the new development was to cons t ruc t a field ve rs ion of the mul t i f requency l idar and to reduce the t ime requ i red to complete a cycle of m e a s u r e m e n t s for obtaining data on the spec t r a l dependence of the optical c h a r a c t e r i s t i c s of the a tmosphe r i c ae roso l . With the L-2 a cycle of m e a s u r e m e n t s involving probing of the a t m o s p h e r e at 5-7 wavelengths takes about 20 min. With the "Glor iya" l ida r it takes 20 sec to r e c o r d informat ion on a t m o s p h e r i c optical p a r a m e t e r s at seven wavelengths . With this appara tus it is thus poss ib le to study the dynamics of va r ious p r o c e s s e s in the a tmosphe re .

The "Glor iya" s y s t e m cons is t s of jus t the l idar and the sepa ra t e control , power supply, and detect ion c i rcui t s (see Fig. 6).

The light source in the "Glor iya" is based on a mul t i f requency dye l a s e r and includes a pumping sy s t em, a dye l a s e r , a sy s t em for moni tor ing and fo rming the spat ia l s t ruc tu re of the radia t ion, and a sy s t em for ad- jus t ing and moni tor ing the d i rec t ion of the t r a n s m i t t e r axis . The dyes a r e excited by the fundamental (X = 0.347 tim) and doubled (h = 0.694 pm) wavelengths . The ruby l a s e r includes a ruby rod, a KS-19 glass pass ive Q-swi tch , and a fu l l - re f lec t ion m i r r o r . One end of the ruby rod is used as an output m i r r o r . The energy at 0.694 tim is 1.7-2 J and the pulsewidth is 30 nsec. The t r a n s m i t t e r design is such that it is imposs ib le for the light at d i f ferent wavelengths to p ropaga te into the a tmosphe re other than para l le l . A te lescope with a magni f i - cation of 5 is used to reduce the d ivergence of the light to 2-3 angular minutes .

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The stand is intended to permi t the apparatus to be rotated in the azimuth and let it stand on a solid base. The stand is a hollow steel cylinder with two radial thrust bal l -bear ings mounted in it and within which an axis with a flange rotates freely. The support f rame of the r ece ive r is bolted to the flange.

The "Gloriya" l idar works at an ambient temperature of - 2 0 to +30~ and at a relative humidity of up to 95% at 20~ With the standard packaging the instrument can be t ranspor ted by any means.

New Developments in Lidars for Multifrequency Probing of the Atmosphere . The development of new special ized mult ifrequency l idars is now being completed at the Institute of Physics of the Belorussian Academy of Sciences, The mult ifrequency "Mirazh" l idar has been developed to study the variabil i ty of the m i c r o s t r u c - ture of an aerosol over extended horizontal paths, to measure the t ransparency of water , and to study water surfaces . OKG IZ-25 and OGM-20 l ase r s are used as sources in this l idar [9]. The main component of the rece iver is an MTO-1000 objective. Interchangeable circui ts make it possible to record the intensity or de- gree of polarization of the signal reflected f rom the atmosphere. A special optical sys tem scans the probe beam from a horizontal plane to the ver t ica l with respec t to the wa te r ' s surface. The design of the apparatus makes it possible to work with the rece iver and t ransmi t te r spaced or adjacent. The l a se r is powered f rom 50- o r 400-Hz grids. The t ransmi t te r operates in single or repeated pulse modes.

A multichannel recording sys tem with pre l iminary data process ing and punched-tape data output has been developed for receiving the reflected signal. This sys tem is intended for operation in intensity measurement and photon counting modes. The maximum time resolution of the signal is 50 nsec. Also included is control of the photomultiplier sensitivity and adjustment of the output signal by t 2, where t is the instantaneous time during the detection process .

A mobile l idar has been developed for studying industrial aerosols and monitoring a tmospher ic purity in industrial regions. OKG IZ-25 and OGM-20 lase rs are used as sources in it. The rece ive r employs reflecting optics. The diameter of the rece iver m i r r o r is 300 ram. Interchangeable circuits in the rece iver make it possible to record the intensity of the signal sca t tered by the a tmosphere at wavelengths of 0.53, 1.06, and 0.694 ~m or the degree of polarizat ion of the scat tered light an at of these wavelengths. The divergence of the laser beam is reduced by a factor of six using a telescope. The l idar is mounted on a type 2-PN-2 auto- mobile t ra i ler . The rece ive r and t r ansmi t t e r a re thermally stabilized. The support f rame for the rece iver is designed so that the l idar can be rotated in the horizontal and ver t ical planes on command f rom the control panel. Switching of neutral density f i l ters in the r ece ive r is also automatic. The power supply, control , cool- ing, and recording apparatus are located in ~ labora tory compar tment on the same t ra i le r as the lidar. The entire apparatus is powered by an independent t~SD-5-T/220 generator or by a 50-Hz three-phase ac line. The l idar is intended for operation at ambient t empera tures of - 20 to + 30~ '

Up to now we have spoken of l idars for studying the s t ructure of the atmosphere. The "Baklan" device (described in detail in [10]) is intended for measur ing the attenuation and absorption coefficients of sea water. It has been used to obtain a large amount of experimental data on the optical charac te r i s t i cs of various seas and oceans [11, 12].

Resea rch is present ly being done to extend the working range of multifrequency l idars into the IR which would make it possible to obtain r icher information on the aerosol and gaseous composition of the a tmosphere .

LITERATURE CITED

1. A . P . Ivanov, A. L. Skrelin, and I. I. Kalinin, Izv. Akad. Nauk SSSR, Ser. Fiz. Atmos. Okeana,_6, No. 9, 889 (1970).

2. A . P. Ivanov and A. L. Skrelin, Zh. Prikl . Spektrosk., 13, No. 6, 1053 (1970). 3. A . P . Ivanov, A. L. Skrelin, L. V. Nikolaev, and I. S. Khutko, Izv. Akad. Nauk SSSR, Fiz. Atmos.

Okeana, 11, No. 4, 370 (1975). 4. E . M . Gitlin, V. E. Matyushkov, S. A. Mikhnov, and V. N. Shumilin, Zh. Prikl . Spektrosk., 23, No. 1,

115 (1976). 5. A.P. Ivanov, A. N. Kozhevnikov, V. S. Korneev, V. M. Orlov, F. P. Osipenko, I. S. Khutko, and

A. P. Chaikovskii, in: Fourth All-Union Symposium on Laser Probing of the Atmosphere, Tomsk (1976),

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L. V. Nikolaev, V. M. Orlov, F. P. Osipenko, and A. P. Chaikovskii, in: Meterologicheskie Issled- ovaniya (Meterological Research) , [in Russian] No. 23, Sovetskoe Radio, Moscow (1977), p. 61.

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P I E Z O S P E C T R O S C O P I C E F F E C T ON Z E R O - P H O N O N

L U M I N E S C E N C E L I N E S OF S I L I C O N

V. D. T k a c h e v a n d A. V. M u d r y i

UDC 535.37

The bombardment of silicon single crystals with high-energy particles (7 quanta, electrons, neutrons, ions} results in the formation of s tructural defects, which are comparatively effective sites for the radiative recombination (luminescence} of nonequilibrium charge ca r r i e r s [1-4]. As the bombardment dose is increased, the intensity of the "intrinsic" luminescence [5] decreases , and a ser ies of bands appears in the impurity r e - gion of the spectrum (< 1.1 eV). At low temperatures (< 100~ the bands have a fine s t ructure , which is char- acterized by the presence of intense zero-phonon lines and their long-wave vibrational repetitions. In the 4.2- 20~ temperature range the lines are comparatively narrow and have a halfwidth of ~ 0.3 MeV (i.e., < kT, where k is Boitzmann's constant and T is the experimental temperature}, making it possible to use them as sensitive indicators for the study of the static and dynamic states of the crystal lattice in the vicinity of the respective emitting defects. Definite information on the propert ies of these centers can be obtained from piezospectro- s copic investigations by examining the splitting of the zero-phonon lines following elastic deformation due to uniaxial compression (or extension} of the crystals. Their anisotropy, i .e. , lower symmetry in comparison with the higher (cubic} symmetry of silicon crystals , should f i rs t of all be manifested. A phenomenological theory of the piezospectroscopic effect has been considered by Kaplyanskii [6], and it is essentially based on the different effects of a strain field on centers oriented differently relative to compression axis P, which cause different displacements of the energy levels of these centers and, therefore, shifts and splitting of the corresponding spectral lines. In the general case, piezospectroscopic experiments make it possible to obtain the following data from the character is t ic splitting patterns of spectral lines following the deformation of a crystal along different crystallographic directions: the number of splitting components of the line, the inten- sity and polarization of each component, and the magnitude Of the energy displacement of the components re la - tive to the position of the spectral line in the unstressed crystal. On the basis of these data it is possible to unequivocally determine the symmetry class of the respective center and the orientation of its principle sym- metry axes in the lattice, the symmetry of the electronic states participating in the optical transition (multi- plicity of the degeneracy, type of irreducible representation}, multipole nature (electric or magnetic nature o f the dipole transition}, and shape of the dipole oscillators. In the present communication we shall examine the results of investigations on the determination of the symmetry of several anisotropic centers in silicon with the aid of piezospectroscopic zero-phonon luminescence lines.

Figure 1 presents the photoluminescence spectra of silicon single crystals , which were grown according to the Czochralski method and irradiated with fast neutrons with a 3.1016 cm -2, as well as the spectra of cal- cined silicon. The investigations were carr ied out at 4.2 to 20~ according to the method described in [3]. Luminescence bands consisting of narrow intense lines CA, W, etc.} and the contiguous long-wave vibrational wings were found both immediately after the irradiation and following isochronous annealing. The A, W, and

Beloruss ian Polytechnic Institute. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 29, No. 6, pp. 1053-1061, December, 1978. Original article submitted June 12, 1978.

0021-9037/78/2912-1485507.50 �9 1979 Plenum Publishing Corporation 1485