A F A S T H I G H - R E S O L U T I O N S P E C T R O G R A P H

E L E C T R O N - O P T I C A L R E C O R D I N G

N. N. M e l ' n i k , V. B . P o d o b e d o v , A . M. P y n d y k , a n d K h . E . S t e r i n

W I T H

UDC 535.853

Fas t record ing of res idua l p a r t s of spec t ra , pa r t i cu la r ly those which a r e low in intensity, cannot be achieved effect ively with high resolu t ion using the usual detectors . Photographic plates: with low quantum e f f i c i ency lose the i r advantage as a many-channe l device, and photoelec t r ic record ing at fas t scanning speeds r e su l t s in a lowered s igna l - to -no i se rat io. In [1] a fas t spec t rograph , capable of record ing a 5 -nm band of the R a m a n spec t rum during a l a s e r pulse (10-3-10 -8 sec) , is descr ibed. The spec t r a l resolu t ion of such a device was l imi ted by the low spa t ia l resolu t ion (~ 3 l i n e s / m m ) of the UM-95 e lec t ron-op t i ca l conve r t e r (EOC) and by the re la t ive ly l a rge l inear d i spe r s ion of the spec t rog raph (0.63 nm/rr~n). The resu l tan t resolut ion was about 0.2 nm in the red region of the spec t rum.

In the p r e se n t work, we desc r ibe a l abora to ry model of a fas t spec t rog raph of high resolut ion (4" 10 -3 nm) w i t h e l e c t r o n - o p t i c a l - r e c o r d i n g s p e c t r a (Fig. 1). Suchh ighreso lu t ionwas ach ievedbecause of the UMI-93 EOC (2) with a spat ia l resolu t ion of about 30 l i n e s / m m (in the s ta t ic regime) and the ISP-67 t h r e e - m e t e r spec t ro - g raph (1). In the la t ter , the p r i s m s were rep laced by a diffract ion grat ing (28) and a Li t t row m i r r o r (27), allowing us to achieve a l inear d i spe r s ion of 0.1 n m / m m in an autocol l imator scheme and 0.06 n m / m m in a Li t t row scheme. We used a gra t ing of 600 l ines / r am, working in second or th i rd o rde r , depending on the spec t r a l region under invest igation. The ISP-67 spec t rog raph (aper ture ra t io 1/30) exceeded the c o m m e r c i a l in s t rument s in d ispers ion , but had one se r ious drawback due to the lens optics - - the necess i ty of r ead jus t - ment upon making a t rans i t ion f r o m one s pec t r a l region to another.

The image of the s pec t r a f r o m the output s c r een of the EOC was t r a n s c r i b e d lust as in [1, 2], with an LI-602 sl i t d i s s ec to r (5) and could be obse rved as intensi ty distr ibution cu rves on the s to rage osci l loscope (9) screen. On the same screen, we could monitor the quality of focusing the spectrum on the photocathode of the EOC. As a pare-phase voltage to scan the dissector, we used the sawtooth voltage from the oscilloscope plates, amplified by the amplifier (8). To avoid nonlinear distortion of the frequency scale, we used only the central part of the dissector photocathode, about i0 mm in size. Using a large portion of the dissector photo- cathode is inconvenient because spatial resolution and sensitivity are strongly decreased toward the edge. The quality of spectral measurements from the EOC screen is thus worsened.

In Fig. 2 we show photometric measurements of the mercury line (~ = 546 nm) using the dissector. The four components of the isotopic s t ruc tu re a r e well reso lved . We can see that l ines spaced by 0.15 c m -1 or 4 . 1 0 -8 inn a re suitably resolved . During the photomet r ic scan of the line, the EOC was constant ly open, i . e . , the EOC was used in the s ta t ic reg ime. The record ing t ime, 10 -4 sec, was de te rmined in the case in question by the d i s s e c t o r sweep t ime.

To demons t r a t e the sensi t iv i ty of the ins t rument , p a r t s of the ro ta t ional Raman s p e c t r a of a i r and of CO 2 at a p r e s s u r e of 1 a tm are shown in Fig. 3. The spec t r a were obtained in 10 -3 sec , using excitat ion as shown in Fig. 1. The "cons t r ic t ion" of the l a s e r beam in the inves t igated gas, produced by a lens (21) and a m i r r o r (20), is imaged on the spec t rog raph sl i t by a lens (22) and a m i r r o r (19). A p r i s m (23) ro ta tes the "cons t r ic t ion" image by 90% The F a b r y - - P e r o t i n t e r f e r o m e t e r (13) al lows moni tor ing of the l a s e r linewidth excit ing Raman sca t t e r ing and a lso allows us to obtain a grooved spec t rum. The la t t e r is ve ry convenient for de te rmin ing such p a r a m e t e r s as the uniformity of the sensi t iv i ty in record ing spec t r a and the l inear i ty of the f requency scale . The n e c e s s a r y region of the spec t rum is p laced on the EOC photocathode using a l i n e - s p e c - t r u m source (26).

T rans l a t ed f r o m Zhurnal Pr ik ladnoi Spektroskopii , Vol. 24, No. 1, pp. 9-12, J anua ry , 1976. Or ig inal a r t i c le submit ted F e b r u a r y 7, 1974; rev i s ion submit ted Ju ly 21, 1975.

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Fig. 1. Schematic drawing of the spectrograph: 1) spectrograph; 2) EOC; 3) magnetic coil; 4) objective of "Jupiter-3" ; 5)dissector; 6) stabilized current source for the magnetic coil; 7) high-voltage source; 8) voltage amplifier of the dissector scan; 9) S1-37 oscillo- scope; 10) laser power supply; 11) laser trigger; 12) oscilloscope synchronization trigger; 13) Fabry--Perot etalon; 14) camera; 15, 16) He--Ne alignment lasers; 17) pulse laser; 18) stop; 19, 20) mi r rors of the illuminating system; 21, 22) objective lenses of the illuminating system; 23) Dove prism; 24) rotating prism; 25) in- candescent lamp; 26) neon gas-discharge lamp; 27) plane mirror ; 28) diffraction grating.

Fig. 2. Shape of the mercury line X = 546 nm.

Fig. 3. Rotat ional s p e c t r a of a i r in the region 40-80 c m -1 (a) and of CO2 in the region 10-50 cm - i (b) (this spec t rum was r eco rded twice). The s p e c t r a l width of the s l i t was 0.5 cm - i and the p r e s s u r e was 1 atm.

A ruby l a s e r (17) was used to exci te Raman sca t te r ing , opera t ing in the f r ee r eg ime (10-3-sec pulses) with pulse energ ies of about 5J and spec t r a l widths of about 0.06 cm -1. To n a r r o w the l a s e r spec t rum, we used a ha l f -concen t r i c r e s o n a t o r with a stop (18) at the l a s e r output.

The s igna l - to -no i se ra t io in the Raman s p e c t r a was somewhat lower than expected. This is due to the spike s t ruc tu re of the l a s e r pu l ses . The d i s s e c t o r sweep t ime was equal to the l a s e r pulse length, while the c h a r a c t e r i s t i c p e r s i s t e n c e t ime of the EOC phosphor was about 100 ~sec. Under these conditions the d i s sec to r s t i l l r e a c t s to the spike s t ruc tu re of the l a s e r pulse. Moreover , there is a pa r t i a l loss of the signal because the l a s e r pulse and d i s s e c t o r sweep t ime exceed the phosphor p e r s i s t e n c e t ime. When the l a s e r pulse is much s h o r t e r than the phosphor p e r s i s t e n c e t ime, the re is no signal loss [1]. The indicated deficiency for record ing t imes g r e a t e r than 10 -5 sec may be avoided if a m e m o r y device, for example , a te levis ion c a m e r a [3-5] or photographic emuls ion, is used to r e a d the spec t r a l image. Photographic emuls ion is even m o r e p r e f e r ab l e for analyzing h igh- reso lu t ion Raman s p e c t r a for exact f requency determinat ion. We should also note that ex t raneous sca t t e r ing inside the spec t r a l i n s t rumen t s r e m a i n s a ve ry significant obs tac le in recording weak Raman sca t ter ing . Many of the diff icul t ies a re l e s sened by using m o r e than one monochroma to r , in our case , a spec t rog raph with a p r e m o n o c h r o m a t o r [6].

The r e s u l t s obtained on the a b o v e - d e s c r i b e d fas t spec t rog raph allow us to a r r i v e at the following con- clusions. Above all, h igh-d i spe r s ion spec t rog raphs coupled with e l ec t ron -op t i ca l record ing allows fas t r e c o r d - ing of l ines with high resolut ion. Also, using fas t record ing with h igh-d i spers ion spec t rographs makes them re la t ive ly insens i t ive to v ibra t ion and t h e r m a l drift . Moreover , s i m i l a r equipment al lows success fu l r e c o r d - ing of Raman s p e c t r a with high resolut ion. Final ly , such ins t rument s allow, in our view, a substant ia l s i m - pl if icat ion in the product ion of r e sonan t f luorescence spec t ra , absorpt ion spec t ra , and Shpol' skii spec t r a with high resolut ion, while the use of f r a m e and l inear sweeps of the EOC allows solutions of new prob lems .

In conclusion, the authors e x p r e s s the i r deep gra t i tude to V. P. Vinogradov for help in making this model of the ins t rument .

1.

2. 3. 4. 5. 6.

LITERATURE CITED I

V. B. Podobedov, A. M. Pyndyk, and Kh. E. Sterin, Prib. Tekh. Eksp., No. I, 190 (1973). Yu. L Malakhov, Prib. Tekh. Eksp., No. 3, 174 (1972). M. Bridoux and M. Delhaye, Nouv. Rev. d'Optique Appliq., i, 23 (1970). M. Delhaye, Appl. Opt., 7, 2195 (1971). C. M. Savage and P. D. Maker, Appl. Opt., I0, 4, 965 (1971). S. Brodersen and J. Bendtsen, J. Raman Speetrosc., i, 97 (1973).