USE O F A L A S E R IN A T O M I C A B S O R P T I O N A N A L Y S I S

E . P . K r i v c h i k o v a a n d V. S. D e m i n UDC 621.375.8

L a s e r s a re a t t rac t ing attention because of the broad poss ibi l i t ies of the i r p rac t i ca l applicat ions. In [1-4] a l a se r was used as an exci tat ion source for an emiss ion spec t r a l ana lys is . It was shown in that s tudy that a l a s e r exci tat ion source has s eve ra l advantages in compar i son with an are or spark .

It was d i scovered in a study of the spec t rum of h igh-chrome s tee l excited by a l a s e r that many ana- lytical lines of impur i t i e s under investigation were absorpt ion lines. This paper pursues the possibi l i ty of using the l a se r b e a m for an a tomic absorpt ion spec t ra l ana lys is . Exper iments were pe r fo rmed with a pul- sed GOR-100M Laser operat ing in the free osci l lat ion reg ime. The l a se r radiation was sharp ly focused on the sample sur face with a lens of focal length f =50 mm.

The physical p r o c e s s e s re la ted to the des t ruct ive action of focused l a se r radiation on meta ls , a re in many respec t s s i m i l a r to t h o s e o c c u r r i n g i n a high-power pulsed d ischarge . For these sources the damage p r o c e s s e s a r e t he rm a l in nature. All the energy is concentrated in a smal l portion of the sample sur face , which leads to its explosive evaporat ion [5-10]. It is well known f rom many studies of the spa rk discharge that vapor iza t ion of the e lec t rode meta l occurs in the fo rm of luminous jets being emit ted pe r - pendicular ly to the e lec t rode surface with a veloci ty reaching s eve ra l thousands of m e t e r s per second [11-15]. The damage of ma te r i a l by l a se r action is a lso accompanied by the format ion of p la sma jets propagat ing perpendicular ly to the sur face of the mate r ia l , and in both sources the p lasma is emi t ted in termi t tent ly . The veloci ty of the p lasma reaches 20 km(sec [9, 10]. Hydrodynamic phenomena play an impor tan t role in the p r o c e s s e s occurr ing in the p lasma jet. These phenomena a r e a lso cha rac te r i s t i c of both sources .

Qualitative studies of the spec t ra of p lasma jets formed by the act ion of l a se r radiation have shown the i r s imi l a r i t y to the spec t r a of pulsed d i scharges [5, 6, 8, 9, 16].

We studied the spec t r a of ruby, Alundum, and h igh-chrome s tee l alloyed with manganese , nickel, and si l icon. The spec t r a of these ma te r i a l s obtained by l a se r beam i r rad ia t ion consis t of continuous background with super imposed absorpt ion and emiss ion lines. The absorpt ion lines belong mainly to neutra l and singly ionized a toms with excitat ion energ3~ 5-7 eV (e.g., the lines Cr I 2408.6 A, Ni I 2325.8 A, oSi I 2506.9 A, Mn I 2794.8 /~in steel ; A I I 2660.4 Aand A I I 2575.1 /~ in Alundum and ruby; Cr I 3578.7 A and Cr I 3593.5 )~ in ruby; and Ti I 2520.5/~ and Ti I 2727.4 A in Alundum). Lines of doubly ionized a toms appeared only in emiss ion . The s t ronges t absorpt ion is observed for the neutra l a tom lines. The absorpt ion lines of ions a re weak. The absorpt ion lines of these ma te r i a l s a re quite diffused. Some lines a r e shifted relat ive to the emiss ion lines of iron as compared to those obtained with an a rc source . This is t rue of both the ab- sorpt ion and emiss ion lines. Thus the quali tat ive study of the spec t ra obtained by i r rad ia t ing the samples by l a se r radiat ion showed considerable r e semblance to the spec t ra in pulsed d i scharges [11-14].

Poss ib le application of a tomic absorpt ion spec t r a of a s t rong pulsed d ischarge in a cap i l l a ry to so lv- ing analyt ical p rob lems was recent ly shown in [11]. We demons t ra ted the possibi l i ty of using a tomic ab- sorpt ion spec t r a obtained in a l a s e r source for analyt ical purposes .

As is well known [17-19], the total ene rgy A absorbed by the line f rom a continuous spec t rum is for weak absorpt ion equal to (Tre2/mc)/fN, where m is the e lec t ron mass , e is the e lec t ron charge, c is the speed of light, l is the length of the absorbing iayer , f is the osc i l l a to r s t rength, and N is the densi ty of a toms . For smal l optical densi t ies the total absorpt ion is propor t ional to /fN (linear par t of the "growth curve") , and for large densi t ies it is propor t ional to ~ . The total absorpt ion in the line is measu red by the a r ea enclosed between the absorpt ion line contour and the background continuum[17-19].

Translated f rom Zhurnai Prikladnoi Spektroskopii , Vol. 14, No. 4, pp. 592-596, April, !971. Origi- nal a r t i c le submit ted April , 24, ~970

�9 1973 Consultants Bureau, a division of Plenum Publishing Corporation, 227 West 17th Street, New York, N. Y. 10011. All rights reserved. This article cannot be reproduced for any purpose whatsoever without permission of the publisher. A copy of this article is available from the publisher for $15.00.

438

Lg I

45

!

Z

/,o

,}

f

Fig. 1

J

AA

45

{ t

0,5 /,0 C,% Fig. 2

Fig. 1. Schematic a r rangement of the laser.. 1) laser; 2) lens, f =50 mm; 3) sample: 4) lens, f =75 ram: 5) ISP-22spec t rograph .

Fig. 2. Calibration graphs for the determination of nickel (1), man- ganese (2), and silicon (3) in steel by the total absorption method.

�9 . o

)0 D ccr,~ Fig. 3. Calibration graphs for the determinat ion of manganese (1), s i l i- con (2), nickel (3), and chromium (4) from optical density measurements .

We made an analysis of impuri t ies in h igh-chrome steel by the total absorption method with laser vaporization of the sample. We determined the elements manganese, nickel, and silicon in the concentrat ion range 0.1-1%, and chromium in the range 9-17%. The optical a r rangement is shown in Fig. 1. The samples were oriented at 45 ~ to the laser beam and to the spectrograph optical axis. The spectra were photographed under identical conditions at a light energy density of 7 �9 10 G W/cm 2. The laser beam was focused on a spot on the sample surface. The amount of mater ia l ejected as a result of the dam- age was the same for all samples and equal to ~3.5 rag. The spectra were recorded with anISP-28 spectrograph. The spec t ro- graph slit was illuminated with a single lens (f =75 mm). A single flash was sufficient to obtain normal blackening of the RF-3 photographic plate with sensitivity 650 units (All-Union State Standard) in the continuous spectrum.

To per fo rm the analysis of the atomic absorption spect rum we selected the lines Ni I 2320.0 A, Cr I 3015.2 A, Mn I 2794.8 /~, and Si I 2516.1 ~. For each element in the steel composi- tion we constructed the line contours and computed the depen-

dence of the total absorption in the line, A, on the content of the element, C, in the sample. This depen- dence is plotted in Fig. 2. It is assumed that the density of atoms in the plasma, N, is proport ional to their concentrat ion, C, in the sample. It is evident from the graph that the points lie on a s traight line. The mean square e r r o r of the data is about 5%. The line contours were recorded with a microphotometer with a recording device containing an automatical ly recording EPP-09 potentiometer. The a reas under the line contours were measured with a planimeter . The advantage of this method lies in the fact that the total absorption is independent of the resolution of the spec t romete r over a broad range [17-1~

In addition to the total absorption method, one other technique was used to determine the impuri t ies in h igh-chrome steel. Knowing the concentration of the element in the sample and measur ing the optical density of the line [20, 21], we can construct a graph in units of log I0/I. To obtain high sensit ivity in this case, it is neces sa ry to work with high resolution. A calibration graph of this dependence is shown in Fig. 3. Over the range of element concentrat ion shown in the graph the optical density increases in direct propor- tion. The experimental points quite sa t i s fac tor i ly lie on a s traight line. However, at high concentrations (their maximum limit is different for each element) the linear dependence breaks down. For example, the deviation from linear dependence for chromium is observed at concentration above 18.5%. The mean square e r r o r of this method is approximately 4%. Table 1 shows the average concentrat ions for nickel, chromium, manganese, and silicon obtained from three measurements with laser-exci ted steel (the concentrations of nickel, chromium, manganese, and silicon were found feom the graphs of the optical density vs concentra- tion). The reproducibi l i ty of the resul ts was verified by photographing the same sample twenty t imes. The ar i thmetic mean e r r o r of the results was about 3%.

439

TABLE i.

Source Resul ts of Analysis Obtained in L a s e r

Element [Introduced, % by [ weight t

Silicon I 0,35 Nickel 1 0,42 Manganese 0,42 Chromium 16

I

Found in laser, % by weight

0,37 0,41 0,42

16,5

To se lec t opt imum conditions for the analys is of admix tures in h igh-chrome s teel we invest igated the dependence of the optical densi ty of the spec t r a l Lines of the impur i t i es on the l a se r radiation flux density. The optical densi ty of the spec t r a l lines i nc rea se s propor t ional ly with energy when the l a se r energy den- s i ty is var ied f rom 5.7 �9 106 to 10 v W / e m 2. Hence it is evident that to inc rease the sens i t iv i ty when analyz- ing smal l concentrat ions of an e lement in the sample , it is n e c e s s a r y to inc rease the l a se r radiation ene rgy density, and to dec rea se it when analyzing high concentra t ions .

In se lec t ing the lines used in the analys is it is n e c e s s a r y to account for the fact that the optical den- s i ty i n c r e a s e s with osc i l l a to r s t rength. For the three resonance Lines of manganese - Mn I 2801.1 A, Mn I 2798.2 ~ , and Mn I 2794.8 ~ - it was calculated that by increas ing the osc i l l a to r s t rength 1.38, 1.45, and 2 t imes the optical densi ty i n c r e a s e s 1.65, 1.76, and 2.92 t imes , respec t ive ly .

The authors thank E. V. Livshchits for his in te res t in the work, and a lso V. E. Kalashnikov for his help with the exper imen t s .

LITERATURE CITED

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(1964) 6. V . V . Zhukovskii and A. A. Yankovskii, Proceedings of the V Be lo russ ian Scientific TechnoLogical

Conference on Spectra l Analysis [in Russian], Minsk (1967), p .33 7. L . I . Grechikhin and L. Ya. Min'ko, Zh. Tekhn. Fiz., 37, 1169 (1967). 8. V . V . Pante leev and A. A. Yankovskii, Zh. Pr ik l . Spektroskopii , 9, 923 (1968). 9. A . I . Korunchikovand A. A. Yankovskii, Zh. Pr ikl . Spekt roskopi i . 5, 586 (1966).

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Nauka, Moscow (1965), I~. 128. 16. G . I . Bakanovich, L. i. Grechikhin, and L. Ya. Min'ko, Proceedings of the XVI Conference on

Spectroscopy [in Russian], Nauka, Moscow (1965), p. 95. 17. A. Ynzol'd, Phys ics of Ste l lar Atmospheres [Russian t ranslat ion] , IL, p. 949. 18. S. ]~. Fr ish , Optical Spectra of Atoms [in Russian], Fizmatgiz , M o s c o w - L e n i n g r a d (1963). 19. B . V . L 'vov, Atomic Absorption Spectral Analysis [in Russian], Nauka, Moscow (1966). 20. A . A . Shishlovskii , Applied Physica l Optics [in Russian], Fizmatgiz , Moscow (1961). 21. V . M . Chulanovskii, Introduction to Molecular Spectral Analysis [in Russian], Gostekhizdat,

Moscow- Leningrad (1950).

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