Stimulated Raman Effect in OrthoPhosphoric AcidVed Parkash and T. S. Jaseja Citation: Journal of Applied Physics 42, 404 (1971); doi: 10.1063/1.1659610 View online: http://dx.doi.org/10.1063/1.1659610 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/42/1?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Stimulated Raman Emissions from Sulfuric Acid J. Chem. Phys. 54, 1419 (1971); 10.1063/1.1674991 Theory of the Stimulated Raman Effect J. Chem. Phys. 49, 3558 (1968); 10.1063/1.1670633 The Stimulated Raman Effect Am. J. Phys. 35, 989 (1967); 10.1119/1.1973774 Raman Spectra of Certain Phosphoric Acids and Their Salts J. Chem. Phys. 17, 1166 (1949); 10.1063/1.1747136 Raman Spectra of Certain Phosphoric Acids and Their Salts J. Chem. Phys. 16, 1163 (1948); 10.1063/1.1746752

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JOURNAL OF APPLIED PHYSICS VOLUME 42, NUMBER 1 JANUARY 1971

Stimulated Raman Effect in Ortho-Phosphoric Acid

VED PARKASH AND T. S. JASEJA

Department of Physics, Birla Institute of Technology and Science, Pilani (Rajasthan), India

(Received 4 March 1970; in final form 25 May 1970)

Stimulated Raman emissions from phosphoric acid solutions of varying concentrations were studied. A 99% concentrated solution produced one Raman line (910 cm-I ). However, the solution of concentra­tion ~85% produced three more sharp lines close to the Raman line (910 cm-I ) which were not observed earlier in the corresponding spontaneous Raman effect. It was also found that no Raman line shifted its position when the concentration of the solution was varied, contrary to the earlier observations on the corresponding spontaneous Raman emission (914 cm-I ). The stimulated Raman emissions were found sharp even under multimode excitations.

The earlier studies (e.g., Ref. 1) of spontaneous Raman effect in the phosphoric acid showed that there was a frequency displacement of the strongest Raman line corresponding to the symmetrical oscilla­tion of P04- group in the tetrahedral structure. The Raman shift 914 cm- I for the strongest line observed in 85% solution was found l to be reduced to 880 cm- I

when the concentration was changed to 83% of the solution.

In our investigations, the stimulated Raman emis­sions (SRE) were studied from the ortho-phosphoric acid solutions with concentrations varying from 99% to 75% (by weight). The solution of 99% concentra­tion produced a sharp stimulated Raman spectral line with frequency shift 910 Cln-I . This Raman line is considered to be produced by the symmetrical vibra­tion of a P04- group in the tetrahedral structure of the molecule H 3P04 •

However, the solution with concentration :::; 85% produced four close sharp lines corresponding to Raman shifts 910, 905, 898, and 893 cm- I (see Fig. 1). The three e:-;:tra Raman lines (905, 898, and 893 cm- I )

have not been reported l earlier from the studies of spontaneous Raman spectra of the acid solutions. Since in the spontaneous Raman effect the Raman lines are usually broader than those in the case of stimulated Raman effect, the resolution in the stimu-lated Raman spectrum is expected to be better than that in the case of spontaneous Raman spectrum. In our studies the linewidths of the Raman lines were found to be quite sharp ('""0.4 cm- I ). Thus, in the corresponding spontaneous Raman spectrum the ob-served four close lines in the SRE might not have been resolved.

To understand the behavior of the three sharp

('""20°C) disappeared when the temperature of the solution was raised to 40°e. From these observations it is believed that the three Raman lines are produced by hydrated molecules of H 3P04 of type H3P04 ,H20.

Another remarkable observation was also made that no Raman line shifted its position when the concen­tration of the acid was varied from 99% to 75%. This is contrary to the observations l made earlier for

FIG. 1. Three orders of anti-Stokes radiations produced from ortho­phosphoric acid of concentration 85% at room temperature. The close Raman lines (910, 90S, 898, and 893 cm- I ) are clearly visible in the 2nd- and 3rd-order anti-Stokes. Here Wo is ruhy laser frequency.

Raman lines (905, 898, and 893 cm- I ) not observed the corresponding main spontaneous Raman emission earlier, the stimulated Raman spectra were recorded (914 cm- I ). In the case of spontaneous Raman effect, at elevated temperatures. At these temperatures, the the variation of Raman shift with dilution may pre­intensity of the SRE decreased with increase of tem- sumably be accounted for by assuming a simple electro­perature. No SRE was observed at temperatures static interaction2 of an oscillating dipole with a sur-2=: 50°e. However, at 40°C the solution of concentra- rounding dielectric fluid whose dielectric constant tion :::; 85% produced only one Raman line with fre- varies with dilution. However, in the case of the quency shift 910 cm- I [see Fig. 2(a)]. The remaining large fields created by the giant pulse laser beams, three Raman lines observed at room temperature the molecules are strongly driven. Therefore, the oscil-

404

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S TIM U L ATE D RAM A N E F FEe T I:'oJ 0 R THO - P H 0 S P H 0 RIC A C I D 405

lating dipole in our case of large fields is strongly coupled with the electromagnetic wave and may be less perturbed by the electrostatic interaction from the neighboring solvent molecules than in the case of low fields. This may qualitatively explain that the stimulated Raman spectral line did not shift its posi­tion when the concentration of the acid was varied.

Giant ruby laser pulses of power 15-25 MW with a pulse duration '" 2 X 10-8 sec focused by a lens of f = 20 cm were used for the generation of SRE. The intensity of SRE was found to decrease with decrease of concentration of the acid. No SRE was observed in the concentration :::; 70%. The stimulated Raman spectra were recorded by a 3-glass prism Carl Zeiss spectrograph. For recording the antis tokes radiations (shown in Figs. 1 and 2), a Corning glass filter No. CS 4-97 was placed in front of the slit of the spectro­graph. In this way, the glaring produced by the strong radiations (laser and 1st- and 2nd-order anti-Stokes) was eliminated and good resolution in the spectra was obtained.

FIG. 2. (a) Three orders of anti-Stokes radiations produced from 85% con­centrated solution when the temperature of the solu­tion is raised to 40°C (compare with Fig. 1). Only one Raman shift 910 em-I was observed; (b) Three orders of anti-Stokes from 99% cone en tra ted solution. Only one Raman shift 910 em-I was ob­served.

(a) (u)

6J +2.~9to o

FIG. 3. Interference fringes obtained with a Fabry-Perot etaloll of the stimulated Stokes radiation (lst order) from a 90% concentrated solution. The interorder spacing is 0.93 em-I.

The SRE from the phosphoric acid was found to be quite sharp. Unfortunately, the threshold for the SRE is high ('" 15 MW). At these high-power levels, the laser beams used in the experiments always had a few strong modes. Even under these multimode ex­citations, the full linewidth of the first-order Stokes radiation was found to be 0.4 cm- l

. The measurements on the linewidths were made with a Fabry-Perot etalon with a spacing of 5 mm. Figure 3 shows a typical fringe pattern of the Stokes radiation produced at a pumping power ",20 MW. From the width of the fringes, the fulliinewidth in this case is 0.46 cm- l .

For comparison, the first-order Stokes radiations pro­duced from benzene under such multimode excitations were a few cm- l broad, although, under single mode excitation, the full linewidth of the Stokes line from benzene was found3 to be 0.4 cm-l •

The authors acknowledge the support of BITS­MIT-Ford Foundation program for the development of laser research laboratory. Thanks are due to Dr. M. K. Dheer and Dr. M. C. Gupta for their assistance in the experiments.

I J. Hibben, The Raman Effect of its Chemical Applications (Reinhold, New York, 1939), p. 382.

2 E. Bauer and M. Magat, Physica 5,718 (1938). 3 For example, see E. Garmire, Ph.D. thesis, M.LT., 1965.

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