DETERMINATION OF PHOTO ACOUSTIC SPECTRA OF DYES-I CONGO RED R. Mathur, G. Pandey To cite this version: R. Mathur, G. Pandey. DETERMINATION OF PHOTO ACOUSTIC SPECTRA OF DYES-I CONGO RED. Journal de Physique Colloques, 1983, 44 (C6), pp.c6-171-c6-175. <10.1051/jphyscol:1983626>. <jpa-00223185> HAL Id: jpa-00223185 https://hal.archives-ouvertes.fr/jpa-00223185 Submitted on 1 Jan 1983 HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.
JOURNAL DE PHYSIQUE Colloque C6, suppibment au nulo, Tome 44, octobre 1983 page C6-171 DETERMINATION OF PHOTO ACOUSTIC SPECTRA OF DYES-I CONGO RED R.N. Mathur and G.C. Pandey University Service and Instrumentation Centre, JawaharZaZ Nehru University, New DeZhi 110067, India msume Gn prgsente les spectres photo acoustiques du Rouge Congo qui est un materiau opaque. On canpare les spectres PA du solide, les spectres de transmission d'une solution dilu&, le spectre PA du colorant dgpos6 sur un papier filtre. ABSTRACT Photo acoustic (PA) spectra of one of the optically opaque material - dye Congc-Red has been reported. The PA spectra of solid dye have been compared with the transmission spectrum of the diluate aquous solution of the dye as well as the PA spectrum of the air dried aquous solution of the dye on a filter paper. PA spectroscopy is one of the very strong analytical tool for investigation of materials not amenable to analysis by the conventional spectroscopy( 1 ) Saturation effects observed in such studies,however limit its utility in the investigation of highly sbserbing compounds like organic dyes and semi-conductors(2). Lin and ~udek/3/ have shown that sample preparation techniques can overcome the P.A. saturation problem. They developed three separate methods for determination of P.A Spectrum of highly absorbing compounds. These are: (i) (ii) Deposition of a thin layer of the sample onto a quartz plate either by vacuum deposition or by smearing with a suitable applicator. Nolecular dispersion of the sample over the surface of a non absorbing medium such as barium sulphate, magnesium oxide, neutral alunina and silica gel, (iii) The sample is coground with a non absorbing substrate in a vibrating mill(mechanica1 dispersion technique) and the mixture used as the sample Lin and ~udek/3/ have successfully used above methods for determination of PA spectra of tetra phenylporphin at different concentrations. However, the presence of diluent or the substrate with the sample may result in deviations and distortions of the actual spectrum. An attempt was,therefore, made to obtain PA Spectra of one of the highly absorbing compolmds-congo-red in solid state(without any substrate) and is reported in the present paper. Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1983626
JOURNAL DE PHYSIQUE According to Rosencwaig-Gersho theory a compound shall be photo accoustically opaque if the ther ma1 diffusion length is less than the optical absorption length 1/B, Since it requires modulation frequencies in excess of 1 o4 HZ to bring these particles out of Saturation( l/b), such samples will appear to be inherently opaque, even from photo accoustic point of view(4). In such cases,theref ore, P.A. Saturation shall prevent recording of a meaningful spectrum. SPECTROSCOPY The PA spectrum of the dye was obtained using the normal settings of the instrument as per details given below: Modulation frequency : As per details given in figures Scan range ; uv-vi sible Scan rate 100 nm/minute Sensitivity As per details given in figures Phase 90% Lamp Current 19 Am The different spectra of the dye recorded have been shown figures 1-4. Figure-I: WAVELENGTH Photo Acoustic Spectrum of Congo Red. Figure-1 shows the P.A Spectra recording with normal ins'nrmental settings at a modulation frequency of 240 HZ A small hump at 470-480m along-with a rapid decrease in P.A signal around 590 nm can be clearly dilineated from this figure. As no comparable spectrum was available in the literature, it was very difficult to analyse the spectm specially the absence of any strong absorption peak.
As already mentioned even modulation frequency of 240 HL is too low to bring the sample out of saturation. The P.8. spectra of the solid dye,theref ore, were recorded again using slit widths four times the normal used for recording of the spectrum in figure-i. A thin layer of the powdered sample Was spread on the bottom surface of the cavity in the sample holder. The P.A. spectra of the sample was then recorded at different modulation frequencies viz. 10,20,40,80,160 and 240 1% reproduced in figure-2.! 10 Hz at 10mV 1100 nmlsec 1 2 20 HZ at lorn" 1100 nm1s.ci I. '0 HZ 01 lam" l,oonmlr=cl ' 80 HZ mt 1 mv 1100nmf S.c 1 5. 20 H..t 10 mv lloonm/se~ 2LoHz 1 m~ 1100om/..c 1 g 2i0 nz I 120 nmf s*c 1 smmpla hold-r 100 P V I ~ ~ ~ ~ ' ~ ' (5111 wlth LX ' 1 WAVELENGTH Figure-2: Photo Acoustic Spectra of Congo-Eied(so1id state) at different modulation frequencies ; The P.A. spectra of the sample houer has been included in the above figure. The spectmun at 240 HZ recorded at two different scan rates( 100 and 2C nm/rainute) to locate any peak which might have been missed at higher scan rate. It ma be noticed from the above figure (dpectra N6s. 182fthat the two spectra are reproducible and thus excludes the possibility of any additional band/peak. The spectra at other modulation frequencies, therefore, were recorded at 100 nm/minute scan rate only. It may also be mentioned that the spectra at different moduiation frequencies are similar except that intensity of the bands(peak height) are reduced due to saturation effects. This study thus shows that the P.A. spectra of optically opaque samples(without any substrate/diluent) can also be recorded by adjustment of certain instrumental settings, Further work along this line is in progress. A corn arison of the PA spectrum of the dye in solid stateyfigure-2) with that of the dilute aquous solution (transmission spectrum reproduced in figme-3).
JOURNAL DE PHYSIQUE CR = Congo rod Ftg 3. Tronsm~ssion spectra of Figure.5: Transmission Spectrum of the aquous solution of the dye. Figure. 3 shows that the broad band around 495 nm present in the transmission spectrum seems to be a combination of the 2 bands at 465 and 495 nm(weak) present in the PA spectrum. The effect of substrate on the PA Spectrwn of the dye was also studied. A piece of 't!hatman(~o. 42) filter paper was soaked in a dilute aquous solution of the dye and dried in air, The spectra of this as well as that of the filter paper as such were recorded(figure-4). The E.A spectrum of Dyes 1 conpo red 2 F0II.r paper 3 *.lhyl=ne blue Yeth~I~~. blue offer OIIosU,S 2 I 1 2W 100 L 00 5W 600 700 800 WAVELENGTH Figure-4: Photo Acoustic Spectrum of the filter paper with and without dye. the dye soaked filter paper gave qnly one very broad band between 420 and 600 nm. It may be mentioned that this spectrum, is comparable to that of the transmission spectrum(figure-3).
Conclusions : The above study indicates that (i) the PA spectrum of highly absorbing compounds can be recorded at lower modulation frequencies. (ii) The P.A. spectrum of the dye in solid state is better resolved than that of the transmission spectrum of the dye solution. (iii)~he P.A. spectrum of the aquous solution of the dye on a substrate(fi1ter paper) seems to replicate its transmission spectrwn. The diluent/substrate appear to affect a change in the P.A. spectrum of the dye. This work is a part of the project supported by Department of Environment, Government of India. 1. P&DhY G.C.,Proc. 3rd International Neeting on Photo accoustic and photo Thermal Spectroscopy, Paris 1983 2. BOBEITCWAIG A., in "Advances in Electronic and Electron Physicsw Vol-46 (ed.~.~orton) Academic press (N.Y.) 1978. 3. LIN J.W. and DUDEK L.P. Anal. Chem. 2 1627(1979] 4. ROSElhCWAIG A., "Photo Accoustics and Photo Accoustics Spectroscopyn John. ~illey(l~.y) 1980 P. 170