Photothermal Experiments

The simple principles of the photothermal experiments of Graham Bell attracted me most because of its potential application to living biological systems. I must say that some of the early promises in this technique were not fulfilled because of saturation of signals and weak signal-to-noise ratio.

The main problem in our experiments was that the signal intensity was rather poor. It took considerable effort to demystify the concepts and get a few photo-acoustic spectra of publishable quality. This was done mainly by me and a large volume of work went into a single publication. It is my usual habit to do some first experiments to understand the technique. I usually generate considerable data by this method. Write a report on which I work later. Much of this preliminary report has little exciting original thoughts or ideas although some of which was publishable and is published (ref 1).

I happened to get a very cheerful and happy student, Mr. T. Somasundaram, to work with me later. With T. Somasundaram we devised and carried out experiments which we thought were exciting. We were the only on-hand experimentalists to work on these systems. We conceptualized and developed new ideas new experiments. Others made only cosmetic contributions (except the paper with K. J. Rao on As2S3 glasses) or were included in the authors’ list for standard science-practice/funding reasons.

The signal-to-noise ratio in the initial stages was rather low despite burning considerable amount of midnight oil (as they say). We were not pioneers but we were working under pioneering conditions and we could have used excuses (as always) with power failures, voltage fluctuations, no running water and so on. We were looking for ways to improve the signal intensity using conventional approaches. They did not work.

One of the important features in our experiments was that we had built a crude quartz cell that could go to 1000 K rather easily. Reproducible results were obtained, however, once we understood our cell well. Using white light we obtained very good signal to noise ratio for those experiments which did not require monochromatic light.  We used this cell to study phase transitions including the antiferromagnetic phase transition of NiO. The detection of the phase transition of NiO was a bit of a surprise since specific heat changes were expected to be small especially because of the high temperatures (> 500 K). I did not get to sit down and solve the problem of why I was able to detect the phase transition of NiO. It rankles in my mind still.

A second interesting feature in the experiments with NaNO2 was that the magnitude of the normalized photothermal anomaly at the phase transition was different for different optical regions of the signal. We attributed this to different origins of the signal:- say, mass flow (because of photo-decomposition which is not dependent on specific heat to first order) and heat flow (dependent on specific heat). This has not been followed up although the potential application of this discovery must be immense.

In many of our studies we found that the photoacoustic signal increased dramatically with temperature instead of decreasing with temperature. It took little time to realize that the effect were due to mass flow into the cell because of evaporation of absorbed water vapour. We therefore did experiments keeping various liquids with different vapour pressure in the cell. The idea was that the vapours of the liquid adsorbed on the surface would contribute to mass flow in a manner proportional to the vapour pressure. We used this aspect to find ways (Refs 5, 6, 7, 9, 12, 18) to enhance the photoacoustic signal several fold. I finally discovered after reading de Gennes’ review article on wetting that the increase in the photoacoustic signal due to introduction of a liquid with high vapour pressure in the photoacoustic cell was due to the formation of a thin liquid layer (greater than its thermal thickness at the chopping frequency) on the surface of the solid. The thermal mismatch of a liquid in equilibrium with its vapour absorbed on a solid is greatly reduced compared to the thermal mismatch at the solid-gas interface.

One of the surprises that we discovered serendipitously was that in our high-temperature experiments we obtained photo-acoustic signals even when the sample cell was open to the atmosphere. For the purpose of changing the ambient gases, we had attached the PA cell to glass stopcocks using a capillary tube (to reduce the volume of the cell). We then used this result to measure photo-acoustic signals under conditions of gas-flow (Ref 14, 15, 17).

Although phoacoustic signals may be measured because of absorption of radiation by the gas-phase molecules, it seemed that the flowing gas will take away that heat liberated intermittently from a solid would be carried away from the surface and the signal intensity should be reduced. It is not clear to me even now why this did not happen even when the thermal diffusion length in the gas phase is comparable to that of the length of the gas-phase above the sample. I undertook a more detailed study of parameters affecting the gas-microphone affected photoacoustic signals in an article published19 in Reviews of Solid State Science in 1988. High-Temperature superconductivity and its attendant scientific climate led to my abandonment of activity in this area.

We did not seriously attempt finding good spectroscopic results applied to systems where photothermal techniques have an advantage. Some attempts on measuring acidities of surfaces using indicators (Ref 2, 3, 13) or on substances spotted on filter paper (Refs 8, 16) were tried. I was told more than twenty years later by Prof. Achwal of Pune university that the experiments with estimation of the amount of 5-methylcytosine in Drosophila melanogaster DNA by amplified ELISA and photoacoustic spectroscopy (ref 8) was well cited and used commercially.


  1. Gas-Microphone-Detected Photoacoustic Signals from Condensed Phases: Comparison of Theory with Experiments, P. Ganguly, Reviews of Solid State Science, 1, 529 (1988) © World Scientific Publishing company.

18.’The importance of an adsorbed liquid layer for the enhancement of photoacoustic signals‘, P. Ganguly and T. Somasundaram, in ” Photoacoustic and photothermal phenomena”, Eds. P. Hess and J. Pelzl, Springer Series in Optical Sciences, v. 58, pp. 316-320, Springer-Verlag, Berlin, 1988.

17.’Influence of flowing gases on the amplitude of gas-microphone detected photoacoustic signals from porous and nonporous solids‘, P. Ganguly and T. Somasundaram, in ” Photoacoustic and photothermal phenomena , Eds. P. Hess and J. Pelzl, Springer Series in Optical Sciences, v. 58, pp. 333-334, Springer-Verlag, Berlin, 1988.

  1. Quantitative Estimation of substances spotted on Filter Paper by Photoacoustic Spectroscopy, T. Somasundaram, S. S. R. Rao, and P. Ganguly, Proc. Indian Acad. Sci. (chem.. Sci) 98, 171- 176 (1987)
  2. In Situ photoacoustic Spectroscopic Studies on heterogeneous catalysts under conditions of Gas Flow, Bull. Materials Sci., 9, 81-87 (1987)
  3. Experiments with Flowing Gases in an open Photoacoustic Cell, P. Ganguly, T. Somasundaram, Proc. Indian Acad. Sci. (Chem.. Sci) 98, 305-309 (1987)
  4. ‘Determination of acidities of zeolites by photoacoustic spectroscopy’, T. Somasundaram, P. Ganguly, and C.N.R. Rao, Zeolites 7, 404-407 (1987).
  5. ‘Studies on the enhancement of photoacoustic signals from non-porous solids in presence of volatile liquids’, P. Ganguly and T. Somasundaram, Appl Phys B43, 43-52 (1987).
  6. Photoacoustic investigation of phase transition in solids‘, T. Somasundaram, P. Ganguly, and C.N. R. Rao, J Phys C: Solid State Phys 19, 2137-2151 (1986).
  7. ‘Investigation of solids and surfaces by photoacoustic spectroscopy’, C.N.R. Rao, P. Ganguly, and T. Somasundaram, J Indian Chem Soc 63, 1-9 (1986).
  8. Enhancement of photoacoustic signals from condensed materials in the presence of volatile liquids: Influence of optical absorption coefficient,particle size, length of gas phase, and chopping frequency‘, T. Somasundaram and P. Ganguly, J Appl Phys 57, 5043-5047 (1985).
  9. Estimation of the amount of 5-methylcytosine in Drosophila melanogaster DNA by amplified ELISA and photoacoustic spectroscopy. C. W. Achawal, P. Ganguly, H. S. Chandra, The EMBO Joumal vol.3 no.2 pp.263 -266, 1984
  10. ‘Influence of adsorbed vapors on the photoacoustic spectra of liquids, T. Somasundaram and P. Ganguly, J Colloid Interface Sci 101, 579-582 (1984).
  11. A novel technique for enhancing photoacoustic signals from solids‘, P. Ganguly and T. Somasundaram, Appl Phys Lett 43, 160-162 (1983).
  12. A novel technique for enhancing photoacoustic signals from solids‘, P. Ganguly and T. Somasundaram, J. Physique Colloque C6, 44, 239-242 (1983).
  13. Photoacoustic spectra of As2S3-As2Se3 glasses‘, T. Somasundaram, P. Ganguly, and K.J. Rao, Proc Indian Acad Sci (Chem Sci) 92, 65-71 (1983).
  14. Study of Surface Acidity of Oxide Catalysts by Photo-acoustic Spectroscopy, K. Jagannathan, P. Ganguly, C. N. R. Rao, J. Catal. 75, 262 (1982)
  15. Study of Solids and Surfaces by Photoacoustic Spectroscopy, C. N. R. Rao, K. Jagannathan, P. Ganguly, J. Mol. Struct., 79, 173 (1982)
  16. Photoacoustic spectroscopy of solids and surfaces, C. N. R. Rao and P. Ganguly, Proc. Indian Acad. Sci. (Chemical), 90, 153 (1981).