C. K. N. Patel

Optoacoustic spectroscopy of liquids

We report a pulsed‐dye‐laser optoacoustic spectroscopy technique for sensitive measurements of weak absorption spectra of liquids. We have used this technique to measure the weak visible absorption band of benzene near 607 nm, due to the sixth harmonic of the C‐H stretch. We obtain a signal‐to‐noise ratio exceeding 102 at the absorption peak. We estimate that with improved data handling, absorption features as small as 10−7 cm−1 should be detectable in liquids. This opens up new possibilities of trace‐constituent detection and pollution monitoring in the liquid phase.

A new optoacoustic cell with improved performance

We report a novel optoacoustic (OA) cell construction which has improved small absorption measurement capability from 10−9 cm−1 to 10−10 cm−1. The new OA cell is ideally suitable for use with planar output lasers, e.g., diode lasers. Now optoacoustic spectroscopy can be extended to include the effects of low temperatures, electric fields, and magnetic field effects.

High overtones of C–H stretch in liquid benzene measured by laser optoacoustic spectroscopy

We present a first measurement of the absorption profile of the eighth harmonic of the C–H stretch in liquid benzene using a recently developed optoacoustic absorption measurement technique involving a pulsed dye laser and submersed piezoelectric transducer. The absorption maximum occurs at 21 040±7cm−1, with a peak absorption coefficient of (9.2±1.5) ×10−5 cm−1. The full linewidth of the absorption profile is 354±9cm−1. We have previously reported similar measurements for the sixth and seventh harmonics. We have also obtained spectra of lower harmonics by conventional spectrophotometry. We make a comparative survey of the studies of this series of overtone absorptions. With our accurate determination of the nth harmonic absorption for large n, we propose an improved anharmonic formula to fit the experimental peak positions. We also note that the absorption linewidth increases linearly with n in liquid benzene.

Measurement of small absorptions in liquids.

We use the recently developed technique of pulsed dye-laser optoacoustic spectroscopy of liquids to study the weak 607-nm absorption band of benzene dissolved in various concentrations in CCl(4). The technique involves the use of submersed piezoelectric transducer and gated detection of the transient acoustic signal. With increasing dilution, the peak of the 607-nm band is observed to be more blue-shifted, and the linewidth and asymmetry of the band decrease. We verify that the present technique can detect absorption coefficients of 10(-6) cm(-1) and absorbed energy of 10(-9) J. With these high sensitivities, we show that nonlinear optical processes such as multiphoton absorption and stimulated Raman scattering in liquids should be readily observable with our optoacoustic method.

High‐resolution optoacoustic spectroscopy of rare‐earth oxide powders

We report high‐resolution absorption spectroscopy of powdered microsamples using a pulsed laser and a gated optoacoustic detection technique. Our method is simple, sensitive, and easily adaptable for cryogenic cooling of the sample. Quantitative data for Ho2O3, Dy2O3, and Er2O3 powdered crystals are obtained, and identification and analysis of the observed spectral lines are given for Dy2O3 and Er2O3.

Ultimate corrosion-resistant optoacoustic cell for spectroscopy of liquids.

We describe a new optoacoustic technique for liquids that utilizes a quartz cuvette to contain the sample and an externally coupled piezoelectric transducer for gated detection of the optoacoustic signal produced by a pulsed dye laser. Such a technique is characterized by ultimate corrosion resistance, absence of contamination of the sample, and ease of sample change. With this technique, we measure quantitatively the weak absorption profiles that are due to the sixth harmonic of the C—H stretch in a series of halogen- and methyl-substituted benzenes. Also, optoacoustic spectra of ions dissolved in a strong acid (H2SO4) are observed for the first time.

Optoacoustic Raman gain spectroscopy of liquids

We report the first optoacoustic Raman gain spectroscopy (OARS) of liquids, using two synchronized pulsed dye lasers, and gated optoacoustic detection of the energy deposited in the liquid due to the stimulated Raman (Stokes) scattering. We demonstrate this sensitive technique with several neat liquids, including benzene, acetone, 1,1,1‐trichloroethane, toluene, and n‐hexane. This technique seems competitive or superior to other techniques like intracavity inverse Raman scattering or coherent anti‐Stokes Raman spectroscopy. OARS can thus be advantageously used in many liquids, including water.

Response of piezoelectric transducers used in pulsed optoacoustic spectroscopy

A general scaling relationship is derived for detector sensitivity in terms of the acoustic properties of the absorber, permitting the transfer of calibration among different materials. A limiting case of an exciting optical-pulse durations being short compared with acoustic relaxation time is considered in detail. A strong temperature dependence of the detector response is predicted for several materials of technical interest.

Quantitative spectroscopy of micron‐thick liquid films

We demonstrate a new sensitive spectroscopic technique for highly transparent micron‐thick liquid films sandwiched between transparent substrates. A pulsed dye laser irradiates the film, producing (via the optoacoustic effect) a transient ultrasonic wave which is launched into the substrate and detected by a piezoelectric transducer bonded to the substrate. This technique is illustrated by measuring the absorption lines of films of aqueous solutions of rare‐earth ion, and absorptions ∼10−5 can be detected presently. This technique should open up new opportunities to study the spectra of chemi‐ or physi‐adsorbed species on surfaces.

Spin‐flip Raman laser at wavelengths up to 16.8 μm

An InSb spin‐flip Raman (SFR) laser is pumped with an optically pumped NH3 laser line at 780.515 cm−1 to obtain tunable first Stokes laser radiation at wavelengths up to 16.8 μm. We report results on the power output, tunability, and preliminary spectroscopy of UF6. Scaling of the primary CO2 pump laser as well as the NH3 laser together with the SFR laser appears possible for potential application of this system to uranium isotope separation.