Infrared Laser-Induced Thermal Lens Calorimetry

Abstract

Thermal lens calorimetry is applied to the measurement of infrared absorption of condensed phase samples. The unique trade offs encountered with longer wavelength lasers in thermal lens measurements are discussed. As a demonstration of the technique, the determination of hydrocarbons using a helium-neon laser operated at 3.39 μm is reported. The minimum detectable sample has an absorbance, A = 5 × 10−4, corresponding to 8 ng of 2,2,4-trimethylpentane within the beam volume in the sample. Interpretation of absorbance at the helium-neon laser transition is aided by the determination of functional group molar absorptivities from a series of normal, cyclic, and branched-chain hydrocarbons. The average specific absorption coefficient for hydrocarbons at this wavelength is found to be 2.09 (±0.09) 1 g−1 cm−1.

Thermal lens spectrometry based on single-laser/dual-beam configuration

“…When the beam is unblocked, the time-dependent defocusing of the beam, resulting from the formation of a "divergent" lens inside the sample, is monitored via the intensity change (drop) at the beam center by an oscilloscope (2)(3)(4)(5). For Sensitive detection of small absorbances, a computer is utilized to evaluate the transient lens signals (7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17). Using a 160-mW Ar+ laser, Dovichi and Harris have demonstrated a minimum measurable absorbance of 7 X 10"8 (10).…”

Thermal lens spectrophotometry using a tunable infrared laser generated by a stimulated Raman effect

“…However, recent findings of an enhanced Raman scattering for various compounds adsorbed at the silver surface (SERS, surface-enhanced Raman scattering) (4,5) give new potential for analytical applications of Raman spectrometry (6). Thus spraying paper chromatograms with silver colloidal solution produces a strong intensity for resonance Raman scatterers from separated dyes (7,8). This surfaceenhanced resonance Raman scattering (SERRS) spectrometry has permitted in situ colored substances down to 2 ng/cm2 to be detected and identified from chromatogram spots (7).…”

“…On the other hand Bialkowski et al demonstrated ultrasensitive detection of hydrocarbons such as dichlorodifluoromethane, chlorotrifluoromethane, and ethanol by a high-power pulsed C02 laser (5-7). Harris et al also reported the use of an infrared He-Ne laser (3.39 µ ) for determination of 2,2,4-trimethylpentane (8). The C02 laser is line-tunable from 9 µ to 11 µ , so that 0003-2700/87/0359-0523501.50/0 the bar graph spectrum can be measured.…”

“…The state-of-the-art Nd:YAG-laser-pumped dye laser produces 150-mJ pulses at 560 nm, which is converted to 14-mJ pulses at 1048 nm by a Raman shifter (12). A more recent publication informs that the conversion efficiency can be improved to 807c by using a Raman laser amplifier system (8). Then, the detection limit can readily be improved more than 2 orders of magnitude by using such an exciting source.…”

Thermal lens measurements of optical computation of the laser beam spot size

“…The thermal lens effect, first reported by Gordon et al (1), has been developed into a sensitive, quantitative method for the determination of trace-level samples (2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20)(21). Efforts to improve the detection capabilities of this method have primarily been directed toward minimizing the effect of laser intensity fluctuations on the measurement.…”