Mira Terzić

Diffraction characteristics of laser‐induced acoustic waves in liquids

The effect of diffraction on the temporal shape of laser‐excited plane acoustic waves propagating in liquids is studied both theoretically and experimentally. Two different boundary conditions corresponding to a constrained and a free surface of the liquid are considered. The main features are the development of a rarefaction zone behind the pure compression pulse in the first case and the appearance of a second compression zone following the original compression/rarefaction signal in the second case. Experiments have been performed in distilled water with a hybrid CO2 laser and piezoelectric detection of the acoustic transients. A good agreement between theory and experiment is found.

Multiple absorption and relaxation processes in SF6CH4 mixtures: an experimental study

Abstract The effects of collisions with nonabsorbing partner molecules (CH4) were investigated in absorption processes in SF6 irradiated by a pulsed CO2 laser. An experimental apparatus was developed for simultaneous absorption measurements both by transmission and photoacoustic spectroscopy. Enhanced absorption cross sections of SF6 in collisions with nonabsorbing CH4 molecules were measured, together with the average number of absorbed photons per molecule. The differential absorption cross sections were also determined for the fluence range of 0.1–0.7 J/cm2, and the CH4 pressure range of 1–140 mbar. The results obtained were analyzed based on the predictions of the generalized two-level model proposed for multiple photon processes. An estimate of the role that rotational-rotational and vibrational-translational relaxation processes play, in inducing the effect of enhanced absorption in SF6CH4 mixtures irradiated by pulsed CO2 laser radiation, is given.

Experimental study of rotational relaxation processes by pulsed photoacoustic technique

The IR pulsed photoacoustic method was used to investigate rotational relaxation processes in MPA. We have examined the bulk rotational relaxation times and the bulk rotational relaxation cross sections , related to R-R and R-T relaxation mechanisms. The saturation intensities for gas molecules have been measured in the gas mixtures with (buffer) molecules, at constant gas pressure (0.47 mbar) and variable gas pressure (1-140 mbar), using the 10P(16) laser line in the fluences range 0.1-0.7 J . It has been found that for proper investigation of the rotational relaxation processes, the buffer gas pressure range must be divided into two parts: the low pressure range (0-20 mbar) and the high pressure range (20-140 mbar), according to the behaviour of the saturation intensities versus buffer gas pressure. Consequently, two different sets of values for rotational relaxation parameters are obtained. The found step-function behaviour for shows that the rotational hole-filling effect during MPA only dominates at low buffer gas pressure ( mbar). At higher buffer gas pressures (between 20-140 mbar) collisional deactivation takes place. We also derive the enhanced absorption cross sections for absorbing gas molecules from the measured saturation intensities data (at zero buffer gas pressure) confirming the expectation that the enhanced absorbing cross sections are not only a function of laser fluence and buffer gas pressure, but are very strongly dependent on laser beam parameters, too.

Buffer-gas pressure influence on multiphoton absorption in SF6-N2 mixtures

In this article the results of pulsed IR photoacoustic spectroscopy measurements of multiphoton absorption and relaxation processes in SF6–N2 mixtures are presented. The total average number of absorbed photons per one absorbing molecule (basic physical quantity which characterized multiphoton processes) during the laser pulse 〈n〉total is used and analyzed with a generalized coupled two-level model. This type of analysis is based on buffer-gas pressure (pbuff) functional behavior of 〈n〉total and calculation of its partial values depending on collisions, 〈n〉coll, and laser fluence, 〈n〉Φ, influence. Using different methods of photoacoustic spectroscopy, collisionaly induced rotational and vibrational to translational relaxation process parameters (τrot and τV-T, respectively) are quantitatively obtained and used to determine partial values of 〈n〉coll, 〈n〉rot, and 〈n〉V-T. It will be shown that a method based on ∂〈n〉total/∂p functional dependence on pbuff and laser fluence Φ can be used to confirm or predict ...

Pulsed photoacoustic measurements of large optical absorption coefficients

A method to determine large optical absorption coefficients of liquids from the amplitude ratio of acoustic waves generated under rigid‐ and free‐boundary conditions is presented. This has been examined both experimentally and theoretically, taking into account the attenuation of the pulse amplitude ratio with the increased acoustic path length due to attenuation effects. Although the proposed method was tested in a narrow wavelength interval and close to the limit of precision of the experimental arrangement, the results show quantitative agreement with the literature values.

Pulsed photoacoustic detection technique applied to the study of multiphoton absorption in molecules

A pulsed laser photoacoustic technique is described. It is optimized for the simultaneous investigations of multiphoton absorption processes as well as rotational relaxation and vibration relaxation studies. The experimental procedure is presented, concerning the calibration of the photoacoustic detector/cell set-up, applying the transmission spectroscopy technique. The obtained relationships, which can be alternatively used for the sensitivity curve S(p, gamma , T) of the apparatus, are analysed. A previously proposed model is confirmed.

Photoacoustic observation of multiphoton absorption and relaxation parameters in gas mixtures

A method is described that allows simultaneous photoacoustic measurements of multiphoton absorption and relaxation parameters in gas mixtures. The “σe” method to determine rotational relaxation parameters was developed. Various approaches for evaluating differential cross sections are discussed.

Comparison of beetroot extracts originating from several sites using time-resolved laser-induced fluorescence spectroscopy

Beetroot (Beta vulgaris) juice contains a large number of fluorophores which can fluoresce. There is a growing interest in beetroot extracts analysis. In contrast, there is only limited information about beetroot obtained without sample preparation and/or extraction of components from the sample. In this work, we continue our previous study (Rabasovic et al 2009 Acta Phys. Pol. A 116 570–2), analyzing and comparing beetroot extracts from several sites, using the time-resolved laser-induced fluorescence technique to measure the fluorescence of samples at different excitation wavelengths (340–470 nm) and for different sample dilutions.

Using lasers to measure pollutants

In recent years, a large number of linear and nonlinear laser-based diagnostic techniques for detection of pollutions in different environments have been developed. Applications of laser spectroscopy constitute a vast field, which is difficult to cover comprehensively in a review. Due to that here are presented only a few spectroscopic methods, chosen to illustrate the power of applied laser spectroscopy in environmental pollution investigation. The paper also gives a brief presentation of main laser spectroscopy methods.

Pulsed Laser Photoacoustic Spectroscopy of Gases: Trace Gas Detection and Multiphoton Absorption Studies

Laser photoacoustic spectroscopy in the midd-infrared spectral region is a powerful spectroscopic technique usable both for highly sensitive trace gas detection (linear regime) and for the study of multiphoton absorption (nonlinear regime). For the latter, high peak power pulsed laser excitation is typically used. The experimental technique is applicable both for atmospheric trace gas monitoring (linear absorption), and for the study of processes leading to the generation of advanced materials (multiphoton spectroscopy).