Laser-induced Grating Technique Research Articles

Collisional relaxation of singlet O2 in neat gas investigated by laser-induced grating technique

Abstract Systematic experimental and theoretical studies of the feasibility to apply the diffraction at laser-induced gratings (LIGs) for getting information on parameters of collisional rotational, electronic and vibrational relaxation in molecular gases have been accomplished for the first time, using as an example the process of collisional relaxation of optically excited metastable O2 ( b 1 Σ + g ) molecules in neat gas. Temporal profiles of LIG signals have been recorded at densities 0.2–160 Amagat and room temperature within a broad range of time intervals (0.1–1000 μs) after ns-scale laser excitation at various rovibronic transitions of the b 1 Σ + g (v ′ =0)←X 3 Σ − g (v ′′ =0 ) band of O2. The experimental data have been analyzed within the framework of a model of a three-stage rotational (R–T), electronic (E–E,V,T + V–V and E–V,T + V–V) and vibrational (V–T) collisional relaxation. The results of the analysis are presented demonstrating temporal and amplitude variations of distinct contributions to the LIG signal within a large range of gas densities. These variations were described using a set of characteristic optical and relaxation parameters of O2. The parameters were derived from the experimental data and compared with the values known from the literature. Reasonable agreement was found between all the values provided by the LIG technique, and, in particular, the time constants of electronic and vibrational relaxation processes involved, and those previously obtained by other methods.

Microscopic Patterning on the Polysilane Films by the Laser Induced Grating Technique

Microscopic patterning on polysilane thin film was observed after photoexcitation with an optical interference pattern using a nanosecond pulsed laser. The patterning processes were monitored by the diffraction of the probe beam. The observed diffraction signals consist of the transient grating component due to the temperature change and the permanent grating component due to a chemical reaction. It was found that the microscopic pattern was destroyed with prolonged laser radiation. The created microscopic pattern was observed by the optical microscope.

Wide-band acoustic spectroscopy of biological material based on a laser-induced grating technique

A laser-induced transient grating technique enables fast noncontact acoustic measurements on transparent biological materials in a frequency range from tens of megahertz to 1 GHz. We have applied this method to the characterization of bovine vitreous and found high-frequency acoustic attenuation values to be close to those of water, with a quadratic dependence on frequency, in contrast to low-frequency data. The potential of the technique for studying other biological materials, such as human stratum corneum, is demonstrated.

Temperature and mixture fraction measurements in gases by laser-induced electrostrictive gratings

We applied time-resolved light scattering from electrostrictive gratings to measure nonintrusively gas compositions and temperatures. The infrared radiation of a pulsed Nd:YAG laser generates a spatially periodic density grating that oscillates in time, and the beam of a cw laser is used to read it out. The oscillation period of the signal depends on the sound velocity in the medium. If the gas composition is known, the measurement of the sound velocity allows the temperature to be deduced. We evaluated this novel technique in a tube furnace for temperatures in air up to about 1400 K. Although the measured temperatures agree within the error bars with the thermocouple readings, there is some evidence that the temperatures determined by the laser-induced grating technique are systematically too high. For singleshot temperature measurements in air, we determined a statistical error of 70 K at a furnace temperature of 1400 K. On the other hand, if the temperature is known, concentrations in isothermal binary mixtures can be determined by the laser-induced grating technique. We performed measurements in methane-nitrogen and hydrogen-nitrogen mixtures. The limits for the minimum detectable variation of the gas composition of 0.5 mole fraction mixtures were found to be 2% and 0.6%, respectively. One advantage of the proposed method is the simple evaluation process that allows for on-line measurements of temperature or gas compositions.

Mass Diffusion Coefficients and Thermal Diffusivity in Concentrated Hydrothermal NaNO3 Solutions

Binary mass diffusion coefficients and thermal diffusivities for hydrothermal sodium nitrate solutions as a function of pressure (270 < P < 1000 bar), temperature (400 < T < 500 °C), and concentration (0.25 < C < 3.0 m) were measured by the laser-induced grating technique. In concentrated hydrothermal NaNO3 systems, the critical slowing down was significant as far as 300 bar from the phase-separation pressure, resulting in binary diffusion coefficients near the critical point that are comparable to values at ambient conditions. Further from the critical point the mass diffusion coefficients plateaued at their ordinary values. Ordinary binary mass diffusion was about 15 times faster than at 25 °C and atmospheric pressure. The ordinary binary mass diffusion coefficients were compared with predictions from hydrodynamic diffusion theory. Experimental results agreed well with predictions from the Stokes−Einstein equation, where the diffusing species was best represented by a hydrated contact ion pair. The Wilk...

Predissociative linewidths of (4pσ) M 2Σ+ (v=1) and (3dσ,π) H 2Σ+, H′ 2Π (v=2) Rydberg states of NO studied by the two-color laser-induced grating technique

The two-color laser-induced grating (TC-LIG) technique has been employed to probe the predissociative Rydberg states (4pσ) M 2Σ+ (v=1) and (3dσ,π) H 2Σ+, H′ 2Π± (v=2) and the non-Rydberg B 2Π (v=26) state of nitric oxide. The ultraviolet pump laser frequency is fixed to a specific rotational line of the A 2Σ+ (v′=1 and 2)←X 2Π3/2 (v″=0) transition of NO. Interference of two pump laser beams crossing at a small angle in the gas sample forms a population grating. Then, the visible probe laser beam is diffracted off the grating as the signal beam when the probe laser frequency is resonant to a transition from the rotational level excited by the pump laser. Term values and rotational assignments of the H 2Σ+, H′ 2Π± (v=2)–B 2Π3/2 (v=26) states have been established for the first time, resulting in the value of ∼1.77 cm−1 for the l-uncoupling constant. Line broadenings due to predissociation are observed in the TC-LIG spectra for the M 2Σ+ (v=1) and H 2Σ+, H′ 2Π+ (v=2) states. The linewidths of the M 2Σ+ (v=1) state show no marked rotational dependence except for the N=4 level which is locally perturbed. On the other hand, the predissociative lifetimes of the H 2Σ+ and H′ 2Π+ (v=2) states exhibit a significant rotational dependence: The linewidth of the former state decreases with N, while the reverse tendency is seen for the latter. The mixing between the H 2Σ+ and H′ 2Π+ states caused by l-uncoupling explains the observed rotational dependence successfully.

Detection of vibrational-overtone excitation in water via laser-induced grating spectroscopy

In this paper we describe a method, based on the laser-induced grating technique, for studying the spectroscopy of vibrational overtone-excited gas-phase water. Two phase-coherent visible laser beams whose frequencies are in the range of the third overtone of the OH stretch in water are crossed in the gas-phase sample. As the wavelength of these excitation beams is scanned through individual rovibrational OH overtone transitions, vibrational energy is deposited into the water in a spatially sinusoidal pattern. A fixed-frequency 266 nm probe laser beam is diffracted from the resultant transmission diffraction grating in water. We show that under collision-free conditions, probe laser diffraction is observed from the initially excited grating, which is a necessary condition for using this technique to study the absorption spectroscopy of the vibrationally excited molecules. Under multiple collision conditions, a probe laser wavelength-independent refractive index grating is formed within the bulk sample. In addition, we observe temporal oscillations in the grating diffraction efficiency arising from excitation of standing acoustic waves.

Measurement of the thermal diffusivity and speed of sound of hydrothermal solutions via the laser-induced grating technique

Hydrothermal processing is being developed as a method for organic destruction for the Hanford Site in Washington. Hydrothermal processing refers to the redox reactions of chemical compounds in supercritical or near-supercritical aqueous solutions. In order to design reactors for the hydrothermal treatment of complicated mixtures found in the Hanford wastes, engineers need to know the thermophysical properties of the solutions under hydrothermal conditions. The author used the laser-induced grating technique to measure the thermal diffusivity and speed of sound of hydrothermal solutions. In this non-invasive optical technique, a transient grating is produced in the hydrothermal solution by optical absorption from two crossed time-coincident nanosecond laser pulses. The grating is probed by measuring the diffraction efficiency of a third laser beam. The grating relaxes via thermal diffusion, and the thermal diffusivity can be determined by measuring the decay of the grating diffraction efficiency as a function of the pump-probe delay time. In addition, intense pump pulses produce counterpropagating acoustic waves that appear as large undulations in the transient grating decay spectrum. The speed of sound in the sample is simply the grating fringe spacing divided by the undulation period. The cell is made from a commercial high pressure fitting and ismore » equipped with two diamond windows for optical access. Results are presented for dilute dye/water solutions with T = 400 C and pressures between 20 and 70 MPa.« less

Excited-state spectroscopy of organic dyes by use of population gratings

Excited-state spectroscopy of organic dyes is demonstrated with a laser-induced grating technique. The basis of this method is the creation of a population grating in the material that is being investigated and the use of a probe detuned from the pump, permitting a great increase in effective contrast. The absorption–emission spectrum of transitions from S1, the first singlet state, were determined from the diffracted signal of the tunable probe laser. The resulting theory of the grating function and the results of experiments on Nile Blue and Rhodamine 640 are presented. In conjunction with other techniques such as flash photolysis, population gratings may provide useful information on excited-state structures of dye molecules.

A two-color laser-induced grating technique for gas-phase excited-state spectroscopy

A new excited−state spectroscopic method is reported. It is a two−color laser−induced grating tecnique for detecting optical transitions of rovibronically excited molecules in the gas phase. (AIP)

Laser-induced surface acoustic waves and photothermal surface gratings generated by crossing two pulsed laser beams

Gigahertz surface acoustic waves have been optically generated and detected on aluminum metallic films and silicon wafers using the laser-induced grating technique. The acoustic velocities were in good agreement with those of Rayleigh waves. Photothermal surface gratings could also be detected as a nonpropagating component of the thermoelastic waves. The relaxation time of the grating was related to the thermal diffusivity of the material. A simplified theory for the generation of surface transient gratings containing acoustic waves is presented in addition to the experimental results.