M. L. Shah

Quantitative elemental analysis of steel using calibration-free laser-induced breakdown spectroscopy

We report the quantitative elemental analysis of a steel sample using calibration-free laser-induced breakdown spectroscopy (CF-LIBS). A Q-switched Nd:YAG laser (532 nm wavelength) is used to produce a plasma by focusing it onto a steel sample in air at atmospheric pressure. The time-resolved spectra from atomic and ionic emission lines of the steel elements are recorded by an echelle grating spectrograph coupled with a gated intensified CCD camera and are used for the plasma characterization and quantitative analysis of the sample. The time delay at which the plasma is in local thermodynamic equilibrium as well as optically thin, necessary for elemental analysis, is deduced. An algorithm for the CF-LIBS relating the experimentally measured spectral intensity values with the basic physics of the plasma is developed and used for the determination of Fe, Cr, Ni, Mg, and Si concentrations in the steel sample. The analytical results obtained from the CF-LIBS technique agree well with the certified values of the elements in the sample, with relative uncertainties of less than 5%.

High-lying even-parity excited levels of atomic samarium

We report the observation of new high-lying even-parity excited levels of samarium in the energy region 34 814.4 cm^-1 to 35 110.9 cm^-1. These levels are identified with two-color laser-induced fluorescence spectroscopy in an atomic beam. The values of the total angular momentum (J) of these levels are assigned. Our results are compared with the results obtained with two-color, three-photon photoionization spectroscopy in the same energy region reported earlier in the literature [J. Opt. Soc. Am. B13, 641 (1996)]. The similarities and differences observed in the spectra by two techniques have been discussed, reaffirming the complementary nature of the two techniques for laser spectroscopy of highly excited levels.

Time-Resolved Emission Spectroscopic Study of Laser-Induced Steel Plasmas

Laser-induced steel plasma is generated by focusing a Q-switched Nd:YAG visible laser (532 nm wavelength) with an irradiance of ~ 1 × 109 W/cm2 on a steel sample in air at atmospheric pressure. An Echelle spectrograph coupled with a gateable intensified charge-coupled detector is used to record the plasma emissions. Using time-resolved spectroscopic measurements of the plasma emissions, the temperature and electron number density of the steel plasma are determined for many times of the detector delay. The validity of the assumption by the spectroscopic methods that the laser-induced plasma (LIP) is optically thin and is also in local thermodynamic equilibrium (LTE) has been evaluated for many delay times. From the temporal evolution of the intensity ratio of two Fe I lines and matching it with its theoretical value, the delay times where the plasma is optically thin and is also in LTE are found to be 800 ns, 900 ns and 1000 ns.

Study of even-parity autoionization resonances of atomic uranium by three-color optogalvanic spectroscopy

We report our investigation on the even-parity autoionization (AI) resonances of atomic uranium in the energy region 52,850–53,350  cm−1, using the three-color optogalvanic spectroscopy technique in a U–Ne hollow cathode discharge lamp with three pulsed dye lasers. To the best of our knowledge, this is the first report of observation of even-parity AI states in atomic uranium lying more than 2000  cm−1 above the ionization limit. We have used four different excitation schemes, starting from the lowest metastable state of uranium at 620  cm−1 (K505). We have identified 102 new even-parity AI resonances in atomic uranium and assigned probable total angular momentum (J) values to these resonances. By observing 25 out of these 102 AI resonances through more than one excitation pathway, the ambiguity in the assignment of their J values has been reduced considerably.

Manifestation of collective effects of laser photo-plasmas in time-of-flight mass spectrometer

The performance of Time-of-Flight Mass Spectrometer (TOF-MS) has been studied with laser produced photoions of various densities using a reflectron TOF-MS built in our laboratory. In this, the source of atoms is a resistively heated atomic beam source. As the atomic species reach the interaction region, the interaction with pulsed laser results in ions. Samarium was used as the source element to observe all its isotopes by resonant non selective excitation of one of the ground state transitions. However, as the temperature of the source is increased, the collective plasma effect of ions and electrons becomes important. As a result of this the Time-of-Flight signal (in linear mode) of Sm isotopes became poorly resolved from a well resolved condition. The number density of the ions produced in these conditions and hence the plasma parameters were calculated. The plasma parameters like Debye length (?D), the number density (ND), and frequency (fpi) confirms the collective effect. In order to avoid this undesired effect, the optimum operating condition with respect to oven temperature and laser intensity was estimated.