The laser optogalvanic effect in radio frequency plasma: Applications in analytical chemistry

The laser optogalvanic effect (LOGE) is the change of electrical impedance of a plasma produced by resonant absorption of laser radiation. Selected excitations can be used to detect specific species by tuning the laser to an appropriate wavelength.; For optimum utilization of LOGE as a detection tool, one must understand the temporal profile of the LOGE signal in terms of the various processes occurring in the plasma. In this work, the characteristics of the LOGE signal and the potential applications of laser optogalvanic spectroscopy (LOGS) have been investigated.; First, by analyzing the LOGE signal profiles of the neon excitations $1s2,4\to 2p1,3,$ we studied the effect of radiation trapping and collisional energy transfer on the generation of LOGE signals. The kinetics of the signal generation has been discussed quantitatively. The ionization rate change (IRC) signal profile has been modeled qualitatively. The dominant transient features were found to be associated with trapping of the $1s2$ state. The temperature of the radio frequency (RF) plasma is $\sim$1000K.; Second, the influence of the distance traveled by the acoustic pulse on the signal profiles has been investigated. It was found that the acoustic wave launched by non-radiative $2pk\to 1sj$ and $1s3,5\to S01$ deexcitations travels at the speed of sound. On the other hand, the acoustic wave launched by non-radiative $1s2,4\to S01$ deexcitations of trapped $1s2,4$ states produces an instantaneous LOGE signal. A model of the photoacoustic (PA) signal formation has been constructed and used to predict the PA signal profiles successfully.; Third, we have studied the capability of LOGS to detect and analyze iodine in an RF plasma. It is shown, by careful selection of the operating conditions, that the signals of iodine atoms and molecules can be distinguished by separating the IRC/PA components. Both air and nitrogen can be used as the filler/carrier gas in the plasma. The detection limit of iodine in nitrogen is approximately one order of magnitude better than that in air. The response of signal intensity to sample concentration is linear in the range of 0.1μg/cm3–2.5μg/cm 3. The detection limit achieved in our experiments is $\sim$10−7μg/cm3 for both iodine atoms and molecules.