The combination of laser-induced breakdown spectroscopy (LIBS) and laser-induced fluorescence (LIF) was investigated to improve the limit of detection (LoD) of trace elements in liquid water, while preserving the distinctive on-line monitoring capabilities of LIBS analysis. The influence of the main experimental parameters, namely the ablation fluence, the excitation fluence, and the inter-pulse delay was studied to maximize the fluorescence signal. The plasma was produced by a 266 nm frequencyquadrupled Q-switched Nd:YAG laser and the trace elements under investigation were then re-excited by a nanosecond optical parametric oscillator (OPO) laser, delivering pulses in the sub-mJ energy range, and tuned to strong absorption lines of the trace elements. The reproducibility of the measurements was improved using a home-made flow-cell, and relative standard deviations as low as 6.7% for a series of 100 shots were attained with a repetition rate of 0.7 Hz. Using the LIBS-LIF technique, we demonstrated LoDs of 39 ppb and 65 ppb for Pb and Fe, respectively, accumulating over 100 laser shots only, which correspond to an improvement of about 500 times with respect to LIBS.

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The principle of measurements using the "microwave resonant cavity technique" applied to rare-earthdoped insulating materials is reviewed and the physical nature of the expected signals is discussed. Experimental results concerning both single crystal and powdered samples of Lu 2 SiO 5 : Ce 3+ are presented and discussed as typical examples. From measurements at various temperatures between 300 and 5K and under different pulsed laser beam powers, it is shown that detailed information on the rare earth photoionization process, photoconductivity dynamics and trapping effects may be gained, in addition to photoconductivity spectra.1 Introduction Photoconductivity studies of rare-earth (RE)-doped insulating materials using the microwave resonant cavity technique (MRCT) were conducted for the first time in our laboratory [1]. The primary goal of these investigations was indeed the determination of the RE photoionization thresholds, with the aim of locating the 4f n levels of RE ions with respect to the host conduction band (CB). As a matter of fact, the energy level diagrams of RE ions usually reported do not take this positioning into account and, consequently, lack the required information to help in understanding the complex processes associated with the absorption of high energy photons by RE-doped materials used as scintillators, ultraviolet lasing media or phosphors for fluorescent tubes and plasma display panels. MRCT was shown to be also helpful in gaining significant information on the photoionization process, photoconductivity dynamics, nature of the photoexcited electrons and trapping effects [2][3][4]. MRCT consists in detecting the dielectric response to a pulsed laser irradiation of a sample placed inside a microwave resonator. Let Q U and f 0 be respectively the cavity quality factor and resonance frequency when both the laser and microwave sources are not operating. Coupling the microwave guide to the cavity lowers its quality factor from Q U (U for unloaded cavity) to Q L (L for loaded cavity) defined by:

The microwave resonant cavity technique ͑MRCT͒ was used to measure the room-temperature photoconductivity spectrum of a CaF 2 :Eu 2+ single crystal between 275 and 450 nm, with the aim of positioning the Eu 2+ levels relatively to the bottom of the host conduction band. A photoconductivity signal was detected at laser wavelengths l Յ 430 nm ͑h l Ն 2.9 eV͒. Its intensity was observed to exhibit a superlinear dependence on the laser mean power for l Ͼ 280 nm and an almost linear one at shorter wavelengths, showing that Eu 2+ photoionization may involve either a one-photon or a two-step two-photon absorption process. The probabilities of both linear and quadratic processes were determined from measurements of the dependences of the photoconductivity signal intensity versus the mean laser power for several laser wavelengths within the spectral range that is under investigation. The Eu 2+ photoionization threshold was estimated at 4.9 eV from the comparison between the MRCT photoconductivity spectrum, the Eu 2+ 4f 6 5d͑e g ͒ excited-state absorption spectrum, and the calculated density of states of the CaF 2 conduction band. In addition, the photoconduction dynamics in two CaF 2 :Eu 2+ samples grown under different experimental conditions was studied. The MRCT signals from the two samples were observed to exhibit different thermal behaviors. This observation is interpreted in terms of differences in trap densities and depths, in connection with thermoluminescence measurements.

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