Cristian Focsa

Experimental and Theoretical Aspects of Aluminum Expanding Laser Plasma

The formation and dynamics of aluminum laser produced plasma are experimentally and theoretically investigated. The visible emitting regions of plasma form two structures of different expansion velocities. Such behavior is in agreement with the transient current recorded by a cylindrical Langmuir probe. Using the hydrodynamic model of scale relativity theory, the plasma dynamics at different time scales are numerically and analytically analyzed.

Photosensitivity of pulsed laser deposited Ge-Sb-Se thin films

Pulsed laser deposition was used to prepare amorphous thin films from (GeSe2)100-x (Sb2Se3)x system, where x is varying from 0 to 60. Fabricated films present a good morphology with no cracks nor breaks and relatively low roughness. To study their photosensitivity under irradiation with energy close to band gap, a comparison of their optical properties (refractive index and band gap energy) before and after irradiation is performed in both, as-deposited and annealed states. In linear regime, annealed films seem to be photostable when x≥30. In nonlinear regime, highest photoinduced threshold intensity values were found for films with x = 10, 16.7 and x = 30, 40. Thus, the highest photostability in both, linear and nonlinear regimes of irradiation, was observed for layers with x = 30 and 40. Finally, the structure of the films is discussed based on Raman scattering spectroscopy results.

Resonant desorption of ice with a tunable LiNbO3 optical parametric oscillator

We report on the use of a pulsed tunable mid-infrared optical parametric oscillator (OPO) for the IR resonant desorption of ice in the 3 lm range. Desorbed neutrals are detected by UV multi-photon ionization and time-of-flight mass spectrometry. Desorption yield is found to increase linearly with the OPO pulse energy in the 0–3 mJ range. The resonant character of the desorption process is very clearly evidenced by the study of the wavelength dependency of the desorption yield. The desorption spectrum peaks at 3.1 lm and is in remarkable agreement with the absorption spectrum of crystalline ice at T ¼ 100 K. This result differs from previous experiments using NIR macro-pulses of a free electron laser. The velocity distribution of the desorption products reveals the presence of at least two components. The fast component is consistent with our previous results obtained on doped ices and indicates a phase explosion process. The observation of high-n (H3O) þ (H2O)n clusters is also reported. 2002 Elsevier Science B.V. All rights reserved.

Oscillatory Langmuir probe ion current in laser-produced plasma expansion

Using the Langmuir probe method to investigate the parameters of laser-produced plasma, our experimental results show an oscillatory structure of the recorded transient ion current. The periodic behavior is analyzed for various targets and probe positions and, after extracting the continuous part, it is found to be connected with the ion frequency and the electron-ion collision frequency. We conclude that theories to describe these oscillations may provide new diagnostic techniques of laser-produced plasma.

Fourier transform emission spectroscopy of the A 2Π–X2Σ+ system of BeH

The A 2Π–X 2Σ+ transition of BeH was observed by Fourier transform emission spectroscopy using a hollow cathode discharge lamp. The 0–0 to 6–6 bands were rotationally analyzed and molecular constants extracted. The equilibrium rotational constants Be and bond lengths were found to be 10.331 21(50) cm−1 and 1.341 68(3) A for the ground state and 10.466 31(27) cm−1 and 1.332 99(2) A in the excited state. In order to link the diagonal bands together and to determine the vibrational constants, the 0–1 to 6–7 bands in an archival arc emission spectrum were also rotationally analyzed. In the X 2Σ+ and A 2Π states, the spectroscopic constants are nearly identical so the Δv=−1 bands were too weak to be seen in our Fourier transform spectra. Franck–Condon factors were calculated for the A 2Π–X 2Σ+ transition from Rydberg–Klein–Rees potential curves. These new rotational analyses now link up with the previous work on the 0–7, 0–8, 0–9, 1–9 and 1–10 bands of the C 2Σ+–X 2Σ+ system [R. Colin, C. Dreze, and M. Steinha...

Can Highly Oxidized Organics Contribute to Atmospheric New Particle Formation

Highly oxidized organic molecules may play a critical role in new-particle formation within Earths atmosphere along with sulfuric acid, which has long been considered as a key compound in this process. Here we explore the interactions of these two partners, using quantum chemistry to find the formation free energies of heterodimers and trimers as well as the fastest evaporation rates of (2,2) tetramers. We find that the heterodimers are more strongly bound than pure sulfuric acid dimers. Their stability correlates well with the oxygen to carbon ratio of the organics, their volatility, and the number of hydrogen bonds formed. Most of the stable trimers contain one sulfuric acid and two organics (1,2), whereas many (2,2) tetramers evaporate quickly, probably due to the stability of (1,2) clusters. This finding agrees with recent experimental studies that show how new-particle formation involving oxidized organics and sulfuric acid may be rate-limited by activation of (1,2) trimers, confirming the importance of this process in the atmosphere.

Periodic Phenomena In Laser‐Ablation Plasma Plumes: A Self‐Organization Scenario

Experimental evidence of the appearance of a proper periodic dynamics in a plasma plume created by pulsed laser ablation is considered as a hint for the presence of a self-organization scenario that explains similar phenomena observed in plasma diodes.

In vivo Real-Time Mass Spectrometry for Guided Surgery Application

Here we describe a new instrument (SpiderMass) designed for in vivo and real-time analysis. In this instrument ion production is performed remotely from the MS instrument and the generated ions are transported in real-time to the MS analyzer. Ion production is promoted by Resonant Infrared Laser Ablation (RIR-LA) based on the highly effective excitation of O-H bonds in water molecules naturally present in most biological samples. The retrieved molecular patterns are specific to the cell phenotypes and benign versus cancer regions of patient biopsies can be easily differentiated. We also demonstrate by analysis of human skin that SpiderMass can be used under in vivo conditions with minimal damage and pain. Furthermore SpiderMass can also be used for real-time drug metabolism and pharmacokinetic (DMPK) analysis or food safety topics. SpiderMass is thus the first MS based system designed for in vivo real-time analysis under minimally invasive conditions.

Parametric study of polycyclic aromatic hydrocarbon laser desorption

We present the use of a combined laser desorption/multi-photon ionization/time-of-flight mass spectrometry technique for the analysis of polycyclic aromatic hydrocarbon (PAH) solid samples. A thorough characterization of the first step (laser desorption) of this experimental technique has been performed. By varying the energy of the laser pulse, a specific response of each PAH has been evidenced for pure and mixed PAH sample desorption. This behaviour has also been studied with respect to the fragmentation processes. Similar studies on PAHs adsorbed on graphite evidenced the possibility of desorbing molecules from the adsorbed phase only, i.e. without a contribution from the graphite substrate. These findings represent important preliminary steps towards the final goal of setting up a completely characterized analytical method for the investigation of the adsorbed phase of soot particles generated in combustion processes.

CO2 Capture Using Semi-Clathrates of Quaternary Ammonium Salt: Structure Change Induced by CO2 and N2 Enclathration

Semi-clathrates of tetrabutylammonium bromide (TBAB) are investigated for their potential application in the CO2 capture context based on hydrate technology. The three-phase lines of semi-clathrates of CO2-TBAB-H2O and N2-TBAB-H2O are established simultaneously with their structure using in situ Raman scattering performed at high pressure. The preferred crystal phase obtained at ambient pressure from solutions of 5 and 40 wt % TBAB initial concentrations is shown to change upon enclathration of CO2 or N2, or by applying a higher pressure on the system. Deep in the stability field, metastable hydrate phases are occurring at the onset of the formation and correspond to the ones expected under ambient pressure conditions. Depending on the pressure, they progressively transformed into the most stable ones when approaching equilibrium and dissociation points. Besides, it is shown that a 5 wt % TBAB original solution forms preferentially a mixed structure of both type B and type A at low gas pressure with CO2 as the guest gas. A new structure is spectroscopically characterized at pressures higher than ∼2 MPa CO2. Type A is demonstrated to be stable at 5 wt % initial TBAB concentration with N2 as the guest molecule and pressure between 8 and 12 MPa. These structural data address new insights on the relationship between the hydrophilic-anion and hydrophobic-cation intercalation with a guest gas producing hydrophobic interaction in a distorted water lattice.