César Costa Vera

A three-point calibration procedure for matrix-assisted laser desorption/ionization mass spectrometry utilizing multiply charged ions and their mean initial velocities

This paper reports the results obtained by using a new calibration procedure for linear time-of-flight matrix-assisted laser desorption/ionization mass spectrometry (MALDI). The procedure takes into account the usual MALDI spectra structure by considering the multiply charged molecular ions and the mean initial velocities as a self calibrating property. The mean velocities for all molecular ions are taken to be the same and the procedure is based on the solution of a number of equations (at least three) which relate time-of-flight to mass. Three times better accuracy was obtained in comparison with the normal internal calibration procedure.

Measurement of thermal diffusivities of silver nanoparticle colloidal suspensions by means of a frequency-resolved thermal lensing approach

A frequency-resolved thermal lensing (TL) approach to measure thermal diffusivity properties of both diluted liquid solutions and silver nanoparticle colloidal suspensions is demonstrated. The experiment is based on a classical two-color pump-probe TL configuration, which is adapted to measure the induced TL signal as a function of the chopping frequency of the pump beam. Because of the thermal diffusivity lengths in the samples, the TL signal decreases exponentially with the increment of the frequency. The exponential decay factor can be associated with the thermal diffusivity of the medium. Measurements are performed on diluted liquid solutions and silver nanoparticles suspended in a PVP solution. A suitable fitting to a theoretical model based on the Fresnel diffraction approximation of the experimental data is obtained. This work demonstrates the feasibility of using this approach for the thermal characterization of nanoparticles in liquid solutions. Thermal diffusivity as low as 0.094×10(-7)  m2  s(-1) can be estimated by using this approach.

The angular distribution of neutral poly-alanine molecules produced in a matrix-assisted laser desorption/ionization process

The angular distribution of desorbed neutral and ionized polyalanine molecules, initially embedded in a matrix of 2,5-dihydroxybenzoic acid, has been measured for the first time. The target was irradiated with a pulsed ultraviolet laser beam (355 mn) inci

Rotating Wave Approximation effects on the nonlinear optical responses of complex molecular systems using a Four-Wave Mixing Signal

The study of a two-level molecular system with permanent dipole moments (PDM) immersed in a thermal bath and interacting with an electromagnetic field depends on the inclusion of the rotating wave approximation (RWA). A methodology based on cumulant expansions to obtain the average in the Fourier components associated with the coherences and populations, calculated by the optical stochastic Bloch equations, is employed. With these equations, we have modeled the optical responses for the four-wave mixing signal and we have demonstrated that inside the RWA the PDM cannot be measured.

Study of the absorptive and dispersive responses of molecular systems under stochastic considerations

Absorptive and dispersive nonlinear optical properties have been studied considering a two-level molecular system with permanent dipole moments immersed in a thermal reservoir interacting with a classical electromagnetic field. Our model is based in a stochastic description, where the system–solvent interaction induces random shifts in the Bohr frequency (ω0) that transforms ω0 into a stochastic variable ξ(t). Our results show that it is necessary to neglect the rotating-wave approximation in order to measure the effect of the permanent dipole moments. The two-photon transitions over those with one-photon are favored when the permanent dipole moments are considered.

Frequency-resolved thermal lensing: An approach for thermal diffusivity measurements in liquid samples

Frequency-resolved thermal lensing (FRTL) is an approach used to measure thermal diffusivity coefficients of liquid samples. The theoretical model of this approach is based on thermal waves and the Fresnel diffraction approximation. This model predicts that only two experimental parameters are needed to estimate the thermal diffusivity of the sample. Experiments performed on pure solvents validate the theoretical predictions and confirm the feasibility of using this approach to thermally characterize liquid samples. The results show that the method is a simple and sensitive approach to determine thermal diffusivity coefficients in liquid samples.