Intense laser matter interactions for explosives detection and tunable spectral blueshift
This dissertation investigates explosives detection and spectral shifts induced in metal surfaces by ultrafast laser pulses at intensities ranging from 1013 to 1014 W/cm2. These ~100 femtosecond (fs) pulses facilitate non-resonant vaporization of explosives, enabling non-contact sampling and detection. When coupled with electrospray post-ionization, fs laser vaporization has successfully detected the explosive signature molecule dinitrotoluene (DNT) on both metallic and dielectric substrates. Systematic investigations reveal that the optimal laser pulse energy for sampling DNT on metal substrates is lower than that on dielectric substrates. To quantify the DNT response, the product of ablation spot area and laser power density was evaluated at constant laser pulse energy, yielding results consistent with experimental measurements. The limit of detection (LoD) for DNT using this non-contact laser electrospray mass spectrometry (LEMS) technique is 15 ng under a laser intensity of 1014 W/cm2 with a 90-μm spot diameter. The noncontact detection of an explosive signature molecule using LEMS represents an advance for explosive detection using mass spectrometry. Femtosecond laser vaporization was further employed to enhance gas-phase analyte concentrations for handheld ion mobility spectrometry (IMS), addressing the persistent challenge presented by the low vapor pressures of explosives. Compared to conventional thermal desorption at 250 °C, fs laser vaporization achieves a 15-fold lower LoD for DNT. The integration of fs laser vaporization with IMS represents a promising step towards a universal, quantitative, and field-deployable method for explosive detection.This dissertation also investigates the spectral modulation of intense fs laser pulses scattering from a metal surface, yielding a tunable blueshift of the carrier frequency. The blueshift reached up to 5.4 nm at 1014 W/cm2, a power level that is 20× lower than that required in air-breakdown plasma. We attribute this blueshift to interaction with a transient free‐electron layer above the metal according to the simulations, distinct from gas‐phase plasma mechanisms. The free electron layer can be thought of as forming a transient capacitor with positive charge residing at the metal surface. The theoretical model presented matches well with the experimental measurements. Multi-pulse enhancement of spectral blueshift and broadening was also demonstrated, reaching a maximum blueshift of ~26 nm and a spectral broadening of ~220 % at 7 × 1014 W/cm2. These findings reveal a novel subwavelength optical medium induced by intense laser-matter interaction and carry promise for ultrafast surface spectroscopy and laser pulse manipulation.
Analytical chemistry, Physical chemistry, Ion mobility spectrometry, Light-matter interaction, Mass spectrometry, Nonlinear optics, Ultrafast laser, Chemistry
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