Compact Solid-State 213 nm Laser Enables Standoff Deep Ultraviolet Raman Spectrometer: Measurements of Nitrate Photochemistry

We describe a new compact acousto-optically Q-switched diode-pumped solid-state (DPSS) intracavity frequency-tripled neodymium-doped yttrium vanadate laser capable of producing ~100 mW of 213 nm power quasi-continuous wave as 15 ns pulses at a 30 kHz repetition rate. We use this new laser in a prototype of a deep ultraviolet (UV) Raman standoff spectrometer. We use a novel high-throughput, high-resolution Echelle Raman spectrograph. We measure the deep UV resonance Raman (UVRR) spectra of solid and solution sodium nitrate (NaNO₃) and ammonium nitrate (NH₄NO₃) at a standoff distance of ~2.2 m. For this 2.2 m standoff distance and a 1 min spectral accumulation time, where we only monitor the symmetric stretching band, we find a solid state NaNO₃ detection limit of ~100 μg/cm². We easily detect ~20 μM nitrate water solutions in 1 cm path length cells. As expected, the aqueous solutions UVRR spectra of NaNO₃ and NH₄NO₃ are similar, showing selective resonance enhancement of the nitrate (NO₃⁻) vibrations. The aqueous solution photochemistry is also similar, showing facile conversion of NO₃⁻ to nitrite (NO₂⁻). In contrast, the observed UVRR spectra of NaNO₃ and NH₄NO₃ powders significantly differ, because their solid-state photochemistries differ. Whereas solid NaNO₃ photoconverts with a very low quantum yield to NaNO₂, the NH₄NO₃ degrades with an apparent quantum yield of ~0.2 to gaseous species.