We developed a state-of-the-art, high-sensitivity, low-stray-light standoff deep-ultraviolet (DUV) Raman spectrometer for the trace detection of resonance Raman-enhanced chemical species. As an excitation source for Raman measurements, we utilized our recently developed, second-generation, miniaturized, diode-pumped, solid-state neodymium-doped gadolinium orthovanadate (Nd:GdVO4) laser that generates quasi-continuous wave 228 nm light. This 228 nm excitation enhances the Raman intensities of vibrations of NOx groups in explosive molecules, aromatic groups in biological molecules, and various aromatic hydrocarbons. Our DUV Raman spectrograph utilizes a custom DUV f/8 Cassegrain telescope with an ∼200 mm diameter primary mirror, high-efficiency DUV transmission gratings, custom DUV mirrors, and a custom 228 nm Rayleigh rejection filter. We utilized our new standoff DUV Raman spectrometer to measure high signal-to-noise ratio spectra of ∼50 μg/cm2 drop-cast explosives: ammonium nitrate (AN), trinitrotoluene, pentaerythritol tetranitrate as well as aromatic biological molecules: lysozyme, tryptophan, tyrosine, deoxycytidine monophosphate, deoxyadenosine monophosphate at an ∼3 m distance within 10–30 s accumulation times. We roughly estimate the average ultraviolet resonance Raman (UVRR) detection limits for the relatively homogeneous drop-cast films of explosives and biological molecules to be ∼1 μg/cm2 when utilizing a continuous raster scanning that averages Raman signal over ∼1 cm2 sample area to avoid quick analyte depletion due to ultraviolet (UV) photolysis. We determined 3 m standoff UVRR detection limits for drop-cast AN films and identified factors impacting UVRR detection limits such as analyte photochemistry and analyte morphology. We found a detection limit of ∼0.5 μg/cm2 for drop-cast AN films on glass substrates when the Raman signal is averaged over ∼0.5 cm2 of sample surface using a continuous raster scan. For a step raster scan, when the probed sample area is limited to the laser spot size, the detection limit is approximately tenfold higher (∼5 μg/cm2) due to the impact of UV photochemistry.

中文翻译：

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用于痕量检测的远距离深紫外拉曼光谱仪

我们开发了最先进的高灵敏度、低杂散光远距离深紫外 (DUV) 拉曼光谱仪，用于共振拉曼增强化学物质的痕量检测。作为拉曼测量的激发源，我们利用了最近开发的第二代小型化二极管泵浦固态掺钕原钒酸钆 (Nd:GdVO4）产生准连续波 228 nm 光的激光器。这种 228 nm 激发增强了 NO 振动的拉曼强度X爆炸性分子中的基团、生物分子中的芳香族基团以及各种芳香烃。我们的 DUV 拉曼光谱仪采用定制的 DUV f/8 卡塞格伦望远镜，配有直径约 200 毫米的主镜、高效 DUV 透射光栅、定制的 DUV 镜和定制的 228 nm 瑞利抑制滤波​​器。我们利用新型远距离 DUV 拉曼光谱仪来测量~50 μg/cm 的高信噪比光谱2滴投炸药：硝酸铵（AN）、三硝基甲苯、季戊四醇四硝酸酯以及芳香族生物分子：溶菌酶、色氨酸、酪氨酸、脱氧胞苷单磷酸、脱氧腺苷单磷酸，在10-30秒的积累时间内，距离约3 m。我们粗略估计炸药和生物分子相对均匀的滴铸薄膜的平均紫外共振拉曼 (UVRR) 检测限为 ∼1 μg/cm2当使用连续光栅扫描时，平均拉曼信号超过 ∼1 cm2样品区域，以避免由于紫外线 (UV) 光解导致分析物快速耗尽。我们确定了滴铸 AN 薄膜的 3 m 远距离 UVRR 检测限，并确定了影响 UVRR 检测限的因素，例如分析物光化学和分析物形态。我们发现检测限为 ∼0.5 μg/cm2当拉曼信号平均超过 ∼0.5 cm 时，适用于玻璃基板上的滴铸 AN 薄膜2使用连续光栅扫描对样品表面进行扫描。对于步进光栅扫描，当探测的样品区域仅限于激光光斑尺寸时，检测限大约高出十倍（∼5 μg/cm2）由于UV光化学的影响。