Meyer Terrence R, Roy Sukesh, Gord James R
Department of Mechanical Engineering, Iowa State University, Ames, IA 50011, USA.
Appl Spectrosc. 2007 Nov;61(11):1135-40. doi: 10.1366/000370207782596996.
There is growing interest in the use of short-pulse lasers for coherent anti-Stokes Raman scattering (CARS) to minimize non-resonant background (NRB) contributions in a variety of applications. Using time-coincident picosecond (ps) pump and Stokes beams and a time-delayed ps probe beam, we show that a three orders of magnitude reduction in NRB interference can be achieved in rich hydrocarbon-air flames while preserving 60% to 80% of the CARS signal. This represents a significant improvement in signal-to-interference ratio compared with previous measurements in room temperature air and is attributable to reduced rates of collisional dephasing and relaxation at flame temperatures. Measurements within the flame zone of a laminar flat-flame burner are used to investigate the characteristics of time-coincident and probe-delayed broadband ps N(2)-CARS spectra for C(2)H(4)-air equivalence ratios of 0.5 to 1.2. Up to three ro-vibrational bands of N(2) are excited with each laser shot using 135 ps pump and 106 ps Stokes beams, and the CARS signal is generated using a 135 ps probe beam delayed by 165 ps. The enhanced signal-to-interference ratio achieved in the current work is one to two orders of magnitude higher than that previously achieved using polarization-selection techniques without sensitivity to the effects of birefringence caused by density gradients or test cell windows. Moreover, the use of a 135 ps laser source in this study enables frequency domain "broadband" CARS with sufficient resolution to extract ro-vibrational spectral features under various flame conditions. The effect of probe delay and NRB suppression on characteristics of these broadband CARS spectra are investigated, and evidence of preferential collisional dephasing and relaxation of different ro-vibrational transitions is not detected. This is a promising but preliminary result to be investigated further in future work.
在各种应用中，使用短脉冲激光进行相干反斯托克斯拉曼散射（CARS）以最小化非共振背景（NRB）贡献的兴趣与日俱增。通过使用时间重合的皮秒（ps）泵浦光和斯托克斯光以及时间延迟的ps探测光，我们表明，在富烃空气火焰中，NRB干扰可降低三个数量级，同时保留60%至80%的CARS信号。与之前在室温空气中的测量相比，这代表着信干比有了显著提高，这归因于火焰温度下碰撞退相和弛豫速率的降低。利用层流平焰燃烧器火焰区内的测量结果，研究了时间重合和探测延迟的宽带ps N(2)-CARS光谱在C(2)H(4)-空气当量比为0.5至1.2时的特性。每次激光脉冲使用135 ps泵浦光和106 ps斯托克斯光激发多达三个N(2)的转动-振动带，并使用延迟165 ps的135 ps探测光产生CARS信号。当前工作中实现的增强信干比比之前使用偏振选择技术实现的高出一到两个数量级，且对由密度梯度或测试池窗口引起的双折射效应不敏感。此外，本研究中使用135 ps激光源能够实现频域“宽带”CARS，具有足够的分辨率以在各种火焰条件下提取转动-振动光谱特征。研究了探测延迟和NRB抑制对这些宽带CARS光谱特性的影响，未检测到不同转动-振动跃迁优先碰撞退相和弛豫的证据。这是一个有前景但仍需在未来工作中进一步研究的初步结果。
