The Raman scattering of several liquids and solid materials has been investigated near the deep ultraviolet absorption features corresponding to the electron energy states of the chemical species present. It is found to provide significant enhancement, but is always accompanied by absorption due to that or other species along the path. We investigate this trade-off for water vapor, although the results for liquid water and ice will be quantitatively very similar. An optical parametric oscillator (OPO) was pumped by the third harmonic of a Nd:YAG laser, and the output frequency doubled to generate a tunable excitation beam in the 215-600 nm range. We use the tunable laser excitation beam to investigate pre-resonance and resonance Raman spectroscopy near an absorption band of ice. A significant enhancement in the Raman signal was observed. The A-term of the Raman scattering tensor, which describes the pre-resonant enhancement of the spectra, is also used to find the primary observed intensities as a function of incident beam energy, although a wide resonance structure near the final-state-effect related absorption in ice is also found. The results suggest that use of pre-resonant or resonant Raman LIDAR could increase the sensitivity to improve spatial and temporal resolution of atmospheric water vapor measurements. However, these shorter wavelengths also exhibit higher ozone absorption. These opposing effects are modeled using MODTRAN for several configurations relevant for studies of boundary layer water and in the vicinity of clouds. Such data could be used in studies of the measurement of energy flow at the water-air and cloud-air interface, and may help with understanding some of the major uncertainties in current global climate models.
We have measured UV resonance Raman near and at the resonance phonon-allowed absorption lines of several liquid species. Resonance absorption with excitation on the symmetry-forbidden but strongly phonon coupled bands in the 230- 290 nm spectral band present enhancement corresponding to the vapor phase absorptions rather than those of the liquid phase. This effect is related to the coherence forced by the internal molecular resonance required to absorb light at this energy. Large resonance gains (~3500x) reflect the narrower vapor phase lines. At the low laser fluence used, bubble formation is observed when the excitation energy corresponds to the maximum in Raman signal generation, not at the wavelength of maximum absorption in the liquid sample, which is several nanometers away.
We have measured UV resonance Raman scattering at and near the resonance absorption lines of liquid benzene and toluene. Resonance occurs for excitation on the symmetry-forbidden but strongly phonon coupled states in the 1B2u band, ~230-270 nm, resulting in enhancements corresponding to the vapor phase absorptions rather than those of the liquid phase. This effect is related to the coherence forced by the internal molecular resonance required to absorb light at this energy. The resonance gains (~1000x) are larger than expected due to the narrower vapor phase lines. Several multiplet and overtone modes are enhanced along with the strongly coupled ring-breathing mode. A contrasting case of resonance Raman of ice is also discussed; in this case resonance is observed for excitation energy corresponding to absorptions that depend upon the final state shielding by the neighbors, and corresponds with the solid phase absorption. This typifies the more common, slow, time dependence of the resonance Raman process.
Raman scattering techniques have long been used as unique identifiers for spectral fingerprints of chemical and
biological species. Raman lidar has been utilized on a routine basis to remotely measure several constituents in the
atmosphere. While Raman scattering is very reliable in uniquely identifying molecules, it suffers from very small
scattering cross sections that diminish its usefulness at increased ranges and decreased concentrations of the species of
interest. By utilizing a resonance Raman technique, where the laser excitation is tuned near an electronic absorption
band, it is possible to increase the Raman scattering cross section. An optical parametric oscillator (OPO) with a UV
tuning range of ~220 nm - 355 nm has been utilized to explore the wavelength dependence of Raman scattering for
diamond, water, benzene, and toluene. Resonance enhancements of the Raman spectra have been studied.
The description of radar propagation in the presence of the evaporation duct has proven to be a difficult problem in both littoral and open ocean environments. To properly characterize the propagation of a radar beam at low elevation angles, the evaporation duct must be located and scattering properties quantified. The two key elements defining an evaporation duct are the gradients in density and specific humidity. The gradients of the neutral density are determined from the rotational Raman temperature profile. The profile of water vapor is measured directly from the vibrational Raman scattered returns. High spatial resolution and high temporal resolution measurements of water vapor and temperature are required to accurately describe the evaporation duct. Raman lidar techniques can provide these measurements continuously with high accuracy and high resolution so the development of the evaporation duct can be studied. A detailed simulation of a Raman lidar has been developed and applied to a near horizontal path, to examine the expected accuracy for high vertical resolution profiles. The simulation also allows various atmospheric scenarios to be investigated and analyzed. The evaporation duct is an atmospheric phenomenon that causes radar propagation to remain trapped in the surface layer. The duct can be thought of as a waveguide that bends and reflects the radar beam along a path effectively trapping it and guiding it over long distances. This is a major problem for radar propagation paths in both littoral and open ocean environments. Moreover, ducting skews details of radar returns such that radar objects are hidden, or are detected at unexpected distances, or may appear with apparent cross-sections and speeds much different than their actual values.
Measurements obtained by the PSU Lidar Atmospheric Profile Sensor (LAPS) Raman lidar, during different periods, provide a comprehensive dataset to characterize cloud properties and aerosol distributions. The PSU Raman lidar measures the profiles of molecular nitrogen, molecular oxygen and the rotational Raman scatter (the mixture of all molecular species) at both visible and ultraviolet wavelengths, which are then used to generate vertical aerosol extinction profiles from the incremental extinction. Since the optical extinction at different wavelengths is strongly dependent on the size distribution of aerosols, variations in the profile of the size distribution can be inferred over an interesting range corresponding to accumulation mode particles, 50 nm to 1μm. The variation in the extinction profiles at different wavelengths is also used along with the water vapor profiles to observe the formation, growth and dissipation of cloud structures. The water vapor concentrations have been seen to decrease in regions surrounding a growing cloud as the particles increase in size by absorbing the water. Also, the water vapor concentration is found to increase as clouds begin to dissipate. The change in the size of the cloud particles during the different stages can also be observed in the multi-wavelength aerosol extinction. Results obtained from different locations, and for a wide range of atmospheric conditions, are used to compare and contrast the aerosol distributions and also to study the physical properties of clouds.