Comparison of Optical Pulse Propagation in Water and Acetonitrile

Abstract

Optical propagation in water has been studied primarily by light sources that have either a constant amplitude in time or have a modulated temporal width greater than a picosecond. Attenuation of light as it propagates through water has been experimentally measured throughout the optical spectrum with these light sources. Quantities such as absorption and scattering have been thoroughly characterized as a function of wavelength of light. Yet with this wealth of measurements, a model that describes the transparency of water in the visible region still does not exist. The reason has been attributed to the complexity in understanding the hydrogen bonding that occurs in water (1). Recently, an experimental study challenged the well established empirical knowledge of attenua- tion in water at near-infrared wavelengths. The study examined propagation of ultrafast optical pulses in water and demonstrated increased transmission of ultrafast light pulses as compared to longer pulses or cw light (2). In these experiments, pulses as short as 60fs with a center wavelength of 800nm were propagated through as much as 6 meters of water. To explain their observations, the authors suggested the possibility of light propagation by optical precursors. The question is: How and why were the optical precursors stimulated in the water? To better understand the propagation of ultrafast pulses of light in water, as demonstrated by Fox and ˜ Osterberg (2), we will compare measurements in water with those in acetonitrile. Both water and acetonitrile are polar solvents, but water molecules exhibit hydrogen bonding while acetonitrile molecules do not. In addition, this study will compare the difierences of the OH bond in water with the CH bond in acetonitrile. Harmonics of these molecular stretching frequencies in the near-infrared region are explored with a femtosecond laser system.