MadSci Network: Chemistry |
As is the case with many science questions, the answer is "... it depends." I'd like to generalize the question a little to include the possibility that the laser might pass through a flammable liquid, too. In general, if a laser "passes through" a liquid with little or no absorption or loss of intensity, nothing much happens. The beam is likely to be bent a little as it goes into and emerges from the liquid, due to the fact that the liquid refractive index will probably be somewhere between 1.3 and 1.6. And the position at which the beam focuses will change, too, for the same reason. But if the laser doesn't lose much intensity, that's probably about it... On the other hand, if the laser interacts strongly with the liquid a tremendous number of phenomena -- many weird and wonderful -- can occur, only a few of which I've listed below: If the beam is absorbed, the liquid will be heated and it will exhibit changes in refractive index due to the density gradients of the different temperature regions of the liquid. If you tried to look through the liquid, it would look "all wavy" and the beam emerging from the liquid -- if any emerges at al -- will also be "all wavy." If the beam is pretty high power and strongly absorbed, the liquid may even boil -- and could even form a steam bubble which is generated so rapidly that it looks like an explosion. More interesting things occur if the beam isn't actually absorbed, but interacts with the liquid through what are called non-linear processes. In the case of Raman scattering in a liquid made up of molecules which have several atoms bound together in their structure, a small fraction of the beam is converted to light which emerges at all angles (not just along the beam direction) but with a wavelength which is shifted by an amount which depends on the particular molecule involved. This is impossible to see under normal conditions, because the conversion efficiency is so small. However, if you use a lens to focus the beam into the liquid to increase the intensity, you can create Stimulated Raman Scattering (SRS) which produces a new beam (not just scattered radiation) at the shifted wavelength -- and you can convert 40 or 50% of the input power to the new wavelength! For example, if you put a focused beam of 0.53 micron green light from a frequency-doubled neodymium YAG laser through benzene or cyclohexane (both very flammable!), the output beam has both residual green and red light beams in it. And, if you focus still further in these liquids, a process called Stimulated Brillouin Scattering takes over and forms a mirror that reflects the input beam back on itself exactly -- not like a mirror, but in a way that causes the focusing beam to diverge into the lens that focuses it so that it's exactly reversed, in a phenomenon known as phase conjugation that looks a lot like "time reversal" (but it isn't). This SBS process can also be 30% or so efficient, and results from the interaction of sound waves generated in the liquid with the beam that created them. The SRS and SBS processes, and others like them, have been used in the past for a number of useful purposes. SRS has been used to convert the very non-eyesafe output of a 1.06 micron neodymium YAG laser to 1.54 microns -- which is relatively safe to observe at relatively high powers. SBS has been used to clean up the output of high power laser 1-pass amplifiers by using the process to "undo" the thermal perturbations imposed on the beam being amplified in the first pass by doing phase conjugation and picking off the final amplified beam after its return reflection through the amplifier. Interestingly, I haven't said much about flammability yet, because it's largely irrelevant. If you focus any really powerful (e.g., multi- megawatt in either short or long pulse) beam hard enough -- into a small enough spot -- either the temperature will rise to a very high value (e.g., 1000's of C) or the localized electrical fields due to the beam will get so high that "something bad" happens. The bad thing is generally boiling and/or decomposition of the material. If this occurs deep in the body of a large volume of an unconstrained liquid, the result is usually pretty non-catastropic: a gas bubble of some sort forms, rises to the surface, and pops. If it's big and the fluid is nasty, it can make a mess and be hazardous as the dickens if it pops in an inhabited area. With some liquids, I suppose it's possible that an explosive, exothermic detonation could occur -- but I can't think of any offhand. More likely, it you have the liquid in a closed or semi-closed container, the shock wave or pressure wave produced by the boiling or decomposition could break the container, exposing the high temperature fluid to air. In this case, if the fluid is flammable, the whole thing could go up in a raging fire if the liquid is above its flash point -- in fact, I did this once with a sealed cell of benzene and made an absolute mess of my lab with raging flames and hideous black smoke filling the room above my optics bench: stupid, embarrassing, dangerous. Alternatively, the broken container could release vapors of the flammable liquid into an area where the focused laser beam exists, again resulting in fire as the vapor suffers an ignition spark in the air/liquid mix soon after the mix is created: very bad, as I can attest from experience. Interestingly, these horrible fires occur in liquids which are quite flammable but which are also generally quite transparent to the laser light -- only when the beam is focused so that it decomposes the liquid or generates high temperatures in the extremely highly localized power (albeit slightly absorbed) focal region do bad things occur. This answer goes a little afield from your question, but I hope that it provides you with some view as to the complexities of light/matter interaction. I've been working with this lasers for about 25 years and have only begun to scratch the surface of the phenomena involved. Before I close, however, I'd like to add a note about lab safety. Unless you are using a very low power laser -- e.g., Class 1, usually under 1 mw - - it's mandatory that you use extreme care to avoid potential eye damage. Also, use of any liquid chemicals other than water should be approached with extreme caution, since many are toxic or carcinogenic -- obtain and consult the Material Safety Data Sheet (MSDS) associated with any materials you may be thinking about using and talk with a good chemist before you actually obtain the materials, to avoid difficulties such as the ones I referenced above.
Try the links in the MadSci Library for more information on Chemistry.