[peptideismylife]

Hi,

Does anybody knows the advantage of using laser instead of UV lamps during photolysis processes?

I guess that laser are more efficient...but does anybody knows why?

[Hunter2]

Wavelength of the light and energy power.

[peptideismylife]

wavelength of the light should depend on the lamp, and I guess that u can use different lamps both for UV lamps and laser. So I don't think that advantage of laser is because of the wavelength...but maybe I am wrong.

[Enthalpy]

Lasers are very inefficent, that's an excellent not to use them for photochemical reactions, since they need to demand a huge amount of light. The best lasers are pumping diodes, with 80% quantum efficiency but 1/2 voltage efficiency, which can provided hundreds or tens of thousands of watts if you pay; these may outperform lamps, but all others are bad, like 10% for CO2 lasers (unusable because of 10.6µm wavelength), 1% for few others, and ppm for most of them.

Dye lasers' wavelength can be fine-tuned to match the reaction, but their power is low.

Any gas discharge lamp outperforms the efficiency and power of a laser. Especially Hg lamps give energetic photons, if needed in UV. But the choice of wavelength is coarse.

Then you have the worry of short wavelengths. At 405nm you get 20mW from DVD laser diodes, at 254nm there are some Hg lamps, and then it's over. Excimer lasers take a room to produce 50W. Doubled and tripled lasers output µW power.

The standard answer is to use a "sensitizer", which enables a too low photon energy.

[fledarmus]

The reason you are using a light source is that there is an electronic transition required for the reaction to proceed. Since energy is quantized, the transition will only absorb photons of the wavelength that provide that amount of energy.

If you are using a tunable laser, you can tune it to just the right wavelength to cause the transition. Every photon produced by the laser will have the appropriate energy to cause the transition you require.

If you are using a broad-band light source, some of the photons will not have enough energy to cause the transition.

So in one sense, the laser is more efficient - every photon going through the reaction will have enough energy to cause the reaction to proceed. In another sense, though, you have to look at the total energy required to generate the laser beam compared to the UV light and the size of the reaction that can be illuminated by the laser. It might be more efficient in terms of total energy or money to just use a broad-band source of illumination.

[Enthalpy]

UV lamps are better than I thought. Excimer improve over the old mercury discharge: shorter monochromatic wavelengths, more varied, some more efficient, and BIG power like 3kWe...
http://old.iupac.org/goldbook/ET07372.pdf